An odorless thia-Michael addition using Bunte salts as thiol surrogates

Abstract

A newly developed C–S bond formation process via acid-catalyzed thia-Michael addition has been demonstrated. The protocol, in which Bunte salts generated from odorless and inexpensive sodium thiosulfate and organic halides are used as the thiol precursors, provides an efficient approach for the synthesis of β-sulfido carbonyl compounds.

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Introduction

Bunte salts are conveniently prepared by the reaction of odorless and inexpensive sodium thiosulfate with alkyl halides, which are easy to handle crystalline solids, even with highly lipophilic organic moieties, and generally have little to no odor.¹ Various nucleophiles such as thiols,² Na₂S,³ cyanide⁴ or Grignard reagents^1c could react with Bunte salts to yield disulfides, trisulfides, thiocyanates and sulfides respectively. Meanwhile, these compounds also exhibit nucleophilicity under acidic conditions to produce thiols.⁵ Therefore, we envisage that electrophiles such as α,β-unsaturated ketones can be attacked by thiol in situ formed by the hydrolysis of Bunte salts under acidic conditions (Scheme 1).

Scheme 1 Working hypothesis: thia-Michael addition using Bunte salts as thiol equivalents.

In the last decades, sulfur-based compounds have received wide-spread attention due to their potential as novel pharmaceutical, agricultural and chemical agents.⁶ Moreover, organic sulfur compounds are essential in materials science, in which the sulfur constituent can have a profound effect on the physical, electronic, and surface properties of the resultant materials.⁷ The construction of sulfur-containing compounds via simple C–S bond-forming reactions is of utmost importance in synthetic and catalytic research fields.⁸ The thia-Michael addition is one of most useful approaches for the construction of C–S bonds, so there has been significant efforts towards the development of methodologies for this transformation.⁹

Nevertheless, most of these strategies require malodorous and expensive thiols as starting materials. Because of their potent stench and air sensitivity, the use of thiol as the starting materials, particularly on a large scale operation, is highly undesirable. In order to solve the issues, several odorless protocols have been reported through the in situ generation of odorless S-alkylisothiouronium salts in place of thiols, which are formed by organic halides and thiourea.^9c,10 Although various electron-deficient alkenes could proceed smoothly under odorless and mild conditions, more sterically hindered substrates such as chalcone fail to yield the desired products, and a stoichiometric amount of base is required.¹⁰

More recently, various attempts have been made to employ odorless disulfides and N-(organothio)succinimides for the synthesis of sulfocompound, but these reagents should be pre-generated from thiols.¹¹ Iranpoor's and our group have reported several transformations for C–S bonds formation using thiourea as an odorless sulfur source,¹² meanwhile a series of sulfur transfer reactions by in situ formation of Bunte salts from sodium thiosulfate and organic halides have also been achieved.¹³ With our interest in exploring novel odorless means for construction of C–S bonds, we found odorless Bunte salts could react with steric α,β-unsaturated ketones under acidic conditions with good yields.

Results and discussion

As a representative example, the sulfur-Michael addition of 1a and 2a was chosen to optimize the reaction conditions (Table 1). After screening different solvent, methanol emerged as the best choice (entry 4). No reaction took place without acid (entry 9). Various Brønsted acids were employed to further improve the yield. Both HN(Tf)₂ (trifluoromethanesulfonimide) and TsOH (p-toluenesulfonic acid) were equally effective in this model reaction (entries 4 and 13). TsOH is much cheaper than HN(Tf)₂, so TsOH was selected as the catalyst for further studies. The amount of TsOH was also optimized, and 20 mol% of TsOH was the best option (entry 15). Finally, satisfactory yield could be obtained via heating the reaction to 80 °C.

Table 1 Optimization of the reaction conditions^a

Entry	Solvent	Acid	Yield^b (%)
a Reaction conditions: 1a 0.60 mmol, 2a 0.50 mmol, acid 0.50 mmol, solvent 1 mL, 6 h, 50 °C.b GC yields.c 0.5 equiv. of TsOH was used.d 0.2 equiv. of TsOH was used.e The reaction temperature is 80 °C, isolated yield.f 0.1 equiv. of TsOH was used.
1	CH2Cl2	TsOH	Trace
2	MeCN	TsOH	63
3	H2O	TsOH	nr
4	MeOH	TsOH	71
5	EtOH	TsOH	68
6	DMSO	TsOH	nr
7	DMF	TsOH	nr
8	Toluene	TsOH	16
9	MeOH	—	nr
10	MeOH	HCl	27
11	MeOH	HClO4	38
12	MeOH	H2SO4	52
13	MeOH	HN(Tf)2	76
14	MeOH	TsOH^c	72
15	MeOH	TsOH^d	69 (88)^e
16	MeOH	TsOH^f	34

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With the optimized conditions in hands, a series of Bunte salts and various α,β-unsaturated ketones were applied in the reaction to establish the scope and generality of the protocol (Scheme 2). Steric α,β-unsaturated ketones including chalcones, benzylidene acetone and (E)-4-(thiophen-2-yl)but-3-en-2-one could react with different Bunte salts to afford corresponding products. Excellent yields would be gained using electron-deficient terminal alkenes to yield the addition products (3d, 3e) at lower temperature (40 °C). Benzyl, allyl and cyclohexyl Bunte salts were also employed in the protocol successfully, and the cyclohexyl Bunte salt leaded to a lower yield (46%) due to sterically hindered effects (3g). It should be noted that ethyloxycarbonylmethyl Bunte salt did not result in the desired product, but generate 3h via an additionally transesterification with MeOH. The substituented groups on chalcones had no obvious influence on the reaction (3k–m).

Scheme 2 The thia-Michael addition of Bunte salts and α,β-unsaturated ketones.^{a,b a}Reaction conditions: 1 0.60 mmol, 2 0.50 mmol, TsOH 0.10 mmol, MeOH 1 mL, 6 h, 80 °C. ^bIsolated yields. ^cReaction conditions: 1 0.60 mmol, 2 0.50 mmol, TsOH 0.10 mmol, CH₂Cl₂ 1 mL, 6 h, 40 °C.

Because sodium S-aryl sulfothioate can't be hydrolyzed under optimized conditions, no desired product was found when aryl Bunte salts such as phenyl and 4-toluene Bunte salts, were employed in the protocol. To further broaden the scope of the approach, thia-Michael additions of Bunte salts with N-substituented maleimides were achieved under optimized conditions (Scheme 3). N-Benzyl, methyl and ethyl maleimide reacted smoothly with Bunte salts in good to excellent yields. In the case of N-phenyl maleimide, no Michael addition occurred. Undesired product generated from the ring-opening of N-phenyl maleimide was observed. Similarly, 3q was formed via Michael addition of ethyloxycarbonylmethyl Bunte salt with N-benzyl maleimide, following a transesterification with MeOH.

Scheme 3 The thia-Michael additions of Bunte salts with N-substituented maleimide.

To further optimize the reaction conditions, attempts were also made to realize thia-Micheal additions via a one-pot process from benzyl chloride, Na₂S₂O₃ and α,β-unsaturated ketones (Scheme 4). Although the results were also satisfactory, the one-pot process required excess benzyl chloride (3 equiv.) and Na₂S₂O₃ (4 equiv.) which leaded to unnecessary waste of substrates.

Scheme 4 Thia-Micheal additions via a one-pot process through in situ formation of Bunte salts.

Finally, a proposed mechanism for the reaction was also illustrated in Scheme 5. TsOH may play a dual role in the reaction: (i) TsOH promotes the hydrolysis of Bunte salts to form thiol in situ;^5a (ii) TsOH actives α,β-unsaturated ketones to form carbonium ion intermediate I, following a nucleophilic attack by thiol to afford the final product.

Scheme 5 A proposed mechanism for thia-Micheal additions with Bunte salts.

Conclusions

In summary, we have described an efficient and odorless thia-Michael addition by in situ formation of thiols from the hydrolysis of Bunte salts under acidic conditions. Steric α,β-unsaturated ketones can also applied in the system successfully. The procedure uses odorless, inexpensive and easy to handle Bunte salts instead of thiols, making it more suitable for large-scale operations.

Experimental section

General

All chemical reagents are obtained from commercial suppliers and used without further purification. All known compounds are identified by appropriate technique such as MS, ¹H NMR, and compared with previously reported data. All unknown compounds are characterized by ¹H NMR, ¹³C NMR, MS and elemental analyses. Analytical thin-layer chromatography are performed on glass plates precoated with silica gel impregnated with a fluorescent indicator (254 nm), and the plates are visualized by exposure to ultraviolet light. Mass spectra are taken on a Finnigan TSQ Quantum-MS instrument in the electrospray ionization (ESI) mode. ¹H NMR and ¹³C NMR spectra are recorded on an AVANCE 500 Bruker spectrometer operating at 500 MHz and 125 MHz in CDCl₃, respectively, and chemical shifts are reported in ppm. Elemental analyses are performed on a Yanagimoto MT3CHN recorder. GC analyses are performed on an Agilent 7890A instrument (column: Agilent 19091J-413: 30 m × 320 μm × 0.25 μm, carrier gas: H₂, FID detection). GC/MS data was recorded on a 5975C Mass Selective Detector, coupled with a 7890A Gas Chromatograph (Agilent Technologies).

General procedures for the synthesis of Bunte salts^1c

A flask is charged with organic halides (50 mmol), sodium thiosulfate pentahydrate (60 mmol), water (10.0 mL) and MeOH (50 mL). The reaction mixture is stirred and heated to 65 °C. After 16 h at 65 °C, the reaction mixture is cooled to rt, and then concentrated on a rotovap at a bath temperature of 60 °C to remove the MeOH and water. The resultant solid is treated with MeOH (100 mL), heated to 50 °C (most solid dissolves), and filtered to remove excess sodium thiosulfate and sodium bromide. The filtrate is concentrated to a white solid, following trituration of this solid with hexanes, filtration, and drying under vacuum at 50 °C to afford the corresponding Bunte salts.

General procedures for thia-Michael additions with Bunte salts and α,β-unsaturated ketones

A mixture of Bunte salts 1 0.60 mmol, α,β-unsaturated ketones 2 0.50 mmol, TsOH 0.10 mmol in MeOH (1.0 mL) is stirred at 80 °C for 6 h. Upon completion, the reaction mixture is diluted with EtOAc (4.0 mL), filtered through a bed of silica gel layered over Celite, The volatiles are removed in vacuo to afford the crude product. Further column chromatography on silica gel affords the pure desired product 3.

General procedures for thia-Michael additions via a one-pot process from benzyl chloride, Na₂S₂O₃ and α,β-unsaturated ketones

A mixture of benzyl chloride 1.5 mmol and Na₂S₂O₃ 2.0 mmol in MeOH (1.0 mL) is stirred at 80 °C for 2 h. Then, α,β-unsaturated ketones 0.50 mmol and TsOH 0.10 mmol are added to the mixture. The reaction proceeds at the same temperature for additionally 6 h. Upon completion, the reaction mixture is diluted with EtOAc (4.0 mL), filtered through a bed of silica gel layered over Celite, the volatiles are removed in vacuo to afford the crude product. Further column chromatography on silica gel affords the pure desired product.

Characterization data for unknown compounds

4-((4-Methylbenzyl)thio)-4-phenylbutan-2-one 3i. ¹H NMR (500 MHz, CDCl₃) δ 2.02 (s, 3H), 2.33 (s, 3H), 2.92–2.94 (m, 2H), 3.41–3.51 (dd, J = 37.0, 8.5 Hz, 2H), 4.19–4.22 (t, J = 7.5 Hz, 1H), 7.09 (brs, 4H), 7.24–7.26 (m, 1H), 7.32–7.33 (m, 4H). ¹³C NMR (125 MHz, CDCl₃) δ 21.2 (1C, Ar-CH₃), 30.6 (1C, O=CCH₃), 35.5 (1C, SCH₂), 44.0 (1C, CH), 50.1 (1C, CH₂C=O), 127.5 (1C, Ar-C), 128.1 (2C, Ar-C), 128.7 (2C, Ar-C), 128.9 (2C, Ar-C), 129.3 (2C, Ar-C), 134.8 (1C, Ar-C), 136.7 (1C, Ar-C), 141.7 (1C, Ar-C), 205.5 (1C, C=O). MS (ESI) m/z: 284.25 [M + H]⁺. Anal. calcd for C₁₈H₂₀OS: C, 76.01%; H, 7.09%. Found: C, 76.36%; H, 7.45%.

4-((4-Nitrobenzyl)thio)-4-phenylbutan-2-one 3j. ¹H NMR (500 MHz, CDCl₃) δ 2.02 (s, 3H), 2.93 (d, J = 7.0 Hz, 2H), 3.48–3.57 (dd, J = 34.0, 8.0 Hz, 2H), 4.17–4.19 (t, J = 7.5 Hz, 1H), 7.21–7.31 (m, 7H), 8.08 (d, J = 8.5 Hz, 2H). ¹³C NMR (125 MHz, CDCl₃) δ 30.7 (1C, CH₃), 35.2 (1C, SCH₂), 44.2 (1C, CH), 49.9 (1C, CH₂C=O), 123.7 (2C, Ar-C), 127.8 (1C, Ar-C), 128.0 (2C, Ar-C), 129.8 (2C, Ar-C), 141.1 (1C, Ar-C), 146.0 (1C, Ar-C), 147.0 (1C, Ar-C), 205.0 (1C, C=O). MS (ESI) m/z: 314.98 [M + H]⁺. Anal. calcd for C₁₇H₁₇NO₃S: C, 64.74%; H, 5.43%; N, 4.44%. Found: C, 64.38%; H, 5.29%; N, 4.75%.

3-(4-Methoxyphenyl)-3-((4-nitrobenzyl)thio)-1-phenylpropan-1-one 3k. ¹H NMR (500 MHz, CDCl₃) δ 3.42 (d, J = 7.0 Hz, 2H), 3.51–3.60 (dd, J = 32, 14 Hz, 2H), 3.79 (s, 3H), 4.39–4.42 (t, J = 7.0 Hz, 1H), 6.84 (d, J = 8.5 Hz, 2H), 7.19–7.36 (m, 7H), 7.81 (d, J = 8.5 Hz, 2H), 8.04 (d, J = 8.5 Hz, 2H). ¹³C NMR (125 MHz, CDCl₃) δ 35.4 (1C, SCH₂), 44.7 (1C, CH), 44.9 (1C, CH₂C=O), 55.6 (1C, OCH₃), 113.9 (2C, Ar-C), 123.7 (2C, Ar-C), 127.7 (1C, Ar-C), 128.1 (2C, Ar-C), 128.8 (2C, Ar-C), 129.7 (1C, Ar-C), 129.8 (2C, Ar-C), 130.5 (2C, Ar-C), 141.6 (1C, Ar-C), 146.1 (1C, Ar-C), 146.9 (1C, Ar-C), 163.8 (1C, Ar-C), 195.0 (1C, C=O). MS (ESI) m/z: 407.13 [M + H]⁺. Anal. calcd for C₂₃H₂₁NO₄S: C, 67.79%; H, 5.19%; N, 3.44%. Found: C, 67.81%; H, 5.32%; N, 3.74%.

1-(4-Bromophenyl)-3-((4-nitrobenzyl)thio)-3-phenylpropan-1-one 3l. ¹H NMR (500 MHz, CDCl₃) δ 3.41–3.42 (dd, J = 7.0, 1.0 Hz, 2H), 3.50–3.59 (dd, J = 32.5, 13.5 Hz, 2H), 4.34–4.37 (t, J = 7.0 Hz, 1H), 7.19–7.32 (m, 7H), 7.51 (d, J = 8.5 Hz, 2H), 7.67 (d, J = 8.5 Hz, 2H), 8.06 (d, J = 8.5 Hz, 2H). ¹³C NMR (125 MHz, CDCl₃) δ 35.4 (1C, SCH₂), 44.4 (1C, CH), 45.2 (1C, CH₂C=O), 123.7 (2C, Ar-C), 127.8 (1C, Ar-C), 128.1 (2C, Ar-C), 128.7 (1C, Ar-C), 128.8 (2C, Ar-C), 129.7 (2C, Ar-C), 129.8 (2C, Ar-C), 132.0 (2C, Ar-C), 135.4 (1C, Ar-C), 141.2 (1C, Ar-C), 145.9 (1C, Ar-C), 147.0 (1C, Ar-C), 195.5 (1C, C=O). MS (ESI) m/z: 455.18 [M + H]⁺. Anal. calcd for C₂₂H₁₈BrNO₃S: C, 57.90%; H, 3.98%; N, 3.07%. Found: C, 57.68%; H, 4.17%; N, 2.86%.

3-((4-Nitrobenzyl)thio)-3-phenyl-1-(p-tolyl)propan-1-one 3m. ¹H NMR (500 MHz, CDCl₃) δ 2.38 (s, 3H), 3.45–3.47 (dd, J = 7.0, 1.5 Hz, 2H), 3.53–3.62 (dd, J = 32, 14 Hz, 2H), 4.40–4.43 (t, J = 7.0 Hz, 1H), 7.19–7.25 (m, 3H), 7.29–7.36 (m, 6H), 7.75 (d, J = 8.5 Hz, 2H), 8.08 (d, J = 8.5 Hz, 2H). ¹³C NMR (125 MHz, CDCl₃) δ 21.7 (1C, Ar-CH₃), 35.4 (1C, CH₂S), 44.6 (1C, CH), 45.2 (1C, CH₂C=O), 123.7 (2C, Ar-C), 127.7 (1C, Ar-C), 128.1 (2C, Ar-C), 128.3 (2C, Ar-C) 128.8 (2C, Ar-C), 129.4 (2C, Ar-C), 129.8 (2C, Ar-C), 134.2 (1C, Ar-C), 141.5 (1C, Ar-C), 144.4, 146.1, 147.0, 196.0. MS (ESI) m/z: 391.20 [M + H]⁺. Anal. calcd for C₂₃H₂₁NO₃S: C, 70.56%; H, 5.41%; N, 3.58%. Found: C, 70.29%; H, 5.17%; N, 3.96%.

3-(Benzylthio)-1-ethylpyrrolidine-2,5-dione 3p. ¹H NMR (500 MHz, CDCl₃) δ 1.18–1.21 (t, J = 7.0 Hz, 3H), 2.39–2.43 (dd, J = 18.5, 1.5 Hz, 1H), 2.94–2.99 (dd, J = 9.0, 4.0 Hz, 1H), 3.49–3.51 (dd, J = 9.0, 3.5 Hz, 1H), 3.55–3.59 (q, J = 7.5 Hz, 2H), 3.87 (d, J = 13.5 Hz, 1H), 4.24 (d, J = 13.5 Hz, 1H), 7.29–7.42 (m, 5H). ¹³C NMR (125 MHz, CDCl₃) δ 13.0 (1C, CH₃), 34.1 (1C, CH₂N), 35.6 (1C, CH₂C=O), 36.0 (1C, SCH₂), 37.4 (1C, CH), 127.7 (1C, Ar-C), 128.8 (2C, Ar-C), 129.3 (2C, Ar-C), 136.9 (1C, Ar-C), 174.7 (1C, C=O), 176.8 (1C, C=O). MS (ESI) m/z: 249.18 [M + H]⁺. Anal. calcd for C₁₃H₁₅NO₂S: C, 62.62%; H, 6.06%; N, 5.62%. Found: C, 62.38%; H, 5.87%; N, 5.37%.

Methyl 2-((1-benzyl-2,5-dioxopyrrolidin-3-yl)thio)acetate 3q. ¹H NMR (500 MHz, CDCl₃) δ 2.50–2.54 (dd, J = 19.0, 4.0 Hz, 1H), 3.11–3.17 (q, J = 9.5 Hz, 1H), 3.34–3.37 (d, J = 16.0 Hz, 1H), 3.74 (s, 3H), 3.90–3.93 (d, J = 16.0 Hz, 1H), 4.01–4.04 (dd, J = 9.5, 3.5 Hz, 1H), 4.62–4.70 (q, J = 9.0 Hz, 2H), 7.28–7.37 (m, 5H). ¹³C NMR (125 MHz, CDCl₃) δ 33.0 (1C, SCH₂), 35.5 (1C, CHCH₂C=O), 38.4 (1C, CH), 42.8 (1C, Ph-CH₂), 52.8 (1C, OCH₃), 128.2 (1C, Ar-C), 128.8 (4C, Ar-C), 135.2 (1C, Ar-C), 170.1 (1C, O–C=O), 174.1 (1C, N–C=O), 176.1 (1C, N–C=O). MS (ESI) m/z: 293.22 [M + H]⁺. Anal. calcd for C₁₄H₁₅NO₄S: C, 57.32%; H, 5.15%; N, 4.77%. Found: C, 57.65%; H, 5.41%; N, 4.59%.

1-Benzyl-3-((4-methylbenzyl)thio)pyrrolidine-2,5-dione 3s. ¹H NMR (500 MHz, CDCl₃) δ 2.34 (s, 3H), 2.40–2.45 (dd, J = 19.0, 1.5 Hz, 1H), 2.95–3.00 (dd, J = 9.0, 4.0 Hz, 1H), 3.81 (d, J = 13.5 Hz, 1H), 4.17 (d, J = 13.5 Hz, 1H), 4.63–4.71 (q, J = 9.0 Hz, 2H), 7.13–7.14 (d, J = 8.0 Hz, 2H), 7.25–7.40 (m, 7H). ¹³C NMR (125 MHz, CDCl₃) δ 21.2 (1C, CH₃), 35.6 (1C, CH₂C=O), 35.7 (1C, CH₂S), 37.6 (1C, CH), 42.7 (1C, NCH₂), 128.1 (1C, Ar-C), 128.8 (4C, Ar-C), 129.2 (2C, Ar-C), 129.5 (2C, Ar-C), 133.7 (1C, Ar-C), 135.5 (1C, Ar-C), 137.4 (1C, Ar-C), 174.5 (1C, C=O), 176.6 (1C, C=O). MS (ESI) m/z: 325.12 [M + H]⁺. Anal. calcd for C₁₉H₁₉NO₂S: C, 70.12%; H, 5.88%; N, 4.30%. Found: C, 70.42%; H, 6.01%; N, 4.41%.

Acknowledgements

We are grateful to Chinese National Natural Science Foundation (21402093, 21476116) and Jiangsu Province Natural Science Foundation (BK20140776, BK20141394) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: More experimental entails, characterization data and copies of ¹H NMR, ¹³C NMR spectra of all products. See DOI: 10.1039/c5ra01381j

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