S. Zakavi et al. / Polyhedron 31 (2012) 368–372
371
Table 4
Table 7
Effect of ImH on the catalytic activity of FeT(n-Pr)P(OAc) in the oxidation of
Oxidation of cyclooctene and styrene with FeT(n-Pr)(OAc) and MnT(n-pr)(OAc).a
cyclohexene with TBAP in dichloromethane.a
Catalyst
Olefin
Yield of epoxide (%)b,c
FeT(n-Pr)P(OAc):ImH Conversion (%)b Epoxide yield (%) Selectivity (%)c
MnT(n-Pr)P(OAc)
FeT(n-Pr)P(OAc)
MnT(n-Pr)P(OAc)
FeT(n-Pr)P(OAc)
cyclooctene
cyclooctene
styrene
33.6
24.7
37.7
33.5
1:0
trace
12.8
8.1
15.3
8.0
trace
2.0
1:10
1:20
1:30
1:50
15.6
26.0
23.5
12.5
2.1
3.6
1.0
styrene
a
b
c
The molar ratio for alkene:oxidant:catalyst is 1000:170:1.
For a reaction time of 4 h, based on the starting TBAP.
The epoxide has been obtained as the sole product.
a
b
c
The molar ratio for alkene:oxidant:catalyst is 1000:170:1.
Based on the starting TBAP.
2-Cyclohexe-1-ol is the other product.
cis- to trans-stilbene oxide is 5.8. Accordingly, the unusual ratio
of cis- to trans-stilbene oxide in competitive reaction catalyzed
by FeT(n-Pr)P(OAc) is probably due to the significant cis–trans
isomerization rather than the preference of trans-stilbene to the
cis-isomer. On the other hand, this remarkable isomerization
seems to be in agreement with the involvement of a high-valent
Fe-oxo species rather than a six coordinate FeT(n-Pr)P(ImH)(IO4)
one as the active oxidant; rotation about the C–C bond is expected
to be more feasible in the presence of an active oxidant with less
steric hindrance about the electrophilic oxygen atom i.e. a metal-
oxo one.
Table 5
Counter ion effect on the efficiency of MnT(n-Pr)P(X) and FeT(n-Pr)P(X) in the
oxidation of cyclohexene with TBAP in dichloromethane at room temperature.a,b
Catalyst
Conversion
(%)
Epoxide yield
(%)
Epoxide selectivity
(%)
FeT(n-Pr)P(Cl)
FeT(n-Pr)P(SCN)
FeT(n-Pr)P(OAc)
MnT(n-Pr)P(Cl)
MnT(n-
Pr)P(SCN)
MnT(n-
Pr)P(OAc)
24.7
9.7
12.9
12.8
11.7
2.3
1.0
2.0
5.0
2.8
9.5
10.3
15.6
39.0
24.0
On the other hand, the remarkably greater cis to trans isomeri-
zation in oxidation of cis-stilbene with TBAP in the presence of
FeT(n-Pr)P(OAc) (81%) compared to MnT(n-Pr)P(OAc) (24%) reveals
31.0
12.0
38.7
a
The molar ratio for alkene:oxidant:ImH:catalyst is 1000:170:10:1.
For a reaction time of 4 h, based on the starting TBAP.
a significantly enhanced cleavage of the carbon–carbon p-bond of
b
the olefin in the transition state of the reaction catalyzed by the
former in comparison with the latter. It should be noted that the
formation of trans-stilbene oxide as the thermodynamically more
stable product [10,23] with respect to the cis-isomer, requires a
rotation about the C@C bond at some intermediate step [24,25].
Table 6
Competetive oxidation of cis- and trans-stilbene with TBAP in the presence of MnT(n-
Pr)P(OAc) and FeT(n-Pr)P(OAc) in dichloromethane at room temperature.a,b
Catalyst
MnT(n-Pr)P(OAc) 24
FeT(n-Pr)P(OAc) 81
Cis to trans isomerization of cis-stilbene (%)c Cis/trans
3.4. Oxidation of styrene and cyclooctene
2.5 (3.6)d
0.3 (5.8)d
Oxidation of styrene and cyclooctene with TBAP in the presence
of FeT(n-Pr)(OAc) and MnT(n-Pr)(OAc) gives the corresponding
epoxide as the sole product (Table 7). As was observed in oxidation
of cyclohexene, the Mn-porphyrin is a more efficient catalyst com-
pared to the iron counterpart. However, in the case of cyclooctene
and styrene, the difference between the efficiency of the two
metalloporphyrins decreases so that in oxidation of styrene
FeT(n-Pr)(OAc) is as efficient as the Mn(III) analogous.
a
The molar ratio for (cis-stilbene, trans-stilbene):oxidant:ImH:catalyst is
(500,500):170:10:1.
Analyzed by 1H NMR.
b
c
The control reaction using cis-stilbene.
The corrected cis/trans ratio considering the cis to trans isomerization of cis-
d
stilbene (see the text).
The results of this study in comparison with other previous
studies [8–12] clearly show that the presence of aryl or alkenyl
[8] substituents at the meso positions of metalloporphyrins, apart
from the electron-donating or electron-withdrawing ability of the
substituents, is the basic electronic requirement for such com-
plexes to act as efficient catalyst precursors for mild oxidation of
organic compounds. Otherwise, the introduction of strong electron
withdrawing groups at the alkyl residues of meso-tetraalkylpor-
phyrins substituents is necessary [26]. In a previous work, we have
shown the higher catalytic performance of Mn(III) complex of a
meso-tetra(alkenyl)porphyrin compared to that of meso-tetraphen-
ylporphyrin in oxidation of olefins with oxone [8].
of cis-stilbene oxide to the trans one in reaction catalyzed by
MnT(n-Pr)P(OAc) suggests that the high-valent Mn-oxo is the pre-
dominant oxidant species in the catalytic cycle. On the other hand,
oxidation of cis-stilbene in the presence of metalloporphyrins with
non-bulky substituents at the ortho position of meso-aryl groups is
usually accompanied with partial cis to trans isomerization of the
product [8,10]. This in turn would result in giving false product ra-
tios in the competitive epoxidation reactions [13,14]. Control reac-
tion with cis-stilbene in the presence of MnT(n-Pr)P(OAc) in the
same reaction conditions showed a cis to trans isomerization of
24% in the products. Accordingly, the correct value for the ratio
of cis to trans-stilbene oxide will be 3.6 instead of 2.5. This ratio
shows the involvement of a six coordinate MnT(n-Pr)P(ImH)(IO4)
species in addition to a high-valent Mn-oxo one as the reactive
intermediates responsible for oxygen atom transfer. In the case
of FeT(n-Pr)P(OAc), the observed ratio (0.3) apparently shows the
unexpected preference of trans-stilbene over cis-stilbene which
has been previously reported in the case of the iron(IV) oxo cation
radical of electron-deficient porphyrins [13]. Interestingly, oxida-
tion of cis-stilbene with TBAP using FeT(n-Pr)P(OAc) as catalyst
in the same reaction conditions gave cis- and trans-stilbene oxide
in 1.0:4.3 molar ratios (Table 6) corresponding to a ca. 81% cis to
trans isomerization. Therefore, the correct value for the ratio of
Further work is now in progress to explore the effect of
branched chain alkyl substituents on the catalytic activity of the
metal complexes of meso-tetraalkylporphyrins in oxidation of ole-
fins with different terminal oxidants.
4. Conclusions
In summary, the catalytic activity of a series of Mn(III) and
Fe(III) complexes of H2T(n-Pr)P bearing various anionic counter
ions in oxidation of olefins with TBAP has been studied and the
influence of different reaction parameters on the efficiency of the