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Статья Paper
Oxidation of Hydrocarbons with Tetrabutylammonium Oxone Catalyzed by p-Tribrominated meso-Tetraphenylporphyrinato-manganese(III) Acetate in the Presence of N-Bases
Akbar Ghaemi,a Saeed Rayati,b@ and Saeed Zakavic
aIslamic Azad University, Saveh Branch, Saveh, 39187-366 Iran bK.N. Toosi University of Technology, Tehran, 16315-1618 Iran
cInstitute for Advanced Studies in Basic Sciences, Gava-Zang Zanjan, 45195-1159 Iran @Corresponding author E-mail: rayati@kntu.ac.ir
P-Tribrominated meso-tetraphenylporphyrinato-manganese(III) acetate (Mn(TPPBr3)OAc) as an electron-deficient metalloporphyrin shows high catalytic activity in the oxidation of less hindered conjugated double bonds and cycloalkenes by tetrabutylammonium monopersulfate.
Keywords: Tetraphenylporphyrin, catalyst, epoxidation, oxidation, alkenes, saturated hydrocarbons.
Introduction
Different manganese porphyrins and related systems such as Schiff bases have been used extensively in chemical modeling of biological monooxygenation reactions. The biomimetic oxidation of hydrocarbons and nitrogen- and sulfur-containing compounds with different oxygen donors such as PhIO, NaOCl, H2O2, periodate, amine N-oxides and nBu4NHSO5 (TBAO) have been extensively studied.[1-20] Iron and manganese porphyrins bearing halogen substituents on the periphery of the porphyrin ring have been shown to be particularly efficient for epoxidation and hydroxylation re-actions.[21-23] Such porphyrins are also more resistant to degradation via free-radical attack or direct oxidation of porphyrin ring than those containing electron-donating substituents.[1,24-25] The employment of nitrogenous donors in metalloporphyrin systems for mimicking the oxygenation function of P-450 has led to marked improvement in selectivities and turnover rates in the epoxidation of alkenes.[18,26-27]
Despite the favorable employment of Oxone® (2KHSO5 KHSO4 K2SO4) in the Mn porphyrin catalyzed oxygenation of hydrocarbons,[28-29] attempts at using TBAO as an oxygen source in association with porphyrin catalysts has been unsuccessful until 2002.[18] Oxidation of hydrocarbons with TBAO in the presence of different electron-rich and electron-deficient Mn111 meso-tetra(halogensubstituted aryl)porphyrins have been reported. The present work is the first report on using electron deficient Mn porphyrins bearing halogen atoms at ^-positions (P-tri-brominated meso-tet-raphenylporphyrinato-manganese(III) acetate, Mn(TPPBr3) OAc, as catalyst for oxidation of hydrocarbons with TBAO in the presence of different nitrogen donors as co-catalyst.
NMR 250 (250 MHz) spectrophotometer. The residual CHCl3 in conventional 99.8 atom% CDCl3 gives a signal at 5 = 7.26 ppm, which was used for calibration of the chemical shift scale. A double beam spectrophotometer (Shimadzu, UV-240) was used for the UV-vis absorption determination. The reaction products of oxidation were determined and analyzed by HP Agilent 6890 gas chromatograph equipped with a HP-5 capillary column (phenyl methyl siloxane 30 m x 320 p,m x 0.25 p,m) and flame-ionization detector.
Preparation of H2TPPBr3. P-Tri-brominated meso-tetraphenylporphyrin (H2TPPBr3) was prepared from H2TPP and freshly recrystalized N-bromosuccinimide (NBS) according to the method reported by Bhyrappa et al. with some modification. [30] H2TPP (300 mg, 0.49 mmol) was dissolved in CHCl3 (80 ml). To this solution, freshly recrystallized NBS (270 mg, 1.47 mmol) was added (recrystallized from hot water and dried at 80°C under vacuum). The reaction mixture was stirred for 24h and then CHCl3 was evaporated to dryness. The residue was washed with methanol (2x20 ml) to remove any soluble succinimide impurities. UV-vis (CH2Cl2) Xmax nm: 427, 523, 599, 658. *H NMR (CDCl3, 250 MHz) 5H ppm: 8.70-8.86 (5H, m, p pyrrole), 8.07-8.21 (8H, m, o-phenyl), 7.73-7.77 (12H, m, m- and p-phenyl).
Preparation of Mn(TPPBr3)OAc. Mn(TPPBr3)OAc was also prepared and purified according to the previously reported method.[31]
Preparation of "BuNHSO.The synthesis of nBu4NHSO5 was based on the procedures given by Compestrini et al.[18,28] Freshly prepared nBu4NHSO5 was a much stronger oxidant than commercially available samples. Since the oxidizing ability of nBu4NHSO5 samples reduces with time, in order to obtain reproducible results, the freshly prepared oxidant was refrigerated and used within three days.
General Oxidation Procedure. The general procedure for oxidation consisted of adding nBu4NHSO5 (0.57 mmol) to a CH2Cl2 solution containing the alkene (0.3 mmol), the catalyst (0.003 mmol) and nitrogenous bases as co-catalysts (0.06 mmol. The reaction solutions were stirred for 2 min at room temperature and analyzed immediately by GLC.
Experimental
Results and Discussion
All of the materials and solvents were purchased from Merck. XH NMR spectra were obtained in CDCL solutions on a Bruker FT-
Mn(TPPBr3)OAc catalyzed oxidation of cyclooctene with nBu4NHSO5 in the presence of imidazole leads to
cyclooctene oxide as the sole product (Table 1). Styrene, a-methylstyrene and para-substituted styrenes are 100% converted within 2 min with 100% selectivity for epoxide products except for a-methylstyrene which gives 25% acetophenone as the by-product. Electron-donating (entries 6, 8 in Table 1) as well as electron-deficient substituents (entry 5) on the para position of the aryl ring of styrene were compatible with the reaction conditions, and gave the corresponding epoxide. With the exception of cyclooctene, terminal double bonds and unconjugated ones (Table 1, entries 2, 3 and 10) are less reactive than the conjugated double bonds and show lower selectivities.
Decreased reactivity of a-methylstyrene relative to styrene (entries 4 and 7) seems to be due to the steric hindrance of methyl group. Also, the comparison of cis- and trans-stilbene (entries 11 and 12) shows the importance of steric effects. The enhanced stability of Mn(TPPBr3)OAc relative to Mn(TPP)OAc towards the oxidant and higher yields of epoxide make this electron-deficient Mn-porphyrin more efficient than Mn(TPP)OAc in the epoxidation of olefins.
Coordination of nitrogen donors to the metal center of metalloporphyrins usually increases the catalytic activity of the metalloporphyrin.[1,9-20] In this work, co-
Table 1. Epoxidation of alkenes with nBu4NHSO5 catalyzed by Mn(TPPBr3)OAc in the presence of ImH.a
Entry
Alkene
Conversion %
Epoxide
Yield% Selectivity % Time (min)
10
11
12
CI-
Ph Ph
Ph
100
100
93
100
100
100
100
100
100
40
100c
70c
O
Ph
100
91
71
97 100 100 75
100
100 30 89
11
70
100
91
76
97
100
100
75b
100
100
75
100
100
The molar ratio for oxidant: alkene:ImH:catalyst is 190:100:20:1. bAcetophenone is the by product.
cThe organic product(s) and unreacted alkenes were determined by 1H NMR spectroscopy.
1
2
2
2
3
2
4
2
5
2
6
2
7
2
8
2
9
2
2
2
2
Mn111 Tetraphenylporphyrinate in Catalytic Oxidation of Hydrocarbons
catalytic activities of a selected series of nitrogen donors, in the presence of Mn(TPPBr3)OAc as catalysts, in the epoxidation of cyclohexene with nBu4NHSO5 were studied (Table 2).
Table 2. Epoxidation of cyclohexene with nBu4NHSO5 catalyzed by Mn(TPPBr3)OAc in the presence of different nitrogen donors.a
Axial Conversion Epoxide Selectivity
Ligand (%)b yield (%)b (%)
ImH 100 92 92
2-MeImH 89 83 93
BzImH 88 79 89
Py 88 83 94
2,6-Cl2Py 30 27 90
4-CNPy 16 16 100
3-CNPy 22 22 100
Piperidine 63 54 85
Et3N 41 32 78
Et2NH 62 50 80
None 9 7 77
aAll reactions were run at room temperature for 2 min. The molar ratio for oxidant: alkene:ImH:catalyst is 190:100:20:1.
It is observed that imidazole with ring size smaller than the other nitrogen donors is the best one. The introduction of n-electron-withdrawing substituents at different positions of pyridine dramatically decreases the activity of pyridine as a nitrogen donor. It seems that the activity of nitrogen donors is strongly depends on the n-donor ability of axial base. In accordance with the explanation, co-catalytic activity of
benzimidazole is almost equal to that of imidazole, although the size of the former is much larger than that of the latter.
It is also observed that simple amines which may be considered as pure c-donors are less effective than those with both c- and n-donor abilities.
Table 3 shows the effect of Im to catalyst molar ratio in the cyclooctene epoxidation. It is observed that an increase in the ImH to catalyst molar ratio up to 20 remarkably improves the epoxidation rate.
Table 3. Effect of various ImH/Mn(TPPBr3)OAc
molar ratios on Cyclooctene epoxidation rate and selectivity
by nBu4NHSO5a.
ImH/Mn(TPPBr3) OAc Ratio(X) Conversion (%)b Epoxide yield (%)b Selectivity (%)
0 30 30 100
5 72 72 100
10 94 94 100
20 100 100 100
40 100 100 100
60 98 98 100
80 96 96 100
100 92 92 100
aAll reactions were run at room temperature for 2 min. The molar ratio for oxidant: alkene:ImH:catalyst is 190:100:X:1.
Oxidation of secondary C-H bonds of saturated hydrocarbons in the same conditions gives alcohols and/or ketones as the products (Table 4).
The influence of the solvent nature in this catalytic system was examined and dichloromethane has been shown to be the best one.
Table 4. Hydroxylation of saturated hydrocarbons with nBu4NHSO5 catalyzed by Mn(TPPBr3)OAc in the presence of ImHa.
Entry
Hydrocarbons
Conversion (%)b
Alcohol
Yield (%)b
Ketone
Yield (%)b
12
18
64
72
58 c
OH
OH
trace
29
12
49
A
18
35
60
trace
aAll reactions were run at room temperature for 2 min. The molar ratio for oxidant: alkane:ImH:catalyst is 190:100:20:1. bGLC yields are based on the starting alkanes. c Yield of 2-adamantanol is 9%.
1
6
6
2
3
4
5
Active Oxidant
Usually, the competitive epoxidation of cis- and trans-stilbenes with different oxygen donors in the presence of metalloporphyrins bearing non-bulky ortho-substituents on the phenyl groups has been carried out to investigate the nature of active oxidant.[932] In this reaction, a low ratio of cis-to trans-stilbene oxide has been proposed to be a supporting evidence for the involvement of a high valent metal-oxo porphyrin species. In Mn(TPPBr3)OAc/Im/TBAO catalytic system, the competitive oxidation of cis- and trans-stilbenes yielded cis- and trans-stilbene oxide in nearly the same ratio which suggests a high valent Mn-oxo porphyrin as the main active oxidant (Scheme 1).
■Mn-
OAc
Scheme 1.
Conclusions
In summary, Mn(TPPBr3)OAc is an efficient catalyst for epoxidation of alkenes in the presence of imidazole. Under the same reaction condition, saturated hydrocarbons were oxidized to alcohol or ketones in only low to moderate yields.
Acknowledgements. We gratefully acknowledge practical support of this study by K.N. Toosi University of Technology, Zanjan Institute for Advanced Studies in Basic Sciences and Islamic Azad University of Saveh Branch.
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Received 20.04.2011 Accepted 05.05.2011
