| Popis: |
The relative rates of acid-catalyzed deoxysilylation of 2-trimethylsilyl-1- hydroxyethane, 4, 1-trimethysilyl-2-hydroxypropane, 5, 1,3-bis(trimethysilyl)- 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 % aqueous-methanol at 40°C. These rates are directly proportional to the sum of the δ + constants of the substituents on the carbon bearing the nucleofuge (ϱ + = − 11). The additive rate-accelerating effect of two trimethylsilyl groups requires equal conjugative stabilization by each trimethylsilyl group, and a mechanism involving a hyper- conjugatively-stabilized carbocation intermediate is proposed. In contrast, the deoxymetalation reactions of the triphenyltin-, triphenyllead-, and iodomercury-analogs exhibits very different structure-reactivity relationships and have been described as proceeding through concerted E 2 -like or bridge-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 E 1 M (carbocation intermediate) mechanism and the other Group IV deoxymetalation reactions more nearly concerted E 2 M-paths. |
