NHC-Catalyzed Annulation of Enals to Cyclic Ketimines

Significance

The NHC-catalyzed addition of ­homoenales 1 to saccharin-derived ketimines 2 is reported. The authors found that cyclic ketimines like 2 are suitable electrophiles to undergo the addition of different aromatic and aliphatic enals in the presence of a triazolium-based NHC. The ­reaction proceeded at low catalyst loadings (usually 0.5 mol%) and under mild conditions to furnish the desired products 3 in high yields. Mechanistic studies indicate that the reaction follows an ene-like mechanism via six-membered transition state A instead of the also plausible homoenolate pathway.