RATES OF SOLVOLYSIS OF THE COMPOUNDS (ME(3)SI)(2)(PH(2)MESI) CSIME(2)X, X=I, OSO2CF3 AND OCN - MECHANISTIC IMPLICATIONS

Comparisons of the rates of reaction of the compounds (Me(3)Si)(3)CSiMe(2)X, with X = I (1a), OSO2CF3 (1b) or OCN (1c), with those of the corresponding compounds (Me(3)Si)(2)(Ph(2)MeSi)CSiMe(2)X (2a-c), point to the following conclusions. (a) With 3:1 (v/v) MeOH-CCl4 the iodide 2a reacts by an ionization process, in which a gamma-Ph group assists leaving of I-, whereas la reacts by a direct bimolecular displacement mechanism. (b) With 5% H2O-dioxane or 6% H2O in 4:1 MeCN-CCl4, reactions of both 2a and 1a involve direct nucleophilic attack at silicon, as does that of the cyanate 2c with 10:1 CD3OD-CDCl3 (containing a little water), towards which 2c is >600 times as reactive as the iodide 2a. (c) With 1:3 CF3CO2H-CDCl3, 1:3 (CF3)(2)CHOH-CDCl3 and 3:1 CF3CH2OH-CDCl3, 2a (which is much more reactive than 1a in all these media) reacts by the ionization mechanism. (d) The triflate 2b probably reacts with all three fluorine-containing media by the ionization mechanism (although much less readily than the iodide 2a) but by the bimolecular mechanism with 3:1 MeOH-CCl4.