Synthesis and Reactivity of Fluorinated Triaryl Aluminum Complexes

The addition of the Grignard 3,4,5-ArFMgBr to aluminum(III) chloride in ether generates the novel triarylalane Al(3,4,5-ArF)3·OEt2. Attempts to synthesize this alane via transmetalation from the parent borane with trimethylaluminum gave a dimeric structure with bridging methyl groups, a product of partial transmetalation. On the other hand, the novel alane Al(2,3,4-ArF)3 was synthesized from the parent borane and trimethylaluminum. Interestingly, the solid-state structure of Al(2,3,4-ArF)3 shows an extended chain structure resulting from neighboring Al···F contacts. Al(3,4,5-ArF)3·OEt2 was then found to be an effective catalyst for the hydroboration of carbonyls, imines, and alkynes with pinacolborane.

S1 Experimental procedure and compound characterization S1.1 General experimental Unless stated otherwise, all reactions were carried out under an atmosphere of dinitrogen using standard Schlenk and glove box techniques. With the exception of THF, Et2O and deuterated solvents, all solvents used were dried by passing through an alumina column incorporated into an MB SPS-800 solvent purification system, degassed and finally stored in an ampoule fitted with a Teflon valve under a dinitrogen atmosphere. THF was dried over molten potassium for three days and distilled over argon, whereas Et2O was dried over sodium wire and benzophenone before being distilled over argon.
Deuterated solvents were dried over calcium hydride, distilled, freeze-pump-thawed degassed and stored over 3 Å molecular sieves in a glove box. Starting materials were purchased from commercial suppliers and used as received. 1 H, 13 C{ 1 H}, 19 F, and 11 B NMR spectra were recorded on a Bruker Avance 400 or 500 MHz spectrometer. Chemical shifts are expressed as parts per million (ppm, δ) and are referenced to CDCl3 (7.26/77.16 ppm) or C6D6 (7.16/128.06 ppm) as internal standards. Multinuclear NMR spectra were referenced to BF3·Et2O/CDCl3 ( 11 B), CFCl3 ( 19 F). The description of signals includes s = singlet, d = doublet, t = triplet, q = quartet, sept = septet, ov dd = overlapping doublet of doublets, and m = multiplet. All coupling constants are absolute values and are expressed in Hertz (Hz). IR-Spectra were measured on a Shimadzu IR Affinity-1 photospectrometer. The description of signals includes s = strong, m = medium, w = weak, sh = shoulder, and br = broad. Mass spectra were measured by the School of Chemistry in Cardiff University on a Waters LCT Premier/XE or a Waters GCT Premier spectrometer.
Note 1: the aluminium compounds prepared in this manuscript are potentially shock and thermally sensitive due to the potential formation of benzyne intermediates. Appropriate care should be taken. Note 2: due to the potential for benzyne formation, lack of in-house elemental analysis (EA) facilities, and the closure of our laboratory as well as the external EA facilities due to the COVID19 pandemic, some elemental analyses of the samples were not performed. For these compounds bulk purity was determined by multinuclear NMR spectroscopy with any residual solvent or starting material accounted for. S3 S1.2 Synthesis of imines General procedure 2 In accordance with the literature known procedure, 1 the necessary aldehyde (10 mmol) was dissolved in CH2Cl2 (10 mL) along with 3 Å molecular sieves. To this the required amine (10 mmol) was added.
The reaction was left at ambient temperature for two hours at which point MgSO4 was added with subsequent filtration. Volatiles were removed in vacuo to leave the pure imine in quantitative yields.

Synthesis of phenylmethanol (6a)
Synthesized in accordance with general procedure 3 using benzaldehyde (21 μL, 200 µmol). The crude material was purified by flash-column chromatography using hexane/ethyl acetate (5:1) as the eluent to afford the title compound as a colorless oil.

Synthesis of N-(napthalen-2-ylmethyl)aniline (8d)
Synthesized in accordance with general procedure 3 using For Al(3,4,5-Ar F )3·OEt2, 5, the coordinated ether group was significantly disordered. This disorder was modelled by parts over two sites in a 50:50 ratio. The DFIX restraint was used for the O-C and C-C bond distances. In addition, two fluorine atoms had elongated thermal ellipsoids and so were modelled by parts. The final refinement gave R1 = 6.9% and wR2 = 19.0%.

Figure S2
Solid-state structure of tris (2,3,4-trifluorophenyl)borane (3). H-atoms omitted for clarity and thermal ellipsoids drawn at 50% probability. S13 Figure S3. Solid-state structure of tris(2,3,4-trifluorophenyl)alane (4). H-atoms omitted for clarity and thermal ellipsoids drawn at 50% probability. S14 Figure S4. Solid-state structure of tris(2,3,4-trifluorophenyl)alane tetrahydrofuran adduct (4·THF). H-atoms omitted for clarity and thermal ellipsoids drawn at 50% probability. S15 Figure S5. Solid-state structure of tris(3,4,5-trifluorophenyl)alane etherate (5). H-atoms omitted for clarity and thermal ellipsoids drawn at 50% probability. S16 basis set on all atoms. 13 After this a vibrational frequency calculation was undertaken to ensure each structure was a minimum on the potential energy landscape. Atomic coordinates are presented in Section S3.4. Natural bond orbital (NBO) analyses were then performed on the optimized geometries using the same functional and basis set described above and presented in Section S3.2. 14 Lastly, fluoride ion affinity (FIA) calculations were then performed. 15 This was done by calculating the enthalpy of the triarylalane/triarylborane, fluoride ion and triarylalane-F /triarylborane-F complex. A counterpoise correction was then performed to give a basis set superposition error (BSSE) value, which was added to the enthalpy of the reaction to give the final FIA value. Note that FIA is the negative of the reaction enthalpy plus BSSE.   Figure S7 1 H NMR (500 MHz, CDCl3, 295 K) spectrum of tris (3,4,5-trifluorophenyl)