Chlorination of αβ-unsaturated carbonyl compounds. Part I. The addition of chlorine to methyl trans-cinnamate in acetic acid
The kinetics and products of the chlorination of methyl trans-cinnamate in acetic acid alone and in acetic acid containing lithium acetate, lithium chloride, lithium perchlorate, or perchloric acid have been studied. The acid has no effect on the rate other than can be attributed to an increase in the ionic strength. The results indicate that the reaction involves the attack of a highly polarised chlorine molecule on the olefin, which leads to an ion-pair that exists in two conformations; these, though readily interconvertible, have the characteristics of diastereoisomers. Methyl threo-αβ-dichloro-β-phenylpropionate is formed by the collapse of one conformation and the erythro-isomer is formed by the collapse of the other. In the presence of lithium chloride, both isomers are also formed by capture of the appropriate conformations by the added chloride ion. The main effect of added electrolytes is to increase the yield of acetoxy-chlorides which are formed by attack of the solvent on the two ion-pair conformations. There is no evidence for a direct chlorination. Dechlorination and deacetoxychlorination of the addition products with chromous sulphate in aqueous dimethylformamide gives methyl trans-cinnamate together with a small amount of the cis-isomer, indicating that this method of dechlorination of these compounds is nonstereospecific.