Metal-free dehydrogenation of amine-boranes by an N-heterocyclic carbene

The dehydrogenation of primary and secondary amine-boranes (RNH2 center dot BH3 and R2NH center dot BH3; R = alkyl groups) was studied using the bulky N-heterocyclic carbene IPr (IPr = [(HCNDipp)C:]; Dipp = 2,6-(Pr2C6H3)-Pr-i) as a stoichiometric dehydrogenation agent. In the case of primary amine-boranes, carbene-bound adducts IPr center dot BH2-NH(R)-BH3 were obtained in place of the desired polymers [RNH-BH2](n). The secondary amine-borane (Pr2NH)-Pr-i center dot BH3 participated in dehydrogenation chemistry with IPr to afford the aminoborane [(Pr2N)-Pr-i=BH2] and the dihydroaminal IPrH2 as products. Attempts to induce H2 elimination from the arylamine-borane DippNH(2)center dot BH3 yielded a reaction mixture containing the known species IPr center dot BH(2)NHDipp, IPr center dot BH2NH(Dipp)-BH3,free DippNH(2) and IPrH2. The new hindered aryl-amine borane adduct Ar*NH2 center dot BH3 [Ar* = 2,6-(Ph2CH)(2)-4-MeC6H2] underwent a reaction with IPr to give IPr center dot BH3 and free Ar*NH2, consistent with the presence of a weaker N-B dative bond in Ar*NH2 center dot BH3 relative to its less hindered amine-borane analogues.