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Metal-free sulfonylation of 3,4-dihalo-2(5H)-furanones (X= Cl, Br) with sodium sulfinates under air atmosphere in aqueous media via a radical pathway Liang Cao, Jianxiao Li, Han-Qing Wu, Kai Jiang, Zhi-Feng Hao, Shihe Luo, and ZhaoYang Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04564 • Publication Date (Web): 14 Jan 2018 Downloaded from http://pubs.acs.org on January 15, 2018

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Metal-free sulfonylation of 3,4-dihalo-2(5H)furanones (X= Cl, Br) with sodium sulfinates under air atmosphere in aqueous media via a radical pathway Liang Cao,a Jian-Xiao Li,*b Han-Qing Wu,a Kai Jiang,a Zhi-Feng Hao,c Shi-He Luoa,b and ZhaoYang Wang*a,b a

School of Chemistry and Environment, South China Normal University; Key Laboratory of

Theoretical Chemistry of Environment, Ministry of Education, 378 Waihuan West Road, University City, Guangzhou 510006, P. R. China. b

Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of

Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, P. R. China. c

School of Chemical Engineering and Light Industry, Guangdong University of Technology,

100 Waihuan West Road, University City, Guangzhou 510006, P. R. China. *E-mail: [email protected]; Fax: (+86)-020-3931-0187; Tel: (+86)-020-3931-0258. *E-mail: [email protected]; Fax: (+86) -020-8711-2906; Tel: (+86)-020-2223-6518.

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ABSTRACT: A convenient and environmentally friendly protocol has been developed for the preparation of various sulfonylated 2(5H)-furanones by the sulfonylation of Csp2-X compounds with sodium sulfinates via a metal-free radical pathway. For low-cost Csp2-Cl substrates, there is also a satisfactory reactivity with moderate to excellent yields as well as high atom economy. Importantly, the application of this method into a gram-scale (even over 10 g) preparation can be accomplished.

KEYWORDS: Metal-free sulfonylation, C-S bond formation, 2(5H)-furanones, Sodium sulfinates, Radical pathway

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INTRODUCTION

Organosulfur architectures are valuable structural motifs for the synthesis of pharmaceutical and naturally active molecules and organic materials, more and more attention has been attracted to the formation of C-S bonds,1-6 especially the synthesis of sulfones identified in a wide range of biologically important molecules.7-12 Generally, the facile sulfone sources can be originated from sulfonyl chlorides, sodium sulfinates, sulfonyl hydrazide, thiols, dimethyl sulfoxide, etc.13-18 Among them, being bench-stable, commercially or readily available, easy to handle,19-21 sodium sulfinates as sulfonylation reagents have been primarily utilized in recent years.22-27 At the same time, due to the low cost, developing Csp2-X compounds as available substrates have gained much attention.28-32 Nowadays, some elegant methods have been undertaken in the synthesis of sulfones through the transition metal-catalyzed coupling reaction between Csp2-X compounds and sodium sulfinates.33,34 For example, Zhang has developed a Cu-catalyzed system for the synthesis of aryl sulfones from aryl iodides and bromides with sodium sulfinates (Scheme 1a).35 On the other hand, Yu has reported an efficient protocol for the synthesis of fivemembered heterocyclic sulfones via a SNAr pathway (Scheme 1b).36 However, more readily available and inexpensive Csp2-Cl compounds were not effective substrates. Therefore, the development of more efficient and environmentally friendly metal-free reaction systems to achieve the sulfonylation of Csp2-X (X = Cl, Br) compounds with sodium sulfinates is still highly desirable.

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Scheme 1. Direct synthesis of sulfones from Csp2-X compounds.

Being interested in 2(5H)-furanones chemistry, we chose 3,4-dihalo-2(5H)-furanones as non-aromatic Csp2-X (X = Br, Cl) substrates instead of aryl halides to investigate their reactions with sodium sulfinates (Scheme 1). 2(5H)-furanones are extremely valuable structural motifs in various natural products and biologically active molecules, which exhibit an impressive array of bioactivities.37-42 Particularly, sulfonated 2(5H)-furanones have attracted the growing synthesis pursuit of chemists because of their excellent biological and pharmaceutical activities.43,44 Based on 3,4-dihalo-2(5H)furanone derivatives as synthons, we have reported the construction of C-C and C-S bonds under the transition metal-catalyzed. And accidentally, a solvent DMSO-promoted metal-free radical pathway to give sulfones has been found (Scheme 2).45-47 As a continuation, a metal-free sulfonylation of 3,4-dihalo-2(5H)-furanones with sodium sulfinates via a radical pathway in the presence of water is discovered this time. In the novel conversion with moderate to excellent yields, a green reaction system without the risk of

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transition metal contamination and the atmosphere of inert gas under mild temperature is established.

Scheme 2. Reactions based on 3,4-dihalo-2(5H)furanone derivatives in the presence/absence of sodium arylsulfinates.

RESULTS AND DISCUSSION Initially, the reaction of 3,4-dibromo-5-methoxy-2(5H)-furanone 1a and p-toluenesulfinate 2a under air atmosphere was chosen as a model reaction to optimize the condition parameters (Table 1). To our delight, the reaction can give the desired product 3a (its chemical structure is confirmed by single-crystal X-ray diffraction analysis, the details can be seen in SI)48 with yields

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of 16-31% using copper salt and 1,10-phenanthroline (Phen.) as the catalytic system at 80 oC in mixed solvent of 1,2-dichloroethane (DCE) and water (DCE/H2O, v/v = 5/1) for 12 h (entries 13). Amazingly, it is not essential for the use of copper catalytic system, because there is a better yield in the absence of copper catalyst (entry 4 vs. entries 1-3). Further optimization was carried out with the use of various solvents (Table 1, entries 5-8). It can be found that the solvent plays a significant role in the success of this process. Among all the tested solvent, including 1,2-dimethoxyethane (DME), DCE-H2O is the most suitable solvent for the present transformation. While trying the reaction only DCE or water as the solvent, no product can be obtained (entries 9 and 10). When the dosage of 2a is increased to 2 equiv., a better yield of 42% can be afforded (entry 11). However, further increasing the amount of 2a to 3 equiv., the isolated yield of product 3a is only 41% (entry 12). As anticipated, phase transfer catalyst (PTC) can be employed in the system to further improve the result, and just 3 mol% nBu4NBr may give an observable yield of 78% (entries 13-15). Additionally, elevating the temperature to make reaction system refluxed is more favorable to the formation of 3a (entry 16 vs. entry 15). Among the examination for reaction time, reflux for 8 h is found to be preferred in terms of the yield (entries 17-19). And under these conditions, the cheaper n-Bu4NBr (entry 18) is proved to be the best among the common PTC such as n-Bu4NI and 18-crown-6 (entries 20 and 21). This indicates that the transformation may be not performed by an ionic reaction mechanism, which will be discussed in the following. Thus, the optimized reaction conditions are identified as 3 mol% of n-Bu4NBr, 1a:2a = 1:2, a 5:1 mixture of DCE and H2O as the solvent at 90 oC under air atmosphere for 8 h (entry 18).

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Table 1. Optimization of reaction conditionsa Br O

Br O 1a

O

+ SO2Na 2a

Cat.

O

Solvent Time

O

S

O Br O

O

3a

Entry Cat. (or additive) (mol%) 1a:2a Organic solvent Time (h) Yield (%)b 1 CuI (10) Phen. (20) 1:1.5 DCE 12 16 2 Cu(OAc)2 (10) Phen. (20) 1:1.5 DCE 12 31 3 Phen. (20) 1:1.5 DCE 12 30 4 1:1.5 DCE 12 36 5 1:1.5 CH3CN 12 26 6 1:1.5 DME 12 24 7 1:1.5 CH3NO2 12 14 8 1:1.5 i-PrOH 12 Trace 9 1:1.5 DCEc 12 Trace 10 1:1.5 H2Od 12 0 11 1:2 DCE 12 42 12 1:3 DCE 12 41 13 n-Bu4NBr (10) 1:2 DCE 12 77 14 n-Bu4NBr (5) 1:2 DCE 12 77 15 n-Bu4NBr (3) 1:2 DCE 12 78 e 16 n-Bu4NBr (3) 1:2 DCE 12 85 17e n-Bu4NBr (3) 1:2 DCE 10 88 e 18 n-Bu4NBr (3) 1:2 DCE 8 92 19e n-Bu4NBr (3) 1:2 DCE 6 90 e 20 n-Bu4NI (3) 1:2 DCE 8 89 21e 18-Crown-6 (3) 1:2 DCE 8 73 a Reaction conditions: 1a (0.30 mmol), 2a (0.45-0.90 mmol) and organic solvent-H2O (3.0 mL, v/v = 5/1, usually only organic solvent is listed) at 80 oC for 12 h under air unless otherwise noted. bIsolated yield. cOnly DCE (3.0 mL). dOnly H2O (3.0 mL). e90 oC (reflux temperature).

Under the optimized reaction conditions, a range of sodium sulfinates 2 were applied in the transformation to establish the scope and generality of this protocol (Table 2). Many sodium arylsulfinates 2 with electron-donating (e.g. alkyl and methoxy) or electron-withdrawing (e.g. halogen) groups can react with 3,4-dibromo-5-methoxy-2(5H)-furanone 1a to produce the corresponding products 3a-3h in moderate to excellent yields. Generally, the substrates 2 containing electron-donating groups perform better than those with electron-withdrawing groups

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(3a vs 3e). For methyl-substituted sodium arylsulfinates 2, though the number or position of methyl is different, all work well under mild conditions, giving the desired products in excellent yields (3i-3k). In addition, polycyclic substituted sodium sulfinates 2 can be transformed into the corresponding products 3l-3n in good to excellent yields. To our delight, heteroaryl sodium sulfinate is also suitable in the protocol with satisfactory result (3o).

Table 2. Substrate scope of various 5-substituted 3,4-dibromo-2(5H)-furanones 1 and sodium sulfinates 2ab

a

Reaction conditions: all reactions were performed with 1 (0.30 mmol), 2 (0.60 mmol) and solvent (3.0 mL, DCE : H2O = 5 : 1) at 90 oC for 8 h under air unless otherwise noted. bYields of isolated products are given. c24 h.

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Moreover, a series of 5-substituted 3,4-dibromo-2(5H)-furanones as substrates 1 were explored in the reaction with sodium p-toluenesulfinate 2a. As expected, this reaction has very good tolerance, different functional groups (e.g. alkyl, aryl) or structures (e.g. cycle, ether, ester and acetal) have little effect on the reactivity (3p-3u). In particular, the reaction of 2(5H)furanone with two different ester structures proceeds smoothly, resulting in the desired product (3u). On the other hand, the steric bulk of the substituent in 5-position of 2(5H)-furanones also has little influence, both sterically hindered substituted 5-cyclohexyl and 5-phenyl work well (3r and 3t). In order to further improve the practicability of this protocol, several 5-substituted 3,4dichloro-2(5H)-furanones were also employed as substrates 1 to react with sodium sulfinates 2 under the optimized conditions (Table 3). Obviously, a variety of sodium arylsulfinates 2, bearing electron-donating groups (e.g. alkyl and methoxy) or electron-withdrawing groups (e.g. halogen and CF3) on the aryl ring, can react smoothly with 3,4-dichloro-5-methoxy-2(5H)furanone 1, affording the corresponding sulfonylated 2(5H)-furanones 4a-4l in moderate to excellent yields. It is worth noting that benzene, naphthalene (including α- and β-naphthalene) and thiophene substituted sodium sulfinates 2 are also suitable reaction partners for this novel transformation (4m-4p). Similarly, 5-alkyl/aryl-substituted 3,4-dichloro-2(5H)-furanones 1 can be successfully transformed into the desired sulfonylation products 4q-4v in good to excellent yields. Of course, each 3,4-dichloro-2(5H)-furanone substrate 1 usually give a slightly lower yield than the corresponding 3,4-dibromo-2(5H)-furanone. The chemical structures of the obtained products 4 have been confirmed by X-ray analysis of 4l and 4t (see the SI for details).48

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Table 3. Substrate scope of various 5-substituted 3,4-dichloro-2(5H)-furanones 1 and sodium sulfinates 2ab

Cl

n-Bu4NBr (3 mol%) 90 C

Cl 2

R1O

+ R

O

O

2

1 R2

S

Cl

S

O

O

O O 4j (83%)

S

Cl O

O 4n (80%)

S

O

R1O O 4q-4u

Cl O

S

S

O

O Cl O

O

O Cl

O

O O 4l (79%) CCDC 1562979 Ph

O Cl

S

O O

O

O

O

O O 4o (82%)

O Cl

O O 4m (75%)

S

Cl

4q, R1 = Et (86%) 4r, R1 = i-Pr (80%) 4s, R1 = Cy (77%) 4t, R1 = Bn (64%) CCDC 1562980 4u, R1 = Ph (63%)

S

4

O O 4k (83%)

O

O R1O

O

O

O

O

(84%) (83%) (84%) (85%) (50%)c (43%)c (35%)c (87%) (29%)c

O

O

O

DCE : H2O = 5:1 Air atmosphere

SO2Na

4a, R2 = Me 4b, R2 = H 4c, R2 = Et O 4d, R2 = t-Bu S Cl 4e, R2 = F O 4f, R2 = Cl O O 4g, R2 = Br O 4h, R2 = OMe 4a-4i 4i, R2 = CF3

R2

S

O Cl

O

O O 4p (65%)

O S

O O

Cl

O

O O 4v (48%)

a

Reaction conditions: all reactions were performed with 1 (0.30 mmol), 2 (0.60 mmol) and solvent (3.0 mL, DCE : H2O = 5 : 1) at 90 oC for 8 h under air unless otherwise noted. bYields of isolated products are given. c24 h.

Finally, several control experiments were carried out to investigate the reaction mechanism. More and more attention has been attached to the development and application of radical strategy recently.49-51 Thus, according to the reported methods,52,53 we conducted the radical trapping experiments by adding 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) or 2,6-di-tert-butyl-4methylphenol (BHT) into the model reaction under the standard conditions (Scheme 3).

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Obviously, the formation of product 3a is suppressed on the whole, and both cases indicate that a free radical pathway should be involved in this process. On the other hand, it can be found that the N2 atmosphere is markedly disadvantageous for the reaction comparing with the air atmosphere. If N2 is replaced with O2, the yield of 90% can be obtained, and the result is basically similar to that in the air atmosphere. However, once oxidant tert-butyl hydroperoxide (TBHP) is added under N2 atmosphere, the reaction yield can be increased to 65%. These suggest that air may play an important role in the transformation.

Scheme 3. Control experiments.

On the basis of the above experimental results and previous reports,54-60 a possible reaction pathway was proposed as Scheme 4. Firstly, an oxygen-centered radical A is produced through the oxidation of sodium sulfinate 2 by air upon heating, and radical A can be resonance-

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stabilized with a sulfonyl radical B. Subsequently, intermediate C is generated through the addition of radical B to the vinyl moiety of 3,4-dihalo-2(5H)-furanone 1. Finally, the desired product 3 is obtained through the rapid leaving of halogen free radical (X·). The resulted X· radical may react with sodium sulfinate 2 to form sodium halide, while compound 2 is oxidized to radical A participating in the next reaction.

Scheme 4. Plausible reaction mechanism.

As an effective and easy-to-operate method, the scale-up of the metal-free sulfonylation may have great practical significance for the large-scale preparation of sulfonylated 2(5H)furanones 3 and 4 in the laboratory, and even in the industry. Using 3,4-dibromo-5-methoxy2(5H)-furanone 1a and p-toluenesulfinate 2a as model substrates for large-scale reactions under the standard conditions, the results are shown in Table 4. It can be found that, with the increase

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of the amount of reactant 1a from 3 to 30 mmol step by step, the yield of product 3a is gradually decreased. However, the yield of product 3a can still reach 80% even when the amount of 1a is expanded by 100 times. Once extending the reaction time to 12 h, even the total weight of the reactant is increased to about 30 g, the yield of 3a is up to 82% (14.2344 g) (entry 4). Similarly, other products, such as 3k, 3n, 4l and 4o, are also can be obtained by large-scale preparation with good yields. Therefore, the metal-free synthesis of different sulfonylated 2(5H)-furanones at the gram-level (even over 10 g) with satisfactory yields becomes possible.

Table 4. Large-scale sulfonylation of 3,4-dihalo-2(5H)furanones

Entry 1 2 3 4b

a

Amount of 1a 0.8157 g (3 mmol) 4.0783 g (15 mmol) 8.1567 g (30 mmol) 13.5945 g (50 mmol)

Amount of 2a 1.0691 g (6 mmol) 5.3454 g (30 mmol) 10.6908 g (60 mmol) 17.8180 g (100 mmol)

Product 3a (g) 0.9269 g 4.4265 g 8.3323 g 14.2344 g

Yield (%)a 89 85 80 82

Isolated yield. b12 h. c Amount of 1 (3 mmol) and 2 (6 mmol).

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CONCLUSION In summary, we have developed a highly efficient method for the preparation of various sulfonylation of Csp2-X (X = Cl, Br) compounds with sodium sulfinates. Fortunately, the protocol is an environmentally friendly metal-free process in good to excellent yields with a wide range of functional group tolerance and a high atom economy. Even its gram-scale preparation can practically be up to 14 g. Not only available Csp2-Cl substrates are suitable, but also this convenient radical pathway may promote the more development and application of radical reactions. Importantly, the introduction of specific functional groups in a regioselective manner can be used to drastically modify the pharmacophoric profile of 2(5H)-furanones. Further applications of this reaction are currently underway in our laboratory.

EXPERIMENTAL SECTION The mixture of 3,4-dihalo-2(5H)-furanone 1 (0.30 mmol), sodium sulfinate 2 (0.60 mmol) and n-Bu4NBr (3 mol %) in DCE : H2O (v : v = 5 : 1, 3 mL) was stirred at 90 °C under air for 8 h. At ambient temperature, the reaction mixture was diluted with H2O (15 mL) and extracted with EtOAc (3 × 15 mL). The organic extracts were dried over anhydrous Na2SO4. After filtration and evaporation of the solvents under reduced pressure, the crude product was purified by column chromatography on silica gel to afford desired product.

ASSOCIATED CONTENT Supporting Information.

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The Supporting Information (experimental procedure and characterization data for compounds 3a-4v, data of single-crystal X-ray analysis, and NMR spectra for all compounds 3a4v) is available free of charge on the ACS Publications website at DOI: xxxxxxxxx.

AUTHOR INFORMATION Corresponding Author * Zhao-Yang Wang. E-mail: [email protected]. * Jian-Xiao Li. E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Financial

support

from

the

Guangdong

Natural

Science

Foundation

(No.

2014A030313429), the National Natural Science Foundation of China (No. 21502055), the Guangzhou Science and Technology Project Scientific Special (General Items, No. 201607010251), Applied Science and Technology Research, Development Special Foundation of Guangdong Province (No. 2016B090930004), the Open Fund of the Key Laboratory of Functional Molecular Engineering of Guangdong Province in SCUT (2017kf01) and Guangdong Provincial Science and Technology Project (No. 2017A010103016) is greatly appreciated.

REFERENCES

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(1) Cui, Y.-M.; Lin, Y.; Xu, L.-W. Catalytic synthesis of chiral organoheteroatom compounds of silicon, phosphorus, and sulfur via asymmetric transition metal-catalyzed C-H functionalization. Coord. Chem. Rev. 2017, 330, 37-52. DOI: 10.1016/j.ccr.2016.09.011. (2) Liu, Y.; Zhang, J.-L.; Song R.-J.; Li, J.-H. Sulfur Incorporation: Copper-catalyzed cascade cyclization of 1,7-enynes with metal sulfides toward thieno[3,4-c]quinolin-4(5H)-ones. Org. Lett. 2014, 16 (22), 5838-5841. DOI: 10.1021/ol5025162. (3) Hou, W. D.; Wei, Q.; Liu, G. S.; Chen, J.; Guo J.; Peng, Y. G. Asymmetric multicomponent sulfa-michael/mannich cascade reaction: synthetic access to 1,2-diamino-3-organosulfur compounds and 2-nitro allylic amines. Org. Lett. 2015, 17 (19), 4870-4873. DOI: 10.1021/acs.orglett.5b02423. (4) Pulis, A. P.; Procter, D. J. C-H coupling reactions directed by sulfoxides: teaching an old functional group new tricks. Angew. Chem. Int. Ed. 2016, 55 (34), 9842-9860. DOI: 10.1002/anie.201601540. (5) Desnoyer A. N.; Love, J. A. Recent advances in well-defined, late transition metal complexes that make and/or break C-N, C-O and C-S bonds. Chem. Soc. Rev. 2017, 46 (1), 197-238. DOI: 10.1039/c6cs00150e. (6) Lian, Z.; Bhawal, B. N.; Yu, P.; Morandi, B. Palladium-catalyzed carbon-sulfur or carbonphosphorus bond metathesis by reversible arylation. Science 2017, 356 (6342), 1059-1063. DOI: 10.1126/science.aam9041. (7) Baunach, M.; Ding, L.; Willing K.; Hertweck, C. Bacterial synthesis of unusual sulfonamide and sulfone antibiotics by flavoenzyme-mediated sulfur dioxide capture. Angew. Chem. Int. Ed. 2015, 54 (45), 13279-13283. DOI: 10.1002/anie.201506541.

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An environmental-friendly protocol for the synthesis of sulfones from Csp2-X compounds and sodium sulfinates via a radical pathway is described.

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