Cathodic Behavior of Aromatic Polythioethers

Abstract

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...

Electrochemical Conversion of Primary Alkyl Halides to Alkenes at Platinum Cathodes

“…Instrumentation used for CV and controlled-potential electrolysis is described in a recent paper. 22 For gas chromatographic analyses of the liquid and gas phases of the electrolysis cell, we used a Varian model 3700 dual-column instrument equipped with flame ionization detectors. Products were separated with a 10 m ϫ 0.53 mm megabore capillary column ͑J & W Scientific͒ that utilizes GS-Q as the stationary phase.…”

Persulfurated Aromatic Compounds

“…Persulfurated arenes are electron acceptors 34. 112 The combination of a reducible hexakis(phenylthio)benzene derivative 111 with a persulfurated benzene core with mixed sulfur ligands and a cation‐coordinating macrobicyclic structure (tetraoxadiazamacrocycle 110 ) was described as a new type of cryptand 112 for some applications as cation sensors with electrochemical detection (Scheme ) 67. A shift of 170 mV (relative to the uncomplexed state) was found in the reduction potential of the cryptand complexed with a potassium ion.…”

Electrochemical Cleavage of Alkyl Carbon-Halogen Bonds at Carbon-Metal and Metal-Carbon Substrates: Catalysis and Surface Modification

“…Being deposited at a GC support (Figure 6), these systems permit one to build a dual carbon electrode. 30 The study of such dual electrodes (a very thin layer of graphite mechanically deposited on smooth GC) permits one to see a new kind of redox catalysis provoked by the acceleration of electron transfers because of easy reduction at graphite compared to that at GC; this process is seen through the cathodic shift of the reduction of the RI compound. Figure 2 aims to emphasize this possibility.…”