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.