1,3-Migration of triorganosilyl groups from carbon to oxygen in highly crowded organosilanols

Abstract

The silanol (Me 3Si)CSiMe 2OH has been shown to isomerize to (Me 3Si) 2CH(SiMe 2OSiMe 3) (which reacts further to give (Me 3Si) 2CH(SiMe 2OMe)) in 0.5 M NaOMe/MeOH, the isomerization being ca. 90% complete after 6 h under reflux. Corresponding isomerizations of (PhMe 2Si) 3CSiMe 2OH (to give (PhMe 2Si) 2CH(SiMe 2OSiMe 2Ph)), (Me 3Si) 2C(SiMePh 2)SiMe 2OH (to give (Me 3Si) 2CH(SiMe 2OSiMePh 2)), and (Me 3Si) 3CSiPh 2OH (to give (Me 3Si) 2CH(SiPh 2OSiMe 3)) take place much more readily, and are complete within ca. 2 min at room temperature in 0.10 M NaOMe/MeOH. The reactions involve 1,3-migration from carbon to oxygen within a silanolate ion to give a carbanion, which rapidly acquires a proton from the solvent. The migration is facilitated by relief of steric strain and stabilization of the forming carbanionic centre by the three attached silyl groups. The relative reactivities of the various silanols may be determined by the magnitude of the release of steric strain, by the inherent ease of nucleophilic attack at the relevant silicon centre, and, at least at low base concentrations, by the acidity of the silanol. Treatment of (Me 3Si) 3CSiPh 2OH with MeLi in Et 2O/THF gives, by the same rearrangement, the organolithium reagent (Me 3Si) 2CLi(SiPh 2OSiMe 3), which on treatment with Me 2SiHCl gives (Me 3Si) 2C(SiMe 2H)(SiPh 2OSiMe 3).