Umpolung catalysts: comparative assessments on reactivities.

Umpolung catalysis is studied by a sequence of model reactions (CPCM in THF, B3LYP/6-31G*) with different aldehydes and catalysts. We involved addition of the catalyst to the aldehyde and 1,2-H-migration to form a carbanionic d1-species, which is the crucial intermediate according to the Lapworth- and Breslow-mechanisms. Cyanide, N-methylthiazol-2-ylidene, and a glycol-based phosphite perform as umpolung catalysts, formaldehyde, acetaldehyde, benzaldehyde, and acrolein are substrates in this study. In these aldehyde substrates, alkyl-substitution disfavors but pi-conjugation favors formation of the carbanionic d1-intermediate. The nucleophilic carbene, N-methylthiazol-2-ylidene, is the strongest umpolung catalyst, while the phosphite is about as active as cyanide.