CLAY-CATALYZED THIOALKYLATION OF THIOPHENES AND BENZO[B]THIOPHENES

A method for the direct thioalkylation of thiophene (1) and benzo[b]th iophene (7) by reaction of either heterocycle with an alkyldisulfide u sing a ZnCl2-modified montmorillonite clay catalyst is described. The clay-catalyzed reaction of 1 with excess dimethyl disulfide (DMDS) ove r 24 hours in refluxing chlorobenzene yielded a mixture of bis-, tris- and tetrakis(methylthio)thiophenes (2-5) from which 2,3,5-tris(methyl thio)thiophene (4a) and tetrakis(methylthio)thiophene (5) could be iso lated in modest yield. Prolonging the reaction time gave (4a) and 5 in isolated yields of 21 and 16% respectively. Carrying out the same rea ction 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 tet ramethylthio-derivative 5 in moderate isolated yield (50%). In a simil ar manner 7 could be converted to 2,3-bis(methylthio)benzo[b] thiophen e (8a) or 2,3,6-tris- and 2,3,6,7-tetrakis(methylthio) derivatives 9 a nd 10a by clay catalyzed reactions with DMDS in halogenated solvents. 7 was also found to react smoothly with diethyl- and di-n-propyl disul fides yielding 2,3-bis(alkylthio)benzo[b]thiophenes (8b-c) in good iso lated yields (73 and 64%).