Sodium iodide-mediated synthesis of vinyl sulfides and vinyl sulfones with solvent-controlled chemical selectivity

Abstract

Vinyl sulfides and vinyl sulfones are ubiquitous structures in organic chemistry because of their presence in natural and biologically active compounds and are very frequently encountered structural motifs in organic synthesis. Herein we report an efficient synthesis of vinyl sulfides and vinyl sulfones via transition metal-free sodium iodide-mediated sulfenylation of alcohols and sulfinic acids with solvent-controlled selectivity.

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Introduction

Functionalized olefins are valuable intermediates in organic synthesis, because of their ability to serve as convenient substrates in transition metal-catalyzed cross-coupling reactions and their presence in natural and biologically active compounds.¹ In this respect, sulfur-containing olefins as important building units or intermediates have been applied in many different fields, such as total synthesis, functional materials science and medicinal chemistry.² For example, vinyl sulfides are employed as precursors for enol substituents,³ Michael acceptors,⁴ in cycloaddition reactions,⁵ in olefin metathesis,⁶ as reagents in cross-coupling,⁷ in Heck reactions,⁸ as equivalents of enolate anions,⁹ and in aldehyde/ketone formation.¹⁰ Vinyl sulfones and their derivatives have been reported to be potent inhibitors of enzymes such as sortase,¹¹ protein tyrosine phosphatases,¹² and cysteine proteases.¹³ In view of their significance, developing more practical and efficient synthetic approaches to construct these compounds is of great interest.¹⁴ Two common methods are frequently utilized for the synthesis of vinyl sulfides, including the copper-catalyzed S-vinylation of thiols with vinyl halides and the addition of thiols to alkynes.¹⁵ Recently, Zhang and co-workers reported a copper-mediated stereospecific C–H oxidative sulfenylation of terminal alkenes with disulfides.¹⁴ For the synthesis of vinyl sulfones, Wittig reaction of α-sulfinyl phosphonium ylides, β-elimination of halosulfones, condensation of aldehydes with sulfonylacetic, and the oxidation of the corresponding sulfides are perhaps the most common synthesis methods.¹⁶ Recently, Zhan's group reported a copper(II)-catalyzed chemo- and stereocontrolled synthesis of vinyl sulfones from terminal alkynes and arylsulfonyl hydrazides.² Kuhakarn and co-workers discovered an efficient method for the synthesis of (E)-vinyl sulfones via decarboxylative sulfonylation of arylpropiolic acids with sulfinic acids.¹⁴ In spite of the great success achieved so far in this area, there are still certain limitations, such as difficulties in controlling stereoselectivities and chemoselectivities, expensive substrates, moderate scope and functional group tolerance. Therefore, a new general, flexible, high stereoselective and chemoselective approach for the synthesis of vinyl sulfides and vinyl sulfones is still necessary.

Sulfinic acids are readily accessible and have been widely used as sulfur sources in medicinal chemistry.¹⁷ Moreover, sulfinic acids can be reduced to disulfides, which are active intermediates that can be used as sulfenylating agents. So we envisioned that sulfinic acids might be served as potential sulfur sources to react with alcohols under certain reaction conditions. To our knowledge, there are no examples describing the reaction of sulfinic acids and alcohols. Herein we report an efficient synthesis of vinyl sulfides and vinyl sulfones via transition metal-free sodium iodide-mediated sulfenylation of alcohols with sulfinic acids.

Results and discussion

To obtain the optimized reaction conditions, several acids (10 mol%) were examined in the model reaction of alcohol 1a with benzenesulfinic acid 2a using tetrabutylammonium iodide as an additive in 1,2-dichloroethane at 80 °C (Table 1, entries 1–4). To our delight, TsOH·H₂O was identified as the acid of choice, which promoted the formation of vinyl sulfide 3a in 61% yield (Table 1, entry 1). Iodine and several iodides such as NaI, KI were investigated to improve the reaction efficiency (Table 1, entries 5–7). Among the above iodides examined, NaI was found to be the most effective additive to give the target product 3a in 70% yield (Table 1, entry 5). Further solvent screening showed interesting results: (1) we observed the generation of vinyl sulfone 4a in MeCN (Table 1, entry 8). (2) MeNO₂ was the best solvent for the formation of 4a, and improved the yield to 59% yield (Table 1, entry 9). (3) DCE was the best solvent for the formation of 3a. These results demonstrate that solvent plays an important role in controlling the chemoselectivity of the reaction, which could be explained by the fact that MeNO₂ is beneficial to the production of free radical intermediates. A high yield was obtained when the loading of sodium iodide was increased to 1.5 equivalents (Table 1, entry 15). Further efforts to increase the yield of vinyl sulfide 3a by increasing the loading of TsOH·H₂O to 0.2 equivalent resulted in a higher yield of 90% (Table 1, entry 16). Similarly, the yield of 4a was increased to 86% in the presence of 1.5 equivalents of NaI and 20 mol% TsOH·H₂O in MeNO₂.

Table 1 Optimization of reaction conditions^a

Entry	Solvent	Additive (eq.)	Acid (eq.)	Yield^b (%) 3a	Yield^b (%) 4a
a Reaction conditions: alcohol 1a (0.20 mmol), benzenesulfinic acid 2a (0.30 mmol), additive, acid, solvent (1.0 mL), 80 °C, 24 h.b Isolated yield.
1	DCE	n-Bu4NI (1.0)	TsOH·H2O (0.1)	61	0
2	DCE	n-Bu4NI (1.0)	HCl (0.1)	57	0
3	DCE	n-Bu4NI (1.0)	TfOH (0.1)	53	0
4	DCE	n-Bu4NI (1.0)	H2SO4 (0.1)	58	0
5	DCE	NaI (1.0)	TsOH·H2O (0.1)	70	0
6	DCE	KI (1.0)	TsOH·H2O (0.1)	63	0
7	DCE	I2 (1.0)	TsOH·H2O (0.1)	0	0
8	MeCN	NaI (1.0)	TsOH·H2O (0.1)	18	44
9	MeNO2	NaI (1.0)	TsOH·H2O (0.1)	Trace	59
10	Toluene	NaI (1.0)	TsOH·H2O (0.1)	42	30
11	DMSO	NaI (1.0)	TsOH·H2O (0.1)	17	21
12	DMF	NaI (1.0)	TsOH·H2O (0.1)	Trace	15
13	Dioxane	NaI (1.0)	TsOH·H2O (0.1)	30	0
14	EtOH	NaI (1.0)	TsOH·H2O (0.1)	Trace	39
15	DCE	NaI (1.5)	TsOH·H2O (0.1)	78	Trace
16	DCE	NaI (1.5)	TsOH·H2O (0.2)	90	Trace
17	MeNO2	NaI (1.5)	TsOH·H2O (0.2)	Trace	86

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Encouraged by our preliminary findings, the substrate scope of alcohols and sulfinic acids for the synthesis of vinyl sulfides was explored, and the results were shown in Table 2. In the presence of 1.5 equivalents of NaI, 20 mol% TsOH·H₂O and DCE as solvent, a broad range of bisbenzylic, monobenzylic and naphthyl alcohols smoothly underwent this kind of transformation, generating structurally diverse vinyl sulfide 3 in moderate to excellent yields with extremely high stereoselectivity (Table 2, entries 1–11). The reactivity of monoaryl alcohols is lower than diaryl alcohol. Notably, this protocol proved useful for the construction of vinyl sulfides containing polycycles such as 3d–f (Table 2, entries 4–6). However, this protocol was not applicable to less reactive alkyl alcohols (Table 2, entry 9) substrate scope is also broad with respect to the sulfinic acids. Various arylsulfinic acids smoothly reacted with alcohol 1a to give the corresponding vinyl sulfides in good to excellent yields (Table 2, entries 12–22). It is noteworthy that both electron-withdrawing and electron-donating groups were introduced into the vinyl sulfides by employing arylsulfinic acid bearing such groups on the aromatic ring. Furthermore, alkylsulfinic acids could also smoothly react with alcohol 1a to give the corresponding vinyl sulfides (Table 2, entries 23–25). But the yield is lower than arylsulfinic acids, this may be due to the low activity of alkylsulfinic acids.

Table 2 Sulfenylation of alcohols with sulfinic acids^a

Entry	1	2	Product 3		Yield^b (%)
a Reaction conditions: alcohol 1 (0.20 mmol), sulfinic acid 2 (0.30 mmol), NaI (0.30 mmol), TsOH·H2O (0.040 mmol), DCE (1.0 mL), 80 °C, 24 h.b Isolated yield.
1	1a	2a		3aa, Ar = Ph	90
2	1b	2a		3ba, Ar = PMP	83
3	1c	2a		3ca, Ar = 4-ClC6H4	89
4	1d	2a		3da, X = (CH2)2	91
5	1e	2a		3ea, X = O	81
6	1f	2a		3fa, X = S	77
7	1g	2a		3ga, R = Ph	65
8	1h	2a		3ha, R = 4-MeC6H4	69
9	1i	2a		3ia, R = Bn	0
10	1j	2a	RCH = CHSPh	3ja, R = Ph	43
11	1k	2a		3ka, R = 2-naphthyl	56
12	1a	2b		3ab, R = 4-MeC6H4	91
13	1a	2c		3ac, R = 4-tBuC6H4	88
14	1a	2d		3ad, R = 4-MeOC6H4	85
15	1a	2e		3ae, R = 4-FC6H4	93
16	1a	2f		3af, R = 4-ClC6H4	89
17	1a	2g		3ag, R = 4-BrC6H4	86
18	1a	2h		3ah, R = 4-IC6H4	81
19	1a	2i		3ai, R = 3-O2NC6H4	90
20	1a	2j		3aj, R = 2,4-Cl2C6H4	83
21	1a	2k		3ak, 1-naphthyl	98
22	1a	2l		3al, 2-naphthyl	99
23	1a	2m		3am, Me(CH2)7	72
24	1a	2n		3an, Bn	76
25	1a	2o		3ao, CH3	67

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Next, the substrate scope of alcohols and sulfinic acids for the synthesis of vinyl sulfones was explored, and the results were shown in Table 3. In the presence of 1.5 equivalents of NaI, 20 mol% TsOH·H₂O and MeNO₂ as solvent, a broad range of bisbenzylic and monobenzylic alcohols were found to react well with benzenesulfinic acids to yield structurally diverse vinyl sulfones derivatives in good to excellent yields with extremely high stereoselectivity (Table 3, entries 1–7). The results represented in Table 3 show that the scope of sulfinic acids is also very general in the sulfonylation of alcohols. A number of arylsulfinic acids bearing either electron-donating groups or electron-withdrawing groups on the aromatic rings were transformed into their corresponding vinyl sulfones at 80 °C in good to excellent yields (Table 3, entries 8–14). Moreover, alkylsulfinic acids could also smoothly react with alcohol 1a to give the corresponding vinyl sulfones (Table 3, entry 15).

Table 3 Sulfonylation of alcohols with sulfinic acids^ab

Entry	1	2	Product		Yield^b (%)
a Reaction conditions: alcohol 1 (0.20 mmol), sulfinic acid 2 (0.30 mmol), NaI (0.30 mmol), TsOH·H2O (0.040 mmol), MeNO2 (1.0 mL), 80 °C, 24 h.b Isolated yield.
1	1a	2a		4aa, Ar = Ph	86
2	1b	2a		4ba, Ar = PMP	93
3	1c	2a		4ca, Ar = 4-ClC6H4	88
4	1d	2a		4da, X = (CH2)2	90
5	1f	2a		4fa, X = S	81
6	1g	2a		4ga	75
7	1j	2a		4ja	67
8	1a	2b		4ab, R = 4-MeC6H4	91
9	1a	2c		4ac, R = 4-tBuC6H4	85
10	1a	2d		4ad, R = 4-MeOC6H4	82
11	1a	2e		4ae, R = 4-FC6H4	73
12	1a	2f		4af, R = 4-ClC6H4	87
13	1a	2k		4ak, 1-naphthyl	90
14	1a	2l		4al, 2-naphthyl	95
15	1a	2m		4am, Me(CH2)7	63

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In order to gain further insight into the reaction mechanism, four control experiments were set up under various reaction conditions. Benzenesulfinic acid was reduced to diphenyldisulfane 2a′ in 94% yield in the presence of 1.5 equivalents of NaI, 20 mol% TsOH·H₂O and DCE as solvent (Scheme 1a). Under the 1.0 equivalents of iodine, diphenyldisulfane 2a′ smoothly reacted with alcohol 1a to give the corresponding vinyl sulfide 3ab in 91% yield (Scheme 1b). Under the standard reaction conditions, alcohol 1a and diphenyldisulfane 2a′ don't react (Scheme 1c). In the presence of 1.5 equivalents of NaI, 20 mol% TsOH and DCE as solvent, alkene 6a was found to react smoothly with benzenesulfinic acids to give vinyl sulfide 3ab in 93% yields (Scheme 1d).

Scheme 1 Control experiments (a–d).

On the basis of the above experimental results and previous relevant mechanism studies,^14,18 we proposed the reaction pathway depicted in Scheme 2. Initially, alcohol 1 undergo elimination reaction to form alkene 6 under TsOH and heating conditions. Meanwhile, 1,2-diphenyldisulfane (2a′) is generated from sulfinic acid 2 in the presence of NaI and TsOH. Then 2a′ reacts with I₂ to give the sulfenyl iodide 5, which is attacked by 6 to give intermediate 7. Finally, elimination of HI from 7 affords the corresponding vinyl sulfide 3.

Scheme 2 Proposed reaction pathway for the synthesis of vinyl sulfides.

We performed a relevant control experiment for vinyl sulfones (Scheme 3). When a stoichiometric amount of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy, a well known radical-capturing species) was added to this reaction mixture, the formation of 4a was completely inhibited in the reaction. Therefore, we reason that a radical process should be involved in this reaction.

Scheme 3 Control experiment for the generation of vinyl sulfone.

On the basis of the preliminary result above and previous relevant mechanism studies,¹⁴ we proposed a plausible mechanism for the generation of vinyl sulfones in Scheme 4. First, alcohol 1 undergo elimination reaction to form alkene 6 under TsOH and heating conditions. Meanwhile, sulfonyl iodide 8 was generated from benzenesulfinic acid in the presence of NaI under air. Then, 8 experienced homolytic cleavage to produce a sulfonyl radical 9, which interacted with alkene 6 to give intermediate 10. Next, intermediate 10 reacts with an iodine radical to give intermediate 11. Finally, elimination of HI from 11 produces the corresponding vinyl sulfone 4.

Scheme 4 Proposed reaction pathway for the synthesis of vinyl sulfones.

Conclusions

In conclusion, we have developed a NaI mediated elimination/coupling reaction of alcohols and sulfinic acids, constructing structurally diverse vinyl sulfides and vinyl sulfones. Solvent is crucial for the chemical selectivity. Solvent-controlled selectivity of the elimination/coupling reaction was realized for divergent synthesis of vinyl sulfides and vinyl sulfones. The described protocol is more convenient for the preparation of vinyl sulfide than vinyl sulfone related skeletons and the starting materials are readily available. Current efforts are directed toward further methodological refinement and synthetic applications.

Experimental

General procedure for the synthesis of vinyl sulfides (Table 2)

To a solution of alcohol 1 (0.20 mmol) in DCE (1.0 mL) under an air atmosphere at room temperature were added sulfinic acid 2 (0.30 mmol), NaI (45.0 mg, 0.30 mmol) and TsOH·H₂O (7.6 mg, 0.040 mmol). The mixture was stirred at 80 °C for 24 h, cooled to room temperature, and directly purified by preparative thin layer chromatography on silica gel, developing with petroleum ether/ethyl acetate (100 : 0 to 20 : 1), to give compound 3.

General procedure for the synthesis of vinyl sulfones (Table 3)

To a solution of alcohol 1 (0.20 mmol) in MeNO₂ (1.0 mL) under an air atmosphere at room temperature were added sulfinic acid 2 (0.30 mmol), NaI (45.0 mg, 0.30 mmol) and TsOH·H₂O (7.6 mg, 0.040 mmol). The mixture was stirred at 80 °C for 24 h, cooled to room temperature, and directly purified by preparative thin layer chromatography on silica gel, developing with petroleum ether/ethyl acetate (20 : 1 to 5 : 1), to give compound 4.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (No. 21502182, 21202154), Key Laboratory of Watershed Geographic Sciences, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences (WSGS2017005), the Cooperation Fund of Energy Research Institute, Nanjing Institute of Technology (CXY201927).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra07086j

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