Adduct Formation, B-H Activation and Ring Expansion at Room Temperature from Reactions of HBcat with NHCs

We report the reactions of catecholborane (HBcat; 1) with unsaturated and saturated NHCs as well as CAAC(Me). Mono-NHC adducts of the type HBcat center dot NHC (NHC=nPr(2)Im, iPr(2)Im, iPr(2)ImMe, and Dipp(2)Im) were obtained by stoichiometric reactions of HBcat with the unsaturated NHCs. The reaction of CAAC(Me) with HBcat yielded the B-H activated product CAAC(Me)(H) Bcat via insertion of the carbine-carbon atom into the B-H bond. The saturated NHC Dipp(2)SIm reacted in a 2: 2 ratio yielding an NHC ring-expanded product at room temperature forming a six-membered -B-C=N-C=C-N- ring via C-N bond cleavage and further migration of the hydrides from two HBcat molecules to the former carbene-carbon atom.