Oxymetallation. Part 23. Peroxymercuriation of bicyclo[n.1.0]alkanes

Abstract

The peroxymercuriations of bicyclo[n.1.0]alkanes, n= 2–4, have been carried out using mercury(II) acetate and a one-fold excess of t-butyl hydroperoxide in dichloromethane with 20 mol% of perchloric acid as catalyst, the products being isolated, after anion exchange, as the organomercury(II) bromides. Small amounts of acetoxymercurials are also formed but are easily removed by silica chromatography.

Bicyclo[2.1.0]pentane reacts by exclusive cleavage of the zero-bridge to give cis-1-bromomercurio-3-t-butylperoxycyclopentane 4, but competing isomerisation to cyclopentene consumes ca. 50% of the cyclopropane. Reaction in neat t-butyl hydroperoxide without perchloric acid gives only the peroxymercurial 4. This has been converted into 2,3-dioxabicyclo[2.2.1]heptane 2a by iododemercuriation then reaction with silver trifluoroacetate.

Bicyclo[4.1.0]heptane reacts by exclusive cleavage of the one-bridge to give cis- and trans-1-bromomercuriomethyl-2-t-butylperoxycyclohexanes 28 and 29, which have been reduced with alkaline sodium borohydride to afford the corresponding 1-methyl compounds 32 and 33.

Bicyclo[3.1.0]hexane reacts by both zero-bridge and one-bridge cleavage to give the γ-peroxymercurials trans-1-bromomercuriomethyl-2-t-butylperoxycyclopentane 10 and 1-bromomercurio-3-t-butylperoxycyclohexane 13. Also formed is a similar amount of the isomeric β-peroxymercurials 1-bromomercuriomethyl-1-t-butylperoxycyclopentane 11 and trans-1-bromomercurio-2-t-butylperoxycyclohexane 14 yet neither starting cyclopropane nor product γ-peroxymercurial is isomerised by perchloric acid. These unusual rearrangements are also obtained with n-butyl hydroperoxide but not with methanol, butanol or acetic acid as nucleophile. They do not take place with other strong acid catalysts, and are inhibited by 2,6-di-t-butyl-4-methylphenol and promoted by di-t-butyl peroxyoxalate. Reaction in neat t-butyl hydroperoxide at 60 °C without perchloric acid gives only the γ-peroxymercurials 10 and cis-13. lododemercuriation of the cis-1,3-cyclohexane derivative cis-13, then reaction with silver trifluoroacetate, gives mainly 3-t-butoxycyclohexanone 27 rather than 6,7-dioxabicyclo[3.2.1]octane.