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 π-accepting ability at the central carbon
of the NHC moiety. This feature is atypical of classic NHCs, which
are strong σ-donors, with only nominal π-accepting ability.
The unanticipated π-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).