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

Abstract

The dehydrogenation of primary and secondary amine–boranes (RNH₂·BH₃ and R₂NH·BH₃; R = alkyl groups) was studied using the bulky N-heterocyclic carbene IPr (IPr = [(HCNDipp)C:]; Dipp = 2,6-ⁱPr₂C₆H₃) as a stoichiometric dehydrogenation agent. In the case of primary amine–boranes, carbene-bound adducts IPr·BH₂–NH(R)–BH₃ were obtained in place of the desired polymers [RNH–BH₂]_n. The secondary amine–borane ⁱPr₂NH·BH₃ participated in dehydrogenation chemistry with IPr to afford the aminoborane [ⁱPr₂NBH₂] and the dihydroaminal IPrH₂ as products. Attempts to induce H₂ elimination from the arylamine–borane DippNH₂·BH₃ yielded a reaction mixture containing the known species IPr·BH₂NHDipp, IPr·BH₂NH(Dipp)–BH₃, free DippNH₂ and IPrH₂. The new hindered aryl-amine borane adduct Ar*NH₂·BH₃ [Ar* = 2,6-(Ph₂CH)₂-4-MeC₆H₂] underwent a reaction with IPr to give IPr·BH₃ and free Ar*NH₂, consistent with the presence of a weaker N–B dative bond in Ar*NH₂·BH₃ relative to its less hindered amine–borane analogues.