Environmentally Friendly Electroorganic Synthesis in Ionic Liquids Toward a Solvent-Free Process

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Solvent free electroorganic synthesis is highly desirable from an environmental aspect. In recent years, room-temperature molten salts, namely ionic liquids, have proved to be a new class of promising solvents because of their good electroconductivity, nonflammability, thermal stability, nonvolatality, and reusability. Therefore, when ionic liquids are used as electrolytic media, organic electrolytic reactions, particularly electroorganic synthesis should be possible without any organic solvents. However, there have been still a limited number of papers dealing with organic electrosynthesis in ionic liquids. In this project, we have achieved the following subjects.1) Anodic fluorodesulfurization of 3-phenylthiophthalide, 4-phenylthio-1,3-dioxolan-2-one, and 1-arylthio-2,3,4,6-tetra-O-acetyl-β-D-glucopyranoside in ionic liquids like Et_3N・nHF (n=4,5) and Et_4NF・nHF (n=4,5) without any solvents provided the corresponding monofluorinated products exclusively in moderate to good yields. In … More sharp contrast, the anodic fluorination in Et_3N・3HF with and without co-solvent resulted in α-fluorination without desulfurization exclusively or preferentially. It was also demonstrated that the electrochemical fluorodesulfurization of 3-phenylthiophthalide under solvent free conditions was achieved repeatedly four times by the reuse of a fluoride salt, Et_4NF・4HF. Thus, we have shown the first example of fluorodesulfurization by the reuse of an ionic liquid fluoride salt. (n=4,5).2) The catalytic effect of a Co(II)salen mediator on the reduction of cyclic vio-dibromide was investigated in ionic liquids by cyclic volteammetry. Macro-scale controlled potential electrolysis gave the corresponding debrominated product in moderate to good yields. The workup after electrolysis in an ionic liquid proved to be much simpler than that in organic solvents. The possibility of reruse of the ionic liquid was demonstrated to some extent.3) Photocatalytic degradation of organic pollutants such as naphthol blue black, Amarance, and 2-chlorophenol (2CP) was carried out using TiO_2 in ionic liquids. The phtocatalytic degradation rate of the former two azo dyes were greatly enhanced in ionic liquids compared with those in water. In sharp contrast, the photodegradation of 2CP was not efficient in an ionic liquid, however, the degradation was enhanced under simultaneous ultrasonication.4) Anodic fluorination of cyclic thiocarbonates like 1,3-oxathiolane-2one was successfully carried out in ionic liquids like Et_4NF・4HF. Less