Alkylaluminum Complexes as Precatalysts in Hydroboration of Nitriles and Carbodiimides

The metathesis reaction of aluminum­(III) dihydride LAlH2 (1) (L = (ArNCMe)2CH, Ar = 2,6-i-Pr2C6H4) with n-BuLi and n-Bu2Mg, respectively, resulted in di-n-butyl aluminum complex LAl­(n-Bu)2 (2). Meanwhile 1 reacted with MeLi producing dimethyl aluminum LAlMe2 (3). Complexes 2 and 3 were characterized by NMR and elemental analysis, and 2 was additionally studied by single-crystal X-ray diffraction. The reported synthesis allows for access of dialkyl­aluminum complexes from aluminum­(III) dihydride compound (1) with excellent yields. The chemoselective double hydroboration of nitriles or single hydroboration of carbodiimides was studied with pinacolborane (HBpin) using LAl­(n-Bu)2 (2) as the precatalyst to afford bis­(boryl)­amines (4a–4m) and N-borylamines (6a–6d) under solvent-free and mild conditions (60 °C). The records show that the catalytic reactions proceed in quantitative yields and short time. Moreover, nearly quantitative yield of di­(boryl)­(silyl)­amine (5) was obtained, when cyanobenzene was reacted with equal amounts of HBpin and phenylsilane in the presence of precatalsyt 2. A comparison of the previously reported hydroboration reactions with nitriles in the literature and our results in the presence of the precatalyst LAl­(n-Bu)2 show convincingly the better results of short reaction time, mild reaction conditions, and quantitative yields.