Electrochemical reduction of hexakis͑phenylthio͒benzene ͑1͒ and pentakis͑phenylthio͒benzene ͑2͒ at glassy carbon cathodes in dimethylformamide containing tetraalkylammonium tetrafluoroborates gives rise to stable radical anions ͑1 •Ϫ and 2 •Ϫ ) which, at more negative potentials, are reduced to their corresponding unstable dianions ͑1 2Ϫ and 2 2Ϫ ). In addition, 1 •Ϫ can undergo elimination and protonation to afford 2. Apparently, 2 2Ϫ behaves similarly; however if tetrakis͑phenylthio͒benzene is formed, it is immediately reduced. Room-temperature electron paramagnetic resonance studies of 1 •Ϫ and 2 •Ϫ have indicated that their halflives are approximately 60 and 15 s, respectively. In the presence of excess alkyl halide, these radical anions engage in a perturbed redox catalysis, giving rise to monoalkylated pentakis-and tetrakis͑phenylthio͒benzene, respectively. These conclusions are supported by mass spectrometric analysis of the products formed in macroelectrolyses.Although thioethers are readily oxidized at platinum and glassy carbon ͑GC͒ anodes to yield sulfoxides, sulfones, and sulfonium and radical cation salts, 1 reduction of thioethers is much more difficult. Normally, only conjugated thioethers ͑e.g., alkyl-aromatic compounds͒ are reduced prior to the solvent or electrolyte. 2 However, due to the negative reduction potentials of arylthio groups, they commonly serve as leaving groups rather than as electrophores. Accordingly, unsaturated compounds result when arylthio groups are vicinal to more strongly electroactive moieties, as shown in Scheme 1
Scheme 1.Reduction of a number of thioalkyl-and poly͑thiophenyl͒benzenes at mercury is reportedly a complex and totally irreversible process, presumably due either to the strong adsorption of thiolates on the electrode or to the formation of bis͑arylthio͒mercury compounds. 3 However, a recent paper by Tucker and co-workers 4 showed that various poly͑arylthio͒arenes are reduced reversibly at platinum, which prompted us to begin an electrochemical study of this new class of -acceptors. Here we focus on the reduction of hexakis͑phenylthio͒benzene ͑1͒ and pentakis͑phenylthio͒benzene ͑2͒, as shown in Scheme 2
Scheme 2.at GC electrodes in dimethylformamide ͑DMF͒ containing tetraalkylammonium salts; results concerning the formation of radical cations are published separately.
ExperimentalReagents.-Thioethers 1 and 2 as well as 1,2-bis͑phenylthio͒benzene were synthesized from hexachloro-, pentachloro-, and 1,2-dichlorobenzene, respectively, according to published procedures. 4,5 DMF ͑ACS spectrophotometric grade, Aldrich, 99.8ϩ%͒ was employed as the solvent. Tetramethylammonium tetrafluoroborate (TMABF 4 ), tetraethylammonium tetrafluoroborate (TEABF 4 ), and tetra-n-butylammonium tetrafluoroborate (TBABF 4 ) were obtained from GFS Chemicals and stored in a vacuum oven at 80°C to remove traces of water. 1-Iodoheptane ͑Al-drich, 98%͒, 1-iodopentane ͑Aldrich, 98%͒, iodomethane ͑Aldrich, 99.5%͒, 1-bromobutane ͑Aldrich, 99%͒, and trimethylsulfonium tetrafluoroborate ͑Lanca...
