Reactions of halothiophenes with metal amides

Abstract

In an attempt to generate and study 2,3-dehydrothiophene a variety of halothiophenes were treated with several metal amides in liquid ammonia. Reaction of 2-bromothiophene with amide ion results in the formation of either 3-aminothiophene or 3-bromothiophene; the product distribution being highly dependent upon the reaction conditions. Four mechanisms for the cine-amination of 2-bromothiophene were considered: (1) a normal substitution, either preceded by the rearrangement of 2- to -3-bromothiophene or followed by rearrangement of 2- to 3-aminothiophene, (11) an elimination- addition (aryne) mechanism, (111) an addition-elimination mechanism, and (iv) a mechanism involving a nucleophilic displacement on a polybromothiophene intermediate which arises from a series of displacements on halogen by thiophene anions. The latter mechanism was found to explain most satisfactorily the results obtained from the various reactions. This mechanism represents a new pathway for the formation of cine-substituted products. A detailed discussion of the mechanism, application of this mechanism to explain cine-substitution observed in other five-membered heterocycles, and a discussion of the synthetic utility of these reactions are presented.