Nucleophilic reactivity of the gold atom in a diarylborylgold(i) complex toward polar multiple bonds

A di(o-tolyl)borylgold complex was synthesized via the metathesis reaction of a gold alkoxide with tetra(o-tolyl)diborane(4). The resulting diarylborylgold complex exhibited a Lewis acidic boron center and a characteristic visible absorption that arises from its HOMO–LUMO excitation, which is narrower than that of a previously reported dioxyborylgold complex. The diarylborylgold complex reacted with isocyanide in a stepwise fashion to afford single- and double-insertion products and a C–C coupled product. Reactions of this diarylborylgold complex with C 
<svg xmlns="http://www.w3.org/2000/svg" version="1.0" width="13.200000pt" height="16.000000pt" viewBox="0 0 13.200000 16.000000" preserveAspectRatio="xMidYMid meet"><metadata>
Created by potrace 1.16, written by Peter Selinger 2001-2019
</metadata><g transform="translate(1.000000,15.000000) scale(0.017500,-0.017500)" fill="currentColor" stroke="none"><path d="M0 440 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z M0 280 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z"/></g></svg>
 O/N double bond species furnished addition products under concomitant formation of Au–C and B–O/N bonds, which suggests nucleophilic reactivity of the gold metal center. DFT calculations provided details of the underlying reaction mechanism, which involves an initial coordination of the CO/N bond to the boron vacant p-orbital of the diarylboryl ligand followed by a migration of the gold atom from the tetracoordinate sp3-hybridized boron center, which is analogous to the reactivity of the conventional sp3-hybridized borate species. The DFT calculations also suggested a stepwise mechanism for the reaction of this diarylborylgold complex with isocyanide, which afforded three different reaction products depending on the applied reaction conditions.

After stirring the reaction mixture at room temperature for 1 min, the resulting solution was concentrated under reduced pressure to dryness. The crude product was recrystallized by a vapor diffusion method using diethyl ether/hexane to give 3a as purple crystals (20.1 mg, 59%). 1
After stirring the reaction mixture at room temperature for 1 min, the color of the resulting reaction mixture changed from orange to red. In the first trial for recrystallization from reaction mixture by a slow evaporation of solvent, single crystals of 3b suitable for single-crystal X-ray diffraction analysis was obtained, however, this recrystallization was not reproducible. During removal of solvent from the reaction mixture in the experiments for reproducibility, 3b seemed to decompose. Therefore, we recorded the NMR spectra of freshly prepared 3b in C6D6 (see following figures). 1

Synthesis of 4
In a glovebox, a benzene (500 µL) solution of 2,6-dimethylphenyl isocyanide (3.8 mg, 29 µmol) was slowly dropped to a benzene (500 µL) solution of 2 (22.6 mg, 29.0 µmol) at room temperature. After stirring the reaction mixture at room temperature for 1 min, the resulting solution was concentrated under reduced pressure and the crude product was recrystallized by a vapor diffusion method using diethyl ether/hexane to give 4 as yellow crystals (22.3 mg, 84%).   Figure S13. The 13 C NMR spectrum of 4. Figure S14. The 11 B NMR spectrum of 4.

Synthesis of 6
In a glovebox, 10 mL J young tube was charged with 5 (29.4 mg, 28.2 µmol) and benzene (2.0 mL). After stirring the solution at 70 °C for 3 h, the resulting reaction mixture was concentrated under reduced pressure and the crude product was recrystallized by a vapor diffusion method with toluene to give 6 as green crystals (11.1 mg, 10.7 µmol, 38%). 1

Synthesis of 6-Mes
In a glovebox, a J young NMR tube was charged with 4 (18.6 mg, 20.5 µmol) and C6D6 (300 µL). To the resulting solution, a solution of Mes-NC (3.0 mg, 21 µmol) in C6D6 was added dropwise at room temperature. After heating the solution at 65 °C for 2.5 h, 1 H NMR spectrum of the crude reaction mixture was measured to check the reaction proceeded. Then the resulting reaction mixture was concentrated under reduced pressure and the crude product was recrystallized by a slow evaporation method with benzene to give 6-Mes as green crystals (14.9 mg, 14.1 µmol, 69%), which are suitable for X-ray crystallographic analysis. However, it was difficult to obtain a sufficient amount of the analytically pure crystals of 6-Mes, therefore, NMR spectra with a small amount of impurities are shown in Figures

Synthesis of 7, [IPrAuCHPh(OB(o-tol)2)]
In a glovebox, benzene (3 mL) was added to a mixture of 2 (60.1 mg, 77.2 µmol) and benzaldehyde (8.2 mg, 77 µmol). After stirring the solution at room temperature for 1 min, the resulting reaction mixture was concentrated under reduced pressure. The crude product was reprecipitated with hexane and diethyl ether to give 7 as white powder (36.8 mg, 54%).

Synthesis of 8, [IPrAuCPh2(OB(o-tol)2)]
In a glovebox, benzene (1 mL) was added to a mixture of 2 (29.9 mg, 38.4 µmol) and benzophenone (7.0 mg, 38 µmol). After stirring the solution at room temperature for 1 min, the resulting reaction mixture was concentrated under reduced pressure and the crude product was recrystallized by a vapor diffusion method using diethyl ether/hexane to give 8 as colorless crystals (26.3 mg, 71%).
Diffraction data were collected on a Rigaku XtaLAB Synergy diffractometer equipped with a HyPix-6000 hybrid pixel detector using MoKa radiation. The Bragg spots were integrated using CrysAlisPro program package. 2 Absorption corrections were applied. All the following procedure for analysis, Yadokari-XG 2009 3 was used as a graphical interface. The structure was solved by a direct method with programs of SHELXS 4 and SIR-2014 5 refined by a full-matrix least squares method with the program of SHELXL-2016 or SHELXL-2018. 4 Anisotropic temperature factors were applied to all non-hydrogen atoms. The hydrogen atoms were put at calculated positions, and refined applying riding models. The detailed crystallographic data have been deposited with the Cambridge Crystallographic Data Centre: Deposition code CCDC 2020132-2020142. A copy of the data can be obtained free of charge via the following URL.

S30
Crystallographic data and structure refinement details for 2, 3a, 3b, 4, 5, 6, 6-Mes, 7, 8, 9, and 10 Figure S43. Molecular structures of 3a with thermal ellipsoids at 50% probability (213 K); hydrogen atoms omitted for clarity. Figure S44. Molecular structures of 3b with thermal ellipsoids at 50% probability (213 K); hydrogen atoms omitted for clarity.  Searching the potential energy surface to find energy diagrams for Scheme 6-10 in the main text All the calculations were performed with the Gaussian 09 program. 13 Geometry optimization was carried out using the PBE0 functional. 7,14 In the DFT calculations, the 6-31G** basis set was used for all the other atoms while the triple-z SDD basis set 9a, 15 with the Stuttgart-Dresden ECP was employed for Au with polarization functions (zf = 1.050) 16 being added. Frequency analysis was performed at the same level of theory to identify the nature of all the stationary points as minima (zero imaginary frequencies) or transition states (one imaginary frequency) and to provide free energies at 298.15 K which include entropic contributions by taking into account the vibrational, S37 rotational, and translational motions of the species under consideration. For each transition state, the intrinsic reaction coordinates (IRC) 17 analysis was conducted to ensure that it indeed connects two relevant local minima.  The results in the PBE0-D3 column are obtained from single-point calculations. Table S2:

Remarks on
One reviewer commented that the PBE0-calculated barriers seem relatively high for a reaction that is complete in under 1 minute. It is likely a result that PBE0 underestimates the dispersion attractive interactions between the diarylborylgold(I) complex and substrate molecules. To verify this reasoning, we carried out single-point energy calculations by including Grimme's D3 dispersion corrections 18 for those species shown in Schemes 7-9. The PBE0-D3 results, given in Table S1, indeed give noticeably smaller barriers, while qualitative conclusions made remain unchanged. However, the data obtained by the PBE0-D3 calculations seem to imply an over-correction by the method.
Since the PBE0 results reproduce and explain well our experimental findings, plus inclusion of the corrections does S38 not alter the relative reactivity of the diarylborylgold(I) complex toward different unsaturated substrates, we did not further spend effort to find a better method to obtain more accurate absolute reaction barriers.