Nucleophilic addition and substitution at coordinatively saturated boron by facile 1,2-hydrogen shuttling onto a carbene donor

Abstract

The reaction of [(cAAC^Me)BH₃] (cAAC^Me = 1-(2,6-ⁱPr₂C₆H₃)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) with a range of organolithium compounds led to the exclusive formation of the corresponding (dihydro)organoborates, Li⁺[(cAAC^MeH)BH₂R]^- (R = sp³-, sp²-, or sp-hybridised organic substituent), by migration of one boron-bound hydrogen atom to the adjacent carbene carbon of the cAAC ligand. A subsequent deprotonation/salt metathesis reaction with Me₃SiCl or spontaneous LiH elimination yielded the neutral cAAC-supported mono(organo)boranes, [(cAAC^Me)BH₂R]. Similarly the reaction of [(cAAC^Me)BH₃] with a neutral donor base L resulted in adduct formation by shuttling one boron-bound hydrogen to the cAAC ligand, to generate [(cAAC^MeH)BH₂L], either irreversibly (L = cAAC^Me) or reversibly (L = pyridine). Variable-temperature NMR data and DFT calculations on [(cAAC^MeH)BH₂(cAAC^Me)] show that the hydrogen on the former carbene carbon atom exchanges rapidly with the boron-bound hydrides.