Mechanistic dichotomy in the asymmetric allylation of aldehydes with allyltrichlorosilanes catalyzed by chiral pyridine N-oxides

Detailed kinetic and computational investigation of the enantio- and diastereoselective allylation of aldehydes with allyltrichlorosilanes, employing the pyridine N-oxides METHOX and QUINOX as chiral organocatalysts, indicate that the reaction can proceed via a dissociative (cationic) or associative (neutral) mechanism: METHOX apparently favors a pentacoordinate cationic transition state, while the less sterically demanding QUINOX is likely to operate via the hexacoordinate neutral complex. In both pathways, only one molecule of the catalyst is involved in the rate- and selectivity-determining step, which is supported by both experimental and computational data.