Spontaneous Ambient Temperature Dehydrocoupling of Aromatic Amine-Boranes

The dehydrocoupling/dehydrogenation behavior of primary arylamineborane adducts ArNH2 center dot BH3 (3a-c; Ar=a: Ph, b: p-MeOC6H4, c: p-CF3C6H4) 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 H2. From reactions of 3a,b in concentrated solutions in THF at 22 degrees C over 24 h cyclotriborazanes (ArNH-BH2)3 (7a,b) were isolated as THF adducts, 7a,b.THF, or solvent-free 7a, which could not be obtained via heating of 3a-c in the melt. The mu-(anilino)diborane [H2B(mu-PhNH)(mu-H)BH2] (4a) was observed in the reaction of 3a with BH3 center dot THF and was characterized in situ. The reaction of 3a with PhNH2 (2a) was found to provide a new, convenient method for the preparation of dianilinoborane (PhNH)2BH (5a), which has potential generality. This observation, together with further studies of reactions of 4a, 5a, and 7a,b, provided insight into the mechanism of the catalyst-free ambient temperature dehydrocoupling of 3a-c in solution. For example, the reaction of 4a with 5a yields 6a and 7a. It was found that borazines (ArN-BH)3 (6a-c) are not simply formed via dehydrogenation of cyclotriborazanes 7a-c in solution. The transformation of 7a to 6a is slowly induced by 5a and proceeds via regeneration of 3a. The adducts 3a-c also underwent rapid dehydrocoupling in the solid state at elevated temperatures and even very slowly at ambient temperature. From anilineborane derivative 3c, the linear iminoborane oligomer (p-CF3C6H4)N[BH-NH(p-CF3C6H4)]2 (11) was obtained. The single-crystal X-ray structures of 3a-c, 5a, 7a, 7b center dot THF, and 11 are discussed.