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Cite this: Org. Biomol. Chem., 2014, 12, 6349 Received 14th June 2014, Accepted 8th July 2014 DOI: 10.1039/c4ob01231c www.rsc.org/obc

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Copper-catalyzed arylsulfonylation of N-arylsulfonyl-acrylamides with arylsulfonohydrazides: synthesis of sulfonated oxindoles† Qingshan Tian,a Ping Hea and Chunxiang Kuang*a,b

A copper-catalyzed arylsulfonylation of N-arylsulfonyl-acrylamides with sulfonylhydrazides through a tandem radical process was developed. This methodology provided an alternative strategy for the synthesis of sulfonated oxindoles by forming C–S, C–N and C–C bonds in a single operation.

Organosulfones play very important roles in organic chemistry. They can be found in a wide range of agrochemicals and pharmaceuticals.1 In addition, the sulfone group is regarded as a highly valuable class of precursors for various chemical transformations in organic synthesis.2 In general, the main synthetic route for the preparation of organosulfones is the oxidation of the corresponding sulfides.3 The reaction of organomagnesium halide compounds with sulfonate esters has also been described.4 Moreover, transition-metal-catalyzed synthesis of organosulfones from aryl halides or triflates has also been developed.5 However, these methods suffer from harsh reaction conditions, isomeric products and poor tolerance of functional groups. Therefore, the development of a more concise and efficient method for organosulfone synthesis is highly desirable. Recently, direct addition of a sulfonyl radical to C–C multiple bonds provided a particularly useful contribution to the synthesis of organosulfones.6 For example, Lei et al.’s group reported the aerobic sulfonylation of alkenes and alkynes using aryl sulfinic acids as sulfonyl radical precursors in the presence of pyridine and O2.7 Recently, Wang et al. also reported a copper-catalyzed oxysulfonylation of alkenes with dioxygen and sulfonylhydrazides, leading to β-ketosulfones in good yields.8 Furthermore, the addition of a sulfonyl radical to C–C double bonds has been extensively studied.9 Oxindoles and their derivatives represent a large class of N-heterocycles, which exhibit advanced pharmaceutical and

a Department of Chemistry, Tongji University, Siping Road 1239, Shanghai 200092, P. R. China. E-mail: [email protected] b Key Laboratory of Yangtze River Water Environment, Ministry of Education, Siping Road 1239, Shanghai 200092, P. R. China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ob01231c

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bioactivities.10 Recently, difunctionalizaton of alkenes provided a powerful strategy for the synthesis of various organic compounds.11 In particular, transition metal catalyzed difunctionalization of alkenes provides some versatile strategies for the synthesis of various functionalized oxindoles. For example, Fe,12 Ag,13 Cu,14 and Pd15 catalyzed oxidative tandem difunctionalization/cyclization reactions of alkenes in arylacrylamides have been independently developed. However, examples of arylsulfonylation of activated alkenes via the radical pathway to prepare sulfonated oxindoles are quite rare. Jiao’s and Li’s groups have independently developed oxidative arylsulfonylation of activated alkenes to synthesize a variety of sulfonated oxindoles through a carbon radical process; however, these methods could not synthesize 6-substituted oxindoles efficiently (Scheme 1a).16 Recently, Nevado’s group developed a new tandem radical strategy for the construction of functionalized oxindoles through an amidyl radical pathway (Scheme 1b).17 Based on these studies, we report an alternative strategy for the synthesis of sulfonated oxindoles; especially, the synthesis of various 6-substituted sulfonated oxindoles was more efficient than previous methods. In this reaction, the sul-

Scheme 1

Difunctionalization of alkenes.

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fonyl radical first formed with the catalysis of Cu(OTf )2 from arylhydrazides using potassium persulfate as the terminal oxidant, and then the sulfonyl radical triggers the tandem radical process to construct sulfonated oxindoles through an amidyl radical intermediate (Scheme 1c). We initiated our studies by examining N-methyl-N-phenylsulfonylmethacrylamide (1a) with tosyl hydrazide (TsNHNH2) in the presence of a catalytic amount of copper(I) chloride and K2S2O8 (3 equiv.) as the oxidant at 80 °C for 16 hours (Table 1). Gratifyingly, the desired product (3a) was isolated in 38% yield (Table 1, entry 1). Inspired by this result, we investigated various copper salts in this reaction, such as CuBr, CuI, Cu(OAc)2, CuSO4, and Cu(OTf )2 (Table 1, entries 2–6). The results showed that Cu(OTf )2 was more efficient than the other copper salts, and the desired product (3a) was isolated in 81% yield (Table 1, entry 6). In addition, decreasing the temperature caused the yield to descend (Table 1, entry 7). No evident improvement in yield was obtained when the reaction was conducted at an elevated temperature (100 °C) (Table 1, entry 8). Various solvents were also examined (CH3CN, H2O, DMF and DMSO) and the result showed that water as a co-solvent is beneficial for this reaction (Table 1, entries 9–12), presumably owing to its ability to improve the solubility of potassium per-

sulfate. No improvement was achieved by increasing catalyst loading to 40 mol% (Table 1, entry 13). In contrast, when catalyst loading decreased to 10 mol%, the yield was reduced to 61% (Table 1, entry 14). The screening results of oxidants, such as DTBP, DCP, and TBHP, indicated that K2S2O8 was the more efficient oxidant in this reaction (Table 1, entries 15–18). The control experiment showed that the presence of Cu(OTf )2 is essential in this reaction (Table 1, entry 19). With the optimized reaction conditions in hand, we next investigated the scope of sulfonohydrazides in this reaction system (Table 2). In most cases, arylsulfonohydrazides were smoothly converted to sulfonated oxindoles in moderate to good yields. Arylsulfonohydrazides with electron-donating groups reacted well and a majority of the products (3a–3c) were obtained in good yields. In addition, arylsulfonohydrazides with electron-withdrawing groups also produced the desired compounds (3i–3k) in moderate yields. Gratifyingly, arylsulfonohydrazides with halogen atoms (F, Cl, Br) were well tolerated and provided the corresponding products in good yields (3d–3h). When the benzene ring of the substrates was changed to a naphthalene ring, the reaction successfully provided the desired product 3l in good yield. However, the

Table 2 Table 1

Copper-catalyzed reaction of 1a with arylsulfonohydrazidesa

Optimization of the reaction conditionsa

Entry

Catalyst (mol%)

Oxidant

Yieldb (%)

1 2 3 4 5 6 7c 8d 9e 10 f 11g 12h 13 14 15 16 17 18 19

CuCl (20) CuBr (20) CuI (20) Cu(OAc)2 (20) CuSO4 (20) Cu(OTf)2 (20) Cu(OTf)2 (20) Cu(OTf)2 (20) Cu(OTf)2 (20) Cu(OTf)2 (20) Cu(OTf)2 (20) Cu(OTf)2 (20) Cu(OTf)2 (40) Cu(OTf)2 (10) Cu(OTf)2 (20) Cu(OTf)2 (20) Cu(OTf)2 (20) Cu(OTf)2 (20)

K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8

38 41 58 20 35 81 49 83 21 37 55 68 80 61 Trace 50 44 53 15

DTBP DCP TBHP K2S2O8

a

Reaction conditions: 1a (0.2 mmol), PhSO2NHNH2 (0.4 mmol), copper catalyst (0.04 mmol) and oxidant (0.6 mmol) in CH3CN–H2O (2/1, 1 mL), 80 °C under Ar for 16 h. b Isolated yields. c Reaction temperature: 60 °C. d Reaction temperature: 100 °C. e CH3CN used as a solvent. f H2O used as a solvent. g DMF used as a solvent. h DMSO used as a solvent. TBHP = tert-butyl hydrogen peroxide (70% in water), DCP = dicumyl peroxide, DTBP = di-tert-butyl peroxide.

6350 | Org. Biomol. Chem., 2014, 12, 6349–6353

a

Reaction conditions: 1a (0.2 mmol), 2 (0.4 mmol), Cu(OTf)2 (0.04 mmol) and K2S2O8 (0.6 mmol) in CH3CN–H2O (2/1, 1.0 mL), stirred at 80 °C for 18 h. Isolated yields are given.

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desired products were not obtained from the reaction of 1a with alkyl-substituted sulfonohydrazides, such as ethyl sulfonohydrazide. We next explored the scope of N-arylsulfonylacrylamides in the reaction system (Table 3). In general, the reaction tolerated a wide variety of substrates with both electron-donating and electron-withdrawing substituents to produce the desired products (4a–4m) in moderate to good yields. Substrates bearing electron-donating groups afforded the desired products in high yields (4a–4c). It was worth noting that halogen atoms (F, Br) on substrates were also tolerated in this reaction (4d, 4e); thus these halogen atoms enabled further elaboration of the products into more complex molecules. In addition, substrates containing electron-withdrawing substituents were also amenable to the reaction conditions, offering the corresponding products (4d, 4f ) in moderate yields. Highly electrondeficient substituents (NO2 and CN) on the aryl moiety did not result in any desired products. When the benzene ring of the substrate was changed to a naphthalene ring, the reaction successfully provided the desired product 4g in good yield. Furthermore, different protecting groups on the nitrogen atom,

Table 3

Copper-catalyzed reaction of tosylhydrazide with 1a

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including butyl, cyclohexyl and benzyl groups, showed good tolerance under the optimized reaction conditions, providing the corresponding oxindoles (4h–4j) in moderate yields. Notably, N-acetyl and N-free substrates were not reactive at all (4k and 4l). Meanwhile, no reaction occurred in the case of the monosubstituted alkene in the substrate (R3 = H) (4m). Several copper-catalyzed difunctionalizations of alkenes are well known to proceed through a radical pathway;8,14 thus, our method for the arylsulfonylation of N-arylsulfonylacrylamides may also proceed through a radical pathway. In order to gain more insights into the reaction mechanism, we used the wellknown radical-trapping reagents TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) and BHT (2,6-di-tert-butyl-4-methylphenol) to elucidate the mechanism. As illustrated in Scheme 2, the addition of 2.5 equiv. of TEMPO and BHT suppressed the arylsulfonylation process, thus suggesting that this transformation might involve a radical process. We also explored the regioselectivity of this radical reaction. Substrate (5) with a benzene ring on the nitrogen atom of N-arylsulfonylacrylamides offered three sites for cyclization (a, b and 5-ipso sites); sites a and b would produce an oxindole derivate (6b) and a 6-member ring product (6c) respectively. Interestingly, the reaction proceeded on the 5-ipso site with high site control and a single product 6a was isolated in 64% yield. Presumably, an amidyl radical in this reaction can undergo hydrogen abstraction from the medium to give amide 6a.18 Based on the experimental results and previous studies, we tentatively proposed a reaction mechanism (Scheme 3).

Scheme 2

Mechanism consideration and regioselectivity experiment.

Scheme 3

A plausible reaction mechanism.

a

Reaction conditions: 1 (0.2 mmol), TsNHNH2 (0.4 mmol), Cu(OTf)2 (0.04 mmol) and K2S2O8 (0.4 mmol) in CH3CN–H2O (2/1, 1.0 mL), stirred at 80 °C for 18 h. Isolated yields are given.

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Initially, the sulfonyl radical 7 was generated from 2 via single electron transfer and the deprotonation process. Then, the sulfonyl radical 7 was added to substrate 1a, leading to a new C–S bond formation and a radical intermediate 8. A 5-ipso cyclization then took place on the aromatic ring, generating a C–C bond and aryl radical 9, which would undergo rapid desulfonylation to form the key amidyl radical 10 with the extrusion of sulfur dioxide. The amidyl radical 10 would then undergo cyclization onto the aromatic ring to produce oxindole 3. In this transformation, the oxidant was required in stoichiometric amounts to complete the final re-aromatisation of the intermediate of 11. In conclusion, we have demonstrated a copper-catalyzed arylsulfonylation of N-arylsulfonylacrylamides with readily available arylhydrazides. This tandem radical reaction provides an alternative strategy to construct three bonds in sulfonated oxindoles via sulfonylation, 5-ipso-cyclization, aryl migration, desulfonylation and amidyl radical cyclization. Especially, the synthesis of various 6-substituted sulfonated oxindoles was more efficient than by previous methods.

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7

8 9

10

11

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Org. Biomol. Chem., 2014, 12, 6349–6353 | 6353

Copper-catalyzed arylsulfonylation of N-arylsulfonyl-acrylamides with arylsulfonohydrazides: synthesis of sulfonated oxindoles.

A copper-catalyzed arylsulfonylation of N-arylsulfonyl-acrylamides with sulfonylhydrazides through a tandem radical process was developed. This method...
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