The hydrolyses of a number of α-metalloidal vinyl ethers were found to undergo a rate-limiting protonation to produce α- silyl, germyl or stannyl carbocations. Comparative rate data leads to the ordering: C>Sn>Ge>Si>H, for the stabilization of such intermediates.
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particular reaction under study. l3 Spectroscop ic methods as well as calculations suggest that sflyl groups can stabilize an a-carbocation. l4 Our systems, which undergo undergo clean, quantitative reactions without rearrangement and which are wholly in accord with the fhm mechanistic criteria established for the analogous non-metalloidal vinyl ethers, would appear to be particula.rly well suited for determlning the role of these metalloidal substituents in a-carbocaUon stabilization. Moreover, as for t-Bu and SiMe,. it seems reasonable to assume that steric hindrance to solvaUon13bo f the transition states leading to 4 would be present in our acetone-water system for the analogous germanium and tin compounds as well. Yet. despite this hindr-ante, all of the compounds, 2a-c as well as the t-Bu derivative, 28, are more reactive than the parent vinyl ether, 1. Thus, the order of stabilization imparted to 4. over and above the major contribution of the methoxy group, by the a substttuent. namely H < SiMe, < GeMe, c SnMe, < CMe,. can be viewed as a large +R ( 6-x: hyperconjugaUon15) only for the t-butyl derivative. 2g. The metalloidal compounds with their longer C-M bond length fail to provide this stabilizaUonsb and an increasing +I (field/inductive) effect with diminished electronegaUvily16[f e. C->Sn) results in a minor stabilization of 4 by the metalloidal groups when compared to a hydrogen atom.” Acknow&cigernent~T he support of the NSF EPSCoR Program is gratefully acknowledged. REFERENCES