Solvolysis of (Z)-5-trimethylstannyl 2-adamantyl p-bromobenzenesulfonate: mechanistic implications of a record-breaking secondary alpha-deuterium kinetic isotope effect for an SN1 substrate

The secondary alpha-deuterium kinetic isotope effect (alpha-kie) for the solvolysis of (Z)-5-trimethylstannyl 2-adamantyl p-bromobenzenesulfonate in 97% w/w aqueous 2,2,2-trifluoroethanol (97T) at 25 degrees C has been measured (k(H)/k(D) = 1.33). The alpha-kie is abnormally high compared to the value of 1.23 for the corresponding limiting S(N)1 solvolysis of 2-adamantyl p-bromobenzenesulfonate, which proceeds via an extended ion-pair mechanism. A novel mechanism for the solvolysis of the tin compound is proposed that accommodates not only the high alpha-kie but also the absence of internal return.