Effect of various substituents on intramolecular 1,1-vinylboration, synthesis of 1-silacyclobutene derivatives

The reaction of 1-boryl-1-alkenyl chlorosilane derivatives with alkynyllithium reagents [Li-C=C-R^3 (R^3 = Ph, SiMe_3) ] at low temperature (–78 ◦C) affords alkenyl(alkyn-1-yl)silanes. These compounds are precursors of 1-silacyclobutene derivatives, which are formed via intramolecular 1,1-vinylboration. This reaction works for various groups at silicon (R^1/R^2: R61 = H,Me, Ph; R^2 = Me, Ph) and at the C=C and C=C units (R/R^3: R= n^Bu, Ph; R^3 = n^Bu, Ph, SiMe_3) . The conversion into 1-silacyclobutene derivatives is incomplete only in the case of R^3 = SiMe_3 . The reactions were monitored by NMR spectroscopy in order to elucidate the reaction mechanism, and the proposed structures of all new compounds follow from consistent sets of NMR parameters ( 1^H-, 13 ^C-, 11^B-, 29^Si-NMR).