1012
S. Zakavi et al. / Inorganic Chemistry Communications 14 (2011) 1010–1013
Table 3
cis to trans isomerisation among the Mn-pors. The low degree of
isomerisation observed in the presence of MnT(2-NO2P)P(OAc) may
be attributed to the steric hindrance caused by the introduction of
bulky groups at the ortho position of aryl substituents which prevent
the rotation around the C―C bond in the intermediate [9]. However,
in the case of MnT(4-NO2P)P(OAc), the electronic effects rather than
the steric ones seem to be involved in the observed low extent of cis-
trans isomerisation.
Oxidation of cis-stilbene with TBAP in dichloromethane catalyzed by various Mn-pors.a.
Mn-pors
Conversion (%)b
Selectivity (%)
Cis-trans
rearrangement
MnT(4-SCH3P)P(OAc)
MnT(4-OCH3P)P(OAc)
MnT(4-NO2P)P(OAc)
MnT(2-NO2P)P(OAc)
MnT(4-SCH3P)PBr4(OAc)
58.0
51.0
21.2
23.0
46.2
86.5c, 13.5d
84.2c, 15.8d
97.0c, 3.0d
95.5c, 4.5d
75.0c, 25.0d
13.5
15.8
3.3
4.5
24.0
It is observed that the relative catalytic activity of a series of
electron-rich and electron-deficient Mn-pors cannot be simply
explained in oxidation of different olefins. In other words, the
observed pattern of catalytic activity of the used Mn-pors in oxidation
of a given olefin cannot be generalized to the rest. In contrast to the
previous reports on the oxidation of olefins with TBAP catalyzed by
electron-rich and electron-deficient Mn-pors [7,9], the results of this
study indicate that the relative catalytic activity of the two classes of
Mn-pors changes when going from an olefin to another one.
In summary, in spite of the complex order of catalytic activity for a
series of electron-rich and electron-deficient Mn-pors in oxidation of
various alkenes, MnT(4-SCH3P)P(OAc) shows the highest catalytic
activity for the oxidation of a wide range of cyclic and acyclic olefins.
While the substitution of β positions of MnT(4-SCH3P)P(OAc) with
four bromine atoms has little or no effect on the oxidative stability of
this Mn-porphyrin, the efficiency of the catalyst has been significantly
decreased for the oxidation of olefins. However, the observed order of
catalytic activity for the used Mn-pors in the oxidation of various
olefins cannot be simply explained by the steric and electronic effects
of the substituents attached to the periphery of porphyrins as well the
olefin double bond. While MnT(2-NO2P)P(OAc) is the most stable
catalyst of the series, MnT(4-NO2P)P(OAc) shows unusual instablilty
towards oxidation with the active oxidant.
a
The molar ratios for Mn-por:imidazole:alkene:oxidant are 1:10:83:167.
Analyzed by 1H NMR.
Cis-stilbene oxide and
b
c
d
trans-stilbene oxide.
described for cyclohexene. However, very similar activity has been
observed for MnT(4-SCH3P)PBr4(OAc) and MnT(4-NO2P)P(OAc).
The following order of catalytic efficiency was observed in the case
of α-Methylstyrene: MnT(4-SCH3P)P(OAc)NMnT(4-OCH3P)P(OAc)~
MnT(2-NO2P)P(OAc) NMnT(4-NO2P)P(OAc) ~ MnT(4-SCH3P)PBr4
(OAc) (Table S3). The change in the position of MnT(2-NO2P)P(OAc)
in the series compared with that in the case of styrene seems to be
due to the increased steric hindrance at the double bond of α-
methylstyrene. It should be noted that the substitution of bulky
substituents at the ortho position of aryl groups limits the range of
porphyrin-aryl group dihedral angle [10] and leads to an increase in
the steric hindrance at the reaction center of the active oxidant [4a]. In
comparison with styrene, there is no difference between the efficiency
of MnT(4-SCH3P)P(OAc) and (4-OCH3P)P(OAc) for the oxidation of
α-methylstyrene.
Oxidation of indene resulted in the following order: MnT(4-
SCH3P)P(OAc) ≥ MnT(4-OCH3P)P(OAc) ~ MnT(2-NO2P)P(OAc) ≫
MnT(4-NO2P)P(OAc)~MnT(4-SCH3P)PBr4(OAc) (Table S4). The cat-
alytic activity of MnT(4-SCH3P)P(OAc), MnT(4-OCH3P)P(OAc) and
MnT(2-NO2P)P(OAc) for the oxidation of indene is nearly the same.
The position of MnT(4-OCH3P)P(OAc) in the series of catalytic activity
is in contrast to that observed in the case of cyclohexene, styrene and
3-chloro-2-methyl-1-propene. However, it seems that the substituent
dependence of the catalytic activity of the Mn-pors for the oxidation
of indene is less than that observed in the case of other olefins.
The order observed in oxidation of 1-hexene is as follows: MnT(4-
SCH3P)P(OAc)~MnT(2-NO2P)P(OAc)≫MnT(4-NO2P)P(OAc)~MnT
(4-OCH3P)P(OAc)NMnT(4-SCH3P)PBr4(OAc) (Table S5). In oxidation
of the terminal olefin, 1-hexene, MnT(4-SCH3P)P(OAc) and MnT(2-
NO2P)P(OAc) show similar catalytic activities. However, these two
Mn-pors are about 2.5 times more efficient than the other ones in this
reaction (see Table S5).
Acknowledgments
Financial support for this work by the Institute for Advanced
Studies in Basic Sciences (IASBS) is acknowledged.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
doi:10.1016/j.inoche.2011.03.057.
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