Palladium-catalyzed direct ortho-sulfonylation of azobenzenes with arylsulfonyl chlorides via C–H activation

Abstract

A highly efficient and practical procedure to direct ortho-sulfonylation of azobenzenes' C–H bond with arylsulfonyl chlorides has been developed. The method was applicable to both electron-rich and electron-deficient substrates and delivered good yields for 21 examples. This reaction provides a convenient access to ortho-sulfonylated azobenzenes under mild conditions.

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Introduction

Transition-metal-catalyzed C–C and C–heteroatom bond formation via C–H bond activation has been broadly explored because it is an efficient method to construct heterocyclic compounds.¹ Recently, more and more effort has been made to develop many techniques to control the reaction selectivity by assistance of a directing group. Among them, examples such as 2-aryloxypyridines,² quinoline N-oxide,³ arylpyrazoles,⁴ triazene azoxybenzenes,⁵ quinoline,⁶ and 2-aryl-1,2,3-triazoles group,⁷ etc. were proved to be versatile directing groups to obtain high regioselectivity on the ortho-C(sp²)–H bond. In recent years, azobenzenes have worked as a directing group to accelerate the C–H activation/functionalization process and have attracted more attention. For example, many groups have developed a palladium-catalyzed regioselective C–H bond activation of azoarenes and related compounds with alcohols,⁸ toluene,⁹ aldehydes,¹⁰ and α-oxocarboxylic acids¹¹ to synthesize ortho-acylazoarenes. But, Sun and co-workers found that they obtained ortho-alkoxyazoarenes when PhI(OAc)₂ was added as the oxidant in this reaction.¹² Also, Hao and Li's group have developed a highly efficient method to synthesize diverse cinnolines and isoquinolines through the rhodium-catalyzed oxidative C–H activation of azobenzenes and ketazines with alkynes.¹³ Similarly, azobenzene derivatives such as ortho-acyloxyazoarenes,¹⁴ ortho-sulfonamideazoarenes,¹⁵ and ortho-arylazoarenes¹⁶ have been synthesized by this method.

Aryl sulfones have attracted considerable interest for they are essential components in medicinal chemistry,¹⁷ synthetic intermediates¹⁸ and advanced organic materials.¹⁹ The increasing applications of sulfones have stimulated investigations on development of efficient processes for the synthesis of these compounds. For example, Xu reported a method of palladium-catalyzed direct sulfonylation of 2-aryloxypyridines on the ortho-position of the benzene ring using 2-pyridyloxyl as the directing group and sulfonyl chlorides as sulfonylation reagents.²⁰ Saidi has developed an efficient meta sulfonation of 2-phenylpyridines in the presence of ruthenium(II) complexes.²¹ Zhao disclosed a method of palladium-catalyzed direct sulfonylation of 2-arylpyridines on the ortho-position of the benzene ring using 2-arylpyridine as the directing group.²² Wu's group reported Pd(II)-catalyzed C–H sulfonylation of azobenzenes with arylsulfonyl chlorides using K₂S₂O₈ as oxidant.²³

As a part of our continuing efforts in C–H bond activation reactions, we have recently developed many methods to form C–S, and C–C bonds.²⁴ Based on these findings, we develop a simple and efficient procedure for the synthesis of various ortho-sulfonylated azobenzenes via palladium-catalyzed direct cross-coupling of azobenzenes with arylsulfonyl chlorides.

Results and discussion

We initiated our investigation on the model reaction of azobenzene (1a) with p-tolylsulfonyl chloride (2a) to optimize the reaction parameters (Table 1). To our delight, the C₂-sulfonylation took place in the presence of Pd(OAc)₂ (10 mol%) and K₂CO₃ (2 equiv.) in DMSO under air for 12 h, affording compound 3a in 41% yield (entry 1, Table 1). Without catalyst, the reaction could not take place at all. Thus, PdCl₂, [PdCl(allyl)]₂, Pd(COD)Cl₂, Pd(CH₃CN)₂Cl₂, CuI and CuCl were tested to catalyze this reaction, in which Pd(CH₃CN)₂Cl₂ gave the best result (entries 1–7, Table 1). K₂CO₃ was superior to other bases, such as Na₂CO₃, KOAc, NaOAc, CsCO₃, NaHCO₃ and KF (entries 8–13, Table 1). The solvent also played an important role in the reaction. Solvents such as DMF, NMP, CH₃CN, 1,4-dioxane, toluene, and DMSO were screened, and 1,4-dioxane was found to be superior to the others (entries 5 and 14–18), affording 3a in 92% yield (entry 17, Table 1). The yield decreased to 73% when the catalyst loading was reduced to 5 mol% from 10 mol% (entry 19, Table 1). After surveying a variety of catalysts, bases, solvents, and catalyst loadings, we found that the combination of 10 mol% of Pd(CH₃CN)₂Cl₂ and 2 equiv. of K₂CO₃ in 1,4-dioxane at 130 °C for 12 h served as the optimal conditions for this transformation. These results indicated that this transformation was facile and practical, as it did not require the use of strong bases, and the oxidants exclusion of air.

Table 1 Optimization of reaction conditions^a

Entry	Catalyst	Base	Solvent	Yield^b (%)
a Reaction conditions: 1a (0.5 mmol), 2a (0.6 equiv.), catalyst (10 mol%), base (2.0 equiv.), 4A MS (100 mg) and solvent (2.0 mL) under air at 130 °C for 12 h, unless otherwise noted.b Isolated yields.c Pd(CH3CN)2Cl2 (5 mol%).
1	Pd(OAc)2	K2CO3	DMSO	41
2	PdCl2	K2CO3	DMSO	33
3	[PdCl(allyl)]2	K2CO3	DMSO	39
4	Pd(COD)Cl2	K2CO3	DMSO	15
5	Pd(CH3CN)2Cl2	K2CO3	DMSO	77
6	CuI	K2CO3	DMSO	N.R.
7	CuCl	K2CO3	DMSO	N.R.
8	Pd(CH3CN)2Cl2	Na2CO3	DMSO	40
9	Pd(CH3CN)2Cl2	KOAc	DMSO	31
10	Pd(CH3CN)2Cl2	NaOAc	DMSO	35
11	Pd(CH3CN)2Cl2	Cs2CO3	DMSO	44
12	Pd(CH3CN)2Cl2	NaHCO3	DMSO	23
13	Pd(CH3CN)2Cl2	KF	DMSO	31
14	Pd(CH3CN)2Cl2	K2CO3	DMF	45
15	Pd(CH3CN)2Cl2	K2CO3	NMP	67
16	Pd(CH3CN)2Cl2	K2CO3	CH3CN	73
17	Pd(CH3CN)2Cl2	K2CO3	Dioxane	92
18	Pd(CH3CN)2Cl2	K2CO3	Toluene	21
19	Pd(CH3CN)2Cl2	K2CO3	Dioxan	73^c

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With the optimized reaction conditions in hand, the reactivities of different arylsulfonyl chlorides as the sulfonylation reagents were investigated. The results are revealed in Table 2, the C–H sulfonylation of azobenzene (1a) with arylsulfonyl chlorides could proceed smoothly and furnish the corresponding ortho-substituted products 3a–n in 63–92% yields (Table 2, entries 1–14). The substrates with a para-electron-donating group afforded the products 3a, 3g, 3n in excellent yields (92%, 84% and 91%). When arylsulfonyl chlorides were substituted at the para position with a electron-withdrawing group (such as 4-F, 4-Br, 4-methoxy, 4-Cl groups) there also afforded the products 3b, 3c, 3d and 3m in good yields (75–86%). But the substrates with a strong electron-withdrawing group (–NO₂) on para and meta position provided the corresponding product 3d, 3f in low yields (67%, 63%). Heteroarysulfonyl chlorides, such as 3,5-dimethyl-isoxazole-4-sulfonyl chloride, thiophene-2-sulfonyl chloride, 5-chloro-3-methylbenzo[b]thiophene-2-sulfonyl chloride, and methyl 2-(chlorosulfonyl)thiophene-3-carboxylate were tested as the substrates. The corresponding ortho-sulfonylation products 3h, 3i, 3j, and 3k were obtained in good yields (73–90%).

Table 2 Preparation of sulfones from various sulfinates^a

Entry	Sulfinates	Product	Yield^b (%)
a Reaction conditions: 1a (0.5 mmol), 2 (0.6 equiv.), catalyst (10 mol%), base (2.0 equiv.), 4A MS (100 mg) and solvent (2.0 mL) under air at 130 °C for 12 h, unless otherwise noted.b Isolated yields.
1			92
2			76
3			86
4			67
5			75
6			63
7			84
8			90
9			87
10			89
11			73
12			90
13			82
14			91

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After screening of different arylsulfonyl chlorides, we explored the scope of differently substituted azobenzenes. Representative azoxybenzenes were firstly synthesized and examined. The results are shown in Table 3. It was found that these electron-rich azobenzenes gave higher yields than those electron-deficient azobenzenes. For instance, the reactions of 4,4′-dimethyl-azoxybenzene, 3,3′-dimethylazoxybenzene, 4,4′-di methoxy azoxybenzene, 3,3′-di methoxy azoxybenzene, provided the corresponding products in 75–86% yields (3o–3r). The reactions of unsymmetrical azobenzenes also proceeded smoothly and gave the products which could be determined by ¹H NMR, were obtained in good yields (71% for 3t; 78% for 3u, respectively).

Table 3 Preparation of sulfones from various azoarenes^a

Entry	Azoarenes	Product	Yield^b (%)
a Reaction conditions: 1a (0.5 mmol), 2 (0.6 equiv.), catalyst (10 mol%), base (2.0 equiv.), 4A MS (100 mg) and solvent (2.0 mL) under air at 130 °C for 12 h, unless otherwise noted.b Isolated yields.
1			86
2			75
3			83
4			85
5			15
6			71
7			81

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A single crystal of 3b was obtained from trichloromethane. Its crystal structure (Fig. 1) exhibited that such palladium-catalyzed direct sulfonylation of azobenzene on the ortho-position. The crystal data, refinement parameters, bond lengths and bond angles are given in the Table S1.†

Fig. 1 ORTEP diagram of catalyst 3b showing atom labelling scheme. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at 30% probability.

In addition, when some radical scavengers such as TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) or HQ (hydroquinone) were used in this C–S coupling reaction, the reaction can be carried out smoothly. So this experiment can exclude the free radical mechanism. On the basis of these results and other previous related studies,^19–22 a plausible reaction mechanism of this palladium-catalyzed sulfonylation of an azobenzene compounds is proposed, as shown in Scheme 1. Step (i), azobenzene 1a firstly reacts with Pd(CH₃CN)₂Cl₂ to form a cyclopalladated(II) intermediate through ortho-C–H bond insertion. In step (ii), the sulfonylation could go through a direct displacement-type reaction to give the final product 3a. Meanwhile, the Pd(II) was regenerated for the next catalytic cycle.

Scheme 1 Plausible reaction mechanism.

Conclusions

In conclusion, we have developed a palladium-catalyzed direct C(sp²)–H sulfonylation of azobenzene with arylsulfonyl Chlorides. The reaction exhibits a good tolerance for a broad range of general functional groups. The present work demonstrated the utility of azobenzene as a removable directing group through the direct C–H bond activation/functionalization and deprotecting group to form ortho-sulfonylated azobenzenes.

Experimental section

General information

All reactions were run under argon in Schlenk tubes using vacuum lines. DMF, NMP, CH₃CN, 1,4-dioxane, toluene, and DMSO, analytical grade were not distilled before use. Commercial arylsulfonyl chlorides and azobenzenes were used without purification. ¹H NMR, ¹³C NMR spectra were recorded using a 500 MHz spectrometer in CDCl₃ and DMSO with shifts referenced to SiMe₄ (δ = 0). IR spectra were recorded on an FTIR spectrophotometer. Melting points were determined by using a local hot-stage melting point apparatus and are uncorrected. Elemental analyses were carried out on a CHN analyzer. Mass spectra were recorded using LC-MS and HRMS (ESI-TOF analyzer) equipment.

General procedure for preparation of azoxybenzenes

All of the azo-compounds were prepared from arylamines, according to the literature.¹ Mix CuBr (4.2 mg, 0.03 mmol), pyridine (8.7 mg, 0.09 mmol), arylamines (93 mg, 1 mmol) in toluene (4 mL) under air (1 atm). The reaction mixture was vigorously stirred at 60 °C for 20 h. After cooling down to room temperature and concentrating in vacuum, the residue was purified by flash chromatography on a short silica gel (eluent: petroleum ether) to afford azo-compound.

General procedure for palladium-catalyzed direct ortho-sulfonylation of azobenzenes with arylsulfonyl chlorides via C–H activation

Mix azoic compound (0.5 equiv.), benzene sulfonyl chloride (0.6 equiv.), Pd(CH₃CN)₂Cl₂ (10 mol%), K₂CO₃ (2 equiv.), 4A MS (100 mg) in 1,4-dioxane (2 mL) under air. The reaction mixture was vigorously stirred at 130 °C for 12 h. After cooling down to room temperature and concentrating in vacuum, the residue was purified by flash chromatography on a short silica gel to afford corresponding product.

Diazene (1a)¹

(E)-1,2-Di-p-tolyldiazene. Obtained as a yellow solid in 90% yield; M.p. 138–140 °C. ¹H NMR (500 MHz, CDCl₃) δ 7.81 (d, J = 8.3 Hz, 4H), 7.30 (d, J = 8.0 Hz, 4H), 2.42 (s, 6H). ¹³C NMR (126 MHz, CDCl₃) δ 150.85, 141.22, 129.73, 122.75, 21.50. HRMS (ESI+): calculated for C₁₄H₁₄N₂: [M + H]⁺ 211.123, found 211.1032.

(E)-1,2-Di-m-tolyldiazene. Obtained as a yellow solid in 87% yield; M.p. 123–124 °C. ¹H NMR (500 MHz, CDCl₃) δ 7.71 (s, 4H), 7.40–7.34 (m, 2H), 7.25 (d, J = 7.0 Hz, 2H), 2.42 (d, J = 3.0 Hz, 6H). ¹³C NMR (126 MHz, CDCl₃) δ 152.87, 139.00, 131.75, 128.95, 122.97, 120.54, 21.43. HRMS (ESI+): calculated for C₁₄H₁₄N₂: [M + H]⁺ 211.123, found 211.1034.

(E)-1,2-Bis(4-ethoxyphenyl)diazene. Obtained as a yellow solid in 92% yield; M.p. 150–151 °C. ¹H NMR (500 MHz, CDCl₃) δ 7.91–7.82 (m, 4H), 6.98 (d, J = 8.9 Hz, 4H), 4.10 (q, J = 7.0 Hz, 4H), 1.44 (t, J = 7.0 Hz, 6H). ¹³C NMR (126 MHz, CDCl₃) δ 161.01, 146.93, 124.36, 114.66, 63.79, 14.80. HRMS (ESI+): calculated for C₁₆H₁₈N₂O₂: [M + H]⁺ 270.1368, found 271.1373.

(E)-1,2-Bis(3-methoxyphenyl)diazene. Obtained as a yellow solid in 85% yield; M.p. 70–71 °C. ¹H NMR (500 MHz, CDCl₃) δ 7.49 (ddd, J = 7.8, 1.6, 0.9 Hz, 2H), 7.40–7.37 (m, 2H), 7.36 (t, J = 8.0 Hz, 2H), 6.98 (ddd, J = 8.2, 2.6, 0.8 Hz, 2H), 3.83 (s, 6H). ¹³C NMR (126 MHz, CDCl₃) δ 159.31, 152.79, 128.76, 116.85, 116.14, 104.69, 54.46. HRMS (ESI+): calculated for C₁₄H₁₄N₂O₂: [M + H]⁺ 243.1128, found 243.0686.

(E)-1-(3-Methoxyphenyl)-2-phenyldiazene. Obtained as a yellow solid in 60% yield; M.p.30–31 °C. ¹H NMR (500 MHz, CDCl₃) δ 7.95–7.89 (m, 2H), 7.57–7.41 (m, 6H), 7.07–7.02 (m, 1H), 3.90 (s, 3H). ¹³C NMR (126 MHz, CDCl₃) δ 160.34, 153.91, 152.61, 131.05, 129.80, 129.11, 122.89, 117.83, 117.14, 105.74, 55.50. HRMS (ESI+): calculated for C₁₃H₁₂N₂O₁: [M + H]⁺ 213.1022, found 213.061.

(E)-1-(3-Methoxyphenyl)-2-(m-tolyl)diazene. Obtained as a yellow liquid in 58% yield. ¹H NMR (500 MHz, CDCl₃) δ 7.73 (d, J = 5.8 Hz, 2H), 7.55 (ddd, J = 7.8, 1.5, 1.0 Hz, 1H), 7.46–7.44 (m, 1H), 7.41 (dd, J = 16.4, 8.3 Hz, 2H), 7.29 (d, J = 7.5 Hz, 1H), 7.03 (ddd, J = 8.2, 2.6, 0.8 Hz, 1H), 3.89 (s, 3H), 2.45 (s, 3H), 2.45 (s, 1H). ¹³C NMR (126 MHz, CDCl₃) δ 160.32, 153.94, 152.68, 138.99, 131.83, 129.76, 128.91, 122.93, 120.55, 117.72, 117.04, 105.69, 55.47, 21.37. HRMS (ESI+): calculated for C₁₄H₁₄N₂O₁: [M + H]⁺ 227.1179, found 227.075.

(E)-1-Phenyl-2-(2-tosylphenyl)diazene (3a). Obtained as a white solid in 78% yield; M.p. 157–158 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.40 (dd, J = 7.7, 1.5 Hz, 1H), 7.86–7.80 (m, 4H), 7.65 (dtd, J = 22.0, 7.5, 1.4 Hz, 2H), 7.58 (dd, J = 7.7, 1.3 Hz, 1H), 7.55–7.50 (m, 3H), 7.16 (d, J = 8.2 Hz, 2H), 2.32 (s, 3H). ¹³C NMR (126 MHz, CDCl₃) δ 152.72, 149.07, 143.91, 139.43, 138.88, 134.35, 131.97, 130.54, 129.35, 129.29, 129.11, 128.20, 123.77, 116.90, 21.51. HRMS (ESI+): calculated for C₂₃H₁₈N₂O₃S: [M + H]⁺ 336.0916, found 337.0989.

(E)-1-(2-((4-Bromophenyl)sulfonyl)phenyl)-2-phenyldiazene (3b). Obtained as a orange solid in 66% yield; M.p. 200–201 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.39 (d, J = 7.8 Hz, 1H), 7.84–7.78 (m, 4H), 7.71 (t, J = 7.6 Hz, 1H), 7.65 (t, J = 7.5 Hz, 1H), 7.60 (d, J = 7.8 Hz, 1H), 7.56–7.52 (m, 3H), 7.52–7.48 (m, 2H). ¹³C NMR (126 MHz, CDCl₃) δ 152.68, 148.96, 141.34, 138.17, 134.82, 132.20, 131.96, 130.71, 129.78, 129.42, 129.23, 128.20, 123.67, 117.06. HRMS (ESI+): calculated for C₁₈H₁₃BrN₂O₂S: [M + Na]⁺ 422.9773, found 422.9178.

(E)-1-(2-((4-Fluorophenyl)sulfonyl)phenyl)-2-phenyldiazene (3c). Obtained as a orange solid in 86% yield; M.p. 107–108 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.40 (dd, J = 7.8, 1.4 Hz, 1H), 8.00–7.95 (m, 2H), 7.83–7.79 (m, 2H), 7.70 (dd, J = 7.7, 1.5 Hz, 1H), 7.65 (td, J = 7.6, 1.4 Hz, 1H), 7.60 (dd, J = 7.8, 1.3 Hz, 1H), 7.57–7.53 (m, 3H), 7.03 (t, J = 8.6 Hz, 2H). ¹³C NMR (126 MHz, CDCl₃) δ 152.69, 148.95, 138.43, 134.70, 132.16, 131.12, 131.05, 130.67, 129.35, 129.21, 123.66, 117.04, 116.01, 115.83. HRMS (ESI+): calculated for C₁₈H₁₃FN₂O₂S: [M + Na]⁺ 341.0755, found 341.0211.

(E)-1-(2-((4-Nitrophenyl)sulfonyl)phenyl)-2-phenyldiazene (3d). Obtained as a pale yellow solid in 67% yield; M.p. 134–135 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.44 (d, J = 7.8 Hz, 1H), 8.19 (d, J = 8.2 Hz, 2H), 8.13 (d, J = 8.5 Hz, 2H), 7.74 (qd, J = 15.1, 7.5 Hz, 4H), 7.65 (d, J = 7.9 Hz, 1H), 7.55 (d, J = 5.8 Hz, 3H). ¹³C NMR (126 MHz, CDCl₃) δ 152.57, 150.15, 148.90, 148.06, 137.29, 135.42, 132.50, 130.92, 129.65, 129.36, 123.90, 123.57, 117.21. HRMS (ESI+): calculated for C₁₈H₁₃N₃O₄S: [M + H]⁺ 368.07, found 368.0128.

(E)-1-(2-((4-Methoxyphenyl)sulfonyl)phenyl)-2-phenyldiazene (3e). Obtained as a white solid in 75% yield; M.p. 139–140 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.37 (d, J = 7.7 Hz, 1H), 7.87 (dd, J = 20.9, 6.6 Hz, 4H), 7.64 (dt, J = 21.9, 7.4 Hz, 2H), 7.55 (dd, J = 12.8, 6.5 Hz, 4H), 6.82 (d, J = 8.4 Hz, 2H), 3.76 (s, 3H). ¹³C NMR (126 MHz, CDCl₃) δ 163.21, 152.75, 149.02, 139.16, 134.23, 133.81, 131.95, 130.54, 129.16, 123.75, 116.92, 113.87, 55.57. HRMS (ESI+): calculated for C₁₉H₁₆N₂O₃S: [M + H]⁺ 353.0954, found 4353.0419.

(E)-1-(2-((3-Nitrophenyl)sulfonyl)phenyl)-2-phenyldiazene (3f). Obtained as a yellow solid in 63% yield; M.p. 162–163 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.92 (s, 1H), 8.46 (d, J = 7.8 Hz, 1H), 8.32 (d, J = 8.1 Hz, 1H), 8.23 (d, J = 7.8 Hz, 1H), 7.79 (dd, J = 3.1, 2.1 Hz, 2H), 7.77–7.68 (m, 2H), 7.64 (d, J = 7.8 Hz, 1H), 7.56 (ddd, J = 9.7, 5.2, 4.3 Hz, 4H). ¹³C NMR (126 MHz, CDCl₃) δ 152.48, 148.85, 147.95, 144.55, 137.37, 135.39, 133.71, 132.55, 130.89, 130.09, 129.61, 129.39, 127.50, 123.75, 123.59, 117.29. HRMS (ESI+): calculated for C₁₈H₁₃N₃O₄S: [M + H]⁺ 368.07, found 368.0146.

(E)-1-(2-(Naphthalen-2-ylsulfonyl)phenyl)-2-phenyldiazene (3g). Obtained as a pale yellow solid in 84% yield; M.p. 139–140 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.64 (s, 1H), 8.64 (s, 1H), 8.57–8.44 (m, 1H), 8.57–8.46 (m, 1H), 7.86 (dd, J = 22.7, 8.8 Hz, 3H), 7.78 (d, J = 8.1 Hz, 3H), 7.70 (ddd, J = 9.2, 6.0, 1.8 Hz, 2H), 7.64–7.57 (m, 2H), 7.57–7.47 (m, 4H). ¹³C NMR (126 MHz, CDCl₃) δ 152.72, 149.00, 138.98, 138.59, 134.97, 134.59, 132.01, 131.95, 130.63, 130.19, 129.46, 129.17, 129.11, 128.96, 127.87, 127.36, 123.76, 123.03, 116.89. HRMS (ESI+): calculated for C₂₂H₁₆N₂O₂S: [M + H]⁺ 373.1005, found 373.0445.

(E)-3,5-Dimethyl-4-((2-(phenyldiazenyl)phenyl)sulfonyl)isoxazole (3h). Obtained as a white solid in 90% yield; M.p. 101–102 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.37 (d, J = 7.9 Hz, 1H), 7.77 (t, J = 7.6 Hz, 1H), 7.73–7.65 (m, 3H), 7.56 (dd, J = 12.3, 7.0 Hz, 4H), 2.50 (s, 3H), 2.19 (s, 3H). ¹³C NMR (126 MHz, CDCl₃) δ 174.00, 157.64, 152.98, 149.64, 137.80, 135.01, 132.19, 130.43, 129.24, 129.13, 123.30, 117.89, 117.64, 13.04, 10.81. HRMS (ESI+): calculated for C₁₇H₁₅N₃O₃S: [M + H]⁺ 342.0907, found 342.0377.

(E)-1-Phenyl-2-(2-(thiophen-2-ylsulfonyl)phenyl)diazene (3i). Obtained as a white solid in 87% yield; M.p. 100–101 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.37 (d, J = 8.0 Hz, 1H), 8.06–7.98 (m, 2H), 7.82–7.77 (m, 1H), 7.76–7.69 (m, 2H), 7.68–7.63 (m, 1H), 7.63–7.53 (m, 4H), 7.00 (t, J = 4.4 Hz, 1H). ¹³C NMR (126 MHz, CDCl₃) δ 152.67, 149.00, 143.44, 139.35, 134.53, 134.38, 134.06, 132.12, 130.82, 129.25, 129.21, 127.23, 124.09, 116.89. HRMS (ESI+): calculated for C₁₆H₁₂N₂O₂S₂: [M + H]⁺ 329.0413, found 329.0336.

(E)-1-(2-((5-Chloro-3-methylbenzo[b]thiophen-2-yl)sulfonyl)phenyl)-2-phenyldiazene (3j). Obtained as a pale yellow solid in 89% yield; M.p. 156–157 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.45 (d, J = 7.7 Hz, 1H), 7.87 (d, J = 7.5 Hz, 1H), 7.76 (t, J = 7.6 Hz, 1H), 7.73–7.63 (m, 1H), 7.52 (q, J = 5.3 Hz, 1H), 7.38 (d, J = 8.7 Hz, 1H), 2.51 (d, J = 0.6 Hz, 1H). ¹³C NMR (126 MHz, CDCl₃) δ 152.46, 149.41, 140.55, 139.83, 138.46, 137.09, 134.91, 132.15, 131.31, 130.53, 129.58, 128.98, 127.89, 124.12, 123.63, 123.21, 117.17, 12.35. HRMS (ESI+): calculated for C₂₁H₁₅N₂O₂S₂: [M + H]⁺ 427.0336, found 426.9721.

(E)-Methyl3-((2-(phenyldiazenyl)phenyl)sulfonyl)thiophene-2-carboxylate (3k). Obtained as a pale yellow solid in 73% yield; M.p. 92–93 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.54 (dd, J = 5.6, 3.6 Hz, 1H), 7.85 (d, J = 5.2 Hz, 1H), 7.79–7.71 (m, 1H), 7.68–7.61 (m, 1H), 7.49 (d, J = 7.6 Hz, 1H), 7.39 (d, J = 5.3 Hz, 1H), 3.71 (s, 1H). ¹³C NMR (126 MHz, CDCl₃) δ 159.44, 154.45, 152.40, 151.33, 146.00, 145.89, 139.06, 134.18, 132.02, 131.79, 130.11, 130.09, 128.90, 123.69, 116.34, 52.59. HRMS (ESI+): calculated for C₁₈H₁₄N₂O₄S₂: [M + H]⁺ 387.0468, found 386.9885.

(E)-1-Phenyl-2-(2-(phenylsulfonyl)phenyl)diazene (3l). Obtained as a orange solid in 90% yield; M.p. 149–150 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.43 (dd, J = 7.8, 1.5 Hz, 1H), 7.95 (dt, J = 6.3, 2.0 Hz, 2H), 7.83–7.75 (m, 2H), 7.68 (dtd, J = 22.5, 7.5, 1.5 Hz, 2H), 7.62–7.58 (m, 1H), 7.54–7.50 (m, 3H), 7.49–7.45 (m, 1H), 7.37 (dd, J = 10.6, 4.9 Hz, 2H). ¹³C NMR (126 MHz, CDCl₃) δ 146.56, 143.29, 142.32, 138.53, 134.56, 132.96, 132.04, 130.59, 129.46, 129.11, 128.66, 128.05, 123.77, 116.92. HRMS (ESI+): calculated for C₁₈H₁₄N₂O₄S₂: [M + H]⁺ 356.0395, found 357.0432.

(E)-1-(2-((4-Chlorophenyl)sulfonyl)phenyl)-2-phenyldiazene (3m). Obtained as a orange solid in 82% yield; M.p. 170–171 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.40 (dd, J = 7.8, 1.4 Hz, 1H), 7.93–7.86 (m, 2H), 7.84–7.76 (m, 2H), 7.71 (td, J = 7.6, 1.5 Hz, 1H), 7.65 (td, J = 7.6, 1.4 Hz, 1H), 7.63–7.58 (m, 1H), 7.57–7.51 (m, 3H), 7.37–7.30 (m, 2H). ¹³C NMR (126 MHz, CDCl₃) δ 152.65, 148.94, 140.76, 139.61, 138.19, 134.79, 132.17, 130.68, 129.69, 129.40, 129.20, 128.95, 123.65, 117.03. HRMS (ESI+): calculated for C₁₈H₁₄N₂O₄S₂: [M + H]⁺ 322.0776, found 323.0832.

(E)-1-(2-((4-(tert-Butyl)phenyl)sulfonyl)phenyl)-2-phenyldiazene (3n). Obtained as a orange solid in 91% yield; M.p. 140–141 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.41 (dd, J = 7.7, 1.5 Hz, 1H), 7.87–7.83 (m, 2H), 7.81–7.74 (m, 2H), 7.70–7.60 (m, 2H), 7.57–7.54 (m, 1H), 7.53–7.49 (m, 3H), 7.38–7.32 (m, 2H), 1.22 (d, J = 3.3 Hz, 9H). ¹³C NMR (126 MHz, CDCl₃) δ 156.81, 152.66, 149.13, 139.31, 138.77, 134.40, 131.98, 130.54, 129.28, 129.07, 127.89, 125.71, 123.78, 116.92, 35.08, 31.00. HRMS (ESI+): calculated for C₁₈H₁₄N₂O₄S₂: [M + H]⁺ 378.1402, found 379.1446.

(E)-1-(4-Methyl-2-tosylphenyl)-2-(p-tolyl)diazene (3o). Obtained as a orange solid in 86% yield; M.p. 174–175 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.23 (s, 1H), 7.87 (d, J = 8.3 Hz, 2H), 7.74 (d, J = 8.2 Hz, 2H), 7.55 (d, J = 8.1 Hz, 1H), 7.47 (dd, J = 8.1, 1.1 Hz, 1H), 7.33 (d, J = 8.1 Hz, 2H), 7.19 (d, J = 8.1 Hz, 2H), 2.54 (s, 3H), 2.48 (s, 3H), 2.34 (s, 3H). ¹³C NMR (126 MHz, CDCl₃) δ 150.91, 147.03, 143.72, 142.46, 141.26, 139.60, 138.53, 134.88, 129.73, 129.62, 129.23, 128.14, 123.73, 116.73, 21.61, 21.53, 21.45. HRMS (ESI+): calculated for C₂₁H₂₀N₂O₂S: [M + H]⁺ 365.1318, found 365.076.

(E)-1-(5-Methyl-2-tosylphenyl)-2-(m-tolyl)diazene (3p). Obtained as a white solid in 75% yield; M.p. 96–97 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.30 (d, J = 8.1 Hz, 1H), 7.86 (d, J = 8.3 Hz, 2H), 7.67 (d, J = 7.9 Hz, 1H), 7.60 (s, 1H), 7.48–7.40 (m, 2H), 7.40–7.32 (m, 2H), 7.20 (d, J = 8.1 Hz, 2H), 2.48 (d, J = 10.1 Hz, 6H), 2.35 (s, 3H). ¹³C NMR (126 MHz, CDCl₃) δ 152.80, 149.06, 145.57, 143.66, 139.72, 138.95, 135.91, 132.69, 130.96, 129.45, 129.24, 128.92, 128.04, 123.82, 121.45, 117.18, 21.62, 21.54, 21.38. HRMS (ESI+): calculated for C₂₁H₂₀N₂O₂S: [M + H]⁺ 365.1318, found 365.0756.

(E)-1-(4-Ethoxy-2-tosylphenyl)-2-(4-ethoxyphenyl)diazene (3q). Obtained as a white solid in 83% yield; M.p. 95–96 °C. ¹H NMR (500 MHz, CDCl₃) δ 7.91 (d, J = 2.7 Hz, 1H), 7.86 (d, J = 8.3 Hz, 2H), 7.79 (d, J = 8.9 Hz, 2H), 7.69 (d, J = 8.9 Hz, 1H), 7.19 (d, J = 8.1 Hz, 2H), 7.16–7.13 (m, 1H), 7.02–6.99 (m, 2H), 4.23 (q, J = 7.0 Hz, 2H), 4.16 (d, J = 7.0 Hz, 2H), 2.34 (s, 3H), 1.52 (d, J = 6.9 Hz, 3H), 1.49 (d, J = 6.9 Hz, 3H). ¹³C NMR (126 MHz, CDCl₃) δ 161.77, 160.33, 147.13, 143.71, 142.76, 140.11, 139.61, 129.22, 128.07, 125.53, 120.75, 118.35, 114.60, 113.75, 64.55, 63.90, 21.53, 14.77, 14.65. HRMS (ESI+): calculated for C₂₃H₂₄N₂O₄S: [M + H]⁺ 425.153, found 425.0923.

(E)-1-(5-Methoxy-2-tosylphenyl)-2-(3-methoxyphenyl)diazene (3r). Obtained as a orange solid in 85% yield; M.p. 139–140 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.33 (d, J = 8.7 Hz, 1H), 7.85 (d, J = 8.3 Hz, 2H), 7.55–7.49 (m, 1H), 7.45 (t, J = 8.0 Hz, 1H), 7.41–7.38 (m, 1H), 7.18 (d, J = 8.1 Hz, 2H), 7.14–7.08 (m, 3H), 3.93 (s, 3H), 3.90 (s, 3H), 2.33 (s, 3H). ¹³C NMR (126 MHz, CDCl₃) δ 164.15, 160.33, 153.79, 150.71, 143.56, 140.06, 131.45, 130.97, 129.82, 129.31, 127.80, 118.86, 118.10, 116.19, 106.73, 101.29, 55.98, 55.56, 21.51. HRMS (ESI+): calculated for C₂₁H₂₀N₂O₄S: [M + H]⁺ 397.1217, found 397.0627.

(E)-1-(3-Methoxyphenyl)-2-(2-tosylphenyl)diazene (3s). Obtained as a orange solid in 15% yield; M.p. 73–74 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.39 (dd, J = 7.7, 1.5 Hz, 1H), 7.85 (d, J = 8.3 Hz, 2H), 7.70–7.61 (m, 2H), 7.58 (dd, J = 7.7, 1.4 Hz, 1H), 7.50–7.42 (m, 2H), 7.37 (dd, J = 5.0, 3.1 Hz, 1H), 7.17 (d, J = 8.0 Hz, 2H), 7.12–7.07 (m, 1H). ¹³C NMR (126 MHz, CDCl₃) δ 160.42, 149.01, 143.93, 139.37, 138.70, 137.02, 134.38, 130.58, 129.80, 129.46, 129.35, 128.12, 118.76, 117.98, 116.93, 106.76, 55.56, 21.55. HRMS (ESI+): calculated for C₂₀H₁₈N₂O₃S: [M + H]⁺ 367.1111, found 367.0548.

(E)-1-(5-Methoxy-2-tosylphenyl)-2-phenyldiazene (3t). Obtained as a orange solid in 71% yield; M.p. 70–71 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.32 (d, J = 8.7 Hz, 1H), 7.84–7.79 (m, 4H), 7.55–7.50 (m, 3H), 7.15 (d, J = 8.1 Hz, 2H), 7.12–7.07 (m, 2H), 3.89 (s, 3H), 2.31 (s, 3H). ¹³C NMR (126 MHz, CDCl₃) δ 164.13, 152.57, 150.71, 143.54, 140.00, 132.05, 131.36, 131.11, 129.23, 129.12, 127.94, 123.84, 116.15, 101.22, 55.97, 21.51. HRMS (ESI+): calculated for C₂₀H₁₈N₂O₃S: [M + H]⁺ 367.1111, found 367.0548.

(E)-1-(5-Methoxy-2-tosylphenyl)-2-(m-tolyl)diazene (3u). Obtained as a orange solid in 81% yield; M.p. 70–71 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.32 (d, J = 8.8 Hz, 1H), 7.83 (d, J = 8.3 Hz, 2H), 7.65 (d, J = 7.8 Hz, 1H), 7.58 (s, 1H), 7.41 (t, J = 7.7 Hz, 1H), 7.34 (d, J = 7.5 Hz, 1H), 7.17 (d, J = 8.3 Hz, 2H), 7.10 (dd, J = 8.8, 2.6 Hz, 1H), 7.07 (d, J = 2.6 Hz, 1H), 3.89 (s, 3H), 2.47 (s, 3H), 2.33 (s, 3H). ¹³C NMR (126 MHz, CDCl₃) δ 164.14, 152.67, 150.79, 143.48, 140.06, 138.98, 132.84, 131.35, 130.95, 129.22, 128.92, 127.89, 123.93, 121.57, 116.04, 101.19, 55.95, 21.53, 21.37. HRMS (ESI+): calculated for C₂₁H₂₀N₂O₃S: [M + H]⁺ 381.1267, found 381.0685.

Acknowledgements

This work was supported by the Zhejiang Provincial Natural Science Foundation of China (no. LZ13B020001), National Natural Science Foundation of China (no. 21376058) and Major scientific and technological innovation projects of Hangzhou City (no. 20122511A43).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR spectra, ¹³C NMR spectrum, GC/MS profile, HRMS profile. CCDC 1057250. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra06474k

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