Hydroboration of Disilyne RSiSiR (R = SiiPr[CH(SiMe3)2]2), Giving Boryl-Substituted Disilenes

The reaction of 1,1,4,4-tetralds[bis-(trimethylsilyl)methyl]-1,4-diisopropyltetrasila-2-yne (1) with hydroboranes afforded boryl-substituted disilenes R(R'B-2)Si=SiHR 3a and 3b (R = (SiPr)-Pr-i[CH(SiMe3)(2)](2), R'B-2 = 9-borabicyclo[3.3.1]nonan-9-yl (3a), catecholboryl (3b)). Spectroscopic and X-ray crystallographic analyses of 3a and 3b showed that 3a has a nearly coplanar arrangement of the Si=Si double bond and the boryl group, allowing pi-conjugation between them, whereas 3b, with a markedly twisted arrangement, exhibits no such conjugation. Theoretical calculations suggest that pi-conjugation between the pi-orbital of the Si=Si double bond and the vacant 2p-orbital on the boron atom is markedly influenced by the dihedral angle between the Si=Si double-bond plane and boryl group plane.