One pot synthesis of trifluoromethyl aryl sulfoxides by trifluoromethylthiolation of arenes and subsequent oxidation with hydrogen peroxide

Abstract

Hydrogen peroxide was used for oxidation of various aryl trifluoromethyl sulfides. Trifluoroacetic acid was used as an activating solvent that enables non-catalyzed oxidation and increases selectivity for sulfoxide formation. As shown by oxidation of thianthrene TFA enhances electrophilic character of the oxidant and further oxidation of sulfoxide group is blocked. We have joined trifluoromethylthiolation of arenes using a modified Billard reagent (p-ClPhNHSCF₃) with oxidation of aryl trifluoromethyl sulfides using 1.2 equiv. of 30% aqueous hydrogen peroxide and this one-pot process has superior yields than would have been obtained in a two step process.

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Introduction

Organofluorine compounds have great potential in the development of new materials, bioactive compounds, agrochemicals, pharmaceutical compounds and in many other areas.^1,2 They are interesting because of their chemical, physical and physiological properties.³ The trifluoromethylsulfanyl group (SCF₃) has a high lipophilicity parameter (1.44) and a strong electron-withdrawing effect.^4,5 It has a slightly weaker electron-withdrawing character (Hammett constant: σ_m = 0.40, σ_p = 0.50) than the trifluoromethyl group (CF₃) (Hammett constant: σ_m = 0.43, σ_p = 0.54). Functional groups SOCF₃ and SO₂CF₃ have even stronger electron-withdrawing character (SOCF₃: σ_m = 0.63, σ_p = 0.69; SO₂CF₃: σ_m = 0.79, σ_p = 0.93) than CF₃.⁶ Organic compounds with SOCF₃ and SO₂CF₃ groups are not as common as SCF₃ compounds due to their problematic synthesis. However, they show pharmacological and biological activities (antibacterial, antimalarial, anti-pneumonia and nervous anorexia treatment) and they are already on the market e.g. fipronil, ponazuril.^7,8

Trifluoromethyl aryl sulfoxides can be prepared directly from different aromatic molecules by sulfinylation of aromatics by triflinate salts in acidic medium,⁹ by CF₃SO₂Cl/PCy₃ reagent¹⁰ and by thia-Fries rearrangement process in the presence of AlCl₃ (Scheme 1).¹¹ In 1999 Langlois reported on the synthetic method for trifluoromethanesulfinates and trifluoromethanesulfinamides using the CF₃SO₂Na/POCl₃ system.¹² They can also be prepared with 1-(trifluoromethylsulfinyl)pyrrolidine-2,5-dione in good yields.¹³ Allylic trifluoromethanesulfenates can be prepared from allylic alcohols using [N–SCF₃]⁺ reagent via a [2,3]-sigmatropic rearrangement.¹⁴ ortho-Trifluoromethanesulfinyl anilines can be synthesized by intermolecular C–N addition of amides and S–N addition of sulfinamides to arynes (Scheme 1).¹⁵ Opposite strategy is introduction of CF₃ group into sulfinic esters or sulfinyl halides using TMSCF₃ reagent and fluoride activator.^16,17

Scheme 1 Different ways for preparation of aryl trifluoromethyl sulfoxides.

Researchers have discovered new processes for the selective oxidation of sulfides to corresponding sulfoxides, which are easy to handle, green and cost-effective. Trifluoromethyl sulfides are less reactive for oxidation and various oxidizing agents such as m-CPBA, CF₃CO₃H, NaIO₄, TCCA, cyclic diacyl peroxide, Oxone, MoO₂Cl₂(OPPH₃)₂/Cu(NO₃)₂ have been used and only two examples with H₂O₂ (F₂₀TPPFe and TFA) were reported (Scheme 1).^3,18–22 As trifluoromethyl sulfides are less reactive, selectivity of oxidation is problematic and overoxidation to sulfones occurs readily. The problem with most oxidants is the formation of toxic waste and environmentally harmful by-products (metal salts, reagent residues).²³ Most of them are expensive, oxidation is very temperature sensitive and overoxidation can lead to the formation of unfavorable by-product (trifluoromethyl sulfone).¹⁸ In recent years, research has focused on the development of an effective, simple and selective route for the synthesis of trifluoromethyl sulfoxides under mild reaction conditions using safe and clean oxidation processes. H₂O₂ is an attractive oxidant, inexpensive, soluble in water and many organic solvents, and environmentally friendly, since water is the only theoretical by-product.^23,24 Oxidation with H₂O₂ is useful for the synthesis of pharmaceuticals and agrochemicals, which requires high chemical purity.^24,25 The problem with hydrogen peroxide is that it reacts slowly with organic compounds and must be activated. There are many reports of the hydrogen peroxide based oxidation of sulfides to corresponding sulfoxides under appropriate activation conditions (nucleophilic, electrophilic and radical activation) using transition metal or organic catalyst,^23,26–28 while H₂O₂ was also activated with fluorinated alcohols²⁹ (Scheme 1).

Studies on oxidation of aromatic trifluoromethyl sulfides with H₂O₂ are limited. In 2019 Yagupolskii reported on the oxidation protocol for the oxidation of CF₃S to the CF₃S(O) group using 15% hydrogen peroxide in trifluoroacetic acid. During the oxidation no further unwanted oxidation to trifluoromethyl sulfone occurred. This method has some disadvantages, such as the incomplete conversion of the trifluoromethyl sulfides to sulfoxides. In a few cases the conversion was complete, in others 87–98%.³⁰ The method for direct synthesis of trifluoromethyl sulfoxides from aromatic molecules by introduction of SCF₃ group followed by oxidation has not yet been investigated. Due to the limited set of available aryl trifluoromethyl sulfides, the one-pot procedure is very desirable. The trifluoromethylsulfanyl group (SCF₃) can be introduced into the aromatic molecules with various electrophilic reagents.^6,31,32 One of the most commonly and widely used reagent is the Billard reagent PhNHSCF₃,^33,34 while reagent p-ClPhNHSCF₃ is suited for the use on wider range of arenes.³⁵ We present in this report effective, simple and highly selective method for the one-pot synthesis of various aryl trifluoromethyl sulfoxides from different activated and deactivated aromatic molecules by trifluoromethylthiolation with the stable and easy to use reagent p-ClPhNHSCF₃,³⁵ followed by oxidation with H₂O₂ in TFA (Scheme 2).

Scheme 2 One-pot synthesis of trifluoromethyl sulfoxides from different aromatic molecules.

Results and discussion

Phenyl trifluoromethyl sulfide 1a was chosen as model substrate for an initial study on the effect of reaction conditions on the selective oxidation of the trifluoromethylsulfanyl group (SCF₃) with hydrogen peroxide as an oxidant. The sulfur atom is deactivated for oxidation because of the electron-withdrawing CF₃ group and must be activated. We examined the effect of different solvents, the concentration of hydrogen peroxide, transition metals, Brønsted and Lewis acids on the oxidation of the sulfur atom.

First, we investigated the oxidation of trifluoromethyl sulfide 1a to the corresponding trifluoromethyl sulfoxides 2a or sulfones 3a with 30% H₂O₂ in different solvents such as non-polar solvent (PhCH₃), polar aprotic solvents (DCM, EtOAc, MeCN), polar protic solvents (EtOH, i-PrOH) and fluorinated alcohols (TFE, HFIP) (Table 1). A solution of trifluoromethyl sulfide 1a in selected solvent and 2 equiv. of 30% aqueous hydrogen peroxide was stirred at room temperature for 24 h. Afterwards the conversion was determined by ¹H NMR spectroscopy of the crude reaction mixture. The experimental results in Table 1 show that a small amount of product 2a was formed only in the HFIP. In other solvents oxidation did not occur.

Table 1 The effect of solvent on oxidation of 1a with H₂O₂^a

Entry	Solvent		Conv.^b [%]
a Reaction conditions: phenyl trifluoromethyl sulfide 1a (0.5 mmol), H2O2 (1.0 mmol, 30–100%), solvent (2 mL), rt, 24 h.b Conversion to product was determined by ^1H NMR.c TFE – 2,2,2-trifluoroethanol, HFIP – 1,1,1,3,3,3-hexafluoropropan-2-ol.d Reflux temperature.
1	Non-polar	PhCH3	0
2	Polar aprotic	DCM, EtOAc, MeCN	0
3	Polar protic	EtOH, i-PrOH	0
4	Fluorinated alcohols^c	TFE	0
5		HFIP (30%)	12
6		HFIP (50%)	15
7		HFIP (100%)	16
8		HFIP^d (30%)	50

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Fluorinated alcohols strongly activate hydrogen peroxide by their high ionizing power, strong hydrogen bond donor ability and weak hydrogen bond acceptor strength. HFIP has a stronger effect than TFE. It is known that the activation of 30% hydrogen peroxide by HFIP is strong enough to selectively oxidize sulfides to sulfoxides.³⁶ Furthermore, HFIP deactivates sulfoxide for further oxidation to sulfone by its interaction with sulfoxide group. In the case of sulfide 1a, the deactivating effect of the trifluoromethyl group outweights activation by HFIP. We have tried to increase the reaction with more concentrated hydrogen peroxide (50% and 100%). Surprisingly, the concentration of H₂O₂ had only a minor influence on the conversion (Table 1, entries 5–7). Only at the reflux temperature 50% of 2a were formed.

Since the activation by HFIP itself was not strong enough to achieve complete oxidation of 1a, we decided to investigate the effect of stronger activators, such as metal catalysts, Brønsted and Lewis acids. We used only 1.2 equiv. of H₂O₂ and 10 mol% catalyst in these reactions to reduce over-oxidation. Strong Brønsted acids catalyzed the oxidation of 1a and complete conversion was observed in 24 hours at room temperature, but trace amounts of sulfone 3a were also formed (Table 2, entries 1–3). TFA – a weaker acid, was not so effective, while no reaction occurred in acetic acid (entries 4 and 5). Activation with Lewis acid BF₃·OEt₂ led to a similar result as with strong acids. Selected metal-catalyzed oxidation depended on the metal H₂WO₄ was the least selective catalyst, MTO did not give complete conversion, while FeCl₃ gave similar results to TFA (Table 2, entries 7–9).

Table 2 Oxidation of 1a with H₂O₂ in presence of different activators^a

Entry	Activator	Conv.^b [%]	Product selectivity [%]
			2a	3a
a Reaction conditions: phenyl trifluoromethyl sulfide 1a (0.5 mmol), H2O2 (0.6 mmol, 30%), catalyst (10 mol%), HFIP (2 mL), rt, 24 h.b Conversion to product was determined by ^1H NMR.c 1 mol% of activator.
1	HCl	100	99	1
2	H2SO4	100	99	1
3	TfOH	100	99	1
4	TFA	11	100	0
5	AcOH	0	0	0
6	BF3·Et2O	99	98	2
7	H2WO4	100	95	5
8	CH3ReO^c, pyridine^c	97	100	0
9	FeCl3, pyridine	10	100	0

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Since TFA had some catalytic activity, we decided to use it as a solvent for oxidation of 1a to 2a. To a solution of trifluoromethyl sulfide 1a in TFA 1.2 equiv. of 30% aqueous hydrogen peroxide was added. The reaction mixture was stirred at room temperature and 1a was completely oxidized within three hours with formation of trifluoromethyl sulfoxide 2a and sulfone 3a in ratio 97 : 3 (Table 3, entry 1). Analogous oxidation of 1a in AcOH was slower and conversion after 48 hours reaction was 80%. Promising results in TFA lead to further optimization. The use of an equimolar amount of H₂O₂ led to a selective formation of sulfoxide 2a with 96% conversion in 3 h at room temperature (Table 3, entry 2), however reaction did not proceed to completion even after prolonged reaction time (Table 3, entry 3). The reaction at 0 °C was slower and a higher conversion was achieved with a higher excess of H₂O₂ (Table 3, entries 4–7). The quantitative conversion was achieved with two equivalents of the oxidant, but a small amount of over-oxidized product 3a was present in the reaction mixture. The quantitative and selective conversion of 1a to 2a was achieved with 1.2 equiv. of H₂O₂ at 0 °C in 6 hours (Table 3, entry 8). Sulfone 3a was difficult to form in TFA and harsher conditions were required – 2.4 equiv. of hydrogen peroxide at 60 °C led to formation of sulfone 3a with a yield of 96% (Table 3, entry 9).

Table 3 The effect of the reaction conditions on oxidation of 1a with H₂O₂ in TFA^a

Entry	Temp.	Equiv. H2O2	Conv.^b [%]	Product selectivity [%]
				2	3
a Reaction conditions: phenyl trifluoromethyl sulfide 1 (0.5 mmol), H2O2 (0.5–1.2 mmol, 30%), TFA (2 mL), 3 h.b Conversion to product was determined by ^1H NMR.c Reaction time: 6 h.d Isolated yield.
1	Rt	1.2	100	97	3
2	Rt	1.0	96	100	0
3	Rt	1.0^c	94	100	0
4	0 °C	1.0	65	100	0
5	0 °C	1.2	67	100	0
6	0 °C	1.5	84	100	0
7	0 °C	2.0	100	99	1
8	0 °C	1.2^c	100	100	0
9	60 °C	2.4	100	4	96 (95%)^d

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We applied the knowledge gained in initial studies to oxidize various aromatic trifluoromethyl sulfides 1. Due to limited commercial availability, a library of trifluoromethyl sulfides 1 was prepared from various activated and deactivated aromatic substrates using our method with p-ClPhNHSCF₃.³⁵ The best conditions for the preparation of 2a (1.2 equiv. of 30% H₂O₂, 0 °C, TFA) were applied. To a solution of trifluoromethyl sulfide 1 in TFA, 1.2 equiv. of 30% H₂O₂ was added and the reaction mixture was stirred at 0 °C until the sulfide was consumed, as determined by GC-MS. In all cases a selective and quantitative transformation to trifluoromethyl sulfoxide 2 was observed and we prepared thirteen trifluoromethyl sulfoxides 2 (Scheme 3) with electron-donating and electron-withdrawing groups on the aromatic ring in 77–95% yield. The oxidation tolerated the presence of hydroxyl, acetyl and hydroxymethyl groups on the aryl ring. On the other hand, the formyl group is not tolerated and p-formylphenyl trifluoromethyl sulfide was converted into a complex mixture of products. The phenanthryl derivative 1m was also oxidized in 85% yield to the corresponding sulfoxide 2m.

Scheme 3 Selective oxidation of aryl trifluoromethyl sulfides 1.

An introduction of the SCF₃ group into heteroaromatic ring is difficult and pyridine, imidazole and benzothiazole failed to react under these reaction conditions. On the other hand, pyrrole was converted into a complex mixture of products. Furthermore, sulfur atom in SCF₃ group is very deactivated for oxidation leading to problems with selectivity of oxidation of substrates with various functional groups. We made a further study on the series – benzofuran, benzothiophene and indole to see how heteroatoms affects the oxidation. Trifluoromethylthiolation of benzofurane did not occur under the classical reaction conditions and only starting compound was recovered. Indole and benzothiophene were selectively transformed to the corresponding trifluoromethylthiolated derivatives 1n and 1o, respectively.³⁵ Indole was selectively and quantitatively trifluoromethylthiolated to give 3-((trifluoromethyl)thio)-1H-indole 1n, which was oxidized under the standard reaction conditions to the desired trifluoromethyl sulfoxide 2n and isolated in 33% yield (Scheme 3). A lower yield is attributed to the formation of a complex mixture of products. 3-((Trifluoromethyl)thio)benzo[b]thiophene 1o has been completely oxidated, but oxidation occurred on both sulfur atoms and a mixture of 2o and 3-((trifluoromethyl)thio)benzo[b]thiophene 1-oxide 2o′ in the ratio 34 : 66 was formed. We isolated both products, however the side product 2o′ is not stable and starts to decompose during isolation procedure into the product 2o′′ (Scheme 4). This surprising reaction could be a result of a hetero Diels–Alder additions followed be a hydride shift. The product 2o′′ also decomposes slowly into a complex mixture of products.

Scheme 4 Oxidation of 3-((trifluoromethyl)thio)benzo[b]thiophene 1o.

TFA as solvent is a very effective activator of H₂O₂. As in template catalysis in fluorinated alcohols,²⁹ formation of hydrogen bonds between ArSCF₃ 1, TFA and H₂O₂ could be the reason for the activation. Furthermore, the interaction between sulfoxide 2 and TFA could reduce the nucleophilicity of the sulfur atom and deactivate the SOCF₃ group for further oxidation to the sulfones.²⁹ To test the specific interaction of TFA and H₂O₂ in selective oxidation, we compared the results of the oxidation of sulfide 1j with H₂O₂ and with meta-chloroperoxybenzoic acid (m-CPBA).³⁷

Table 4 shows data for the oxidation of sulfide 1j to sulfoxide 2j or sulfone 3j with m-CPBA and with 30% H₂O₂. 1j is oxidized selectively and quantitatively with 1.2 equiv. of 30% H₂O₂ in TFA in 2.5 hours at 0 °C. An analogous reaction with m-CPBA is less effective and less selective in TFA. Under the same reaction conditions the conversion is only 22%, while a significant amount of sulfone 3j is already formed.

Table 4 Selectivity of oxidation of 1j with different H₂O₂ and m-CPBA^a

Entry	Oxidant	Conv.^b [%]	Product selectivity [%]
			2j	3j
a Reaction conditions: 1j (0.5 mmol), oxidant (1.0 mmol), TFA (5 mL), 2.5 h, 0 °C.b Conversion to product was determined by ^1H NMR.
1	H2O2	100	100	0
2	m-CPBA	22	68	32

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To further elucidate the role of trifluoroacetic acid in this reaction we have turned to thianthrene 4 as a model substrate. Adam and his coworkers have studied the oxidation of thianthrene 5-oxide (SO) as a mechanistic probe to determine the electronic character of various oxidising agents. Electrophilic oxidants oxidize SO at the sulfide S-atom to give sulfoxide SOSO, while nucleophilic oxidants react preferentially at the sulfoxide S-atom to afford sulfone SSO₂.^38,39

We compared selectivity of oxidation of thianthrene 4 with m-CPBA and with H₂O₂ in three solvents – DCM, MeCN and TFA. As presented in Fig. 1, oxidation with 2 equiv. of m-CPBA in DCM and MeCN proceeded at room temperature with 100% conversion. The main product was sulfoxide SO 5, while some sulfone SSO₂ 7 already formed. Reaction in TFA was performed at 0 °C and the main reaction channel was oxidation of both sulfur atoms with formation of SOSO 6 with the major stereoisomer being cis-SOSO 6a. The result is in accordance with the effect of TFA.⁴⁰ No formation of SSO₂ 7 was observed. Oxidation of thianthrene S 4 with H₂O₂ proceeds only in TFA. Contrary to oxidation in MeCN and DCM, cis-SOSO 6a was the major product in reaction in TFA and the selectivity for 6a/6b was higher at 0 °C. Even after prolonged reaction time at room temperature no sulfone 7 is formed. Results show that TFA promotes oxidation through electrophilic process, which is evident by higher selectivity towards formation of sulfoxide products.

Fig. 1 Selectivity of oxidation of thianthrene 4 (reaction conditions: 4 (0.5 mmol), oxidant (1.0 mmol), solvent (5 mL), time. Product distribution was determined by ¹H NMR spectroscopy).

TFA therefore plays an important role in the activation of hydrogen peroxide for the oxidation of ArSCF₃ and in the deactivation of further oxidation to sulfone. TFA could activate the electrophilic character of H₂O₂ for oxidation by hydrogen bonding, while the same interaction with the oxygen atom of the sulfoxide group would make the S-atom in SOCF₃ more electrophilic and thus less reactive.

The same activation by TFA could also be effective in the reaction of trifluoromethylthiolation of aromatic molecules with the ArNHSCF₃ reagent. Our goal was to combine both reactions – the introduction of SCF₃ group and its oxidation – in a one-pot process to incorporate SOCF₃ group into the aromatic ring.

With these optimized reaction conditions in hand, we have investigated the possibility of selective and quantitative synthesis of trifluoromethyl sulfoxides 2 from different aromatic and heteroaromatic molecules 1′ in one step. The reaction conditions for the preparation of trifluoromethyl sulfides 1 were taken from the reported reaction conditions.³⁵ A solution of substrate 1′, reagent p-ClPhNHSCF₃, activator TfOH and solvent was stirred for 20 h at room temperature. The choice of the solvent depended on the reactivity of the arene (DCM, hexane or TFA). After completion of the reaction, TFA was added to the reaction mixture and cooled to 0 °C. 1.2 equiv. of 30% aqueous hydrogen peroxide were added and the reaction time was followed by GC-MS.

Ten different products were prepared by one-pot process and the reactions were quantitative and selective as in both separate processes (Scheme 5). In general, this one-pot process generated the corresponding products in high yields (78–93%), with the exception of heteroaromates 2n (37%) and 2o (25%). Trifluoromethyl sulfoxides 2 were obtained in quantitative yields, without any trace of trifluoromethyl sulfone 3. Compared to the classical two-step process, the one-pot method is faster, cheaper, easier to carry out and gives better yields (yields were 2 to 13% higher than in the classical two-step process).

Scheme 5 Isolated yields of the trifluoromethyl sulfoxides 2. (1) (Method A: 1a : Cl/SCF₃ : TfOH = 1 : 1.3 : 1.3, solvent: DCM; method B: 1a : Cl/SCF₃ : TfOH = 1 : 1.3 : 1.3, stepwise addition of Cl/SCF₃, solvent: DCM; method C: 1a : Cl/SCF₃ : TfOH = 1 : 2 : 3; stepwise addition of Cl/SCF₃ and TfOH, solvent: ¹hexane/²TFA), rt, 20 h. (2) H₂O₂ (30%), TFA (5 mL), t, 0 °C.

Conclusions

In summary, we have developed the highly selective oxidation of aryl trifluoromethyl sulfides to the corresponding sulfoxides under metal-free conditions with 30% H₂O₂. The oxidation was performed in the presence of 30% aqueous hydrogen peroxide as oxidant and TFA as solvent and activator. TFA proved to be effective in activating hydrogen peroxide and increasing selectivity by deactivating further oxidation to sulfone. Furthermore, we have investigated a new method for the direct introduction of SOCF₃ functional group into various aromatic molecules. This method allows a simple and efficient synthesis of different trifluoromethyl sulfoxides from aromatic molecules by one-pot trifluoromethylthiolation with p-ClPhNHSCF₃ reagent followed by oxidation with 30% H₂O₂. Mild reaction conditions and generally excellent yields were observed (78–93%).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support of the Slovenian Research Agency (P1-0134, P1-0230) is greatly appreciated. The authors are grateful to the stuff of The Centre for Research Infrastructure at the Faculty of Chemistry and Chemical Technology (IC UL FCCT).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra04621c

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