Anchimeric assistance by γ-aryl groups in reactions of organosilicon iodides

Abstract

The rates of reaction of the iodides (Me3Si)2C(SiMe2C6H4Y)(SiMe2I), 1(Y =p-OMe, p-Me, H, p-Cl, or m-CF3), with (CF3)2CHOH and CF3CH2OH (both containing CDCl3), MeOH (containing CCl4), and H2O (mixed with dioxane and CCl4) have been measured. In the reaction with the two fluorinated alcohols the spread of rates is very large, 1, Y =p-OMe being 1.9 × 105 and 6.5 × 104 times, respectively as reactive as 1, Y =m-CF3, showing that the aryl group provides anchimeric assistance to the leaving of the iodide ion in the rate-determining step, thought to be formation of the 1,3-aryl-bridged cation II. Unexpectedly, since this process is analogous to an electrophilic aromatic substitution, the substituent effects show excellent correlations with σ-constants. In the methanolysis and hydrolysis of 1 such an SN1 process appears to operate alongside an SN2 process involving direct nucleophilic displacement of iodide by the solvent, the SN2 process being facilitated by electron-withdrawal and the SN1 by electron-release by Y; estimated values of the rate constants for the SN1 processes show reasonable correlation with σ+-constants. Rates of the SN2 reactions of 1 with KSCN in MeCN have also been measured, and the substituent effects, involving an increase in rate by a factor of only 2.3 on going from 1, Y =p-OMe to 1, Y =m-CF3, are found to be consistent with normal through-bond transmission of electronic effects.