Chemoselective oxidation of sulfides to sulfoxides

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Inorganic Chemistry Communications 40 (2014) 82–86

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Chemoselective oxidation of sulfides to sulfoxides with urea hydrogen peroxide (UHP) catalyzed by non-, partially and fully β-brominated meso-tetraphenylporphyrinatomanganese(III) acetate Saeed Rayati a,⁎, Fatemeh Nejabat a, Saeed Zakavi b,⁎ a b

Department of Chemistry, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran 15418, Iran Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran

a r t i c l e

i n f o

Article history: Received 13 October 2013 Accepted 26 November 2013 Available online 4 December 2013 Keywords: Oxidation Sulfides Urea hydrogen peroxide (UHP) Manganese porphyrin

a b s t r a c t Selective oxidation of sulfides to sulfoxides with urea hydrogen peroxide in the presence of the manganese complex of non-, partially and fully brominated meso-tetraphenylporphyrin, (MnTPPBrx(OAc) (x = 0, 2, 4, 6 and 8)) is reported. Although, the maximum conversion was achieved in the case of MnTPPBr4(OAc), little difference was found between the catalytic activity of MnTPP(OAc), MnTPPBr2(OAc) and MnTPPBr4(OAc). MnTPPBr8(OAc) showed an unusually very low catalytic efficiency compared to the other manganese porphyrins. The presence of small amounts of acetic acid was shown to have significant effect on the total conversion and the oxidative stability of the catalyst. © 2013 Elsevier B.V. All rights reserved.

Organic sulfoxides are useful and active intermediates in both laboratory and industry, organic synthesis and therefore the chemoselective oxidation of organic sulfides to the corresponding sulfoxides has been the subject of various studies for the past two decades [1–3]. An additional challenge in this respect is to make such processes environmentally friendly, by strategies such as using nontoxic solvents, green oxidants and energy-efficient catalytic methods [2–5]. Metalloporphyrins as model catalysts of cytochrome P450 have been used extensively for biomimetic oxidation of organic compounds [6–10]. On the other hand, in the past two decades great interest has been focused on using clean procedures for oxidation reactions catalyzed by metalloporphyrins [11–13]. In this regard, hydrogen peroxide and its derivatives such as UHP (urea hydrogen peroxide) and polyvinylpyrrolidone-supported hydrogen peroxide (PVP–H2O2) as cheap and environmentally friendly oxidants which only produce water and oxygen as side products are attractive oxidants for oxidation of organic compounds [2–5,11–14]. Also, UHP has the advantage that the vacuum dried reagent may be used as a nearly water-free peroxide source. It is noteworthy that in the oxidation of organic compounds catalyzed by manganese porphyrins, the presence of H2O leads to the formation of a high-valent Mn-oxo species as the active oxidant [11,12]. In the oxidation of sulfides, higher oxidizing ability of the high valent manganese oxo species compared to the corresponding manganese(III) species, i.e. (porphyrin)Mn(III)(oxidant)(axial base) is possible to direct the reaction towards the formation of sulfone as the major product [15]. In other words, further oxidation of sulfoxide to sulfone may be prevented by the removal of water from the reaction ⁎ Corresponding authors. Tel.: +98 21 22850266; fax: +98 21 22853650. E-mail addresses: [email protected], [email protected] (S. Rayati). 1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.11.036

mixture. The released urea upon the reaction of UHP with organic substrates is then available to form strong hydrogen bonds with the water molecules formed in the oxidation reaction. Herein, a green and simple method for selective oxidation of sulfides to sulfoxides with UHP catalyzed by (MnTPPBrx(OAc) (x = 0, 2, 4, 6 and 8)), in the presence of imidazole (ImH) in ethanol is reported. Also, the influence of different parameters on the efficiency of the catalysts was investigated. The free base porphyrins and the manganese complexes were prepared and purified as reported previously [16]. Oxidation of methyl phenyl sulfide with UHP catalyzed by MnTPPBrx (OAc) (x = 0, 2, 4, 6 and 8) gave methyl phenyl sulfoxide as the major product. In a search for suitable reaction conditions to achieve the maximum conversion and highest selectivity for sulfoxide, the effect of different parameters including solvent, temperature, amount of oxidant and ImH and the presence of acetic acid (HOAc) was studied. Oxidation of methyl phenyl sulfide with UHP was carried out in the presence of MnTPPBrx(OAc) (x = 0, 2, 4, 6 and 8) (Table 1) and the highest conversion was observed in the presence of MnTPPBr4(OAc). However, little difference was found between the catalytic activity of MnTPPBrx(OAc) with no, two and four bromine atoms. Also, MnTPPBr8 showed very low catalytic activity. Along with our previous work [16], the present results confirmed that the partially brominated Mn-porphyrins have a higher catalytic activities compared to the fully ß-brominated one. The reaction was performed in dichloromethane, methanol, ethanol and the mixture of dichloromethane and the non-chlorinated solvents (Table 2). Although the highest conversion was achieved in mixture of CH2Cl2:MeOH or methanol, ethanol is a more convenient solvent for

S. Rayati et al. / Inorganic Chemistry Communications 40 (2014) 82–86

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Table 1 Oxidation of methyl phenyl sulfide with UHP catalyzed by MnTPPBrx(OAc) (x = 0, 2, 4, 6 and 8) in the presence of ImH in CH3OH:CH2Cl2 (9:1) at room temperature.a Catalyst

Conversion%

Sulfoxide%

Sulfone%

Sulfoxide selectivity%

None MnTPP(OAc) MnTPPBr2(OAc) MnTPPBr4(OAc) MnTPPBr6(OAc) MnTPPBr8(OAc)

Trace 44 46 48 20 5

– 44 44 48 20 5

– – 2 – – –

– 100 96 100 100 100

a

The molar ratios for catalyst:ImH:sulfide:UHP are 1:10:20:60. Reaction time: 5 min.

Table 2 Effect of various solvents (in different molar ratios) on the oxidation of methyl phenyl sulfide in the presence of ImH and MnTPPBr4 at room temperature.a Entry

Time

Solvent

Conversion%

Sulfoxide%

Sulfone%

Selectivity (sulfoxide)%

1 2 3 4 5 6 7

5 5 5 5 5 15 20

CH2Cl2 CH2Cl2:MeOH (1:1) CH2Cl2:MeOH (9:1) CH2Cl2:MeOH (1:9) MeOH CH2Cl2:ethanol (1:9) Ethanol

3.6 26.6 21.0 34.7 33.0 12.0 5.4

3.6 26.6 21.0 34.7 33.0 12.0 5.4

– – – – – – –

100 100 100 100 100 100 100

a

The molar ratios for MnTPPBr4:ImH:MePhS:UHP are 1:10:20:40.

Table 3 The effect of various conditions on the oxidation of methyl phenyl sulfide catalyzed by MnTPPBr4 in the presence of ImH in ethanol.a Entry

Time (min)

Temperature

HAC/cat

ImH/cat

UHP/cat

Conversion%

Sulfoxide%

Sulfone%

Sulfoxide selectivity%

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

20 20 5 10 10 5 5 5 5 5 10 10 10

25 25 25 25 25 25 25 25 25 0 0 0 0

– – – – 45 45 30 15 55 30 30 45 55

10 10 30 30 30 30 30 30 30 30 30 30 30

40 60 60 60 60 40 40 40 40 40 60 60 60

5.4 6.2 19.0 26.7 96.2 56.7 67.6 42.2 40.0 42.5 80.4 90.8 73.6

5.4 6.2 19.0 24.2 39.1 46.7 54.2 37.4 40.0 35.9 61.6 55.1 40.3

– – – 2.5 57.1 10.0 13.4 4.7 0 6.8 18.8 35.7 33.3

100 100 100 90 41 82 80 87 100 85 77 60 55

a

See the footnotes of Table 1.

environmentally friendly conversion of sulfides to sulfoxides [17–19] and therefore in further optimization of the catalytic system ethanol was used as solvent (Table 3). UHP is very insoluble in dichloromethane, due to the strong hydrogen bonds between H2O2 and urea. Solvents such as methanol or ethanol increase the solubility of UHP and consequently hydrogen peroxide.

The selective oxidation of sulfides to sulfoxides has been an important challenge in synthetic organic chemistry [1–3,20]. Accordingly, the optimum conditions for the formation of sulfoxide as the major product have been investigated. The results for the oxidation of methyl phenyl sulfide in various conditions are summarized in Table 3. As the results show, in the absence of HOAc, very low conversions

70 60

yield%

50 40

conversion

30

sulfoxide sulfone

20 10 0

10

30

40

50

ImH/Catalyst Fig. 1. Effect of molar ratio of the ImH:MnTPPBr4 on the oxidation of methyl phenyl sulfide with UHP in ethanol and 10 μl HOAc at room temperature. The molar ratios for MnTPPBr4:ImH:MePhS:UHP are 1:X:20:40.

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Table 4 Oxidation of various sulfides with UHP catalyzed by MnTPPBr4 in the presence of ImH and 10 μl of HOAc at 0 °C in ethanol.a Entry

Conversion%

Selectivity%

1

80.4 (96.2)b

77 (41)

2

91.4 (89.5)

77 (69)

3

45.2 (33.9)

57 (67)

a b

Sulfides

The molar ratios for MnTPPBr4:ImH:MePhS:UHP are 1:30:20:60. The reaction carried out in EtOH and 15 μl of HOAc at room temperature.

Table 5 The effect of HOAc on the oxidation of methyl phenyl sulfide with UHP in ethanol at room temperature.a,b Entry

HOAc/UHP

Conversion%

Selectivity to sulfoxide%

1 2

0 0.7

0 8.5

0 100

a b

The molar ratios for sulfide:UHP are (20:60). For reactions performed in the absence of MnTPPBr4.

(5.4–26.7%) were obtained and increases in ImH concentration or reaction time have little effect on the conversion (Table 3, Entries1–4). Addition of 15 μl of HOAc increased the conversion to 96.2%, but the selectivity for sulfoxide decreased (Table 3, Entry 5). The formation of

high valent manganese oxo species has been shown to be facilitated in the presence of alcohols [21]. The decreased selectivity for sulfoxide seems to be due to the higher oxidizing ability of the high valent metal oxo species compared to the six coordinated Mn(III)–H2O2 one. Acceptable conversions and selectivity for sulfoxide may be achieved by changing the molar ratio between HOAc, ImH and the oxidant. The presence of nitrogenous base has been shown to have significant effect on the catalytic activity of manganese porphyrins [22–27]. Oxidation of methyl phenyl sulfide was carried out using various molar ratios of ImH/catalyst (Fig. 1) and the 1:30 molar ratio was found to be the optimized one. Further increase in the molar ratio catalyst to ImH, beyond the 1:40 one led to a dramatic decrease in the conversion. This observation may be due to the formation of an inactive six coordinate species, i.e. Mn(porphyrin)(ImH)2 [27]. Oxidation of various sulfides with UHP in the presence of catalytic amounts of MnTPPBr4 and ImH gave a mixture of sulfoxide and sulfones as the products (Table 4). In the absence of MnTPPBr4, the oxidation of methyl phenyl sulfide with UHP did not proceed even after 45 min. Also, in the presence of acetic acid a conversion of ca. 8.5% was obtained (Table 5). These observations suggest a catalytic cycle containing the manganese porphyrin as catalyst and peracetic acid formed by the reaction of hydrogen peroxide and HOAc [28] as oxidant (Scheme 1, I) [29–31]. Increasing the rate of the reaction by addition of HOAc suggests the formation of a manganese-acylperoxo species (Scheme 1, II) [32–34] which facilitates the cleavage of the oxygen–oxygen bond to generate a manganese-oxo intermediate (Scheme 1, III) [35,36]. On the other hand, oxidative stability of MnTPPBr4(OAc) has been found to be higher

Scheme 1. Proposed catalytic cycle.

S. Rayati et al. / Inorganic Chemistry Communications 40 (2014) 82–86 Table 6 The effect of acetic acid on catalyst stability in the oxidation of methyl phenyl sulfide with UHP as oxidant.a Entry

1 2 3 4

Time (min)

3 6 9 12

Catalyst degradation% With HOAc

Without HOAc

4.3 33.6 37.1 37.1

63.3 71.1 73.5 78.5

a The molar ratios for MnTPPBr4:ImH:MePhS:UHP are 1:30:20:60,the reaction carried out in ethanol and 15 μl HOAc or without HOAc at room temperature.

(a)

(b)

Fig. 2. Hydrogen bond formation between HOAc and the possible active oxidant.

in the presence of HOAc (Table 6), possibly due to the formation of hydrogen bonds (Fig. 2) between HOAc and the active oxidant(s) [21]. In summary, the catalytic activity of a series of non-, partially and fully brominated meso-tetraphenylporphyrin, (MnTPPBrx(OAc) (x = 0, 2, 4, 6 and 8)) for the oxidation of sulfides to sulfoxides with UHP in the presence of ImH has been studied. Although, the maximum conversion was achieved in the case of MnTPPBr4(OAc), the comparable catalytic activity of MnTPP(OAc), MnTPPBr2(OAc) and MnTPPBr4(OAc) showed that the β bromination of MnTPP(OAc) cannot be used as an efficient strategy to improve the catalytic performance of the manganese porphyrin. In this regard, MnTPPBr8(OAc) showed an unusually very low catalytic efficiency compared to the other manganese porphyrins. The addition of small amounts of HOAc was demonstrated to have significant effect on the total conversion of methyl phenyl sulfide to the oxidation products and the oxidative stability of the catalyst for the reaction catalyzed by MnTPPBr4(OAc). Also, the product distribution as well as the total conversion was found to be significantly influenced by the molar ratio of catalyst, ImH, sulfide, HOAc and UHP. Furthermore, the activation of peracetic acid formed by the reaction of hydrogen peroxide and acetic acid on the manganese porphyrin has been proposed as the possible mechanism for the oxidation reactions. Acknowledgments The financial support of this work by K.N. Toosi University of Technology research council is acknowledged. References [1] S.S. Kim, G. Rajagopal, Efficient and mild oxidation of sulfides to sulfoxides by iodosobenzene catalyzed by Cr(salen) complex, Synthesis 16 (2003) 2461. [2] M. Bagherzadeh, M. Zare, Oxidation of sulfides with urea-hydrogen peroxide catalyzed by iron–salen complexes, J. Sulfur Chem. 32 (2011) 335–343. [3] M.M. Khodaei, K. Bahrami, Oxidation of sulfides to sulfoxides with H2O2/HNO3 reagent system, J. Sulfur Chem. 31 (2010) 83–88. [4] M. Bagherzadeh, L. Tahsini, R. Latifi, Efficient oxidation of olefins and sulfides catalyzed by manganese(III)–tridentate Schiff base complex using UHP as oxidant, Catal. Commun. 9 (2008) 1600–1606. [5] X.T. Zhou, H.B. Ji, H.C. Xu, H.L.X. Pei, L.F. Wang, X.D. Yao, Enzymatic-like mediated olefins epoxidation by molecular oxygen under mild conditions, Tetrahedron Lett. 48 (2007) 2691–2695.

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