Sustainable synthesis of sulfonamides via oxidative chlorination in alternative solvents: a general, mild, and eco-friendly strategy

Abstract

A general, mild, and environmentally friendly method for synthesizing sulfonamides in sustainable solvents was developed. Sodium dichloroisocyanurate dihydrate (NaDCC·2H₂O) served as an efficient oxidant for converting thiols to sulfonyl chlorides, which then reacted in situ with various amines to afford sulfonamides in good to excellent yields in water, EtOH, glycerol and the DES ChCl/glycerol: 1/2. The process features simple conditions and a solvent-free workup involving only filtration, highlighting its potential as a green and practical approach to sulfonamide synthesis.

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Introduction

Sulfonamides were the first synthetic compounds implemented as effective antibacterial drugs.¹ Discovered in the 1930s, these compounds marked a significant breakthrough in medical science, paving the way for the development of modern antibiotics.² Sulfonamides work by inhibiting bacterial growth through the interference with folic acid synthesis, a vital nutrient for bacteria. They are particularly effective against a range of Gram-positive and Gram-negative bacteria,³ though their use has diminished due to the rise of resistance^4,5 and the advent of newer antibiotics. Despite this, sulfonamides remain an important part of medical history and continue to be used in certain situations, particularly in the treatment of urinary tract infections.⁶ Their development not only revolutionized the treatment of bacterial infections but also laid the groundwork for the future of antimicrobial therapy including their re-evaluation for their priority in clinical practice.^7,8

Sulfonamides are synthesized through a variety of chemical methods, each allowing for different modifications to the sulfonamide structure.^9,10 Among these, the most used involve S–N bond formation, typically through the reaction of amino compounds with sulfonyl chlorides in the presence of a base in organic solvents.¹¹ With the aim of improving the sustainability of this methodology,¹² a series of greener procedures have been developed, including the use of alternative solvents like water,^13,14 aqueous mixtures,¹⁵ PEG-400,¹⁶ ionic liquids,¹⁷ Deep Eutectic Solvents (DES),^18,19 the use of neat conditions,^20–22 the employment of reusable supported catalysts,^23,24 and photocatalytic methods.²⁵

The use of sulfonyl chlorides poses significant challenges. These electrophiles are highly reactive and can be corrosive, posing risks during handling, storage, and transport. They can also release toxic gases, such as hydrogen chloride, upon decomposition. These compounds are sensitive to moisture and can degrade over time, making them difficult to store for extended periods without special precautions. Moreover, very often undesirable disulfonamides are obtained when primary amines are used, reducing the efficiency and selectivity of the process. As a result, significant recent efforts have focused on developing mild methods for synthesizing sulfonamides from in situ generated sulfonyl chlorides, with many of these methods utilizing oxidative chlorination of thiols and disulfides with different oxidants such as sodium hypochlorite/HCl,²⁶ ammonium nitrate/HCl,²⁷ 1,3-dichloro-5,5-dimethylhydantoin (DCH)/BnMe₃NCl,²⁸ trichloroisocyanuric acid (TCCA)/BnMe₃NCl,²⁹ TMSCl/H₂O₂,³⁰ generally using volatile organic compounds (VOC) or organic co-solvents as reaction media. Regarding alternative solvents, N-aryl and N-alkyl sulfonamides have been synthesized from thiols and disulfides using TCCA as oxidative chlorinating reagent in water and large excess of reagents.³¹

As can be demonstrated, to date there is no general method for the synthesis of sulfonamides from thiols via oxidative chlorination that can be employed with equal efficiency across different sustainable media. In this work, we present our studies on the synthesis of sulfonamides by reaction of amines with in situ generated sulfonyl chlorides by oxidative chlorination of thiols in alternative solvents such as water, alcohols, and DES.

Results and discussion

The investigation began with the study of the reaction between thiophenol (1a) and morpholine (2a) by testing different conditions for the oxidative chlorination, using the DES choline chloride/glycerol (ChCl/Gly): 1/2 as the reaction medium at rt. As depicted in Scheme 1, various oxidants were tested: TMSCl/H₂O₂, NaOCl/with or without HCl, TCCA (trichloroisocyanuric acid)/HCl, and NaDCC (sodium dichloroisocyanurate)/HCl. Yields for sulfonamide formation ranged from 3% to 12%, with the TMSCl/H₂O₂ system being the most effective. Overall, the results were poor across all tested oxidants, due to the instability of the in situ generated sulfonyl chloride and the DES which is not able to maintain its nature under the used aqueous conditions.^32,33

Scheme 1 Synthesis of sulfonamide 3aa in ChCl/Gly: 1/2.

To reduce or eliminate water content in the amide formation process from thiols, a critical factor for stability and scalability, we initially performed an optimization of the reaction conditions using the NaOCl/HCl mixture as the oxidative chlorinating agent. Notably, previous literature reports have made limited efforts to minimize the presence of water and organic co-solvents in this process. As observed in entry 1 of Table 1, when using the previously reported conditions²⁵ [1a (1 mmol), HCl (5 mL of a 1 M aqueous soln., 5 mmol, 5 equiv.), aqueous soln. of NaOCl (3.6 mmol, 3.6 equiv.), 2a (6 mmol, 6 equiv.) but in the absence of the organic cosolvent (CH₂Cl₂)], a 70% isolated yield of sulfonamide 3aa was obtained. Halving the amount of NaOCl resulted in a significant decrease in yield (entry 2), whereas reducing HCl to 2 equiv. had minimal impact in the yield, unless running the reaction at 5 °C which afforded 3aa in an 84% isolated yield (entry 3). Upon scaling the reaction to 10 mmol of 1a, a marked decrease in yield was observed, reaching only 30%. The detection of the sulfonyl chloride intermediate in the crude mixture suggests that extended reaction times may be required to achieve complete conversion under higher scale conditions. Further reduction of the HCl led to a lower 46% yield while in its absence, only the corresponding disulfide was obtained (Table 1, entry 5). Finally, no product formation was observed with concentrated 12 M HCl (entry 6), while the use of 6 M HCl allowed, after further optimization, to use only 1.2 equiv. of acid with a twentyfold reduction in volume, affording a 65% isolated yield of 3aa at rt and 70% at 5 °C (Table 1, entry 7).

Table 1 Reaction conditions optimization for the synthesis of 3aa using NaOCl/HCl as the oxidative chlorinating agent^a

Entry	HCl ([ ], equiv.)	NaOCl (equiv.)	2a (equiv.)	3aa yield^b (%)
a Reaction conditions: thiophenol (1 mmol), HCl (1–12 M, 0.5–5 equiv.), 15% aq. NaOCl (1.8–4.5 equiv.), morpholine (1.5–6 equiv.).b Isolated yield after precipitation from the crude reaction mixture.c Reaction performed at 5 °C starting from 2 mmols of 1a.d Reaction performed at 5 °C starting from 10 mmols of 1a.e Only disulfide was observed by ^1HNMR analysis of the crude reaction mixture.f Similar yields were always obtained when using 4, 5, 6 or 7 equiv. of amine.g Reaction performed in ChCl/glycerol: 1/2 as solvent.
1	1 M, 5	3.6	6	70
2	1 M, 5	1.8	6	5
3	1 M, 2	3.6	6	68 (84)^c (30)^d
4	1 M, 0.5	3.6	6	46
5	—	3.6	6	<5^e
6	12 M, 2.5	3.6	6	<5
7	6 M, 1.2	3.6	6	65 (70)^c
8	1 M, 2	2	6	20
9	1 M, 2	3	6	62
10	1 M, 2	4	6	72
11	1 M, 2	4.5	6	59
12	1 M, 2	4	1	33
13	1 M, 2	4	1.5	63
14	1 M, 2	4	2	73
15	1 M, 2	4	3	71
16	1 M, 2	4	4^f	73
17	1 M, 2	4	4	6^g

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These initial experiments demonstrated that no organic solvent was required, as the reaction proceeds efficiently in aqueous HCl/NaOCl through an exothermic formation of HOCl, which oxidizes the thiol firstly to the disulfide (the initial formation of white solids corresponding to this intermediate is observed) to in situ sulfonyl chloride, which appears as an immiscible liquid ranging from colourless to slightly yellow. Subsequent addition of morpholine yields sulfonamide 3aa as a white solid, isolated by filtration. Reaction efficiency is highly dependent on the order and rate of reagent addition; optimal results were obtained by premixing PhSH and HCl, followed by dropwise addition of NaOCl in two portions with a 10 minute interval to control exothermicity, adding the amine 10 minutes after the NaOCl addition.

Optimization of NaOCl (15% soln.) was next continued using the 1 M HCl soln. As depicted in Table 1 (entries 3 and 8–11) the highest isolated yield for 3aa (72%) was obtained using 4 equivalents of NaOCl (entry 10). Next, the amount of amine was also optimized (Table 1, entries 12–16), with 2 equivalents of base being established as optimal. Finally, under the optimized reaction conditions [1a (1 mmol), 1 M HCl (2 mmol), 15% NaOCl (4 mmol), 2a (2 mmol)], an additional test was conducted using ChCl/glycerol: 1/2 as the reaction medium at rt (Table 1, entry 17) affording a 6% yield for 3aa.

Although the synthesis of sulfonamides from thiols using the NaOCl/HCl mixture had been optimized, the conditions proved still inadequate for application in other alternative solvents such as DES. Then, we subsequently explored other oxidants such as trichloroisocyanuric acid (TCCA; H₂O solubility 10 g l⁻¹ at 25 °C), sodium dichloroisocyanurate (NaDCC; H₂O solubility 236.8 g l⁻¹ at 25 °C), and sodium dichloroisocyanurate dihydrate (NaDCC·2H₂O; H₂O solubility 236.8 g l⁻¹ at 25 °C) (Fig. 1), as HOCl precursors for using in alternative solvents.

Fig. 1 TCCA and salts used as alternative oxidants.

The study began by applying the previously optimized reaction conditions (Table 1, entry 14), substituting the HOCl/HCl mixture with the corresponding TCCA derivatives. As shown in Table 2, when the reactions were performed in the presence of 1 M aqueous HCl as solvent resulted in moderate yields (20–56%) of sulfonamide 3aa. Replacing the acidic medium with water alone led to a significant decrease in yields across all three oxidants tested (Table 2, entries 4–6). Subsequently, a study of the reaction was conducted in the DES ChCl/glycerol: 1/2 as the solvent. As shown in Table 2 (entries 7–9), the highest yield (65%) was achieved using NaDCC·2H₂O as the oxidant. Utilizing this reagent, a broad range of DESs were evaluated in the model reaction (see Table 1 in the SI for the complete dataset), all of which resulted in lower yields, except for ChCl/d-sorbitol: 1/1, which afforded compound 3aa in a 58% isolated yield (Table 2, entries 11–15). Finally, some other sustainable solvents such as, 2-MeTHF, H₂O, and different alcohols were also studied as reaction media (Table 2, entries 16–23) affording 3aa in an 81% isolated yield when using EtOH.

Table 2 Reaction conditions optimization for the synthesis of 3aa using TCCA and derivatives as oxidation agents^a

Entry	Oxidant (equiv.)	Solvent^b	Yield^c (%)
a Reaction conditions: 1a (0.5 mmol), solvent (1 mL) oxidant, morpholine (2 equiv.).b ChCl: choline chloride; Gly: glycerol; EG: ethylene glycol; GA: glycolic acid.c Isolated yield after column chromatography (silica, hexane/EtOAc).d A 25% of the sulfonyl chloride was also detected by ^1HNMR in the crude reaction mixture.e A similar yield (60%) was obtained when using 81% technical grade NaDCC·2H2O.f A saturated aqueous soln. of the polyol was used as reaction medium.
1	TCCA (2)	1 M HCl	20
2	NaDCC (2)	1 M HCl	56
3	NaDCC·2H2O (2)	1 M HCl	46^d
4	TCCA (2)	H2O	15
5	NaDCC (2)	H2O	40
6	NaDCC·2H2O (2)	H2O	45
7	TCCA (2.3)	ChCl/Gly: 1/2	58
8	NaDCC (2.3)	ChCl/Gly: 1/2	31
9	NaDCC·2H2O (2.3)	ChCl/Gly: 1/2	65^e
10	NaDCC·2H2O (2.3)	ChCl/urea: 1/2	5
11	NaDCC·2H2O (2.3)	ChCl/EG: 1/2	8
12	NaDCC·2H2O (2.3)	ChCl/AcOH: 1/1	16
13	NaDCC·2H2O (2.3)	ChCl/GA: 1/1	12
14	NaDCC·2H2O (2.3)	GA/H2O: 1/4	23
15	NaDCC·2H2O (2.3)	ChCl/d-sorbitol: 1/1	58
16	NaDCC·2H2O (2.3)	2-MeTHF	34
17	NaDCC·2H2O (2.3)	H2O	70
18	NaDCC·2H2O (2.3)	Glycerol	70
19	NaDCC·2H2O (2.3)	EG	62
20	NaDCC·2H2O (2.3)	MeOH	63
21	NaDCC·2H2O (2.3)	EtOH	81
22	NaDCC·2H2O (2.3)	Inositol^f	67
23	NaDCC·2H2O (2.3)	Erythritol^f	70

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With the best conditions established and the most effective sustainable solvents selected, the methodology was evaluated across a broad substrate scope, including aromatic and aliphatic thiols, as well as primary, secondary, and aromatic amines (Table 3). As depicted, similar results were obtained with all the solvents studied (H₂O, EtOH, glycerol, and ChCl/Gly: 1/2) none of them outperforming significantly among the others. Nevertheless, the best results overall for products 3 were obtained in ethanol, standing out products obtained from aromatic thiophenols with yields up to 93% for sulfonamide 3da in EtOH (Table 3, entry 29).

Table 3 Sulfonamide synthesis through oxidative chlorination of thiols 1a–e and subsequent reaction with amines 2a–c in sustainable solvents^a

Entry	Solvent^b	R^1 (1a–e)	R^2, R^3 (2a–c)	Product 3	Yield^c (%)
a Reaction conditions: 1 (0.5 mmol), NaDCC·2H2O (1.15 mmol), 2 (2 mmol), solvent (1 mL).b ChCl: choline chloride; Gly: glycerol.c Isolated yield after after column chromatography (silica, hexane/EtOAc).d The reaction was scaled up to 10 mmols of 1a.e A 10% yield of 4-[(4-chloro-1-methyl-1H-imidazol-2-yl)sulfonyl]morpholine was also obtained.
1	H2O	Ph (1a)	(CH2CH2)2O (2a)	3aa	68
2	H2O	Bn (1b)	(CH2CH2)2O (2a)	3ba	22
3	H2O	Cy (1c)	(CH2CH2)2O (2a)	3ca	30
4	H2O	Ph (1a)	H, Cy (2b)	3ab	54
5	H2O	Ph (1a)	H, Ph (2c)	3ac	76
6	H2O	Ph (1a)	(CH2)5 (2d)	3ad	80 (60)^d
7	H2O	Ph (1a)	H, Bu^t (2e)	3ae	11
8	EtOH	Ph (1a)	(CH2CH2)2O (2a)	3aa	82
9	EtOH	Bn (1b)	(CH2CH2)2O (2a)	3ba	24
10	EtOH	Cy (1c)	(CH2CH2)2O (2a)	3ca	45
11	EtOH	Ph (1a)	H, Cy (2b)	3ab	89
12	EtOH	Ph (1a)	H, Ph (2c)	3ac	66
13	EtOH	Ph (1a)	(CH2)5 (2d)	3ad	92
14	EtOH	Ph (1a)	H, Bu^t (2e)	3ae	52
15	Gly	Ph (1a)	(CH2CH2)2O (2a)	3aa	69
16	Gly	Bn (1b)	(CH2CH2)2O (2a)	3ba	15
17	Gly	Cy (1c)	(CH2CH2)2O (2a)	3ca	45
18	Gly	Ph (1a)	H, Cy (2b)	3ab	49
19	Gly	Ph (1a)	H, Ph (2c)	3ac	66
20	Gly	Ph (1a)	(CH2)5 (2d)	3ad	49
21	Gly	Ph (1a)	H, Bu^t (2e)	3ae	36
22	ChCl/Gly: 1/2	Ph (1a)	(CH2CH2)2O (2a)	3aa	57
23	ChCl/Gly: 1/2	Bn (1b)	(CH2CH2)2O (2a)	3ba	15
24	ChCl/Gly: 1/2	Cy (1c)	(CH2CH2)2O (2a)	3ca	60
25	ChCl/Gly: 1/2	Ph (1a)	H, Cy (2b)	3ab	52
26	ChCl/Gly: 1/2	Ph (1a)	H, Ph (2c)	3ac	41
27	ChCl/Gly: 1/2	Ph (1a)	(CH2)5 (2d)	3ad	18
28	ChCl/Gly: 1/2	Ph (1a)	H, Bu^t (2e)	3ae	25
29	EtOH	4-MeOC6H4 (1d)	(CH2CH2)2O (2a)	3da	93
30	Gly	4-MeOC6H4 (1d)	(CH2CH2)2O (2a)	3da	73
31	ChCl/Gly: 1/2	4-MeOC6H4 (1d)	(CH2CH2)2O (2a)	3da	58
32	EtOH	3-ClC6H4 (1e)	(CH2CH2)2O (2a)	3ea	76
33	Gly	3-ClC6H4 (1e)	(CH2CH2)2O (2a)	3ea	83
34	ChCl/Gly: 1/2	3-ClC6H4 (1e)	(CH2CH2)2O (2a)	3ea	60
35	EtOH	1-Methyl-1H-imidazol-2-yl (1f)	(CH2CH2)2O (2a)	3ef	21^e
36	Gly	1-Methyl-1H-imidazol-2-yl (1f)	(CH2CH2)2O (2a)	3ef	<5
37	ChCl/Gly: 1/2	1-Methyl-1H-imidazol-2-yl (1f)	(CH2CH2)2O (2a)	3ef	<5

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While solvent effects showed no clear trend, more discernible correlations emerged with the nature of thiols 1 and amines 2. The lowest yields are observed when using benzylic thiol 1b, obtaining from 15 to 24% yields (Table 3, entries 2, 9, 16, and 23). Aliphatic cyclohexanethiol (1c) was well tolerated, with the highest yield of sulfonamide 3ca (60%) obtained using ChCl/Gly: 1/2 as reaction solvent (Table 3, entry 24). The nature of the amine had a lesser impact on reaction efficiency compared to the thiol, as similar yields were observed when using primary cyclohexylamine, secondaries morpholine and piperidine or aniline. Nevertheless, reactions with thiophenol generally gave good yields with morpholine (57–82%; Table 3, entries 1, 8, 15, and 22), piperidine (18–92%; Table 3, entries 6, 13, 20, and 27), and cyclohexylamine (49–89%; entries 4, 11, 18 and 25). In the case of morpholine a good 60% yield was obtained when scaling the reaction to 10 mmols of starting 1a (Table 3, entry 6). Due to its steric hindrance, tert-butylamine provided low yields in the reaction with thiophenol. Among the solvents tested, EtOH proved to be the most effective, affording sulfonamide 3ae in an isolated yield of 52% (Table 3, entry 14).

Good to excellent yields were achieved for sulfonamides 3da and 3ea using substituted thiophenols and morpholine in EtOH, Gly, and ChCl/Gly: 1/2 (Table 3, entries 29–34). 4-Methoxythiophenol afforded 3da in 58–93% yield, with EtOH providing the highest. In contrast, 3-chlorothiophenol afforded the best result in Gly, yielding 3ea in 83% isolated yield (Table 3, entry 33). Finally, the reaction of the heterocyclic thiol 1-methyl-1H-imidazole-2-thiol with morpholine proceeded effectively only in ethanol, affording the sulfonamide derivative 3ef in 21% yield, along with the corresponding chlorinated analogue, 4-[(4-chloro-1-methyl-1H-imidazol-2-yl)sulfonyl]morpholine, in 10% yield (Table 3, entry 37).

Regarding the reaction mechanism, sulfonamides are formed via the reaction of the corresponding amine with an in situ generated sulfonyl chloride. Based on previous studies involving TCCA-derived oxidants,^31,34 and supported by the observation of disulfide formation in our system, the sulfonyl chloride intermediate arises from the thiol through sequential chlorination, formation of sulfenic acid, dimerization to the symmetric disulfide, and subsequent oxidation via a thiosulfonate intermediate. The reaction proceeds heterogeneously in all tested solvents following the addition of the oxidant; however, the specific role of each solvent, particularly DES, requires further consideration. Previous studies¹⁸ have indicated that certain DES can slow the hydrolysis of sulfonyl chloride intermediates, while additional effects, such as reaction acceleration through intermediate stabilization and subtle catalytic contributions, may also be involved. The exact influence of the solvent remains complex and is currently under investigation by our research group.

Conclusions

An environmentally benign method has been developed for the direct synthesis of sulfonamides from thiols and amines. This one-pot protocol employs sodium dichloroisocyanurate dihydrate (NaDCC·2H₂O) as an oxidative chlorination agent, demonstrating broad applicability across different sustainable solvents in short reaction times. Overall, the method aligns with several principles of green chemistry and offers a practical approach for the synthesis of sulfonamides in good to excellent yields.

Author contributions

All the authors equally contributed to this work. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI.

SI is available, containing the general procedures for sulfonamide synthesis, complete characterization data for the isolated products, and the ¹H and ¹³CNMR spectra for all synthesized compounds. See DOI: https://doi.org/10.1039/d5su00405e.

Acknowledgements

This research has been funded by Generalitat Valenciana (Project AICO 2021/013), the State Research Agency of the Spanish Ministry of Science, Innovation and Universities (project PID2021-127332NB-I00), and the University of Alicante (Project VIGROB-173 and grants UAUSTI 2023). F. J. S.-M. acknowledges Alicante University for a pre-doctoral grant (UAFPU22-27). C. A. C.-C. acknowledges CIC-UMSNH for financial support of this project and A. G.-H. thanks CONAHCYT (grant no. 773141) for a graduate fellowship.

Notes and references

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