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

The reaction of [(cAAC)BH] (cAAC = 1-(2,6-PrCH)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) with a range of organolithium compounds led to the exclusive formation of the corresponding (dihydro)organoborates, Li[(cAACH)BHR] (R = sp-, sp-, 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 MeSiCl or spontaneous LiH elimination yielded the neutral cAAC-supported mono(organo)boranes, [(cAAC)BHR]. Similarly the reaction of [(cAAC)BH] with a neutral donor base L resulted in adduct formation by shuttling one boron-bound hydrogen to the cAAC ligand, to generate [(cAACH)BHL], either irreversibly (L = cAAC) or reversibly (L = pyridine). Variable-temperature NMR data and DFT calculations on [(cAACH)BH(cAAC)] show that the hydrogen on the former carbene carbon atom exchanges rapidly with the boron-bound hydrides.