Acyclic Aminocarbenes in Catalysis

Acyclic aminocarbenes have received much less attention as ancillary ligands in homogeneous catalysis compared with their cyclic relatives (i.e., N-heterocyclic carbenes, NHCs), despite having a longer history and greater structural variety. Although these species are generally more fragile than NHCs, recent advances in the synthesis and catalytic application of metal complexes of acyclic carbenes have brought increased attention to these underutilized ligands. It is increasingly clear that acyclic carbenes possess unique properties that distinguish them from other ligand classes and make them potentially valuable for catalysis. These include exceptional donor ability, conformational flexibility, and wide N–C(carbene)–N angles that can place bulky or chiral substituents near catalytic substrate binding sites. This purpose of this Perspective is to review and critically assess recent progress in this forefront area of catalyst design. Syntheses and ligand properties of acyclic diaminocarbenes, aminooxycarbenes, and other aminocarbenes are reviewed with a view toward catalytic relevance. A special focus is to highlight catalytic reactions in which acyclic carbene ligands confer unusual selectivity or activity on metal catalysts compared with conventional ligand types. Particularly promising catalytic results have been obtained with acyclic carbene complexes of gold, including some highly enantioselective catalytic transformations.