Chlorination of α,β-unsaturated ketones and esters in the presence of acid scavengers

Abstract

The chlorination of a series of α,β-unsaturated ketones and esters by Cl₂ in CH₃OH, with and without acid scavengers such as N-chlorosuccinimide (NCS), pyridine and 2,6-lutidine, is described. Methyl vinyl ketone and cyclohex-2-enone have also been chlorinated in ethanol. Mixtures of Markovnikov(M) and anti-Markovnikov(AM) methoxy chlorides and dichlorides are formed in most cases; phenyl vinyl ketone gives no M products in the absence of pyridine, M methoxy chloride is not formed with (E-)-4-chlorobut-3-en-2-one under any conditions, pyridine has no effect on the product ratios and methyl 3-chlorobut-2-enoate forms only dichloride. Chlorination of the ketones in the presence of the pyridines results in a significant increase in the M regioisomer (except for methyl isopropenyl ketone and the ketones mentioned), giving M:AM ratios which are similar to the corresponding esters. Ratios for the esters are not affected significantly by pyridine. We ascribe the effect of the pyridine bases to the elimination of acid and the acid-catalysed mechanism, permitting the chlorination to occur via a carbon–carbon π-bond (chloronium ion) mechanism. The rate of chlorination of methyl vinyl ketone is retarded by pyridine but is still considerably faster than methyl acrylate. NCS, in contrast to N-bromosuccinimide (NBS) reported previously, has no effect on the M:AM ratio. The chlorination of methyl vinyl ketone with NCS and HCl gives markedly different results from Cl₂.