Mild Copper–TBAF-Catalyzed N-Arylation of Sulfoximines with Aryl

Aug 21, 2014 - as availability, low toxicity, ease of handling, high stability, and environmental benignity under mild reaction conditions, thus openi...
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Mild Copper−TBAF-Catalyzed N‑Arylation of Sulfoximines with Aryl Siloxanes Jaeeun Kim, Jinpyo Ok, Sanghyuck Kim, Wonseok Choi, and Phil Ho Lee* Department of Chemistry, Kangwon National University, Chuncheon 200-701, Republic of Korea S Supporting Information *

ABSTRACT: An efficient copper−TBAF-catalyzed C−N bond formation of sulfoximines with arylsiloxanes in dichloromethane at room temperature, affording the desired N-aryl sulfoximines in good to excellent yields under an oxygen atmosphere, is reported. This method complements the existing synthetic approaches due to some advantageous properties of arylsiloxanes such as availability, low toxicity, ease of handling, high stability, and environmental benignity under mild reaction conditions, thus opening a new approach to practical C−N bond formation.

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ecause sulfoximines are valuable privileged structures that have been applied as biologically active compounds1 and chiral auxiliaries and ligands in asymmetric synthesis,2 the development of novel synthetic methodologies for preparing sulfoximines and their functionalizations is extremely desirable. Thus, functionalization of sulfoximines via the reaction with easily available starting materials has been continuously investigated.3 Since 1998, Bolm and co-workers have reported a variety of N-arylation of sulfoximines via transition-metalcatalyzed cross-coupling reactions (Scheme 1): N-arylation of

However, because sulfoximines exhibit lower nucleophilic character at the nitrogen than amines and imines, development of a new C−N bond formation of sulfoximines still represents a formidable challenge. Recently, Cheng and co-workers reported an efficient C−N bond formation via Cu−TBAF-catalyzed arylation of amines and amides with aryl trimethoxysilane.8 Therefore, we envisioned that organosilicon reagents would be excellent coupling partners in transition-metal-catalyzed Narylation of sulfoximines due to their significant advantages involving low toxicity, ease of handling, high stability, and environmental benignity.9 Recently, we were interested in a transition-metal-catalyzed C−N bond formation.10 These results prompted us to investigate the feasibility of an organosilicon reagent in reaction with sulfoximines for C−N bond formation. In this paper, we report an efficient copperTBAF-catalyzed N-arylation of sulfoximines with aryl siloxanes under mild conditions (eq 5). First, we started our investigation with an N-arylation reaction of sulfoximine (1a) with trimethoxyphenylsilane (2a) under an oxygen atmosphere (Table 1, see the Supporting Information). When CuI and tetrabutylammonium fluoride trihydrate (TBAF·3H2O) (10 mol % each) as catalyst were used, dichloromethane (DCM) gave the best result (84%) among solvents such as dichloroethane (DCE), toluene, MeOH, DMSO, and DCM (entries 1−5). CuI provided most the desirable result among the catalysts such as CuI, CuBr, CuCl, CuOAc, and Cu(OAc)2 in the presence of (TBAF· 3H2O) (10 mol %) in DCM (entries 5−9). The N-arylation reaction did not take place without (TBAF·3H2O) (entry 10). The use of CuI is critically significant for a successful Narylation (entry 12). The use of molecular oxygen was also essential for the reaction to proceed. Hence, the N-arylation did

Scheme 1. Metal-Catalyzed Synthesis of N-Arylsulfoximines

sulfoximines using aryl triflates, nonaflates, tosylates, iodides, and bromides under palladium, copper, nickel, and iron catalysis (eq 1) 4 and copper-catalyzed N-arylation of sulfoximines using aryl boronic acids (eq 2).5 Recently, Varma and co-workers described the copper-catalyzed Narylation of sulfoximines using diaryliodonium triflates and ultrasound (eq 3).6 Moreover, Bolm and Miura have developed N-arylation of sulfoximines via copper-catalyzed C−H activation with azoles and polyfluoroarenes (eq 4).7 © 2014 American Chemical Society

Received: July 23, 2014 Published: August 21, 2014 4602

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Table 1. Optimization of N-Arylation of Sulfoximinesa

entry

cat.

solvent

yieldb (%)

1 2 3 4 5 6 7 8 9 10 11d 12 13f 14g

CuI, TBAF·3H2O CuI, TBAF·3H2O CuI, TBAF·3H2O CuI, TBAF·3H2O CuI, TBAF·3H2O CuOAc, TBAF·3H2O CuBr, TBAF·3H2O CuCl, TBAF·3H2O Cu(OAc)2, TBAF·3H2O CuI CuI, TBAF·3H2O TBAF·3H2O CuI, TBAF·3H2O CuI, TBAF·3H2O

DCE toluene MeOH DMSO CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2

78 0 0 42 84 72 50 61 57 (36)c 0 91 (90)e 0 0 0

Scheme 2. N-Arylation of Substituted Sulfoximines with Trimethoxyphenylsilanea

a Reactions were carried out with 1a (0.2 mmol), 2a (0.3 mmol, 1.5 equiv), TBAF·3H2O, and Cu catalyst (10 mol % each) in solvent (1.0 mL) under O2 at room temperature for 12 h. bNMR yield using CH2Br2 as an internal standard. cNMR yield of 1a. d2a (0.35 mmol, 1.75 equiv) was used. eIsolated yield. fUnder N2. gUnder air.

not occur under nitrogen and air atmosphere (entries 13 and 14). The optimal reaction conditions could be obtained from the reaction of 1a (1 equiv) with 2a (1.75 equiv) in the presence of CuI and (TBAF·3H2O) (10 mol % each) in DCM for 12 h at room temperature under an oxygen atmosphere (1 atm), leading to the formation of 3aa in 90% isolated yield (entry 11). On the basis of the optimal reaction conditions, we next examined the scope and limitations of the N-arylation with respect to sulfoximines 1 to react with trimethoxyphenylsilane (2a) in Scheme 2. Electronic variation of a wide range of substituents on the arene moiety of sulfoximines 1 had little effect on the reaction efficiency. In fact, sulfoximines (1b, 1c, and 1d) having electron-donating p-, m-, and o-methyl groups on the phenyl ring were smoothly transformed to the desirable S-aryl-N-phenylsulfoximines (3ba, 3ca, and 3da) in good to excellent yields under mild conditions. 3,5-Dimethylsulfoximine (1e) was less reactive and then provided 3ea in 76% yield with CuI (20 mol %). 4- and 3-methoxyphenyl-substituted sulfoximines (1f and 1g) underwent the N-arylation reaction, producing N-phenylsulfoximines (3fa and 3ga) in 78% and 81% yields, respectively. Sulfoximines bearing chloro (1h and 1i) and bromo (1j and 1k) groups were all smoothly N-arylated to afford 3ha−ka in high yields ranging from 84% to 89%. The tolerance of chloro and bromo groups is especially significant, as sequentially catalytic cross-coupling reactions are possible. Despite the presence of strong electron-withdrawing nitro and trifluoromethyl groups, the N-phenylated sulfoximines (3la and 3ma) were obtained in 88% and 89% yields, respectively. NPhenylation also took place in S-2-naphthyl sulfoximine (1n), leading to the formation of S-2-naphthyl-N-phenylsulfoximine (3na) in 76% yield. Likewise, phenyl ethyl sulfoximine (1o) worked well with CuI (20 mol %). S,S-Dimethylsulfoximine (1p) also was N-arylated, producing the desired product 3pa in 81% yield. To demonstrate the applicability of the present method to a larger scale processes, a 2.0 mmol scale reaction of phenylsulfoximine 1a (0.31 g) was undertaken with trimethoxyphe-

a

Reaction conditions: 1a (0.2 mmol), 2 (1.75 equiv), CuI, TBAF· 3H2O (10 mol % each), and CH2Cl2 (1 mL) under O2 at room temperature for 12 h. bReaction scale is 2 mmol. cCuI (20 mol %) was used for 24 h.

nylsilane 2a (1.75 equiv) in the presence of CuI and TBAF· 3H2O (10 mol % each) under the optimal conditions, producing the N-arylated compound 3aa in 86% yield (0.40 g). Next, we turned our attention to the N-arylation of S-methylS-phenylsulfoximine 1a with a broad range of aryltrimethoxysilanes 2, and the results are summarized in Scheme 3. The electronic nature of aryl substituents in aryltrimethoxysilanes Scheme 3. N-Arylation of Sulfoximine with Various Organosilanes

a

CuI (20 mol %) was used for 24 h. Recovery yield of starting material.

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lane 2a was subjected to S-4-methoxyphenyl-S-methylsulfoximine 1f and S-methyl-S-4-nitrophenylsulfoximine 1l to provide the N-arylation products 3fa (34%) and 3la (72%). A competition experiment between aryltrimethoxysilanes (2d and 2g) having 4-methoxy and 4-fluoro groups produced the N-aryl-S-methyl-S-phenylsulfoximines 3ad (37%) and 3ag (59%) (b). These results indicate that electron-deficient sulfoximines and arylsiloxanes are more reactive in C−N bond formation. Because the catalytic cycle of copper and TBAF is unclear at the present stage, the elucidation of the detailed reaction mechanism must wait further study. In conclusion, we have developed an efficient copper− TBAF-catalyzed coupling reaction of sulfoximines with arylsiloxanes in dichloromethane at room temperature to produce the desired N-arylsulfoximines in good to excellent yields under an oxygen atmosphere. The present method complements the previously reported synthetic approaches due to some advantageous properties of aryl siloxanes such as availability, low toxicity, ease of handling, high stability, and environmentally benignity under mild conditions, thus opening a new approach to practical C−N bond formation.

had little effect on the reaction efficiency. Thus, the N-arylation products were produced in good to excellent yields from aryltrimethoxysilanes having not only electron-donating methyl and methoxy but also electron-withdrawing chloro and fluoro groups. It was noteworthy that the chloro group in 4chlorophenyltrimethoxysilane 2f could be tolerated under the reaction conditions, enabling further functional group transformation. On the other hand, an o-methyl group at the phenyl ring of trimethoxy-2-methylphenylsilane 2c slightly decreased the reactivity, presumably due to steric reasons, and then produced 3ac in 66% yield together with 1a (15%) with 20 mol % of CuI. Trimethoxy-2-thienylsilane 2h could also be used as the cross-coupling partner and delivered the N-2-thienylated product 3ah in quantitative yield. The scope and limitations of the N-arylation reaction with respect to a myriad of S-aryl-S-methylsulfoximines 1 and aryltrimethoxysilanes 2 were next investigated under optimal conditions (Scheme 4). When trimethoxy-4-methylphenylsilane Scheme 4. N-Arylation of Substituted Sulfoximines with Organosilanes

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures, characterization data, and 1H and 13C NMR spectra for new compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

2b was treated with 4-methyl-, 4-methoxy-, and 3-chlorosubstituted sulfoximines, the N-arylation proceeded smoothly to afford the desired products (3bb, 3fb, and 3ib). Likewise, aryltrimethoxysilanes (2f and 2g) having chloro and fluoro substituents underwent effectively the N-arylation reaction, thus producing the corresponding products in good yields ranging from 75% and 88%. Competition experiments between S-aryl-S-methylsulfoximines 1 were examined [Scheme 5, (a)]. Trimethoxyphenylsi-

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2014001403).

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Scheme 5. Competitive Reaction Profiles

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Reaction conditions: 1f (0.2 mmol), 1l (0.2 mmol), 2a (1.75 equiv), CuI, TBAF·3H2O (10 mol % each), and CH2Cl2 (1 mL) under O2 at room temperature for 12 h. NMR yield using CH2Br2 as an internal standard. bNMR yield of 1. cReaction conditions: 1a (0.2 mmol), 2d (1.75 equiv), and 2g (1.75 equiv). 4604

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