General and practical method of synthesis of 2-disubstituted-1-chloro- and 1-bromoenamines

Disubstituted-alpha-chloroenamines are useful synthetic intermediates which bad earlier been prepared by the reaction of tertiary amides with phosgene. The toxicity of the latter led us to systematically investigate new synthetic routes towards alpha-chloro- and alpha-bromoenamines. The reactions of various halogenating agents (SOCl2, diphosgene, triphosgene, OPCl3, OPBr3) with tertiary amides followed by the addition of triethylamine have been studied. Thionyl chloride was found unsuitable for the preparation of alpha-chloroenamines. Of the other halogenating agents, OPCl3 and OPBr3 were found the most practical. The generality of the method is illustrated by the synthesis of fifteen alpha-chloroenamines and six alpha-bromoenamines. (C) 1998 Elsevier Science Ltd. All rights reserved.