Carbodicarbenes: Unexpected π-Accepting Ability during Reactivity with Small Molecules

An investigation of carbodicarbenes, the less explored member of the carbenic complex/ligand family has yielded unexpected electronic features and concomitant reactivity. Observed 1,2-addition of E-H bonds (E = B, C, Si) across the carbone central carbon and that of the flanking N-heterocyclic carbene (NHC) fragment, combined with single-crystal X-ray studies of a model Pd complex strongly suggests a significant level of pi-accepting ability at the central carbon of the NHC moiety. This feature is atypical of classic NHCs, which are strong sigma-donors, with only nominal pi-accepting ability. The unanticipated pi-acidity is critical for engendering carbodicarbenes with reactivity more commonly observed with frustrated Lewis pairs (FLPs) rather than the more closely related NHCs and cyclic (alkyl)(amino)carbenes (CAACs).