On the Mechanism of Asymmetric Allylation of Aldehydes with Allyltrichlorosilanes Catalyzed by QUINOX, a Chiral Isoquinoline N-oxide

Allylation of aromatic aldehydes 1a-m with allyl- and crotyl-trichlorosilanes 2- 4, catalyzed by the chiral N-oxide QUINOX (9), has been found to exhibit a significant dependence on the electronics of the aldehyde, with p-(trifluoromethyl)benzaldehyde 1g and its p-methoxy counterpart 1h affording the corresponding homoallylic alcohols 6g, h in 96 and 16% ee, respectively, at -40 degrees C. The kinetic and computational data indicate that the reaction is likely to proceed via an associative pathway involving neutral, octahedral silicon complex 22 with only one molecule of the catalyst involved in the rate- and selectivity-determining step. The crotylation with (E) and (Z)-crotyltrichlorosilanes 3 and 4 is highly diastereoselective, suggesting the chairlike transition state 5, which is supported by computational data. High-level quantum chemical calculations further suggest that attractive aromatic interactions between the catalyst 9 and the aldehyde 1 contribute to the enantiodifferentiation and that the dramatic drop in enantioselectivity, observed with the electron-rich aldehyde 1h, originates from narrowing the energy gap between the (R)- and (S)-reaction channels in the associative mechanism (22). Overall, a good agreement between the theoretically predicted enantioselectivities for 1a and 1h and the experimental data allowed to understand the specific aspects of the reaction mechanism.