Oxidation of hydrocarbons with tetra-n-butylammonium peroxy monosulfate catalyzed by β-tetrabromo-meso-tetrakis(4-methoxyphenyl)-and β-tetrabromo-meso-tetraphenylporphyrinatomanganese(III)

Article abstract of DOI:10.3906/kim-1310-35

β-Tetrabromo-meso-tetrakis(4-methoxyphenyl)porphyrin, H₂ T(4-OCH₃ P)PBr₄, was synthesized and characterized by UV-Vis and ¹H NMR spectroscopy. Oxidation of alkanes and olefins with tetra-n-butylammonium peroxymonosulfate (n-Bu₄NHSO₅) was studied in the presence of MnT(4-OCH₃ P)PBr₄ (OAc) and MnTPPPBr₄ (OAc) (TPP = meso-tetraphenylporphyrin). While significance differences were observed between the catalytic activities of the title complexes in the oxidation of alkanes, the 2 manganese porphyrins showed comparable activities in oxidation of most of the olefins used. However, the latter showed greater catalytic performance in the oxidation of the hydrocarbons. Moreover, the oxidative degradation of the former (60%) was greater than that of the latter (45%) in the oxidation of cyclooctene. TUeBITAK.

Full text of DOI:10.3906/kim-1310-35

Turk J Chem
Turkish Journal of Chemistry
(2014) 38: 611 – 616
¨
http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1310-35
Research Article
Oxidation of hydrocarbons with tetra-n-butylammonium peroxy monosulfate
catalyzed by β-tetrabromo-meso-tetrakis(4-methoxyphenyl)- and
β-tetrabromo-meso-tetraphenylporphyrinatomanganese(III)
Saeed RAYATI1,∗, Saeed ZAKAVI2,∗, Hossein VALINEJAD1
1Department of Chemistry, K.N. Toosi University of Technology, Tehran, Iran
2Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, Iran
Received: 12.10.2013
•
Accepted: 13.01.2014
•
Published Online: 11.06.2014
•
Printed: 10.07.2014
Abstract: β−Tetrabromo-meso-tetrakis(4-methoxyphenyl)porphyrin, H2 T(4-OCH3 P)PBr4 , was synthesized and char-
acterized by UV-Vis and 1 H NMR spectroscopy. Oxidation of alkanes and olefins with tetra-n-butylammonium per-
oxymonosulfate (n-Bu4 NHSO5) was studied in the presence of MnT(4-OCH3 P)PBr4 (OAc) and MnTPPPBr4 (OAc)
(TPP = meso-tetraphenylporphyrin). While significance differences were observed between the catalytic activities of
the title complexes in the oxidation of alkanes, the 2 manganese porphyrins showed comparable activities in oxidation of
most of the olefins used. However, the latter showed greater catalytic performance in the oxidation of the hydrocarbons.
Moreover, the oxidative degradation of the former (60%) was greater than that of the latter (45%) in the oxidation of
cyclooctene.
Key words: β -Tetrabromo-meso-tetrakis(4-methoxyphenyl)porphyrin, β -tetrabromo-meso-tetraphenylporphyrin, oxi-
dation, catalyst, alkenes, alkanes, oxone, manganese porphyrins
1. Introduction
The catalytic activity of the metal complexes of porphyrins with different substituents at the meso and β
positions has been extensively studied as model complexes of cytochrome p-450 enzymes.1−3 In spite of the
general agreement on the better catalytic performance of the metalloporphyrins with electronegative substituents
on the periphery of the porphyrin macrocycle, the lack of close correlation between the catalytic activity of the
manganese and iron porphyrins and the known electronic properties of the meso and β substituents has been
evident from recent studies on the oxidation of organic compounds with weak, moderate, and strong terminal
oxidants, catalyzed by the metalloporphyrins.4−16 Herein the oxidation of olefins, alkanes, and sulfides with
tetra-n-butylammonium peroxymonosulfate (n-Bu4 NHSO5) in the presence of β -tetrabromo-meso-tetrakis(4-
methoxylphenyl)porphyrinatomanganese(III) acetate is studied and compared with that of the corresponding
complex of β -tetrabromo-meso-tetrakis(4-methylphenyl)porphyrin.
2. Results and discussion
Catalytic oxidation of alkanes and olefins with tetra-n-butylammonium peroxymonosulfate (n-Bu4 NHSO5) was
studied in the presence of MnT(4-OCH3 P)PBr4 (OAc) and MnTPPPBr4 (OAc) (TPP = meso-tetraphenylpor-
∗Correspondence: rayati@kntu.ac.ir, zakavi@iasbs.ac.ir
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phyrin). Different solvents were examined for the oxidation reaction and ethanol was found to be the best
one. The results of oxidation of alkanes with n-Bu4 NHSO5 in ethanol catalyzed by MnTPPBr4 (OAc) and
MnT(4-OCH3 P)PBr4 (OAc) are summarized in Table 1.
Table 1. Oxidation of different alkanes with n-Bu4 NHSO5 catalyzed by MnTPPBr4 (OAc) and MnT(4-OCH3 P)PBr4 (OAc)
(the data in parentheses) in ethanol.a,b
alkane
Conversion(%)
8.0 (-)c
alcohol
ketone
(-)
OH
8 (-)
OH
O
37.0 (3.4)
0.7
28.0 (2.7)
OH
83.0 (39.2)
46.0 (23.0)
O
37 (16.2)
2.0 (-)b
O
OH
0.5 (-)
1.5 (-)
OH
O
73.0 (19.4)
63.0 (16.7)
30.0 (7.4)
43(12.0)
O
OH
13.0 (2.7)
50 (14.0)
a The molar ratios of catalyst: ImH:olefin:n-Bu4 NHSO5 are 1:10:100:200. b All reactions were repeated 3 times and
c
averaged. No reaction.
According to the data in Table 1, the oxidation of alkanes gives the corresponding alcohol and ketones
as the major and minor products, respectively. MnTPPBr4 (OAc) is significantly more efficient than MnT(4-
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RAYATI et al./Turk J Chem
OCH3 P)PBr4 (OAc). The result is in accord with the general agreement on the better catalytic activity of
electron-deficient manganese porphyrins relative to electron-rich ones.17
Oxidation of olefins (Table 2), on the other hand, gives the corresponding epoxide as the major
product. In spite of the better catalytic activity of MnTPPBr4 (OAc) in comparison with that of MnT(4-
OCH3 P)PBr4 (OAc), with the exception of 1-octene, there is no significant preference for the former over the
latter. The extent of degradation of metalloporphyrins in the presence of cyclooctene was measured on the basis
of the absorbance changes (∆A) at the λmax of the complexes (∆A/A). The results show a degradation of
45% and 60% for MnTPPBr4 (OAc) and MnT(4-OCH3 P)PBr4 (OAc), respectively. Moreover, the catalytic ac-
tivity of MnT(4-OCH3 P)PBr4 (x) with different counter anions in the oxidation of cyclooctene was studied and
the complex with acetate showed the highest catalytic activity (Table 3). However, little difference was found
between the catalytic activity of MnT(4-OCH3 P)PBr4 (x) with OAc counter anion and that of the manganese
porphyrin with Br, IO3 , N3 , and SCN one. The presence of counter anions with better leaving ability including
OAc, Br, IO3 , N3 , and SCN led to higher catalytic activity of the manganese porphyrin. This behavior may
be due to the necessity of cleavage of the Mn-counter anion bond in the catalytic cycle.7,18
Interestingly, while little difference has been found between the catalytic efficiency of the manganese por-
phyrins with phenyl- and 4-methoxylphenyl at the meso positions of nonbrominated meso-tetraphenylporphyrins,16
large differences have been observed between the catalytic efficiency of the tetra-brominated counterparts.
3. Experimental
Synthesis and purification of H2 T(4-OCH3)PP and H2 TPP (Figure) were carried out according to the
literature.17 Freshly distilled pyrrole (56 mL, 0.8 mol) and benzaldehyde (or 4-methoxy benzaldehyde) were
added to 3 L of refluxing reagent grade propionic acid. After refluxing for 30 min, the solution was cooled
to room temperature and filtered, and the filter cake was washed thoroughly with methanol. After a hot wa-
ter wash, the resulting purple crystals were air dried and finally dried in a vacuum to remove adsorbed acid.
H2 TPPBr4 and H2 T(4-OCH3)PBr4 (Figure) were prepared according to the literature.19,20 Free base por-
phyrin (0.49 mmol) was dissolved in CHCl3 (80 mL). To this solution was added freshly recrystallized NBS
(360 mg, 1.96 mmol) (recrystallized 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 methanol (2
× 20 mL) to remove any soluble succinimide impurities. The products, H2 TPPBr4 and H2 T(4-OCH3)PBr4 ,
were recrystallized from CHCl3 /n-hexane (3:1, v/v) and dried under vacuum.
UV-Vis (CH2 Cl2) (H2 T(4-OCH3)PBr4): λmax (nm): 445, 544, 708 nm. 1 H NMR (250 MHz CDCl3 ,
TMS) (δ ppm): –2.72 (2H, s, NH); 4.09 (12H, s, –OCH3); 7.31–7.34 (8H, d, 3 J (HH)= 7.5 Hz, Hm); 8.09–8.12
(8H, d, 3 J (HH)= 7.5 Hz, Ho); 8.69 (4H, s, Hβ). The absorption bands of MnTPPBr4 (OAc) prepared using
the method of Adler et al. appear at 492 (λmax), 604, and 656 nm. UV-Vis and 1 H NMR spectral data
of H2 TPPBr4 and the manganese complex have been reported previously.19 Moreover, the λmax of MnT(4-
CH3 P)PBr4 (X) with X = F, Cl, Br, N3 , IO3 , and SCN prepared by the method of Oghoshi et al.20 was found
at 486, 489, 502, 502, 499, and 499 nm, respectively.
In a general oxidation procedure, stock solutions of the catalyst (0.003 M) and nitrogenous bases (0.5 M)
were prepared in CH2 Cl2 . In a 10-mL round-bottom flask the reagents were added in the following order: alkene
(0.25 mmol), catalyst (0.003 mmol, 1.0 mL), and imidazole (0.03 mmol, 60 µL). Tetrabutylammonium oxone
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(0.5 mmol, 0.206 g) was then added to the reaction solutions at 25 ◦ C. The mixture was stirred thoroughly for
2 min at ambient temperature. Formation of epoxide was detected by GC analysis. All reactions were repeated
at least 3 times, analyzed by GC, and averaged.
Table 2.
Oxidation of different olefins with n-Bu4 NHSO5 catalyzed by MnT(4-OCH3 P)PBr4 (OAc) and
MnTPPBr4 (OAc) (the data in parentheses) in ethanol.a
Olefins
Conversion(%)
Epoxide
Epoxide
selectivity (%)
o
100
100
70 (100)
80 (100)
o
Cl
Cl
o
100
86 (100)
88 (100)
OH3C
H3CO
24 (75)b
CH3
CH3
o
100
100 (83)b
100
66 (95)
47 (92)
o
o
o
84 (97)
o
35
5 (51)
30 (24)d
100
o
a
c
d
See the footnotes of Table 1. b Acetophenone was the other product. The products of allylic oxidation.
Exo or endo
Table 3. Effect of counteranion on the catalytic activity of MnT(4-OCH3 P)PBr4 (X) in oxidation of cyclooctene with
n-Bu4 NHSO5 in ethanol at room temperature.a
X
OAc
F
Cl
Br
IO3 N3
SCN
Conversion (%)b 66.0 47.0 55.0 60.0 64.0 62.0 61.0
a,bSee the footnotes of Table 1.
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X
Br
Br
NH
N
X
X
HN
N
Br
Br
X
X=
H
H2TPPBr4
X=OCH3 H2T(4-OCH3)PBr4
Figure. The porphyrins used in this study.
4. Conclusions
In summary, the catalytic activity of MnT(4-CH3 P)PBr4 (X) (X = OAc, F, Cl, Br, N3 , IO3 , and SCN) and
MnTPPPBr4 (OAc) in oxidation of alkanes and olefins with n-Bu4 NHSO5 in ethanol as the best solvent of the
oxidation reaction was studied and compared. In spite of the better catalytic activity of the latter relative to
the former, there was no large difference between the catalytic performances of the 2 catalysts in the oxidation
of most of the olefins used. However, the catalytic activity of MnTPPPBr4 (OAc) was significantly greater than
that of MnT(4-CH3 P)PBr4 (OAc) in the oxidation of alkanes. Furthermore, the oxidative degradation of the
former was smaller than that of the latter.
Acknowledgment
The financial support of this work by KN Toosi University of Technology research council is acknowledged.
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