Mechanistic Study of B(C 6 F 5 ) 3 -Catalyzed Transfer Hydrogenation of Aldehydes/Ketones with PhSiH 3 and Stoichiometric Water.

A DFT study was performed on the mechanisms of B(C 6 F 5 ) 3 -catalyzed transfer hydrogenation of aldehydes/ketones, using PhSiH 3 and stoichiometric water. Path B2 includes a stepwise Piers S N 2-Si process, H - transfer, and hydrolysis desilylation of siloxane, in which the hydrolysis desilylation step is rate-determining. Path C1 is first determined, involving a B(C 6 F 5 ) 3 -catalyzed concerted addition step of 2H 2 O to carbonyl generating R 1 R 2 C(OH) 2 , a subsequent S N 2-Si dehydroxylation step of R 1 R 2 C(OH) 2 giving R 1 R 2 C=OH + and (C 6 F 5 ) 3 B-H - , and final H - transfer producing the respective alcohol R 1 R 2 CHOH. A B(C 6 F 5 ) 3 -catalyzed H 2 generation process (Path H0) is determined. Path B2 is the only mechanism for the stepwise method. Using a one-time one-pot feeding method, alkyl/aryl aldehydes, dialkyl ketones, and alkyl aryl ketones ( 1a - g ) can be reduced into alcohols chemoselectively and effectively at room temperature. More than 1 equiv of water over substrates is necessary. Herein, Path C1 is the dominant transfer hydrogenation pathway, and the H 2 generation is efficiently inhibited, by the competitive advantage of Path C1 and initial dominant existence of the complexes IM0 and IM1-x. The diaryl ketones ( 1h , 1i ) cannot be efficiently reduced into the respective alcohols using the one-time feeding one-pot method. The barriers of C-TS1-h/i are obviously higher than those of C-TS1-a-g, attributed to the electron-donating and space effects of the two aryls on carbonyl C. The possible Paths B2 and C1 of transfer hydrogenation have no competitive advantage with Path H0. The DFT results are consistent with the experiments.