Clay-catalyzed thioalkylation of thiophenes and benzo[b]thiophenes

A method for the direct thioalkylation of thiophene (1) and benzo[b]thiophene (7) by reaction of either heterocycle with an alkyldisulfide using a ZnCl2-modified montmorillonite clay catalyst is described. The clay-catalyzed reaction of 1 with excess dimethyl disulfide (DMDS) over 24 hours in refluxing chlorobenzene yielded a mixture of bis-, tris- and tetrakis(methylthio)thiophenes (2-5) from which 2,3,5-tris(methylthio)thiophene (4a) and tetrakis(methylthio)thiophene (5) could be isolated in modest yield. Prolonging the reaction time gave (4a) and 5 in isolated yields of 21 and 16% respectively. Carrying out the same reaction in a sealed autoclave at 150 degrees C with an air overpressure resulted in a clean and rapid (5 h) conversion of thiophene to the tetramethylthio-derivative 5 in moderate isolated yield (50%). In a similar manner 7 could be converted to 2,3-bis(methylthio)benzo[b] thiophene (8a) or 2,3,6-tris- and 2,3,6,7-tetrakis(methylthio) derivatives 9 and 10a by clay catalyzed reactions with DMDS in halogenated solvents. 7 was also found to react smoothly with diethyl- and di-n-propyl disulfides yielding 2,3-bis(alkylthio)benzo[b]thiophenes (8b-c) in good isolated yields (73 and 64%).