cine-Substitution in the thiophen series. Mechanism of the reaction of 3,4-dinitrothophen with sodium arenethiolates in methanol

Abstract

The mechanism of cine-substitution of 3,4-dinitrothiophen (1) with sodium arenethiolates in methanol, which yields 2-arylthio-4-nitrothiophens (2), has been studied. By t.l.c. and ¹H n.m.r. techniques it was found that, besides the final sulphide (2), 3-nitrothiophen, diaryl disulphide, and three other compounds were present in amounts depending on the molar ratio of the reactants and on the experimental conditions. Using sodium 2,4,6-trimethylbenzenethiolate as nucleophile it was possible to isolate the other three components of the mixture, which were identified as two stereoisomers [(3) and (4)] of 4-nitro-2,3,5-tris-(2,4,6-trimethylphenylthio)tetrahydrothiophen and as 4-nitro-2,3-bis-(2,4,6-trimethylphenylthio)-2,3-dihydrothiophen (5). Evidence was found that compounds (3)—(5) are intermediates in the transformation of the substrate (1) into the final product. A mechanism involving a set of consecutive addition–elimination steps is suggested to account for the cine-substitution process.