Effects of the substituents Y in reactions of compounds of the type (Me3Si)3CSiH(C6H4Y-p)I. Evidence against the SN2(intermediate) mechanism for methanolysis of (Me3Si)3CSiMe2I and (Me3Si)3CSiHPhI

Article abstract of DOI:10.1016/0022-328X(87)80243-7

The rates of methanolysis of the iodides TsiSiH(C₆H₄Y-p)I (Y MeO, Me, H, Cl, or CF₃) in 1/1 v/v MeOH/dioxane have been shown to be increased by electron withdrawal by Y and correspondingly decreased by electron release. This is taken to imply that the methanol is covalently involved in the transition state, and thus that, contrary to an earlier suggestion, the reaction cannot have an S_N2(intermediate) mechanism. No explanation can at present be offered for the fact that methanolysis of TsiSiHPhI (like that of TsiSiMe₂X with X I, OClO₃, or OSO₂CF₃) is not accelerated by NaOMe whereas that of some other TsiSiHPhX compounds (e.g. X Br, ONO₂, or OSO₂Me) is so accelerated, with its implications of a duality of mechanism within an S_N2 range. The reactions of the iodides TsiSiH(C₆H₄Y-p)I with KSCN in MeCN are also accelerated by electron withdrawal by Y, whereas those with AgOAc in MeCO₂H are accelerated by electron release.