Tris(hydridosilylethyl)boranes: highly reactive synthons for polymeric silicon compounds

Two methods for the synthesis of tris(hydridosilylethyl)boranes B[C2H4Si(CH3)nH3−n]3 (C2H4=CHCH3, CH2CH2; n=0, 1, 2; 5a–c) are reported. In the first route, tris(chlorosilylethyl)boranes B[C2H4Si(CH3)nCl3−n]3 (n=0, 1, 2; 3a–c) are reacted with LiAlH4 in diethyl ether solution. Compounds 3a–c are prepared by hydroboration of the respective vinyl chlorosilanes (H2CCH)Si(CH3)nCl3−n (n=0, 1, 2; 1a–c) with borane dimethylsulfide, BH3·S(CH3)2. Hydroboration of hydrido vinylsilanes (H2CCH)Si(CH3)nH3−n (n=0, 1, 2; 6a–c) with BH3·S(CH3)2 represents an alternative route, which produces 5a–c in much higher yield. The intermediates 6a–c are obtained from 1a–c and LiAlH4 in diethyl ether solution and, because of their difficult handling, reacted in situ. Detailed NMR studies prove boron addition to the olefinic moieties to be not regioselective, since resonance signals for products formed by α- and β-boron addition to the vinyl functions are observed. The elemental constitution of 5a–c is established by high-resolution electron ionization mass spectrometry for both the molecular ion peak and a series of fragment ions. Major fragments arise from CC bond formation in the gas phase between two of the boron side chains in the ion dipole complex.