An efficient route to substituted 1-silacyclopent-2-enes and 1-silacyclohex-2-enes via consecutive 1,2-hydroboration and 1,1-organoboration

The reaction of alkyn-1-yl(vinyl)silanes R2Si(Ctriple bond; length of mdashCR1)CHdouble bond; length as m-dashCH2 [R = Me (1), Ph (2); R1 = tBu (a), Ph (b), SiMe3 (c)] with 9-borabicyclo[3.3.1]nonane in a 1:1 ratio affords the 1-silacyclopent-2-ene derivatives 4a–c (R = Me) and 5a–c (R = Ph) as a result of selective intermolecular 1,2-hydroboration of the vinyl group, followed by intramolecular 1,1-organoboration of the alkynyl substituent. The analogous reaction sequence converts the alkyn-1-yl(allyl)dimethylsilanes 3a,c into the 1-silacyclohex-2-ene derivatives 7a,c. All reactions were monitored by 29Si NMR spectroscopy and the structural assignment of the final products was based on multinuclear magnetic resonance data (1H, 11B, 13C and 29Si NMR). The molecular structure of 6a was determined by X-ray analysis.