A radical coupling reaction of DMSO with sodium arylsulfinates in air : mild utilization of DMSO as C₁ resource for the synthesis of arylsulfonyl dibromomethane

Abstract

A radical coupling reaction of DMSO with sodium arylsulfinates under air atmosphere to afford arylsulfonyl dibromomethane is described. This transformation provides a novel approach for the utilization of DMSO as a C₁ resource with mild temperature without the need for an anaerobic atmosphere.

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Dimethyl sulfoxide (DMSO) is an important organic solvent¹ and oxidant.² What's more, as a cheap material as a source of CH₃,³ CH₃S,⁴ O,^4e,g,5 CH₃SO,⁶ its application in organic synthesis is increasingly attractive. In particular, using DMSO as a one-carbon synthon, which can be transformed into not only CH₃,³ but also CHO,^7–9 and CN,¹⁰ even be utilized to build heterocyclic compounds,¹¹ is much more useful. However, these transformations usually involve metal catalysts,⁷ or high temperature,^7b,8 even sometimes the harsh or special conditions are necessary.^7c,9

Sodium sulfinates have been also widely applied in organic synthesis.¹² Recently, Jiang's group reported some important breakthroughs, including the copper-catalyzed coupling reaction of oxime acetates with sodium sulfinates for the synthesis of sulfonylvinylamine.¹³ According to the reaction type, many radical reactions of sodium sulfinates catalyzed by metal or other catalysts have aroused wide concern among the chemists.^13c,d,14 However, the radical coupling reaction without any catalyst is still less investigated,^13c,15 which urgently needs to be developed.

On the basis of our previous work on 2(5H)-furanone chemistry,¹⁶ recently we investigated palladium-catalyzed desulfitative arylation of 5-alkoxy-3,4-dibromo-2(5H)-furanone with sodium arylsulfinates.¹⁷ During its condition optimization, the reaction of sodium arylsulfinates 1, 3,4-dibromo-5-hydroxy-2(5H)-furanone 2 and DMSO in air was accidentally detected to afford arylsulfonyl dibromomethane 3 (Scheme 1). To the best of our knowledge, there is no report about the reaction of sodium sulfinates using DMSO as the CH source.

Scheme 1 Coupling reaction of DMSO and sodium arylsulfinates.

Furthermore, many arylsulfonyl dihalomethanes have important bioactivities, and some similar compounds can be used as a kind of very useful organic intermediates, which make their syntheses and applications be under the spotlight recently.¹⁸ Especially, arylsulfonyl dibromomethanes are developed too.¹⁹ Thus, this new reaction reported herein not only has theoretical significance for the utilization of DMSO in synthetic chemistry, but also has a certain practical value.

In our initial studies, we took the reaction of 2 and 1a as our model reaction to optimize the reaction condition (Table 1). From entries 1–5, it can be seen that the suitable time should be 12 h (entry 3). Comparing entry 3 with entries 6–8, the suitable temperature should be 100 °C.

Table 1 Optimization of reaction conditions^a

Entry	Time (h)	Temp. (°C)	Solvent	Yield^b (%)
a Reaction conditions: all reactions were performed with 1a (0.55 mmol), 2 (0.50 mmol) and solvent (3.0 mL) at the designed oil bath temperature under air.b GC yield.c Isolated yield.d In order to avoid the volatilization of acetone, the experiments were investigated using different temperatures or using a sealed reaction system filled with oxygen (or air) at 100 °C.
1	6	100	DMSO	76
2	9	100	DMSO	84
3	12	100	DMSO	92 (84^c)
4	15	100	DMSO	92
5	18	100	DMSO	92
6	12	60	DMSO	68
7	12	80	DMSO	74
8	12	120	DMSO	87
9	12	100	Toluene	0
10	12	100	1,4-Dioxane	0
11	12	100	DMF	0
12	12	100	NMP	0
13	12	100^d	Acetone	0
14	12	56^d	Acetone	0

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We also investigated the influence of different solvents, including some potential one-carbon synthons, such acetone, N,N-dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP),¹¹ while only DMSO was found to be the effective solvent (entries 9–14 vs. entry 3, Table 1). This indicates DMSO is not only used as a solvent in this reaction, but also participates in the reaction as a one-carbon synthon via its peculiar mechanism, which will be discussed in the following.

With the optimal reaction conditions in hand, we evaluated the scope of sodium arylsulfinates 1 and the results were summarized in Table 2. Different substituents on the benzene ring of sodium arylsulfinates 1 such as methyl, methoxy, fluoro, chloro, bromo were well tolerated and the corresponding products could be formed in good to excellent yields (69–90%). And the substrates with other aryl such as naphthyl, thienyl also could afford the products with satisfactory yields under the optimum reaction conditions (3m and 3n).

Table 2 Substrate scope of various sodium arylsulfinates^a,b

a Reaction conditions: 1 (0.55 mmol), 2 (0.50 mmol), DMSO (3.0 mL), under air atmosphere, 100 °C for 12 h.b Isolated yields.

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Interestingly, electron-donating groups exhibited positive effect on this transformation (3a, 3l vs. 3b). Contrarily, electron-withdrawing groups were adverse to reaction (3g–3k vs. 3b). Due to the steric hindrance, some products (3c–3f) were obtained with slightly lower yields than 3b, though there were alkyl groups in the substrates. Thereby, the lowest yield was given for 3f (74%). Steric effect was also reflected in the meta-substitution of benzene ring (3k vs. 3g). The combined influences of both electronic effect and steric hindrance on reaction may be related to the reaction mechanism (which will be discussed in the following).

The structure of compound 3 was confirmed by NMR, MS and single crystal X-ray analysis (Fig. 1).²⁰ To gain more insight into the mechanism of the process, we conducted several control experiments (Scheme 2). According to the results of optimization, we have found that the reaction can be carried out only in the solvent DMSO (Table 1, entry 3 vs. entries 9–12). We speculated that some carbon atoms in DMSO should be involved in the reaction. Further experiments were carried out with other sulfoxides, and the results of GC-MS detection and product purification all demonstrated that this reaction was only performed very well with methyl sulfoxide (Scheme 2, eqn (1) vs. eqn (2)). Especially, due to only one methyl per molecule, the yield of methylsulfinylbenzene (eqn (1)) was significantly lower than the normal yield under standard conditions (Table 2, 3a, 84%). Thus, the CH group in product 3 should be stemmed from the CH₃ group in DMSO,²¹ and not be related to the reactant 2.

Fig. 1 The molecular structure of 3a.

Scheme 2 Control experiments.

When the amount of the reactant 2 was reduced by half under our standard conditions, the yield of the target product 3a was only 41% (Scheme 2, eqn (3)). This datum was almost a half of the original yield (Table 2, 3a, 84%). Therefore, it was determined that the bromine source of the product 3 was from the reactant 2. However, the expected product 3a was not obtained by using Br₂ instead of the reactant 2 (eqn (4)).²² This suggests that the reactant 2 may be involved in the reaction as the form of bromine radical.

It has been reported that sodium arylsulfinate could generate sulfone radical in the presence of some oxidants, including DMSO or O₂.^{13c,d,14a,b,g} Thus, serial radical control experiments were designed. When the reaction was carried out under N₂ atmosphere, the yield of product 3a was greatly decreased from 84% to 12% (Scheme 2, eqn (5)). This indicates that DMSO also has certain oxidizing ability for the formation of free radicals, but obviously less than O₂. Thus, we further added free radical trapping agent 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) into the standard reaction system. Then, only trace product 3a was detected by GC-MS, but too less to be isolated (eqn (6)). The result confirmed that the reaction should be a radical coupling of DMSO with 1.

On the basis of the above experiments and previous reports about the formation of radical from DMSO,^7a,b a possible mechanism was illustrated in Scheme 3. In the steps therein, both DMSO and O₂ (especially O₂ by comparing eqn (5) with eqn (6) in Scheme 2) were important for the formation of different radicals, such as radical A and Br radical. Subsequently, Br radical reacted with DMSO to give intermediate I. Then, intermediate I was broken down into radical B and radical C, which (especially radical C) might be transformed into radical D by the oxidative substitution²³ of Br radical in the presence of O₂. Finally, radical D was coupled by radical A to form the product 3.

Scheme 3 Possible reaction mechanism.

The intermediate I which is stemmed from DMSO and 2 in Scheme 3 was proved via the verification experiment (Scheme 4, eqn (1)) and indeed detected by GC-MS (Fig. S1, see ESI†).

Scheme 4 Verification experiments.

According to the most reported synthetic methods of arylsulfonyl dibromomethanes (Scheme S1, eqn (1), see ESI†),¹⁹ the radical reaction of halogenating agents with methyl aryl sulfone also may give compound 3. However, two plausible reactions were not successful indeed (Scheme 4, eqn (2) and (3)). There are also a few reports about the synthesis of arylsulfonyl dihalomethanes in the presence of KOH from haloform and sodium arylsulfinates (Scheme S1, eqn (2), see ESI†)²⁴ or arylsulfonyl monohalomethanes.²⁵ However, haloform was not reacted with sodium arylsulfinate 1a under our standard conditions (Scheme 4, eqn (4)).²⁶ And especially, the pH of our reaction system for the reaction of 1a and 2 was basically maintained between 5–7 within 0–12 h. Therefore, both positive and negative verification experiments in Scheme 4 also demonstrated the mechanism in Scheme 3.

What's more, the radical mechanism in Scheme 3 could well explain the influences of sodium arylsulfinates 1 on the yields mentioned above. When radical A was produced from the substrate 1, due to the conjugate effect, the benzene ring bearing electron-donating groups easily made radical A more stable, which was benefit for the radical coupling. Contrarily, when the benzene ring was substituted by electron-withdrawing groups, radical A easily became more unstable, which made the yield decreased. Obviously, the bigger steric hindrance was adverse for both the stability of radical A and the coupling reaction with radical D, so the yield was reduced.

In conclusion, without the needs of anaerobic atmosphere, the utilization of DMSO as one-carbon synthon was developed for the first time. In addition, the temperature is not high, the condition is mild. The detailed mechanistic studies showed that this transformation was involved in the radical coupling reaction of DMSO with sodium arylsulfinates. This protocol may provide new opportunities in organic synthesis for the comprehensive utilization of organic sulphur oxides, such as sulfoxides²⁷ and sodium sulfinates. Further study on this topic is currently undergoing in our laboratory.

Acknowledgements

The authors thank Guangdong NSF (No. 2014A030313429, S2011010001556), the 3rd Talents Special Funds of Guangdong Higher Education (No. Guangdong-Finance-Education [2011]431) and NNSFC (No. 20772035) for financial support.

Notes and references

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20. ESI.†.
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Footnotes

† Electronic supplementary information (ESI) available: Experimental section, characterization of all compounds, Fig. S1, copies of ¹H and ¹³C NMR spectra for all target compounds. CCDC [1062338]. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra01393g

‡ These authors contributed equally to this work.

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