Halogenation of unsaturated esters—III : Reactions of bromine chloride with chlorinated methyl 2-butenoates

Additions of Cl₂, Br₂; and BrCl to methyl Z-2-chloro-2-(1), E-3-chloro-2-(2), Z-3-chloro-2-(3) and E-4-chloro-2-butenoates (4) in CCl₄ both in the dark and light have been investigated. The product compositions are compared with those reported previously for methyl monochloropropenoates. The chlorine substitution was found to affect the product distributions in the reactions with the BrCl reagent. Under the ionic conditions Br₂ adducts were the main products from 1 and 2, whereas 3 reacted like methyl Z-3-chloropropenoate giving only negligible amount of the Br₂ addition. The Cl₂ addition amounted, as in the reactions of methyl monochloropropenoates, to only a few percent. Compared with the parent esters the chlorine substitutions in 2, 3 and 4 also affected the regiochemistry of the BrCl addition. The main regioisomer in the dark, i.e. the 2-bromo-3-chloro adduct, was obtained from 3, as from methyl Z-3-chloropropenoate, with almost complete regioselectivity. Compound 4 also showed greater regioselectivity than methyl E-2-butenoate. Substrate 2 was, however, found to produce more 3-bromo-2-chloro adduct than the unchlorinated parent ester. Mechanisms presented for the additions of BrCl to methyl 3-chloropropenoates are reconsidered on the basis of the results from the present study.