Synthesis, reactivity, and catalytic studies of transition metal complexes of chelating cyclic and acyclic diaminocarbene ligands

Abstract

Since the first report of a stable diaminocarbene by Arduengo in 1991, carbene ligands have been widely explored in organometallic chemistry due to their favorable features in catalysis. In order to find broad applications of carbenes in catalysis, the preparation of electronically and sterically tunable carbene transition metal complexes are needed. Two major approaches used in synthesizing chelating bis(carbene) complexes of catalytically important transition metals will be discussed. The first approach involves the synthesis of a sterically bulky silver bis(N-heterocyclic carbene) complex which is effective for transferring the bis(carbene) ligand to other transition metals. The second approach is based on nucleophilic attack of secondary diamines or hydrazines on palladium-bound arylisocyanide precursors to generate bis(acyclic diaminocarbene) (ADC) palladium(II) complexes. This reaction shows similarities to a self-assembly process in that it is highly selective and produces no by-products. This route has been utilized to synthesize bis(ADC) palladium complexes with different chelating ring sizes and chiral backbones. Bis(ADC) palladium complexes have been successfully used as electrophilic catalysts for enantioselective aza-Claisen rearrangements of allylic imidates.