A Rigid Cyclic (Alkyl)(amino)carbene Ligand Leads to Isolation of Low-Coordinate Transition-Metal Complexes

摘要

In the last decade the use of diamino N-heterocyclic carbenes (NHCs) A as ancillary ligands for transition-metal catalysts and as organic catalysts on their own has proven very fruitful.The efficiency of NHCs is attributed to their strong o-donor properties and sterically demanding structure.Not only have NHCs yielded improved transition metal catalysts,but they have also lead to the isolation of unusual low-coordinate metal complexes,which often play a key role in catalytic processes.Recently,we reported the synthesis of stable cyclic (alkyl)(amino)carbenes (CAACs) B and demonstrated that CAACs can be even stronger a donors than NHCs,and lead to highly efficient palladium catalysts for the alpha-arylation of ketones and aldehydes.Here we report that the peculiar steric and electronic properties of rigid CAAC ligands allow the preparation of low-coordinate metal complexes,hitherto not isolable with any other ligands.