Transposed N-heterocyclic olefins from the reaction of Eschenmoser hydrazones with N-heterocyclic carbenes

N-heterocyclic olefins (NHOs) are strongly polarized basic and nucleophilic alkenes that have found new roles as catalysts or ligands for transition metals and main group species. NHOs are also important intermediates in organocatalytic cycles implicating the umpolung of Michael acceptors by N-heterocyclic carbenes (NHCs). Though NHOs are most often prepared by the deprotonation of alkylazolium salts or the alkylation of NHCs, the reaction of NHCs with Eschenmoser hydrazones provides a new approach to obtain NHOs. This study shows that these carbenogenic hydrazones exhibit dual reactivity: Whereas N-aziridinyl aldimines directly react with NHCs as nucleophiles to provide NHOs, N-aziridinyl ketimines first react with NHCs as bases, initiating a reaction sequence leading to the formation, in high yields and under mild reaction conditions, of 1,2-transposed α-imino NHOs that are not accessible through reported synthetic methods. A mechanistic pathway leading to the formation of the 1,2-transposed α-imino NHOs, which is not feasible using other carbene precursors such as toluenesulfonyl hydrazones, was proposed on the basis of a deuterium-labeling experiment.