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

The reaction of [(cAAC(Me))BH(3)] (cAAC(Me) = 1-(2,6-(i)Pr(2)C(6)H(3))-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(Me)H)BH(2)R](–) (R = sp(3)-, sp(2)-, 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 Me(3)SiCl or spontaneous LiH elimination yielded the neutral cAAC-supported mono(organo)boranes, [(cAAC(Me))BH(2)R]. Similarly the reaction of [(cAAC(Me))BH(3)] with a neutral donor base L resulted in adduct formation by shuttling one boron-bound hydrogen to the cAAC ligand, to generate [(cAAC(Me)H)BH(2)L], either irreversibly (L = cAAC(Me)) or reversibly (L = pyridine). Variable-temperature NMR data and DFT calculations on [(cAAC(Me)H)BH(2)(cAAC(Me))] show that the hydrogen on the former carbene carbon atom exchanges rapidly with the boron-bound hydrides.