Selective approach to thioesters and thioethers via sp³ C–H activation of methylarenes

Abstract

Novel C–S cross-dehydrogenative coupling (CDC) approaches for the selective synthesis of thioesters and thioethers have been developed via sp³ C–H activation of methylarenes and subsequent functionalization. The reaction of methylarenes with thiols resulted in thioesters in the presence of a FeBr₂/TBHP system, while treatment of methylarenes with thiols in the Pd(OAc)₂/O₂/TBHP system led to the formation of thioethers. Both the green protocols demonstrate good functional group tolerance and satisfactory yields.

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Introduction

The direct C–H activation path is now being extensively investigated as a short step and greener alternative to the traditional transition-metal-catalyzed couplings.¹ One such strategy, cross-dehydrogenative coupling (CDC), is of immense importance because it does not require substrate prefunctionalisation and enjoys the advantage of atom economy.² Until now the CDC based protocols for the construction of a C–C,³ C–O,⁴ C–N⁵ bonds have been easily achieved using transition metal catalysts in combination with various oxidants. However there is still a dearth of information on C–S bond formation.⁶ In this regard, C–S bond formation and eventual synthesis of thioesters and thioethers, involving C–H bond activation of methylarenes remains the cherished target.

Sulfur-containing compounds, especially thioesters and thioethers, represent a class of important synthetic intermediates due to their ease of transformation.⁷ Moreover, they are excellent building blocks for chemical biology, and hence are frequently found in a number of biologically active and medicinal agents.⁸ Traditionally, thioethers were formed by coupling reactions between halides and thiols (or sulfur surrogates),⁹ the SNAr reactions,¹⁰ substitution reactions,¹¹ whereas thioesters were commonly prepared via acylation with thiols (or sulfur surrogates) and carboxylic acids,¹² carboxylic acid halides,¹³ carboxylic acid anhydrides¹⁴ or aldehydes.¹⁵ Although many elegant protocols for the synthesis of thioethers and thioesters have emerged, in accordance with their importance, the synthesis of thioethers or thioesters by unconventional approaches was always appreciable, particularly through the functionalization of inert C–H bonds.

Methylarenes, derived from the oil industry, constitute the most inexpensive and readily available feed stocks for chemistry. Of late, the sp³ C–H activation and subsequent oxidative functionalization of methylarenes has drawn scientist's attention.^2e,16 Through this strategy, methylarenes, especially methylbenzenes, are found to be useful precursors and have been used as ArCOO–,¹⁷ ArCO–,^3h,5b,18 ArCH₂O–¹⁹ and ArCH₂– surrogates.²⁰ Considering the availability and economy of methylarenes and the few reported C–S formation examples via CDC protocol,^6a,21 herein we wish to report two novel methods for the synthesis of thioesters and thioethers via sp³ C–H activation of methylarenes. Thus, the use of more traditional coupling partners that require prefunctionalisation, including aryl halide, acyl halide, aryl acid, and aryl aldehyde, are avoided.

Results and discussion

Encouraged by recent Fe-catalyzed synthesis of thioesters from thiols and aldehydes,^15k we initiated our work using toluene and 4-chlorobenzenethiol under similar conditions (Fe, TBHP system). The aimed thioesters were observed in low yield by GC-MS along with the formation of benzaldehyde and disulfide²² (Table 1, entry 7). In the process to optimize the reaction conditions, it was found that small amount of thioether was formed when Pd(OAc)₂ was used (Table 1, entry 6). Promising to selectively obtain thioesters and thioethers, we optimized both the reactions.

Table 1 Screening for optimal conditions^a

Entry	Change from the “standard conditions”	Yield^b
a Standard conditions: toluene 1.5 mmol, 4-chlorobenzenethiol 0.5 mmol, FeBr2 0.01 mmol, TBHP (70% in water) 1.5 mmol, 2 wt% SDS–H2O 1 mL, 100 °C, 24 h.b The yield was determined by GC.c Thiol was added in a dropwise fashion.d The yield of thioether.e The yield of disulfide.
1	None	65(80^c)
2	No FeBr2	<1
3	FeCl2 instead of FeBr2	62
4	Fe(acac)3 instead of FeBr2	43
5	Cu(OAc)2 instead of FeBr2, no surfactant	8
6	Pd(OAc)2 instead of FeBr2, no surfactant	<1(7^d)
7	No surfactant	12(80^e)
8	2 wt% Prij L23 instead of 2 wt% SDS	10
9	2 wt% Trion X100 instead of 2 wt% SDS	11
10	0.1 eq. TBAI instead of 2 wt% SDS	25(69^e)
11	BPO instead of TBHP	0(95^e)
12	DTBP instead of TBHP	0(10^e)

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As the reaction was conducted in water, surfactants may accelerate the reaction process. Screening nonionic and ionic surfactants revealed that nonionic surfactants such as Trion-X 100, and Prij were ineffective, whereas in the presence of TBAI, the yield increased to 25%. The addition of 2 wt% SDS–water boosted the yield to 65% (Table 1, entries 7–10). Next, we examined the metal catalysts in a quest to improve the yield (Table 1 entries 3–6). Among all the catalysts tested, FeBr₂ was still found to be the best choice, and oxidants were also checked. In the presence of BPO, 4-chlorobenzenethiol converted to the corresponding disulfide quickly without the formation of desired thioester (Table 1, entry 11), while in the case of DTBP, most starting materials were not converted (Table 1, entry 12). Among all the conditions, the formation of disulfide seemed inevitable. When the thiol was added in a dropwise fashion, the yield of thioester could increase to 80% (Table 1, entry 1).

Having established the optimized reaction conditions, the present oxidative esterification reactions were then implemented on the reaction between toluene and a series of substituted thiophenols and thiols. As shown in Table 2, thiophenols containing electron-donating or electron-withdrawing groups reacted smoothly with toluene, giving thioesters in moderate to good yields (Table 2, 3a–e). There was a decrease of yield when thiophenols bear methyl group at different positions (Table 2, 3b and 3c). It was found that the methyl group can also be activated with the small amount of mercaptobenzaldehyde and mercapto thioesters that were observed. Alkyl thiols can also successfully couple with toluene (Table 2, 3f–k, 3m and 3o). In general, the product yields for the alkyl thiols are lower than those of thiophenols. Especially, sterically demanding substrates yield the product in a lower level (Table 2, 3j). It is worthy to note that thiols bearing fluorous tail can also undergo this coupling, which may be useful in fluorine chemistry²³ (Table 2, 3p). We further investigated heterocyclic thiophenols, and to our delight, the reaction between toluene and benzo[d]thiazole-2-thiol afforded the aimed product (Table 2, 3n) in moderate yield. Unfortunately, the reaction failed to proceed in the case of 1H-benzo[d]imidazole-2-thiol, with a collection of byproducts, including the open ring product and the C–N coupling product being obtained instead of the aimed thioester (Table 2, 3q).

Table 2 Scope of thiols in the oxidative esterification^a

Entry	Product			Yield^b (%)
a Reaction conditions: toluene 1.5 mmol, thiol 0.5 mmol, FeBr2 0.01 mmol, TBHP (70% in water) 1.5 mmol, 2 wt% SDS–H2O 1 mL, 100 °C, 24 h.b Isolated yield.c Reaction conditions: toluene 1.5 mmol, thiol 0.25 mmol, FeBr2 0.02 mmol, TBHP (70% in water) 3 mmol, 2 wt% SDS–H2O 1 mL, 100 °C, 36 h.
1		R = 4-Cl	3a	82
2		R = 2-CH3	3b	45
3		R = 4-CH3	3c	56
4		R = 4-F	3d	66
5		R = 4OCH3	3e	84
6		R = 4-F	3f	64
7		R = H	3g	71
8		R = 4-CH3	3h	72
9			3i	65
10			3j	10
11			3k	35
12			3l	0
13			3m	52
14			3n	46
15			3o	42^c
16			3p	45
17			3q	0

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After that we turned our attention to the functional group compatibility of methylarenes. Methylarenes containing a methyl-, chloro-, bromo- or methoxy-substituent were all tolerated under the reaction conditions (Table 3, 4a–d, 4f and 4g). However, methylarene bearing a –N(CH₃)₂ substituent failed to couple with the thiophenol. The demethylation of the N–CH₃ was observed under the standard conditions. The steric hindrance had little influence with similar yields obtained with 4b, 4g. When there are more than one methyl groups in the benzene ring (for example 4k), the “dithioester” could be afforded when more TBHP was added and the reaction time extended. Accompanied with 4k, S-(p-tolyl)-3-formylbenzothioate 4k′ was also observed. Remarkably, naphthalene-containing methylarenes could also serve as coupling partners with thiols (Table 3, 4j). However, unfortunately, only trace amount of the aimed product was observed when pyridine-containing substrate was used.

Table 3 Scope of methylarenes in the oxidative esterification^a

Entry	Product			Yield^b (%)
a Reaction conditions: toluene 1.5 mmol, thiol 0.5 mmol, FeBr2 0.01 mmol, TBHP (70% in water) 1.5 mmol, 2 wt% SDS–H2O 1 mL, 100 °C, 24 h.b Isolated yield.c Reaction conditions: toluene 1.5 mmol, thiol 1 mmol, FeBr2 0.02 mmol, TBHP (70% in water) 3 mmol, 2 wt% SDS–H2O 1 mL, 100 °C, 36 h.
1		R = 4-CH3	4a	69
2		R = 2,4-Cl	4b	79
3		R = 3-Br	4c	77
4		R = 4-OCH3	4d	83
5		R = 4-N(CH3)2	4e	Trace
6		R = 4-NO2	4f	52
7		R = 4-Cl	4g	80
8		R = 4-CF3	4h	Trace
9			4i	Trace
10			4j	75
11^c			4k	45
			4k′	15

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As mentioned above, thioether instead of thioester was obtained when FeBr₂ was replaced by Pd(OAc)₂. Aiming for the selective preparation of thioethers, we further explored the palladium-catalyzed CDC thiolation. Details of optimizations are shown in Table 4. The yield of the desired thioether slightly increased when the reaction was moved from the argon atmosphere to the open air, albeit in a low yield (Table 4, entry 9). This indicated that oxidants may play a key role in the CDC process, thus we placed the reaction in a pure oxygen atmosphere (Table 4, entry 1) and increased the equivalent of TBHP (Table 4, entry 11). The addition of excess TBHP had no positive effect to the present reaction but produced more benzaldehyde and sulfur oxidation products in contrast. When the reaction was conducted under oxygen atmosphere, the yield increased to 86%. In view of this result, TBHP may not be necessary; however, the yield dropped sharply in the absence of TBHP (Table 4, entry 7). It is known that thioether may be oxidised by TBHP under heated conditions. However, only trace amount of sulfur oxidation products was observed with the present conditions. Similar to the above reaction, surfactant was also used to accelerate the reaction, but it did not show any improvement (Table 4, entry 6). Finally, some commercially available palladium catalysts and copper catalysts were examined. Still, Pd(OAc)₂ was the best choice (Table 4, entries 3–5), and a decrease in catalyst loading (5 mol%) had an adverse effect on product yield (Table 4, entry 12).

Table 4 Optimal conditions for palladium-catalyzed CDC thiolation^a

Entry	Change from the “standard conditions”	Yield^b
a Standard conditions: toluene 1.5 mmol, 4-chlorobenzenethiol 0.5 mmol, Pd(OAc)2 0.05 mmol, TBHP (70% in water) 0.5 mmol, 1 atom O2, 115 °C, 24 h.b The yield was determined by GC.c The yield of benzaldehyde.d The yield of thioester.
1	None	86
2	No Pd(OAc)2	0
3	Cu(OAc)2 instead of Pd(OAc)2	0
4	Pd(PPh3)2Cl2 instead of Pd(OAc)2	55
5	Pd(dppf)Cl2 instead of Pd(OAc)2	37
6	The addition of SDS (0.02 eq.) as a surfactant	84
7	No TBHP, 48 h	16
8	Under Ar atmosphere	8(12^c)
9	In the air	30
10	The addition of FeBr2	26(35^d)
11	3 eq. TBHP	21(55^c, 9^d)
12	Pd(OAc)2 loading decreased to 5%	62

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Then, these optimized conditions were applied to the reaction of structurally different methylarenes and thiophenols or thiols. A series of unsymmetrical aryl alkyl thioethers could be successfully prepared from substituted methylarenes whereby electronic effects showed no significant association with the yield (Table 4, 5a–g). Unlike the thioesterification described above, the steric hindrance affected the product yield significantly (Table 4, 5b and 5g). Like the above esterification reaction, the “di-thioether” could be obtained under harsher reaction conditions (Table 5, 5q). We have also investigated the applicability of our method using different thiols or thiophenols. As expected, thiophenols could easily convert to the aimed products for all the electron-donating and the electron-withdrawing substituents (Table 5, 5i–l). Alkyl thiols, including benzyl thiol and cyclohexanethiol were also applied for the coupling. Symmetric thioether was obtained in good yield with benzyl thiol (Table 5, 5n), while cyclohexanethiol reacted sluggishly and a 55% yield was obtained by prolonging the reaction time (Table 5, 5m). At last, heterocyclic thiols were tested. Similarly, benzo[d]thiazole-2-thiol underwent the present reaction smoothly while benzo[d]imidazole-2-thiol failed this reaction (Table 5, 5o and 5p).

Table 5 Scope of methylarenes in the oxidative thioetherification^a

Entry	Product			Yield^b (%)
a Reaction conditions: methylarenes 1.5 mmol, thiol or thiophenol 0.5 mmol, Pd(OAc)2 0.05 mmol, TBHP (70% in water) 0.5 mmol, 1 atm O2, 115 °C, 24 h.b Isolated yield.c The yield of 1,4-bis(((4-chlorophenyl)thio)methyl)benzene.d Reaction time 48 h.e Reaction conditions: m-xylene 1.5 mmol, thiophenol 1 mmol, Pd(OAc)2 0.1 mmol, TBHP (70% in water) 1 mmol, 1 atm O2, 120 °C, 36 h.
1		R = –H	5a	84
2		R = 4-F	5b	77
3		R = 4-CH3	5c	62(4^c)
4		R = 4-NO2	5d	58
5		R = 4-CN	5e	62
6		R = 4-Br	5f	83
7		R = 2-F	5g	71
8			5h	8
9		R = 2-CH3	5i	72
10		R = 4-CH3	5j	77
11		R = 4-OCH3	5k	70
12		R = 4-F	5l	46
13			5m	55^d
14			5n	80
15			5o	80
16			5p	0
17			5q	28^e

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To gain insight into the catalytic pathway of the two reactions, we conducted some mechanistic experiments (Scheme 1). In the presence of TBHP, thiophenol could easily convert to the disulfide.

Scheme 1 Controlled experiments for preliminary mechanistic studies.

Thus, disulfide was used for the developed two reactions to replace the thiophenol. Under the standard condition, both the thioesterification and thioetherification could proceed, but with lower yields. This means thiophenol is more appropriate for the two reactions, although the formation of disulfide is inevitable. During the reaction of toluene with FeBr₂/TBHP, both aldehyde and benzoic acid were observed while in the Pd(OAc)₂/O₂ conditions, and only trace amount of aldehyde was observed. Thus, we further placed benzaldehyde and 4-chlorobenzoic acid simultaneously into the thioesterification reaction. The crossover experiment showed that benzaldehyde reacted with 4-chlorobenzenethiol under the present experimental conditions, which ruled out the usual esterification of acid and thiol. Moreover, the thioesterification reaction was treated with a radical inhibitor. When excess TEMPO (5 equivalents) was added, no thioester could be observed, which indicated that the iron-catalyzed thioesterification may follow a radical mechanism.

Based on the above results and in combination with the already known mechanism in the literature,^15j a plausible mechanism was proposed as shown in Scheme 2. In the iron-catalyzed reaction, toluene was first oxidized to form benzaldehyde, which further reacted with t-BuOO˙ to give an acyl radical. On the other hand, iron salts promoted TBHP to form t-BuOO˙ and the FeBr₂ converted to intermediate A, which further reacts with the thiol or disulfide to give an iron(III) thiolate complex B and release HBr. After that the complex B reacted with acyl radical which provide the aimed thioester and complex C, and C reacted with HBr to regenerate FeBr₂ to complete the circle.

Scheme 2 Proposed mechanism for the Fe-catalyzed thioesterification and Pd-catalyzed thioetherification via sp³ C–H activation of methylarenes.

As for the Pd-catalyzed cycle, the catalytic cycle starts with Pd^II-mediated C–H cleavage to form benzyl Pd complex A′. The benzylation products may result from nucleophilic attack on the benzylic carbon by thiophenol (Path A). Alternatively, they can undergo ligand exchange to afford benzyl Pd(II)carboxylate B′, and subsequent reductive elimination yields thioether products (Path B). The Pd⁰ species generated in Path A or B is oxidised to Pd^II by O₂ to continue the catalytic cycle. It should be noted that the existing TBHP may accelerate the C–H cleavage and the palladium oxidation process (Pd⁰–Pd^II).

Experimental section

Melting points are uncorrected. All commercial materials were used without further purification. Thiols were synthesized according to the literature;²⁴ TBHP used was 70% TBHP in water. GC-MS analyses were performed on an Agilent 7890A-5975C instrument (Column: DB-5 MS). NMR was recorded on Bruker DRX 500 and tetramethylsilane (TMS) was used as a reference. Elemental analysis was performed on a Yanagimoto MT3CHN recorder. Preparative high performance liquid chromatography was performed on an Agilent Technologies 1200 Column: XDB-C18 9.6 × 250 mm.

General procedure for synthesis of thioesters

A 5 mL vial with condenser was charged with toluene (1.5 mmol), FeBr₂ (0.01 mmol), TBHP (1.5 mmol), 2 wt% SDS–H₂O (2 mL). Thiol (or thiophenol, 0.5 mmol) was then added dropwise (in portion for solids) at 100 °C. The reaction mixture was stirred at 100 °C for 24 h. Upon completion, the reaction mixture was then cooled, extracted with ethyl acetate, and dried over anhydrous Na₂SO₄. After filtration, the organic solutions were concentrated and the residue was purified by column chromatography on silica gel to give the pure product (hexane–ethyl acetate = 20/1).

General procedure for synthesis of thioethers

A mixture of toluene (1.5 mmol), Pd(OAc)₂ (0.05 mmol), TBHP (1.5 mmol), and thiol (thiophenol) (0.5 mmol) was introduced in a pressure reactor. The suspension was stirred at 115 °C for 24 h under 1 atm of oxygen. After this, the pressure was released and the mixture was extracted with ethyl acetate, and dried over anhydrous Na₂SO₄. After filtration, the organic solutions were concentrated and the residue was purified by column chromatography on silica gel to give the pure product (hexane–ethyl acetate = 50/1).

Conclusions

In summary, we developed two facile and efficient protocols, by which thioesters and thioethers can be selectively prepared via the Fe-catalyzed C–H functionalization thioesterification and Pd-catalyzed C–H functionalization thioetherification, respectively. The major advantages of these methods include the use of toluene as the benzylation or benzoylation reagent and oxygen or TBHP as the green oxidant, which has high atom economy, avoids the prefunctionalization of substrates and reduces the production of chemical wastes. In addition, the two rarely reported protocols demonstrate good functional group tolerance and moderate to good yields.

Acknowledgements

We gratefully acknowledge the Natural Science Foundation of Jiangsu Province (BK 20131346) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures; spectral data for all reported compounds. See DOI: 10.1039/c4ra09450f

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