Base cleavage of R–Si bonds of silanols RSiMe2OH. A proposed new mechanism of substitution at silicon

Abstract

Cleavage of R–Si bonds has been found to occur unexpectedly rapidly in solutions of RSiMe₂OMe (R =m-ClC₆H₄CH₂ or PhC⋮C) in 5 vol-% H₂O–MeOH containing relatively small concentrations of NaOMe, but with a levelling off of the rate at high concentrations of the base. The behaviour is attributed to the formation of RSiMe₂OH and hence RSiMe₂O^–, and it is suggested that unimolecular dissociation of the silanolate anion RSiMe₂O^– to give R^– and the silianone Me₂SiO (both of which react rapidly with the solvent) plays a significant role. The silanols RSiMe₂OH are roughly estimated to have pK_a values of 11 (R =m-ClC₆H₄CH₂) and 9.8 (R = PhC⋮C) in water.