Chlorination of trans-1,2-diarylethenes in chloroform and acetic acid

When chlorinated in chloroform, (E)-stilbenes bearing electron-donor substi tuents in the para position of the benzene ring give threo-1,2-diaryl-1,2-d ichloroethanes, while meta-substituted (E)-stilbenes, predominantly erythro isomers, irrespective of the nature of the substituent. The reactions in a cetic acid result in preferential formation of threo-1,2-dichloro- and thre o-1-acetoxy-1,2-diaryl-2-chloroethanes.