Vinyl ether hydrolysis. 25. Effect of α‐ and β‐trimethylsilyl substitution

Rates of hydrolysis catalyzed by dilute perchloric acid were measured for the vinyl ethers MeOC(SiMe 3 )CH 2 , EtOC(SiMe 3 )CH 2 , cis ‐EtOCHCHSiMe 3 , MeOC( t ‐Bu)CH 2 and EtOC( t ‐Bu)CH 2 in wholly aqueous solution and for MeOCHCH 2 , MeOC(SiMe 3 )CH 2 and MeOC( t ‐Bu)CH 2 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 < SiMe 3 < t ‐Bu, with a greater spread (10 3 ) in wholly aqueous solution than in the mixed solvent (10 2 ). The β‐trimethylsilyl substituent shows a modest 100‐fold acceleration over hydrogen; the diminished magnitude of this effect relative to the 10 12 ‐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.