A detailed NMR- and DFT-based study of the Sakurai-Hosomi-Yamamoto asymmetric allylation reaction

A Lewis acid complex between benzaldehyde and the silver catalyst was detected by (31)P NMR and shown to be the direct precursor to allylation within the Sakurai-Hosomi-Yamamoto reaction. Structural and thermochemical hybrid-DFT calculations indicated that benzaldehyde predominantly formed an η(1)-σ-complex with the catalyst; however, two other competing conformers involving different coordination modes were found, including an activated μ(2)-bound complex. The differences in (31)P NMR shifts upon complexation were calculated by the gauge-independent atomic orbital (GIAO-DFT) method for each conformer. The minimum energy conformer was found to correlate well with chemical shift trends observed experimentally, and an analysis of Mullikan charge populations revealed that the carbonyl carbon of the highest-energy conformer was the most electron-deficient. Furthermore, one minor and three major silicon intermediates were detected by (29)Si NMR and, with the aid of (1)H-(29)Si HSQC, were assigned by comparison with parent compounds and GIAO-DFT calculations. Finally, a tentative mechanism was proposed based on these findings.