Comparison of the reactivities of the compounds (Me3Si)2C(SiMe2OMe)(SiMe2Cl) and (MeOMe2Si)3C(SiMe2Cl). Crystal structure of (MeOMe2Si)3CSiPh2H

Abstract

The relative rates of methanolysis of (MeOMe₂Si)(Me₃Si)₂CSiMe₂Cl (1), (MeOMe₂Si)₂(Me₃Si)CSiMe₂Cl (2) and (MeOMe₂Si)₃CSiMe₂Cl (3) in 1 : 3 v/v MeOH–dioxane at 35 °C have been found to be ca. 1 : 9 : 160. The smallness of the reactivity increases on introduction of the second and third OMe groups on going from 1, to 2, to 3, compared with the >10⁶-fold increase on going from (Me₃Si)₃CSiMe₂Cl to 1, is consistent with a mechanism for methanolysis of 1 involving rate-determining intramolecular displacement of Cl^– by the OMe group. Possible reasons for the rate enhancements by the second and third OMe groups are discussed. In reaction with KSCN in MeCN, a direct bimolecular nucleophilic displacement, 3 is only 11 times as reactive as 1. The hydride (MeOMe₂Si)₃CSiMe₂H reacts more readily than (MeOMe₂Si)(Me₃Si)₂CSiMe₂H with silver salts. The crystal structure of the hydride (MeOMe₂Si)₃CSiPh₂H has been determined, revealing that one OMe group is in an ideal position to carry out (the notional) backside displacement of H^–, and a second such group in a good position to initiate frontside (equatorial) displacement.