catalysts

Article The Selective Oxidation of Sulfides to Sulfoxides or Sulfones with Hydrogen Peroxide Catalyzed by a Dendritic Phosphomolybdate Hybrid

Qiao-Lin Tong †, Zhan-Fang Fan †, Jian-Wen Yang, Qi Li, Yi-Xuan Chen, Mao-Sheng Cheng and Yang Liu * Key Laboratory of Structure-Based Drug Design & Discovery of Ministry of Education, School of Pharmaceutical Engineering, Shenyang Pharmaceutical University, Shenyang 110016, China; [email protected] (Q.-L.T.); [email protected] (Z.-F.F.); [email protected] (J.-W.Y.); [email protected] (Q.L.); [email protected] (Y.-X.C.); [email protected] (M.-S.C.) * Correspondence: [email protected] Qiao-Lin Tong and Zhan-Fang Fan contributed equally to this work. †


Received: 6 September 2019; Accepted: 20 September 2019; Published: 22 September 2019


Abstract: The oxidation of sulfides to their corresponding sulfoxides or sulfones has been achieved using a low-cost poly(amidoamine) with a first-generation coupled phosphomolybdate hybrid as the catalyst and aqueous hydrogen peroxide as the oxidant. The reusability of the catalyst was revealed in extensive experiments. The practice of this method in the preparation of a smart drug Modafinil has proved its good applicability.

Keywords: dendritic phosphomolybdate hybrid; sulfides; selective oxidation; hydrogen peroxide; reusability

1. Introduction Many biologically and chemically active molecules are constructed from sulfoxides and sulfones [1–5]. The oxidation of sulfides is a fundamental reaction as one of the most straightforward methods to afford sulfoxides and sulfones [6]. Many reagents, including peracids and halogen derivatives, are used in the common approaches of sulfoxidation reactions [7,8]. However, these common reactions often suffer from the formation of many environmentally unfriendly byproducts and low oxygen atom efficiency [6,9]. The development of environmentally benign catalysts and oxidants has become a research hotspot in recent green chemistry pursuits. Among various oxidants, aqueous hydrogen peroxide, which is inexpensive, is readily available, has a high effective oxygen content, is safe in operation and storage, and yields stoichiometric amounts of environmentally friendly water as a byproduct, is generally considered one of the most environmentally benign “green oxidants”. These characteristics of aqueous hydrogen peroxide have led to the development of many valuable methods for the oxidation of sulfides to sulfoxides and sulfones with various transition metal catalysts, including Ti(SO4)2/GOF [10], PDDA-SiV2W10 [11], the cobalt(III)–salen ion [12], TiO /AA/MoO [13], a copper–Schiff base complex [14], (C H N) [MoO(O ) (C O )] H O[6], 2 2 19 42 2 2 2 2 4 · 2 vanadium Schiff base complexes [15], and so on [16–19]. Polyoxometalates (POMs), which are composed of anionic transition metal–oxygen clusters [20–23], have been widely studied in the field of catalysis due to their excellent catalytic characteristics, including efficient catalytic activity and selectivity, controllable redox potential and acidity through the choice of the countercations and constituent elements [24,25]. Among these POMs, Keggin-type phosphomolybdic acid (H3Mo12O40, HPMo) has shown excellent catalytic performance in oxidation reactions with aqueous hydrogen peroxide [26–29].

Catalysts 2019, 9, 791; doi:10.3390/catal9100791 www.mdpi.com/journal/catalysts Catalysts 2019, 9, x FOR PEER REVIEW 2 of 10 Catalysts 2019, 9, 791 2 of 10 Dendritic poly(amidoamine) (PAMAM), which contains many amino groups, is an excellent organicDendritic modifier poly(amidoamine) for POMs [30–32]. (PAMAM), PAMAMs can which be functionalized contains many to amino couple groups, to various is an POMs, excellent and theorganic dendritic modifier structure, for POMs which [30 –is32 different]. PAMAMs from can the be common functionalized organic to modifier, couple to can various provide POMs, a local and microenvironmentthe dendritic structure, for the which POMs, is diwhichfferent may from result the commonin a help organicful “dendritic modifier, effect” can on provide the reaction a local [microenvironment33,34]. PAMAM-G for1 (poly(amidoamine) the POMs, which may with result the in first a helpful generation) “dendritic dendrimers effect” on have the reaction been used [33,34 in]. industrialPAMAM-G1 production (poly(amidoamine) and are relatively with the inexpensive. first generation) Moreover, dendrimers the amino have groups been used of the in PAMAM industrial- G1production dendrimer and can are be relatively protonated inexpensive. by HPMo, Moreover, and the PMo the amino anions groups and PAMAM of the PAMAM-G1-G1 can strongly dendrimer bond togethercan be protonated through electrostatic by HPMo, and interactions the PMo anions [35]. and PAMAM-G1 can strongly bond together through electrostaticHerein, interactions we report a [35 heterogeneous,]. simple, recyclable, efficient, and green protocol for the oxidationHerein, of sulfides we report to sulfoxides a heterogeneous, and sulfones simple, with recyclable, 30 wt% H2 eOffi2 undercient, andmild greenconditions protocol in excellent for the yield, catalyzed by a composite based on PAMAM-G1 and HPMo (denoted as PAMAM-G1-PMo). To oxidation of sulfides to sulfoxides and sulfones with 30 wt% H2O2 under mild conditions in excellent theyield, best catalyzed of our knowledge, by a composite this basedis the first on PAMAM-G1 report of PAMAM and HPMo-G1-PMo (denoted as the as catalyst PAMAM-G1-PMo). used in the Tooxidation the best of of sulfides our knowledge, to sulfoxides this and is the sulfones. first report of PAMAM-G1-PMo as the catalyst used in the oxidation of sulfides to sulfoxides and sulfones. 2. Results and Discussion 2. Results and Discussion In this oxidation system, PAMAM-G1-PMo is utilized as the catalyst, and 30 wt% H2O2 is used as theIn oxidant. this oxidation We selected system, thioanisole PAMAM-G1-PMo (1a) as a ismodel utilized substrate as the catalyst,and found and that 30 less wt% than H2O 5%2 is of used 1a couldas the be oxidant. directly We oxidized selected by thioanisole 30 wt% H2O (1a)2 alone as a, even model when substrate the reaction and found time lasted that less to 6 than hours 5% (Table of 1a 1could, entry be 1) directly. However, oxidized when by 50 30 mg wt% of HPAMAM2O2 alone,-G1PMo even whenwas added the reaction to the timeabove lasted mixture, to 6 hsulfoxide (Table1, (entry1b) was 1). generated However, whensoon as 50 the mg oxidation of PAMAM-G1PMo product. In wasaddition, added control to the experiments above mixture, were sulfoxide performed (1b) wasto assess generated the individually soon as the catalytic oxidation activity product. of HPMo In addition, and PAMAM control-G1 experiments dendrimer. were Regrettably, performed only to 8%assess of starting the individually material could catalytic be activityoxidized of by HPMo 30 wt% and H PAMAM-G12O2 catalyzing dendrimer. by HPMo alone Regrettably, (Table only1, entry 8% 2),of startingand much material less yield could was be oxidizedobtained bywhen 30 wt% catalyzing H2O2 catalyzing by PAMAM by- HPMoG1 dendrimer alone (Table alone1, ( entryTable 2), 1, entryand much 3). When less yield the reaction was obtained was carried when catalyzing out in the by presence PAMAM-G1 of PAMAM dendrimer-G1- alonePMo, (Tableup to 185%, entry of 3).1a Whencould be the oxidized reaction within was carried 4 hours out (Table in the 1, presenceentry 4), indicating of PAMAM-G1-PMo, that PAMAM up-G1 to- 85%PMo ofpla 1ayed could a very be significantoxidized within role in 4 prompting h (Table1, entry the process 4), indicating of this thatoxidation PAMAM-G1-PMo effectively. played a very significant role in prompting the process of this oxidation effectively. Table 1. Control experiments with different components as catalyst a. Table 1. Control experiments with different components as catalyst a.

b Entry Catalyst Time (h) Temperature (◦C) Yield (%) Entry Catalyst Time (h) Temperature (°C ) Yield b (%) 1 — 6.0 25 4 1 — 6.0 25 4 2 HPMo 8.0 25 8 32 PAMAM-G1HPMo 8.0 8.0 25 258 0.5 43 PAMAM-G1-PMoPAMAM-G1 8.0 4.0 25 250.5 85 4 PAMAM-G1-PMo 4.0 25 85 a a Reaction conditions: 1a (62 mg, 0.5 mmol), solvent (8 mL), catalyst (50 mg), 30 wt% H2O2 (63 mg, 0.55 mmol); bReactionIsolated yield.conditions: HPMo: 1a Keggin-type (62 mg, 0.5 phosphomolybdicmmol), solvent (8 acid. mL), PAMAM-G1:catalyst (50 mg), poly(amidoamine) 30 wt% H2O2 with(63 mg, the 0.55 first mmol);generation. b Isolated PAMAM-G1-PMo: yield. HPM compositeo: Keggin based-type on phosphomolybdic PAMAM-G1 and HPMo. acid. PAMAM-G1: poly(amidoamine)

with the first generation. PAMAM-G1-PMo: composite based on PAMAM-G1 and HPMo. TToo optimize the conditions for the oxidation of sulfides sulfides to sulfoxides,sulfoxides, we firstfirst investigated the effectseffects of the solvent in thethe PAMAM-G1-PMo-catalyzedPAMAM-G1-PMo-catalyzed reactionreaction system.system. The The results results are listed in Table1 1.. After screening different di fferent solvents, solvents, we we observed observed that that when when MeOH MeOH or or95% 95% EtOH EtOH was was used used as theas the solvent solvent in the in the presence presence of PAMAM of PAMAM-G1-PMo,-G1-PMo, we we obtained obtained the the desired desired sulfoxide sulfoxide products products (1b (1b)) in relativelyin relatively high high yields yields of 85 of– 85–90%90% and and a minor a minor amount amount of starting of starting material material (1a) (1a)remained remained,, without without any sulfonesany sulfones (Table (Table 2, entries2, entries 1–2). 1–2). However, However, the conversion the conversion was wasrelatively relatively poor poor when when the reaction the reaction was was carried out in other solvents, including EtOH, CH COCH , CH COOC H and i-PrOH (Table2, carried out in other solvents, including EtOH, CH3COCH3 3, 3 CH3COOC3 2H25, 5,and i-PrOH (Table 2, entries 3–6).3–6). After After comprehensive comprehensive consid considerationserations of of various various factors, we selected environmentally friendly and low low-cost-cost 95% EtOH as the the solvent. solvent. Next, Next, t thehe temperature temperature of of the the reaction reaction was was optimized. optimized.

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As shown in Table 2, when the temperature increased from 25 °C to 30 °C, the yield of the desired Asproduct shown increased in Table to2, more when than the temperature90% and reaction increased time reduced from 25 to◦C 2 tohours, 30 ◦ C,but the when yield the of temperature the desired productincreased increased to 35 °C , tothe more yield than of the 90% sulfoxide and reaction appeared time to reduced decrease to due 2 h, to but the when increase the in temperature byproduct increasedsulfone formation to 35 ◦C, (Ta theble yield2, entries of the 2, sulfoxide 7–9). Hence, appeared 30 °C was to decrease the optimal due temperature to the increase for inthe byproduct oxidation sulfoneof sulfides formation to sulfoxides (Table 2in, entriesthe system 2, 7–9). of 30 Hence, wt% 30H2O◦C2/PAMAM was the optimal-G1-PMo. temperature for the oxidation of sulfides to sulfoxides in the system of 30 wt% H2O2/PAMAM-G1-PMo. Table 2. Oxidation of sulfide to sulfoxide in different conditions a. Table 2. Oxidation of sulfide to sulfoxide in different conditions a.

b Entry Solvent Time (h) Temperature (◦C) Yield (%) Entry Solvent Time (h) Temperature (°C ) Yield b (%) 1 MeOH 4.0 25 90 1 MeOH 4.0 25 90 2 95%EtOH 4.0 25 85 2 395%EtOH EtOH4.0 4.0 2525 60 85 3 4EtOH CH3COCH3 4.0 4.0 2525 45 60 4 5CH3COCH CH33COOC 2H5 4.0 4.0 2525 40 45 6 i-PrOH 4.0 25 52 5 CH3COOC2H5 4.0 25 40 7 95%EtOH 2.0 25 72 6 i-PrOH 4.0 25 52 8 95%EtOH 2.0 30 91 7 995%EtOH 95%EtOH 2.0 2.0 3525 88 72 8 95%EtOH 2.0 30 91 a 9Reaction conditions:95%EtOH 1a (62 mg, 0.5 mmol), solvent2.0 (8 mL), catalyst (50 mg),35 30 wt% H2O2 (63 mg, 0.5588 mmol); b Isolated yield. a Reaction conditions: 1a (62 mg, 0.5 mmol), solvent (8 mL), catalyst (50 mg), 30 wt% H2O2 (63 mg, 0.55 mmol); b Isolated yield. Furthermore, 1a was still chosen to be the model substrate to optimize the conditions for the oxidationFurthermore, of sulfides 1a to was sulfones. still chosen In our experience, to be the model the formation substrate of sulfonesto optimize needs the higher conditions concentration for the ofoxidation the oxidant. of sulfides The contrast to sulfones. experiments In our demonstrated experience, that the30 formation wt% H2O 2 ofitself, sulfones even ifneeds the amount higher wasconcentration increased of to the 300 oxidant. mol%, couldThe contrast not complete experiment thes oxidation. demonstrated However, that 30 once wt% more,H2O2 itself, this process even if wasthe amount greatly promotedwas increased by the to 300 catalyst mol% PAMAM-G1-PMo., could not complete The the temperature oxidation. However, of the reaction once wasmore, firstly this optimized.process was As greatly shown promoted in Table3 by, when the catalyst the temperature PAMAM increased-G1-PMo. fromThe temperature 30 ◦C to 40 ◦ofC the in the reaction system was of 30first wt%ly optimized. H2O2/PAMAM-G1-PMo, As shown in Table the yield3, when of thethe desiredtemperature sulfone increased product from (1c) 30 increased, °C to 40but °C whenin the thesystem temperature of 30 wt% increased H2O2/PAMAM to 50 ◦C,-G1 the-PMo, yield the of sulfoneyield of did the notdesired notably sulfone change product (Table 3(1c, entries) increased, 1–3). Hence,but when 40 ◦theC was temperature the optimal increased temperature to 50 for°C, the the oxidation yield of sulfone of sulfides did to not sulfones notably in change the system (Table of 303, wt%entries H2 1O–23)./PAMAM-G1-PMo. Hence, 40 °C was Inthe addition, optimal thetemperature effects of 30for wt% the Hoxidation2O2 in the of reactionsulfides wereto sulfones investigated. in the Whensystem the ofamount 30 wt% of H 302O wt%2/PAMAM H2O2 increased-G1-PMo. fromIn addition 300 mol%, the to 400effects mol%, of 30 there wt% was H no2O2 major in the di reactionfference inwere the investigated. yield of the desiredWhen the product amount (Table of 303 ,wt% entries H2O 2,4).2 increased However, from if the300 amountmol% to of 400 30 mol%, wt% H there2O2 decreasedwas no major to 200 difference mol%, the in yield the ofyield the desiredof the desired product product decreased (Table accordingly 3, entries (Table 2,4).3, However, entry 5). Hence, if the 300amount mol% of was 30 an wt% optimal H2O2 quantity decreased of 30 to wt% 200 H mol%,2O2 for the this yield catalytic of the oxidation desired reaction. product decreased accordinglyNext, we (Table studied3, entry thescope 5). Hence, of application 300 mol% of was this an system optimal by testingquantity a diverseof 30 wt% range H2 ofO2 sulfides,for this andcatalytic the resultsoxidation are reaction. presented in Table4. Notably, we observed that most of the substrates tested in the oxidation system were converted to their corresponding products in excellent yields under a the optimized reactionTable conditions. 3. Oxidation However, of sulfide we to found sulfone that in different (2-chloro-4-nitrophenyl)(phenyl)sulfane conditions . (5a), with a strong electron-deficient phenyl group, did not successfully convert to 2-chloro-4-nitro-1- (phenylsulfinyl)benzene (5b) (Table4, entry 5) and 2-chloro-4-nitro-1-(phenylsulfonyl)benzene (5c) (Table4, entry 12). Additionally, (2-chlorophenyl)(methyl)sulfane (6a) did not successfully convert to 1-chloro-2-(methylsulfonyl)benzene (6c) (Table4, entry 13), partially because the ortho-substituted chlorine on the benzene ring is also an electron-withdrawing group and has a certain steric hindrance.

Entry 30 wt% H2O2 (mol%) Time (h) Temperature (°C ) Yield b (%) 1 300 3.0 30 60 2 300 3.0 40 93

Catalysts 2019, 9, x FOR PEER REVIEW 3 of 10

As shown in Table 2, when the temperature increased from 25 °C to 30 °C, the yield of the desired product increased to more than 90% and reaction time reduced to 2 hours, but when the temperature increased to 35 °C , the yield of the sulfoxide appeared to decrease due to the increase in byproduct sulfone formation (Table2, entries 2, 7–9). Hence, 30 °C was the optimal temperature for the oxidation of sulfides to sulfoxides in the system of 30 wt% H2O2/PAMAM-G1-PMo.

Table 2. Oxidation of sulfide to sulfoxide in different conditions a.

Entry Solvent Time (h) Temperature (°C ) Yield b (%) 1 MeOH 4.0 25 90 2 95%EtOH 4.0 25 85 3 EtOH 4.0 25 60 4 CH3COCH3 4.0 25 45 5 CH3COOC2H5 4.0 25 40 6 i-PrOH 4.0 25 52 7 95%EtOH 2.0 25 72 8 95%EtOH 2.0 30 91 9 95%EtOH 2.0 35 88

a Reaction conditions: 1a (62 mg, 0.5 mmol), solvent (8 mL), catalyst (50 mg), 30 wt% H2O2 (63 mg, 0.55 mmol); b Isolated yield. Furthermore, 1a was still chosen to be the model substrate to optimize the conditions for the oxidation of sulfides to sulfones. In our experience, the formation of sulfones needs higher concentration of the oxidant. The contrast experiments demonstrated that 30 wt% H2O2 itself, even if the amount was increased to 300 mol%, could not complete the oxidation. However, once more, this process was greatly promoted by the catalyst PAMAM-G1-PMo. The temperature of the reaction was firstly optimized. As shown in Table 3, when the temperature increased from 30 °C to 40 °C in the system of 30 wt% H2O2/PAMAM-G1-PMo, the yield of the desired sulfone product (1c) increased, but when the temperature increased to 50 °C, the yield of sulfone did not notably change (Table 3, entries 1–3). Hence, 40 °C was the optimal temperature for the oxidation of sulfides to sulfones in the system of 30 wt% H2O2/PAMAM-G1-PMo. In addition, the effects of 30 wt% H2O2 in the reaction were investigated. When the amount of 30 wt% H2O2 increased from 300 mol% to 400 mol%, there was no major difference in the yield of the desired product (Table 3, entries 2,4). However, if the amount of 30 wt% H2O2 decreased to 200 mol%, the yield of the desired product decreased accordingly (Table3, entry 5). Hence, 300 mol% was an optimal quantity of 30 wt% H2O2 for this CatalystsCatalysts 20192019,, 99,, xx FORFOR PEERPEER REVIEWREVIEW 44 ofof 1010 Catalysts 20192019,, 99,, xx FORFOR PEERPEER REVIEWREVIEW 44 of 1010 catalyticCatalystsCatalysts 20192019 oxidation,, 99,, xx FORFOR reaction. PEERPEER REVIEWREVIEW 44 ofof 1010 CatalystsCatalystsCatalysts2019 20192019,,,,9 99,,,, 791 xxx FORFORFOR PEERPEERPEER REVIEWREVIEWREVIEW 444of ofof 10 1010 CatalystsCatalysts 20192019,, 99,, xx FORFOR PEERPEER REVIEWREVIEW 44 ofof 1010 CatalystsCatalysts33 20192019,,, 99,,, xxx FORFORFOR PEERPEERPEER REVIEWREVIEWREVIEW300300 3.03.0 5050 9292 44 ofof 1010 3 300 3.0 50 a 92 33 Table300300 3. Oxidation of sulfide to3.03.0 sulfone in different conditions5050 . 9292 3443 300400400300 3.03.0 50404050 92939392 4343 Table400300400300 3. Oxidation of sulfide to3.03.0 sulfone in different conditions40504050 a. 93929392 4545 400200400200 3.03.0 4040 93819381 454 400200400 3.03.0 4040 938193 545 aa 200400200 3.03.0 4040 bb 819381 55 aa ReactionReaction conditions:conditions:200200 1a1a (62(62 mg,mg, 0.50.5 mmol),mmol), 95%95%3.03.0 EtOH EtOH (10(10 mL),mL), catalystcatalyst4040 (150(150 mg);mg); bb IsolatedIsolated yield.yield.8181 55 aa Reaction conditions:200200 1a (62(62 mg,mg, 0.50.5 mmol),mmol), 95%95%3.03.0 EtOH EtOH (10(10 mL),mL), catalystcatalyst4040 (150(150 mg);mg); bb IsolatedIsolated yield.yield.8181 5 aa ReactionReaction conditions:conditions:200 1a1a (62(62 mg,mg, 0.50.5 mmol),mmol), 95%95%3.0 EtOH EtOH (10(10 mL),mL), catalystcatalyst40 (150(150 mg);mg); bb IsolatedIsolated yield.yield.81 aa Reaction Reaction conditions:conditions: 1a1a (62(62 mg,mg, 0.50.5 mmol),mmol), 95%95% EtOHEtOH (10(10 mL),mL), catalystcatalyst (150(150 mg);mg); bb Isolated Isolated yield.yield. aa ReactionReaction conditions:conditions: 1a1a (62(62 mg,mg, 0.50.5 mmol),mmol), 95%95% EtOHEtOH (10(10 mL),mL), catalystcatalyst (150(150 mg);mg); bb IsolatedIsolated yield.yield. Next,Next,aa ReactionReaction wewe studiedstudied conditions:conditions: thethe 1a 1ascopescope (62(62 mg,mg, ofof application0.5application0.5 mmol),mmol), 95%95% ofof EtOHthEtOHthisis systemsystem (10(10 mL),mL), byby catalystcatalyst testingtesting (150(150 aa diversediverse mg);mg); bb IsolatedIsolated rangerange yield.yield.ofof sulfides,sulfides, Next,Next, wewe studiedstudied thethe scopescope ofof applicationapplication ofof ththisis systemsystem byby testingtesting aa diversediverse rangerange ofof sulfides,sulfides, andand thetheNext,Next, resultsresults wewe studiedstudied areare presentedpresented thethe scopescope inin TableofTableof applicationapplication 44.. Notably,Notably, ofof ththweweisisis observedobservedsystemsystem byby thatthat testingtesting mostmost aa of ofdiversediverse thethe substratessubstrates rangerange ofof testedtestedsulfides,sulfides, inin andand thetheNext,Next, resultsresults wewe studiedstudied areare presentedpresented thethe scopescope inin TableTableofof applicationapplication 44.. Notably,Notably, ofof ththweweisis observedobservedsystemsystem byby thatthat testingtesting mostmost aa ofofdiversediverse thethe substratessubstrates rangerange ofof testedtestedsulfides,sulfides, inin andtheandthe oxidation oxidationthethe resultsresults systemsystem areare presentedpresented werewere convertedconverted inin TableTable to to44.. . the theNotably,Notably,Notably,irir correspondingcorresponding wewewe observedobservedobserved productsproducts thatthatthat mostmostmost inin excellentofexcellentofof thethethe substratessubstratessubstrates yieldsyields underunder testedtestedtested thethe ininin andthetheand oxidation oxidationthethe resultsresults systemsystem areare presentedpresented werewere convertedconverted inin TableTable to to44. . thetheNotably,Notably,irir correspondingcorresponding wewe observedobserved productsproducts thatthat mostmost inin excellentexcellentofof thethe substratessubstrates yieldsyields underunder testedtested thethe inin theandthe oxidationoxidationthe resultsEntry systemsystem are presented were 30were wt% convertedconverted H inO Table(mol%) toto4. . thetheNotably,Notably,irir correspondingcorresponding Time wewe (h) observedobserved Temperature productsproducts thatthat mostmost inin ( C) excellentexcellentofof thetheYield substratessubstrates yieldsyieldsb (%) underunder testedtested thethe inin theoptimizedtheoptimized oxidationoxidation reactionreaction systemsystem conditions.conditions. werewere convertedconverted However,2However,2 toto thewetheweir irfoundfound correspondingcorresponding thatthat (2(2--chlorochloro productsproducts--44--nitrophenyl)(phenyl)sulfanenitrophenyl)(phenyl)sulfane inin◦ excellentexcellent yieldsyields underunder (( 5a5athethe)),, optimizedthetheoptimizedE ntry oxidationoxidation reactionreaction systemsystem30 wt% conditions.conditions. werewere H2O converted2converted (mol%) However,However, toto thewetheweTime irir foundfound correspondingcorresponding (h) thatthat (2(2--chlorochloroTemperature productsproducts--44--nitrophenyl)(phenyl)sulfanenitrophenyl)(phenyl)sulfane inin excellent(excellent°C ) yieldsyieldsYield underunder b (%) (( 5a5athethe)),, optimizedwithwithoptimized a a strong strong reactionreaction 1 electron electron conditions.conditions.--deficientdeficient 300 However,However, phenyl phenyl we group,we group, foundfound did did 3.0 thatthat not not (2(2 - successfully- successfullychlorochloro--44-- 30nitrophenyl)(phenyl)sulfanenitrophenyl)(phenyl)sulfane convert convert to to 22-- 60chlorochloro--44--nitronitro ((5a5a--11))-,-,, withoptimizedwithoptimized1 a a strong strong reactionreaction electron electron conditions.conditions.--deficient300deficient However,However, phenyl phenyl we group,we group, foundfound3.0 did did thatthat not not (2(2 - successfully- successfullychlorochloro--44--nitrophenyl)(phenyl)sulfanenitrophenyl)(phenyl)sulfane30 convert convert to to 22--chlorochloro60--44-- nitronitro ((5a5a--11))-,-, with(phenylsulfinyl)benzenewith(phenylsulfinyl)benzene a a strong strong 2 electron electron--deficientdeficient ((5b5b)) (Table(Table 300 phenyl phenyl 44,, entryentry group, group, 5)5) and and did did 3.0 2 2 not not--chlorochloro successfully successfully--44--nitronitro 40--11 -- convert convert(phenylsulfonyl)benzene(phenylsulfonyl)benzene to to 22-- 93chlorochloro--44--nitronitro ((5c-5c-11-)-) with(phenylsulfinyl)benzene(phenylsulfinyl)benzenewith a a strong strong electron electron--deficientdeficient ((5b)) (Table(Table phenyl phenyl 4,, entryentry group, group, 5)5) and and did did 2 2 not not--chlorochloro successfully successfully--4--nitro--1 -- convert convert(phenylsulfonyl)benzene(phenylsulfonyl)benzene to to 22--chlorochloro--44--nitronitro ((5c--11)-)- (phenylsulfinyl)benzenewith(phenylsulfinyl)benzene2 a strong 3 electron--deficient300 ((5b5b )) (Table(Table 300 phenyl 44,, entryentry group, 5)5)3.0 and and did 3.0 22 not--chlorochloro successfully--44--nitronitro 5040--11 -- convert (phenylsulfonyl)benzene(phenylsulfonyl)benzene to 2-- 92chloro93--4-- nitro ((5c5c--1)-)- (phenylsulfinyl)benzene(Table(Table(phenylsulfinyl)benzene 44,, entryentry 12).12). Additionally,Additionally, ((5b5b)) (Table(Table (2(2- - chlorophenyl)(methyl)sulfane4chlorophenyl)(methyl)sulfane4,, entryentry 5)5) andand 22--chlorochloro--44--nitronitro ((6a-6a-11-)-)(phenylsulfonyl)benzene (phenylsulfonyl)benzene diddid notnot successfullysuccessfully convertconvert ((5c5c)) (Table(phenylsulfinyl)benzene(phenylsulfinyl)benzene(Table 44,, entryentry4 12).12). Additionally,Additionally, ((5b5b)) (Table(Table 400 (2(2-- chlorophenyl)(methyl)sulfane4chlorophenyl)(methyl)sulfane4,, entryentry 5)5) andand 3.0 22--chlorochloro--44--nitronitro 40((6a-6a-11-)-)(phenylsulfonyl)benzene (phenylsulfonyl)benzene diddid notnot successfullysuccessfully 93 convertconvert ((5c5c)) (Tabletoto(Table 11--chlorochloro 44,,, entryentryentry--22--(methylsulfonyl)benze(methylsulfonyl)benze 12).12).12). Additionally,Additionally,Additionally, (2(2(2-ne-nechlorophenyl)(methyl)sulfanechlorophenyl)(methyl)sulfane ((6c6c)) (Table(Table 44,, entryentry 13),13), partiallypartially ((6a6a)) becausebecausediddid notnot thesuccessfullythesuccessfully orthoortho--substitutedsubstituted convertconvert to(Table(Tableto 11--chlorochloro 44,, entryentry--22--(methylsulfonyl)benze(methylsulfonyl)benze5 12).12). Additionally,Additionally, 200 (2(2--nenechlorophenyl)(methyl)sulfanechlorophenyl)(methyl)sulfane ((6c6c)) (Table(Table 3.044,, entryentry 13),13), partiallypartially 40((6a6a)) becausebecausediddid notnot thesuccessfullythesuccessfully ortho 81ortho--substitutedsubstituted convertconvert tochlorinetochlorine 11--chlorochloro onon-- 2the2the--(methylsulfonyl)benze(methylsulfonyl)benze benzenebenzene ringring isis alsoalso anannene electronelectron ((6c6c)) (Table(Table--withdrawingwithdrawing 44,,, entryentryentry 13),13),13), groupgroup partiallypartially andand has hasbecausebecause aa certaincertain thethe ortho orthostericsteric--substituted substituted hindrance.hindrance. tochlorinetochlorine 11--chlorochloro on-- 2the2--(methylsulfonyl)benze(methylsulfonyl)benze benzene ring is also annene electron ((6c6c)) (Table(Table--withdrawing 44,, entryentry 13),13), group partiallypartially and hasbecausebecause a certaincertain thethe ortho orthostericsteric--substituted substituted hindrance.hindrance. chlorinechlorine onona Reaction thethe benzenebenzene conditions: ringring 1a isis (62alsoalso mg, anan 0.5 electronelectron mmol), 95%--withdrawingwithdrawing EtOH (10 mL), groupgroup catalyst andand (150 hashas mg); aab certaincertainIsolated stericsteric yield. hindrance.hindrance. chlorinechlorine onon thethe benzenebenzene ringring isis alsoalso anan electronelectron--withdrawingwithdrawing groupgroup andand hashas aa certaincertain stericsteric hindrance.hindrance. chlorineTableTable on 4.4. the TheThe benzene ooxidationxidation ring ofof sulfidessulfides is also toanto sulfoxidessulfoxides electron-- andwithdrawingand sulfonessulfones withwith group 3030 wt%wt% and HH has22OO22 catalyzed catalyzeda certain bybysteric PAMAMPAMAM hindrance.–– Table 4. The oxidation of sulfides to sulfoxides and sulfones with 30 wt% H22O22 catalyzed by PAMAM– Table 4. The oxidation of sulfides to sulfoxides and sulfones with 30 wt% H22O22 catalyzed by PAMAM– TableTable 4.4. TheThea,ba,b ooxidationxidation ofof sulfidessulfides toto sulfoxidessulfoxides andand sulfonessulfones withwith 3030 wt%wt% HH22OO22 catalyzed catalyzed byby PAMAMPAMAM–– G1TableG1––PMoPMo 4. The a,ba,b.. oxidation of sulfides to sulfoxides and sulfones with 30 wt% H22O22 catalyzed by PAMAM– TableG1Table–PMo 4. 4. The The a,ba,b.. ooxidationxidation ofof sulfidessulfides toto sulfoxidessulfoxides andand sulfonessulfones withwith 3030 wt%wt% HH22OO22 catalyzed catalyzed byby PAMAMPAMAM–– TableTableG1Table–PMo 4. 4.4.The TheThe a,ba,b.. oxidation ooxidationxidation of ofof sulfides sulfidessulfides to toto sulfoxides sulfoxidessulfoxidesand andandsulfones sulfonessulfones with withwith 30 3030 wt% wt%wt% H HH222OOO222 catalyzed catalyzed by by PAMAM PAMAM–PAMAM–– TableG1G1––PMoPMo 4. The a,ba,b.. oxidation of sulfides to sulfoxides and sulfones with 30 wt% H22O22 catalyzedcatalyzed byby PAMAMPAMAM– G1G1––PMoPMo a,b a,ba,b... G1–PMoG1G1––PMoPMo a,ba,b.. cc G1G1––PMoPMo a,ba,b... E Entryntry SSubstratesubstrates PProductsroducts TTimeime (h)(h) YYieldield cc (%)(%) Entry Substrates Productsroducts Timeime (h)(h) Yieldield cc (%)(%) EEntryntry SSubstratesubstrates PProductsroducts TTimeime (h)(h) YYieldield cc (%)(%) EEntryntry SSubstratesubstrates PProductsroducts TTimeime (h)(h) YYieldield cc c (%)(%) EntryEEntryntry SS Substratesubstratesubstrates PP Productsroductsroducts TT Timeimeime (h) (h) YY Yieldieldield cc (%)(%)(%) EEntryntry11 SSubstratesubstrates PProductsroducts TTimeime2.02.0 (h) (h) YYieldield9191 (%)(%) 1 11aa 2.0 91 11 1a 1b1b 2.02.0 9191 11 11aa 1b 2.02.0 9191 111 11aa 1b1b 2.02.02.0 9191 91 1a11aa 1b1b 22 1b1b 1.51.5 9090 2 1.5 90 22 2a2a 2b 1.51.5 9090 22 2a2a 2b 1.51.5 9090 222 2a2a 2b2b 1.51.51.5 9090 90 2 2a2a2a 2b2b 1.5 90 2a2a 2b2b 33 2b 3.03.0 8585 3 3.0 85 33 3a3a 3b3b 3.03.0 8585 333 3a3a 3b 3.03.03.0 8585 85 33 3a3a 3b3b 3.03.0 8585 3a3a3a 3b3b 3a 3b3b 44 3.53.5 8585 44 4a4a 4b 3.53.5 8585 444 4a4a 4b 3.53.53.5 8585 85 44 4a4a4a 4b4b 3.53.5 8585 4a4a 4b4b 4a 4b4b 55 5.05.0 tracetrace 55 5a5a 5.05.0 tracetrace 555 5a 5b5b 5.05.0 tracetrace trace 55 5a5a5a 5b 5.05.0 tracetrace 5 5a5a 5b5b 5.0 trace 5a5a 5b5b 66 5b 6.06.0 9393 6 6.0 93 666 6a6a 6b6b 6.06.06.0 9393 93 66 6a6a6a 6b 6.06.0 9393 66 6a6a 6b6b 6.06.0 9393 6a6a 6b6b 6a 6b6b 77 4.54.5 9393 777 7a7a 7b 4.54.54.5 9393 93 77 7a7a7a 7b 4.54.5 9393 77 7a7a 7b7b 4.54.5 9393 7a7a 7b7b 88 7b7b 3.53.5 9393 88 1a1a 3.53.5 93 93 88 1a1a 1c1c 3.53.5 9393 88 1a1a 1c1c 3.53.5 9393 88 1a1a 1c1c 3.53.5 9393 1a1a 1c1c 99 1c 2.02.0 9494 9 9 2.02.0 94 94 99 2a2a2a 2c2c 2.02.0 9494 99 2a2a 2c2c 2.02.0 9494 99 2a2a 2c2c 2.02.0 9494 2a2a 2c2c 2a 2c2c 101010 4.04.04.0 9292 92 1010 3a3a 4.04.0 9292 1010 3a3a 3c3c 4.04.0 9292 1010 3a3a 3c 4.04.0 9292 10 3a3a 3c3c 4.0 92 3a3a 3c3c 111111 3c3c 4.04.04.0 9393 93 11 4a 4.0 93 1111 4a4a 4c4c 4.04.0 9393 1111 4a4a 4c 4.04.0 9393 1111 4a4a 4c4c 4.04.0 9393 4a4a 4c4c 4a 4c4c 1212 5.05.0 NANA dd 12 5.0 NA dd 1212 5a5a 5.05.0 NANA dd 1212 5a 5c5c 5.05.0 NANA dd 1212 5a5a 5c 5.05.0 NANA dd 1212 5a5a 5c5c 5.05.0 NANA dd 5a5a 5c5c 5a 5c5c

Catalysts 20192019,, 99,, xx FORFOR PEERPEER REVIEWREVIEW 44 of 1010

3 300 3.0 50 92 4 400 3.0 40 93 5 200 3.0 40 81 aa Reaction conditions: 1a (62(62 mg,mg, 0.50.5 mmol),mmol), 95%95% EtOHEtOH (10(10 mL),mL), catalystcatalyst (150(150 mg);mg); bb IsolatedIsolated yield.yield. Next, we studied the scope of application of ththisis systemsystem byby testingtesting aa diversediverse rangerange ofof sulfides,sulfides, and the results are presented in Table 4.. Notably,Notably, wewe observedobserved thatthat mostmost ofof thethe substratessubstrates testedtested inin thethe oxidationoxidation systemsystem werewere convertedconverted toto thetheirir correspondingcorresponding productsproducts inin excellentexcellent yieldsyields underunder thethe optimized reaction conditions. However, we found that (2--chloro--4--nitrophenyl)(phenyl)sulfane ((5a)),, with a strong electron--deficient phenyl group, did not successfully convert to 2--chloro--4--nitro--1-- (phenylsulfinyl)benzene(phenylsulfinyl)benzene ((5b)) (Table(Table 4,, entryentry 5)5) andand 22--chloro--4--nitro--1--(phenylsulfonyl)benzene(phenylsulfonyl)benzene ((5c)) (Table(Table 4,, entryentry 12).12). Additionally,Additionally, (2(2--chlorophenyl)(methyl)sulfane ((6a)) did not successfully convert toto 11--chloro--2--(methylsulfonyl)benze(methylsulfonyl)benzene ((6c)) (Table(Table 4,, entryentry 13),13), partially because thethe ortho--substitutedsubstituted chlorine on the benzene ring is also an electron--withdrawing group and has a certain stericsteric hindrance.hindrance.

Table 4. The oxidation of sulfides to sulfoxides and sulfones with 30 wt% H22O22 catalyzedcatalyzed byby PAMAMPAMAM– G1–PMo a,ba,b..

Entry Substrates Productsroducts Timeime (h)(h) Yieldield cc (%)(%)

1 2.0 91 1a 1b

2 1.5 90 2a 2b

3 3.0 85 3a 3b

4 3.5 85 4a 4b

5 5.0 tracetrace 5a 5b

6 6.0 93 6a 6b

7 4.5 93 7a 7b

8 3.5 93 1a 1c

9 2.0 94 2a 2c Catalysts 2019, 9, 791 5 of 10 10 4.0 92 3a 3c Table 4. Cont.

11 4.0 93 c Entry Substrates4a Products4c Time (h) Yield (%)

Catalysts 2019, 9, x FOR PEER REVIEW 5 of 10 Catalysts 2019,, 9,, xx FORFOR1212 PEER PEER REVIEWREVIEW 5.05.0 NANA ddd 5 of 10 5a 5a 5c 13 5.0 trace 1313 6a 6c 5.05.0 tracetrace trace 6a6a 6c 14 4.0 91 1414 7a 4.04.0 91 91 14 7a7a 7c 4.0 91 7a 7c7c a Reaction conditions of sulfides to sulfoxides: sulfide (0.5 mmol),7c 95% EtOH (8 mL), catalyst (50 mg), a aa Reaction conditions of sulfides to sulfoxides: sulfide (0.5 mmol), 95% EtOH (8 mL), catalyst (50 mg), 30 wt% H O Reaction conditions of sulfides to sulfoxides:b sulfide (0.5 mmol), 95% EtOH (8 mL), catalyst (50 mg),2 2 30(63 wt% mg, 0.55H2O mmol),2 (63 mg, 30 C;0.55b Reaction mmol), conditions 30 °C; Reaction of sulfides conditions to sulfones: of sulfide sulfides (0.5 mmol),to sulfones: 95% EtOH sulfide (10 mL),(0.5 2 2 ◦ bb 30 wt% H22O22 (63(63 mg,mg, 0.550.55 mmol),mmol), 3030 °C;°C; Reaction conditionsc of sulfidesd to sulfones: sulfidec (0.5 mmol), mmol),catalyst (15095% mg),EtOH 30 (10 wt% mL), H2O 2catalyst(170 mg, (150 1.5 mmol),mg), 30 40 wt%◦C; HIsolated2O2 (170 yield; mg, 1.5Not mmol), available. 40 °C; Isolated yield; c d 95% EtOH (10 mL), catalyst (150 mg), 30 wt% H22O22 (170(170 mg,mg, 1.51.5 mmol),mmol), 4040 °C;°C; cc IsolatedIsolated yield;yield; d Not d 95%Not available.EtOH (10 mL), catalyst (150 mg), 30 wt% H2O2 (170 mg, 1.5 mmol), 40 °C; Isolated yield; Not available. ToTo study the recyclability of the PAMAM-G1-PMo catalyst, we again chose the oxidation of of To study the recyclability of the PAMAM--G1--PMo catalyst, we again chose the oxidation of thioanisolethioanisoleTo study (1a) (1a) theto to (methylsulfinyl)benzene (methylsulfinyl)benzene recyclability of the PAMAM (1b) (1b) or or-G1 (methylsulfonyl)benzene (methylsulfonyl)benzene-PMo catalyst, we again (1c) (1c) chose as as model model the oxid reactions. reactions.ation of thioanisolethioanisole ((1a)) toto (methylsulfinyl)benzene(methylsulfinyl)benzene ((1b)) oror (methylsulfonyl)benzene(methylsulfonyl)benzene ((1c)) as model reactions. Thethioanisole catalyst catalyst PAMAM-G1-PMo(1a PAMAM-G1-PMo) to (methylsulfinyl)benzene was was easily easily recovered recovered(1b) or (methylsulfonyl)benzene by byfiltration filtration after after the thereaction, ( reaction,1c) as washed model washed withreactions. with95% The catalyst PAMAM--G1--PMo was easily recovered by filtration after the reaction, washed with 95% EtOH,95%The EtOH,catalyst and dried and PAMAM driedin a vacuum in-G1 a- vacuumPMo at roomwas at easily temperature. room recovered temperature. After by filtrationthe After above the after treatment, above the treatment, reaction, the catalyst washed the catalyst was with reused was95% EtOH,, and dried in a vacuum at room temperature. After thethe above treatment, tthe catalyst was reused forreusedEtOH subsequent, forand subsequent dried experiments in a vacuum experiments (up at room to (up five temperature. to runs), five runs), and After andno major nothe major above differences di treatment,fferences in in tthehe the catalystyield yield or orwas required required reused forfor subsequent subsequent experiments experiments ( (up to fivefive runs), runs), and and no no major major differences differences in in the the yield yield or or required required reactionreactionfor subsequent times times were were experiments observed, (as up summarized to five runs), in Figure and no1 1.. major differences in the yield or required reaction times were observed, as summarized in Figure 1.

Figure 1. Study of the PAMAM-G1-PMo recyclability for the oxidation of thioanisole to sulfoxide (left ) Figureand sulfone 1. Study (right of). the PAMAM-G1-PMo recyclability for the oxidation of thioanisole to sulfoxide Figure 1. Study of the PAMAM--G1--PMo recyclability for the oxidation of thioanisole to sulfoxide (left) (left)Figure and 1. sulfone Study of (right). the PAMAM -G1-PMo recyclability for the oxidation of thioanisole to sulfoxide (left) Afterand sulfone being (right). used for five times and separated from the reaction system, the catalysts were characterizedAfter being by used Fourier-transform for five times infrared and separated spectroscopy from (FT-IR,the reaction Figure system,2), X-ray the di ffcatalystsraction (XRD,were After being used for five times and separated from the reaction system, the catalysts were characterizedFigureAfter3), and being scanningby Fourier-transform used electron for five microscopy times infrared and (SEM, separated spectros Figurecopy from4). (FT-IR, As the shown reaction Figure in Figure 2), system, X-ray2, the thediffraction IR catalysts frequencies (XRD, were characterized by Fourier--transformtransform infraredinfrared spectroscopyspectroscopy1 (FT(FT--IR,IR, FigureFigure 22),), XX--ray diffraction (XRD, Figureofcharacterized reused 3), and PAMAM-G1-PMo scanning by Fourier electron-transform (958, microscopy 3373,infrared 3100 (SEM, spectroscopy cm Figure− ) are 4). consistent(FT As- shownIR, Figure within Figure 2), those X-ray 2, the of diffraction fresh-preparedIR frequencies (XRD, Figure 3)),, and scanning electron microscopy (SEM, Figure 4).. As shown in Figure 2, the IRIR frequenciesfrequencies ofPAMAM-G1-PMo,Figure reused 3), andPAMAM-G1-PMo scanning indicating electron (958, the microscopy PMo3373, anions3100 (SEM, cm are −Figure1) stillare successfully4)consistent. As shown with immobilizedin Figure those 2,of the fresh-prepared on IR protonatedfrequencies of reused PAMAM--G1--PMo (958, 3373, 3100 cm−1−1)) are are consistent with thosethose of of freshfresh--prepared PAMAM-G1-PMo,PAMAM-G1.of reused PAMAM As shown indicating-G1- inPMo Figure the (958,3 ,PMo the 3373, XRD anions 3100 patterns cmare )st for ill are bothsuccessfully consistent fresh-prepared withimmobilized those PAMAM-G1-PMo of onfresh protonated-prepared and PAMAM--G1--PMo, indicatingindicating thethe PMo PMo anions are still successfully immobilized on protonated PAMAM-G1.reusedPAMAM one-G1 show -AsPMo, shown a broadindicating in bandFigure the in 3, the the PMo range XRD anions patterns of 2 θ are= 14forstill 40both successfully◦, whichfresh-prepared confirms immobilized PAMAM-G1-PMo the high on dispersion protonated and of PAMAM--G1. As shown in Figure 3, thethe XRD patterns forfor− bothboth freshfresh--prepared PAMAM--G1--PMo and reusedthePAMAM PMo one anions-G1. show As on ashown broad PAMAM-G1 inband Figure in support.the 3, therange XRD Atof patterns2 last,θ = 14 it can− 40°,for be both which observed fresh con-preparedfi viarms the the SEM highPAMAM spectradispersion-G1 (Figure-PMo of theand4) reused one show a broad band in the range of 2θ = 14−40°, which confirms the high dispersion of the PMothatreused fresh-preparedanions one showon PAMAM-G1 a broad PAMAM-G1-PMo band support. in the rangeAt and last, reusedof it 2θ can = 14−40°, onebe observed have which similar via confirms the micromorphology, SEM the spectra high disper (Figure suchsion 4) as of that the the PMo anions on PAMAM--G1 support. At lastlast,, iitt can be observed via the SEM spectra (Figure 4) that fresh-preparedtinyPMo projections anions on PAMAM-G1-PMo PAMAM on the amorphous-G1 support. and surface. Atreused last, The oneit can resultshave be observedsimilar of all themicromorphology, via above the SEM detections spectra such prove(Figu as rethe that 4) tiny that the freshfresh--prepared PAMAM--G1--PMo and reused one have similar micromorphologyicromorphology,, suchsuch asas thethe tinytiny projectionsPAMAM-G1-PMo,fresh-prepared on the PAMAM amorphous no matter-G1 fresh-prepared- PMosurface. and The reused results or reused,one of haveall hasthe similar aabove higher detectionsmicromorphology surface-to-volume prove that, such ratio the PAMAM-as and the more tiny projections on the amorphous surface. The results of all the above detections prove that the PAMAM-- G1-PMo,reactiveprojections cavities. no onmatter the Besides, amorphous fresh-prepared there surface. are noor reused, noticeable The results has di a of ffhighererences all the surface-to-volume above between detections the fresh-prepared pratiorove and that more the catalysts PAMAM reactive and - G1--PMo, no matter fresh--prepared or reused, has a higher surface--toto--volume ratio and more reactive cavities.theG1- reusedPMo, Besides, no ones, matter demonstrating there fresh are-prepared no noticeable that theor reused, catalysts differenc has have aes higher goodbetween stability surface the- intofresh-prepared- thevolume process ratio of oxidationcatalystsand more and reaction. reactive the cavities. Besides, there are no noticeable differences between the fresh--prepared catalysts and the reusedcavities. ones, Besides, demonstrating there are thatno noticeable the catalysts differences have good between stability the in thefresh proces-prepareds of oxidation catalysts reaction. and the reused ones, demonstrating that the catalysts have good stability in the process of oxidation reaction.

Catalysts 2019, 9, x FOR PEER REVIEW 5 of 10

13 5.0 trace 6a 6c

14 4.0 91 7a 7c a Reaction conditions of sulfides to sulfoxides: sulfide (0.5 mmol), 95% EtOH (8 mL), catalyst (50 mg),

30 wt% H2O2 (63 mg, 0.55 mmol), 30 °C; b Reaction conditions of sulfides to sulfones: sulfide (0.5 mmol), 95% EtOH (10 mL), catalyst (150 mg), 30 wt% H2O2 (170 mg, 1.5 mmol), 40 °C; c Isolated yield; d Not available. To study the recyclability of the PAMAM-G1-PMo catalyst, we again chose the oxidation of thioanisole (1a) to (methylsulfinyl)benzene (1b) or (methylsulfonyl)benzene (1c) as model reactions. The catalyst PAMAM-G1-PMo was easily recovered by filtration after the reaction, washed with 95% EtOH, and dried in a vacuum at room temperature. After the above treatment, the catalyst was reused for subsequent experiments (up to five runs), and no major differences in the yield or required reaction times were observed, as summarized in Figure 1.

Figure 1. Study of the PAMAM-G1-PMo recyclability for the oxidation of thioanisole to sulfoxide (left) and sulfone (right).

After being used for five times and separated from the reaction system, the catalysts were characterized by Fourier-transform infrared spectroscopy (FT-IR, Figure 2), X-ray diffraction (XRD, Figure 3), and scanning electron microscopy (SEM, Figure 4). As shown in Figure 2, the IR frequencies of reused PAMAM-G1-PMo (958, 3373, 3100 cm−1) are consistent with those of fresh-prepared PAMAM-G1-PMo, indicating the PMo anions are still successfully immobilized on protonated PAMAM-G1. As shown in Figure 3, the XRD patterns for both fresh-prepared PAMAM-G1-PMo and reused one show a broad band in the range of 2θ = 14−40°, which confirms the high dispersion of the PMo anions on PAMAM-G1 support. At last, it can be observed via the SEM spectra (Figure 4) that fresh-prepared PAMAM-G1-PMo and reused one have similar micromorphology, such as the tiny projections on the amorphous surface. The results of all the above detections prove that the PAMAM- G1-PMo, no matter fresh-prepared or reused, has a higher surface-to-volume ratio and more reactive Catalysts 2019, 9, 791 6 of 10 cavities. Besides, there are no noticeable differences between the fresh-prepared catalysts and the reused ones, demonstrating that the catalysts have good stability in the process of oxidation reaction.

CatalystsCatalysts 20192019,, 99,, xx FORFOR PEERPEER REVIEWREVIEW 66 ofof 1010

FigureFigure 2.2. TheThe IRIR spectraspectra ofof fresh-preparedfresh-prepared PAMAM-PAMAM-G1-PMoG1-PMo (left)(left) andand reusedreused PAMAM-G1-PMoPAMAM-G1-PMo (right).(right). Figure 2. The IR spectra of fresh-prepared PAMAM-G1-PMo (left) and reused PAMAM-G1-PMo (right).

Figure 3. The XRD patterns of fresh-prepared PAMAM-G1-PMo (left) and reused PAMAM-G1- FigureFigure 3.3. TheThe XRDXRD patternspatterns ofof fresh-preparedfresh-prepared PAMAM-G1-PMoPAMAM-G1-PMo (left)(left) andand reusedreused PAMAM-G1-PMoPAMAM-G1-PMo PMo (right). (right).(right).

Figure 4. The SEM spectra of fresh-prepared PAMAM-G1-PMo (top) and reused PAMAM-G1- FigureFigure 4.4. TheThe SEMSEM spectraspectra ofof fresh-preparedfresh-prepared PAMAM-PAMAM-G1-PMoG1-PMo (top)(top) andand reusedreused PAMAM-G1-PMoPAMAM-G1-PMo PMo (bottom). (bottom).(bottom).

TheThe presentpresent catalyticcatalytic systemsystem waswas alsoalso appliedapplied toto thethe preparationpreparation ofof aa well-knownwell-known smartsmart drugdrug ModafinilModafinil (2-[(diphenylmethyl)sulfinyl]acetamide,(2-[(diphenylmethyl)sulfinyl]acetamide, 9),9), whichwhich isis aa clinicallyclinically wakefulness-promotingwakefulness-promoting agentagent toto treattreat disordersdisorders suchsuch asas narcolepsynarcolepsy andand exexcessivecessive daytimedaytime sleepiness.sleepiness. Traditionally,Traditionally, ModafinilModafinil waswas producedproduced byby thethe oxidatiooxidationn ofof 2-[(diphenylmethyl)thio]2-[(diphenylmethyl)thio]acetamideacetamide (8)(8) withwith 3030 wt%wt% HH22OO22 inin aceticacetic acid.acid. However,However, aa considerableconsiderable amountamount ofof sulfonesulfone asas thethe mainmain by-productby-product waswas oftenoften generatedgenerated andand difficultdifficult toto bebe removedremoved byby recrystallizationrecrystallization duedue toto itsits similarsimilar physicalphysical propertyproperty withwith ModafinilModafinil [36].[36]. InspiredInspired byby thethe selectiveselective oxidationoxidation propertyproperty ofof thethe 3030 wt%wt% HH22OO22/PAMAM-G1-PMo/PAMAM-G1-PMo system,system, wewe practicedpracticed itsits applicationapplication forfor thethe oxidationoxidation processprocess inin preparationpreparation ofof Modafinilf.Modafinilf. TheThe finalfinal productproduct waswas

Catalysts 2019, 9, 791 7 of 10

The present catalytic system was also applied to the preparation of a well-known smart drug Modafinil (2-[(diphenylmethyl)sulfinyl]acetamide, 9), which is a clinically wakefulness-promoting agent to treat disorders such as narcolepsy and excessive daytime sleepiness. Traditionally, Modafinil was produced by the oxidation of 2-[(diphenylmethyl)thio]acetamide (8) with 30 wt% H2O2 in acetic acid. However, a considerable amount of sulfone as the main by-product was often generated and difficult to be removed by recrystallization due to its similar physical property with Modafinil [36]. Inspired by the selective oxidation property of the 30 wt% H2O2/PAMAM-G1-PMo system, we practiced itsCatalysts application 2019, 9, x forFOR the PEER oxidation REVIEW process in preparation of Modafinilf. The final product was obtained2 of 10 in a good yield of 89% and excellent purity of 99.79% after the crude product was collected and obtained in a good yield of 89% and excellent purity of 99.79% after the crude product was collected recrystallized with acetone (Scheme1). and recrystallized with acetone (Scheme 1).

Scheme 1. The oxidation process in preparation of Modafinil. Scheme 1. The oxidation process in preparation of Modafinil. 3. Materials and Methods 3. Materials and Methods All reactants and solvents were directly obtained from commercial sources and used without furtherAll purification. reactants and1H solvents and 13C NMRwere spectradirectly were obtained recorded from with commercial a Bruker Avance-IIIsources and 600 used spectrometer without (Bruker,further purification. Zürich, Switzerland). 1H and 13 TheC NMR purity spectra of the products were recorded was determined with a Bruker by HPLC Avance with areas-III 600 of peakspectrometer normalization (Bruker, method Zürich, (pump: Switzerland waters). 1525,The purity detector: of the waters products 2489. was Chromatographic determined by column: HPLC WondaSilwith areas C-18, of monitoring peak normalization wavelength used method 210 nm). (pump: waters 1525, detector: waters 2489. ChromatographicInfrared spectra column: were WondaSil obtained fromC-18, Fourier-transformmonitoring wavelength infrared used spectroscopy 210 nm). (FT-IR, Thermo 1 ScientificInfrared IS5) spectra at universal were attenuated obtained from total Fourier reflection-transform (ATR) mode infrared in the spectroscopy range 400–4000 (FT- cmIR,− Thermo, using catalystScientific samples IS5) at universal supported attenuated on KBr wafers. total reflection X-ray diff ractometer(ATR) mode (XRD, in the PERSEE range 400 XD-3,–4000 Beijing cm−1, Persee using instrumentcatalyst samples co. LTD, supported Beijing, China.)on KBr wafers. with monochromated X-ray diffractometer Cu–Kα (XRD,radiation PERSEE source XD at- 303, kVBeijing and Persee 30 mA wasinstrument used to co. investigate LTD, Beijing the crystal, China structure.) with monochromated change between Cu fresh-prepared–Kα radiation and source reused at catalysts,30 kV and and 30 X-raymA w diasff usedraction to investigate patterns were the crystal obtained structure in a scan change range between 2θ = 10–80 fresh◦ with-prepared a scan and rate reused of 4◦/min. catalysts, SEM spectraand X-ray was diffraction obtained frompatterns Scanning were obtained electron microscopein a scan range (SEM, 2θ FEIQ45, = 10–80° Hitachi, with a Tokyo,scan rate Japan.). of 4°/min. SEM spectra was obtained from Scanning electron microscope (SEM, FEIQ45, Hitachi, Tokyo, Japan.). 3.1. Preparation and Characterization of PAMAM-G1-PMo 3.1. PreparationIn a typical and experiment, Characterization PAMAM-G1 of PAMAM (1.0 g,-G1 2.0-PMo mmol) was dissolved in deionized water (35 mL) underIn room a typical temperature, experiment, and PAMAM Keggin-type-G1 (1.0 H3 g,PMo 2.0 12mmol)O40 (7.3 was g, dissolved 4.0 mmol) in was deionized added whilewater stirring.(35 mL) Theunder resulting room temperature, mixture was stirredand Keggin for 48-type h and H3 filtered.PMo12O The40 (7.3 solids g, 4.0 were mmol) washed was withadded deionized while stirring water. andThe ethanol,resulting and mixture dried was under stirred vacuum for 48 at h 40 and◦C forfiltered 12 h. toThe aff solidsord an were aqua washed powder with of PAMAM-G1-PMo. deionized water and ethanol, and dried under vacuum at 40 °C for 12 h to afford an aqua powder of PAMAM-G1- 3.2. Catalytic Reactions PMo. 3.2.1. General Experimental Procedure for the Oxidation of Sulfides to Sulfoxides 3.2. Catalytic Reactions A 50 mL, three-necked flask was charged with sulfide (0.5 mmol), the catalyst (50 mg), and 95%3.2.1. EtOH General (8 mL).Experimental The resulting Procedure solution for was the stirredOxidation at 30 of◦ SC,ulfides and 30 to wt% Sulfoxides H2O2 (63 mg, 0.55 mmol) was added slowly into the above mixture. The reaction was monitored by TLC (petroleum ether:ethyl A 50 mL, three-necked flask was charged with sulfide (0.5 mmol), the catalyst (50 mg), and 95% acetate = 7:3). After the reaction was finished, the catalyst was separated from the mixture by filtration, EtOH (8 mL). The resulting solution was stirred at 30 °C, and 30 wt% H2O2 (63 mg, 0.55 mmol) was washed with 95% EtOH, and dried in a vacuum at room temperature overnight for recycle use. The added slowly into the above mixture. The reaction was monitored by TLC (petroleum ether:ethyl filtrate and the washing solution were combined, concentrated under reduced pressure, followed by a acetate = 7:3). After the reaction was finished, the catalyst was separated from the mixture by filtration, freeze-drying. The crude products were purified by column chromatography on a silica gel column washed with 95% EtOH, and dried in a vacuum at room temperature overnight for recycle use. The filtrate and the washing solution were combined, concentrated under reduced pressure, followed by a freeze-drying. The crude products were purified by column chromatography on a silica gel column with n-hexane/ethyl acetate (2:1) as eluent to afford the desired products. The 1H NMR and 13C NMR data and spectra for corresponding products are available online at Supplementary Materials.

3.2.2. General Experimental Procedure for the Oxidation of Sulfides to Sulfones A 50 mL, three-necked flask was charged with sulfide (0.5 mmol), the catalyst (150 mg), and 95% EtOH (10 mL). The resulting solution was stirred at 40 °C, and 30 wt% H2O2 (170 mg, 1.5 mmol) was added slowly into the above mixture. The reaction was monitored by TLC (petroleum ether:ethyl acetate = 7:3). After the reaction was finished, the catalyst was separated from the mixture by filtration, washed with 95% EtOH, and dried in a vacuum at room temperature overnight for recycle use. The

Catalysts 2019, 9, 791 8 of 10 with n-hexane/ethyl acetate (2:1) as eluent to afford the desired products. The 1H NMR and 13C NMR data and spectra for corresponding products are available online at Supplementary Materials.

3.2.2. General Experimental Procedure for the Oxidation of Sulfides to Sulfones A 50 mL, three-necked flask was charged with sulfide (0.5 mmol), the catalyst (150 mg), and 95% EtOH (10 mL). The resulting solution was stirred at 40 ◦C, and 30 wt% H2O2 (170 mg, 1.5 mmol) was added slowly into the above mixture. The reaction was monitored by TLC (petroleum ether:ethyl acetate = 7:3). After the reaction was finished, the catalyst was separated from the mixture by filtration, washed with 95% EtOH, and dried in a vacuum at room temperature overnight for recycle use. The filtrate and the washing solution were combined, concentrated under reduced pressure, followed by a freeze-drying. The crude products were purified by column chromatography on a silica gel column with n-hexane/ethyl acetate (2:1) as eluent to afford the desired products. The 1H NMR and 13C NMR data and spectra for corresponding products are available online at Supplementary Materials.

3.2.3. Synthesis of Modafinil A 250 mL, three-necked flask was charged with 2-[(diphenylmethyl)thio]acetamide (13 g, 50 mmol), the catalyst (5 g), and 95% EtOH (80 mL). The resulting solution was stirred at 30 ◦C, and 30 wt% H2O2 (6.3 g, 55 mmol) was added slowly into the reaction mixture. The mixture was allowed stirring for 2 h at 30 ◦C. After the reaction was finished, the catalyst was separated from the mixture by filtration, washed with 95% EtOH, and treated for recycle use. The filtrate and washings were combined and transferred to a 1 L beaker containing 500 mL deionized water to form white solid, which was collected by filtration and purified by a recrystallization with 30 mL of acetone to provide 12.3 g of desired product with a 1 HPLC purity of 99.79%. H NMR (400 MHz, DMSO-d6) δ 7.66–7.49 (m, 5H), 7.44–7.28 (m, 7H), 5.34 13 (s, 1H), 3.36 (d, J = 13.7 Hz, 1H), 3.22 (d, J = 13.6 Hz, 1H). C NMR (151 MHz, CDCl3) δ 165.22, 133.37, 133.06, 128.46, 128.37, 127.93, 127.85, 127.79, 127.66, 70.50, 50.56. The 1H NMR, 13C NMR and HPLC spectra for Modafinil are available online at Supplementary Materials.

4. Conclusions In summary, we developed a PAMAM-G1-PMo-catalyzed method for the oxidation of sulfides to sulfoxides or sulfones via an efficient and mild procedure. This system is environmentally benign since 30 wt% H2O2 and 95% EtOH are used as the oxidant and solvent, respectively, and the catalysts are recyclable. Moreover, the catalysts are inexpensive, stable during the reaction, and easily recovered by filtration after the reaction and for reuse in additional runs without a noticeable loss of catalytic activity. This is the first report of PAMAM-G1-PMo as the heterogeneous catalyst used in the oxidation of sulfides to sulfoxides or sulfones, which will help the enrichment and development of future research on the dendrimers’ hybrid.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/9/10/791/s1, Figure S1: XRD patterns of HMo. Author Contributions: Y.L. and M.-S.C. supervised the whole experiment and provided technical guidance. Q.-L.T. and Z.-F.F. performed the preparation and identification of the catalysts, and the investigation of the oxidation conditions. J.-W.Y., Q.L. and Y.-X.C. assisted in the synthesis work. Funding: The financial supports from the Program for Liaoning Innovative Talents in University (LR2017043) and the Career Development Support Plan for Young and Middle-aged Teachers in Shenyang Pharmaceutical University (ZQN2016005) are gratefully appreciated. Conflicts of Interest: The authors declare no conflict of interest. Catalysts 2019, 9, 791 9 of 10

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