Gold(III)-versus GoId(I)-Induced Cyclization: Synthesis of Six-Membered Mesoionic Carbene and Acyclic (Aryl)(Heteroaryl) Carbene Complexes

摘要

In the last decade, gold complexes have been widely used as catalysts for promoting a variety of organic transformations. We have recently demonstrated that the gold-induced cyclization of heteroatom-substituted alkynes A and B provides the corresponding gold complexes D-[M] and E-[M], respectively (Scheme 1). These complexes feature a recently discovered type of carbon-based ligand, namely a mesoionic carbene (MIC), in which a positive charge is delocalized throughout the ring atoms, and the formal negative charge is associated with a carbon atom of the ring. In contrast to their N-heterocyclic carbene (NHC) cousins, which are known to exist as 4-to 8-membered rings, MICs have been mostly limited so far to the 5-membered series. We report herein the selective preparation of a cyclic 6-membered mesoionic carbene gold complex, as well as a very rare example of an acyclic (aryl)(heteroaryl)carbene gold complex. Both compounds are prepared from the same precursor by an unusual gold induced 6-endo-dig ring closure process and by a more classical 5-exo-dig cyclization, respectively. The selectivity observed is dictated by the oxidation state of the gold promoter.