Synthesis of silyl substituted organoboranes by hydroboration of vinylsilanes

Hydroboration reactions of dichloroborane-, monochloroborane- and borane-dimethylsulfide with dichloromethylvinylsilane and trichlorovinylsilane were investigated. The proposed structures of the produced organochlorosilylboranes 1-4 were verified by NMR spectroscopic measurements and in the case of [alpha-(dichloromethylsilyl)ethyl] dichloroborane-dimethylsulfide (4) the molecular structure was determined by single-crystal X-ray diffraction (L.M. Ruwisch, R. Riedel, U. Klingebiel, M. Noltemeyer, Z. Naturforsch., Teil B 54 (1999) 624). Following the Markovnikov rule, the first addition appears strictly regioselective in the alpha-position to silicon, producing one chiral methine group between silicon and dichloroborane in compound 4. The second addition of borane- or monochloroborane-dimethylsulfide at the vinyl groups of dichloromethylvinylsilane and trichlorovinylsilane also takes place in the alpha-position to silicon, forming a second chiral methine group. In the case of borane-dimethylsulfide the third addition occurs in the beta-position (anti-Markovnikov) owing to steric hindrance to boron in tris[( dichloromethylsilyl)ethyl]borane (1). A stepwise substitution of chlorine bonded at boron in compounds 2 and 3 using hexamethyldisilazane produces bis[alpha-(dichloromethylsilyl)ethyl]boryl-trimethylsilylamine (5) and bis[alpha-(trichlorosilyl)ethyl]boryltrimethylsilylamine (6), respectively, under release of chlorotrimethylsilane. The remaining trimethylsilylamine group in 6 can be replaced by further reaction with 3 forming tetrakis [alpha-(trichlorosilyl)ethyl]diborylamine (7). This reaction resembles a selective amino condensation at boron. In a similar condensation reaction of 4 with equivalent amounts of hexamethyldisilazane, tris [alpha-( dichloromethylsilyl)ethyl]borazine (8) can be obtained. (C) 2000 Elsevier Science Ltd All rights reserved.