DEOXYMETALATION REACTIONS. THE MECHANISMS OF DEOXYSILYLATION OF MONO-TRIMETHYLSILYL-AND BIS-TRIMETHYLSILYL-SUBSTITUTED ALCOHOLS AND A COMPARISON TO THE MECHANISM OF DEOXYSTANNYLATION AND DEOXYPLUMBYLATION

Article abstract of DOI:10.1016/S0022-328X(00)93462-4

The relative rates of acid-catalyzed deoxysilylation of 2-trimethylsilyl-1-hydroxyethane, 4, 1-trimethylsilyl-2-hydroxypropane, 5, 1,3-bis(trimethylsilyl)-2-hydroxypropane, 6, and 1-trimethylsilyl-2-methyl-2-hydroxypropane, 7, were found to be 1 : 10^3.30 : 10^5.92 : 10^6.77, respectively, in 9 vol percent aqueous-methanol at 40 deg C.These rates are directly proportional to the sum of the ?⁺ constants of the substituents on the carbon bearing the nucleofuge (ρ⁺ = -11).The additive rate-accelerating effect of two trimethylsilyl groups requires equal conjugativen bystabilization by each trimethylsilyl group, and a mechanism involving a hyperconjugatively-stabilized carbocation intermediate is proposed.In contrast, the deoxymetalation reactions of the triphenyltin-, triphenyllead-, and iodomercury-analogs exhibit very different structure-reactivity relationships and have been described as proceeding through concerted E₂-like or bridged-ion mechanisms.These mechanistic regimes are reconciled by considering the conjugative interactions, electrofugalities and nucleophilic solvent assistance at the organometal-leaving groups in terms of Thornton's Reacting Bond Rules.This analysis suggests a spectrum of merging mechanisms, the acid-catalyzed deoxysilylation representing one extreme, the E₁M (carbocation intermediate) mechanism and the other Group IV deoxymetalation reactions more nearly concerted E₂M-paths.