An Organodiselenide with Dual Mimic Function of Sulfhydryl

Letter Cite This: Org. Lett. 2018, 20, 6274−6278

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An Organodiselenide with Dual Mimic Function of Sulfhydryl Oxidases and Glutathione Peroxidases: Aerial Oxidation of Organothiols to Organodisulfides Vandana Rathore, Aditya Upadhyay, and Sangit Kumar* Department of Chemistry, Indian Institute of Science Education and Research (IISER) Bhopal, Bhopal By-pass Road, Bhauri, Bhopal 462 066, Madhya Pradesh India

Org. Lett. 2018.20:6274-6278. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/05/18. For personal use only.

S Supporting Information *

ABSTRACT: A novel organodiselenide, which mimics sulfhydryl oxidases and glutathione peroxidase (GPx) enzymes for oxidation of thiols by oxygen and hydrogen peroxide, respectively, into disulfides has been presented. The developed catalyst oxidizes an array of organothiols into respective disulfides in practical yields by using aerial O2 to avoid any reagents/additives, base, and light source. The synthesized diselenide also catalyzes the reduction of hydrogen peroxide into water by following the GPx enzymatic catalytic cycle with a reduction rate of 49.65 ± 3.7 μM·min−1.

T

Heavier organochalcogens also catalyze oxidation reactions,11−13 particularly, oxidation of organothiols into disulfides. However, these require strong oxidizing reagents, namely peroxides, or are performed under UV irradiation.13 Consequently, many organothiols with complex functionalities have not been converted into their respective disulfides. Moreover, peptide and protein modification using reagents and subsequent purification steps are undesirable. Therefore, a mild protocol for the oxidation of thiols is needed. In living cells, the sulfhydryl oxidase (SOX) family of flavoenzymes catalyzes the direct and facile oxidation of thiols into disulfides (Scheme 1, eq 1).14 Disulfide bond formation occurs first with thiol−disulfide exchange reaction followed by shuttling of electrons from reduced thiols to the oxidized disulfide member of the protein disulfide isomerase (PDI) family. This results in the reduction of conserved cysteine−S−

he disulfide bond plays a vital role in biological functions of large molecules such as peptides and proteins.1 The structure and stability of various antibodies; human immunoglobulin G1 antibody is also associated with the disulfide linkage.2 The formation and transfer of disulfide bond is an important event in the maturation of the proteins present in both eukaryotic and prokaryotic cells.3 Besides its vital role in living cells, the disulfide bond is an important parameter for the construction of novel proteins in the laboratory by chemical synthesis and semisynthesis. Moreover, the disulfide bonds or corresponding thiols in proteins have become an important component of therapeutic agents for the treatment of several diseases such as cancer and autoimmune diseases.4 The formation of the disulfide S−S bond is a fundamental step toward accessing a wide range of organic molecules, which are prevalent in the industry, namely pharmaceuticals, agrochemicals, and pesticides.5 Furthermore, disulfide bonds have been used as ligands for the isolation of soft metals, particularly Hg, Cd, Zn, etc.6 Because of their significance in chemistry and biology, numerous transition-metal (TM)-catalyzed7 or TM-free8−10 methods have been developed for the oxidation of organothiols into disulfides. Recently, Noël and co-workers have reported aerobic oxidation of organothiols by using visible-light irradiation and a sensitizer eosin Y.10a Subsequently, a visible-light-induced photocatalytic aerobic oxidation of thiols has been achieved by the same group utilizing titanium dioxide nanoparticle as a heterogeneous photocatalyst.10b Nonetheless, high catalyst loading or specially designed ligands are essential to achieve an optimum yield of the disulfide due to its inherent chalcophilic nature, which poisons the TM catalysts. © 2018 American Chemical Society

Scheme 1. Routes to Organodisulfides

Received: August 28, 2018 Published: September 24, 2018 6274

DOI: 10.1021/acs.orglett.8b02756 Org. Lett. 2018, 20, 6274−6278

Letter

Organic Letters Table 1. Optimization of the Reaction Conditionsa

S−cysteine motifs in the PDIs. Subsequently, the PDIs are reoxidized by the flavoenzyme endoplasmic reticulum oxidase 1, which transfers electrons via flavin adenine dinucleotide (FAD) onto oxygen leading to de novo disulfide generation and concomitant release of hydrogen peroxide (Figure S2). The selenium-containing glutathione peroxidase (GPx) selenoenzyme and related mimics catalyze the reduction of hydrogen peroxide and organic hydroperoxide to water and alcohols, respectively, by using glutathione GSH as the stoichiometric reductant in this process (Scheme 1, eq 2).15 Here, in continuation of our work on organochalcogens,16 we report an organodiselenide 1a that exhibits the function of sulfhydryl oxidases and glutathione peroxidases enzymes depending on the available substrate. The developed enzyme mimic catalyzes the oxidation of organothiols into organodisulfides by aerial oxygen and peroxide with the concomitant release of hydrogen peroxide and water, respectively. In addition, mechanistic understanding of the reaction was investigated by 77Se NMR, CV, mass spectrometry, control experiments, EPR spectroscopy, and DFT. The diselenide catalyst 1a was synthesized from 2-aminophenyl diselenide 1b (Scheme 2). 2-Aminophenyl diselenide

entry

catalyst

solvent

time (h)

convb and yieldc (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14e

1a, 5 mol % 1a 1b 1c 1d 1e 1f 1g 1h, 1i 1j

CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN+H2O CH3CN

7 7 7 7 12 12 12 12 12 12 14 7 7 7

99 and 90 99 and 97, 96d 20 35 12 9 15 15 ND 7 ND 10 92 trace

Scheme 2. Synthesis of Diselenide Catalyst 1a

1k 1a 1a

a

The reactions were carried out using thiol 0.1 mmol and catalyst (0.01 mmol, 1 mol %) in an open NMR tube for indicated time. b Determined by 1H NMR spectroscopy. cIsolated. ND: not detected. d Yield at 2 mmol scale. eReaction was carried out under inert atmosphere.

tested organoselenium compounds were effective for the aerial oxidation of thiols (entries 3−9, Table 1). To our surprise, organoditelluride 1j20 also provided only 7% oxidation of thiols (entry 10, Table 1). Next, aerial oxidation of p-toluenethiol was studied in the presence of 1k lacking selenium, 2aminophenyl diselenide 1b, Schiff base 1c, and 2-hydroxyphenyl diselenide 1f lacking one or both functionalities among phenolic, amino, and selenium. 2-Aminophenyl diselenide 1b provided 20% oxidation of thiol into disulfide 2a (entry 3, Table 1), and Schiff base 1c lacking NH functionality and 2-hydroxyphenyl diselenide 1f afforded 35 and 15% of the oxidation product respectively (entries 4 and 7, Table 1). In the absence of a selenium catalyst, oxidation of ptoluenethiol could not be realized (entry 11, Table 1). The radical quencher 1k afforded only 10% conversion of ptoluenethiol (entry 12, Table 1). The aerial oxidation of thiol can also be carried out in water and acetonitrile solvent mixture (entry 13, Table 1). The conversion of thiol into disulfide 2a was not observed under inert atmosphere, and only traces of disulfide 2a were observed as expected (Table 1, entry 14). Next, various organothiols were tested for aerial oxidation in the presence of 1 mol % of the enzyme mimic 1a. A variety of aromatic thiols, namely thiophenols containing OH, OMe, NH2, CH2OH, Cl, Br, F, thionaphthols, heteroaryl thiols 2mercaptobenzothiazole, 2-mercaptopyridine, 2-mercaptothiophene, and alkyl thiols, were oxidized into respective disulfides

1b was obtained in gram quantity by following the reported procedure16a by careful variation in the stoichiometry of reagents, providing the diselenide 1b in isolated 70% yield. Next, the addition of 2-aminophenyl diselenide 1b to osalicylaldehyde in toluene in the presence of a catalytic amount of acetic acid under reflux conditions in a Dean−Stark apparatus provided the Schiff base 1c17 quantitatively. The reduction of the CN bond in 1c by an excess of NaBH4 provided the desired diselenide catalyst 1a in 85% yield. Diselenides 1d, 1e,18a and 1f,18a selenides 1h−1i,18b Spector’s catalyst 1g,19 and ditelluride 1j,20 used in this study, were synthesized by following the methods described in the literature. During our 1H NMR study on the reaction of diselenide 1a with p-toluenethiol, complete oxidation of ptoluenethiol into p-toluene disulfide 2a was observed along with the formation of H2O (entry1, Table 1). Moreover, 1 mol % of diselenide 1a was sufficient for the oxidation of ptoluenethiol and led to 96% isolated yield of 2a (entry 2, Table 1). Next, Spector’s catalyst 1g and other diselenides 1b−f, which act as efficient GPx mimics19 for the reduction of hydrogen peroxides using thiol substrate, were studied under the same conditions for the purpose of comparison (entries 3− 8, Table 1). Similarly, monoselenides 1h and 1i, which reduced the peroxyl radicals21 in a catalytic manner exploiting thiol as a sacrificial cosubstrate reductant, have been studied under similar conditions (entry 9, Table 1). Surprisingly, none of the 6275

DOI: 10.1021/acs.orglett.8b02756 Org. Lett. 2018, 20, 6274−6278

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Organic Letters

Scheme 4. 77Se NMR Study on the Reaction of Diselenide 1a with p-Toluenethiol and O2

2b−2y practically (Scheme 3). The biologically relevant thiols such as homocysteine, cysteine, N-acetylcysteine, and glutaScheme 3. Aerial Oxidation of Various Organothiolsa,b

Next, a solution of diselenide 1a with 2 equiv of ptoluenethiol in CD3CN was purged with oxygen for 7 h, and the resulting reaction mixture was studied by 77Se NMR in which a peak at 747 ppm was observed and seems to correspond to formation of selone 3c, and additional an unidentified peak at 818 ppm was also observed (see the Supporting Information). Next, an EPR study was carried out on air exposed diselenide 1a in the solid phase and an oxygen-purged (15 min) solution of 1a (Figure 1). The EPR spectrum shows a a The reaction was carried out using 2 mmol of organothiol and 1a (0.02 mmol) in CH3CN (5 mL) at rt. bReaction was conducted in CH3CN + H2O mixture (1:1). Isolated yields.

thione were studied next in the series. The reaction, realized to be sluggish in acetonitrile, resulted in the formation of the impure product. The reaction progressed smoothly in a water/ acetonitrile mixture (1:1), thus providing the corresponding disulfides 2z−ac in substantial yields. Next, we showed that the newly synthesized mimic can reduce hydrogen peroxide oxidant into water by benzenethiol cosubstrate into benzene disulfide by following the first-order reaction with a reduction rate of 49.65 ± 3.7 μM·min−1, which is comparable with the studied GPx mimic phenyl diselenide 1d (reduction rate = 24 ± 1.86 μM·min−1) and Spector’s catalyst 1g (reduction rate = 128.34 ± 13.3 μM·min−1).20 The reaction of diselenide mimic 1a with p-toluenethiol was monitored by 77Se NMR spectroscopy as 77Se nuclei is NMR active, sensitive to the electronic environment, and helpful in establishing the intermediates involved in the reaction. 77Se NMR of diselenide 1a exhibits a signal at 400 ppm (Scheme 4). The addition of an excess of p-toluenethiol to the catalyst 1a produced a new weak signal at 478 ppm, which corresponds to the selenenyl sulfide 3a, and the expected signal due to selenol 3b was not observed presumably due to the oxidation into diselenide 1a. The generation of selenol 3b was indirectly established by trapping it with methyl iodide, which results in the formation of RSeCH3 (δ 109 ppm). Methyl selenide 3ba has also been isolated (see the Supporting Information). Attempts to isolate selenenyl sulfide 3a were unsuccessful and seem to undergo disproportionation diselenide 1a and ptoluene disulfide 2a (see the Supporting Information).

Figure 1. (a) EPR spectrum of 1a after O2 purged in CH3CN. (b) Cyclic voltammogram of 1a (2 mM) in CH3CN using SCE, scan rate 10 mV s−1 in TBAPF6.

signal at 3354 G. The reaction of p-toluenethiol with N-tertbutyl-α-phenylnitrone (PBN), a radical-trapping reagent, under similar conditions shows an EPR signal (Figure S13), which infers that oxidation of p-toluenethiol proceeds via a radical pathway. The cyclic voltammogram of 1a shows a redox potential of +0.44 V (Figure 1b). The observed oxidation potential of 1a is +0.50 V, and oxygen has a reduction potential Ered (O2/H2O2) of 0.65 V (vs SCE), which suggests that O2 is strong enough for the oxidation of 1a by +0.15 V. DFT calculations were performed to predict the feasible path for aerial oxidation of thiol by diselenide 1a. Several paths were considered for diselenide 1a (see the Supporting Information), and the tentative mechanism of the reaction is depicted in Scheme 5. Diselenide reacts with 1 equiv of the thiol to produce selenenyl sulfide 3a and selenol 3b. Selenenyl sulfide 3a may further react with an additional molecule of thiol to produce selenol 3b, expectedly. The oxidation of 3b by O2 would afford selone 3c and hydrogen peroxide by electron transfer followed by proton 6276

DOI: 10.1021/acs.orglett.8b02756 Org. Lett. 2018, 20, 6274−6278

Letter

Organic Letters

Aditya Upadhyay: 0000-0003-4072-062X Sangit Kumar: 0000-0003-0658-8709

Scheme 5. Mechanism for 1a-Catalyzed Oxidation of Thiol by Air and Peroxidea

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from DST-SERB New Delhi (EMR/2015/000061). V.R. [23/12/2012(ii)EU-V] and A.U. acknowledge UGC and IISER Bhopal, respectively, for fellowships. We thank Professor Deepak Chopra and the editorial office (proofreading of the manuscript), Miss Evelin L. Verghese and Mr. Satya R. Jena (CV study), and Mr. Saurav K. Veedu (isolation of intermediate).

Calculated relative Gibb’s free energy (ΔG° in kJ mol−1) obtained at the DFT-B3LYP(6-311+G(d,p)) level using the CPCM model in acetonitrile. a

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(1) (a) Heras, B.; Kurz, M.; Shouldice, S. R.; Martin, J. L. Curr. Opin. Struct. Biol. 2007, 17, 691. (b) Mcauley, A.; Jacob, J.; Kolvenbach, C. G.; Westland, K.; Lee, H. J.; Brych, S. R.; Rehder, D.; Kleemann, G. R.; Brems, D. N.; Matsumura, M. Protein Sci. 2008, 17, 95. (c) Arai, K.; Takei, T.; Okumura, M.; Watanabe, S.; Amagai, Y.; Asahina, Y.; Moroder, L.; Hojo, H.; Inaba, K.; Iwaoka, M. Angew. Chem., Int. Ed. 2017, 56, 5522. (2) Hagihara, Y.; Saerens, D. Biochim. Biophys. Acta, Proteins Proteomics 2014, 1844, 2016. (3) Sevier, C. S.; Kaiser, C. A. Nat. Rev. Mol. Cell Biol. 2002, 3, 836. (4) Trivedi, M. V.; Laurence, J. S.; Siahaan, T. J. Curr. Protein Pept. Sci. 2009, 10, 614. (5) (a) Whitham, G. H.; Organosulfur Chemistry; Oxford University Press: New York, 1995. (b) Lyons, T. W.; Sanford, M. S. Chem. Rev. 2010, 110, 1147. (6) Dénès, F.; Pichowicz, M.; Povie, G.; Renaud, P. Chem. Rev. 2014, 114, 2587. (7) (a) Dhakshinamoorthy, A.; Alvaro, M.; Garcia, H. Chem. Commun. 2010, 46, 6476. (b) Zhang, J.-W.; Zhang, H.-T.; Du, Z.-Y.; Wang, X.; Yu, S.-H.; Jiang, H.-L. Chem. Commun. 2014, 50, 1092. (c) Dong, W.; Liu, X.; Shi, W.; Huang, Y. RSC Adv. 2015, 5, 17451. (d) Dhakshinamoorthy, A.; Alvaro, M.; Puche, M.; Fornes, V.; Garcia, H. ChemCatChem 2012, 4, 2026. (e) Li, S.; Liu, X.; Chai, H.; Huang, Y. TrAC, Trends Anal. Chem. 2018, 105, 391. (f) Corma, A.; Ródenas, T.; Sabater, M. J. Chem. Sci. 2012, 3, 398. (g) Chauhan, S. M. S.; Kumar, A.; Srinivas, K. A. Chem. Commun. 2003, 2348. (h) Dou, Y.; Huang, X.; Wang, H.; Yang, L.; Li, H.; Yuan, B.; Yang, G. Green Chem. 2017, 19, 2491. (8) (a) Garcia Ruano, J. L.; Parra, A.; Aleman, J. Green Chem. 2008, 10, 706. (b) Singh, D.; Galetto, F. Z.; Soares, L. C.; Rodrigues, O. E. D.; Braga, A. L. Eur. J. Org. Chem. 2010, 2010, 2661. (c) Rattanangkool, E.; Krailat, W.; Vilaivan, T.; Phuwapraisirisan, P.; Sukwattanasinitt, M.; Wacharasindhu, S. Eur. J. Org. Chem. 2014, 2014, 4795. (9) Arai, K.; Dedachi, K.; Iwaoka, M. Chem. - Eur. J. 2011, 17, 481. (10) (a) Talla, A.; Driessen, B.; Straathof, N. J. W.; Milroy, L.-G.; Brunsveld, L.; Hessel, V.; Noël, T. Adv. Synth. Catal. 2015, 357, 2180. (b) Bottecchia, C.; Erdmann, N.; Tijssen, P. M. A.; Milroy, L.-G.; Brunsveld, L.; Hessel, V.; Noël, T. ChemSusChem 2016, 9, 1781. (11) Organoselenium catalysts in oxidation reactions: Freudendahl, D. M.; Santoro, S.; Shahzad, S. A.; Santi, C.; Wirth, T. Angew. Chem., Int. Ed. 2009, 48, 8409. (12) Diselenide for oxidative protein folding: (a) Metanis, N.; Hilvert, D. Angew. Chem., Int. Ed. 2012, 51, 5585. (b) Sai Reddy, P.; Metanis, N. Chem. Commun. 2016, 52, 3336. (c) Selenol-catalyzed interchange of dithiols and disulfides: Singh, R.; Whitesides, G. M. J. Org. Chem. 1991, 56, 6931. (13) (a) Oba, M.; Tanaka, K.; Nishiyama, K.; Ando, W. J. Org. Chem. 2011, 76, 4173. (b) Lutkus, L. V.; Irving, H. E.; Davies, K. S.; Hill, J. E.; Lohman, J. E.; Eskew, M. W.; Detty, M. R.; McCormick, T. M. Organometallics 2017, 36, 2588.

transfer from N−H and Se−H bonds unprecedentedly, and the ΔGo for the process is −68.17 kJmol−1 (cycle I). Selenenyl sulfide 3a could also react with oxygen leading to selone, HO2•, and PhS• radicals, and the latter one would dimerize to RSSR. The nucleophilic addition of sulfur from RSH to the selenium of selone 3c followed by proton transfer would provide the selenenyl sulfide 3a and thus complete the catalytic cycle. The selenol 3b and selenenyl sulfide 3a also catalyze the oxidation of the thiol by the oxidant hydrogen peroxide oxidant by following the GPx-enzymatic triads 3a, 3b, and 3d (cycle II). The selenol reacts with peroxide to form selenenic acid 3d, which is reduced by another molecule of RSH to afford water and selenenyl sulfide. In summary, a diorganodiselenide that mimics sulfhydryl oxidases and GPx enzymatic activities has been presented. The synthesized diselenide possesses features that can activate atmospheric oxygen toward oxidation and thus substantially impact the oxidation catalysis and the chemistry of organoselenium. This study also demonstrates the first utility of organoselenium in the activation of oxygen toward oxidation in a catalytic manner, which is contrary to the report that organoselenium decomposes peroxides. The facile activation of oxygen toward oxidation of organothiols with diverse functionalities into respective disulfides in the presence of low catalyst loading will encourage further exploration such as selective oxidation in sensitive substrates and biological studies, particularly on pathogens and malignant cells that rely on the thiols’ defense system.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02756. Experimental details, 1H, 13C, and 77Se NMR, HRMS spectra, EPR spectroscopy, and cyclic voltammetry (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Vandana Rathore: 0000-0001-7815-6513 6277

DOI: 10.1021/acs.orglett.8b02756 Org. Lett. 2018, 20, 6274−6278

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DOI: 10.1021/acs.orglett.8b02756 Org. Lett. 2018, 20, 6274−6278