Anchimeric Assistance in Hydrogen Atom Transfer Reactions on the Side Chains of Amioo Acid Derivatives

摘要

Only a limited range of examples of neighboring group participation in atom transfer reactions have been reported, Anchimeric assistance has been observed in hydrogen atom abstractions, in the vicinal bromination of alkyl bromides, and in reactions offerf-butoxy radical with EttSi, EuGe, and Eu-Sn. Results -of studies by Wilt et al., of reactions of beta-haloalkylsilanes with stannanes, have also been shown to illustrate neighboring group participation in halogen atom transfer reactions. In each ofthese systems alkyl radical production is facilitated by a substituent on carbon adjacent to the incipient radical center, through 1,3-participation. We now report strong evidence for anchimeric assistance by an amido substituent, in hydrogen atom transfer reactions, through 1,4-participation. The present work stems from our earlier observations that nucleophilic substitution reactions of 3a—f to give alcohols are substantially affected through neighboiing group participation by the ester and amide groups, particularly in the latter case where the amido substituent can interact more extensively with an electron deficient center developing in a reaction transition state. In that work, 3a—f were prepared, each as a 1:1 mixture of the diastereomers, bytreatment of la—f with NBS. The reverse transformations, of 3a—f to la—f, have now been accomplished using PhsSnH. As these reactions may be assumed to proceed via hydrogen and halogen atom transfer, respectively, to give the corresponding radicals 2a—f, they provided the opportunity to probe for aEchimeric assistance in atom transfer reactions.