Photocatalytic Deoxygenation of Sulfoxides Using Visible Light: Mechanistic Investigations and Synthetic Applications

The photocatalytic deoxygenation of sulfoxides to generate sulfides facilitated by either Ir[(dF(CF3)ppy)2(dtbbpy)]PF6 or fac-Ir(ppy)3 is reported. Mechanistic studies indicate that a radical chain mechanism operates, which proceeds via a phosphoranyl radical generated from a radical/polar crossover process. Initiation of the radical chain was found to proceed via two opposing photocatalytic quenching mechanisms, offering complementary reactivity. The mild nature of the radical deoxygenation process enables the reduction of a wide range of functionalized sulfoxides, including those containing acid-sensitive groups, in typically high isolated yields.


General procedures General procedure A -Sulfoxide preparation
To a solution of sulfide (1.00 mmol) in anhydrous CH2Cl2 (3 mL) at 0 °C was added 3-chloroperbenzoic acid (1.20 mmol) portionwise. The reaction mixture was left to gradually warm to RT and stirred overnight. The reaction was quenched by the addition of sat. aq. NaHCO3 (1 mL) and stirred for 5 min. Following this, the reaction mixture was concentrated in vacuo and purified by column chromatography to afford the sulfoxide product.

General procedure B -Sulfoxide reduction
To an 8 mL vial equipped with a PTFE septum and a magnetic stirrer bar was added [Ir{dF(CF3)ppy}2(dtbpy)]PF6 (0.003 mmol, 0.01 equiv.), PPh3 (0.36 mmol, 1.2 equiv.) and sulfoxide (if solid, 0.30 mmol, 1.0 equiv.). The reaction vessel was purged by alternating vacuum and argon three times before anhydrous and degassed CH2Cl2 (1.5 mL) was added. Sulfoxide (if liquid, 0.30 mmol, 1.0 equiv.) was added and the septum additionally sealed with paraffin film. The reaction was irradiated with a 60 W blue LED floodlight for 24 h, with rapid stirring and cooling from a small fan to maintain an ambient temperature. The reaction mixture was then directly poured on to silica and purified by column chromatography affording the desired sulfide product.

4-(Methylthio)phenol (13b)
To an 8 mL vial equipped with a PTFE septum and a magnetic stirrer bar was added

Cyclic Voltammetry Information
The cyclic voltammograms were performed using a 5 mL electrochemical cell vial containing a glassy carbon disk working electrode, platinum counter electrode and Ag/AgCl reference electrode, all from the IKA ElectraSyn range.
The cell lid was modified in-house to permit connection to an EmStat potentiostat and the data was collected using the complementary PSTrace software.
The same procedure was followed for experiments conducted on both PPh3 and sulfoxide, 8a. First, 77 mg of tetrabutylammonium hexafluorophosphate (0.2 mmol Bu4NPF6, Acros Organics, 98%) was added to the cell vial, the lid was attached and then the vial was purged with N2 for approximately 5 min via the access port. After this, 2 mL anhydrous CH2Cl2 was added then three control "background" cyclic voltammograms were recorded over a

F NMR Spectra
Examination of the reaction mixture for the conversion of 8a into 8b using PC2 revealed that PC2 remained unchanged. All 19 F NMR signals corresponding to pure PC2 (bottom, red) are observed in the 19 F NMR spectrum of the reaction mixture for the conversion of 8a into 8b (top, blue), suggesting that the catalyst does not degrade significantly or form aggregates during the course of the reaction. Figure S2. Overlaid 19 F NMR spectra of the reaction mixture for the conversion of 8a into 8b using PC2 (top, blue) and pure PC2 (bottom, red).