Amino acid and water-driven tunable green protocol to access S–S/C–S bonds via aerobic oxidative coupling and hydrothiolation

Abstract

A green methodology utilizing a natural supplement such as L-arginine in conjunction with water and oxygen led to oxidative coupling of thiols into disulfides (S–S bond) whereas thiol–yne coupling to access vinyl sulfides (C–S bond) was facilitated in a nitrogen atmosphere. The tunable protocol offers several advantages such as low catalyst loading, high yields, clean reaction, no over-oxidation of the S–S bond besides being metal/base/waste-free. The synthesis of ubiquitous cystine and glutathione disulfide in the same catalytic system is an added advantage and the catalytic system has been recycled up to seven times.

---

Introduction

Nature¹ has nurtured mankind with an infinite repository of diverse^1a types of biochemicals and carries out innumerable biochemical processes utilizing the native solvent of cells, i.e., water.^1b Additionally water in conjunction with air leads to the complexity of the biochemical world. The use of water as a new example to innovate eco-sustainable chemical reactions is also gaining momentum because either its release^1c as a by-product or its use as a solvent,^1d,e will clearly have the least impact on the environment. Nevertheless, conducting reactions in water is still challenging because most of the organic reactants are insoluble in water. Furthermore, water may deteriorate the catalytic activity, as well as disrupting polar interaction among catalysts and substrate molecules. Therefore, the development of a water-compatible organocatalyst and its application to chemical reactions using water as the reaction medium is of considerable interest. Furthermore, it is also highly desirable to develop environmentally benign chemical^1f processes that use oxygen from the air in aqueous medium without requiring any metal oxidant.

Besides water, L-amino acids also dominate the natural world² and display a central theme in biological systems.³ Recently, they have been efficiently employed as organocatalysts in triggering a variety of chemical reactions⁴ such as the synthesis⁵ of Pfizer's anti-glaucoma drug: latanoprost^5a and the anticancer drug carboplatin.^5b Interestingly, under physiological conditions, sulfur containing amino acids such as cysteine and glutathione, undergoes oxidative coupling to form the ubiquitous cystine and glutathione disulfide (GSSG) having disulfide (S–S) bonds which are of great practical value.⁶ Likewise, an alicyclic disulfide known as lipoic acid^6a also acts as an essential cofactor of mitochondrial enzymatic complexes which highlights the importance of the S–S bond. Some of the medicinally significant examples of disulfide bond^6b,c are shown in Fig. 1.

Fig. 1 Medicinally significant disulfides.

The disulfide bond is widely found in flora and fauna.⁷ It also plays a substantial role in the design of rechargeable lithium batteries, in a number of chemical reactions⁸ and catalysis.⁹ Furthermore, many of the next generation¹⁰ pharmaceuticals incorporating disulfide bonds have already hit the market. In this context, a large number of oxidative coupling protocols^11–13 mediated by gold nanoparticles^11a or rhenium^13b metal as an effective catalyst have emerged for the synthesis of disulfides. However, most of the approaches suffer from disadvantages such as low selectivity because of over oxidation^13a of the final product into sulfoxides/sulfones, the expensive nature of the catalyst with poor recovery and finally laborious work-up procedures. Recently, environmentally benign approaches to synthesizing heterocyclic disulfides^13g have also emerged, which use a cocktail of enzymes, buffer and methanol. As well as these supported iron oxide nanoparticles^13h have also been used to synthesize disulfides but these methods do not truly fall into the category of green reactions because of the use of organic solvent and the expensive nature of reagents. So there is a strong need to develop such a catalytic system for the chemoselective oxidation of thiols to disulfides, which has all the properties required, that is, a green catalyst, solvent, and reagent besides recyclability of the catalytic system and waste-free, green approach in comparison to relentless conventional protocols^13b–f (which utilize metal oxidant/base/organic solvent).

Apart from making disulfide (S–S) bond, thiols are also known to participate in hydrothiolation¹⁴ with alkynes via thiol–yne coupling (TYC) click reactions^14a to generate vinyl sulfide (C–S bond) which has many miscellaneous medicinal and physicochemical applications.^14b Some of the medicinally significant examples of vinyl sulfides^14c are shown in Fig. 2.

Fig. 2 Important drug intermediates with C–S bond.

In this paper, we disclose for the first time, a waste free and atom economical protocol for the direct oxidative coupling of thiols to disulfides utilizing quasi natural compounds^14d (amino acid–water–air) but precluding any oxidant, base or metal. Among the various amino acids tested, arginine (Arg) provided chemoselective access to disulfides including ubiquitous cystine and GSSG with an ample yield and with recyclability of the catalytic system for up to seven cycles. Additionally, the zwitterionic character of Arg also worked as a driving force towards rapid hydrothiolation of alkynes, i.e., TYC, thus leading to an environmentally benign synthesis of vinyl sulfides (C–S bond). Interestingly, our tunable catalytic system also clearly demonstrated that C–S bond formation is faster than S–S bond formation in a nitrogen atmosphere (Fig. 3).

Fig. 3 Arginine–H₂O catalysed oxidation of thiols and hydrothiolation of alkynes.

Results and discussion

In continuation of our interest in the development¹⁵ of greener approaches,^15a–c our group has recently reported on the amphiphillicity (cationic and anionic character) of 1-hexyl-3-methyl imidazolium bromide ([hmim]Br), a neutral ionic liquid,^15d and this characteristic was found to be imperative for oxidative coupling of thiophenol into disulfides (S–S bond). Encouraged by this, we now attempted to perform oxidative coupling of thiols to disulfides in histidine, a versatile amino acid¹⁶ with an imidazolium ring analogous to the heterocyclic core of [hmim]Br, having zwitterionic characteristics^16a in water (Fig. 4) and which is more environmentally benign, safe and abundant in comparison to ionic liquids.

Fig. 4 Diagram showing the analogy between zwitterionic histidine and ambiphillic [hmim]Br.

Initially, 4-chlorothiophenol (0.25 mmol), water (0.6 mL) and L-histidine (50 mol%) were placed in a round-bottomed flask and stirred at room temperature for 12 h. The desired 4,4′-dichlorodiphenyl disulfide (1b) was formed with a yield of 64% [measured with gas chromatography-mass spectroscopy (GC-MS)]. It is worth noting that our green oxidative coupling process comprising histidine–water–air might be confirming a biochemical process where an intracellular^16b disulfide bond forms in presence of amino acids such as cysteine. However, the amino acid, L-proline (Table 1, entry 2) gave a poor yield (37%) of 1b. Thereafter, a series of neutral, acidic and basic amino acids were screened (Table 1), out of which Arg, a basic amino acid with a pK_a of 2.488 gave a good yield of 1b (70%, 12 h) in air (Table 1, entry 10) whereas a reaction using an O₂ balloon provided 1b in same yield in the shorter reaction time of 5 h (Table 1, entry 15).

Table 1 Screening of different amino acids for the formation of disulfide^a

Entry	Amino acid	Yield^b (%)	Entry	Amino acid	Yield^b (%)
a The reaction conditions involved usage of 0.25 mmol of 1a, atmospheric O2, 50 mol% of L-amino acid, 0.6 mL of high-performance liquid chromatography (HPLC) grade water.b Yields determined using GC-MS.c In an O2 atmosphere the reaction was completed in 5 h.d N2 atmosphere.e Ethanol was used as solvent.f Chloroform was used as solvent.g No Arg used.
1	Histidine	64	11	Glycine	42
2	Proline	37	12	Aspartic acid	15
3	Serine	33	13	Glutamic acid	14
4	Phenylalanine	23	14	Cysteine	Traces
5	Glutamine	33	15	Arginine^c	70
6	Valine	61	16	Arginine^d	Traces
7	Leucine	60	17	Arginine^e	Nd
8	Alanine	25	18	Arginine^f	Nd
9	Isoleucine	42	19^g	—	Nd
10	Arginine	70

---

As expected, 1b was obtained in traces in a nitrogen atmosphere (Table 1, entry 16) whereas when the water was replaced with ethanol or chloroform 1b was not formed (entry 17–18). This is possibly because of the lack of formation of hydrogen bonding and inefficient generation of a zwitterion in organic solvents. With this information, the amount of Arg used and the effect of temperature for the synthesis of 1b was optimized (Table 2).

Table 2 Optimization of the reaction conditions for the formation of 1b under an O₂ atmosphere

S. no.	Amount of organocatalyst (L-arginine)	Temp in °C	Time	% Yield (on GC basis)
1	50 mol%	R.T.	5 h	92
2	100 mol%	R.T.	5 h	91
3	30 mol%	R.T.	5 h	92
4	20 mol%	R.T.	5 h	93
5	10 mol%	R.T.	8 h	70
6	20 mol%	50	15 min	98
7	20 mol%	40	90 min	90
8	20 mol%	70	20 min	92
9	20 mol%	80	15 min	80

---

Thus, a decreased catalyst load of 20 mol% in the presence of O₂ was found to be the most efficient for promoting the synthesis of 1b in 15 min at 50 °C (Table 2, entry 6). A further impact of the contemporary tools namely, a microwave and an ultrasonicator was evaluated (see ESI†). Unfortunately, the reaction conducted in a microwave^12g (50 °C, 80 W) had no profound effect on the efficiency of the synthesis of 1b in a shorter reaction time (up to 45 min) as it led to several side products. Interestingly, the reaction conducted in the ultrasonicator^12f provided the desired yield of 1b (98%) in 10 min (see ESI†). However, because of the malodorous smell¹⁷ of thiols, the oxidative coupling was conducted under conventional conditions in a well ventilated hood.

Finally, optimized conditions using 20 mol% of Arg in 0.6 mL of water, 0.25 mmol of 1a and 1/2O₂ resulted in a yield of 96% 1b (isolated) at 50 °C in 15 min (Table 3, 1b). The scope for using the substrate was explored using electronically versatile thiols which provided diverse types of disulfides (Table 3, 1b–11b) in moderate to excellent yields (up to 96%) and their nuclear magnetic resonance (NMR) results agreed well with those reported in previous experiments.^11,15d After the synthesis of aromatic and alicyclic disulfides (Table 3, 1b–11b) the synthesis of a heterocyclic disulfide (12b–15b) was inspired by a previous paper by Abdel-Mohsen et al.^13g which utilizes laccases/buffer/methanol for oxidative coupling of thiophenols into a series of heterocyclic disulfides. The heterocyclic compounds have numerous added benefits over carbocyclic ones. The catalytic system provided heterocyclic disulfides in up to 70% yield. Unfortunately, 16b or 17b (Table 3) could not be obtained which was possibly because of the feeble zwitterionic interaction of Arg with thiol as well as electron rich hydroxy or amine groups. Thereafter, we decided to work towards establishing the mechanism for the synthesis of disulfides using the Arg–O₂–H₂O catalytic system.

Table 3 Substrate scope of oxidative coupling of thiols in water–Arg–oxygen^a

a Reaction conditions involved use of 0.25 mmol of thiol, 20 mol% of L-Arg, 0.6 mL of HPLC grade water, in O₂.b Isolated yields.c nd = not detected.

---

Mechanism

It is known that in aqueous medium Arg interacts with water through cation–π interactions,¹⁸ which in turn interact with aromatic charged residues of thiophenol. The guanidinium group of Arg is the most polar of all the common amino acid side chains and plays a vital role in the binding of negatively charged substrates.^18b The water provides an enhanced π-stacking as a result of the hydrophobic effect.^18c The amino acid side chain of zwitterionic¹⁹ Arg consists of a three-carbon aliphatic straight chain in which the distal end is capped by a positively charged complex guanidinium^19a group. The delocalized positive charge on the guanidinium group enables the formation of multiple hydrogen bonds with water (Fig. 5).

Fig. 5 The probable mechanism of the formation of diphenyl disulfide via oxidative coupling of thiophenol.

Subsequently, in the aqueous charged^19b pool, hydrogen atoms of one molecule of thiophenol start interacting with the negatively charged carboxyl terminal of the arginine whereas the sulfur atom of another molecule of thiophenol interacts with the positively charged guanidinium group. This complex interplay of subtle non-covalent interactions led to the efficient union of two molecules of 4-chlorothiophenol and was studied using a UV spectrophotometer (Fig. 6). The study revealed that the λ_max of 4-chlorothiophenol (1a) and Arg appeared at 246.5 nm and 207.5 nm, respectively. However, when the substrates were stirred, the spectral analysis clearly indicated that 1a and an aqueous solution of zwitterionic Arg led to the immediate initiation of complexation which was evident from the bathochromic shift from λ_max 246.5 nm to 274.6 nm. It was observed that complexation between the iminium ion of arginine and the heteroatom (S) of thiophenol exponentially increased up to 9 min leading to a chromic shift which is probably indicative of an increase in the concentration of complex “C” (Fig. 5). Diminution in the absorbance (hypochromic shift) of C appeared after 12 min with the initiation of formation of 1b which was finally completed in 110 min at room temperature (see ESI† for details).

Fig. 6 Absorption profiles of thiophenol, Arg, complexation and 4,4′-diphenyl disulfide at 0 min and at the completion of the reaction (after 110 min).

Recyclability of the catalytic system is an important factor in the context of green chemistry as it has both economical as well as ecological benefits. In this experiment, 0.25 mmol of 1a, 20 mol% of Arg, O₂ and 0.6 mL of water were heated at 50 °C, and the resulting product 1b was formed in 20 min. The aqueous solution containing Arg and 1b was extracted using ethyl acetate. The ethyl acetate layer was vacuum evaporated to obtain 1b, whereas the same aqueous solution containing Arg was used repeatedly for carrying out the oxidative coupling for a number of times. The product could be easily recovered from the aqueous medium merely by extracting with ethyl acetate. The remaining aqueous solution of Arg was reused for up to seven consecutive cycles (98–91%) (Fig. 7). Thereafter, there was a considerable decrease in the yield of disulfide. Interestingly, the addition of 5 mol% of Arg in the previously described catalytic cycle further increased the yield of 1b up to 96% (for up to ten cycles) because of the rejuvenation of the organocatalytic system.

Fig. 7 Recyclability studies using the same catalytic water–Arg–oxygen system for up to seven cycles.

It is well known that GSSG with cisplatin is a well-tolerated therapeutic adjuvant called NOV-002 in standard anticancer therapy.^20a Likewise cystine is also a physiological disulfide of clinical importance. Recent biocatalytic protocol highlights disulfides catalyzed by laccases^13g but does not disclose the principles of the synthesis of physiological disulfides, i.e., cystine and GSSG. Thus, because of their medicinal significance and to test the rigor of our protocol, we attempted to synthesize physiological disulfides, i.e., cystine and GSSG under similar conditions documented in Table 3. It is prudent to state that oxidations of cysteine and related compounds have been extensively studied, utilizing metal phthalocyanines^20b and expensive noble metals,^13a however, their synthesis utilizing perfectly benign conditions sill remains a significant challenge.²¹ Interestingly, the organocatalytic system of “Arg–water–oxygen” that was developed successfully led to the formation of dimers (18b and 19b) of aminothiols namely, L-cysteine and L-glutathione in appreciable yields of 71% and 76%, respectively, at 50 °C for longer reaction times of 4 h (Fig. 8).

Fig. 8 Synthesis of cystine (18b) and GSSG (19b) using natural compounds such as Arg–water–oxygen.

Like S–S bonds, C–S²² bonds are another critical^22a–f and ubiquitous^22g constituent of many pharmaceuticals^22h,i and natural products,^22j e.g., allicin, the antihelminthic drug – levamisole, and antibiotics such as penicillins and so on. To construct the C–S bonds, popular ‘click’-procedures such as TYC or hydrothiolation have become an outstanding tool for the functionalization²³ of molecules. The TYC unarguably yields useful synthetic intermediates which have applications in organic^24a transformations including Diels–Alder, thio-Claisen, acting as Michael acceptors and as versatile synthons in olefin metathesis. Certain reports reveal the synthesis of vinyl sulfides using a strong base^24b and ammonia.^24c

There are also numerous reported methods²⁵ for the hydrothiolation of terminal alkynes to form vinyl sulfides using various transition metals such as Wilkinson's catalyst, lanthanides, late transition metal catalysts and various free radical initiators (Fig. 9). However, a method employing a mild, environmentally compatible, efficient^25c and in particular, selective route towards the synthesis of the C–S bond has yet to be found. Therefore, in the light of its expected use,²⁶ hydrothiolation between alkyne and thiophenol in Arg–water to generate vinyl sulfides was carried out in a nitrogen atmosphere (replacing the oxygen atmosphere to prevent the formation of the S–S bond). However, GC analysis of the crude product confirmed the formation of vinyl sulfide (20b) at a yield of 20% along with the unwanted S–S bond (30–35%) and unreacted thiol. Interestingly, the sequence of addition of reactants was found to be crucial, thus, addition of thiophenol to a well stirred mixture of phenylacetylene and water–Arg in a N₂ atmosphere efficiently provided vinyl sulfide (C–S bond) (20b; Table 4) in 45 min with a yield of 81% as an E : Z isomeric mixture.^23e

Fig. 9 Comparison of the usual protocols with our method for TYC.

Table 4 The scope for using the substrate for alkenylative cross-coupling of alkynes and thiophenols to form vinyl sulfides^a

S. no.	Thiol	Alkyne	Alkenyl sulfide (Z/E)	Yield^b
a Reaction conditions involved use of 50 mg of thiol, alkyne (1.1 equiv.), N2, 20 mol% of L-Arg, 0.6 mL of HPLC grade water.b Yields of isolated product.c E : Z ratio obtained using NMR. (see ESI† for details).
1	R1 = 4-OCH3			81
2	R1 = H			71
3	R1 = H			75
4	R1 = OCH3			85
5	R1 = 4-CH3			80

---

The catalytic system has been successfully extended for anti-Markovnikov products (21b–24b)^23e with variable appendages attached to the aromatic ring of thiophenol and alkyne in varying yields up to 85%. The catalytic system also led to the synthesis of 1-phenyl-3-p-tolylsulfanyl-propenone (24b) a unique and significant^26c scaffold with α,β-unsaturated carbonyl moiety (O=C–C=C–S)^26e,f which might be useful for applications in biomedicine, materials chemistry and bioconjugation chemistry.^26g The probable mechanism for the formation of E/Z vinyl sulfides is presented in Fig. 10. Here it is presumed that zwitterionic Arg in water interacts with the thiophenol by the carboxylate terminal and the negatively charged sulfur atom interacts with the terminal alkyne resulting in the formation of vinyl sulfide. The exact mechanism of the formation of E/Z vinyl sulfides is currently under investigation.

Fig. 10 Probable mechanism for the formation of vinyl sulfides.

Conclusions

In conclusion, we have developed an ecologically compatible, tunable catalytic system comprising water and sub-stoichiometric amounts of the organocatalyst Arg which is efficient enough in promoting the chemoselective disulfide (S–S bond) synthesis via oxidative coupling of thiol in the presence of oxygen as well as carrying out hydrothiolation of alkynes/thiol to form their alkenyl sulfides (C–S bond) under a nitrogen atmosphere without requiring any metallic salts and a basic medium. Moreover, aerobic oxidative coupling of L-cysteine and L-glutathione into dimeric cystine and GSSG render our green oxidative process highly practical and appealing for its application in industrial processes. Further, synthesis of vinyl sulfides (E : Z isomeric mixture) having α,β-unsaturated carbonyl moiety (O=C–C=C–S) emphatically demonstrates the synthetic viability of the catalytic system. Therefore, because the quest for such a novel type of tunable catalytic system is high, the method could be used for applications in the construction of aromatic (hetero)/alicyclic/aliphatic disulfides as well as vinyl sulfides by concurrently maintaining the criteria of catalysis, greenness, recyclability and selectivity.

Experimental section

General procedure for the synthesis of disulfide

Initially 0.25 mmol of thiophenol (1a) was added to HPLC grade water (0.6 mL) containing Arg (20 mol%), and heated for 15 min at 50 °C in the presence of O₂. After complete consumption of the starting material (determined by thin layer chromatography (TLC)), the reaction mixture was extracted with ethyl acetate (2 × 10 mL). The combined organic layers were dried over Na₂SO₄, filtered and vacuum evaporated. The GC-MS analysis confirmed the formation of disulfide (1b) at a yield of 98% (isolated 96%). The residue was purified using column chromatography (silica gel 60–120) with ethylacetate–hexane (0.5 : 9.5) as eluent to give the corresponding disulfide. The results of the analysis of the disulfide (1b) obtained using ¹H-NMR and ¹³C-NMR agreed well with the reported values.

General procedure for the synthesis of vinyl sulfides

20 mol% of Arg was stirred for 5 minutes under nitrogen in HPLC grade water (0.6 mL) followed by addition of phenyl acetylene (1.1 equiv.). Next, substituted thiophenol (0.25 mmol) was added to the reaction mixture and stirred at 50 °C under a N₂ atmosphere for 2 h until complete consumption of the starting material (determined by TLC) was achieved. The crude reaction mixture was extracted with ethylacetate (2 × 10 mL) and the combined organic layers were dried over Na₂SO₄, filtered and vacuum evaporated. The residue was purified using column chromatography (silica gel 60–120) with ethylacetate–hexane (1 : 9) as eluent to give the corresponding vinyl sulfide (E : Z isomeric mixture).

Acknowledgements

AS and RK thank CSIR for award of SRF. The authors gratefully acknowledge Directors of CSIR-IHBT, Palampur and CSIR-CDRI, Lucknow for their kind cooperation and encouragement. Some parts of the work were carried out at IHBT, Palampur – CDRI/IHBT Communication No. 8722/3418.

References

1. (a) J. K. Weng, R. N. Philippe and J. P. Noel, Science, 2012, 336, 1667; (b) S. D. Colson and T. H. Dunning Jr, Science, 2012, 265, 43; (c) R. Kumar and E. V. V. Eycken, Chem. Soc. Rev., 2013, 42, 1121; (d) A. Rahmati and K. Vakili, Amino Acids, 2010, 39, 911; (e) D. A. Alonso and C. Najera in Water in Organic Synthesis in Science of Synthesis, ed. S. Kobayashi, 2012; (f) R. A. Sheldon in Green oxidation in water Handbook of Green Chemistry, ed. P. T. Anastas, By Wiley-VCH Verlag, 2010.
2. H. Park, K. M. Kim, A. Lee, S. Ham, W. Nam and J. Chin, J. Am. Chem. Soc., 2007, 129, 1518.
3. H. W. Roesky, M. G. Walawalkar and R. Murugavel, Acc. Chem. Res., 2001, 34, 201.
4. (a) Z. Chai and G. Zhao, Catal. Sci. Technol., 2012, 2, 29; (b) A. C. Evans, A. Lu, C. Ondeck, D. A. Longbottom and R. K. O'Reilly, Macromolecules, 2010, 43, 6374.
5. (a) G. Coulthard, W. Erb and V. K. Aggarwal, Nature, 2012, 489, 278; (b) K. J. Barnham, M. I. Djuran, P. S. Murdoch, J. D. Ranford and P. J. Sadler, Inorg. Chem., 1996, 35, 1065.
6. (a) J. K. Howie, J. J. Houts and D. T. Sawyer, J. Am. Chem. Soc., 1977, 99, 6323; (b) J. Chen, C. S. Jiang, W. Mab, L. Gao, J. Gong, J. Li, J. Li and Y. Guo, Bioorg. Med. Chem. Lett., 2013, 23, 5061; (c) T. C. McMahon, S. Stanley, E. Kazyanskaya, D. Hung and J. L. Wood, Org. Lett., 2012, 14, 4534.
7. (a) R. Munday, Chem. Res. Toxicol., 2012, 25, 47; (b) D. Huang, J. Chen, W. Dan, J. Ding, M. Liu and H. Wu, Adv. Synth. Catal., 2012, 354, 2123; (c) J. West, R. Kevin and S. Otto, Curr. Drug Discovery Technol., 2005, 2, 123; (d) D. J. Craik, Nat. Chem., 2012, 4, 600; (e) A. R. Stefankiewicz and J. K. M. Sanders, Chem. Commun., 2013, 49, 5820.
8. A. Mijovilovich, L. G. M. Pettersson, F. M. F. de Groot and B. M. Weckhuysen, J. Phys. Chem. A, 2010, 114, 9523.
9. R. Knowles and E. N. Jacobsen, Proc. Natl. Acad. Sci. U. S. A., 2010, 48, 20678.
10. G. Saito, J. A. Swanson and K. D. Lee, Adv. Drug Delivery Rev., 2003, 55, 199.
11. (a) A. Corma, T. Ródenas and M. J. Sabater, Chem. Sci., 2012, 3, 398; (b) Y. Liu, H. Wang, C. Wang, J.-P. Wanand and C. Wen, RSC Adv., 2013, 3, 21369–21372.
12. (a) D. Singh, F. Z. Galetto, L. C. Soares, O. E. D. Rodrigues and A. L. Braga, Eur. J. Org. Chem., 2010, 2661; (b) T. Chatterjee and B. C. Ranu, RSC Adv., 2013, 3, 10680; (c) P. Attri, S. Gupta and R. Kumar, Green Chem. Lett. Rev., 2012, 5, 33; (d) M. Sridhar, S. K. Udhel and U. T. Bhalerao, Synth. Commun., 1998, 28, 1499; (e) A. Christoforou, G. Nicolaou and Y. Elemes, Tetrahedron Lett., 2006, 47, 9211; (f) J. L. G. Ruano, A. Parra and J. Aleman, Green Chem., 2008, 10, 706; (g) M. K. Mohammadi, S. Ghammamy and M. H. Farjam, Int. J. Chem. Eng. Appl., 2011, 2, 221; (h) D. M. L. Cabrera, F. M. Líbero, D. Alves, G. Perin, E. J. Lenardão and R. G. Jacob, Green Chem. Lett. Rev., 2012, 5, 329.
13. (a) M. Hung and D. M. Stanbury, Inorg. Chem., 2005, 44, 3541; (b) J. Holguin, J. Am. Chem. Soc., 1997, 119, 9309; (c) H. Golchoubian and F. Hossienpoor, Catal. Commun., 2007, 8, 697; (d) A. Dhakshinamoorthy, M. Alvaro and H. Garcia, Chem. Commun., 2010, 46, 6476; (e) A. Saxena, A. Kumar and S. Mozumdar, J. Mol. Catal. A: Chem., 2007, 269, 35; (f) C. C. Silveira and S. R. Mendes, Tetrahedron Lett., 2007, 48, 7469; (g) H. T. Abdel-Mohsen, K. Sudheendran, J. Conrad and U. Beifuss, Green Chem., 2013, 15, 1490; (h) F. Rajabi, T. Kakeshpour and M. R. Saidi, Catal. Commun., 2013, 40, 13.
14. (a) V. T. Huynh, G. Chen, P. de Souza and M. H. Stenzel, Biomacromolecules, 2011, 12, 1738; (b) C. E. Hoyle and C. N. Bowman, Angew. Chem., Int. Ed., 2010, 49, 1540; (c) H. W. Lam, P. A. Cooke, G. Pattenden, W. M. Bandaranayake and W. A. Wickramasinghe, J. Chem. Soc., Perkin Trans. 1, 1999, 847; (d) T. Y. Chen, S. F. Chen, H. S. Sheu and C. S. Yeh, J. Phys. Chem. B, 2002, 106, 9717.
15. (a) R. Kumar, A. Shard, R. Bharti, Y. Thopate and A. K. Sinha, Angew. Chem., Int. Ed., 2012, 51, 2636; (b) U. K. Sharma, N. Sharma, R. Kumar, R. Kumar and A. K. Sinha, Org. Lett., 2009, 11, 4846; (c) A. Shard, N. Sharma, R. Bharti, S. Dadhwal, R. Kumar and A. K. Sinha, Angew. Chem., Int. Ed., 2012, 51, 12250; (d) R. Kumar, N. Sharma, U. K. Sharma, A. Shard and A. K. Sinha, Adv. Synth. Catal., 2012, 354, 2107; (e) R. Kumar, Richa, N. H. Andhare, A. Shard and A. K. Sinha, Chem.–Eur. J., 2013, 19, 14798.
16. (a) H. Ohno and K. Fukumoto, Acc. Chem. Res., 2007, 40, 1122; (b) M. Cornad and H. Sato, Amino Acids, 2012, 42, 231.
17. M. M. Coulter, K. G. M. Kou, B. Galligan and V. M. Don, J. Am. Chem. Soc., 2010, 132, 13117.
18. (a) D. Shukla and B. L. Trout, J. Phys. Chem. B, 2010, 114, 13426; (b) C. L. Borders, J. A. Broadwater, P. A. Bekeny, J. E. Salmon, A. S. Lee, A. M. Eldridge and V. B. Pett, Protein Science, Cambridge University Press, printed in the USA, 1994, p. 3541; (c) C. J. Li, Angew. Chem., Int. Ed., 2003, 42, 4856.
19. (a) J. L. Alonso, E. J. Cocinero, A. Lesarri, M. E. Sanz and J. C. Lopez, Angew. Chem., Int. Ed., 2006, 45, 3471; (b) G. T. Brink, I. W. C. E. Arends and R. A. Sheldon, Science, 2000, 287, 1636.
20. (a) M. Onofrj, L. Bonanni and A. Thomas, Expert Opin. Invest. Drugs, 2008, 17, 1115; (b) X. Zhao, A. Kong, X. Zhang, C. Shan, H. Ding and Y. Shan, Catal. Lett., 2010, 135, 291.
21. E. G. Jampel and M. Therisod, J. Chem. Soc., Perkin Trans. 1, 1999, 3067.
22. (a) R. Munday, Chem. Res. Toxicol., 2012, 25, 47; (b) X. Guo and F. C. Szoka Jr, Acc. Chem. Res., 2003, 36, 335; (c) S. Otto, R. L. E. Furlan and J. K. M. Sanders, Science, 2002, 297, 590; (d) A. Martineza, I. Iglesias, R. Lozanob, J. M. Teijonc and M. D. Blancoc, Carbohydr. Polym., 2011, 83, 1311; (e) M. Oba, K. Tanaka, K. Nishiyama and W. Ando, J. Org. Chem., 2011, 76, 4173; (f) M. Hung and D. M. Stanbury, Inorg. Chem., 2005, 44, 3541; (g) An Introduction to Organosulfur Chemistry, R. J. Cremlyn, John Wiley & Sons, Chichester, 1996; (h) J. A. Cabeza, I. D. Rio, S. G. Granda, V. Riera and M. Suarez, Eur. J. Inorg. Chem., 2002, 2561; (i) J. E. Baldwin, E. P. Abraham, R. M. Adlington, J. A. Murphy, N. B. Green, H. H. Ting and J. J. Usher, Chem. Commun., 1983, 1319; (j) D. P. Ilic, V. D. Nikolic, L. B. Nikolic, M. Z. Stankovic and L. P. Stanojevic, Hem. Ind., 2010, 64, 85.
23. (a) T. T. Le, N. T. T. Chau, T. T. Nguyen, J. Brien, T. T. Thai, A. Nourry, A. S. Castanet, K. P. Phung Nguyen and J. Mortier, J. Org. Chem., 2011, 76, 601; (b) A. Kondoh, K. Takami, H. Yorimitsu and K. Oshima, J. Org. Chem., 2005, 70, 6468; (c) D. Konkolewicz, A. Gray-Weale and S. Perrier, J. Am. Chem. Soc., 2009, 131, 18075; (d) I. P. Beletskaya and V. P. Ananikov, Chem. Rev., 2011, 111, 1596; (e) R. Singh, D. S. Raghuvanshi and K. N. Singh, Org. Lett., 2013, 15, 4202.
24. (a) A. D. Giuseppe, R. Castarlenas, J. J. P. Torrente, M. Crucianelli, V. Polo, R. Sancho, F. J. Lahoz and L. A. Oro, J. Am. Chem. Soc., 2012, 134, 8171; (b) N. K. Gusarova, N. A. Chernysheva, S. V. Yas'ko and B. A. Trofimov, Russ. Chem. Bull., 2013, 62, 438; (c) A. N. Volkov, K. A. Volkova, E. P. Levanova and B. A. Trofimov, Izv. Akad. Nauk SSSR, Ser. Khim., 1983, 1, 212.
25. (a) D. Konkolewicz, A. Gray-Weale and S. Perrier, J. Am. Chem. Soc., 2009, 131, 2062; (b) S. Shoai, P. Bichler, B. Kang, H. Buckley and A. Jennifer, Organometallics, 2007, 26, 5778; (c) C. J. Weiss, S. D. Wobser and T. J. Marks, Organometallics, 2010, 29, 6308; (d) J. S. Yadav, B. V. Subba Reddy, A. Raju, K. Ravindar and G. Baishya, Chem. Lett., 2007, 36, 1474; (e) L. Benati, L. Capella, P. C. Montevecchi and P. Spagnolo, J. Org. Chem., 1995, 60, 7941; (f) C. J. Weiss, S. D. Wobser and T. J. Marks, J. Am. Chem. Soc., 2009, 131, 2062; (g) A. Corma, C. G. Arellano, M. Iglesias and F. Sánchez, Appl. Catal., A, 2010, 49.
26. (a) A. Kondoh, K. Takami, H. Yorimitsu and K. Oshima, J. Org. Chem., 2005, 70, 6468; (b) B. Meunier, S. P. de Visser and S. Shaik, Chem. Rev., 2004, 104, 3947; (c) T. R. Swaroop, R. Roopashree, H. Ila and K. S. Rangappa, Tetrahedron Lett., 2013, 54, 147; (d) R. Sarma, N. Rajesh and D. Prajapati, Chem. Commun., 2012, 48, 4014; (e) B. Cavalchi, D. Landini and F. Montanari, J. Chem. Soc. C, 1969, 9, 1204; (f) T. Nishio and Y. Omote, J. Chem. Soc., Perkin Trans. 1 (1972–1999), 1981, 934; (g) A. Massi and D. Nanni, Org. Biomol. Chem., 2012, 10, 3791.

---

Footnote

† Electronic supplementary information (ESI) available: Detailed UV-based mechanistic studies and NMR spectral values of compounds are provided. See DOI: 10.1039/c4ra02909g

---