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 1 with allyltrichlorosilanes 5, employing the pyridine N-oxides METHOX (9) and QUINOX (10) as chiral organocatalysts, indicate that the reaction can proceed through 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 a 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.

METHOX: a new pyridine N-oxide organocatalyst for the asymmetric allylation of aldehydes with allyltrichlorosilanes.

[reaction: see text]. Allylation of aromatic aldehydes with allyltrichlorosilane is catalyzed by the new terpene-derived pyridine N-oxide (+)-METHOX (< or =5 mol %) in MeCN with high enantioselectivities (< or =96% ee) and conversion rates; this catalyst retains high selectivity even at room temperature.