Isothiazole chemistry. VI. Reactions of carbanions with N-Ethyl-3-isothiazolone

Abstract

N-Ethyl-3-isothiazolone undergoes cleavage of the S-N bond when subjected to nucleophilic attack by carbanions, yielding a series of cis-3-alkylmercapto-acrylamides. Under basic conditions the reaction is shown to be reversible, the stability of the product being apparently dependent upon the degree of substitution at the carbanion site. Resonance-stabilized carbanions derived from primary carbon acids E-CH₃ (E = electrophile) react with two moles of the isothiazolone to give E- CH(S-CH=CH-CONHEt)₂ as well as E-CH₂-S-CH=CH-CONHEt, whereas those derived from secondary carbon acids (E-CH₂R) give only the product E- CHR-S-CH=CH=CONHEt. When carbanions from tertiary carbon acids E-CHR₂ are employed, no stable product incorporating the carbon acid is obtained; a competing side reaction of the base on N-ethyl-3-isothiazolone itself results in the formation of a dimer (C₅H₇ONS)₂.     It is proposed that the formation and subsequent stability of the products of carbanion attack depend upon their ability to form a carbanion adjacent to the sulphur atom, and that, in the absence of such a carbanion site, reversion to starting materials will be brought about by bases. Factors relevant to this are discussed.