Aryl diazonium salt and thioacetamide: a catalyst free, efficient blend of an inexpensive arylating agent with “S” surrogate for sulphide synthesis

Abstract

Novel, facile C–S and S–S bond coupling reactions were achieved by using an aryl diazonium salt as an arylating agent and thioacetamide as a sulphur surrogate. The reaction proceeds smoothly at room temperature without using any transition metal catalyst, ligand or base. Aryl diazonium salts undergo rapid reactions with thioacetamide at room temperature to give the desired products in a much shorter period than the previously reported metal catalysed protocols.

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Introduction

Carbon–sulphur and sulphur–sulphur bond formation reactions play an important role in synthetic organic chemistry, in pharmaceuticals, as biologically active materials, and for the synthesis of sulphur containing heterocyclic compounds.¹ Thioethers and disulphides are widely used in quite a few clinical applications such as in the synthesis of anti-diabetic, anti-HIV, anti-inflammatory and anti-cancer drugs.^2,3 Various transition metals such as Cu,^4–14 Pd,^15–17 Co,¹⁸ Ni,^19–22 Pt,²³ Fe,^24,25 Rh,²⁶ In,^27,28 along with bases were preferably used for the synthesis of thioethers and disulphides. Although, good yields and selectivity for thioethers or disulphide were achieved by these methods, they often suffer from some shortcomings such as requirement of high temperature, longer time, use of costly transition metal catalysts as well as ligands, and strong base.²⁹ Another approach involves reduction of sulphone and sulphoxide which were essentially performed by using strong reducing agents.^30,31 To overcome these problems, Migita et al. in 1980 developed a simple palladium complex catalysed coupling reaction between thiols and aryl halides at mild reaction conditions using polar aprotic solvent.^32,33 Diaryl disulphides are synthesized by oxidation of thiophenol.^34–41 The use of thiophenols for these transformations is limited due to its foul smell, volatile nature and high cost. The most important drawback of this method is the ability to coordinate reactants with metal catalyst which in turn resulted in poisoning the catalyst. Various methods for the preparation of diaryl sulphides using different sulphur sources like potassium thiocyanate,^42–44 thiourea,^45,46 thioacetamide,⁴⁷ ethyl potassium xanthogenate,⁴⁸ sodium sulphide⁴⁹ and carbon disulphide⁵⁰ have been reported. It clearly indicates that use of thiols is completely avoided in these methods. Since last few decades, aryl diazonium salts have emerged as an attractive and easily available source of arylating agent. They serve as an inexpensive and highly reactive substitute for expensive arylating agents such as aryl halides and triflates (Scheme 1).

Scheme 1 Previous reports and present work.

They are widely used in many coupling reactions such as Mizoroki–Heck reaction,^51–53 Suzuki Miyaura,^54–57 Stille⁵⁸ and Sonogashira.⁵⁹ Diazonium salts were also used for C–S and S–S coupling reactions. Chandrashekhar et al. have reported use of diazonium salts with sulphur and tetrabutyl amine molybdenium sulphur to give C–S and S–S coupling reaction at 0 °C. However, the process used for the preparation of the salts is quite tedious and these salts are also expensive than the other surrogates.⁶⁰ Benati et al. reported synthesis of disulphides using CS₂ with diazonium salts in the presence of NaI, but the reaction gives lower yields due to the formation of diphenyl trithiocarbonate.⁶¹ Batanero et al.⁶² also synthesized diaryl disulphide by electrochemical method using aryl diazonium salt. Aryl diazonium salts and dichalcogenides photosynthesized to yield unsymmetrical dichalcogenides.^63–65 Ranu and co-workers reported synthesis of unsymmetrical diaryl sulphide with aryl diazonium salts using microwave and ball milling techniques.^66–68 Synthesis of unsymmetrical diaryl sulphide using ascorbic acid and aryl diazonium salt is also reported.⁶⁹ Most of these above reported methods give the account of synthesis of unsymmetrical diaryl sulphides. Therefore, a mild, efficient, and simple protocol for the synthesis of symmetrical diaryl disulphide and diaryl sulphides needs to be addressed by the researchers.

In continuation of our previous work on aryl diazonium salts,^70,71 here we report a simple, protocol for the synthesis of C–S and S–S coupling reactions by using various aryl diazonium salts with thioacetamide as sulphur surrogate. To our knowledge, this is the first report on the synthesis of diaryl disulphides and aryl sulphides using diazonium salts and thioacetamide without using any catalyst, ligand or base.

Results and discussion

The reaction conditions were optimized by taking aryl diazonium salt of aniline 1a (1 mmol) as a key substrate, sulphur surrogate (2 mmol) and DMSO as solvent. Initially the reaction was optimized with respect to sulphur surrogates at room temperature (30 °C). The effect of other parameters such as, solvents, temperature, and loading of sulphur surrogate and diazonium salt was investigated and the results are summarized in Table 1. The screening of sulphur surrogate shows that all surrogates give mixture of diaryl disulphides (2a) and diaryl sulphides (3a), but the best results were obtained for thioacetamide which gave 90% yield of 2a along with 8% yield of 3a in 2 h (Table 1, entry 1). The other surrogates such as thiourea and thiobenzamide gave poor to moderate yields of 2a in 5 h at room temperature (Table 1, entries 2, 3). The present protocol was found to be unsuccessful for potassium thiocyanide which gave only traces of product formation after 5 h (Table 1, entry 4). The other polar solvents such as methanol, acetonitrile, DMF, and 1,4-dioxane were also tested under similar experimental conditions (Table 2, entries 1–4). DMF was the only solvent apart from DMSO, which gave higher yield (80%) for 2a with 9% yield of 3a in 5 h at 30 °C (Table 2, entry 2). In order to study the effect of reaction temperature on the yield of 2a, we carried out the reaction at 0 °C to 40 °C (Table 2, entries 5, 6). At 0 °C, only 75% yield of 2a was obtained in 5 h (Table 2, entry 5), whereas the temperature was increased to 40 °C 92%, yield of 2a was obtained in 2 h. This shows that there is no much improvement in the yield even through the reaction was carried out at higher temperature (Table 2, entry 6). The optimization study on loading of thioacetamide and diazonium salt demonstrated that, these two parameters play a major role in deciding the nature of the product. To study the effect of thioacetamide on the nature of product, we kept amount of diazonium salt constant to 1 mmol and varied thioacetamide loading (Table 2, entries 7–9). It was observed that, as the thioacetamide loading increased from 1.1 mmol to 1.5 mmol, yield of 2a also increased from 61% to 74% with decreasing that of 3a from 39% to 26% (Table 2, entries 7, 8). Higher yield for 2a was obtained when 2 mmol of thioacetamide were used (Table 1, entry 1). Further increase in thioacetamide loading to 2.5 mmol does not show any substantial improvement in the yield of 2a (Table 2, entry 9). On the other hand, when we varied diazonium salt loading from 3 mmol to 4 mmol, by keeping that of thioacetamide constant to 1 mmol, yield of 3a was found to increase from 60% to 75% respectively (Table 2, entries 10, 11). Increase in the diazonium salt loading beyond 5 mmol does not show much variation in the yield of 3a (Table 2, entry 12). Thus, from the optimization data it was clear that, when 1 mmol diazonium salt (1a) was treated with 2 mmol of thioacetamide in DMSO at room temperature, diaryl disulphide (2a) was obtained as a major product. However, when 4 mmol of diazonium salt was treated with 1 mmol of thioacetamide under similar conditions, diaryl sulphide (3a) was obtained as a major product in 2 h.

Table 1 Screening of sulphur surrogate for synthesis of diaryl disulphides and aryl sulphides^a

Entry	Sulphur surrogate	Solvent	Time (h)	Yield^b (%)
				2a	3a
a Reaction conditions: 1a (1 mmol); sulphur surrogate (2 mmol), solvent (2 mL); temperature 30 °C.b Isolated yield.
1	Thioacetamide	DMSO	2	90	08
2	Thiobenzamide	DMSO	5	58	9
3	Thiourea	DMSO	5	27	2
4	KSCN	DMSO	5	—	—

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Table 2 Optimizations of reaction conditions^a

Entry	1a (mmol)	Thioacetamide (mmol)	Time (h)	Yield^b (%)
				2a	3a
a Reaction conditions: solvent (2 mL); temperature 30 °C.b GC yield.c Solvent methanol.d Solvent DMF.e Solvent 1,4-dioxane.f Solvent ACN.g Temperature 0 °C.h Temperature 40 °C.
1^c	1	2	5	51	21
2^d	1	2	5	80	9
3^e	1	2	5	59	7
4^f	1	2	5	34	23
5^g	1	2	5	75	15
6^h	1	2	2	92	7
7	1	1.1	5	61	39
8	1	1.5	5	74	26
9	1	2.5	2	91	9
10	3	1	5	40	60
11	4	1	2	8	75
12	5	1	2	5	76

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The optimized conditions were applied to various aryl diazonium salts bearing electron donating as well as withdrawing substituents to synthesize diaryl disulphide compounds (Table 3). Highest yield was obtained for diaryl disulphide prepared from diazonium salt of aniline (Table 3, entry 2a). The diazonium salts bearing electron donating substituents such as 2-methyl, 4-methyl, and 4-methoxy gave good yield of corresponding diaryl disulphide derivatives (Table 3, entries 2b–2d). However, diazonium salts with electron withdrawing substituents such as 3-nitro and 4-nitro gave 67% and 68% yields of corresponding diaryl disulphides (Table 3, entries 2e, 2f). The diazonium salts substituted with halogens such as 3-chloro, 4-chloro, and 4-bromo gave 71%, 74%, and 77% yield of respective diaryl disulphide (Table 3, entries 2g–2i).

Table 3 Synthesis of diaryl disulphides^a

a Reaction conditions: 1 (1 mmol), thioacetamide (2 mmol), DMSO (2 mL); temperature 30 °C; reaction time: 2 h; yield: isolated yield.

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Further, the optimized conditions of diaryl sulphide were applied to various diazonium salts and the results obtained were summarized in Table 4. The aryl diazonium salts without any substituent, as well as electron donating and withdrawing substituents gave moderate yields of corresponding diaryl sulphides at 30 °C. Diazonium salts with electron donating substituents such as 2-methyl, 4-methyl, and 4-methoxy gave 71%, 72%, and 68% yields of their respective diaryl sulphides (Table 4, entries 3b–3d). Similarly, diazonium salts with electron withdrawing groups such as 3-nitro and 4-nitro in 2 h gave 63% and 66% yields respectively (Table 4, entries 3e, 3f). The 2-chloro, 3-chloro, and 4-bromo derivatives of diazonium salts gave 65%, 69%, and 71% yields respectively (Table 4, entries 3g–3i).

Table 4 Synthesis of diaryl sulphides^a

a Reaction conditions: 1 (1 mmol), thioacetamide (2 mmol), DMSO (2 mL); temperature 30 °C; reaction time: 2 h; yield: isolated yield.

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Conclusions

In summary, aryl diazonium salts were found to be efficient arylating agents with thioacetamide as sulphur surrogate for the synthesis of symmetrical diaryl disulphides and diaryl sulphides, without using any catalyst, ligand, or base. The nature of product was found to vary with amounts of aryl diazonium salts and thioacetamide. Various diazonium salts bearing electron donating as well as withdrawing substituents react with thioacetamide to offer corresponding products in moderate to good yields. The reaction can be said as energy efficient as it proceeds at room temperature only. Thus, the present work offers a mild and effective alternative to all metal catalysed protocols for the synthesis of diaryl disulphides and aryl sulphides.

Selected spectral data

1. 1,2-Diphenyl disulfide (2a)⁶⁰

White solid; yield: 0.097 g (90%); mp 52–57 °C; ¹H NMR: (400 MHz, CDCl₃) δ 7.48 (d, J = 7.4 Hz, 4H), 7.28 (t, J = 7.6 Hz, 4H), 7.25–7.20 (m, 2H); GC-MS m/z (% relative intensity): 154 (M⁺, 30), 152 (100), 77 (60).

2. 1,2-Bis(3-nitrophenyl) disulfide (2e)³¹

Yellow solid; yield: 0.102 g (67%) mp 80–81 °C; ¹H NMR: (400 MHz, CDCl₃) δ 8.36–8.31 (m, 2H), 8.07 (d, J = 7.2, 5.1 Hz, 2H), 7.85–7.76 (m, 2H), 7.51 (d, J = 8.1 Hz, 2H); LC-MS m/z (% relative intensity): 308.

3. 1,2-Bis(4-nitrophenyl) disulfide (2f)⁶¹

Yellow solid; yield: 0.104 g (68%); mp 171–174 °C; ¹H NMR: (400 MHz, CDCl₃) δ 7.46–7.35 (m, 4H), 7.26 (d, J = 7.3 Hz, 4H); LC-MS m/z (% relative intensity): 308.

4. 1,2-Bis(4-chlorophenyl) disulfide (2h)⁶⁰

White solid; yield: 0.106 g (74%); mp 69–72 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.46–7.35 (m, 4H), 7.26 (d, J = 7.3 Hz, 4H); GC-MS m/z (% relative intensity) 286 (M⁺, 65), 143 (100), 251 (5), 222 (6), 108 (62).

5. Diphenyl sulphide (3a)⁴⁴

Yellow liquid; yield: 0.137 g (75%); ¹H NMR (400 MHz, CDCl₃) δ 7.36–7.22 (m, J = 7.6 Hz, 10H); GC-MS m/z (% relative intensity): 186 (M⁺, 100), 152 (8), 77 (10).

6. Bis(4-methoxyphenyl) sulphide (3c)⁴⁴

Yellow solid; yield: 0.177 g (72%); mp 46–47 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.25 (d, J = 8.8 Hz, 4H), 6.81 (d, J = 8.7 Hz, 4H), 3.77 (s, 6H); GC-MS m/z (% relative intensity): 246 (M⁺, 100), 231 (40), 128 (7), 203 (11).

7. Bis(2-methylphenyl) sulphide (3d)¹¹

White solid; yield: 0.142 g (68%); mp 63–65 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.25–7.20 (m, 2H), 7.19–7.12 (m, 2H), 7.10 (d, J = 7.8 Hz, 2H), 7.05 (t, J = 6.7 Hz, 2H), 2.37 (s, 6H); GC-MS m/z (% relative intensity): 214 (M⁺, 100), 199 (11), 184 (6), 153 (2), 122 (33), 105 (10), 91 (13).

8. Bis(3-nitrophenyl) sulphide (3e)⁴⁴

Yellow solid; yield: 0.174 g (63%); mp 57–60 °C; ¹H NMR: (400 MHz, CDCl₃) δ 8.18 (s, 2H), 8.15 (d, J = 8.2 Hz, 2H), 7.68–7.63 (m, 2H), 7.53 (t, J = 8.0 Hz, 2H).

9. Bis(4-bromophenyl) sulphide (3i)⁴⁴

White solid; yield: 0.24 g (71%); mp 112–114 °C; ¹H NMR: (400 MHz, CDCl₃) δ 7.41 (d, J = 8.5 Hz, 4H), 7.16 (d, J = 8.5 Hz, 4H); GC-MS m/z (% relative intensity): 344 (M⁺, 100), 342 (35), 267 (33), 183 (14), 155 (20), 77 (20).

Acknowledgements

The authors are thankful to UGC-SAP New Delhi (India) for providing financial assistance and Dr Vinod Parab Loba chemie, India for providing a gift samples.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ¹H, GCMS and LCMS. See DOI: 10.1039/c6ra12557c

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