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Organophotoredox-Catalyzed Intermolecular Oxa-[4+2] Cycloaddition Reactions

  • Kenta Tanaka
    Kenta Tanaka
    Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
    More by Kenta Tanaka
  • Daichi Omata
    Daichi Omata
    Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
    More by Daichi Omata
  • Yosuke Asada
    Yosuke Asada
    Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
    More by Yosuke Asada
  • Yujiro Hoshino*
    Yujiro Hoshino
    Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
    *E-mail: [email protected] (Y.H.).
  • , and 
  • Kiyoshi Honda*
    Kiyoshi Honda
    Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
    *E-mail: [email protected] (K.H.).
Cite this: J. Org. Chem. 2019, 84, 17, 10669–10678
Publication Date (Web):July 19, 2019
https://doi.org/10.1021/acs.joc.9b01156

Copyright © 2019 American Chemical Society. This publication is licensed under CC-BY-NC-ND.

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Abstract

An intermolecular oxa-[4+2] cycloaddition reaction promoted by a thioxanthylium photoredox catalyst under irradiation with green light has been developed. The reaction of ortho-quinone methides with styrenes smoothly affords the desired cycloadducts. Especially styrenes bearing electron-donating groups are efficiently transformed in this reaction. This method represents a sustainable way to carry out oxa-[4+2] cycloaddition reactions using only a catalytic amount of a photocatalyst and visible light.

Introduction

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Photoinduced chemical transformations represent a powerful synthetic strategy to generate heterocycles and polycyclic compounds. (1) Over the past few decades, O-heterocycles have attracted increasing interest due to the relevance of their structural units in medicinal chemistry, materials science, and natural products. Among the variety of synthetic routes to O-heterocycles, the oxa-[4+2] cycloaddition reaction is particularly attractive on account of its high atom economy and as it provides efficient access to various synthetically useful heterocycles. Current reaction methods focus mainly on the use of high-temperature conditions or the presence of catalytic/stoichiometric amounts of metals (Scheme 1a). (2,3) On the other hand, only few photoinduced reactions have been reported and these require high-energy (ultraviolet) light sources. (4) Therefore, the development of cost-effective and milder synthetic routes to O-heterocycles remains highly desirable, especially when a low environmental impact is targeted.

Scheme 1

Scheme 1. Representative Examples of Oxa-[4+2] Cycloaddition Reactionsa

aPMP: p-methoxyphenyl.

To overcome these shortcomings, organophotoredox-promoted chemical transformations using visible light as an energy source represent a promising approach. (5) A variety of organophotoredox catalysts, including Acr+-Mes, eosin Y, 4CzIPN, DPZ, and TPT, have been employed in various reactions. (5,6) Recently, we have reported the design and synthesis of thioxanthylium organophotoredox catalysts, which are active under irradiation with green light. (7) During the course of that study, we found that these thioxanthylium photocatalysts efficiently oxidize styrene derivatives such as trans-anethole and promote radical cation Diels–Alder reactions. Among the numerous photoredox reactions, hetero-[4+2] cycloadditions have been reported by several groups. (8) However, most of these reactions are aza-[4+2] cycloadditions, while only a few examples of oxa-[4+2] cycloadditions exist. Recently, the intramolecular oxa-[4+2] cycloaddition of tethered bis(enones) in the presence of a Ru photocatalyst has been reported by Yoon and co-workers. (9) Yet, to the best of our knowledge, studies on intermolecular oxa-[4+2] cycloadditions catalyzed by photoredox chemistry have not been reported. Thus, the development of efficient synthetic strategies for catalytic oxa-[4+2] cycloadditions remains an attractive research target.
ortho-Quinone methides are key reactive intermediates for oxa-cyclic compounds such as benzopyrans and benzofurans. (10) Based on the varied reactivity of ortho-quinone methides, a series of attractive strategies have been developed. (10,11) On the other hand, the hitherto reported examples involving photoirradiation require the use of high-energy irradiation such as ultraviolet light. (4,12) Recently, we have developed the acid-catalyzed generation of ortho-quinone methides from salicylaldehyde in the presence of trimethyl orthoformate. (13) During the course of this study, we found that the inverse-electron-demand [4+2] cycloaddition reaction of 1,1-diphenylethylenes with in situ generated electron-withdrawing ortho-quinone methides affords a series of 2-phenylchromanes. (14) Based on the above and to expand the generality of oxa-[4+2] cycloaddition reactions, we herein report the thioxanthylium organophotoredox-catalyzed intermolecular oxa-[4+2] cycloaddition reaction of ortho-quinone methides under irradiation with green light (Scheme 1b).

Results and Discussion

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Initially, we screened the reaction of ortho-quinone methide (1a) with 4-methoxystyrene (2a) in the presence of the thioxanthylium catalyst (TXT) at room temperature (Table 1). Medium to nonpolar solvents, such as tetrahydrofuran (THF), toluene, and 1,2-dichloroethane (DCE), afforded the desired product (3a) in moderate yield (entries 1–3), while polar solvents such as CH3NO2, CF3CH2OH, and AcOEt provided the desired product in high yield (entries 4–6). In particular, AcOEt effectively increased the product yield to 88% (cis/trans = 3:1). Although solvent effects are not obvious, polar solvents may stabilize potentially emerging radical cation intermediates. (15) Fortunately, the adduct 3a was crystallized to give a single crystal suitable for X-ray diffraction analysis, which allowed to determine the relative stereochemistry of chromane 3a as cis form. (16) Eosin Y and rose bengal are the most widely used organic photoredox catalysts under green light irradiation, and these can be employed in a wide variety of organic transformations. (17) However, using these catalysts under the optimal conditions established in this study was not successful due to the low reduction potentials of their excited states [E (C*/C) = +0.81 and +0.88 V vs SCE, respectively] compared to that of 2a (Eox = +1.47 V vs SCE) (entries 7 and 8). (18) Acr+-Mes and TPT, which are typical organophotoreodox catalysts having high excited-state reduction potentials, are also not suitable for this reaction due to their inefficient absorption of green light and low solubility in AcOEt (entries 9–12). On the other hand, when CH3NO2 was used as a solvent, which can dissolve Acr-Mes and TPT, the desired product was obtained in 32 and 34% yields with moderate diastereoselectivities, respectively (entries 13 and 14). Then, it is found that representative photoredox catalysts having high excited-state reduction potentials can also be applied to the reaction while TXT can be dissolved in various solvents and can use AcOEt as a solvent. All blank experiments, i.e., in the absence of a catalyst, light source, or air, afforded low yields of 3a (entries 15–17). Based on these results, we concluded that the combination of the photoredox catalyst TXT with a light source in the presence of air effectively promotes the present [4+2] cycloaddition. The reaction could be performed on a large scale to furnish the product in moderate yield (entry 18). When TEMPO was used as a radical scavenger, the reaction did not proceed, suggesting that the transformation could occur via a radical mechanism (entry 19). Interestingly, the reaction was carried out at reflux in the absence of catalyst and green light to afford the product in low yield (entry 20). Thus, the photocatalytic system has good advantages to afford the oxa-[4+2] cycloadducts under mild conditions.
Table 1. Optimization of the Reaction Conditions for the Catalytic Intermolecular Oxa-[4+2] Cycloadditiona
entrycatalystsolventyield (%)
1TXTTHF35 (dr 4:1)
2TXTtoluene37 (dr 3:1)
3TXTDCE60 (dr 3:1)
4TXTCH3NO282 (dr 5:1)
5TXTCF3CH2OH86 (dr 6:1)
6TXTAcOEt88 (dr 3:1)
7eosin YAcOEt0
8rose bengalAcOEt0
9Acr+-MesAcOEt0
10TPTAcOEt0
11bAcr+-MesAcOEt0
12bTPTAcOEt0
13bAcr+-MesCH3NO232 (dr 5:1)
14bTPTCH3NO240 (dr 5:1)
15no catalystAcOEt10
16cTXTAcOEt9
17dTXTAcOEt13
18eTXTAcOEt46
19fTXTAcOEt0
20gno catalystAcOEt15 (dr 7:1)
a

All reactions were carried out for 3 h using 1a (0.375 mmol), 2a (0.125 mmol), and TXT (1.0 mol %) in the specified solvent (2.0 mL) at room temperature under green light irradiation.

b

Blue light was used as a light source.

c

No light.

d

Under N2.

e

1a (15.3 mmol) and 2a (5.1 mmol) were used as substrates.

f

Tempo (3.0 equiv) was added to the reaction.

g

The reaction temperature was increased to reflux in the absence of catalyst and green light irradiation. PMP: p-methoxyphenyl.

With the optimized conditions in hand, we next examined the reaction of various ortho-quinone methides and alkenes (Table 2). Dienophiles bearing ethoxy, isopropoxy, or tert-butyl-dimethyloxy moieties were well tolerated in this reaction (3bd). However, a benzyloxy-functionalized derivative afforded the corresponding product in poor yield, while the starting material was recovered in 62% yield (3e). We have rationalized this in terms of the mild electron-donating properties of the benzyloxy substituent, which may not sufficiently promote the oxidation of the dienophile. On the other hand, when the catalyst loading was increased to 5.0 mol % and the reaction time prolonged to 24 h, the product yield improved to 66%, which means that the dienophile is effectively oxidized by the photocatalyst. A dibenzyloxy-functionalized derivative also readily underwent this transformation (3f). Dienophiles with multiple methoxy groups (3gj) were well tolerated in this reaction. Moreover, 1,1-diaryl-functionalized substrates readily afforded the corresponding products (3k–l). Importantly, 1,1-diphenylethylene did not engage in the reaction, despite having a lower oxidation potential (Eox = +1.54 V vs SCE) (19) than the photocatalyst TXT (E0′ (C*/C) = +1.76 V vs SCE). This result indicates that electron-donating groups effectively increase the stability of radical cation intermediates (3m). (20) In contrast, the reaction with styrene, which exhibits a relatively high oxidation potential (Eox = +1.97 V vs SCE), (19) did not proceed due to the lower reduction potential of the excited state of TXT (3n). Ethoxy vinyl ether and phenyl vinyl ether also afforded the corresponding products in moderate yields (3o and 3p). Moreover, ortho-quinone methides bearing dimethoxy or diethoxy groups were suitable for this reaction (3qs). We also found that this reaction can be applied to various types of styrenes, such as mono-, di-, and tri-substituted styrenes.
Table 2. Scope of the TXT-Catalyzed Intermolecular Oxa-[4+2] Cycloaddition
a

TXT (5.0 mol %) was used and the reaction was conducted for 24 h. PMP: p-methoxyphenyl.

A plausible reaction mechanism for organophotoredox-catalyzed intermolecular oxa-[4+2] cycloaddition is represented in Scheme 2. Excitation of the photocatalyst (PC*; E0′ (C*/C) = +1.76 V vs SCE) (18) under irradiation with visible light enables the oxidation of 4-methoxystyrene 2a (Eox = +1.47 V vs SCE). According to Stern–Volmer experiments (Figure 1), the electron transfer from 4-methoxystyrene to the photocatalyst should occur smoothly. Since the reaction seems to require oxygen to promote the catalytic cycle (Table 1, entry 12), the reduced photocatalyst (PC•–) would be regenerated into the original photocatalyst (PC) via single electron transfer from O2. ortho-Quinone methide 1a could undergo an oxa-[4+2] cycloaddition with the resulting radical cation A via an endo-TS, thus furnishing radical cation B. (21) According to previous reports, (20) an aromatic substituent with an electron-donating group on the dienophile would effectively increase the stability of such a radical cation intermediate. Finally, transfer of a single electron from the superoxide radical (O2•–), 4-methoxystyrene 2a, or the reduced photocatalyst (PC•–) would afford the desired cycloadduct 3a. Since the reaction does not proceed efficiently under N2 (Table 1, entry 12), O2 must be a key mediator in this reaction. To confirm the radical chain processes in the catalysis, we determined the quantum yield of the reaction (0.21). (22) Accordingly, it seems likely that a closed radical mechanism is the dominating reaction pathway, even though the possibility of a radical chain propagation mechanism cannot be excluded at this point.

Figure 1

Figure 1. Stern–Volmer plots for 4-methoxystyrene 2a (blue line) and ortho-quinone methide 1a (red line).

Scheme 2

Scheme 2. Proposed Reaction Mechanisma

aPMP: p-methoxyphenyl.

Conclusions

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We have developed an oxa-[4+2] cycloaddition reaction that is promoted by an organophotoredox catalyst under green light irradiation. When ortho-quinone methides are reacted with styrenes in the presence of a TXT photoredox catalyst under green light irradiation, the reaction smoothly affords the desired cycloadducts in good yield. In particular, styrenes bearing electron-donating groups such as methoxy moieties are efficiently transformed in this reaction. The reaction can also be applied to mono- and disubstituted ethylenes. The present transformation thus provides a sustainable approach to oxa-[4+2] cycloaddition reactions using only catalytic amounts of a photoredox catalyst and visible light.

Experimental Section

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General Information

Infrared (IR) spectra were recorded on a JASCO FT/IR-4100. 1H NMR spectra were recorded on a JEOL ECA-500 (500 MHz) spectrometer or a Bruker DRX-500 (500 MHz) spectrometer with tetramethylsilane (TMS) as internal standard. Chemical shifts are reported in ppm from TMS. Data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet), coupling constants, integration. 13C NMR spectra were recorded on a Bruker DRX-500 (126 MHz) spectrometer with complete proton decoupling. Chemical shifts are reported in ppm from TMS with the solvent resonance as the internal standard (CDCl3: δ 77.0). 19F NMR spectra were recorded on a JEOL JNM AL-400 (376 MHz) spectrometer with hexafluorobenzene (C6F6: δ −164.9) as internal standard. High-resolution mass spectra were obtained with Hitachi Nanofrontier LD Spectrometer (ESI/TOF). Column chromatography was carried out with Cicareagent silica gel 60 N (spherical, particle size 63–210 mm). Thin-layer chromatography (TLC) was carried out with Merck TLC plates with silica gel 60 F254. Unless otherwise noted, reagents were commercially available and were used without purification. Single-crystal X-ray diffraction analysis was performed at 223 K using a Rigaku XtaLAB P200 diffractometer with a graphite monochromatic Cu Kα radiation source (l 1/4 1.54187 Å). The UV absorption spectra were measured with a JASCO V-630 spectrometer. The fluorescence spectra were obtained on a JASCO FP-8500 spectrofluorometer, which was used as the light source for the reaction quantum yield measurement. Cyclic voltammetry measurements were carried out with a computer-controlled potentiostat Model 660C (ALS Co., Ltd.). Photochemical reaction was carried out in the borosilicate vial under visible light by a Beamtec 7 W Green LED (LDA7G-C50) or a Beamtec 7 W Blue LED (LB1526B) at room temperature. The sample was placed at an approximate distance of 5 cm from the lamp. The emission spectrum of the LED was measured with a miniature fiber-optic spectrometer (FLAME-S-XR1-ES, Ocean Optics).

General Procedure for the Synthesis of Starting Materials

For the Synthesis of 2-(4-Methoxybenzyl)-4,5-methylene-dioxyphenol (23)

To a solution of ascorbic acid (925 mg, 5.25 mmol) in an aqueous solution of citric acid (90 mL, 2%) were added sesamol (4.96 g, 35.9 mmol) and 4-methoxybenzyl alcohol (4.96 g, 35.9 mmol). The mixture was heated to reflux for 17 h. Upon cooling, crystals precipitated from the reaction mixture. Filtration of the precipitate followed by recrystallization from toluene formed the desired pure product (7.75 g, 84% yield). 1H NMR (500 MHz, CDCl3): δ 7.12 (d, J = 9.0 Hz, 2H), 6.84 (d, J = 8.5 Hz, 2H), 6.58 (s, 1H), 6.39 (s, 1H), 5.87 (s, 2H), 4.49 (s, 1H), 3.82 (s, 2H), 3.77 (s, 3H).

For the Synthesis of 6-(4-Methoxybenzylidene)-3,4-methylenedioxy-cyclohexa-2,4-dienone (1a) (23)

A solution of 2-(4-methoxybenzyl)-4,5-methylenedioxyphe-nol (3.88 g, 15 mmol) in ether (300 mL) was heated under reflux with silver oxide (11.6 g, 50 mmol) for 3.5 h and filtered. The orange crystals were separated by filtration. The solution was concentrated to 150 mL, cooled, and the colored product was collected. The ether filtrate was diluted to 200 mL and treated once again with silver oxide (5.8 g, 25 mmol) for 2 h to give an additional quantity of the orange product (1.56 g, 41% yield). 1H NMR (500 MHz, CDCl3): δ 7.89 (s, 1H), 7.49 (d, J = 8.5 Hz, 2H), 6.96 (d, J = 9.0 Hz, 2H), 6.71 (s, 1H), 5.97 (s, 1H), 5.89 (s, 2H), 3.86 (s, 3H).

For the Synthesis of 4,5-Dimethoxy-2-(4-methoxybenzyl)phenol (23)

To a solution of ascorbic acid (430 mg, 2.44 mmol) in an aqueous solution of citric acid (45 mL, 2%) were added 3,4-dimethoxyphenol (2.44 g, 15.8 mmol) and 4-methoxybenzyl alcohol (2.18 g, 15.8 mmol). The mixture was heated to reflux for 17 h. When the reaction solution was decanted, an oily product was obtained. The crude was chromatographed by silica gel column to give a brown solid (4.32 g, 99% yield). 1H NMR (500 MHz, CDCl3): δ 7.13 (d, J = 14.5 Hz, 2H), 6.84 (d, J = 14.5 Hz, 2H), 6.64 (s, 1H), 6.45 (s, 1H), 4.52 (s, 1H), 3.87 (s, 2H), 3.82 (s, 3H), 3.80 (s, 3H), 3.78 (s, 3H).

For the Synthesis of 3,4-Dimethoxy-6-(4-methoxyphenyl-methylidene)cyclohexa-2,4-dien-1-one (23)

To a solution of 4,5-dimethoxy-2-(4-methoxybenzyl)phenol (4.32 g, 15.8 mmol) in ether (262 mL) was added silver oxide (14.6 g, 63 mmol) and stirred for 12 h. The solution was filtered, then the filtrate was washed with dichloromethane. The solvent was removed under reduced pressure, and red crystals were collected (2.14 g, 50% yield). 1H NMR (500 MHz, CDCl3): δ 7.87 (s, 1H), 7.53 (d, J = 9.0 Hz, 2H), 6.98 (d, J = 9.0 Hz, 2H), 6.52 (s, 1H), 5.85 (s, 1H), 3.87 (s, 3H), 3.86 (s, 3H), 3.82 (s, 3H).

For the Synthesis of 4,5-Diethoxy-2-(4-methoxybenzyl)-phenol

To a solution of ascorbic acid (157 mg, 0.89 mmol) in an aqueous solution of citric acid (20 mL, 2%) were added 4-methoxybenzyl alcohol (764 μL, 6.15 mmol) and 3,4-diethoxyphenol (1.12 g, 6.15 mmol). The reaction was heated to reflux for 24 h. On cooling, an oily product was obtained. The crude was chromatographed by silica gel column to give yellow oil (1.13 g, 61% yield). 1H NMR (500 MHz, CDCl3): δ 7.10 (d, J = 9.0 Hz, 2H), 6.79 (d, J = 8.5 Hz, 2H), 6.63 (s, 1H), 6.34 (s, 1H), 5.69 (s, 1H), 3.95 (q, J = 7.0 Hz, 2H), 3.82 (m, 4H), 3.73 (s, 3H), 1.32 (t, J = 7.0 Hz, 3H), 1.28 (t, J = 7.0 Hz, 3H); 13C{1H} NMR (126 MHz, CDCl3): δ 158.2, 148.5, 148.1, 142.4, 131.8, 129.4, 118.1, 117.6, 114.1, 102.9, 65.8, 64.5, 55.3, 35.2; IR (ATR): 3451, 2978, 1509, 1193, 1103, 1032, cm–1; HRMS (ESI+) m/z calcd for C18H22O4Na ([M + Na]+): 325.1410, found: 325.1394.

For the Synthesis of 3,4-Diethoxy-6-(4-methoxyphenyl-methylidene)cyclohexa-2,4-dien-1-one

To a solution of 4,5-diethoxy-2-(4-methoxybenzyl)phenol (500 mg, 1.65 mmol) in ether (25 mL) was added silver oxide (1.5 g, 6.47 mmol), then stirred for 12 h. The solution was filtered, and the solvent was removed under reduced pressure. Recrystallization from dichloromethane formed the desired pure product (220 mg, 46% yield). 1H NMR (500 MHz, CDCl3): δ 7.84 (s, 1H), 7.51 (d, J = 8.5 Hz, 2H), 6.97 (d, J = 8.5 Hz, 2H), 6.49 (s, 1H), 5.81 (s, 1H), 4.04 (q, J = 7.0 Hz, 2H), 3.98 (q, J = 7.0 Hz, 2H), 1.48–1.43 (m, 6H); 13C{1H} NMR (126 MHz, CDCl3): δ 185.6, 163.7, 160.6, 148.1, 140.1, 132.4, 129.3, 128.5, 114.4, 104.5, 102.1, 64.8, 64.1, 55.4, 14.3, 14.0; IR (ATR): 2977, 1508, 1172, 1104, 1030, 570 cm–1; HRMS (ESI+) m/z calcd for C18H21O4 ([M + H]+): 301.1434, found: 301.1424.

For the Synthesis of 9-(o-Tolyl)-1,3,6,8-tetra-methoxythioxanthylium trifluoromethanesulfonate (7,24)

(1)

(25) To a solution of 1-bromo-3,5-dimethoxybenzene (4.3 g, 20 mmol) in THF (30 mL) at −78 °C was dropwise added tert-butyllithium (34 mL, 50 mmol, 1.49 M in hexane). After the reaction was stirred at −78 °C for 1 h, a solution of iodine (7.6 g, 60 mmol) in THF (20 mL) was added via cannula. The resulting mixture was stirred at −78 °C for 1 h and then was warmed to room temperature, neutralized with water, and diluted with CH2Cl2. The organic layer was washed with saturated aqueous Na2S2O3 and brine. It was dried over MgSO4, filtered, and concentrated in vacuo. The crude mixture was purified by flash chromatography (hexane/ethyl acetate = 10:1) to afford 1-iodo-3,5-dimethoxybenzene (4.70 g, 89% yield) as a white solid.

1H NMR (500 MHz, CDCl3): δ 6.85 (d, J = 2.3 Hz, 2H), 6.40 (t, J = 2.3 Hz, 1H), 3.76 (s, 6H).

(2)

(26) A mixture of 1-iodo-3,5-dimethoxybenzene (6.6 g, 25 mmol), carbon disulfide (1.5 mL, 25 mmol), CuI (480 mg, 2.5 mmol), and DBU (7.5 mL, 50 mmol) in toluene (40 mL) was stirred under N2 at reflux for 12 h. After H2O was added, the solution was extracted with CH2Cl2. The organic layer was dried over MgSO4 and concentrated in vacuo. The crude was purified by column chromatography on silica gel (hexane/ethyl acetate = 10:1) to provide bis(3,5-dimethoxypheny)-sulfane (2.38 g, 62% yield).

1H NMR (500 MHz, CDCl3): δ 3.74 (s, 12H), 6.34 (t, J = 2.3 Hz, 2H), 6.52 (d, J = 2.3 Hz, 4H).

(3)

(7) A solution of bis(3,5-dimethoxyphenyl)sulfane (75 mg, 0.25 mmol) and benzoyl chloride (98 μL, 0.75 mmol) in chlorobenzene (5.0 mL) was placed in a 50 mL recovery flask under N2. Trifluoromethanesulfonic acid (66 μL, 0.75 mmol) was slowly added to the solution, which was heated to 120 °C for 2 h. It was cooled to room temperature and excess Et2O was added to precipitate a solid. After stirring for 1 h, the mixture was filtered. The solid was washed with Et2O and dried in vacuo, affording 9-(2-methylphenyl)-1,3,6,8-tetramethoxy-thioxanthylium trifluoromethanesulfonate (TXT). Brown solid (110.7 mg, 80% yield).

1H NMR (500 MHz, CDCl3): δ 7.50 (d, J = 2.2 Hz, 2H), 7.28 (dd, J = 7.3, 1.3 Hz, 1H), 7.26–7.23 (m, 1H), 7.23–7.17 (m, 1H), 6.73 (d, J = 7.3 Hz, 1H), 6.54 (d, J = 2.2 Hz, 2H), 4.15 (s, 6H), 3.40 (s, 6H), 2.03 (s, 3H); 13C{1H} NMR (126 MHz, CDCl3): δ 168.3, 166.0, 165.5, 147.7, 142.1, 134.2, 128.0, 127.5, 125.1, 124.0, 116.7, 102.0, 101.3, 57.7, 56.9, 20.1; 19F NMR (376 MHz, CDCl3): δ −81.3; IR (ATR): 1584, 1220, 1151, 1028, 637 cm–1; HRMS (ESI+) m/z calcd for C24H23O4S+: 407.1312, found: 407.1324.

General Procedure for [4+2] Cycloaddition with Photocatalyst under Visible Light Irradiation

The alkene (2) (0.125 mmol), o-quinone methide (1) (0.375 mmol), TXT (1.0 mol %), and AcOEt (2.0 mL) were added into a 8 mL borosilicate vial. The resulting solution was stirred at room temperature under air and green LED irradiation. The desired cycloadduct (3) was isolated by column chromatography on silica gel.

Procedure for [4+2] Cycloaddition with Photocatalyst under Visible Light Irradiation in Large-Scale Reaction

o-Quinone methide (1a) (3.93 g, 15.3 mmol), 4-methoxystyrene (2a) (0.69 g, 5.1 mmol), TXT (0.029 g, 1.0 mol %), and AcOEt (82 mL) were added into a 200 mL recovery flask. The resulting solution was stirred at room temperature under air and irradiation with two 7 W green LEDs. After the reaction was stirred for 72 h, the solution was concentrated by rotary evaporation. The resulting residue was purified by column chromatography on silica gel (hexane/ethyl acetate = 10:1) to afford the desired product 3a (0.92 g, 46% yield, dr 3:1) as a yellow solid.

(6S*,8R*)- and (6R*,8R*)-6,8-bis(4-Methoxyphenyl)-7,8-dihydro-6H-[1,3]dioxolo[4,5-g]chromene (3a)

4-Methoxystyrene (16.5 mg, 0.123 mmol), o-quinone methide 1a (96.0 mg, 0.375 mmol), TXT (0.71 mg, 0.00125 mmol), and AcOEt (2.0 mL) were used. Yellow solid (42.7 mg, 88% yield, dr 3:1). 1H NMR (500 MHz, CDCl3) (major): δ 7.37 (d, J = 9.0 Hz, 2H), 7.12 (d, J = 9.0 Hz, 2H), 6.91 (d, J = 9.0 Hz, 2H), 6.85 (d, J = 8.5 Hz, 2H), 6.46 (s, 1H), 6.21 (s, 1H), 5.82 (dd, J = 8.0, 1.5 Hz, 2H), 5.05 (dd, J = 11.5, 1.5 Hz, 1H), 4.19 (dd, J = 12.0, 6.5 Hz, 1H), 3.80 (s, 3H), 3.79 (s, 3H), 2.33–2.29 (m, 1H), 2.21–2.14 (m, 1H); (minor): δ 7.24 (d, J = 9.0 Hz, 2H), 7.06 (d, J = 9.0 Hz, 2H), 6.87–6.84 (m, 4H), 6.52 (s, 1H), 6.39 (s, 1H), 5.87 (dd, J = 4.0, 1.0 Hz, 2H), 4.91 (dd, J = 10.5, 2.0 Hz, 1H), 4.06 (dd, J = 5.5, 3.0 Hz, 1H), 3.79 (s, 3H), 3.78 (s, 3H), 2.41–2.36 (m, 1H), 2.13–2.09 (m, 1H); 13C{1H} NMR (126 MHz, CDCl3) (mixture): δ 159.4, 159.2, 158.4, 158.1, 150.4, 150.2, 147.1, 146.6, 141.6, 141.5, 138.4, 136.8, 133.4, 133.3, 129.5, 129.3, 127.5, 127.4, 117.6, 114.8, 114.1, 113.9, 113.8, 113.8, 109.0, 108.4, 100.9, 100.8, 98.5, 98.5, 77.9, 72.8, 55.3, 55.3, 55.2, 55.2, 42.7, 40.6, 39.6, 38.3; IR (ATR) (mixture): 2918, 1612, 1476, 1253, 1146, 1030, 830, 543 cm–1; HRMS (ESI+) m/z calcd for C24H23O5 ([M + H]+): 391.1540, found: 391.1523.

(6S*,8R*)- and (6R*,8R*)-6-(4-Ethoxyphenyl)-8-(4-methoxyphenyl)-7,8-dihydro-6H-[1,3]dioxolo[4,5-g]-chromene (3b)

4-Ethoxystyrene (18.5 mg, 0.125 mmol), o-quinone methide 1a (93.0 mg, 0.363 mmol), TXT (0.74 mg, 0.00133 mmol), and AcOEt (2.0 mL) were used. Yellow oil (50.6 mg, >99% yield, dr 2.5:1). 1H NMR (500 MHz, CDCl3) (major): δ 7.36 (d, J = 8.5 Hz, 2H), 7.12 (d, J = 8.5 Hz, 2H), 6.90 (d, J = 8.5 Hz, 2H), 6.85 (d, J = 8.5 Hz, 2H), 6.46 (s, 1H), 6.21 (s, 1H), 5.82 (dd, J = 8.5, 1.5 Hz, 2H), 5.05 (d, J = 10.5 Hz, 1H), 4.19 (dd, J = 11.5, 6.0 Hz, 1H), 4.03 (q, J = 7.0 Hz, 2H), 3.79 (s, 3H), 2.33–2.29 (m, 1H), 2.22–2.14 (m, 1H), 1.40 (t, J = 6.5 Hz, 3H); (minor): δ 7.22 (d, J = 9.0 Hz, 2H), 7.06 (d, J = 9.0 Hz, 2H), 6.85 (d, J = 8.5 Hz, 2H), 6.85 (d, J = 8.5 Hz, 2H), 6.52 (s, 1H), 6.39 (s, 1H), 5.87 (d, J = 3.0 Hz, 2H), 4.90 (d, J = 8.5 Hz, 1H), 4.11 (dd, J = 13.0, 6.5 Hz, 1H), 4.01 (q, J = 7.0 Hz, 2H), 3.79 (s, 3H), 2.41–2.35 (m, 1H), 2.13–2.04 (m, 1H), 1.39 (t, J = 6.5 Hz, 3H); 13C{1H} NMR (126 MHz, CDCl3) (mixture): δ 158.8, 158.6, 158.4, 158.1, 150.4, 150.2, 147.1, 146.6, 141.6, 141.5, 138.5, 136.8, 133.3, 133.1, 129.5, 129.3, 127.4, 127.4, 117.6, 114.8, 114.5, 114.4, 114.0, 113.8, 109.0, 108.4, 100.8, 100.8, 98.5, 98.5, 78.0, 72.8, 63.5, 63.5, 55.3, 42.7, 40.5, 39.6, 38.3, 29.7, 14.8, 14.8; IR (ATR) (mixture): 2963, 1610, 1477, 1260, 1091, 1032, 800 cm–1; HRMS (ESI+) m/z calcd for C25H25O5 ([M + H]+): 405.1697, found: 405.1678.

(6S*,8R*)- and (6R*,8R*)-6-(4-Isopropoxyphenyl)-8-(4-methoxyphenyl)-7,8-dihydro-6H-[1,3]dioxolo[4,5-g]-chromene (3c)

4-Isopropoxystyrene (20.1 mg, 0.128 mmol), o-quinone methide 1a (94.4 mg, 0.369 mmol), TXT (0.74 mg, 0.00142 mmol), and AcOEt (2.0 mL) were used. Yellow oil (45.9 mg, 72% yield, dr 3:1). 1H NMR (500 MHz, CDCl3) (major): δ 7.35 (d, J = 8.5 Hz, 2H), 7.12 (d, J = 8.5 Hz, 2H), 6.89 (d, J = 9.0 Hz, 2H), 6.86–6.83 (m, 2H), 6.46 (s, 1H), 6.21 (s, 1H), 5.83 (dd, J = 8.5, 1.5 Hz, 2H), 5.04 (dd, J = 11.0, 1.0 Hz, 1H), 4.57–4.49 (m, 1H), 4.19 (dd, J = 12.0, 6.0 Hz, 1H), 3.79 (s, 3H), 2.34–2.29 (m, 1H), 2.22–2.15 (m, 1H), 1.33 (s, 3H), 1.32 (s, 3H); (minor): δ 7.21 (d, J = 8.5 Hz, 2H), 7.06 (d, J = 8.5 Hz, 2H), 6.86–6.83 (m, 4H), 6.52 (s, 1H), 6.39 (s, 1H), 5.87 (dd, J = 4.0, 1.0 Hz, 2H), 4.89 (dd, J = 11.0, 2.5 Hz, 1H), 4.57–4.49 (m, 1H), 4.07 (dd, J = 5.5, 3.0 Hz, 1H), 3.79 (s, 3H), 2.42–2.37 (m, 1H), 2.13–2.09 (m, 1H), 1.32 (s, 3H), 1.30 (s, 3H); 13C{1H} NMR (126 MHz, CDCl3) (mixture): δ 158.4, 158.1, 157.7, 157.5, 150.4, 150.2, 147.0, 146.6, 141.6, 141.5, 138.5, 136.8, 133.2, 133.0, 129.5, 129.3, 127.5, 127.4, 117.6, 115.9, 115.7, 114.7, 114.0, 113.8, 108.9, 108.4, 101.3, 100.8, 100.7, 98.5, 98.4, 77.9, 72.8, 69.9, 69.8, 55.2, 42.7, 40.5, 39.6, 38.2, 22.0; IR (ATR) (mixture): 2976, 1610, 1477, 1242, 1146, 1035, 829 cm–1; HRMS (ESI+) m/z calcd for C26H26O5 ([M + H]+): 419.1853, found: 419.1834.

(6S*,8R*)- and (6R*,8R*)-6-(4-(tert-Butyldimethyl-silyloxy)phenyl)-8-(4-methoxyphenyl)-7,8-dihydro-6H-[1,3]-dioxolo[4,5-g]-chromene (3d)

4-(tert-Butyldimethylsiloxy)styrene (30.0 mg, 0.128 mmol), o-quinone methide 1a (97.4 mg, 0.380 mmol), TXT (0.73 mg, 0.00131 mmol), and AcOEt (2.0 mL) were used. Yellow oil (55.2 mg, 88% yield, dr 4:1). 1H NMR (500 MHz, CDCl3) (major): δ 7.31 (d, J = 8.5 Hz, 2H), 7.12 (d, J = 8.5 Hz, 2H), 6.85 (d, J = 8.5 Hz, 2H), 6.84 (d, J = 8.5 Hz, 2H), 6.47 (s, 1H), 6.21 (s, 1H), 5.83 (dd, J = 8.5, 1.0 Hz, 2H), 5.05 (dd, J = 11.5, 1.5 Hz, 1H), 4.17 (dd, J = 12.0, 6.0 Hz, 1H), 3.79 (s, 3H), 2.33–2.29 (m, 1H), 2.20–2.13 (m, 1H), 0.98 (s, 9H), 0.19 (s, 6H); (minor): δ 7.17 (d, J = 8.5 Hz, 2H), 7.06 (d, J = 9.0 Hz, 2H), 6.85 (d, J = 8.5 Hz, 2H), 6.79 (d, J = 8.5 Hz, 2H), 6.53 (s, 1H), 6.39 (s, 1H), 5.88 (dd, J = 4.5, 1.5 Hz, 2H), 4.89 (dd, J = 10.5, 1.5 Hz, 1H), 4.06 (dd, J = 5.0, 3.0 Hz, 1H), 3.79 (s, 3H), 2.40–2.35 (m, 1H), 2.12–2.10 (m, 1H), 0.97 (s, 9H), 0.18 (s, 6H); 13C{1H} NMR (126 MHz, CDCl3) (mixture): δ 158.4, 158.1, 155. 5, 155.3, 150.4, 150.2, 147.1, 146.6, 141.6, 141.5, 138.5, 136.8, 134.0, 133.9, 129.5, 129.3, 127.3, 127.3, 120.1, 119.9, 117.7, 114.8, 114.1, 113.8, 109.0, 108.4, 100.9, 100.8, 98.6, 98.5, 78.0, 72.9, 55.3, 42.7, 40.7, 39.6, 38.3, 29.7, 25.7, 25.5, 18.2, −4.44; IR (ATR) (mixture): 2929, 1509, 1247, 1146, 1037, 910, 830, 780 cm–1; HRMS (ESI+) m/z calcd for C29H35O5Si ([M + H]+): 491.2248, found: 491.2239.

(6S*,8R*)- and (6R*,8R*)-6-(4-(Benzyloxy)phenyl)-8-(4-methoxyphenyl)-7,8-dihydro-6H-[1,3]dioxolo[4,5-g]-chromene (3e)

4-(Benzyloxy)styrene (24.1 mg, 0.115 mmol), o-quinone methide 1a (97.8 mg, 0.382 mmol), TXT (0.75 mg, 0.00133 mmol), and AcOEt (2.0 mL) were used. Yellow oil (18.3 mg, 34% yield, dr 4:1; 35 mg, 60% yield, dr 4:1 (5.0 mol % of TXT)). 1H NMR (500 MHz, CDCl3) (major): δ 7.43–7.35 (m, 7H), 7.12 (d, J = 8.5 Hz, 2H), 6.98 (d, J = 9.0 Hz, 2H), 6.86 (d, J = 13.5 Hz, 2H), 6.45 (s, 1H), 6.21 (s, 1H), 5.83 (dd, J = 8.5, 2.0 Hz, 2H), 5.07 (s, 2H), 5.05 (s, 1H), 4.19 (dd, J = 12.0, 6.0 Hz, 1H), 3.79 (s, 3H), 2.33–2.29 (m, 1H), 2.21–2.16 (m, 1H); (minor); δ 7.43–7.35 (m, 5H), 7.23 (d, J = 9.0 Hz, 2H), 7.06 (d, J = 8.5 Hz, 2H), 6.93 (d, J = 9.0 Hz, 2H), 6.86 (d, J = 13.5 Hz, 2H), 6.52 (s, 1H), 6.39 (s, 1H), 5.88 (dd, J = 8.5, 2.0 Hz, 2H), 5.07 (s, 2H), 4.90 (d, J = 8.0 Hz, 2H), 4.06 (dd, J = 5.5, 3.0 Hz, 1H), 3.79 (s, 3H), 2.38–2.35 (m, 1H), 2.14–2.12 (m, 1H); 13C{1H} NMR (126 MHz, CDCl3) (mixture): δ 158.6, 158.4, 158.4, 158.1, 150.4, 150.2, 147.1, 146.6, 141.6, 141.5, 138.4, 136.9, 136.8, 133.7, 133.6, 129.5, 129.3, 129.1, 128.7, 128.6, 127.9, 127.5, 127.4, 117.6, 114.9, 114.8, 114.7, 114.0, 113.8, 109.0, 108.4, 100.9, 100.8, 98.5, 98.5, 77.9, 72.8, 70.0, 70.0, 55.3, 42.7, 40.6, 39.6, 38.3, 29.7, 29.3; IR (ATR) (mixture): 2920, 1608, 1509, 1244, 1146, 1034, 827 cm–1; HRMS (ESI+) m/z calcd for C30H27O5 ([M + H]+): 467.1853, found: 467.1831.

(6S*,8R*)- and (6R*,8R*)-6-(3,4-bis(Benzyloxy)phenyl)-8-(4-methoxyphenyl)-7,8-dihydro-6H-[1,3]dioxolo[4,5-g]-chromene (3f)

1,2-bis(Benzyloxy)-4-vinylbenzene (39.6 mg, 0.125 mmol), o-quinone methide 1a (98.3 mg, 0.384 mmol), TXT (0.71 mg, 0.00128 mmol), and AcOEt (2.0 mL) were used. White solid (32.9 mg, 46% yield, dr 2.5:1; 72.0 mg, >99% yield, dr 2.5:1 (5.0 mol % of TXT)). 1H NMR (500 MHz, CDCl3): δ 7.46–7.29 and 7.12–6.79 (m, 17H) (mixture), 6.51 (s, 1H) (minor), 6.45 (s, 1H) (major), 6.37 (s, 1H) (minor), 6.21 (s, 1H) (major), 5.88 (dd, J = 3.0, 1.5 Hz, 2H) (minor), 5.83 (dd, J = 9.5, 1.5 Hz, 1H) (major), 5.17–5.12 (m, 4H) (mixture), 5.00 (dd, J = 11.0, 1.0 Hz, 1H) (major), 4.86 (dd, J = 10.5, 2.0 Hz, 1H) (minor), 4.16 (dd, J = 12.0, 6.0 Hz, 1H) (major), 4.02 (dd, J = 5.0, 3.5 Hz, 1H) (minor), 3.79 (s, 3H) (mixture), 2.35–2.31 (m, 1H) (minor), 2.30–2.26 (m, 1H) (major), 2.16–2.10 (m, 1H) (major), 2.09–2.06 (m, 1H) (minor); 13C{1H} NMR (126 MHz, CDCl3) (mixture): δ 158.4, 158.1, 150.2, 150.0, 149.1, 149.0, 148.8, 148.6, 147.1, 146.6, 141.6, 141.5, 138.3, 137.3, 137.2, 137.2, 136.7, 134.7, 134.5, 129.5, 129.3, 128.4, 128.4, 127.8, 127.8, 127.7, 127.5, 127.4, 127.2, 119.4, 119.2, 117.6, 115.1, 115.0, 114.8, 114.3, 114.0, 113.8, 113.3, 109.0, 108.4, 100.9, 100.8, 98.5, 98.5, 78.0, 72.9, 71.5, 71.4, 71.4, 71.3, 55.3, 42.6, 40.6, 39.4, 38.2, 29.7, 29.6, 22.7, 22.6, 14.2, 14.1; IR (ATR) (mixture): 2919, 1509, 1477, 1259, 1146, 1091, 1033, 804 cm–1; HRMS (ESI+) m/z calcd for C37H32O6Na ([M + Na]+): 595.2091, found: 595.2109.

(6S*,8R*)- and (6R*,8R*)-6-(3,4-Dimethoxyphenyl)-8-(4-methoxyphenyl)-7,8-dihydro-6H-[1,3]dioxolo[4,5-g]-chromene (3g)

3,4-Dimethoxystyrene (19.3 mg, 0.118 mmol), o-quinone methide 1a (97.8 mg, 0.382 mmol), TXT (0.78 mg, 0.0014 mmol), and AcOEt (2.0 mL) were used. Yellow oil (50.1 mg, >99% yield, dr 4:1). 1H NMR (500 MHz, CDCl3) (major): δ 7.13 (d, J = 9.0 Hz, 2H), 6.99–6.98 (m, 1H), 6.88–6.83 (m, 4H), 6.48 (s, 1H), 6.22 (s, 1H), 5.84 (dd, J = 8.5, 1.5 Hz, 2H), 5.06 (dd, J = 11.0, 1.5 Hz, 1H), 4.21 (dd, J = 12.0, 6.0 Hz, 1H), 3.91 (s, 3H), 3.88 (s, 3H), 3.80 (s, 3H), 2.35–2.31 (m, 1H), 2.23–2.17 (m, 1H); (minor): δ 7.07 (d, J = 9.0 Hz, 2H), 6.99–6.98 (m, 3H), 6.88–6.83 (m, 2H), 6.54 (s, 1H), 6.40 (s, 1H), 5.89 (dd, J = 4.5, 1.0 Hz, 2H), 4.89 (dd, J = 10.5, 2.5 Hz, 1H), 4.09 (dd, J = 6.0, 3.0 Hz, 1H), 3.88 (s, 3H), 3.86 (s, 3H), 3.86 (s, 3H), 3.80 (s, 3H), 2.43–2.39 (m, 1H), 2.14–2.11 (m, 1H); 13C{1H} NMR (126 MHz, CDCl3) (mixture): δ 158.4, 158.1, 150.3, 150.1, 149.1, 148.9, 148.8, 148.6, 147.1, 146.6, 141.6, 141.5, 138.4, 136.7, 133.8, 133.6, 129.5, 129.3, 118.6, 118.5, 117.6, 114.7, 114.0, 113.7, 111.0, 111.0, 109.5, 109.3, 109.0, 108.3, 100.9, 100.8, 98.5, 98.5, 78.2, 73.0, 55.9, 55.9, 55.8, 55.8, 55.2, 42.7, 40.6, 39.6, 38.3; IR (ATR) (mixture): 2930, 1609, 1477, 1240, 1144, 1027, 831 cm–1; HRMS (ESI+) m/z calcd for C25H25O6 ([M + H]+): 421.1646, found: 421.1625.

(6S*,8R*)- and (6R*,8R*)-6-(2,4-Dimethoxyphenyl)-8-(4-methoxyphenyl)-7,8-dihydro-6H-[1,3]dioxolo[4,5-g]-chromene (3h)

2,4-Dimethoxystyrene (20.6 mg, 0.125 mmol), o-quinone methide 1a (98.6 mg, 0.385 mmol), TXT (0.79 mg, 0.0014 mmol), and AcOEt (2.0 mL) were used. Yellow oil (53.8 mg, >99% yield, dr 10:1). 1H NMR (500 MHz, CDCl3) (major): δ 7.46 (d, J = 8.5 Hz, 1H), 7.12 (d, J = 8.0 Hz, 2H), 6.84 (m, 2H), 6.53 (dd, J = 8.5, 2.5 Hz, 1H), 6.46 (s, 1H), 6.45 (d, J = 2.0 Hz, 1H), 6.21 (s, 1H), 5.82 (d, J = 8.0 Hz, 2H), 5.43 (d, J = 11.5 Hz, 1H), 4.20 (dd, J = 12.5, 6.0 Hz, 1H), 3.80 (s, 6H), 3.78 (s, 3H), 2.36–2.31 (m, 1H), 2.08–2.01 (m, 1H); (minor): δ 7.32 (d, J = 8.0 Hz, 1H), 7.08 (d, J = 8.5 Hz, 2H), 6.84 (m, 2H), 6.49 (d, J = 2.5 Hz, 1H), 6.46 (s, 1H), 6.40 (d, J = 2.0 Hz, 1H), 6.38 (s, 1H), 5.86 (d, J = 4.0 Hz, 2H), 5.29 (d, J = 7.5 Hz, 1H), 4.12 (dd, J = 9.5, 5.0 Hz, 1H), 3.79 (s, 6H), 3.63 (s, 3H), 2.30–2.27 (m, 1H), 2.20–2.16 (m, 1H); 13C{1H} NMR (126 MHz, CDCl3) (mixture): δ 160.4, 160.2, 158.3, 158.0, 157.1, 157.0, 150.7, 150.4, 146.9, 146.5, 141.3, 141.3, 138.2, 137.0, 129.5, 129.3, 127.2, 127.1, 122.3, 122.2, 117.9, 115.4, 113.9, 113.6, 109.0, 108.4, 104.4, 104.1, 100.8, 100.7, 98.5, 98.4, 72.3, 68.5, 60.3, 55.4, 55.3, 55.3, 55.3, 55.2, 42.7, 39.5, 39.4, 36.6, 29.7; IR (ATR) (mixture): 2959, 1612, 1477, 1247, 1148, 1035, 830 cm–1; HRMS (ESI+) m/z calcd for C25H24O6([M + H]+): 421.1646, found: 421.1625.

(6S*,8R*)- and (6R*,8R*)-8-(4-Methoxyphenyl)-6-(2,3,4-trimethoxyphenyl)-7,8-dihydro-6H-[1,3]dioxolo[4,5-g]-chromene (3i)

2,3,4-Trimethoxystyrene (21.3 mg, 0.110 mmol), o-quinone methide 1a (98.0 mg, 0.383 mmol), TXT (0.85 mg, 0.00153 mmol), and AcOEt (2.0 mL) were used. White solid (32.7 mg, 66% yield, dr 2:1). 1H NMR (500 MHz, CDCl3) (major): δ 7.21 (d, J = 9.0 Hz, 1H), 7.14–7.11 (m, 2H), 6.85 (m, 2H), 6.72 (d, J = 8.5 Hz, 1H), 6.45 (s, 1H), 6.22 (s, 1H), 5.83 (dd, J = 9.5, 1.5 Hz, 2H), 5.38 (dd, J = 11.5, 1.5 Hz, 1H), 4.22 (dd, J = 12.0, 6.0 Hz, 1H), 3.93 (s, 3H), 3.87 (s, 3H), 3.86 (s, 3H), 3.79 (s, 3H), 2.33–2.28 (m, 1H), 2.16–2.14 (m, 1H); (minor): δ 7.12 (m, 1H), 7.09 (d, J = 9.0 Hz, 2H), 6.85 (m, 2H), 6.68 (d, J = 8.5 Hz, 1H), 6.51 (s, 1H), 6.43 (s, 1H), 5.88 (s, 2H), 5.20 (dd, J = 11.0, 2.0 Hz, 1H), 4.06 (dd, J = 5.5, 3.0 Hz, 1H), 3.84 (s, 3H), 3.82 (s, 3H), 3.79 (s, 3H), 3.59 (s, 3H), 2.37–2.33 (m, 1H), 2.13–2.11 (m, 1H); 13C{1H} NMR (126 MHz, CDCl3) (mixture): δ 158.3, 158.0, 153.4, 153.2, 151.1, 150.9, 150.5, 150.4, 147.0, 146.5, 142.0, 142.0, 141.5, 141.4, 138.5, 136.9, 129.4, 129.3, 127.5, 127.2, 121.1, 117.8, 115.0, 114.0, 113.6, 109.1, 108.4, 107.5, 107.4, 100.8, 100.7, 98.5, 98.4, 72.8, 68.3, 61.4, 60.9, 60.7, 60.7, 56.0, 55.9, 55.2, 55.2, 42.8, 39.9, 39.8, 37.6; IR (ATR) (mixture): 2962, 1607, 1477, 1260, 1092, 1034, 801 cm–1; HRMS (ESI+) m/z calcd for C26H27O7 ([M + H]+): 451.1751, found: 451.1761.

(6S*,8R*)- and (6R*,8R*)-8-(4-Methoxyphenyl)-6-(2,4,6-trimethoxyphenyl)-7,8-dihydro-6H-[1,3]dioxolo[4,5-g]-chromene (3j)

2,4,6-Trimethoxystyrene (21.6 mg, 0.111 mmol), o-quinone methide 1a (94.7 mg, 0.369 mmol), TXT (0.78 mg, 0.0014 mmol), and AcOEt (2.0 mL) were used. White solid (35.0 mg, 70% yield, dr 1.5:1). 1H NMR (500 MHz, CDCl3) (major): δ 7.15 (d, J = 8.5 Hz, 2H), 6.86 (d, J = 8.5 Hz, 2H), 6.43 (s, 1H), 6.19 (s, 1H), 6.15 (s, 2H), 5.80 (dd, J = 10.0, 1.5 Hz, 2H), 5.70 (dd, J = 11.5, 2.0 Hz, 1H), 4.14 (dd, J = 12.0, 6.0 Hz, 1H), 3.81 (s, 3H), 3.80 (s, 3H), 3.79 (s, 3H), 3.68 (s, 3H), 3.01–2.93 (m, 1H), 2.06–2.02 (m, 1H); (minor): δ 7.09 (d, J = 8.5 Hz, 2H), 6.84 (d, J = 9.0 Hz, 2H), 6.49 (s, 1H), 6.44 (s, 1H), 6.09 (s, 2H), 5.85 (s, 2H), 5.51 (dd, J = 12.0, 2.0 Hz, 1H), 4.11 (d, J = 5.0 Hz, 1H), 3.79 (s, 3H), 3.78 (s, 3H), 3.77 (s, 3H), 3.68 (s, 3H), 3.16–3.10 (m, 1H), 1.84–1.81 (m, 1H); 13C{1H} NMR (126 MHz, CDCl3) (mixture): δ 161.2, 161.1, 160.1, 160.0, 158.2, 157.8, 151.3, 151.2, 146.8, 146.3, 141.0, 140.9, 139.1, 137.4, 129.5, 129.4, 118.2, 115.0, 113.9, 113.5, 109.4, 109.2, 109.2, 108.4, 100.7, 100.6, 98.6, 98.6, 91.5, 91.4, 71.0, 65.9, 56.0, 55.9, 55.3, 55.3, 55.2, 55.2, 43.4, 40.6, 36.1, 34.0; IR (ATR) (mixture): 2932, 1593, 1475, 1146, 1035, 940, 811, 608 cm–1; HRMS (ESI+) m/z calcd for C26H27O7 ([M + H]+): 451.1751, found: 451.1743.

rac-6,6,8-tris(4-Methoxyphenyl)-7,8-dihydro-6H-[1,3]-dioxolo[4,5-g]chromene (3k)

1,1-Bis(4-methoxyphenyl)ethylene (30.2 mg, 0.125 mmol), o-quinone methide 1a (96.5 mg, 0.375 mmol), TXT (0.79 mg, 0.00142 mmol), and AcOEt (2.0 mL) were used. Yellow solid (58.4 mg, 94% yield). 1H NMR (500 MHz, CDCl3): δ 7.37 (d, J = 8.5 Hz, 2H), 7.33 (d, J = 9.0 Hz, 2H), 7.09 (d, J = 9.0 Hz, 2H), 6.85 (d, J = 9.5 Hz, 2H), 6.84 (d, J = 9.0 Hz, 2H), 6.80 (d, J = 9.5 Hz, 2H), 6.62 (s, 1H), 6.06 (s, 1H), 5.80 (dd, J = 14.0, 1.0 Hz, 2H), 3.78 (s, 3H), 3.77 (s, 3H), 3.74 (s, 3H), 3.71 (dd, J = 12.5, 6.0 Hz, 1H), 2.93 (dd, J = 14.0, 5.5 Hz, 1H), 2.50 (dd, J = 14.0, 12.5 Hz, 1H); 13C{1H} NMR (126 MHz, CDCl3): δ 158.7, 158.4, 158.4, 148.9, 146.7, 141.4, 138.4, 136.6, 135.5, 129.6, 127.5, 127.0, 117.3, 114.0, 113.9, 113.5, 108.2, 100.8, 98.7, 81.6, 55.3, 55.2, 55.1, 42.4, 39.5; IR (ATR): 2957, 1508, 1477, 1243, 1175, 1033, 828 cm–1; HRMS (ESI+) m/z calcd for C31H29O6 ([M + H]+): 497.1959, found: 497.1953.

(6S*,8R*)- and (6R*,8R*)-6,8-bis(4-Methoxyphenyl)-6-phenyl-7,8-dihydro-6H-[1,3]dioxolo[4,5-g]chromene (3l)

1-(4-Methoxyphenyl)-1-phenylethylene (25.5 mg, 0.121 mmol), o-quinone methide 1a (98.9 mg, 0.386 mmol), TXT (0.77 mg, 0.00138 mmol), and AcOEt (2.0 mL) were used. Yellow oil (32.0 mg, 57% yield, dr 1:1; 46.6 mg, 80% yield, dr 1:1 (5.0 mol % of TXT)). 1H NMR (500 MHz, CDCl3) δ 7.47–7.39 (m, 3H) (mixture), 7.36–7.18 (m, 4H) (mixture), 7.10–7.07 (m, 2H) (mixture), 6.87–6.79 (m, 2H) (mixture), 6.07 and 6.05 (s, 1H) (mixture), 5.81 and 5.78 (s, 2H) (mixture), 3.78 (s, 3H) (mixture), 3.76 and 3.74 (s, 3H) (mixture), 3.68–3.65 and 0.95–0.87 (m, 1H) (mixture), 3.00–2.97 and 2.55–2.47 (m, 2H) (mixture); 13C{1H} NMR (126 MHz, CDCl3) (mixture): δ 158.7, 158.5, 158.4, 148.9, 148.8, 146.8, 146.7, 146.1, 143.6, 141.4, 141.4, 138.0, 136.6, 135.3, 132.5, 131.9, 129.7, 129.6, 128.5, 128.2, 127.5, 127.2, 127.0, 126.9, 126.2, 125.6, 117.4, 117.3, 114.0, 113.9, 113.5, 113.4, 108.2, 108.2, 100.7, 98.7, 98.6, 81.8, 81.7, 60.4, 55.4, 55.2, 55.2, 55.1, 42.3, 42.1, 39.5, 39.4; IR (ATR) (mixture): 2963, 1509, 1262, 1097, 1023, 801, 691 cm–1; HRMS (ESI+) m/z calcd for C30H27O5 ([M + H]+): 467.1853, found: 467.1840.

(6S*,8R*)- and (6R*,8R*)-6-Ethoxy-8-(4-methoxyphenyl)-7,8-dihydro-6H-[1,3]dioxolo[4,5-g]chromene (3o)

Ethyl vinyl ether (9.0 mg, 0.125 mmol), o-quinone methide 1a (98.3 mg, 0.384 mmol), TXT (0.69 mg, 0.00124 mmol), and AcOEt (2.0 mL) were used. White solid (17.8 mg, 44% yield, dr 10:1). 1H NMR (500 MHz, CDCl3) δ 7.13–7.11 (m, 2H) (minor), 7.10–7.08 (m, 2H) (major), 6.87–6.86 (m, 2H) (minor), 6.86–6.83 (m, 2H) (major), 6.43 (s, 1H) (major), 6.41 (s, 1H) (minor), 6.19 (s, 1H) (minor), 6.15 (s, 1H) (major), 5.83–5.81 (m, 2H) (mixture), 5.22 (t, J = 2.86 Hz, 1H) (minor), 5.17 (dd, J = 8.3, 2.6 Hz, 1H) (major), 4.07–4.00 (m, 2H) (mixture), 3.79 (s, 3H) (mixture), 3.67–3.61 (m, 1H) (mixture), 2.33 (ddd, J = 13.3, 6.2, 2.3 Hz, 1H) (major), 2.19 (ddd, J = 13.3, 5.9, 3.2 Hz, 1H) (minor), 2.12–2.04 (m, 1H) (mixture), 1.26 (t, J = 7.2 Hz, 3H) (major), 1.21 (t, J = 7.2 Hz, 3H) (minor); 13C{1H} NMR (126 MHz, CDCl3) (mixture): δ 158.4, 147.9, 146.7, 146.6, 146.5, 141.8, 136.7, 136.3, 129.6, 129.4, 128.0. 118.1, 118.0, 114.0, 113.9, 110.7, 108.4, 108.0, 100.9, 99.6, 98.7, 98.5, 96.5, 64.3, 63.8, 55.2, 40.5, 37.6, 36.5, 36.1, 15.2; IR (ATR) (mixture): 2907, 1613, 1477, 1248, 1136, 1035, 905 cm–1; HRMS (ESI+) m/z calcd for C19H21O5 ([M + H]+): 329.1384, found: 329.1375.

(6S*,8R*)- and (6R*,8R*)-8-(4-Methoxyphenyl)-6-phenoxy-7,8-dihydro-6H-[1,3]dioxolo[4,5-g]chromene (3p)

Vinyloxybenzene (15.5 mg, 0.129 mmol), o-quinone methide 1a (95.3 mg, 0.372 mmol), 5.0 mol % of TXT (3.53 mg, 0.00634 mmol), and AcOEt (2.0 mL) were used. White solid (21.3 mg, 44% yield, dr 2.5:1). 1H NMR (500 MHz, CDCl3) (major): δ 7.33–7.26 (m, 2H), 7.20–7.01 (m, 5H), 6.88–6.86 (m, 2H), 6.45 (s, 1H), 6.23 (s, 1H), 5.89–5.85 (m, 1H), 5.85 (s, 2H), 4.13 (dd, J = 9.8, 6.6 Hz, 1H), 3.82 (s, 3H), 2.56–2.49 (m, 1H), 2.40–2.33 (m, 1H); (minor): δ 7.33–7.26 (m, 2H), 7.20–7.01 (m, 5H), 6.90 (m, 2H), 6.40 (s, 1H), 6.22 (s, 1H), 5.85 (s, 2H), 5.80 (m, 1H), 4.33 (dd, J = 11.8, 5.5 Hz, 1H), 3.82 (s, 3H), 2.44–2.40 (m, 1H), 2.23–2.16 (m, 1H); 13C{1H} NMR (126 MHz, CDCl3) (mixture): δ 158.5, 158.4, 156.7, 157.0, 147.2, 146.9, 146.7, 145.9, 142.2, 142.0, 136.1, 129.7, 129.6, 129.5, 129.4, 122.4, 122.3, 117.9, 117.7, 117.1, 116.6, 114.1, 113.9, 108.2, 108.1, 101.0, 100.9, 98.9, 98.7, 97.0, 95.2, 55.3, 39.7, 36.9, 36.2, 36.0; IR (ATR) (mixture): 1477, 1237, 1146, 1034, 916, 754 cm–1; HRMS (ESI+) m/z calcd for C23H21O5 ([M + H]+): 377.1384, found: 377.1365.

(2S*,4R*)- and (2R*,4R*)-6,7-Dimethoxy-4-(4-methoxy-phenyl)-2-phenoxychromane (3q)

Vinyloxybenzene (15.8 mg, 0.131 mmol), 3,4-dimethoxy-6-(4-methoxyphenylmethylidene)cyclohexa-2,4-dien-1-one (102.7 mg, 0.39 mmol), 5.0 mol % of TXT (3.83 mg, 0.00690 mmol), and AcOEt (2.0 mL) were used. White solid (23.1 mg, 45% yield, dr 4:1). 1H NMR (500 MHz, CDCl3) (major): δ 7.34–7.28 (m, 2H), 7.15 (d, J = 8.5 Hz, 2H), 7.14–7.00 (m, 3H), 6.89–6.85 (m, 2H), 6.49 (s, 1H), 6.29 (s, 1H), 5.92–5.87 (m, 1H), 4.19 (dd, J = 9.5, 6.6 Hz, 1H), 3.83 (s, 3H), 3.82 (s, 3H), 3.63 (s, 3H), 2.55–2.51 (m, 1H), 2.41–2.35 (m, 1H); (minor): δ 7.34–7.28 (m, 2H), 7.20 (d, J = 8.5 Hz, 2H), 7.14–7.00 (m, 3H), 6.90 (m, 2H), 6.44 (s, 1H), 6.27 (s, 1H), 5.92–5.87 (m, 1H), 4.37 (dd, J = 11.7, 6.0 Hz, 1H), 3.82 (s, 3H), 3.81 (s, 3H), 3.61 (s, 3H), 2.43–2.41 (m, 1H), 2.23–2.17 (m, 1H); 13C{1H} NMR (126 MHz, CDCl3) (mixture): δ 158.4, 158.3, 157.0, 156.7, 148.9, 148.7, 145.5, 146.5, 143.7, 143.5, 136.3, 136.2, 129.7, 129.5, 129.4, 122.3, 122.2, 117.0, 116.4, 115.9, 114.0, 113.8, 112.1, 112.0, 101.0, 100.9, 96.8, 95.1, 56.3, 55.9, 55.8, 55.2, 39.4, 37.1, 36.4, 35.9; IR (ATR) (mixture): 1509, 1219, 1123, 1020, 918, 728 cm–1; HRMS (ESI+) m/z calcd for C24H24O5Na ([M + Na]+): 415.1516, found: 415.1511.

(2S*,4R*)- and (2R*,4R*)-6,7-Dimethoxy-2,4-bis(4-methoxyphenyl)chromane (3r)

4-Methoxystyrene (16.0 mg, 0.120 mmol), 3,4-dimethoxy-6-(4-methoxyphenylmethylidene)cyclohexa-2,4-dien-1-one (103.6 mg, 0.380 mmol), TXT (0.88 mg, 0.00158 mmol), and AcOEt (2.0 mL) were used. White solid (35.7 mg, 71% yield, dr 3:1). 1H NMR (500 MHz, CDCl3) (major): δ 7.39 (d, J = 8.5 Hz, 2H), 7.13 (d, J = 8.5 Hz, 2H), 6.91 (d, J = 9.0 Hz, 2H), 6.87–6.85 (m, 2H), 6.51 (s, 1H), 6.27 (s, 1H), 5.08 (dd, J = 11.5, 1.0 Hz, 1H), 4.27 (dd, J = 12.0, 6.0 Hz, 1H), 3.82 (s, 3H), 3.80 (s, 3H), 3.79 (s, 3H), 3.61 (s, 3H), 2.34–2.30 (m, 1H), 2.23–2.15 (m, 1H); (minor): δ 7.25 (d, J = 8.5 Hz, 2H), 7.06 (d, J = 8.5 Hz, 2H), 6.87–6.85 (m, 4H), 6.53 (s, 1H), 6.46 (s, 1H), 4.89 (dd, J = 11.0, 2.0 Hz, 1H), 4.13 (dd, J = 5.5, 2.5 Hz, 1H), 3.86 (s, 3H), 3.79 (s, 3H), 3.78 (s, 3H), 3.72 (s, 3H), 2.46–2.40 (m, 1H), 2.13–2.09 (m, 1H); 13C{1H} NMR (126 MHz, CDCl3) (mixture): δ 159.4, 159.2, 158.3, 158.0, 149.7, 149.5, 148.9, 148.6, 143.3, 143.1, 138.5, 136.7, 133.4, 133.3, 129.5, 129.3, 127.5, 127.4, 116.3, 113.9, 113.9, 113.8, 113.7, 113.5, 112.9, 112.5, 100.8, 100.7, 77.9, 72.7, 56.4, 56.3, 55.8, 55.3, 55.2, 55.2, 55.2, 42.3, 40.9, 39.3, 38.6; IR (ATR) (mixture): 2835, 1610, 1508, 1246, 1169, 1024, 829 cm–1; HRMS (ESI+) m/z calcd for C25H27O5 ([M + H]+): 407.1853, found: 407.1835.

(2S*,4R*)- and (2R*,4R*)-6,7-Diethoxy-2,4-bis(4-methoxyphenyl)chromane (3s)

4-Methoxystyrene (17.8 mg, 0.133 mmol), 3,4-diethoxy-6-(4-methoxyphenylmethylidene)cyclohexa-2,4-dien-1-one (113 mg, 0.375 mmol), TXT (0.81 mg, 0.00146 mmol), and AcOEt (2.0 mL) were used. Yellow solid (33.0 mg, 55% yield, dr 3:1, 50.0 mg, 93% yield, dr 3:1 (5.0 mol % of TXT)). 1H NMR (500 MHz, CDCl3) (major): δ 7.38 (d, J = 9.0 Hz, 2H), 7.12 (d, J = 8.5 Hz, 2H), 6.91 (d, J = 8.5 Hz, 2H), 6.87–6.83 (m, 2H), 6.49 (s, 1H), 6.30 (s, 1H), 5.08 (dd, J = 11.5, 1.5 Hz, 1H), 4.22 (dd, J = 12.0, 6.0 Hz, 1H), 4.07–4.01 (m, 2H), 3.87–3.82 (m, 1H), 3.80 (s, 3H), 3.79 (s, 3H), 2.33 (m, 1H), 2.22–2.14 (m, 1H), 1.42 (t, J = 7.5 Hz, 3H), 1.27 (t, J = 7.0 Hz, 3H); (minor): δ 7.25 (d, J = 9.0 Hz, 2H), 7.05 (d, J = 8.5 Hz, 2H), 6.87–6.83 (m, 4H), 6.55 (s, 1H), 6.49 (s, 1H), 4.88 (dd, J = 10.5, 2.0 Hz, 1H), 4.10 (dd, J = 5.5, 2.5 Hz, 1H), 4.07–4.01 (m, 2H), 3.95–3.90 (m, 1H), 3.79 (s, 3H), 3.78 (m, 3H), 2.45–2.39 (m, 1H), 2.12–2.09 (m, 1H), 1.45 (t, J = 7.5 Hz, 3H), 1.34 (t, J = 7.0 Hz, 3H); 13C{1H} NMR (126 MHz, CDCl3) (mixture): δ159.4, 159.2, 158.3, 158.0, 150.0, 149.8, 148.9, 148.8, 142.7, 142.4, 138.6, 136.9, 133.6, 133.4, 129.5, 129.3, 127.5, 127.4, 116.5, 116.0, 115.9, 114.3, 114.1, 113.9, 113.8, 113.7, 103.2, 102.1, 102.0, 77.9, 72.7, 65.6, 65.4, 64.2, 64.1, 55.3, 55.2, 55.2, 42.3, 40.9, 39.2, 38.6, 14.9, 14.8, 14.7; IR (ATR) (mixture): 2928, 1602, 1509, 1245, 1173, 1031, 825 cm–1; HRMS (ESI+) m/z calcd for C27H31O5 ([M + H]+): 435.2166, found: 435.2148.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.9b01156.

  • Crystallography data for 3a (CIF)

  • Characterization for TXT catalyst, photophysical and redox properties of TXT and representative substrates, Stern–Volmer plot of TXT, reaction quantum yield, NMR spectra of new compounds, detailed crystallography, and detailed experimental procedures (PDF)

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Author Information

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  • Corresponding Authors
    • Yujiro Hoshino - Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, JapanOrcidhttp://orcid.org/0000-0002-8373-8013 Email: [email protected]
    • Kiyoshi Honda - Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Email: [email protected]
  • Authors
    • Kenta Tanaka - Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, JapanOrcidhttp://orcid.org/0000-0001-8253-3561
    • Daichi Omata - Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
    • Yosuke Asada - Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors thank Prof. Mahito Atobe of Yokohama National University for cyclic voltammetry measurements.

References

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This article references 26 other publications.

  1. 1
    (a) Zhao, Y.; Lv, Y.; Xia, W. Synthesis of Cyclic Compounds via Photoinduced Radical Cyclization Cascade of C═C bonds. Chem. Rec. 2019, 19, 424439,  DOI: 10.1002/tcr.201800050
    (b) Xuan, J.; Lu, L.-Q.; Chen, J.-R.; Xiao, W.-J. Visible-Light-Driven Photoredox Catalysis in the Construction of Carbocyclic and Heterocyclic Ring Systems. Eur. J. Org. Chem. 2013, 67556770,  DOI: 10.1002/ejoc.201300596
    (c) Liu, Y.; Song, R.; Li, J. The cycloaddition reaction using visible light photoredox catalysis. Sci. China Chem. 2016, 59, 161170,  DOI: 10.1007/s11426-015-5516-5
  2. 2
    (a) Pałasz, A. Recent Advances in Inverse-Electron-Demand Hetero-Diels–Alder Reactions of 1-Oxa-1,3-Butadienes. Top. Curr. Chem. 2016, 374, 24,  DOI: 10.1007/s41061-016-0026-2
    (b) Hall, D. G.; Rybak, T.; Verdelet, T. Multicomponent Hetero-[4 + 2] Cycloaddition/Allylboration Reaction: From Natural Product Synthesis to Drug Discovery. Acc. Chem. Res. 2016, 49, 24892500,  DOI: 10.1021/acs.accounts.6b00403
    (c) Neely, J. M.; Rovis, T. Pyridine synthesis by [4 + 2] cycloadditions of 1-azadienes: hetero-Diels Alder and transition metal-catalysed approaches. Org. Chem. Front. 2014, 1, 10101015,  DOI: 10.1039/C4QO00187G
    (d) Rizzacasa, M. A.; Pollex, A. The hetero-Diels–Alder approach to spiroketals. Org. Biomol. Chem. 2009, 7, 10531059,  DOI: 10.1039/b819966n
  3. 3
    (a) Pałasz, A.; Bogdanowicz-Szwed, K. Hetero-Diels-Alder reaction of propenenitriles with enol ethers: a convenient approach to functionalized 3,4-dihydro-2H-pyrans. Monatsh. Chem. 2008, 139, 647655,  DOI: 10.1007/s00706-007-0824-x
    (b) Pałasz, A.; Pałasz, T. Knoevenagel condensation of cyclic ketones with benzoylacetonitrile and N,N′-dimethylbarbituric acid. Application of sterically hindered condensation products in the synthesis of spiro and dispiropyrans by hetero-DielseAlder reactions. Tetrahedron 2011, 67, 14221431,  DOI: 10.1016/j.tet.2010.12.053
    (c) Hanessian, S.; Compain, P. Lewis acid promoted cyclocondensations of α-ketophosphonoenoates with dienes—from Diels–Alder to hetero Diels–Alder reactions. Tetrahedron 2002, 58, 65216529,  DOI: 10.1016/S0040-4020(02)00662-2
    (d) Davies, H. M. L.; Dai, X. Lewis Acid-Catalyzed Tandem Diels-Alder Reaction/Retro-Claisen Rearrangement as an Equivalent of the Inverse Electron Demand Hetero Diels-Alder Reaction. J. Org. Chem. 2005, 70, 66806684,  DOI: 10.1021/jo050821s
    (e) Yang, Y.; Liu, H.; Peng, C.; Wu, J.; Zhang, J.; Qiao, Y.; Wang, X.-N.; Chang, J. AlCl3-Catalyzed Annulations of Ynamides Involving a Torquoselective Process for the Simultaneous Control of Central and Axial Chirality. Org. Lett. 2016, 18, 50225025,  DOI: 10.1021/acs.orglett.6b02480
  4. 4
    (a) Arumugam, S.; Popik, V. V. Light-Induced Hetero-DielsAlder Cycloaddition: A Facile and Selective Photoclick Reaction. J. Am. Chem. Soc. 2011, 133, 55735579,  DOI: 10.1021/ja200356f
    (b) Fujiwara, M.; Sakamoto, M.; Komeyama, K.; Yoshida, H.; Takaki, K. Convenient Synthesis of 2-Amino-4H-chromenes from Photochemically Generated o-Quinone Methides and Malononitrile. J. Heterocycl. Chem. 2015, 52, 5966,  DOI: 10.1002/jhet.1964
    (c) Nakatani, K.; Higashida, N.; Saito, I. Highly from Efficient Photochemical Generation of o-Quinone Methide Mannich Bases of Phenol Derivatives. Tetrahedron Lett. 1997, 38, 50055008,  DOI: 10.1016/S0040-4039(97)01071-X
    (d) Zhang, X. Mechanistic study on the intramolecular oxa-[4 + 2] cycloaddition of substituted o-divinylbenzenes. J. Mol. Model. 2019, 25, 14,  DOI: 10.1007/s00894-018-3883-5
    (e) Liu, Q.; Wang, J.; Li, D.; Gao, G.-L.; Yang, C.; Gao, Y.; Xia, W. Synthesis of Oxatricyclooctanes via Photoinduced Intramolecular Oxa-[4+2] Cycloaddition of Substituted o-Divinylbenzenes. J. Org. Chem. 2017, 82, 78567868,  DOI: 10.1021/acs.joc.7b01055
  5. 5
    (a) Yan, D.-M.; Chen, J.-R.; Xiao, W.-J. New Roles for Photoexcited Eosin Y in Photochemical Reactions. Angew. Chem., Int. Ed. 2019, 58, 378380,  DOI: 10.1002/anie.201811102
    (b) Romero, N. A.; Nicewicz, D. A. Organic Photoredox Catalysis. Chem. Rev. 2016, 116, 1007510166,  DOI: 10.1021/acs.chemrev.6b00057
    (c) Nicewicz, D. A.; Nguyen, T. M. Recent Applications of Organic Dyes as Photoredox Catalysts in Organic Synthesis. ACS Catal. 2014, 4, 355360,  DOI: 10.1021/cs400956a
  6. 6
    (a) Le Vaillant, F.; Garreau, M.; Nicolai, S.; Grynova, G.; Corminboeuf, C.; Waser, J. Fine-tuned organic photoredox catalysts for fragmentation-alkynylation cascades of cyclic oxime ethers. Chem. Sci. 2018, 9, 58835889,  DOI: 10.1039/C8SC01818A
    (b) Hloušková, Z.; Tydlitat, J.; Kong, M.; Pytela, O.; Mikysek, T.; Klikar, M.; Almonasy, N.; Dvorak, M.; Jiang, Z.; Ruzicka, A.; Bures, F. Structure-Catalytic Activity in a Series of Push-Pull Dicyanopyrazine/Dicyanoimidazole Photoredox Catalysts. ChemistrySelect 2018, 3, 42624270,  DOI: 10.1002/slct.201800719
    (c) Srivastava, V.; Singh, P. P. Eosin Y catalysed photoredox synthesis: a review. RSC Adv. 2017, 7, 3137731392,  DOI: 10.1039/C7RA05444K
    (d) Fukuzumi, S.; Kotani, H.; Ohkubo, K.; Ogo, S.; Tkachenko, N. V.; Lemmetyinen, H. Electron-Transfer State of 9-Mesityl-10-methylacridinium Ion with a Much Longer Lifetime and Higher Energy Than That of the Natural Photosynthetic Reaction Center. J. Am. Chem. Soc. 2004, 126, 16001601,  DOI: 10.1021/ja038656q
    (e) Alfonzo, E.; Alfonso, F. S.; Beeler, A. B. Redesign of a Pyrylium Photoredox Catalyst and Its Application to the Generation of Carbonyl Ylides. Org. Lett. 2017, 19, 29892992,  DOI: 10.1021/acs.orglett.7b01222
    (f) Weiser, M.; Hermann, S.; Penner, A.; Wagenknecht, H.-A. Photocatalytic nucleophilic addition of alcohols to styrenes in Markovnikov and anti-Markovnikov orientation. Beilstein J. Org. Chem. 2015, 11, 568575,  DOI: 10.3762/bjoc.11.62
  7. 7
    Tanaka, K.; Sukekawa, M.; Kishimoto, M.; Hoshino, Y.; Honda, K. Green-light-driven thioxanthylium-based organophotoredox catalysts: Organophotoredox promoted radical cation Diels-Alder reaction. Tetrahedron Lett. 2018, 59, 33613364,  DOI: 10.1016/j.tetlet.2018.07.058
  8. 8
    (a) Yadav, A. K.; Yadav, L. D. S. Visible light photoredox catalysis with N-hydroxyphthalimide for [4+2] cyclization between N-methylanilines and maleimides. Tetrahedron Lett. 2017, 58, 552555,  DOI: 10.1016/j.tetlet.2016.12.077
    (b) Santacroce, V.; Duboc, R.; Malacria, M.; Maestri, G.; Masson, G. Visible-Light, Photoredox-Mediated Oxidative Tandem Nitroso-Diels–Alder Reaction of Arylhydroxylamines with Conjugated Dienes. Eur. J. Org. Chem. 2017, 20952098,  DOI: 10.1002/ejoc.201601492
    (c) Frazier, C. P.; Palmer, L. I.; Samoshin, A. V.; Alaniz, J. R. Accessing nitrosocarbonyl compounds with temporal and spatial control via the photoredox oxidation of N-substituted hydroxylamines. Tetrahedron Lett. 2015, 56, 33533357,  DOI: 10.1016/j.tetlet.2015.01.024
    (d) Hu, X.; Zhang, G.; Bu, F.; Lei, A. Selective Oxidative [4+2] Imine/Alkene Annulation with H2 Liberation Induced by Photo-Oxidation. Angew. Chem., Int. Ed. 2018, 57, 12861290,  DOI: 10.1002/anie.201711359
  9. 9
    Hurtley, A. E.; Cismesia, M. A.; Ischay, M. A.; Yoon, T. P. Visible light photocatalysis of radical anion hetero-DielseAlder cycloadditions. Tetrahedron 2011, 67, 44424448,  DOI: 10.1016/j.tet.2011.02.066
  10. 10
    Van De Water, R. W.; Pettus, T. R. R. o-Quinone methides: intermediates under developed and underutilized in organic synthesis. Tetrahedron 2002, 58, 53675405,  DOI: 10.1016/S0040-4020(02)00496-9
  11. 11
    (a) Bai, W.-J.; David, J. G.; Feng, Z.-G.; Weaver, M. G.; Wu, K.-L.; Pettus, T. R. R. The Domestication of ortho-Quinone Methides. Acc. Chem. Res. 2014, 47, 36553664,  DOI: 10.1021/ar500330x
    (b) Singh, M. S.; Nagaraju, A.; Anand, N.; Chowdhury, S. ortho-Quinone methide (o-QM): a highly reactive, ephemeral and versatile intermediate in organic synthesis. RSC Adv. 2014, 4, 5592455959,  DOI: 10.1039/C4RA11444B
    (c) Willis, N. J.; Bray, C. D. ortho-Quinone Methides in Natural Product Synthesis. Chem. - Eur. J. 2012, 18, 91609173,  DOI: 10.1002/chem.201200619
    (d) Jaworski, A. A.; Scheidt, K. A. Emerging Roles of in Situ Generated Quinone Methides in Metal-Free Catalysis. J. Org. Chem. 2016, 81, 1014510153,  DOI: 10.1021/acs.joc.6b01367
  12. 12
    Very recently, blue-light-driven generation of ortho-quinone methides in the presence of Iridium catalyst was reported.Zhou, F.; Cheng, Y.; Liu, X.-P.; Chen, J.-R.; Xiao, W.-J. A visible light photoredox catalyzed carbon radical-mediated generation of ortho-quinone methides for 2,3-dihydrobenzofuran synthesis. Chem. Commun. 2019, 55, 31173120,  DOI: 10.1039/C9CC00727J
  13. 13
    (a) Tanaka, K.; Kishimoto, M.; Ohtsuka, N.; Iwama, Y.; Wada, H.; Hoshino, Y.; Honda, K. Highly Selective One-Pot Synthesis of Polysubstituted Isoflavanes using Styryl Ethers and Electron-Withdrawing ortho-Quinone Methides Generated In Situ. Synlett 2019, 30, 189192,  DOI: 10.1055/s-0037-1611361
    (b) Tanaka, K.; Hoshino, Y.; Honda, K. Development of Regioselective Inverse-Electron-Demand [4+2] Cycloaddition with Electron-Rich Arylalkynes for Access to Multi-Substituted Condensed Oxapolycyclic Compounds. J. Synth. Org. Chem., Jpn. 2018, 76, 13411351,  DOI: 10.5059/yukigoseikyokaishi.76.1341
    (c) Tanaka, K.; Sukekawa, M.; Shigematsu, Y.; Hoshino, Y.; Honda, K. Highly regioselective synthesis of 2,3-disubstituted 2H-1-benzopyrans: Brønsted acid catalyzed [4+2] cycloaddition reaction with a variety of arylalkynes via ortho-quinone methides. Tetrahedron 2017, 73, 64566464,  DOI: 10.1016/j.tet.2017.09.045
    (d) Tanaka, K.; Hoshino, Y.; Honda, K. A direct synthesis of 2,2-disubstituted 3-silylchromenes by [4+2] cycloaddition of in situ generated o-quinonemethides with electron-rich alkynes. Heterocycles 2017, 95, 474486,  DOI: 10.3987/COM-18-S(F)5
    (e) Tanaka, K.; Hoshino, Y.; Honda, K. A novel synthesis of polysubstituted chromenes from various salicylaldehydes and alkynes under mild conditions. Tetrahedron Lett. 2016, 57, 24482450,  DOI: 10.1016/j.tetlet.2016.04.086
    (f) Tanaka, K.; Shigematsu, Y.; Sukekawa, M.; Hoshino, Y.; Honda, K. Regioselective one-pot synthesis of 2,3-diaryl-2H-1-benzopyrans via Brønsted acid-catalyzed [4+2] cycloaddition of salicylaldehydes with diarylacetylenes. Tetrahedron Lett. 2016, 57, 59145918,  DOI: 10.1016/j.tetlet.2016.11.076
    (g) Inoue, S.; Wang, P.; Nagao, M.; Hoshino, Y.; Honda, K. One-Pot Stereoselective Synthesis of Pyrano[3,2-c]benzothiopyrans: A New Generation and [4+2] Cycloaddition of ortho-Thioquinonemethides. Synlett 2005, 3, 469472,  DOI: 10.1055/s-2005-862379
    (h) Miyazaki, H.; Honda, Y.; Honda, K.; Inoue, S. Facile synthesis and desulfurization of 5-(phenylthio)pyrano-[3,2-c][1]benzopyrans starting from 5-phenylthio-4-penten-1-ols and salicylaldehyde via in situ intramolecular cycloaddition of substituted o-quinonemethides. Tetrahedron Lett. 2000, 41, 26432647,  DOI: 10.1016/S0040-4039(00)00236-7
    (i) Miyazaki, H.; Honda, K.; Asami, M.; Inoue, S. Stereoselective Synthesis of Pyrano[3,2-c]benzopyrans via Intramolecular Cycloaddition of o-Quinonemethides Generated from Salicylaldehydes and Unsaturated Alcohols under Very Mild Conditions. J. Org. Chem. 1999, 64, 95079511,  DOI: 10.1021/jo991132h
    (j) Tanaka, K.; Sukekawa, M.; Hoshino, Y.; Honda, K. The Ring-contraction Reaction of Electron-deficient 3-Silylchromene to 2-Benzylbenzofuran under Mildly Basic Conditions. Chem. Lett. 2018, 47, 440443,  DOI: 10.1246/cl.171124
    (k) Tanaka, K.; Sukekawa, M.; Kishimoto, M.; Hoshino, Y.; Honda, K. CsF-Promoted Desilylation and Ring-Contraction Reaction of Electron-Deficient 3-Silyl-2H-chromenes to 2-Benzylbenzofurans. Heterocycles 2019, 99, 145170,  DOI: 10.3987/COM-18-S(F)5
  14. 14
    Tanaka, K.; Kishimoto, M.; Hoshino, Y.; Honda, K. Temperature-controlled divergent synthesis of 4-alkoxy- or 4-alkenylchromanes via inverse electron-demand cycloaddition with in situ generated ortho-quinone methides. Tetrahedron Lett. 2018, 59, 18411845,  DOI: 10.1016/j.tetlet.2018.03.090
  15. 15

    A radical cation Diels-Alder reaction that can be conducted in polar solvents such as CH3NO2 has been reported to proceed via a cationic intermediate:

    (a) Stevenson, S. M.; Shores, M. P.; Ferreira, E. M. Photooxidizing Chromium Catalysts for Promoting Radical Cation Cycloadditions. Angew. Chem., Int. Ed. 2015, 54, 65066510,  DOI: 10.1002/anie.201501220
    (b) Lin, S.; Ischay, M. A.; Fry, C. G.; Yoon, T. P. Radical Cation Diels-Alder Cycloadditions by Visible Light Photocatalysis. J. Am. Chem. Soc. 2011, 133, 1935019353,  DOI: 10.1021/ja2093579
    (c) Higgins, R. F.; Fatur, S. M.; Shepard, S. G.; Stevenson, S. M.; Boston, D. J.; Ferreira, E. M.; Damrauer, N. H.; Rappé, A. K.; Shores, M. P. Uncovering the Roles of Oxygen in Cr(III) Photoredox Catalysis. J. Am. Chem. Soc. 2016, 138, 54515464,  DOI: 10.1021/jacs.6b02723
    (d) Alpers, D.; Gallhof, M.; Stark, C. B. W.; Brasholz, M. Photoassisted oxidation of ruthenium(II)-photocatalysts Ru(bpy)32+ and Ru(bpz)32+ to RuO4: orthogonal tandem photoredox and oxidation catalysis. Chem. Commun. 2016, 52, 10251028,  DOI: 10.1039/C5CC08994H
    (e) Zhao, Y.; Antonietti, M. Visible-Light-Irradiated Graphitic Carbon Nitride Photocatalyzed Diels–Alder Reactions with Dioxygen as Sustainable Mediator for Photoinduced Electrons. Angew. Chem., Int. Ed. 2017, 56, 93369340,  DOI: 10.1002/anie.201703438
    (f) Pitre, S. P.; Scaiano, J. C.; Yoon, T. P. Photocatalytic Indole Diels–Alder Cycloadditions Mediated by Heterogeneous Platinum-Modified Titanium Dioxide. ACS Catal. 2017, 7, 64406444,  DOI: 10.1021/acscatal.7b02223
  16. 16
    (a) The details of the X-ray diffraction analysis can be found in the Supporting Information.
    (b) Farrugia, L. J. WinGX and ORTEP for Windows: an update. J. Appl. Crystallogr. 2012, 45, 849854,  DOI: 10.1107/S0021889812029111
  17. 17
    (a) Srivastava, V.; Singh, P. P. Eosin Y catalysed photoredox synthesis: a review. RSC Adv. 2017, 7, 3137731392,  DOI: 10.1039/C7RA05444K
    (b) Hari, D. P.; Koenig, B. Synthetic applications of eosin Y in photoredox catalysis. Chem. Commun. 2014, 50, 66886699,  DOI: 10.1039/C4CC00751D
    (c) Sun, D.; Zhang, R. Transition-metal-free, visible-light-induced oxidative cross-coupling for constructing β-acetylamino acrylosulfones from sodium sulfinates and enamides. Org. Chem. Front. 2018, 5, 9297,  DOI: 10.1039/C7QO00729A
    (d) Vila, C.; Lau, J.; Rueping, M. Visible-light photoredox catalyzed synthesis of pyrroloisoquinolines via organocatalytic oxidation/[3 + 2] cycloaddition/oxidative aromatization reaction cascade with Rose Bengal. Beilstein J. Org. Chem. 2014, 10, 12331238,  DOI: 10.3762/bjoc.10.122
  18. 18

    The cyclic voltammograms of the photocatalyst and substrates are shown in the Supporting Information.

  19. 19
    Roth, H. G.; Romero, N. A.; Nicewicz, D. A. Experimental and Calculated Electrochemical Potentials of Common Organic Molecules for Applications to Single-Electron Redox Chemistry. Synlett 2016, 27, 714723,  DOI: 10.1055/s-0035-1561297
  20. 20
    Okada, Y.; Chiba, K. Redox-Tag Processes: Intramolecular Electron Transfer and Its Broad Relationship to Redox Reactions in General. Chem. Rev. 2018, 118, 45924630,  DOI: 10.1021/acs.chemrev.7b00400
  21. 21

    The stereoselectivity of radical cation [4+2] cycloaddition is low; for details, see: ref 13 (13) and

    (a) Bellville, D. J.; Bauld, N. L. Selectivity Profile of the Cation Radical Diels-Alder Reaction. J. Am. Chem. Soc. 1982, 104, 26652667,  DOI: 10.1021/ja00373a069
    (b) Bellville, D. J.; Wirth, D. D.; Bauld, N. L. The Cation-Radical Catalyzed Diels-Alder Reaction. J. Am. Chem. Soc. 1981, 103, 718720,  DOI: 10.1021/ja00393a061
    (c) Bauld, N. L. Cation Radical Cycloadditions And Related Sigmatropic Reactions. Tetrahedron 1989, 45, 53075363,  DOI: 10.1016/S0040-4020(01)89486-2
    (d) Bauld, N. L.; Yang, J.; Gao, D. Diels–Alder cycloadditions of the N-vinylcarbazole radical cation. J. Chem. Soc., Perkin Trans. 2 2000, 2, 207210,  DOI: 10.1039/a908659e
  22. 22

    The details regarding the experimental determination of the reaction quantum yield can be found in the Supporting Information.

  23. 23
    Jurd, L. Quinone and quinone-methides. Tetrahedron 1977, 33, 163168,  DOI: 10.1016/0040-4020(77)80122-1
  24. 24
    Tanaka, K. Development of pericyclic reaction for environmental loading reduction. Ph.D. Thesis, Yokohama National University, Japan, March, 2017.
  25. 25
    Alberico, D.; Rudolph, A.; Lautens, M. Synthesis of Tricyclic Heterocycles via a Tandem Aryl Alkylation/Heck Coupling Sequence. J. Org. Chem. 2007, 72, 775781,  DOI: 10.1021/jo0617868
  26. 26
    (a) Zhao, P.; Yin, H.; Gao, H.; Xi, C. Cu-Catalyzed Synthesis of Diaryl Thioethers and S-Cycles by Reaction of Aryl Iodides with Carbon Disulfide in the Presence of DBU. J. Org. Chem. 2013, 78, 50015006,  DOI: 10.1021/jo400709s
    (b) Cavanagh, C. W.; Aukland, M. H.; Hennessy, A.; Procter, D. J. Iron-mediated C–H coupling of arenes and unactivated terminal alkenes directed by sulfur. Chem. Commun. 2015, 51, 92729275,  DOI: 10.1039/C5CC02676H

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  26. Fan-Xiao Meng, Ruo-Nan Wang, Hong-Li Huang, Shu-Wen Gong, Qian-Li Li, Shao-Liang Zhang, Chun-Lin Ma, Chen-Zhong Li, Ji-Yuan Du. Lewis acid-catalyzed tandem cyclization of in situ generated o -quinone methides and arylsulfonyl hydrazides for a one-pot entry to 3-sulfonylbenzofurans. Organic Chemistry Frontiers 2019, 6 (24) , 3929-3933. https://doi.org/10.1039/C9QO01196J
  27. Kenta Tanaka, Yuta Tanaka, Mami Kishimoto, Yujiro Hoshino, Kiyoshi Honda. Friedel–Crafts approach to the one-pot synthesis of methoxy-substituted thioxanthylium salts. Beilstein Journal of Organic Chemistry 2019, 15 , 2105-2112. https://doi.org/10.3762/bjoc.15.208
  • Abstract

    Scheme 1

    Scheme 1. Representative Examples of Oxa-[4+2] Cycloaddition Reactionsa

    aPMP: p-methoxyphenyl.

    Figure 1

    Figure 1. Stern–Volmer plots for 4-methoxystyrene 2a (blue line) and ortho-quinone methide 1a (red line).

    Scheme 2

    Scheme 2. Proposed Reaction Mechanisma

    aPMP: p-methoxyphenyl.

  • References

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    Jump To

    This article references 26 other publications.

    1. 1
      (a) Zhao, Y.; Lv, Y.; Xia, W. Synthesis of Cyclic Compounds via Photoinduced Radical Cyclization Cascade of C═C bonds. Chem. Rec. 2019, 19, 424439,  DOI: 10.1002/tcr.201800050
      (b) Xuan, J.; Lu, L.-Q.; Chen, J.-R.; Xiao, W.-J. Visible-Light-Driven Photoredox Catalysis in the Construction of Carbocyclic and Heterocyclic Ring Systems. Eur. J. Org. Chem. 2013, 67556770,  DOI: 10.1002/ejoc.201300596
      (c) Liu, Y.; Song, R.; Li, J. The cycloaddition reaction using visible light photoredox catalysis. Sci. China Chem. 2016, 59, 161170,  DOI: 10.1007/s11426-015-5516-5
    2. 2
      (a) Pałasz, A. Recent Advances in Inverse-Electron-Demand Hetero-Diels–Alder Reactions of 1-Oxa-1,3-Butadienes. Top. Curr. Chem. 2016, 374, 24,  DOI: 10.1007/s41061-016-0026-2
      (b) Hall, D. G.; Rybak, T.; Verdelet, T. Multicomponent Hetero-[4 + 2] Cycloaddition/Allylboration Reaction: From Natural Product Synthesis to Drug Discovery. Acc. Chem. Res. 2016, 49, 24892500,  DOI: 10.1021/acs.accounts.6b00403
      (c) Neely, J. M.; Rovis, T. Pyridine synthesis by [4 + 2] cycloadditions of 1-azadienes: hetero-Diels Alder and transition metal-catalysed approaches. Org. Chem. Front. 2014, 1, 10101015,  DOI: 10.1039/C4QO00187G
      (d) Rizzacasa, M. A.; Pollex, A. The hetero-Diels–Alder approach to spiroketals. Org. Biomol. Chem. 2009, 7, 10531059,  DOI: 10.1039/b819966n
    3. 3
      (a) Pałasz, A.; Bogdanowicz-Szwed, K. Hetero-Diels-Alder reaction of propenenitriles with enol ethers: a convenient approach to functionalized 3,4-dihydro-2H-pyrans. Monatsh. Chem. 2008, 139, 647655,  DOI: 10.1007/s00706-007-0824-x
      (b) Pałasz, A.; Pałasz, T. Knoevenagel condensation of cyclic ketones with benzoylacetonitrile and N,N′-dimethylbarbituric acid. Application of sterically hindered condensation products in the synthesis of spiro and dispiropyrans by hetero-DielseAlder reactions. Tetrahedron 2011, 67, 14221431,  DOI: 10.1016/j.tet.2010.12.053
      (c) Hanessian, S.; Compain, P. Lewis acid promoted cyclocondensations of α-ketophosphonoenoates with dienes—from Diels–Alder to hetero Diels–Alder reactions. Tetrahedron 2002, 58, 65216529,  DOI: 10.1016/S0040-4020(02)00662-2
      (d) Davies, H. M. L.; Dai, X. Lewis Acid-Catalyzed Tandem Diels-Alder Reaction/Retro-Claisen Rearrangement as an Equivalent of the Inverse Electron Demand Hetero Diels-Alder Reaction. J. Org. Chem. 2005, 70, 66806684,  DOI: 10.1021/jo050821s
      (e) Yang, Y.; Liu, H.; Peng, C.; Wu, J.; Zhang, J.; Qiao, Y.; Wang, X.-N.; Chang, J. AlCl3-Catalyzed Annulations of Ynamides Involving a Torquoselective Process for the Simultaneous Control of Central and Axial Chirality. Org. Lett. 2016, 18, 50225025,  DOI: 10.1021/acs.orglett.6b02480
    4. 4
      (a) Arumugam, S.; Popik, V. V. Light-Induced Hetero-DielsAlder Cycloaddition: A Facile and Selective Photoclick Reaction. J. Am. Chem. Soc. 2011, 133, 55735579,  DOI: 10.1021/ja200356f
      (b) Fujiwara, M.; Sakamoto, M.; Komeyama, K.; Yoshida, H.; Takaki, K. Convenient Synthesis of 2-Amino-4H-chromenes from Photochemically Generated o-Quinone Methides and Malononitrile. J. Heterocycl. Chem. 2015, 52, 5966,  DOI: 10.1002/jhet.1964
      (c) Nakatani, K.; Higashida, N.; Saito, I. Highly from Efficient Photochemical Generation of o-Quinone Methide Mannich Bases of Phenol Derivatives. Tetrahedron Lett. 1997, 38, 50055008,  DOI: 10.1016/S0040-4039(97)01071-X
      (d) Zhang, X. Mechanistic study on the intramolecular oxa-[4 + 2] cycloaddition of substituted o-divinylbenzenes. J. Mol. Model. 2019, 25, 14,  DOI: 10.1007/s00894-018-3883-5
      (e) Liu, Q.; Wang, J.; Li, D.; Gao, G.-L.; Yang, C.; Gao, Y.; Xia, W. Synthesis of Oxatricyclooctanes via Photoinduced Intramolecular Oxa-[4+2] Cycloaddition of Substituted o-Divinylbenzenes. J. Org. Chem. 2017, 82, 78567868,  DOI: 10.1021/acs.joc.7b01055
    5. 5
      (a) Yan, D.-M.; Chen, J.-R.; Xiao, W.-J. New Roles for Photoexcited Eosin Y in Photochemical Reactions. Angew. Chem., Int. Ed. 2019, 58, 378380,  DOI: 10.1002/anie.201811102
      (b) Romero, N. A.; Nicewicz, D. A. Organic Photoredox Catalysis. Chem. Rev. 2016, 116, 1007510166,  DOI: 10.1021/acs.chemrev.6b00057
      (c) Nicewicz, D. A.; Nguyen, T. M. Recent Applications of Organic Dyes as Photoredox Catalysts in Organic Synthesis. ACS Catal. 2014, 4, 355360,  DOI: 10.1021/cs400956a
    6. 6
      (a) Le Vaillant, F.; Garreau, M.; Nicolai, S.; Grynova, G.; Corminboeuf, C.; Waser, J. Fine-tuned organic photoredox catalysts for fragmentation-alkynylation cascades of cyclic oxime ethers. Chem. Sci. 2018, 9, 58835889,  DOI: 10.1039/C8SC01818A
      (b) Hloušková, Z.; Tydlitat, J.; Kong, M.; Pytela, O.; Mikysek, T.; Klikar, M.; Almonasy, N.; Dvorak, M.; Jiang, Z.; Ruzicka, A.; Bures, F. Structure-Catalytic Activity in a Series of Push-Pull Dicyanopyrazine/Dicyanoimidazole Photoredox Catalysts. ChemistrySelect 2018, 3, 42624270,  DOI: 10.1002/slct.201800719
      (c) Srivastava, V.; Singh, P. P. Eosin Y catalysed photoredox synthesis: a review. RSC Adv. 2017, 7, 3137731392,  DOI: 10.1039/C7RA05444K
      (d) Fukuzumi, S.; Kotani, H.; Ohkubo, K.; Ogo, S.; Tkachenko, N. V.; Lemmetyinen, H. Electron-Transfer State of 9-Mesityl-10-methylacridinium Ion with a Much Longer Lifetime and Higher Energy Than That of the Natural Photosynthetic Reaction Center. J. Am. Chem. Soc. 2004, 126, 16001601,  DOI: 10.1021/ja038656q
      (e) Alfonzo, E.; Alfonso, F. S.; Beeler, A. B. Redesign of a Pyrylium Photoredox Catalyst and Its Application to the Generation of Carbonyl Ylides. Org. Lett. 2017, 19, 29892992,  DOI: 10.1021/acs.orglett.7b01222
      (f) Weiser, M.; Hermann, S.; Penner, A.; Wagenknecht, H.-A. Photocatalytic nucleophilic addition of alcohols to styrenes in Markovnikov and anti-Markovnikov orientation. Beilstein J. Org. Chem. 2015, 11, 568575,  DOI: 10.3762/bjoc.11.62
    7. 7
      Tanaka, K.; Sukekawa, M.; Kishimoto, M.; Hoshino, Y.; Honda, K. Green-light-driven thioxanthylium-based organophotoredox catalysts: Organophotoredox promoted radical cation Diels-Alder reaction. Tetrahedron Lett. 2018, 59, 33613364,  DOI: 10.1016/j.tetlet.2018.07.058
    8. 8
      (a) Yadav, A. K.; Yadav, L. D. S. Visible light photoredox catalysis with N-hydroxyphthalimide for [4+2] cyclization between N-methylanilines and maleimides. Tetrahedron Lett. 2017, 58, 552555,  DOI: 10.1016/j.tetlet.2016.12.077
      (b) Santacroce, V.; Duboc, R.; Malacria, M.; Maestri, G.; Masson, G. Visible-Light, Photoredox-Mediated Oxidative Tandem Nitroso-Diels–Alder Reaction of Arylhydroxylamines with Conjugated Dienes. Eur. J. Org. Chem. 2017, 20952098,  DOI: 10.1002/ejoc.201601492
      (c) Frazier, C. P.; Palmer, L. I.; Samoshin, A. V.; Alaniz, J. R. Accessing nitrosocarbonyl compounds with temporal and spatial control via the photoredox oxidation of N-substituted hydroxylamines. Tetrahedron Lett. 2015, 56, 33533357,  DOI: 10.1016/j.tetlet.2015.01.024
      (d) Hu, X.; Zhang, G.; Bu, F.; Lei, A. Selective Oxidative [4+2] Imine/Alkene Annulation with H2 Liberation Induced by Photo-Oxidation. Angew. Chem., Int. Ed. 2018, 57, 12861290,  DOI: 10.1002/anie.201711359
    9. 9
      Hurtley, A. E.; Cismesia, M. A.; Ischay, M. A.; Yoon, T. P. Visible light photocatalysis of radical anion hetero-DielseAlder cycloadditions. Tetrahedron 2011, 67, 44424448,  DOI: 10.1016/j.tet.2011.02.066
    10. 10
      Van De Water, R. W.; Pettus, T. R. R. o-Quinone methides: intermediates under developed and underutilized in organic synthesis. Tetrahedron 2002, 58, 53675405,  DOI: 10.1016/S0040-4020(02)00496-9
    11. 11
      (a) Bai, W.-J.; David, J. G.; Feng, Z.-G.; Weaver, M. G.; Wu, K.-L.; Pettus, T. R. R. The Domestication of ortho-Quinone Methides. Acc. Chem. Res. 2014, 47, 36553664,  DOI: 10.1021/ar500330x
      (b) Singh, M. S.; Nagaraju, A.; Anand, N.; Chowdhury, S. ortho-Quinone methide (o-QM): a highly reactive, ephemeral and versatile intermediate in organic synthesis. RSC Adv. 2014, 4, 5592455959,  DOI: 10.1039/C4RA11444B
      (c) Willis, N. J.; Bray, C. D. ortho-Quinone Methides in Natural Product Synthesis. Chem. - Eur. J. 2012, 18, 91609173,  DOI: 10.1002/chem.201200619
      (d) Jaworski, A. A.; Scheidt, K. A. Emerging Roles of in Situ Generated Quinone Methides in Metal-Free Catalysis. J. Org. Chem. 2016, 81, 1014510153,  DOI: 10.1021/acs.joc.6b01367
    12. 12
      Very recently, blue-light-driven generation of ortho-quinone methides in the presence of Iridium catalyst was reported.Zhou, F.; Cheng, Y.; Liu, X.-P.; Chen, J.-R.; Xiao, W.-J. A visible light photoredox catalyzed carbon radical-mediated generation of ortho-quinone methides for 2,3-dihydrobenzofuran synthesis. Chem. Commun. 2019, 55, 31173120,  DOI: 10.1039/C9CC00727J
    13. 13
      (a) Tanaka, K.; Kishimoto, M.; Ohtsuka, N.; Iwama, Y.; Wada, H.; Hoshino, Y.; Honda, K. Highly Selective One-Pot Synthesis of Polysubstituted Isoflavanes using Styryl Ethers and Electron-Withdrawing ortho-Quinone Methides Generated In Situ. Synlett 2019, 30, 189192,  DOI: 10.1055/s-0037-1611361
      (b) Tanaka, K.; Hoshino, Y.; Honda, K. Development of Regioselective Inverse-Electron-Demand [4+2] Cycloaddition with Electron-Rich Arylalkynes for Access to Multi-Substituted Condensed Oxapolycyclic Compounds. J. Synth. Org. Chem., Jpn. 2018, 76, 13411351,  DOI: 10.5059/yukigoseikyokaishi.76.1341
      (c) Tanaka, K.; Sukekawa, M.; Shigematsu, Y.; Hoshino, Y.; Honda, K. Highly regioselective synthesis of 2,3-disubstituted 2H-1-benzopyrans: Brønsted acid catalyzed [4+2] cycloaddition reaction with a variety of arylalkynes via ortho-quinone methides. Tetrahedron 2017, 73, 64566464,  DOI: 10.1016/j.tet.2017.09.045
      (d) Tanaka, K.; Hoshino, Y.; Honda, K. A direct synthesis of 2,2-disubstituted 3-silylchromenes by [4+2] cycloaddition of in situ generated o-quinonemethides with electron-rich alkynes. Heterocycles 2017, 95, 474486,  DOI: 10.3987/COM-18-S(F)5
      (e) Tanaka, K.; Hoshino, Y.; Honda, K. A novel synthesis of polysubstituted chromenes from various salicylaldehydes and alkynes under mild conditions. Tetrahedron Lett. 2016, 57, 24482450,  DOI: 10.1016/j.tetlet.2016.04.086
      (f) Tanaka, K.; Shigematsu, Y.; Sukekawa, M.; Hoshino, Y.; Honda, K. Regioselective one-pot synthesis of 2,3-diaryl-2H-1-benzopyrans via Brønsted acid-catalyzed [4+2] cycloaddition of salicylaldehydes with diarylacetylenes. Tetrahedron Lett. 2016, 57, 59145918,  DOI: 10.1016/j.tetlet.2016.11.076
      (g) Inoue, S.; Wang, P.; Nagao, M.; Hoshino, Y.; Honda, K. One-Pot Stereoselective Synthesis of Pyrano[3,2-c]benzothiopyrans: A New Generation and [4+2] Cycloaddition of ortho-Thioquinonemethides. Synlett 2005, 3, 469472,  DOI: 10.1055/s-2005-862379
      (h) Miyazaki, H.; Honda, Y.; Honda, K.; Inoue, S. Facile synthesis and desulfurization of 5-(phenylthio)pyrano-[3,2-c][1]benzopyrans starting from 5-phenylthio-4-penten-1-ols and salicylaldehyde via in situ intramolecular cycloaddition of substituted o-quinonemethides. Tetrahedron Lett. 2000, 41, 26432647,  DOI: 10.1016/S0040-4039(00)00236-7
      (i) Miyazaki, H.; Honda, K.; Asami, M.; Inoue, S. Stereoselective Synthesis of Pyrano[3,2-c]benzopyrans via Intramolecular Cycloaddition of o-Quinonemethides Generated from Salicylaldehydes and Unsaturated Alcohols under Very Mild Conditions. J. Org. Chem. 1999, 64, 95079511,  DOI: 10.1021/jo991132h
      (j) Tanaka, K.; Sukekawa, M.; Hoshino, Y.; Honda, K. The Ring-contraction Reaction of Electron-deficient 3-Silylchromene to 2-Benzylbenzofuran under Mildly Basic Conditions. Chem. Lett. 2018, 47, 440443,  DOI: 10.1246/cl.171124
      (k) Tanaka, K.; Sukekawa, M.; Kishimoto, M.; Hoshino, Y.; Honda, K. CsF-Promoted Desilylation and Ring-Contraction Reaction of Electron-Deficient 3-Silyl-2H-chromenes to 2-Benzylbenzofurans. Heterocycles 2019, 99, 145170,  DOI: 10.3987/COM-18-S(F)5
    14. 14
      Tanaka, K.; Kishimoto, M.; Hoshino, Y.; Honda, K. Temperature-controlled divergent synthesis of 4-alkoxy- or 4-alkenylchromanes via inverse electron-demand cycloaddition with in situ generated ortho-quinone methides. Tetrahedron Lett. 2018, 59, 18411845,  DOI: 10.1016/j.tetlet.2018.03.090
    15. 15

      A radical cation Diels-Alder reaction that can be conducted in polar solvents such as CH3NO2 has been reported to proceed via a cationic intermediate:

      (a) Stevenson, S. M.; Shores, M. P.; Ferreira, E. M. Photooxidizing Chromium Catalysts for Promoting Radical Cation Cycloadditions. Angew. Chem., Int. Ed. 2015, 54, 65066510,  DOI: 10.1002/anie.201501220
      (b) Lin, S.; Ischay, M. A.; Fry, C. G.; Yoon, T. P. Radical Cation Diels-Alder Cycloadditions by Visible Light Photocatalysis. J. Am. Chem. Soc. 2011, 133, 1935019353,  DOI: 10.1021/ja2093579
      (c) Higgins, R. F.; Fatur, S. M.; Shepard, S. G.; Stevenson, S. M.; Boston, D. J.; Ferreira, E. M.; Damrauer, N. H.; Rappé, A. K.; Shores, M. P. Uncovering the Roles of Oxygen in Cr(III) Photoredox Catalysis. J. Am. Chem. Soc. 2016, 138, 54515464,  DOI: 10.1021/jacs.6b02723
      (d) Alpers, D.; Gallhof, M.; Stark, C. B. W.; Brasholz, M. Photoassisted oxidation of ruthenium(II)-photocatalysts Ru(bpy)32+ and Ru(bpz)32+ to RuO4: orthogonal tandem photoredox and oxidation catalysis. Chem. Commun. 2016, 52, 10251028,  DOI: 10.1039/C5CC08994H
      (e) Zhao, Y.; Antonietti, M. Visible-Light-Irradiated Graphitic Carbon Nitride Photocatalyzed Diels–Alder Reactions with Dioxygen as Sustainable Mediator for Photoinduced Electrons. Angew. Chem., Int. Ed. 2017, 56, 93369340,  DOI: 10.1002/anie.201703438
      (f) Pitre, S. P.; Scaiano, J. C.; Yoon, T. P. Photocatalytic Indole Diels–Alder Cycloadditions Mediated by Heterogeneous Platinum-Modified Titanium Dioxide. ACS Catal. 2017, 7, 64406444,  DOI: 10.1021/acscatal.7b02223
    16. 16
      (a) The details of the X-ray diffraction analysis can be found in the Supporting Information.
      (b) Farrugia, L. J. WinGX and ORTEP for Windows: an update. J. Appl. Crystallogr. 2012, 45, 849854,  DOI: 10.1107/S0021889812029111
    17. 17
      (a) Srivastava, V.; Singh, P. P. Eosin Y catalysed photoredox synthesis: a review. RSC Adv. 2017, 7, 3137731392,  DOI: 10.1039/C7RA05444K
      (b) Hari, D. P.; Koenig, B. Synthetic applications of eosin Y in photoredox catalysis. Chem. Commun. 2014, 50, 66886699,  DOI: 10.1039/C4CC00751D
      (c) Sun, D.; Zhang, R. Transition-metal-free, visible-light-induced oxidative cross-coupling for constructing β-acetylamino acrylosulfones from sodium sulfinates and enamides. Org. Chem. Front. 2018, 5, 9297,  DOI: 10.1039/C7QO00729A
      (d) Vila, C.; Lau, J.; Rueping, M. Visible-light photoredox catalyzed synthesis of pyrroloisoquinolines via organocatalytic oxidation/[3 + 2] cycloaddition/oxidative aromatization reaction cascade with Rose Bengal. Beilstein J. Org. Chem. 2014, 10, 12331238,  DOI: 10.3762/bjoc.10.122
    18. 18

      The cyclic voltammograms of the photocatalyst and substrates are shown in the Supporting Information.

    19. 19
      Roth, H. G.; Romero, N. A.; Nicewicz, D. A. Experimental and Calculated Electrochemical Potentials of Common Organic Molecules for Applications to Single-Electron Redox Chemistry. Synlett 2016, 27, 714723,  DOI: 10.1055/s-0035-1561297
    20. 20
      Okada, Y.; Chiba, K. Redox-Tag Processes: Intramolecular Electron Transfer and Its Broad Relationship to Redox Reactions in General. Chem. Rev. 2018, 118, 45924630,  DOI: 10.1021/acs.chemrev.7b00400
    21. 21

      The stereoselectivity of radical cation [4+2] cycloaddition is low; for details, see: ref 13 (13) and

      (a) Bellville, D. J.; Bauld, N. L. Selectivity Profile of the Cation Radical Diels-Alder Reaction. J. Am. Chem. Soc. 1982, 104, 26652667,  DOI: 10.1021/ja00373a069
      (b) Bellville, D. J.; Wirth, D. D.; Bauld, N. L. The Cation-Radical Catalyzed Diels-Alder Reaction. J. Am. Chem. Soc. 1981, 103, 718720,  DOI: 10.1021/ja00393a061
      (c) Bauld, N. L. Cation Radical Cycloadditions And Related Sigmatropic Reactions. Tetrahedron 1989, 45, 53075363,  DOI: 10.1016/S0040-4020(01)89486-2
      (d) Bauld, N. L.; Yang, J.; Gao, D. Diels–Alder cycloadditions of the N-vinylcarbazole radical cation. J. Chem. Soc., Perkin Trans. 2 2000, 2, 207210,  DOI: 10.1039/a908659e
    22. 22

      The details regarding the experimental determination of the reaction quantum yield can be found in the Supporting Information.

    23. 23
      Jurd, L. Quinone and quinone-methides. Tetrahedron 1977, 33, 163168,  DOI: 10.1016/0040-4020(77)80122-1
    24. 24
      Tanaka, K. Development of pericyclic reaction for environmental loading reduction. Ph.D. Thesis, Yokohama National University, Japan, March, 2017.
    25. 25
      Alberico, D.; Rudolph, A.; Lautens, M. Synthesis of Tricyclic Heterocycles via a Tandem Aryl Alkylation/Heck Coupling Sequence. J. Org. Chem. 2007, 72, 775781,  DOI: 10.1021/jo0617868
    26. 26
      (a) Zhao, P.; Yin, H.; Gao, H.; Xi, C. Cu-Catalyzed Synthesis of Diaryl Thioethers and S-Cycles by Reaction of Aryl Iodides with Carbon Disulfide in the Presence of DBU. J. Org. Chem. 2013, 78, 50015006,  DOI: 10.1021/jo400709s
      (b) Cavanagh, C. W.; Aukland, M. H.; Hennessy, A.; Procter, D. J. Iron-mediated C–H coupling of arenes and unactivated terminal alkenes directed by sulfur. Chem. Commun. 2015, 51, 92729275,  DOI: 10.1039/C5CC02676H
  • Supporting Information

    Supporting Information

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    The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.9b01156.

    • Crystallography data for 3a (CIF)

    • Characterization for TXT catalyst, photophysical and redox properties of TXT and representative substrates, Stern–Volmer plot of TXT, reaction quantum yield, NMR spectra of new compounds, detailed crystallography, and detailed experimental procedures (PDF)


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