factors influencing the catalytic activity of β-tetrabrominated meso -tetra( para...

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Journal of Porphyrins and PhthalocyaninesJ. Porphyrins Phthalocyanines 2011; 15: 131–139

DOI: 10.1142/S108842461100301X

Published at http://www.worldscinet.com/jpp/

Copyright © 2011 World Scientific Publishing Company

INTRODUCTION

The monooxygenation of substrates by cytochromes P450 has been reproduced by using metalloporphyrins model systems, the most efficient catalysts being Fe(III) or Mn(III) tetraarylporphyrins bearing electron-withdrawing substituents at the β-pyrrole positions [1]. Introduction of electron-withdrawing substituents at the periphery of por-phyrin ring is among the main strategies have been used to reduce degradation of metalloporphyrins in oxidative conditions [2]. The effect of degree of β-bromination of porphyrin core on the catalytic efficiency of meso- tetrakis (4-carbomethoxyphenyl)porphyrinatomanganese(III) chloride in oxidation of cyclohexene has been studied by Silva et al. [3]. Tetra-n-butylammonium hydrogen

monopersulfate (TBAHS) have been utilized as very efficient oxidant in oxidation of olefins and organosul-furs in the presence of Mn-porphyrins as catalyst and imidazole (ImH) as the nitrogen donor ligand [4–8]. Sul-foxides are important intermediates in organic synthesis which are useful in the synthesis of natural products and biologically significant molecules [9, 10]. Accordingly, the selective oxidation of sulfides to sulfoxides has been an important challenge in synthetic organic chemistry [11–15]. In oxidation of sulfides with TBAHS in dichlo-romethane, sulfone has been obtained as the sole product using Mn-porphyrin/ImH catalytic system [4–8, 16, 17] which limits its application to the oxidation of sulfides to the corresponding sulfone. In the present work, it has been shown that by changing the reaction parameters, the chemoselectivity of this very fast reaction may be significantly altered towards the formation of sulfoxide as a considerable product compared to the sulfone one. On the other hand, due to the environmental problems

Factors influencing the catalytic activity of b-tetrabrominated meso-tetra(para-tolyl)porphyrinatomanganese(III) for oxidation of sulfides and olefins with Oxone

Saeed Rayati*a, Saeed Zakavi*b and Hossein Kalantarib,c

a Department of Chemistry, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran 15418, Iran b Department of Chemistry, Institute for Advanced Studies in Basic Sciences, Gava Zang, Zanjan 45137-66731, Iran c Department of Chemistry, University of Kurdestan, P.O. Box 66135-416, Sanandaj, Iran

Received 13 December 2010Accepted 27 January 2011

ABSTRACT: Effect of different reaction parameters on the catalytic activity of β-tetrabrominated meso-tetra(para-tolyl)porphyrinatomanganese(III), MnT(4-CH3P)PBr4(OAc), for oxidation of different sulfides and hydrocarbons with tetra-n-butylammonium hydrogen monopersulfate (TBAHS) has been studied. In oxidation of sulfides, the chemoselectivity of reaction has been significantly changed in THF as the solvent compared with the common organic solvents. Also, using nitrogenous bases bearing electron-withdrawing groups (-Cl or -CN) clearly increased the ratio of sulfoxide to sulfone relative to the electron-donating ones. Catalytic oxidation of olefins with TBAHS was conducted in protic and aprotic solvents and acetonitrile has been found as the best solvent. A significantly large difference was found between the co-catalytic activity of imidazole (ImH) and pyridine in comparison with that observed in dichloromethane. The competitive oxidation of cis- and trans-stilbene suggests the presence of a high valent manganese oxo as well as a six coordinate (ImH)MnT(4-CH3P)PBr4(HSO5) species as the active oxidants in acetonitrile.

KEYWORDS: porphyrin, sulfide, sulfone, alkene, catalyst.

SPP full member in good standing

*Correspondence to: Saeed Rayati, email: [email protected], fax: +98 21-22853650, tel: +98 21-23064221

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http://dx.doi.org/10.1142/S108842461100301X

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Copyright © 2011 World Scientific Publishing Company J. Porphyrins Phthalocyanines 2011; 15: 132–139

132 S. RayatI et al.

associated with the use of halogenated solvents [18] the optimized reaction conditions for oxidation of olefins and organosufurs in a green solvent have been investi-gated. In the present work, factors influencing the cata-lytic activity of β-tetrabrominated meso-tetra(para-tolyl)porphyrinatomanganese(III) as a partially β-brominated meso-tetraarylporphyrin for oxidation of hydrocarbons and organosulfur compounds with TBAHS have been studied.

RESULTS AND DISCUSSION

Oxidation of sulfides

Oxidation of methyl phenyl sulfide with TBAHS cata-lyzed by MnT(4-CH3P)PBr4(OAc) gave methyl phenyl sulfone as the major product. In a search for suitable reac-tion conditions to achieve the maximum conversion and product selectivity, effect of different parameters includ-ing solvent, type and amount of co-catalyst and amount of TBAHS were studied in details.

Solvent effect. In order to obtain the best reaction media, protic and aprotic solvents with different dielec-tric constants [19] have been used (Table 1). According to Table 1, maximum conversion was obtained in chlo-roform, ethanol and dichloromethane while the highest selectivity for sulfone formation was achieved in ethanol. Consequently, ethanol with highest ability to sulfone for-mation and as green solvent was selected for the oxida-tion of sulfides.

Co-catalyst effects of nitrogen donor. Presence of a nitrogen base, for example ImH, as a co-catalyst increased not only the conversion of methyl phenyl sulfide but also selectivity for the formation of sulfone product. The results of various nitrogen bases on the oxi-dation of methyl phenyl sulfide are illustrated in Table 2. The highest yield of sulfone relative to the sulfoxide was obtained in the case of ImH (Table 2, Entry 1) with a strong π and σ-donation ability to the metal center. We expect that 1-MeImH and 2-EtImH (Entry 2, 3) with Me

and Et electron donor substituents show better co-cata-lytic activity but the lower yield for sulfone formation is due to the existence of the steric strain. Diethyamine and triethylamine (Entry 4, 5) with pure σ-donor ability show lower efficiency than those of ImH. The much lower co-catalytic activity of diethylamine than triethylam-ine in spite of the lower steric hindrance of diethylam-ine is due to the intermolecular formation of hydrogen bonding in the former and caused the lower coordina-tion of the diethylamine to the metal center. In the case of pyridine and methyl-substituted pyridines (Entry 7–9), the observed order of co-catalytic activities of 4-MePy > 3-MePy > 2-MePy > Py seems to be related to both steric and electronic effect of methyl substituent. The higher co-catalytic activity of the methyl-substituted pyridines than pyridine is due to their better electron donation. The lower activity of the 2-methyl pyridine respect to the 3- and 4-methyl pyridines seems to be due to the existence of steric hindrance between methyl group and catalyst. Cyano pyridines (Entry 10, 11) with an electron–withdrawing CN substituent show lower co-catalytic activity. 2-amino pyri-dine (Entry 12) with a lone pair on the amino-substituent that makes the nitrogen atom better σ- and π-donor shows catalytic activity similar to the 2-methyl pyridine. This may be due to the steric effect of the substituent on the ortho position of the pyridine. 2,6-dichloropyridine with two substituents on the ortho position shows lower activity than 2-methyl pyridine or 2-amino pyridine.

The effect of ImH concentration. The effect of ImH concentration on the oxidation of methyl phenyl sulfide has also been studied (Table 3). Addition of ImH up to 5:1 relative to the catalyst led to an increase in the con-version of reaction with no effect on the ratio of sulfoxide to sulfone. Beyond this ratio, a significant decrease in the ratio of sulfoxide to sulfone has been observed.

The effect of TBAHS concentration. Since oxidation of axial base can take place in the presence of an oxidant, optimization of the amount of TBAHS as oxygen source is necessary. Different molar ratios of methyl phenyl sul-fide to the oxidant were used (Table 4) and 1:2 molar ratio has been found to be the optimized one.

Table 1. Oxidation of methyl phenyl sulfide with TBAHS catalyzed by 1 in the presence of ImH in different solvents at room temperaturea

Entry Solvent Conversionb (%) Yield (%) Yield (%) Sulfoxide/ (sulfoxide) (sulfone) Sulfone ratio

1 Acetone 86 26 60 0.43

2 Acetonitrile 96 17 79 0.21

3 Chloroform 100 35 65 0.53

4 Tetrahydrofuran 94 46 48 # 1

5 Ethanol 100 18 82 0.22

6 Dichloromethane 100 17 83 0.20

a The molar ratios for oxidant:sulfide:ImH:catalyst are 200:100:10:1. b For 2 min reaction time.

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CatalytIC aCtIvIty Of β-tetRabROmInateD meso-tetRa(Para-tOlyl)PORPhyRInatOmanganeSe(III) 133

Oxidation of different sulfides. To investigate the general scope of sulfide oxidation a series of sulfides such as diallyl, dipropyl, diphenyl and phenyl butyl were used for oxidation with TBAHS in the presence of catalytic amount of 1 under optimized conditions, catalyst:ImH:sulfide:TBAHS (1:10:100:200). Table 5

shows that oxidation of various sulfides gives the cor-responding sulfone as the major product. In the case of diallyl sulfide (Table 5, Entry 2), no oxidation at carbon-carbon double bond was observed. This observation may be due to the facile oxidation of sulfides with respect to double bonds. Comparison of the sulfone product in the methyl phenyl sulfide and diphenyl sulfide (Table 5, Entry 1, 5) reflect the steric effects of phenyl substituents in the diphenyl sulfide. On the other hand, the higher amount of sulfone in the dipropyl sulfide than diphenyl sulfide clearly demonstrates σ-electron donation effects of alkyl groups with respect to the phenyl substituents.

Stability of the catalyst against the degradation with oxidant. Electronic spectra of a dichloromethane solu-tion of the catalyst in the presence of sulfide (Fig. 1A) and in the absence of sulfide (Fig. 1B) were recorded. It is observed that in the presence of sulfide the catalyst is more stable than the time that in the absence of sulfide.

Proposed mechanism for oxidation of sulfides. Sta-bility of the catalyst in the presence of sulfide (up to 4 h)

Table 2. Oxidation of methyl phenyl sulfide with TBAHS catalyzed by 1 in the presence of different nitrogen donors in ethanol at room temperaturea

Entry Axial ligand Conversion (%) Yield (%) Yield (%) Time (min) (sulfoxide) (sulfone)

1 ImH 100 17 83 2

2 2-EtImH 100 24 76 2

3 1-MeImH 100 21 79 2

4 NHEt2 100 22 78 2

5 NEt3 100 19 81 2

6 Py 100 29 71 2

7 2-MePy 100 25 75 2

8 3-MePy 100 23 77 2

9 4-MePy 100 20 80 2

10 3-CNPy 100 39 61 2

11 4-CNPy 100 40 60 2

12 2-NH2Py 100 25 75 2

13 2,6-Cl2Py 100 31 69 2

14 none 86 35 51 2

a The molar ratios for oxidant:sulfide:ImH:catalyst are 200:100:10:1.

Table 3. Effect of various ImH/catalyst molar ratios on oxidation of methyl phenyl sulfide with TBAHS in ethanol at room temperaturea,b

Entry

Conversion (%) Yield (%) Yield (%) Sulfoxide/

[ ]

[ .]

lm H

Cat (sulfoxide) (sulfone) Sulfone

1 0 86 35 51 0.69

2 5 100 41 59 0.69

3 10 100 17 83 0.20

4 15 100 18 82 0.22

5 20 100 25 75 0.33

a The molar ratios for oxidant:sulfide:ImH:catalyst are 200:100:X:1. b For 2 min reaction time.

Table 4. Effect of various TBAHS/cat. molar ratios on oxida-tion of methyl phenyl sulfide in the presence of ImH in etha-nole at room temperaturea

TBAHS/ Conversion Yield (%) Yield (%) Cat. (%) (sulfoxide) (sulfone)

100 32 3 29

150 76 24 52

200 100 17 83

250 100 14 86

a The molar ratios for oxidant:sulfide:ImH:catalyst are X:100:10:1.

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supposed the formation of a six coordinate intermedi-ate instead of Mn-oxo species. We have not ruled out Mn-oxo formation because the absence of a Mn-oxo band in the electronic spectra may be due to the facile oxidation of sulfides in the presence of this species. Therefore, it seems that TBAHS coordinated to the Mn-center via less steric hindrance peroxo oxygen (Scheme 1) and activation of this oxygen will occurr on the surface of the catalyst.

Oxidation of olefins

Solvent effect in the epoxidation of cyclooctene with TBAHS. In order to obtain the best reaction solvent, the influence of various solvents in the catalytic epoxida-tion of cyclooctene with TBAHS in the catalytic amount

of MnT(4-CH3P)PBr4(OAc) were examined. It was observed that catalytic activity decreases in the order acetonitrile (94% conversion) ≈ dichloromethane (92%) > ethanol (81%) > acetone (76%) > chloroform (70%) >> tetrahydrofuran (15%) and acetonitrile was selected as the best reaction solvent for the oxidation reactions. The observed order of catalytic activity in different sol-vents cannot be simply rationalized on the basis of the polarity of solvents. It seems that various factors such as hydrogen bond ability of solvent molecules, steric effects and their coordination ability to the metal center are involved in the observed solvent effect [6, 20].

Oxidation of different alkenes. Oxidation of various olefins with TBAHS was carried out in the presence of Mn(T(4-CH3P)PBr4)OAc (Table 6). Reactivity of

Table 5. Oxidation of sulfides with TBAHS catalyzed by 1 in the presence of ImH in ethanol at room temperaturea

Entry Substrate Conversion (%) Yield (%) (sulfoxide) Yield (%) (sulfone) Time (min)

1 S 100 17 83 2

2 S 100 - 100 2

3 S 100 21 79 2

4 S 99 5 94 2

5 S 95 30 65 2

a The molar ratios for oxidant:sulfide:ImH:catalyst are 200:100:10:1.

Fig. 1. UV-vis spectra of (A) a solution of Mn(T(4-CH3P)PBr4)OAc in CH2Cl2 in the presence of ImH (a), ImH/TBAHS and methyl phenyl sulfide after 2 min (b), ImH/TBAHS and the sulfide after 60 min (c) and ImH/TBAHS and the sulfide after 240 min (d). Figure B shows the same spectra in the absence of methyl phenyl sulfide

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Mn

OAc

TBAO, Im

CH2Cl2, rtMn

ImH

O

OSO3H

Mn

ImH

O

S

Mn

ImH

O

OSO3HS

S O

S O+ TBAO +

S O

O

Mn

ImH

OS

Overoxidation of sulfoxide

Scheme 1. Proposed mechanism for oxidation of sulfides

Table 6. Epoxidation of alkenes with TBAHS catalyzed by MnT(4-CH3P)PBr4(OAc) in the presence of ImH in acetonitrilea

Entry Alkene Conversion (%) Epoxide yield Selectivity (%)

1 95 95 100

2 59 59 100

3 CH3 91 91 100

4 52 52 100

5 100 100 100

6 CH3 99.6 38b 38

7

MeO

100 100 100

8

Cl

76 76 100

9 44 44 100

10 93 93 100

11 100 88.5c 88.5

12 64 64c 100

a The molar ratio for oxidant:alkene:ImH:catalyst is 200:100:10:1. b Acetophenone is the by-product. c Determined by 1H NMR spectroscopy.

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cyclohexene is less than cyclooctene (Table 6, Entry 1 and 2), because the oxidation product of cyclooctene (cyclooctene oxide) is more stable than cyclohexene oxide. The presence of electron donating groups usually increases the reactivity of olefins [21]. Therefore, 1-methyl cyclohexene and 4-methoxy styrene were oxidized better than cyclohexene and styrene, respectively (Table 6, Entry 2, 3 and 5, 7). On the other hand, electron-deficient sub-stituents at the double bond decrease conversion and con-sequently epoxide yield (Table 6, Entry 8). Styrene and their para-substituents derivatives give selectively epoxide yield as the sole product but α-methylstyrene gives aceto-phenone as the by product in addition to epoxide (Table 6, Entry 6). So, due to steric strain, as well as electron-donat-ing effects of methyl substituent over oxidation of epoxide to acetophenone will occur. Epoxidation of 1-octene and norbornene proceeds in moderate yield (52% and 44%) and due to the observed steric in the norbornene cycle, it shows the least reactivity than the other alkene (Table 6, Entry 4, 9). Since double bond acts as nucleophile in these oxidation reactions, it seems that conjugated double bonds were reactive than the others. The more reactivity of non-terminal alkenes with respect to terminal alkenes, in spite of more stability of non-terminal alkenes demonstrates the higher nucleophilic nature of these alkenes rather than ter-minal double bonds.

Due to the sterically demanding properties of trans-stilbene, its reactivity is less than cis isomer, and its oxidation product is trans-stilbene oxide while less hin-dered isomer (cis-stilbene) results in a mixture of cis and trans isomer. Apparently, before formation of epoxide in cis isomer (with less steric strain), a free rotation about the alkene double bond will occur.

Co-catalytic effects of nitrogen donors. Oxidation of cyclooctene with TBAHS was carried out in the presence of different nitrogenous bases as co-catalyst (Table 7). The results demonstrate the importance of π-rather than σ-interactions. Pyridines are as a class of nitro-gen donors with weak π-donating ability and also show weaker σ-donating ability with respect to linear amines. Pyridines with a methyl group on the 2, 3 or 4 position (Table 7, Entry 2–4) display the higher co-catalytic activ-ity than pyridine (Table 7, Entry 1) and also pyridines with an electron-withdrawing CN substituent show lower catalytic activity than pyridine (Table 7, Entry 5, 6). The improved conversion for 2 and 4-MePy than 3-MePy may result in the presence of electron-donating methyl group at ortho or para position with respect to the nitrogen and increasing electron density on the nitrogen atom.

Although 2-MePy with one methyl group near to the nitrogen donor atom, hindering its coordination to the metal center, but shows similar catalytic activity than 4-MePy that may be related to the hyperconjuga-tion effects of the methyl group. It is observed that the difference between the co-catalytic activity of ImH and pyridines significantly increases in acetonitrile compared with the dichloromethane as the solvent [22].

2-NH2Py (Table 7, Entry 7) with a lone pair on the amino group (as a π electron donation substituent) dis-plays better catalytic activity than Py. On the other hand 2,6-Cl2Py with two electron-withdrawing Cl substituents (Table 7, Entry 8) essentially displays lower catalytic activity than Py. The highest co-catalytic activity observed in the case of ImH (Table 7, Entry 9) with the strongest π-donor ability and also smallest size. A decrease in the conversion in the 1-MeImH and 2-EtImH (Table 7, Entry 10,11) displays the steric hindrance effects of methyl and ethyl substituents. Pure σ-donor amines (diethylamine and triethylamine) display relatively poor co-catalytic activities (Table 7, Entry 12,13). Et2NH which is less sterically hindered and a stronger base than Et3N, shows lower co-catalytic activity. It seems that the formation of intramolecular hydrogen bonding in the Et2NH and consequently the lower coordination ability towards metal center is the dominant factor in determining the co-catalytic activitiy of Et2NH.

The effect of co-catalyst/catalyst ratio. The co- catalytic activitiy of ImH in the presence of MnT(4-CH3P)PBr4(OAc) at various ImH/catalyst ratios are presented in Fig. 2 and 10:1 ratio was selected for the oxidation of cyclooctene. A decrease in cyclooctene conversion observed in ImH/catalyst ratio ≥50 may be due to the for-mation of inactive six coordinate [ImH2: MnT(4-CH3P)PBr4(OAc)] species.

Oxidant/catalyst molar ratio effects. The amount of TBAHS in the oxidation of cyclooctene in the presence of MnT(4-CH3P)PBr4(OAc) in acetonitrile has been tested. Since a higher amount of oxidant leading to a degrada-tion of catalyst and also oxidation of nitrogenous bases, therefore the amount of oxidant should be optimized.

Table 7. Oxidation of cyclooctene with TBAHS catalyzed by 1 in the presence of different nitrogen donors in acetonitrile at room temperaturea

Entry Axial ligand Conversion (%)

1 Py 49

2 2-MePy 76

3 3-MePy 51

4 4-MePy 77

5 3-CNPy 32

6 4-CNPy 36

7 2-NH2Py 82

8 2,6-Cl2Py 27

9 ImH 95

10 1-MeImH 85

11 2-EtImH 68

12 NHEt2 25

13 NEt3 56

a The molar ratios for oxidant:cyclooctene:ImH:catalyst are 200:100:10:1.

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Different TBAHS:catalyst malar ratios (150:1, 200:1 and 250:1) were used and the 200:1 (TBAHS:catalyst) ratios has been found to be the optimized one for the oxida-tion of the cyclooctene in the presence of MnT(4-CH3P)PBr4(OAc).

Counter ion effects. The catalytic efficiency of MnT-(4-CH3P)PBr4X with different counter ions (Table 8) for oxidation of cyclooctene has been investigated and the following order has been observed: MnT (4-CH3P)PBr4OAc > MnT(4-CH3P)PBr4SCN ~ MnT(4-CH3P)PBr4N3 ~ MnT(4-CH3P)PBr4IO3 ~ MnT(4-CH3P)PBr4Br > MnT(4-CH3P)PBr4Cl > MnT(4-CH3P)PBr4F. Acetonitrile is an aprotic solvent which does not par-ticipate in hydrogen bonding with the counter ion of MnT(4-CH3P)PBr4X. It is observed that the efficiency of catalyst is greatly lower in the case of Cl or F as the counter ion.

In order to obtain active oxidant, counter ion should be released from the metal center and oxidant should be attached to the metal center. Therefore, soft anions such as acetate or thiocyanate can remove from the catalyst (Mn(III) act as a hard acid) more facile than hard anions. On the

other hand, acetonitrile as a solvent with low dielectric constant cannot solvate anions and consequently anions with extended charge are better leaving group.

Proposed mechanism. Competitive epoxidation reactions of cis- and trans-stilbene with TBAHS in the presence of MnT(4-CH3P)PBr4OAc using similar molar ratios of the reagents may be used as an indirect method to elucidate the nature of active oxidant [21].

The observed ratio [Table 9] of cis- to trans-stilbene oxide (5.12) suggests the presence of a high valent man-ganese oxo species (Scheme 2, I) in equilibrium with a six coordinate one, i.e. (ImH)MnT(4-CH3P)PBr4(HSO5) (Scheme 2, II) [20].

Fig. 2. Co-catalytic activities of ImH in the presence of MnT(4-CH3P)PBr4(OAc) as a function of different co-catalyst/catalyst molar ratio

Table 8. The effect of different anions in oxidation of cyclooctene in acetonitrile

MnT(4-CH3P)PBr4X Conversion (%) Selectivity (%) Time (min)

MnT(4-CH3P)PBr4F 64 100 2

MnT(4-CH3P)PBr4Cl 76 100 2

MnT(4-CH3P)PBr4Br 83 100 2

MnT(4-CH3P)PBr4IO3 86 100 2

MnT(4-CH3P)PBr4N3 87 100 2

MnT(4-CH3P)PBr4SCN 89 100 2

MnT(4-CH3P)PBr4OAc 5 100 2

Table 9. Oxidation of cis- and trans-stilbene with MnT(4-CH3P)PBr4(OAc)

cis trans cis to trans ratio

14.0 2.7 5.2

The molar ratios for MnT(4-CH3P)PBr4OAc:ImH:(cis- and trans-stilbene):TBAHS are 1:10:(500 and 500):200.

Mn

OAc

TBAO, Im

CH2Cl2, rtMn

ImH

O

OSO3H

Mn

ImH

O

Mn

ImH

O

OSO3H

Mn

ImH

O

O

III

Scheme 2. Proposed catalytic cycle for oxidation of alkenes with TBAHS in acetonitrile

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EXPERIMENTAL

Instruments and reagents1H NMR spectra were obtained in CDCl3 solutions

with a Bruker FT-NMR 250 (250 MHz) spectrometer. The residual CHCl3 in conventional 99.8 atom % CDCl3 gives a signal at d = 7.26 ppm, which was used for calibration of the chemical shift scale. The electronic absorption spectra were recorded on a double beam spectrophoto meter (Shimadzu, UV-240) in CH2Cl2. The reaction products were analyzed by gas chromatography using a HP Agilent 6890 gas chromatograph equipped with a HP-5 capillary column (phenyl methyl silox-ane 30 m × 320 mm × 0.25 mm) and a flameionization detector.

The free base meso-tetrakis(4-methylphenyl)porphy-rin was prepared and purified as reported previously [23] Chemicals were purchased from Merck or Fluka chemi-cal companies. n-Bu4NHSO5 was prepared according to the literature [7].

Synthesis of H2T(4-CH3P)PBr4 and MnT(4-CH3P)PBr4(OAc)

H2T(4-CH3P)PBr4 (Fig. 1) was prepared in accord with the literature [24]. H2T(4-CH3P)P (247 mg, 0.36 mmol) was dissolved in CHCl3 (100 mL). To this solution, freshly recrystallised NBS (262 mg, 1.47 mmol) was added (recrystallised from hot water and dried at 80 °C under vacuum). The reaction mixture was stirred for 24 h and then CHCl3 was evaporated to dryness. The residue was washed with a mixture of methanol-water (1:1, v/v) (2 × 20 mL) to remove any soluble succinimide impuri-ties. UV-vis (CH2Cl2): λmax (nm): 434, 531, 680. 1H NMR (CDCl3, 250 MHz): δ, ppm 8.68 (s, 4H, β-pyrrole), 8.05–8.08 (d, 8, o-phenyl), 7.56–7.60 (d, 8, m-phenyl), -2.78 (s, 2, NH). Mn(III) complex of H2T(4-CH3P)PBr4 was prepared using the methods of Adler et al. [25]. The Soret band of MnT(4-CH3P)PBr4(OAc) appears at 484 nm in dichloromethane.

General oxidation procedure

Stock solution of the catalyst (0.003 M) and nitrog-enous bases (0.5 M) were prepared in CH2Cl2. In a 10 mL round-bottom flask, the reagents were added in the fol-lowing order: substrate (0.3 mmol), catalyst (0.003 mmol, 1.0 mL), nitrogenous bases (0.03 mmol, 60 μL). Tetrabutylammonium oxone (0.6 mmol, 0.247 g) (0.75 mmol, 0.308 g, in the case of 2) was then added to the reaction solution at 25 °C. The reaction solutions were analyzed immediately by GC after stirring for 2 min. With the exception of cis and trans-stilbene, all data have been obtained using GC. However, all experiments were repeated three times and the data show the average values with an error of 1–5%.

CONCLUSION

In summary, the optimized conditions for oxidation of olefins and sulfides with TBAHS in the presence of a par-tially β-brominated meso-tetraarylporphyrin and nitrogen donors in nonhalogenated solvents have been reported. Using THF as solvent direct the chemoselectivity of reac-tion towards the formation of sulfoxide and sulfone in ca. 1:1 molar ratio. Also, using pyridines with electron-with-drawing groups such as -CN increases the ratio of sulfoxide to sulfone compared with pyridines bearing electron-do-nating groups and imidazoles. Indirect evidence obtained from competitive oxidation of cis- and trans-stilbene sug-gests the involvement of a high valent Mn-oxo and a six-coordinate Mn(III)-porphyrin species in the catalytic cycle of oxidation of olefins in acetonitrile as solvent.

Acknowledgements

This work has been supported by the Iran National Science Foundation (INSF) (Grant no. 87040848).

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factors influencing the catalytic activity of β-tetrabrominated meso -tetra( para...

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