Facile synthesis of 4,4'-bis-sydnones

An efficient method is developed for direct dehydrogenative C–C coupling of 3-arylsydnones in the presence of palladium(II) acetate catalyst under oxygen. The reaction proceeds efficiently and leads to the formation of bis-sydnones in high yields


Introduction
Sydnones, unique, dipolar, and five-membered heterocycles, 1 are of interest because of their diverse biological activities, such as antimicrobial, 2 anti-inflammatory, 3 antitumor, 4 antimalarial, 5 analgesic, and antipyretic. 6 These compounds are also known for their potential applications in therapeutic agents, 7-8 liquid crystals, [9][10] and electrolytic solvents. 11 Previous research on the synthesis of bis-sydnone focused on the use of 4-halosydnone or mercuric chloride derivatives of sydnone as starting materials, [12][13] which often need to be synthesized. These methods leads generally to low yields. Specifically, the multi-step synthesis of bis-(3phenylsydnone) which involved the use of butyllithium to form unstable lithiosydnone species and the addition of copper (I) bromide or copper (II) chloride under harsh condition (−78 °C) resulted in low yield (25%) products. 14 Recently, a series of new synthetic methods for the direct functionalization of 3-arylsydnones have been developed. [15][16][17] However, there is still no literature reporting the preparation of bis-sydnones. Therefore, it is desirable to develop a convenient and practical synthetic route for the synthesis of them.
C-H bond activation catalyzed by transition metals and C-C bond formation, which features high atom economy and bond formation efficiency, have attracted much attention in recent years. [18][19][20][21] Direct cross-dehydrogenative coupling of C-H bonds, which avoids prefunctionalization of the substrates has been successfully used to construct organic molecules. [22][23] In this paper, we report a novel method for the synthesis of bis-sydnones (2) via direct crossdehydrogenative coupling of 3-arylsydnones (1) in the presence of Pd (II) catalyst under oxygen (Scheme 1).

Results and Discussion
To establish the optimal reaction conditions, we selected 3-(4-methoxyphenyl)sydnone (1a) as the model substrate. A mixture of 1a (0.3 mmol), Pd(OAc) 2 (0.03 mmol), and Cu(OAc) 2 (0.6 mmol) was mixed with 2 mL of DMSO in a sealed tube and stirred in an oil bath at 120 °C. After 16 h, product 2a was obtained in 93% yield (Table 1, entry 1). The effects of different catalysts and oxidants in different solvents at various reaction temperatures on the formation of 2a were investigated. Table 1 shows that using toluene, 1,2-dichloroethane (DCE), 1,4-dioxane, N,Ndimethylformamide (DMF), and acetonitrile as solvents, instead of DMSO, the yield of 2a (Table 1, entries 2-6) was decreased. Thus, DMSO was determined to be the best solvent. Oxidant screening showed that an improved chemical yield could be obtained by using O 2 (1 atm) as the oxidant, whereas other oxidants, such as K 2 S 2 O 8 , Ag 2 CO 3 , AgOAc, and benzoquinone were less effective ( Table 1, entries [8][9][10][11][12]. When no oxidant was used in the reaction, the yield decreased to 63% (Table 1, entry 7). Therefore, O 2 (1 atm) was chosen as the ideal oxidant. The effect of various dosage of Pd(OAc) 2 was further investigated. When the dosage of Pd(OAc) 2 was reduced to 0.05 and 0.02 eq., the yield was decreased ( Table 1, entries [13][14]. Different reaction temperatures were also examined. When the reaction temperature was decreased, the yield dropped dramatically ( Entry A variety of 3-arylsydnone (1) substrates were also investigated under the optimized conditions. Table 2 shows that bis-sydnones (2) were efficiently generated with moderate to good yields. The reaction proceeded efficiently when the aryl group of 3-arylsydnone (1) is a phenyl that carries an electron-donating group such as methoxy (1a) or methyl (1b and 1c), less so with an electron-withdrawing substituent such as halogen (1e-1h). The presence of an electron-donating group in the phenyl group of 3-arylsydnone (1a-c) increased the reaction yield compared with 1d ( Table 2, entries 1-3). In contrast, the presence of one or two electronwithdrawing substituents in the phenyl group (1e-1h) generated relatively low yields of product 2 ( Table 2, entries 5-8). Furthermore, higher yields were obtained when the substrates contained an electron-donating substituent at the para position than that at the meta position. This finding could be attributed to the ability of the electron-donating group at the para position to exert a stronger electron-donating inductive effect than the group at the meta position (Table 2, entries 2,3). This behavior was contrary to that of substrates (1) containing electron-withdrawing groups ( Table 2, entries 6,7). We have also tested the reactivity of 3-(4-nitrophenyl)sydnone under the optimal conditions, but the desired product 3,3'-bis(4-nitrophenyl)-4,4'-bis-sydnone (2i) was not found (Table 2, entry 9). It appears that the strong electron-withdrawing inductive effect of the nitro group greatly reduced its reactivity. A plausible reaction mechanism for the synthesis of bis-sydnones (2) is shown in Scheme 2. The initial electronic attack of Pd (II) on 3-arylsydnone (1) and the subsequent deprotonation form species A. [24][25] The resulting species A reacted with another 3-arylsydnone molecule to generate the corresponding intermediate B. 26 The intermediate was then subjected to reductive elimination to synthesize the product 2 and generate Pd 0 . Finally, Pd(II) was regenerated through the oxidation of Pd 0 .

Conclusions
In summary, we synthesized bis-sydnones in moderate to high yields through direct crossdehydrogenative coupling of 3-arylsydnones catalyzed by Pd under oxygen. It was convenient, efficient, and environment friendly and considered an atom-and step-economical process.

Experimental Section
General. All commercially available reagents and solvent were obtained from the commercial providers and used without further purification. Melting points were recorded using a WRS-2A melting point apparatus. IR spectra were obtained on a Nexus FT-IR spectrophotometer. 1 H NMR and 13 C NMR spectra were recorded using a Varian 400 MHz spectrometer. Chemical shifts were reported relative to internal tetramethylsilane (0.00 ppm) for 1 H and CDCl 3 (77.0 ppm) for 13 C. High-resolution mass spectra were obtained on a Finnigan-NAT GC/MS/DS 8430 spectrometer. 3-Arylsydnones 1 were prepared according to literature procedures. [27][28][29] General procedure for the synthesis of 2. A mixture of 3-arylsydnone (0.3 mmol) and Pd(OAc) 2 (0.03 mmol) in dimethyl sulfoxide (DMSO, 2 mL) was placed in a tube. The tube was equipped with a balloon filled with O 2 (1 atm). The mixture was stirred in an oil bath at 120 °C for 16 h. After the reaction was completed (as monitored by TLC), the mixture was cooled to room temperature. The reaction mixture was then added with 30 mL of water and extracted with EtOAc three times (20 mL of each extraction). The combined organic layers were washed with saturated brine, dried with anhydrous Na 2 SO 4 , and concentrated under reduced pressure. The resulting residue was purified through flash column chromatography (petroleum ether/ethyl acetate = 2:1, v/v) to yield bis-sydnones (2).