Stereochemical Evidence for an Alkylated Perepoxide Intermediate

Article abstract of DOI:10.1021/jo00382a027

Peroxymercuration of (Z)-pent-2-ene afforded single stereoisomers of 2-(bromomercurio)-3-(tert-butylperoxy)pentane (M1) and 3-(bromomercurio)-2-(tert-butylperoxy)pentane (M2), which were separated by medium pressure liquid chromatography.Iodinolysis of each of these gave a pair of epimeric β-iodopentyl tert-butyl peroxides (I1a and I1B from M1, and I2A and I2B from M2), which were similarly separated.When treated with silver trifluoroacetate, the regioisomers I1A and I2A each yielded the same 5:3 mixture of 3-(tert-butylperoxy)-2-(trifluoroacetoxy)pentane (T1) and 2-(tert-butylperoxy)-3-(trifluoroacetoxy)pentane (T2).Independent experiments showed that the starting iodides and the product trifluoroacetates were stereochemically stable under the reaction conditions.Hence, the results are taken to provide compelling evidence for the intermediacy of a tert-butylated perepoxide (P_A) that is sufficiently long-lived to be attacked at each ring carbon atom.However, the epimeric regioisomers I1B and I2B each reacted with silver trifluoroacetate to afford a single, new, trifluoroacetate with retention of both regio- and stereochemistry.This is taken to provide evidence for a new mechanism of substitution involving a six-centered cyclic transition state.Product correlations for similar substitutions with analogous bromo peroxides for which stereochemistries are identified by assuming trans addition for peroxymercuration and retention of configuration during bromodemercuration indicate that the alkylated perepoxide (P_A) has the structure with cis-alkyl groups.This was confirmed by identifying the stereochemistry of T1 and T2 by LiAlH4 reduction to threo-pentane-2,3-diol.