"Spontaneous" ambient temperature dehydrocoupling of aromatic amine-boranes

The dehydrocoupling/dehydrogenation behavior of primary arylamine-borane adducts ArNH(2)?BH(3) (3?a-c; Ar = a: Ph, b: p-MeOC(6)H(4), c: p-CF(3)C(6)H(4)) has been studied in detail both in solution at ambient temperature as well as in the solid state at ambient or elevated temperatures. The presence of a metal catalyst was found to be unnecessary for the release of H(2). From reactions of 3?a,b in concentrated solutions in THF at 22?°C over 24?h cyclotriborazanes (ArNH-BH(2))(3) (7?a,b) were isolated as THF adducts, 7?a,b?THF, or solvent-free 7?a, which could not be obtained via heating of 3?a-c in the melt. The μ-(anilino)diborane H(2)B(μ-PhNH)(μ-H)BH(2)] (4?a) was observed in the reaction of 3?a with BH(3)?THF and was characterized in situ. The reaction of 3?a with PhNH(2) (2?a) was found to provide a new, convenient method for the preparation of dianilinoborane (PhNH)(2)BH (5?a), which has potential generality. This observation, together with further studies of reactions of 4?a, 5?a, and 7?a,b, provided insight into the mechanism of the catalyst-free ambient temperature dehydrocoupling of 3?a-c in solution. For example, the reaction of 4?a with 5?a yields 6?a and 7?a. It was found that borazines (ArN-BH)(3) (6?a-c) are not simply formed via dehydrogenation of cyclotriborazanes 7?a-c in solution. The transformation of 7?a to 6?a is slowly induced by 5?a and proceeds via regeneration of 3?a. The adducts 3?a-c also underwent rapid dehydrocoupling in the solid state at elevated temperatures and even very slowly at ambient temperature. From aniline-borane derivative 3?c, the linear iminoborane oligomer (p-CF(3)C(6)H(4))NBH-NH(p-CF(3)C(6)H(4))](2) (11) was obtained. The single-crystal X-ray structures of 3?a-c, 5?a, 7?a, 7?b?THF, and 11 are discussed.