Rationalization of an Unusual Solvent-Induced Inversion of Enantiomeric Excess in Organocatalytic Selenylation of Aldehydes

An unusual solvent-induced inversion of the sense of enantioselectivity obsd. in the α-selenylation of aldehydes catalyzed by a diphenylprolinol silyl ether catalyst is correlated to the presence of intermediates formed subsequent to the highly selective C-Se bond-forming step in the catalytic cycle. This work provides support for a mechanistic concept for enamine catalysis and includes a general role for "downstream intermediates" in selectivity outcomes in organocatalysis. [on SciFinder(R)]