COMPARISON OF THE REACTIVITIES OF THE COMPOUNDS (ME3SI)2C(SIME2OME)-(SIME2CL) AND (MEOME2SI)3C(SIME2CL) - CRYSTAL-STRUCTURE OF (MEOME2SI)3-CSIPH2H

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-degrees-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 > 10(6)-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.