Late‐Stage Diversification of Chiral N‐Heterocyclic‐Carbene Precatalysts for Enantioselective Homoenolate Additions

A library of chiral triazolium salts has been prepared by late‐state diversification of a triazolium amine salt. By utilizing a primary amine as a functional handle, a single triazolium salt can be transformed into a variety of chiral N‐heterocyclic carbene precatalysts. This approach makes the preparation of chiral N‐heterocyclic carbenes possible by a single‐step modification of a triazolium salt, rather than the usual need for multistep organic synthesis and challenging heterocycle formation for each member of a catalyst library. We have screened these catalysts for control of diastereo‐ and enantioselectivity in a γ‐lactam‐forming reaction between α,β‐unsaturated aldehydes and cyclic ketimines.