Oxidation of Thiol Using Ionic Liquid-Supported Organotelluride As a Recyclable Catalyst

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 halogen1-methyl-3-(4-(phenyltellanyl)benzyl)-1ation with thionyl chloride followedH-imidazol-3-ium by basic hydrolysis. chloride (4 Then,) was 1obtained-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 werereaction 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 manner (Scheme 1). Using (4-(hydroxymethyl)phenyl)boronic acid (1) as the starting material, (4- (phenyltellanyl)phenyl)methanol (2) was produced in 84% yield from the coupling reaction with diphenyl ditelluride. Next, (4-(chloromethyl)phenyl)(phenyl)tellane (3) was prepared in 94% yield via halogenation with thionyl chloride followed by basic hydrolysis. Then, 1-methyl-3-(4- (phenyltellanyl)benzyl)-1H-imidazol-3-ium chloride (4) was obtained in 75% yield by reacting the compound 3 with methyl imidazole. Since this IL-supported telluride 4 was extremely hygroscopic, the anion was subsequently converted to PF6− to produce hydrophobic IL-supported diphenyl telluride (5) in 96% yield, which was insoluble in low polarity organic solvents and water.

Scheme 1. Synthesis of ionic liquid-supported diphenyl telluride.

Initially, we investigated the catalytic oxidation of thiol using IL-supported diphenyl telluride in various ILs employing thiophenol as the model substrate. An IL solution of the thiol, IL-supported catalyst, and rose bengal as a photosensitizer were stirred in an open flask and irradiated with a 500- W halogen lamp. After 3 h, the produced diphenyl disulfide was isolated by extracting with diethyl ether. The yields are compiled in Table 1. The catalytic activities of the IL-supported diphenyl tellurides 4 and 5 were similar to or higher than that of free diphenyl telluride (Entry 7), whereas the reaction was significantly retarded in the absence of the catalyst (Entry 8). The presence of oxygen is also essential for this transformation. In fact, the reaction under nitrogen atmosphere resulted in significant yield reduction (Entry 9). Although the reaction in [bmim](CF3SO2)2N and [bmim]MeSO4 reached completion, the isolated yields were slightly lowered owing to the phase separation problems (Entries 1 and 2). The best result was obtained using a hydrophobic IL, [bmim]PF6, as a solvent (Entry 3). Diphenyl disulfide was also isolated in quantitative yields in Entries 4 and 6, however, the IL [bmim]BF4 and the IL-supported catalyst 4 were unsuitable for reuse because of their hygroscopicCatalysts 20192020, 910 nature., x, 398FOR PEER REVIEW 3 of 8

a Table 1.2 Oxidation of thiophenol5 in the presence[bmim]MeSO of organotellurium4 90 catalysts . Table 1. Oxidation of thiophenol in the presence of organotellurium catalysts a. 3 5 [bmim]PF6 quant. 4 5 [bmim]BF4 quant. 5 5 [bmim]Br 79 6 4 [bmim]PF6 quant. b EntryEntry7 Tellurium PhTePh Catalyst catalyst [bmim]PFSolvent Solvent 6 Yield84 (%)Yield (%) b 18 none5 [bmim](CF[bmim]PF3SO62 )2N 7847 1 5 [bmim](CF3SO2)2N 78 9 c 5 [bmim]PF6 23 2 5 [bmim]MeSO4 90 a Condition: thiol3 (1 mmol), tellurium5 catalyst (0.2 mmol),[bmim]PF rose bengal6 (0.05 mmol),quant. solvent (10 mL), irradiated with4 a 500-W halogen lamp5 under aerobic conditions,[bmim]BF 3 h; 4b isolated yield,quant. c under a nitrogen atmosphere. 5 5 [bmim]Br 79 6 4 [bmim]PF6 quant. Although the7 active species PhTePh for this oxidation reaction [bmim]PF could6 not be identified84 at the present stage, a possible8 catalytic cycle was none proposed according [bmim]PF to our6 previous paper47 (Figure 1) [14]. c 5 Namely, singlet 9 oxygen oxidation of the telluride catalyst[bmim]PF 5 gave6 the corresponding23 telluroxide a (and/orCondition: tellurone), thiol which (1 mmol), under telluriumwent catalyst a nucleophilic (0.2 mmol), rose attack bengal by (0.05 thiol mmol), to afford solvent (10an mL),adduct irradiated A. Then, with the a 500-W halogen lamp under aerobic conditions, 3 h; b isolated yield, c under a nitrogen atmosphere. adduct A reacted with another thiol to give the disulfide along with regeneration of the catalyst 5.

Figure 1. PlausiblePlausible reaction reaction mechanism mechanism for the catalytic oxidation of thiols thiols.. With the optimized conditions in hand, the substrate scope of the oxidation reaction using With the optimized conditions in hand, the substrate scope of the oxidation reaction using IL- IL-supported catalyst 5 was evaluated with various thiols. The results are summarized in Table2. supported catalyst 5 was evaluated with various thiols. The results are summarized in Table 2. Dodecanethiol (Entry 1), cyclohexanethiol (Entry 2), and benzyl mercaptane (Entry 6) were converted to Dodecanethiol (Entry 1), cyclohexanethiol (Entry 2), and benzyl mercaptane (Entry 6) were converted their corresponding disulfides in high yields. However, the yield of the oxidation reaction of tert-butyl to their corresponding disulfides in high yields. However, the yield of the oxidation reaction of tert- mercaptane decreased to 73% owing to the steric hindrance (Entry 3). Under the employed oxidation butyl mercaptane decreased to 73% owing to the steric hindrance (Entry 3). Under the employed conditions, hydroxyl and ester groups remained untouched to produce their corresponding disulfides oxidation conditions, hydroxyl and ester groups remained untouched to produce their corresponding in excellent yields (Entries 4 and 5). The reaction also proceeded with the aromatic nitro and chloro disulfides in excellent yields (Entries 4 and 5). The reaction also proceeded with the aromatic nitro functional groups intact (Entries 8 and 9). Furthermore, oxidation of heterocyclic 4-mercaptopyridine and chloro functional groups intact (Entries 8 and 9). Furthermore, oxidation of heterocyclic 4- produced 4,4 -dipyridyl disulfide in quantitative yield without affecting the pyridine ring (Entry 10). mercaptopyridine0 produced 4,4′-dipyridyl disulfide in quantitative yield without affecting the Finally, we investigated the reusability of the IL-supported diphenyl telluride 5 employing the pyridine ring (Entry 10). conditions shown in Entry 7 of Table2. After completing the thiophenol oxidation, the resulting diphenyl disulfideTable 2. wasCatalytic isolated oxidation via extraction of various with thiols diethyl using ether,IL-supported and the diphenyl remaining telluride [bmim]PF 5 a. 6 solution containing IL-supported catalyst 5 and rose bengal could be reused at least four times in the subsequent reactions with only a slight decrease in the product yield (97% after four times recycling, Figure2).

Catalysts 2019, 9, x FOR PEER REVIEW 3 of 8

2 5 [bmim]MeSO4 90 3 5 [bmim]PF6 quant. 4 5 [bmim]BF4 quant. 5 5 [bmim]Br 79 6 4 [bmim]PF6 quant. 7 PhTePh [bmim]PF6 84 8 none [bmim]PF6 47 9 c 5 [bmim]PF6 23 a Condition: thiol (1 mmol), tellurium catalyst (0.2 mmol), rose bengal (0.05 mmol), solvent (10 mL), irradiated with a 500-W halogen lamp under aerobic conditions, 3 h; b isolated yield, c under a nitrogen atmosphere. Although the active species for this oxidation reaction could not be identified at the present stage, a possible catalytic cycle was proposed according to our previous paper (Figure 1) [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.

Figure 1. Plausible reaction mechanism for the catalytic oxidation of thiols.

With the optimized conditions in hand, the substrate scope of the oxidation reaction using IL- supported catalyst 5 was evaluated with various thiols. The results are summarized in Table 2. Dodecanethiol (Entry 1), cyclohexanethiol (Entry 2), and benzyl mercaptane (Entry 6) were converted to their corresponding disulfides in high yields. However, the yield of the oxidation reaction of tert- butyl mercaptane decreased to 73% owing to the steric hindrance (Entry 3). Under the employed oxidation conditions, hydroxyl and ester groups remained untouched to produce their corresponding disulfidesCatalysts in 2019 excellent, 9, x FOR PEERyields REVIEW (Entries 4 and 5). The reaction also proceeded with the aromatic4 of 8nitro and chloro functional groups intact (Entries 8 and 9). Furthermore, oxidation of heterocyclic 4- mercaptopyridine produced 4,4′-dipyridyl disulfide in quantitative yield without affecting the pyridineCatalysts 2020 ring, 10 ,(Entry 398 E10nt).r y Substrate Product Yield (5) b 4 of 8 Catalysts 2019, 9, x FOR PEER REVIEW 4 of 8 CatalystsCatalystsCatalysts 20192019,, 2019 99,, xx FOR,FOR 9, x PEERFORPEER PEER REVIEWREVIEW REVIEW 44 ofof 88 4 of 8 CatalystsCatalysts 2019, 20199, x FOR, 9, x PEERFOR1 PEER REVIEW REVIEW quant. a 4 of 8 4 of 8 CatalystsCatalysts 2019,Table 92019, x FOR, 92., x CatalyticPEER FOR PEERREVIEW o xidationREVIEW of various thiols using IL-supported diphenyl telluride 5 . 4 of 8 4 of 8 CatalystsCatalysts 20192019Table,, 2019920199,, xx FORFOR,, 9 2.9,, xxCatalytic PEER PEERFORFOR PEER PEER REVIEWREVIEW oxidation REVIEWREVIEW of various thiols using IL-supported diphenyl telluride 5 a. 44 of 88 4 4 of 88 CatalystsCatalysts 2019 ,2019 9, x ,FOR 9, x FORPEER PEER REVIEW REVIEW 4 of 84 of 8 Catalysts 2019, 9, x FOR PEER REVIEW 4 of 8 Catalysts 2019, 9, x FOR PEER REVIEW b 4 of 8 CatalystsCatalysts 20192019,, 99,, xx FORFOR PEERPEERE2 n REVIEWtREVIEWry Substrate Product 94 Yieldb (5) 44 ofof 88 Catalysts 2019, 9E,E xnn FORttrryEy n PEERtry SREVIEWSuubbsSstturraab ttseet rate PP rrooddPuurccottd uct YYiieelldYd i ((e55l))d b (5) b 4 of 8 b b EntrEy ntry SubsSturabtset rate P rodPurcotd uct YieldY i(e5l)bd (5)b EEnntttrrryEyE n nttrryy SSuubbssSStttrururaabbttteses tt rraattee PPr rrooddPuPurcrcoottt d d uucctt YYiiieelllddYY ( ii((5ee5)ll)) dd bb ((55)) bb 3 1 7 3 quabnt. b En11t r Eyn 1tr y SubSsutrbastetr ate P rodPurocdt uct YiqqeuulYdaain neq(t5tlu..d) a bn( 5t). Entry Substrate Product Yield (5) b 1 Ent1r y Substr ate Produ c t quYaineqtlu.d a b(n5t). b EntrEyn try SubSsutrbastetr ate ProdPurocdt uc t YielYdi e(5ld) b ( 5) b b Entry En11 t rEy n11t ry SSubstrateubsSturabtset r ate ProdP Producturcotd uc t YqiqeuuladaYnn iqYield(qtet5t.u.u .l ) da an n (t5t (5).. ) 1 1 quaqnut.a nt. 1 4 2 98q uant. 94 22 12 9944q u9a4n t. 121 12 qquu9aa4nnq ttu.. 9 a4n t. 2 2 94 94 1 2 2 94 quant.94 2 5 2 quant.9 4 94 32 3 7934 73 32 23 7934 9743 2 23 23 9743 974394 3 33 73 7733 3 6 3 91 73 73 3 3 73 73 3 4 73 98 44 34 9988 7938 3 343 34 797383 793873 4 44 98 9988 4 7 4 q uant9. 8 98 4 4 98 98 4 54 5 qu9a8n qtu. a98nt. 54 45 qu9a8nq tu.9 a8n t. 45 45 qu9a8n qtu.9 a8n t. 55 8 55 q uaqqnuuta.a nnqqtt.u.u aanntt.. 5 5 quanqtu. ant. 5 5 5 quaqnut.quant.a nt. 65 6 qu9a1n t. 91 6 56 91q u9a1n t. 565 56 qquu9aa1nnq ttu.. 9 a1n t. 6 9 66 71 91 9911 6 6 91 91 6 6 6 91 9191 6 7 91 quant. 77 67 qquuaannqttu..9 a1n t. 676 10 67 quanqtu9.9 a11n q tu.9 a1n t. 7 77 quanqqt.uu aanntt.. 7 7 7 quanqttu..quant. ant. 7 78 quaqnqutu.a anntt.. 878 78 qquuaannqtttu... antt.. 77 7 q quuaanqntut.. a nt. a Condition: thiol8 (0.578 mmol), 5 (0.1 mmol), rose bengal (0.025 mmol), [bmim]PF 6 q(5u mL),anqqtuu. airradiatedanntt.. 8 88 88 qquuaannqqtt.u.u quant. aanntt.. 8 8 b q uanqtu. ant. with a 500-W halogen lamp under aerobic conditions, 3 h; isolated yield. 8 89 quaqnut.a 7n1 t. 989 89 qu77a11nq tu.7 a1n t. Finally, we 8investigated8 8 the reusability of the IL-supported diphenyl telluride qquuaanqn 5tut. . employinga nt. the 9 9 89 71q u7a171n t. 99 99 7711 7711 conditions shown in 9E ntry 7 of Table 2. After completing the thiophenol oxidation 7, 1 the resulting 9 91 0 71 qu71a nt. diphenyl disulfide11900 was190 isolated via extraction with diethyl ether, and the remainingqquu7aa1nnq ttu..7 a1n t [bmim]PF. 6 10 1990 919 0 qu77a11n q tuquant..77 1a1 n t. solution containing10 IL10-supported catalyst 5 and rose bengal could be reused atq leastuanq tfour.u an ttimes. in the a 10 10 q uanqttu.. antt.. subsequentCondition: thiolreactions (0.5 mmol),1 0with only5 (0.1 a mmol), slight rosedecrease bengal in (0.025 the product mmol), [bmim]PF yield (97% 6 (5 after mL), irradiatedfourq utimesant with. recycling a 500-W, halogen lamp under10 aerobic conditions, 3 h; b isolated yield. quant. a 10 10 quanqtu. ant. Figurea a Condition:2). 1 0thiol 10 (0.5 mmol), 5 (0.1 mmol), rose bengal (0.025 mmol), [bmim]PFquaqnut.a 6n (5t. mL), irradiated a Condition:Condition: Condition: thiolthiol10 thiol (0.5(0.510 mmol), mmol),(0.5 mmol), 55 (0.1(0.1 5 mmol),mmol),(0.1 mmol), roserose rosebengal bengal bengal (0.025(0.025 (0.025 mmol),mmol), mmol), [bmim]PF[bmim]PF [bmim]PFqua66 n (5q(5tu. mL),mL),a6 n(5t. mL), irradiatedirradiated irradiated a a b Condition:with Condition: a 500 thiol-W thiol (0.5halogen mmol),(0.5 lampmmol), 5 (0.1under 5 mmol),(0.1 aerobic mmol), rose conditions, rosebengal bengal (0.025b 3 h (0.025; mmol), isolated mmol), [bmim]PF yield. [bmim]PF 6 (5 mL),6 (5 mL),irradiated irradiated a aa b b awith awithCondition: awith a a Condition: Condition: 500500 a--W W500thiol halogenhalogen-W thiol(0.5thiol halogen mmol), lamp(0.5lamp(0.5 mmol),lampmmol), underunder 5 (0.1 under aerobic5 aerobic5mmol), (0.1(0.1 aerobic mmol),mmol), conditions,conditions, rose conditions, bengalroserose bengal 3bengal3 h(0.025h;; 3 ii solated hsolated (0.025(0.025 ;mmol), isolated mmol), yield.mmol),yield. [bmim]PF yield. [bmim]PF[bmim]PF 6 (5 mL),66 (5(5 mL),irradiatedmL), irradiatedirradiated Condition: Condition: thiol thiol(0.5 mmol),(0.5 mmol), 5 (0.1(0.1 5 mmol),mmol),(0.1 mmol), roserose rosebengalbengal bengal (0.025(0.025b (0.025 mmol),mmol),b mmol), [bmim]PF[bmim]PF [bmim]PF66 (5(5 mL),mL),6 (5 mL), irradiatedirradiated irradiated witha witha 500 a-W 500 halogen-W halogen lamp lamp under under aerobic aerobic conditions, conditions, 3 h; 3 i solatedh; isolated yield. yield. a Condition: thiol (0.5 mmol), 5 (0.1 mmol), rose bengal b (0.025bb mmol), [bmim]PF6 6 (5 mL), irradiated witha Condition: awithwithFinally, 500 -aaW 500500 thiolhalogen we--WW investigated (0.5halogenhalogen lamp mmol), lamplampunder 5 the(0.1 underunder aerobic reusabilitymmol), aerobicaerobic conditions, rose conditions,conditions, ofbengal the 3 h IL;(0.025 bb -3i3supportedsolated hh;; bmmol), iisolatedsolated yield. [bmim]PFdiphenyl yield.yield. telluride(5 mL), irradiated 5 employing the withFinally, Condition: awitha 500 we a--W 500 thiolinvestigated halogen-W (0.5halogen lampmmol), lampthe under 5 reusability (0.1under aerobic mmol), aerobic conditions, ofrose theconditions, bengal IL -3supported h (0.025;; 3 ii solatedsolatedh; mmol), isolated diphenyl yield.yield. [bmim]PF yield. telluride 6 (5 mL), 5 employing irradiated the Finally,a aFinally, Condition: we investigatedwe thiolinvestigated (0.5 mmol),the reusabilitythe 5 reusability(0.1 mmol), of the ofrose ILthe bengal-supported IL-supported (0.025b diphenyl mmol), diphenyl [bmim]PF telluride telluride6 (5 5 mL),employing 5 employing irradiated the the a Condition:a Condition: thiol thiol (0.5 (0.5mmol), mmol), 5 (0.1 5 (0.1mmol), mmol), rose rose bengal bengal (0.025b (0.025 mmol), mmol), [bmim]PF [bmim]PF6 (5 mL),6 (5 mL), irradiated irradiated withFinally, Condition:with Finally,a Condition: 500 wea- 500W thiolinvestigated halogenwe-W thiolhalogen(0.5investigated mmol),lamp(0.5 lamp mmol), theunder 5 reusability(0.1underthe aerobic5 mmol),reusability(0.1 aerobic mmol), conditions, roseof conditions,the rosebengalof ILthe -bengal3supported hIL(0.025; 3b- supportedihsolated (0.025; mmol), isolated diphenyl mmol),yield. [bmim]PF diphenylyield. [bmim]PF telluride 6 (5telluride mL),6 (55 employingmL),irradiated 5 employingirradiated the the conditionswith a 500-W shown halogen in lamp Entry under 7 of aerobic Table conditions, 2. After completing 3 h; isolatedb the yield. thiophenol oxidation, the resulting conditionsFinally,conditionsFinally,withFinally, shown we a 500 showninvestigated wewe - inW investigated investigatedhalogenE ntry in E ntry 7 thelamp of reusability 7 Table the theunder of reusability Tablereusability 2. aerobic After 2.of Afterthe conditions, completing ofof IL thethe - completingsupported ILIL --3bsupportedsupported h the; b isolated diphenylthiophenol the thiophenoldiphenyldiphenyl yield. telluride oxidation telluridetelluride oxidation 5 employing, the 55 employingemploying, resulting the resultingthe thethe conditionsFinally,withwithFinally, a 500 shown we a- 500W investigated we halogen- W in investigated halogenEntry lamp 7 lampthe ofunder reusability Tablethe under aerobic reusability 2.aerobic After conditions, of theconditions, completingof ILthe-- supportedsupported3 ILh; -b3supported ihsolated ; the b isolated thiophenoldiphenyldiphenyl yield. diphenyl yield. telluridetelluride oxidation telluride 5 employing, the5 employing resulting the the conditionsdiphenylconditionswith witha 500 shown a- disulfideW 500 shown halogen -W in halogenE ntryin waslamp E ntry 7 isolatedlamp under of 7 Table under of aerobic Tablevia 2.aerobic extractionAfter conditions, 2. After conditions, completing completing with3 h; 3 i diethylsolatedh the; isolated thiophenol the yield. ether, thiophenol yield. and oxidation the oxidation remaining, the, resulting the [bmim]PF resulting 6 diphenylFinally,Finally, disulfide we investigatedwe wasinvestigated isolated the thereusabilityvia reusabilityextraction of the of with theIL- supported diethylIL-supported ether, diphenyl diphenyl and thetelluride remainingtelluride 5 employing 5 [bmim]PFemploying the6 the6 conditionsconditionsdiphenylconditionsdiphenylconditionsFinally, disulfide shown shown we disulfide shown shown investigated in in was EE ntryntry in in was isolated EE ntryntry 7 7 isolatedthe of of 7Table 7 Tablereusability via of of Table Table viaextraction 2. 2. AfterAfterextraction 2. 2.of AfterAfter the completing completing with IL completing completing with- supported diethyl diethyl the the ether, thiophenol thiophenolthiophenol the thediphenyl ether, thiophenolthiophenol and and thetelluride oxidationoxidation theremaining oxidationoxidation remaining 5 ,,employing , the the the [bmim]PF, , resulting resulting resultingthe the [bmim]PF resulting resulting the 6 diphenylsolutiondiphenylFinally, disulfide containing disulfide we wasinvestigated IL was isolated-supported isolated thevia reusability extractioncatalystvia extraction 5 andof with the rose with diethylIL bengal-supported diethyl ether, could ether, diphenyl and be reused and the remainingthetelluride at remainingleast 5four employing [bmim]PF times [bmim]PF in6 thethe6 diphenylsolutionconditionsconditionsdiphenylsolutionFinally,Finally, containingFinally, disulfide shown wecontaining we disulfide shown investigated investigatedwe in wasIL investigated E -supported in ntry wasILisolated E-ntrysupported 7 isolated the the of 7 viareusability Tablecatalystreusabilitythe of Tableextraction reusabilityviacatalyst 2. 5extractionAfter 2.and ofof After5 the theroseand withcompletingof IL ILthe completingrosebengal with-- diethylsupportedsupported IL bengal -supported diethyl could the ether, could thethiophenol diphenyldiphenyl be ether, thiophenol and reuseddiphenyl be andreusedthe telluridetelluride atoxidation remaining the leasttelluride oxidationat remaining least four55 employing,employing the four5times [bmim]PF ,employing the resulting times [bmim]PF in resulting thethe in6 the6 diphenylconditionssolutiondiphenyl containing disulfide shown disulfide in was IL E-supported ntry was isolated 7 isolated of Tableviacatalyst extractionvia 2. extraction5After and rosecompleting with bengal with diethyl diethyl could the ether, thiophenol be ether, andreused and the oxidationat remaining theleast remaining four, the times [bmim]PF resulting [bmim]PF in the66 66 solutionconditionsssolutionubsequent containing containing shown reactions IL - insupported IL Ewith-ntrysupported only 7 catalyst of a Tableslight catalyst 5 2.anddecrease After5 androse completingrose bengalin the bengal product could thecould be yieldthiophenol reused be (97%reused at afterleast oxidation at least four fourtimes, the times inrecycling resulting the in the, solutionsconditionsdiphenylconditionsubsequentdiphenylsolutionsconditionsubsequent containing disulfide shown shownreactions containing disulfide shownreactions inIL in was -withEsupportedE ntry inntry wasIL isolated with -Eonlysupported ntry 7 7isolated of ofonly a 7catalyst Tableslight Tablevia of a Tableviaslightcatalystextraction decrease 2. 2. 5 extractionAfter Afterand decrease 2. 5After rose and completingin completing with the bengalrose completing within diethylproduct the bengal diethyl couldproduct the the ether,yield couldthiophenol thethiophenol be ether, yield reused thiophenol(97% and be reused and(97% theafter at oxidationoxidation theleast remainingafter four atoxidation remaining leastfour fourtimes,, the thefourtimes times [bmim]PF ,recycling resulting theresulting times [bmim]PFin recycling resultingthe in ,6 the6, solutionsolutiondiphenylsubsequentsolutionsolution containingcontaining disulfide reactions containingcontaining ILIL was with--supportedsupported ILIL isolated- -onlysupportedsupported a catalystslightcatalystvia catalystextractioncatalyst decrease 5 and 5 roseand in with the rosebengal diethylproduct bengal could ether, yieldcould be reused (97% and be reused theafter at remainingleast fourat least four times fourtimes [bmim]PF recycling times in thethe in6, thethe subsequentdiphenylsubsequent reactions disulfide reactions with was with only isolated only a slight a viaslight decrease extraction decrease in the within product the diethyl product yield ether, yield (97% and(97% after the after four remaining fourtimes times recycling [bmim]PF recycling, 6, solutiondiphenylsolutiondiphenylFigure containing disulfide 2 containing). disulfide IL was-supported IL was isolated-supported isolated catalystvia catalystviaextraction extraction5 and 5 and rose with rose bengal with diethyl bengal diethyl could ether, could be ether, reused andbe reused and the at the remainingleast at remainingleast four four times [bmim]PF times [bmim]PF in the in66 the ssFiguresdiphenylsolutionFigureubsequentubsequentsFiguresdiphenylubsequentubsequent 22). ).containing disulfide reactions2reactions). disulfide reactionsreactions wasIL withwith-supported was isolated withwith onlyonly isolated only aonlya slightslight viacatalyst aa slightslightextractionvia decreasedecrease 5extraction decreaseanddecrease inrose in with thethe inbengalin with product diethylproduct thethe diethylproductproduct could ether,yield yield be ether, yieldyield (97% (97%reused and (97% (97% andafter theafter at remaining theafterleastafter fourfour remaining fourtimes fourtimes timestimes [bmim]PF recycling recyclingrecycling [bmim]PF recyclinginrecycling the,, , ,6, FiguresolutionsFigureubsequent 2). 2 containing). reactions IL with-supported only a slightcatalyst decrease 5 and rose in the bengal product could yield be reused(97% after at least four four times times recycling in the, FiguresolutionssolutionubsequentsFigureFiguresolutionubsequent 2). containing containing 22 reactions). ).containing reactions ILIL with--supportedsupported IL with -onlysupported only a slight catalystcatalyst a slight catalyst decrease 55 decrease andand 5 androse rosein the rose inbengalbengal productthe bengal product couldcould yield could bebe yield reused(97%reused be (97%reused after atat after leastleast four at least four four fourtimes fourtimes timestimes recycling times recycling inin thethe in, the, FiguresubsequentsFigureubsequent 2).). 2 reactions). reactions with with only only a slight a slight decrease decrease in the in productthe product yield yield (97% (97% after after four four times times recycling recycling, , sFiguresubsequentubsequentFiguressubsequentubsequent 2). 2 reactions).reactions reactionsreactions withwith withwith onlyonly onlyonly aa slightslight aa slightslight decreasedecrease decreasedecrease inin thethe inin product theproductthe productproduct yieldyield yieldyield (97%(97% (97%(97% afterafter afterafter fourfour four four timestimes timestimes recyclingrecycling recyclingrecycling,, ,, FigureFigure 2). 2). FigureFigureFigureFigure 22).). 22).).

Run 1 (fresh) 2 3 4 5 Yield (%) a quant. quant. 98 97 97 a Isolated yield.

Figure 2. Recycling experiment of IL-supported diphenyl telluride catalyst 5. Condition: thiol (1 mmol), tellurium catalyst (0.2 mmol), rose bengal (0.05 mmol), solvent (10 mL), irradiated with a 500-W halogen lamp under aerobic conditions, 3 h.

Run 1 (fresh) 2 3 4 5 RunRun Run 11 (fresh)(fresh)1 (fresh) 22 2 33 344 455 5 Run 1a (fresh) 2 3 4 5 YieldRun a(%) a 1quant. (fresh) quant.2 983 974 975 YieldYieldRunRunYield (%)(%) Run Run a(%) 11 quant.(fresh)quant.(fresh) 11 quant. (fresh)(fresh) quant.quant. 22 quant. 22 398983 9843973974 9745974975 9755 Yield (%) a quant.a a quant. 98 97 97 RunYieldRun a (%) 1a a (fresh) a 1 (fresh)quant.a Isolated 2 yieldquant.2 .3 3984 4975 597 YieldYieldRunYieldYield (%)(%) aa (%)(%) 1 quant. quant.a (fresh) a IsolatedIsolatedquant.quant. Isolated quant. quant. yieldyield 2 quant.quant.yield.. 9898.3 979898974 97975 9797 YieldYield (%)Run (%) quant. a 1quant. (fresh)a quant. quant. 2 98 98397 97497 975 RunRun a 1a a(fresh) Isolated1 (fresh)a Isolated yield 2 yield. 2 .3 34 45 5 YieldRunYield (%)Run (%)a 1 quant. (fresh)a Isolated1quant. (fresh)a Isolated yieldquant. 2 quant..yield 2 .98 3 989734 979745 975 Yield (%) aquant.a Isolateda Isolated yieldquant. .yield 98. 97 97 Yield (%)a a Isolatedquant. Isolated yield quant.yield. . 98 97 97 YieldYield (%) a(%) aquant. a quant.a quant. quant. 98 9897 9797 97 YieldYield (%) (%) quant. a Isolatedquant. Isolated quant.yield yieldquant.. .98 9897 9797 97 Isolateda yield. a a Isolated yield. a Isolateda Isolated yield yield. . Isolated Isolated yield yield. .

Catalysts 2020, 10, 398 5 of 8

3. Materials and Methods

3.1. General All reagents and solvents were commercially sourced and of reagent grade and were used without puriﬁcation. The reactions were monitored using aluminum thin layer chromatography plates with silica gel 60 F254 (Merck, (Darmstadt, Germany)). Column chromatography was performed using silica gel 60 (Kanto Chemical, Japan, Tokyo). 1H, 13C, and 125Te NMR spectra were measured on a Bruker Advance DRX 500 (1H: 500 MHz, 13C: 125 MHz, 125Te: 159 MHz spectrometer. All chemical 1 13 shifts are reported in parts per million (ppm) relative to TMS (0 ppm for H), CHCl3 (77 ppm, for C), 13 125 DMSO (39 ppm for C), and PhTeTePh (419 ppm in CDCl3, 422ppm in DMSO for Te). Mass analyses were performed using a JEOL AccuTOF LC-plus JMS-T100LP spectrometer (Japan, Tokyo).

3.2. (4-(Hydroxymethyl)Phenyl)(Phenyl)Telluride (2) A solution of diphenyl ditelluride (81.9 mg, 0.200 mmol), (4-hydroxymethyl)phenyl)boronic acid (1) (66.9 mg, 0.440 mmol), 1,10-phenthroline H O (2.20 mg, 12.0 mol), and CuSO (19.0 mg, · 2 4 12.0 mol) in ethanol (0.6 mL) was stirred at room temperature for 1 min. Then, a 5% Na2CO3 aqueous solution (0.1 mL) was added to the solution and the mixture was stirred at room temperature for 5 h. The progress of the reaction was monitored by 1H NMR spectroscopy. After the reaction was completed, the mixture was dried over MgSO4, and the solvent was evaporated. The residue was puriﬁed via dry column chromatography to produce product 2 (0.104 g, 84%) as a colorless solid. 1 Mp 62–63 ◦C; H-NMR (500 MHz, CDCl3): δ = 7.69–7,67 (m, 4H), J = 6, 4H), 7.21–7.29 (m, 5H), 4.66 13 (s, 2H), 1.80 (br, 1H); C-NMR (125 MHz, CDCl3): δ = 140.7, 138.3, 137.9, 129.5, 128.1, 127.9, 114.7, 125 + 113.7, 65.0; Te-NMR (159 MHz, CDCl3): δ = 690.7; HRMS (APCI): m/z [M–OH] calcd. for C13H11Te: 296.9917, found: 296.9874.

3.3. (4-(Chloromethyl)Phenyl)(Phenyl)Telluride (3) To a solution of (4-(hydroxymethyl)phenyl)(phenyl)telluride (2) (2.32 g, 7.45 mmol) and triethylamine (1.50 mL, 10.4 mmol) in CH2Cl2 (4.6 mL), a solution of thionyl chloride (2.41 mL, 33.5 mmol) in CH2Cl2 (7.4 mL) was added dropwise at 0 ◦C. The resulting mixture was stirred at room temperature for 1 h. Then CH2Cl2 was removed by evaporation, THF (36.9 mL) and a 2 M NaOH solution (25.2 mL) were added, and the mixture was stirred at room temperature for 19 h. After the reaction was completed, the mixture was evaporated and extracted with ethyl acetate. The organic layer was washed with water, dried over MgSO4, and evaporated. The residue was puriﬁed via 1 column chromatography to yield product 3 (2.32 g, 94%) as a yellow oil. H-NMR (500 MHz, CDCl3): 13 δ = 7.72 (d, J = 8.0, 2H), 7.63 (d, J = 8.0, 2H), 7.33–7.21 (m, 5H), 4.55 (s, 2H); C-NMR (125 MHz, CDCl3): 125 δ = 138.5, 137.8, 137.1, 129.6, 128.1 (overlapped), 115.3, 114.3, 45.9; Te-NMR (159 MHz, CDCl3): + δ = 690.6; HRMS (APCI): m/z [M–Cl] calcd. for C13H11Te: 296.9917, found: 296.9951.

3.4. 1-Methyl-3-(4-(Phenyltellanyl)Benzyl)-1H-Imidazol-3-Ium Chloride (4) A solution of phenyl(4-chloromethylphenyl)tellane (3) (2.32 g, 7.02 mmol) and N-methylimidazole (0.692 g, 8.43 mmol) in CH3CN (63.6 mL) was heated to reflux for 19 h. Then, the mixture was evaporated, and the residue was purified via column chromatography to yield product 4 (2.18 g, 75%) as a yellow oil. 1 H-NMR (500 MHz, DMSO-d6): δ = 9.42 (s, 1H), 7.84 (s, 1H), 7.75 (s, 1H), 7.71 (d, J = 8.0, 2H), 7.66 (d, J = 8.0, 13 2H), 7.37–7.27 (m, 5H), 5.45 (s, 2H), 3.86 (s, 3H); C-NMR (125 MHz, DMSO-d6): δ = 138.7, 137.9, 137.24, 125 135.0, 130.3, 129.9, 128.7, 124.8, 122.8, 116.6, 115.0, 51.8, 36.3; Te-NMR (159 MHz, DMSO-d6): δ = 705.3; + HRMS (APCI): m/z M calcd. for C17H17N2Te: 379.0448, found: 379.0462. Catalysts 2020, 10, 398 6 of 8

3.5. 1-Methyl-3-(4-(Phenyltellanyl)Benzyl)-1H-Imidazol-3-Ium Hexaﬂuorophosphate (5) A solution of 1-methyl-3-(4-(phenyltellanyl)benzyl)-1H-imidazol-3-ium chloride (4) (2.16 g, 5.24 mmol) and KPF6 (0.965 g, 5.24 mmol) in methanol (10 mL) was stirred at 30 ◦C for 17 h. Then, the mixture was ﬁltered, and the solution was dried over MgSO4 and evaporated to yield 1 product 5 (2.63 g, 96%) as a yellow oil. H-NMR (500 MHz, DMSO-d6): δ = 9.18 (s, 1H), 7.77–7.65 13 (m, 6H), 7.38–7.26 (m, 5H), 5.39 (s, 2H), 3.84 (s, 3H); C-NMR (125 MHz, DMSO-d6): δ = 138.7, 137.9, 125 137.2, 134.9, 130.3, 129.9, 128.7, 124.5, 122.8, 116.6, 115.0, 51.9, 36.3; Te-NMR (159 MHz, DMSO-d6): + δ = 707.0; HRMS (APCI): m/z M calcd. for C17H17N2Te: 379.0448, found: 379.0491.

3.6. Oxidation of Thiols

Typically, [bmim]PF6 solution (10 mL) of thiophenol (0.110 g, 1.00 mmol), IL-supported diphenyl telluride 5 (0.104 g, 0.200 mmol), and rose bengal (50.9 mg, 0.0500 mmol) were vigorously stirred in an open flask and irradiated using a 500-W halogen lamp for 3 h. The resulting mixture was extracted with diethyl ether and evaporated to yield diphenyl disulfide (0.114 g, quant.). The catalytic oxidation of other thiols was similarly conducted and the products were identified by comparison of physical and spectral data with published values.

3.6.1. Diphenyl Disulﬁde

1 Pale pink solids, mp 55–56 ◦C (mp 59–61 ◦C[14]); H-NMR (500 MHz, CDCl3): δ = 7.50 (d, J = 8.5, 13 4H), 7.30 (t, J = 7.8, 4H), 7.26 (t, J = 8.5, 2H); C-NMR (125 MHz, CDCl3): δ = 137.0, 129.1, 127.5, 127.2.

3.6.2. Didodecyl Disulﬁde

1 Pale pink solids, mp 33–40 ◦C (mp 31–32 ◦C[14]); H-NMR (500 MHz, CDCl3): δ = 2.70 (t, J = 7.5, 4H), 1.68 (quin., J = 7.5, 4H), 1.39 (m, 4H), 1.26 (br, 32H), 0.88 (t, J = 6.8, 6H); 13C NMR (125 MHz, CDCl3): δ = 39.20, 31.90, 29.68, 29.66, 29.60 29.50, 29.40, 29.27, 29.25, 28.60, 22.70, 14.10.

3.6.3. Dicyclohexyl Disulﬁde

1 Orange oil; H-NMR (500 MHz, CDCl3): δ = 2.67–2.68 (m, 2H), 2.03–2.05 (m, 4H), 1.77–1.79 (m, 13 4H), 1.56–1.62 (m, 2H), 1.25–1.31 (m, 10H); C NMR (125 MHz, CDCl3): δ = 50.00, 32.90, 26.10, 25.70.

3.6.4. Di-Tert-Butyl Disulﬁde

1 13 Pink oil; H-NMR (500 MHz, CDCl3): δ = 1.31 (s, 18H); C NMR (125 MHz, CDCl3): δ = 46.20, 30.60.

3.6.5. 2-Hydroxyethyl Disulﬁde

1 13 Pink oil; H-NMR (500 MHz, CDCl3): δ = 3.92 (t, J = 5.8, 4H), 2.89 (t, J = 5.8, 4H); C NMR (125 MHz, CDCl3): δ = 60.40, 41.20.

3.6.6. Dimethyl 3,30-Dithiodipropionate 1 13 Pink oil; H-NMR (500 MHz, CDCl3): δ = 3.71 (s, 6H), 2.93 (t, J = 7.0, 4H), 2.76 (t, J = 7.3, 4H); C NMR (125 MHz, CDCl3): δ = 172.2, 51.90, 33.90, 33.10 [21].

3.6.7. Dibenzyl Disulﬁde

1 Colorless solids, mp 65–70 ◦C (mp 70–71 ◦C[14]); H-NMR (500 MHz, CDCl3): δ = 7.29–7.33 (m, 13 4H), 7.26–7.28 (m, 4H), 7.22–7.24 (m, 2H), 3.59 (s, 1H); C NMR (125 MHz, CDCl3): δ = 137.40, 129.47, 128.53, 127.48, 43.300. Catalysts 2020, 10, 398 7 of 8

3.6.8. 1,2-Bis(4-Nitrophenyl)Disulfane

1 Yellow solids, mp 168-173 ◦C (mp 178-180 ◦C[22]); H-NMR (500 MHz, CDCl3): δ = 8.20 (d, 13 J = 9 Hz, 4H), 7.61 (d, J = 9 Hz, 4H); C NMR (125 MHz, CDCl3): δ = 147.30, 144.38, 126.70, 124.79.

3.6.9. 1,2-Bis(4-Chlorophenyl)Disulfane

1 Colorless solids, mp 63–65 ◦C (mp 65–66 ◦C[14]); H-NMR (500 MHz, CDCl3): δ = 7.39 (d, 13 J = 8.5 Hz, 4H), 7.27 (d, J = 8.5 Hz, 4H); C NMR (125 MHz, CDCl3): δ = 135.44, 133.94, 131.08.

3.6.10. 4,40-Dipyridyl Disulﬁde 1 Colorless solids, mp 59–64 ◦C (mp 72–74 ◦C[23]); H-NMR (500 MHz, CDCl3): δ = 8.56–8.50 (m, 13 4H), 7.46–7.24 (m, 4H); C NMR (125 MHz, CDCl3): δ = 150.4, 149.9, 121.8.

4. Conclusions We demonstrated the synthesis of the IL-supported organotelluride reagent and its application as a recyclable catalyst for thiol oxidation. A novel diphenyl telluride catalyst 5 carrying an imidazolium hexafluorophosphate moiety as an IL support was successfully synthesized. The synthesis was very easy, requiring only four steps. The IL-supported diphenyl telluride 5 was found to be an efficient catalyst for aerobic oxidation of various thiols under photosensitized conditions, affording the corresponding disulfides in excellent yields. Although the oxidation proceeds smoothly in various IL solutions, [bmim]PF6 is best suited for efficient phase separation. The produced disulfides can be isolated and purified via simple extraction with diethyl ether. After removal of the product, the resulting IL solution containing IL-supported catalyst (5) and rose bengal can be reused at least four times without a considerable loss in catalytic activity (97% yield in the fifth run).

Author Contributions: Conceptualization, S.K., and M.O.; synthetic methodology, A.M., S.K., and M.O.; investigation, M.M., Y.S., and A.M.; writing—original draft preparation, A.M.; writing—review and editing, A.M. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: We would like to thank Yoshiki Oda and Yoshimi Kanie for helpful inputs related to this work. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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