An electron donor–acceptor photoactivation strategy for the synthesis of S-aryl dithiocarbamates using thianthrenium salts under mildaqueous micellar conditions?

Ho Xu,Xufeng Li,Jie M,Junze Zuo,Xiuyn Song,Jin Lv,Doshn Yng,c,?

a Key Laboratory of Optic-electric Sensing and Analytical Chemistry for Life Science,MOE,College of Chemistry and Molecular Engineering,Qingdao University of Science and Technology,Qingdao 266042,China

b Zhejiang Wansheng Co.,Ltd.,Linhai 317000,China

c Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology (Ministry of Education),Department of Chemistry,Tsinghua University,Beijing 100084,China

Keywords:S-Aryl dithiocarbamates Visible-light EDA complex Thianthrenium salts Micellar photocatalysis

ABSTRACT An eco-friendly and convenient method is developed herein for the synthesis of S-aryl dithiocarbamates via visible-light-induced SET process of an EDA complex between thianthrenium salt functionalized arenes and dithiocarbamate anions under mild aqueous micellar conditions.This strategy indirectly realizes the method for constructing S-aryl dithiocarbamates through site-selective C-H functionalization of arenes.Most importantly,the reaction proceeded smoothly without addition of any photocatalyst,and the by-product thianthrene is recycled in quantity,ultimately minimizing the production of chemical waste.This protocol provides a promising synthesis candidate for the construction of valuable S-aryl dithiocarbamates,which also opens up a new avenue for micellar photocatalysis.

The dithiocarbamate skeleton is one of the most important sulfur-containing organic frameworks,showing various biological activities such as anticancer [1,2],anti-infection [3],antibacterial [4],and antitumor properties (Scheme 1A) [5].Furthermore,dithiocarbamates are also widely used as versatile synthetic building blocks in organic synthesis [6].In light of their widespread utilities,the development of novel and convenient methods for the synthesis of these useful sulfur-containing compounds continues to motivate a large number of scientists.The classical approaches towardS-aryl dithiocarbamates are the reaction of amines with thiophenols and thiophosgene or the treatment of isothiocyanates with thiophenols [7,8].Recently,copper-catalyzed Ullmann-type coupling between iodoarenes or aryl boronic acids with sodium dialkyldithiocarbamates or tetraalkylthiuram disulfides has also been developed for the preparation ofS-aryl dithiocarbamates [9–13].For example,Bolm and Dong demonstrated an elegant coppercatalyzed system towardsS-aryl dithiocarbamates using commercially available thiuram disulfide reagents [14].Nevertheless,most of these methods still involve the use of toxic metal catalysts,special ligands,multi-step synthetic sequences,or harsh reaction conditions,which would limit their practical utility.Therefore,the development of an efficient,facile,and eco-friendly approach toSaryl dithiocarbamates is still highly desirable.As a practical organic synthesis strategy,multicomponent reactions (MCRs) have attracted extensive attention because it improves the atomic economy and step economy of the reaction process,thus minimizing pollution and reducing costs [15,16].In 2008,Ranu and co-workers reported an efficient method for the synthesis ofS-aryl dithiocarbamates through copper nanoparticle-catalyzed three component condensation of arylhalides,carbon disulfide and amines in water (Scheme 1B) [17].In 2016,Qietal.demonstrated a similar process utilizing aryl boronic acids as the aryl reagents by copper catalysis (Scheme 1B) [18].Very recently,Murarka and Singh developed an elegant multicomponent coupling between diaryliodonium triflates,amines,and carbon disulfide for the preparation ofS-aryl dithiocarbamates under mild conditions (Scheme 1B) [19].Furthermore,Biju and Jindal reported a facile method for the synthesis ofS-aryl dithiocarbamates by three-component coupling of arynes,CS2and aliphatic amines under metal-free conditions (Scheme 1B) [20].Although great achievements have been made,developing an alternative approach toS-aryl dithiocarbamates with high efficiency and generality is still of great interest.

Scheme 1.(A) Biologically relevant S-aryl dithiocarbamates,(B) Classical methods toward S-aryl dithiocarbamates.

In recent years,the direct C-H functionalization of unreactive C-H bonds provides a valuable tool for the construction of C-C bonds and C-heteroatom bonds [21–25].However,the main challenge for this straightforward strategy is the site-selectivity.The existence of the directing group together with transition-metal catalysis could effectively solve this problem,but the removal or transformation of the directing groups is a intractable task [26].Notably,Ritter and co-workers demonstrated an efficient and convenient site-selective C-H functionalization of arenes by thianthrenation process affording many chances for late-stage functionalization of arenes without a directing group (Scheme 2A) [27].Recently,desulfurization couplings using thianthrenium salts for the construction of C-C bonds and C-heteroatom bonds have been extensively studied [28–46].Undoubtedly,the C-H bond functionalization is the most efficient and direct method to constructS-aryl dithiocarbamates (Scheme 2C).

Scheme 2.(A) Late-stage C-H functionalization of arens;(B) The reaction mode of micellar catalysis;(C) Our strategy for the synthesis of S-aryl dithiocarbamates from thianthrenium salt functionalized arenes.

Over the past decade,visible-light photoredox catalysis has emerged as an attractive strategy for organic transformationsviaphotoinduced energy transfer (EnT),hydrogen atom transfer (HAT),or electron transfer (SET) processes as well as their combinations[47–56].In this regard,electron donor-acceptor (EDA) complexes which possess typical red-shifted charge-transfer transition bands,can effectively induce inner-sphere electron transfer upon their photoexcitation to generate radicals [57–59].Considering the special properties of thianthrenium salts and their susceptibility to undergo single electron reduction,we envisage thatS-aryl dithiocarbamates could be constructed starting from readily available arenesviaindirect C-H bond functionalization (Scheme 2C).

On the other hand,water is an ideal green non-toxic reaction medium,which is environmentally friendly and safe to handle.However,performing organic reactions in water is often challenging due to the immiscibility of organic molecules in aqueous media,and this has brought to light the use of surfactants as micellar catalytic approach (Scheme 2B) [60–63].Micelles can effectively accumulate reactants and catalysts in their inner region,forming a higher local concentration,which can facilitate organic transformations effectively.Over the past few decades,tremendous progress has been made in “micellar catalysis” since it affords a sustainable alternative to various conventional organic synthesis [64–67].As a continuous interest of our research in sulfur chemistry and photochemical reactions [68–79],herein,we would like to report a novel and efficient visible-light-induced SET process of an EDA complex between thianthrenium salt functionalized arenes and dithiocarbamate anions under mild aqueous micellar conditions for the synthesis ofS-aryl dithiocarbamates (Scheme 2C).

We firstly embarked on optimization studies through reacting aryl thianthrenium salt1a,carbon disulfide2and pyrrolidine3ain DMSO by using K2CO3as a base at room temperature with irradiation of a 20 W blue LED lamp.To our delight,the desiredS-aryl dithiocarbamate4jcould be obtained in 70% isolated yield(Table 1,entry 1).Encouraged by this result,further optimization of different reaction parameters including the solvents,bases and surfactnts was carried out.Initially,seven solvents,such as NMP,1,4-dioxane,THF,MeCN,DMF,DCM,and DCE were evaluated,and none of these solvents are as effective as DMSO.In addition,the yield of product4jwas not improved with the increase of the water content in DMSO (Table 1,entry 9).We found that the yieldof4jreached 65% when water was used as the solvent (Table 1,entry 10).Inspired by micellar catalysis,various surfactants were screened in order to achieve the optimum conditions.To our delight,dodecyl trimethyl ammonium chloride (DTAC)Ashow the highest activity,delivering the product4jin 82% isolated yield(Table 1,entries 11–15).Next,we examined different bases including K2CO3,Na2CO3,Cs2CO3,DBU,DABCO,NaOH,DMAP,Et3N,and K2CO3was more efficient (Table 1,compare entries 11,and 17-24).Furthermore,control experiments indicated that light irradiation is an essential factor in the present transformation (Table 1,entry 16).In addition,control experiments had shown that adding additional photocatalysts did not improve the reaction efficiency (entries 25-27,Table 1).Finally,controlled experiments showed that raising the reaction temperature in the absence of light resulted in lower yields (entries 28 and 29,Table 1).

Table 1Screening for the optimal conditions.a

After establishing the optimal conditions (Table 1,entry 11),the scope and generality of the present reaction was investigated(Scheme 3).To our delight,various aryl thianthrenium salts1which were directly synthesized from diverse arenes reacted efficiently with carbon disulfide2and secondary amines3,affording the target products4in moderate to excellent yields (4a–4af).The secondary amines including aliphatic and aromatic ones were well tolerated under the standard conditions,showing no obvious difference in reactivity.There is no significant difference in the electron-effect of the substituted groups in secondary aromatic amines,including electron-rich,and -neutral ones (compare4a,4c,4dand4h).In addition,a series of cyclic and linear amines coupled with carbon disulfide and aryl thianthrenium salts to produce the desired products in good yields.Subsequently,the scope of aryl thianthrenium salts was explored.To our satisfaction,the present photochemical reaction tolerated an array of structurally and electronically diverse aryl thianthrenium salts.Notably,the electronwithdrawing groups (-Cl,-Br) or electron-donating groups (-Me,-Et,-OMe) present at any position of the phenyl group of arylsulfonium salts showed similar reactivity.Most noteworthy is the EDA complex photoactivation method was also applied to the latestage functionalization of pharmaceuticals or their derivatives (4ab,4ac,4aeand4af).A series of valuable functional groups including alkoxy,alkyl,ester,halides,and imidazole are well tolerated under the standard conditions leaving ample room for further modifications.To our delight,alkyl thianthrenium salt was also well tolerated in the present tranformaiton,leading to theS-alkyl dithiocarbamate4agin 91% isolated yield.

Scheme 3.The substrate scope.Reaction conditions: 1 (0.2 mmol),2 (0.6 mmol),3 (0.4 mmol),K2CO3 (0.4 mmol),2 wt% sulfactant/H2O (2 mL) at room temperature under irradiation with a 20 W blue LED (455 nm) for 12 h.Isolated yield.

To show the synthetic application of this EDA complex photoactivation method,a gram-scale reaction was carried out by using substrates1a(9 mmol),CS2(27 mmol),and pyrrolidine (3a) under irradiation of two 20 W blue LED lamps (Scheme 4a).To our delight,the reaction proceeded well by delivering4jin 63% isolated yield,suggesting that this photochemical system is easy to scale up.Importantly,thianthrene5could be well recovered which subsequently was oxidized into thianthrene 5-oxide6in quantitative yield (Scheme 4a) [80,81].Furthermore,a sunlight-promoted experiment also afforded4jin 60% isolated yield (Scheme 4b).The sustainability of the reaction media was also evaluated using the model reaction under the optimized conditions.After completion of the reaction,the mixture was extracted with ethyl acetate and only fresh reagents (1a,CS2and3a) were added to the DTAC/H2O solution for the next photochemical reaction cycle.Excitingly,the chemical process showed no significant loss of activity after five cycles as illustrated in Scheme 4c.Notably,the reaction also proceeded smoothly when 4-chlorophenol was used as the substrate,and the thioether product4ahwas obtained in 80% isolated yield(Scheme 4d).Therefore,this developed protocol can be well applied to the synthesis of thioethers.These experimental results indicate the potential value of this synthesis strategy.

Scheme 4.Synthetic applications.

To illuminate the mechanism of the reaction,radical trapping experiments were conducted (Scheme 5).When radical-trapping reagents 2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO) and 1,1-diphenylethylene were added into the present reaction system,the transformation was significantly suppressed,which implied that the reaction might proceedviaa free radical pathway.In addition,the formation of trapping product of aryl radical was detected by HRMS analysis,suggesting the generation of aryl radicals in the EDA complex photoactivation transformation.

Scheme 5.Radical trapping experiment.

In order to verify the formation of an EDA complex in the current transformation,some preliminary experiments were performed (Fig.1).When dithiocarbamate anion8generated in situ was added to a solution of aryl thianthrenium salt1a,the solution transformed from pale colourless to yellow color.Furthermore,the UV-vis absorption of different combinations of the reactants were measured.As shown in Fig.1a,neither aryl thianthrenium salt1a,carbon disulfide2+pyrrolidine3a,nor1a+2+3ashowed obvious absorption beyond 400 nm,but a mixture of thianthrenium salt1a,carbon disulfide2,pyrrolidine3aand K2CO3showed a significant bathochromic shift.In addition,a molar ratio of 1:1 between1aand8was observed by using Job’s method of continuous variations (Fig.1b).Finally,1H NMR titration experiments were performed as shown in Fig.1c.Upon addition of8generated in situ to1ain CDCl3,the proton signals of1ashowed significant upfield shift,indicating the formation of an EDA complex between1aand8in the photochemical process.

Fig.1.Mechanistic studies.

Based on the above preliminary finding and previous related reports [82–85],a plausible mechanism hypothesis for the EDA complex photoactive transformation is thus presented in Scheme 6.Initially,carbon disulfide2reacts with amines3in the presence of K2CO3under mild aqueous micellar conditions to give the corresponding dithiocarbamate anionA.Subsequently,the electron-rich dithiocarbamate anionAand the electron-poor aryl thianthrenium salt1associate to form an EDA complexB.Upon irradiation with visible-light,a SET (single electron transfer) transformation fromAto1to give an aryl radicalD,a thiyl radicalE,and thianthrene5.Finally,the aryl radicalDcouples with the thiyl radicalE,delivering the coupling product4.

Scheme 6.Possible reaction pathway.

In conclusion,we have successfully developed a novel and efficient visible-light-induced EDA complex process for the synthesis ofS-aryl dithiocarbamates under mild aqueous micellar conditions.The corresponding products were obtained in moderate to good yields with excellent functional group tolerance.Some important features of the present protocol involve: (a) using water as an environmentally friendly reaction medium;(b) using readily available thianthrenium salts,carbon disulfide and amines as the starting materials;(c) external photocatalyst-free;(d) gramscale synthesis;and (e) ease of operation.The advantages of the developed method meet the requirements of sustainable and green chemistry.Thus,we expect that the present study will be found wide applications in pharmaceutical chemistry and organic synthesis.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No.22271170),and the Scientific Research Foundation of Qingdao University of Science and Technology.