catalysts Communication Oxidation of Thiol Using Ionic Liquid-Supported Organotelluride as a Recyclable Catalyst Aya Mihoya 1, Shinichi Koguchi 1,*, Yuga Shibuya 1, Minato Mimura 1 and Makoto Oba 2,* 1 Department of Chemistry, Tokai University, 4-1-1 Kitakaname, Hiratsuka, Kanagawa 259-1292, Japan; [email protected] (A.M.); [email protected] (Y.S.); [email protected] (M.M.) 2 Graduate School of Science and Technology, Tokai University, 3-20-1 Orido, Shimizu-ku, Shizuoka 424-8610, Japan * Correspondence: [email protected] (S.K.); [email protected] (M.O.); Tel.: +81-46-358-1211 (ext. 3752) (S.K.); +81-54-334-0411 (ext. 3210) (M.O.) Received: 16 March 2020; Accepted: 3 April 2020; Published: 4 April 2020 Abstract: Organotellurium compounds are known to be useful oxidation reagents. For developing a recoverable and reusable reagent, this paper describes the use of an ionic liquid (IL) support for the organotellurium reagent and its application as a recyclable catalyst for thiol oxidation. We have successfully prepared a novel diphenyl telluride derivative 5 bearing an imidazolium hexafluorophosphate group in its structure. It is found that the IL-supported diphenyl telluride 5 efficiently catalyzed the aerobic oxidation of various thiols in [bmim]PF6 solution under photosensitized conditions to provide the corresponding disulfides in excellent yields. The product can be isolated by simple ether extraction. The IL-supported catalyst 5 remaining in the ionic liquid phase can be reused for five successive runs while retaining high catalytic activity (97% yield even in the fifth run). Keywords: ionic liquid; recyclable catalyst; organotellurium compound; aerobic oxidation; thiol; photosensitized oxygenation 1. Introduction Recently, organotellurium oxides have been recognized as useful oxidation reagents [1]. In particular, diaryl telluroxides, tellurones, and aryltellurinic acid derivatives have been identified as versatile and effective oxidants for alcohols, phosphines, thiols, thiocarbonyl compounds, and so on [2–11]. Generally, these oxidation reactions require stoichiometric amounts of organotellurium reagents. From synthetic, economic, and environmental perspectives, the development of a catalytic process is desirable. Herein, we have developed organotelluride-catalyzed oxidation of phosphites to phosphates [12], silanes to silanols [13], and thiols to disulfides [14] while employing aerobic oxygen as a terminal oxidant under photosensitized conditions, where the in situ generation of tellurium oxide species by singlet oxygen oxidation is expected. However, the protocol is not without disadvantages, for instance, the product requires isolation by chromatographic purification, and the organotelluride catalyst is rarely reusable. To overcome these issues, we envisioned to immobilize the organotelluride catalyst on an ionic liquid (IL) support. IL-supported organic synthesis and catalysis have been extensively studied in recent years [15]. Due to its high polarity, the IL support offers the advantages of easy product isolation and catalyst recycling via simple phase separation. We have previously reported the synthesis of IL-supported 18-crown-6 ether [16], ascorbate-based IL [17], and IL-supported benzyl chloride [18] for Huisgen click chemistry, IL-supported hypervalent iodine reagent [19] for alcohol oxidation, and IL-supported 1,3-dimethylimidazolidin-2-one for halogenation [20]. Herein, we describe the synthesis of IL-supported Catalysts 2020, 10, 398; doi:10.3390/catal10040398 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 398 2 of 8 diphenyl telluride as a recoverable and reusable oxidation catalyst. The catalytic activity and recyclability of the reagent are evaluated via aerobic oxidation of thiols under photosensitized Catalysts 2019, 9, x FOR PEER REVIEW 2 of 8 conditions. The transformation of thiols to disulfides is of interest from the viewpoint of organic and biologicaland recyclability processes. of the reagent are evaluated via aerobic oxidation of thiols under photosensitized conditions. The transformation of thiols to disulfides is of interest from the viewpoint of organic and 2.biological Results andprocess Discussiones. The IL-supported diphenyl telluride was prepared in the following manner 2. Results and Discussion (Scheme1). Using (4-(hydroxymethyl)phenyl)boronic acid ( 1) as the starting material, (4- (phenyltellanyl)phenyl)methanolThe IL-supported diphenyl telluride (2) was was prepared produced in the following in 84% manner yield (Scheme from 1). the Using coupling (4-(hydroxymethyl)phenyl)boronic acid (1) as the starting material, (4- reaction with diphenyl ditelluride. Next, (4-(chloromethyl)phenyl)(phenyl)tellane (3) was (phenyltellanyl)phenyl)methanol (2) was produced in 84% yield from the coupling reaction with prepared in 94% yield via halogenation with thionyl chloride followed by basic hydrolysis. diphenyl ditelluride. Next, (4-(chloromethyl)phenyl)(phenyl)tellane (3) was prepared in 94% yield Then,via halogen 1-methyl-3-(4-(phenyltellanyl)benzyl)-1ation with thionyl chloride followedH-imidazol-3-ium by basic hydrolysis. chloride (4Then,) was 1 obtained-methyl-3 in-(4 75%- yield by(phenyltellanyl)benzyl) reacting the compound-1H-3imidazolwith methyl-3-ium imidazole.chloride (4)Since was obtained this IL-supported in 75% yield telluride by reacting4 was the extremely hygroscopic,compound 3 thewith anion methyl was imidazole. subsequently Since this converted IL-supported to PF telluride6− to produce 4 was extremely hydrophobic hygroscopic, IL-supported diphenylthe anion telluride was subsequently (5) in 96% yield,converted which to wasPF6− insolubleto produce in hydrophobic low polarity IL organic-supported solvents diphenyl and water. telluride (5) in 96% yield, which was insoluble in low polarity organic solvents and water. SchemeScheme 1. 1. SynthesisSynthesis of ionic of ionic liquid liquid-supported-supported diphenyl diphenyl telluride. telluride. Initially,Initially,we we investigatedinvestigated the the catalytic catalytic oxidation oxidation of ofthiol thiol using using IL-supported IL-supported diphenyl diphenyl telluride telluride in variousin various ILs ILs employing employing thiophenol thiophenol as the the model model substrate. substrate. An IL An solution IL solution of the ofthiol, the IL thiol,-supported IL-supported catalyst,catalyst, and and roserose bengalbengal as as a a pho photosensitizertosensitizer were were stirred stirred in an in open an open flask flask and irradiated and irradiated with a with 500- a 500-W halogenW halogen lamp. lamp. After After 3 h, 3 theh, the produced produced diphenyl diphenyl disulfidedisulfide was isolated isolated by by extracting extracting with with diethyl diethyl ether. ether. The yields are compiled in Table 1. The catalytic activities of the IL-supported diphenyl The yields are compiled in Table1. The catalytic activities of the IL-supported diphenyl tellurides 4 tellurides 4 and 5 were similar to or higher than that of free diphenyl telluride (Entry 7), whereas the andreaction5 were was similar significantly to or retarded higher than in the that absence of free of the diphenyl catalyst telluride(Entry 8). The (Entry presence 7), whereas of oxygen the is reaction wasalso significantly essential for retardedthis transformation in the absence. In fact, of the the reaction catalyst under (Entry nitrogen 8). The atmosphere presence resulted of oxygen in is also essentialsignificant for yield this transformation.reduction (Entry 9). In Although fact, the reaction the reaction under in [bmim](CF nitrogen atmosphere3SO2)2N and [bmim]MeSO resulted in significant4 yieldreached reduction completion, (Entry the 9). isolated Although yields the were reaction slightly in [bmim](CF lowered owing3SO2) 2toN the and phase [bmim]MeSO separation4 reached completion,problems (Entries the isolated 1 and yields2). The werebest result slightly was lowered obtained owing using toa thehydrophobic phase separation IL, [bmim]PF problems6, as a (Entries solvent (Entry 3). Diphenyl disulfide was also isolated in quantitative yields in Entries 4 and 6, 1 and 2). The best result was obtained using a hydrophobic IL, [bmim]PF6, as a solvent (Entry 3). however, the IL [bmim]BF4 and the IL-supported catalyst 4 were unsuitable for reuse because of their Diphenyl disulfide was also isolated in quantitative yields in Entries 4 and 6, however, the IL [bmim]BF4 andhygroscopic the IL-supported nature. catalyst 4 were unsuitable for reuse because of their hygroscopic nature. AlthoughTable the active 1. Oxidation species of forthiophenol this oxidation in the presence reaction of organotellurium could not be catalysts identified a. at the present stage, a possible catalytic cycle was proposed according to our previous paper (Figure1)[ 14]. Namely, singlet oxygen oxidation of the telluride catalyst 5 gave the corresponding telluroxide (and/or tellurone), which underwent a nucleophilic attack by thiol to afford an adduct A. Then, the adduct A reacted with another thiol to give the disulfide along with regeneration of the catalyst 5 . Entry Tellurium catalyst Solvent Yield (%) b 1 5 [bmim](CF3SO2)2N 78 Catalysts 2019, 9, x FOR PEER REVIEW 2 of 8 and recyclability of the reagent are evaluated via aerobic oxidation of thiols under photosensitized conditions. The transformation of thiols to disulfides is of interest from the viewpoint of organic and biological processes. 2. Results and Discussion The IL-supported diphenyl telluride was prepared in the following