“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 )N[BH‐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.