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

摘要

The dehydrogenation of primary and secondary amine-boranes (RNH _2·BH_3 and R_2NH·BH_3; R = alkyl groups) was studied using the bulky N-heterocyclic carbene IPr (IPr = [(HCNDipp)C:]; Dipp = 2,6-~iPr_2C_6H_3) as a stoichiometric dehydrogenation agent. In the case of primary amine-boranes, carbene-bound adducts IPr·BH_2-NH(R)-BH_3 were obtained in place of the desired polymers [RNH-BH_2]_n. The secondary amine-borane ~iPr_2NH·BH_3 participated in dehydrogenation chemistry with IPr to afford the aminoborane [~iPr_2NBH_2] and the dihydroaminal IPrH _2 as products. Attempts to induce H_2 elimination from the arylamine-borane DippNH_2·BH_3 yielded a reaction mixture containing the known species IPr·BH_2NHDipp, IPr·BH_2NH(Dipp)-BH_3, free DippNH_2 and IPrH_2. The new hindered aryl-amine borane adduct Ar*NH _2·BH_3 [Ar* = 2,6-(Ph_2CH) 2-4-MeC_6H_2] underwent a reaction with IPr to give IPr·BH_3 and free Ar*NH_2, consistent with the presence of a weaker N-B dative bond in Ar*NH_2· BH_3 relative to its less hindered amine-borane analogues.