Oxymetallation. Part VI. Halogenodemercuration of peroxymercurials derived from αβ-unsaturated esters and ketones

Abstract

Conditions are described for the halogenolysis of the β- and α-carbonyl-β-mercurio-peroxides RCH(OOBu^t)·-CH(HgX)·COR′ and XHgCH₂·CMe(OOBu^t)·COR′, formed by peroxymercuration of αβ-unsaturated esters and ketones, to afford the previously unknown β- and α-carbonyl-β-halogeno-peroxides, respectively; eleven compounds have been characterised by elemental analysis and ¹H n.m.r. spectroscopy.

The peroxymercuration of methyl cis- and trans-cinnamate is shown to be stereospecific whereas both cis- and trans-chalcone give a mixture of diastereoisomers of the compound PhCH(OOBu^t)·CH(HgOAc)·COPh in the same proportions (1 : 4). In methanol, halogenodemercuration of the methyl cinnamate peroxymercurial, and probably also of the chalcone derivative, involves extensive racemisation, suggesting a radical or an S_E1 mechanism as the major pathway.

A correlation is suggested between J_αβ(and hence configuration) and the chemical shifts of t-butyl or methyl groups in the diastereoisomers of compounds of the type RCH^β(OOBu^t)·CH^αZ·COR′ or RCH^β(OMe)·CH^αZ·COR′. On the basis of this it is tentatively concluded that the greater stereoselectivity found for bromodemercuration of the compound PhCH(OOBu^t)·CH(HgBr)·CO₂Me in dichloromethane as opposed to methanol is unexpectedly due to predominant inversion of configuration.