Tetrahedron Letters 55 (2014) 1028–1030

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Tetrahedron Letters

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Copper–Schiff base complex catalyzed oxidation of sulfideswith hydrogen peroxide

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⇑ Corresponding author. Tel.: +91 9435374128; fax: +91 3842 224797.E-mail address: barmanpranjit@yahoo.co.in (P. Barman).

Table 1Oxidation of 2-nitrophenylbenzyl sulfide (2b) using 30% aqueous H2O2 cby 1a

Entry Catalyst (mol %) Solvent Time(h)

Sulfoxideb

(%)Su(%

1 1 (0.5) CH3CN 6 95 52 1 (0.05) CH3CN 10 68 103 1 (1.0) CH3CN 6 90 84 1 (0.5) CH3COCH3 6 70 105 1 (0.5) CH2Cl2 8 90 86 1 (0.5) CH3OH 8 84 77 CuCl2 (0.5) CH3CN 8 70 128 Cu(C5H7O2)2 (1.0) CH3CN 7 75 10

9c [CoL]H2O (0.5) CH3CN 8 78 1810c [NiL]H2O (0.5) CH3CN 8 57 15

a Reaction condition: substrate (2 mmol), complex 1 (0.5 mol %), and 3(5 mmol) were stirred at room temperature in solvent (2 mL).

b Isolated yield.c Where ligand L, N-[(2-benzylthio)-phenyl] salicylaldimine was pre

reported in our earlier work.19

Prasanta Gogoi, Mukul Kalita, Tirtha Bhattacharjee, Pranjit Barman ⇑Department of Chemistry, National Institute of Technology, Silchar 788010, Assam, India

a r t i c l e i n f o

Article history:Received 25 October 2013Revised 17 December 2013Accepted 19 December 2013Available online 30 December 2013

Keywords:Copper catalystSulfide oxidationH2O2

a b s t r a c t

A straightforward, efficient, and selective oxidation of sulfide to sulfoxide with 30% H2O2 catalyzed bycopper(II)–Schiff base complex is described. The reactions proceed under mild conditions in acetonitrileat room temperature to provide a variety of aryl and alkyl sulfoxides in excellent yield.

� 2013 Elsevier Ltd. All rights reserved.

atalyzed

lfoneb

)

0% H2O2

pared as

Sulfoxides are a common functionality found in numerouspharmaceutically active compounds.1 They also have widespreadutilization as convenient intermediates or reagents in organic syn-theses.2 Indeed, a number of drugs in therapeutic areas such asantiulcerative, cardiotonic, antihypertensive as well as psychoton-ics and vasodilators contain sulfoxide functionality.3 Traditionalmethod for the synthesis of sulfoxides which gives good yieldscould be obtained from different catalytic systems based on gold,4

titanium,5 iron,6 vanadium,7 palladium,8 manganese,9 copper,10

cobalt,11 magnesium,12 molybdenum,13 zinc,14 zirconium,15 andruthenium.16 Recent developments using non-metal catalysts andeven, a neat condition without any catalyst can give a good yieldand selectivity for common sulfides.17 Therefore, it is of currentinterest to develop economically and environmentally more sus-tainable procedures for the preparation of sulfoxides.20

On the contrary, the Schiff base metal catalyzed preparation ofsulfoxide from sulfide has been widely developed.18 In this Letter,we wish to describe the protocol in which H2O2, an ideal terminaloxidant, has been used in the presence of Schiff base–copper com-plex 1 for the oxidation of sulfides to their sulfoxides; a processdelivering a high yield and desirable reaction time.

To check the feasibility of our envisioned route to oxidation ofsulfides, a series of experiments were carried out. Initially, 2-nitro-phenylbenzyl sulfide (2b) was treated with 30% H2O2 in the pres-ence of copper–complex 1 (0.5 mol % based on the sulfide) inCH3CN (Table 1). After stirring the reaction mixture at room tem-perature for 6 h, the 2-nitrophenylbenzylsulfide (2b) was com-pletely converted to the corresponding sulfoxide and sulfone in

good chemoselectivity (95:5) (Table 1, entry 1). On decreasingthe catalyst loading under this condition, the reaction time wasprolonged for the satisfied conversion but the ratio of the sulfoxideand sulfone was decreased (Table 1, entry 2). However, use of morethan 0.5 mol % catalyst 1 did not improve the yield (Table 1, entry3). Using 0.5 mol % copper–catalyst 1, the chemoselectivity of 95:5could be achieved in 6 h (Table 1, entry 1). In comparison to0.5 mol % catalyst loading possessed the higher catalytic efficiencydue to the short reaction time. Among the other tested catalysts(entries 7–10), catalyst 1 was found to be the most effective andthe product 3b (Fig. 1, Scheme 1) was obtained in 95% yield. With

Table 2Copper-complex 1 catalyzed selective oxidation of sulfide to sulfoxidesa

Entry Product (3) Time(h)

Select. of sulfoxideb

(%)

1

3a

4 91

2

3b

6 95

3

3c

5 87

4

3d

5 88

5

3e

4 90

Table 2 (continued)

Entry Product (3) Time(h)

Select. of sulfoxideb

(%)

6

3f

5 90

7

3g

6 85

8

3h

9 82

9

3i

7 81

10

3j

7 87

11

3k

2 89

12

3l

2.5 92

13

3m

4 90

14

3n

2 95

15

3o

3 85

16

3p

4 91

17

3q

6 82

(continued on next page)

Scheme 1. Copper–Schiff base complex catalyzed oxidation of 2-nitrophenylbenzylsulfide with hydrogen peroxide.

Figure 1. ORTEP view of 3b.

P. Gogoi et al. / Tetrahedron Letters 55 (2014) 1028–1030 1029

Table 2 (continued)

Entry Product (3) Time(h)

Select. of sulfoxideb

(%)

18

3r

5 84

19

3s

5 83

20

3t

7 87

a Reaction condition: substrate (2 mmol), complex 1 (0.5 mol %), and 30 % H2O2

(5 mmol) were stirred at room temperature in CH3CN (2 mL).b Isolated yield.

1030 P. Gogoi et al. / Tetrahedron Letters 55 (2014) 1028–1030

the initial results in hand, a series of solvents were employed forthe reaction, including acetone, dichloromethane, and methanolgiving good yields but a slightly longer reaction time (Table 1, en-tries 5 and 6). A widely used solvent, dichloromethane had aremarkable effect on the oxidation reaction, while acetonitrilewas the best among the solvents (Table 1).

With the optimal conditions established (Table 1, entry 1), weexplored the scope to study the oxidation of other sulfides(Table 2). The other substrates, dibenzyl, aryl benzyl, aryl alkyl, arylallyl, and dialkyl sulfides, could be oxidized to the correspondingsulfoxides. The reactivity and conversion were dependent on thenature of the substituent. In the case of benzylic sulfides (entries1–3) no oxidation was observed at the benzylic C–H bond. Simi-larly, allyl sulfides, aryl allyl sulfides, and aryl vinyl sulfide (entries4–7) could be oxidized to the corresponding sulfoxides withoutaffecting the carbon–carbon double bond. Diallyl and dialkyl sul-fides were moderately reactive providing the correspondingsulfoxides.

Catalyst 1 and substrate 2b preparation: The catalyst 1 and sub-strate 2b were prepared as reported in our earlier work.19

In summary, the results indicate that complex 1 is an efficientcatalyst for the oxidation of both aryl and alkyl sulfides to the cor-responding sulfoxides with hydrogen peroxide. The product 3b isnew, the catalytic processes are clean, safe, and most of the startingsulfides are commercially available.

Acknowledgements

P.G. acknowledges Professor C. Bolm, RWTH Aachen University,Germany and Selina Rawal, Slough, Lonza Biologics plc, UK. Wealso thank the Director of NIT, Silchar for financial assistance andCIF of Tezpur and SAIF of Gauhati University, India for extendingthe facilities for characterization of the samples and XRD analysis,respectively.

Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.tetlet.2013.12.073.

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20. A typical procedure: A 30% hydrogen peroxide solution (5 mmol) was added to asolution containing the sulfide 2 (2 mmol), complex 1 (0.5 mol %) and 2 mLCH3CN. The reaction mixture was stirred at room temperature untilcompletion of reaction as monitored by TLC. After complete conversion ofthe reactant, the product was extracted with EtOAc and washed with water.The organic layer was dried over anhydrous Na2SO4. The solvent was removedunder vacuum and the residue was purified by chromatography (eluting with1:1 hexane/EtOAc).Nitrophenyl benzyl sulfoxide (3b): White crystalline solid, mp 140 �C. 1H NMR(400 MHz, CDCl3) d 8.23 (d, J = 9.0 Hz, 1H), 7.95 (d, J = 7.8 Hz, 1H), 7.83 (t,J = 8.2 Hz, 1H), 7.54 (t, J = 7.9 Hz, 1H), 7.32–7.21 (m, 5H), 4.11 (d, J = 12.1 Hz,1H), 3.88 (d, J = 12.1 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 144.2, 141.7, 134.1,133.8, 129.3, 129.1, 128.5, 128.2, 127.4, 125.1, 61.5; Anal. Calcd forC13H11NO3S: C, 59.76; H, 4.24; N, 5.36. Found: C, 59.70; H, 4.34; N, 5.21.