Oxymetallation. Part 16. Modified procedures for reducing peroxymercurials: conversion of non-terminal alkenes into secondary alkyl t-butyl peroxides

Abstract

Various procedures for reducing PhCH(OOBu^t)CH₂HgO₂CCF₃ or PhCMe(OOBu^t)CH₂HgOAc have been investigated in an attempt to optimize yields of the mercury-free peroxides. Effects of varying the concentration of borohydride and the mode of mixing, and of including a phase transfer catalyst, have been assessed for the two phase system of alkaline NaBH₄ and dichloromethane and reductions have been carried out in a single phase by using Bu₄NBH₄ in dichloromethane.

The t-butyl peroxymercurials [RCH(OOBu^t)CH(HgX)R] derived from cis- and trans-but-2-ene, cis- and trans-hex-3-ene, cyclopentene, cyclohexene, and norbornene have been reduced under the optimum conditions of slowly adding base-pretreated peroxymercurials in dichloromethane to a large excess of alkaline NaBH₄. Product analyses support the idea that the reactions proceed viaβ-peroxyalkyl radicals [RCH(OOBu^t)ĊHR], but show that, except for the norbornyl derivative, epoxides are the dominant products; cyclopentyl, cyclohexyl, and exo-norbornyl t-butyl peroxides have been isolated in yields of 2, 13, and 37%, respectively. Better, but still modest, yields (24–36%) have been obtained for butan-2-yl, hexan-3-yl, and cyclopentyl t-butyl peroxides by silver trifluoroacetate-assisted t-butyl perhydrolysis of the alkyl bromides, but competing dehydrobromination lowered the yield (6%) of the cyclohexyl compound. Vastly improved yields (ca. 60%) of butan-2-yl, cyclopentyl, and cyclohexyl t-butyl peroxides have been achieved by reduction of the corresponding peroxymercurials with neat Bu₃SnH.