A general method for the synthesis of alkylenedithio- and bis(alkylenedithio)tetraselenafulvalenes

A general synthetic method toward a series of alkylenedithio-and bis(alkylenedithio)tetraselenafulvalenes, i.e., methylenedithio- (MDT-TSF, 1a), ethylenedithio- (EDT-TSF, 1b), propylenedithio-PDT-TSF, 1c), bis(methylenedithio)- (BMDT-TSF, 2a), bis(ethylenedithio)-(BETS, 2b), and bis(propylenedithio)tetraselenafulvalene (BPDT-TSF, 2c), as superior electron donors for organic conductors has been developed. This method is advantageous to ready access to a series of compounds from common synthetic intermediates, 2-methylthio-3-(2-methoxycarbonylethylthio)tetraselenafulvalene (6) and 2,6(7′)-bis(methylthio)-3,7(6′)-bis(2-methoxycarbonylethylthio) tetraselenafulvalene (7), for the asymmetrical alkylenedithio- and symmetrical bis(alkylenedithio)-TSFs, respectively. These key intermediates are readily prepared by phosphite-promoted coupling reactions of 4-methylthio-5-(2-methoxycarbonylethylthio)-1,3-selenole-2-selone (5) or by a reaction of TSF with LDA and methyl 3-thiocyanatopropionate. The latter method provides not only the successful conversion of TSF to these heterocycle derivatives but also a generally acceptable route to them, since TSF is accessible without the toxic and less easily available CSe₂.