Abstract
AbstractRates of hydrolysis catalyzed by dilute perchloric acid were measured for the vinyl ethers MeOC(SiMe3)CH2, EtOC(SiMe3)CH2, cis‐EtOCHCHSiMe3, MeOC(t‐Bu)CH2 and EtOC(t‐Bu)CH2 in wholly aqueous solution and for MeOCHCH2, MeOC(SiMe3)CH2 and MeOC(t‐Bu)CH2 in ethanol—water (4:1), in order to assess the effects of silyl substitution on the stability of the carbocations formed in the rate‐determining step of these reactions. The results for α‐substitution give the reactivity order H < SiMe3 < t‐Bu, with a greater spread (103) in wholly aqueous solution than in the mixed solvent (102). The β‐trimethylsilyl substituent shows a modest 100‐fold acceleration over hydrogen; the diminished magnitude of this effect relative to the 1012‐fold acceleration found recently in a cyclohexyl solvolysis reaction is attributed to the imposition of a conformation in the transition state of vinyl ether hydrolysis that is far from optimum for hyperconjugative electron donation.
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