Reactivity studies of biomimetic catalytic epoxidation of alkenes with tetrabutylammonium periodate in the presence of various manganese porphyrins and nitrogen donors: Significant axial ligand π-bonding effects

Article abstract of DOI:10.1039/b310909g

Highly selective epoxidation (> 95%) of unfunctionalized alkenes was performed by tetrabutylammonium periodate in the presence of six different phenyl substituted manganese(III) meso-tetraphenylporphyrins [Mn(Por)] and imidazole in CH₂Cl_2<

Full text of DOI:10.1039/b310909g

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P A P E R
Reactivity studies of biomimetic catalytic epoxidation of alkenes with
tetrabutylammonium periodate in the presence of various manganese
porphyrins and nitrogen donors: significant axial ligand
p-bonding effects
Daryoush Mohajer,*a Gholamreza Karimipoura and Mojtaba Bagherzadehb
a
Department of Chemistry, Shiraz University, Shiraz 71454, Iran.
E-mail: dmohajer@chem.susc.ac.ir; Fax: þ98-711-2280926; Tel: þ98-711-2284822
b
Department of Chemistry, Sharif University of Technology, Tehran 11365-9516, Iran.
E-mail: Bagherzadehm@Sharif.edu; Fax: þ98-21-6005718; Tel: þ98-21-6005718
Received (in Montpellier, France) 7th September 2003, Accepted 16th January 2004
First published as an Advance Article on the web 14th May 2004
Highly selective epoxidation ( > 95%) of unfunctionalized alkenes was performed by tetrabutylammonium
periodate in the presence of six different phenyl substituted manganese(III) meso-tetraphenylporphyrins
[Mn(Por)] and imidazole in CH2Cl2 . Electron-withdrawing and bulky substituents on the phenyl groups
lowered the catalytic activities of the corresponding Mn(Por). Less bulky alkenes with electron -rich double
bonds showed greater reactivity in the epoxidation. Co-catalytic activities of four different classes of axial
nitrogen donors are compared in the presence of various Mn(Por). In general no direct correlation was found
between co-catalytic activities and the pKa values of the nitrogen donors. Strong s-donor amines (pKa ¼ 10.6
to 11.123) and weak p-donor pyridines (pKa ¼ 5.25 to 6.65) showed comparable co-catalytic activities. Strong
p-donor aminopyridines (pKa ¼ 7 to 9.71) and imidazoles (pKa ¼ 6.95 to 7.86) are generally much better
co-catalysts than pure s-donors, suggesting the importance of p-bonding interactions of the nitrogenous
donors. N–H imidazoles with the possibility for N–Hꢀ ꢀ ꢀB hydrogen bonding are much more effective
co-catalysts than 1-methylimidazole. Methylpyridines and aminopyridines with substituents at the 2 or 2,6
positions showed particularly high co-catalytic activities toward manganese(III) meso-tetrakis(pentafluoro-
phenylporphyrin) acetate. This ‘‘unusual’’ observation suggests the occurrence of some attractive hydrogen
bonding interactions between ortho-fluorines of the manganese porphyrin and the substituents of these donors.
A simple qualitative molecular orbital diagram is presented to describe the possible p-interactions of (dxz , dyz)
metal orbitals with axially coordinated ImH and periodate in six-coordinate Mn(Por)(ImH)(IO4), as the active
oxidizing species. A catalytic cycle is postulated in which the intermediate involves complexation of an alkene
to the coordinated periodate, and attempts have been made to visualize the orbital interactions for this
intermediate species.
Mn(TPFPP)OAc, for the first time, is suggestive of inter-
Introduction
molecular hydrogen bonding between ortho-C–F groups of
Mn(Por) and the 2,6-substituents of the pyridines. A simple
molecular orbital scheme is presented as an attempt to
clarify features of the possible p-interactions that may be
operative along the unique molecular axis containing the
axial ligands in six-coordinate Mn(Por)(ImH)(IO4). Based on
our experimental results a mechanism is proposed for the
epoxidation.
Synthetic manganese and iron porphyrins have long been of
great interest because of their potential as functional models
of cytochrome P-450 and peroxidases.1 It has been shown that
the steric and electronic properties of metalloporphyrins and
alkenes substantially affect the product distribution, selectivity
and the rate of the oxygenation.2 The employment of nitrogen-
ous donors in metalloporphyrin systems for mimicking the
oxygenation function of P-450 has led to marked improve-
ments in selectivities and turnover rates in the epoxidation of
alkenes.3
In this work epoxidation of different alkenes by tetrabutyl-
ammonium periodate (n-Bu4NIO4) in the presence of phenyl
substituted manganese(III) meso-tetraphenylporphyrin cata-
lysts (Fig. 1) was studied. Particular focus was placed on
exploring the co-catalytic effects of various classes of nitrogen
donors. The key role played by the steric effects of all the spe-
cies involved in the epoxidation is described. It is also demon-
strated that Mn(Por) catalysts with high electron density on
their phenyl groups, electron-rich alkenes and p-donor ligands,
especially imidazoles (ImH) with hydrogen bonding ability,
dramatically increase the overall rate of epoxidation. The
‘‘unusual’’ co-catalytic activities of 2,6-dimethylpyridine
and 2,6-diaminopyridine observed only in the presence of
Fig. 1 Manganese(III) meso-tetraphenylporphyrin catalysts used in
this work.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
740
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 4 0 – 7 4 7

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three different para-substituted Mn(Por) in the epoxidation
of styrene and a-methylstyrene, provides additional evidence
for this viewpoint (Table 2). Presuming that these Mn(Por)
have very similar steric environments at their Mn centers, it
may be concluded that the lower activity of Mn[T(4-NO2P)-
P]OAc is due to the presence of the electron-withdrawing
NO2 substituent. On the other hand, the higher catalytic activ-
ity of Mn[T(4-OCH3P)P]OAc compared to Mn(TPP)OAc is
presumably caused by the p-resonance effects of its p-OCH3
groups.
Results and discussion
Catalytic alkene epoxidation by Mn(Por)/ImH/n-Bu4NIO4
systems
Epoxidation of various alkenes by n-Bu4NIO4 in the presence
of different Mn(Por) and ImH proceeded quantitatively in
CH2Cl2 . The only exception was oxidation of a-methylstyrene
with Mn(TMP)OAc and Mn(TPFPP)OAc catalysts, where
ꢁ5% acetophenone by-product was obtained (Table 1). The
electron-rich dialkylsubstituted cis-2-hexene displayed a much
greater reactivity than 1-octene with all the catalysts. Also, it
was observed that a-methylstyrene was more reactive than
the less electron-rich styrene in the presence of Mn(TPP)OAc,
and Mn(TPFPP)OAc. However, when the bulky Mn(TMP)-
OAc was used the steric factors clearly became dominant
and styrene displayed a greater reactivity than the hindered
a-methylstyrene. It appears that the steric and electronic fac-
tors are balanced for Mn(TDCPP)OAc with which styrene
and a-methylstyrene show similar reactivities. The greater
reactivity of cyclohexene than the electron-rich and sterically
hindered 1-methycyclohexene, with all the Mn(Por), again
demonstrates the importance of steric effects in the alkenes.
The higher reactivity of cyclooctene relative to cyclohexene
might be due to the greater flexibility of the former. In the
epoxidation of cis- and trans-stilbenes the steric effects are par-
ticularly quite distinct. The sterically demanding trans-stilbene
is much less reactive than the cis isomer and its oxidation gives
only trans-stilbene oxide with all the catalysts. Similarly, oxi-
dation of cis-stilbene in the presence of bulky Mn(TMP)OAc
and Mn(TDCPP)OAc is 100% stereoselective and leads to for-
mation of the cis isomer as the sole product. In contrast the
oxygenation of cis-stilbene with less hindered catalysts [i.e.,
Mn(TPP)OAc and Mn(TPFPP)OAc] results in a mixture of
cis and trans isomers. Apparently, formation of the thermo-
dynamically more stable trans-stilbene oxide requires a free
rotation about the alkene C–C bond at some intermediate
step;4 such a rotation is expected to be more feasible using
catalysts with less steric strain, as observed.
Co-catalytic effects of nitrogen donors
To better understand the role of nitrogenous bases in activat-
ing Mn(Por) catalysts, we investigated the co-catalytic effects
of four classes of nitrogen donors with different stereoelectro-
nic properties. The influence of such ligands in improving
Mn(Por)-mediated epoxidation is especially high.3d,5 A draw-
back to the study of nitrogenous axial ligand effects is their oxi-
dation. Pyridine can be oxidized by Mn(Por) catalytic systems
containing iodosylbenzene (PhIO),6 potassium monopersulfate
(KHSO5)7 and (ClOꢂ).8 In the present Mn(Por)/n-Bu4NIO4
epoxidation systems pyridines and imidazole are essentially
robust. It is assumed that the epoxidation yields (%) or the
turnover numbers (TON) for the oxygenation of cyclooctene
are directly related to the co-catalytic activities of the nitro-
gen donors, since in the absence of such donors virtually no
epoxidation occurred (Table 3).
Amines. Pure s-donor amines, with very large pKa values,
are relatively poor co-catalysts in the epoxidation of cyclo-
octene (Table 3). Within this group, it appears that steric pro-
perties rather than pKa values are the dominant factor in
determining their co-catalytic activities. However, the least
steric s-donor, piperidine, with the highest pKa value, shows
the greatest co-catalytic activity with all the Mn(Por) catalysts.
Quinuclidine, which is less sterically hindered and a weaker
base than Et2NH and Et3N, is in the second place in the series.
However, the importance of the size of these s-donors
becomes more pronounced when their relative activities are
examined in the presence of the sterically demanding
Mn(TMP)OAc and Mn(TDCPP)OAc catalysts. Et3N shows
very little or no ability to promote the catalytic activities of
the bulky Mn(Por).9
Electronic effects of Mn(Por)
The much longer reaction times required for the
Mn(TDCPP)OAc and Mn(TPFPP)OAc catalysts compared
to Mn(TPP)OAc and Mn(TMP)OAc (24 h vs. 4 h for the for-
mer) definitely reflect the negative effects of electron-withdraw-
ing substituents on the phenyl groups upon their catalytic
activities (Table 1). Comparison of the catalytic properties of
Pyridines. Pyridine and methyl-substituted pyridines with
weak p-donating ability10 and pKa values much smaller than
Table 1 Epoxidation of alkenes with Mn(Por)/ImH/n-Bu4NIO4 catalytic systemsa
% Conversionb (TONc )
Substrate
Mn(TPP)OAc (4 h)
Mn(TMP)OAc (4 h)
Mn(TDCPP)OAc (24 h)
Mn(TPFPP)OAc (24 h)
cis-2-Hexene
1-Octene
58 (48.3)
14 (11.7)
16 (13.3)
8 (6.7)
37 (30.8)
21 (17.5)
59 (49.1)
57 (47.5)
46 (38.3)
25 (20.8)
60 (50.0)
67 (55.8) cis
26 (21.6)
18 (15.0)
51d (42.5)
45 (37.5)
29 (24.1)
a-Methylstyrene
Styrene
80 (66.6)
66 (55)
51d (42.5)
58 (48.3)
40 (33.3)
22 (18.3)
48 (40.0)
47 (39.1) cis
Cyclohexene
1-Methylcyclohexene
Cyclooctene
cis-Stilbene
78 (65.0)
62 (51.6)
92 (77.0)
21 (17.5)
47 (39.1)
50 (41.7) cis
11 (9.2) trans
16 (13.3) trans
39 (32.5) cis
8 (6.7) trans
18 (15.0) trans
trans-Stilbene
<5 (4.1) trans
10 (8.3) trans
a
The molar ratios for Mn(Por):imidazole:substrate:oxidant are 1:10:83:167. The imidazole ratio for Mn(TDCPP)OAc is 1.5 rather than 10, since
under the latter conditions, Mn(TDCPP)OAc loses almost all its activity. The reactions were performed in CH2Cl2 with [Mn(Por)] ¼ 3 ꢃ 10ꢂ3
M
under air at 25 ꢄ 2 ꢅC with 100% selectivity. b The GLC conversion (%) or epoxide yield (%) are measured relative to the starting alkenes. c Turn-
over number (TON) is the ratio of the number of moles of produced epoxide to the number of moles of catalyst. d Selectivities for the epoxidations
are 100% except for a-methylstyrene, which gives 5% acetophenone by-product.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 4 0 – 7 4 7
741
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4

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Table 2 Effect of para-substituents on the catalytic activity of
Mn(Por) in the epoxidation of styrene and a-methylstyrene (a-Mesty)a
2,6-Me2Py, and to a lesser extent that of 2-MePy, toward
Mn(TPFPP)OAc is of considerable interest. This observation,
with no precedence in the literature as far as we know, suggests
the occurrence of some attractive interactions between 2,6-
Me2Py or 2-MePy and Mn(TPFPP)OAc, which is not present
for the other manganese porphyrin catalysts. We propose that
formation of C–Hꢀ ꢀ ꢀF–C hydrogen bonds13 between the
methyl groups of 2,6-Me2Py or 2-MePy and the ortho-fluorines
of the coordinated Mn(TPFPP)OAc might be responsible for
this behavior. The lower co-catalytic activity of 2-MePy than
2,6-Me2Py may reflect the fewer number of C–Hꢀ ꢀ ꢀF–C hydro-
gen bonds in the former, leading to its less efficient coordina-
tion as a nitrogen donor to the Mn center.
% Conversion (TON)b
Substrate
Mn(TPP)OAc
Mn[T(4-OCH3P)P]OAc
Mn[T(4-NO2P)P]OAc
Styrene
a-Mesty
70 (58.3)
91 (75.8)
76 (63.3)
95 (79.1)
51 (42.5)
72 (60.0)
a
The molar ratios and general reaction conditions are the same as in Table 1,
b
with 5 h reaction time. The turnover number is defined in Table 1.
those of s-donor amines generally show co-catalytic activities
similar to those of amines (Table 3). These results may suggest
the importance of p- rather than s-interactions of the nitro-
genous donors with Mn(Por). The observed order of co-cata-
lytic activities of 4-MePy > Py ꢆ 4-CNPy seems to be
directly related to both the s- and p-donor abilities of these
nitrogen donors. The weak base 4-CNPy (pKa ¼ 1.86), with
an electron-withdrawing CN substituent, essentially displays
no co-catalytic activity. The higher co-catalytic activity of 4-
MePy than Py is apparently due to its better s- and p- dona-
tion, caused by hyperconjugation of the methyl group.11 The
steric properties of alkylpyridines play an important part in
determining the extent of their interactions with various
Mn(Por).12 The lower activity of 4-t-BuPy than Py apparently
indicates the effect of steric bulk of the 4-substituent. 2-MePy
with one methyl group near to the nitrogen donor atom, and
particularly 2,6-Me2Py with two such groups, hindering their
coordination to the Mn(Por), are poor co-catalysts. They
actually show no co-catalytic activity in the presence of
Mn(TMP)OAc. The remarkable co-catalytic activity of
Aminopyridines. These nitrogenous bases are in general
much better co-catalysts than pyridines and amines, unless
some steric effects are involved. Comparison of pKa values of
aminopyridines with those of pure s-donor amines confirms
the fact that no direct correlation exists between their co-cata-
lytic activities and simple s-donor strength. Apparently, the p-
resonance effect of the lone pair(s) on the amino substituent(s)
makes the nitrogen donor site of the aminopyridines both
good s- and p-donors. While both 4-NH2Py and the sterically
demanding 4-NMe2Py show similar co-catalytic activities
toward Mn(TPP)OAc (25 vs. 23), they act differently in the
presence of the bulky Mn(TMP)OAc (22 vs. 12) and
Mn(TDCPP)OAc (30 vs. 22). Varied co-catalytic activities of
3,4-, 2,3- and 2,6-(NH2)2Py clearly illustrate the importance
of their steric properties (Table 3). As the amino substituents
are placed closer to the nitrogen donor site, hindering its coor-
dination, the corresponding aminopyridine demonstrates a
lower co-catalytic activity. Thus, 2,6-(NH2)2Py is the least
Table 3 Epoxidation yields (co-catalytic activities) for nitrogenous donors in the epoxidation of cyclooctene by Mn(Por) catalystsa
% Epoxidation
Nitrogenous basesb
pKa (BHþ)c
Mn(TPP)OAc (4 h)
Mn(TMP)OAc (8 h)
Mn(TDCPP)OAc (24 h)
Mn(TPFPP)OAc (24 h)
None
–
2
9
1
10
8
<2
12
6
2
12
9
Piperidine
Quinuclidine
Et2NH
11.123
10.6
11.02
10.75
5.25
1.86d
6.02
5.99d
5.97
6.65
9.114
9.71d
9.08d
7e
7
7
7
5
9
Et3N
Py
5
3
2
6
7
8
10
2
10
3
4-CNPy
4-MePy
2
2
10
4
11
5
15
6
15
12
12
15
26
19
22
16
20
30
32
16
17
20
12
3
4-t-BuPy
2-MePy
6
2
6
2,6-Me2Py
4-NH2Py
4-NMe2Py
3,4-(NH2)2Py
2,3-(NH2)2Py
2,6-(NH2)2Py
ImH
5
1
4
25
23
23
14
7
22
12
15
9
30
22
9
6
–
5
4
6.953
7.52f
7.86f
7.86g
5.532
6.95
ꢂ2h
50
47
18
17
19
13
6
42
40
7
38
35
16
13
19
14
3
4(5)-MeImH
2-MeImH
2-EtImH
BzImH
6
12
5
1-MeIm
DMF
4
a
Mn(Por):nitrogenous base:cyclooctene:oxidant molar ratios are: 1:10:90:100 [Mn(TPP)OAc], 1:10:83:83 [Mn(TMP)OAc and Mn(TPFPP)OAc],
1:1.5:83:83 [Mn(TDCPP)OAc] with [Mn(Por)] ¼ 3 ꢃ 10ꢂ3 M in CH2Cl2 at 25 ꢄ 2 ꢅC. b Abbreviations: Et2NH, diethylamine; Et3N, triethylamine;
Py, pyridine; 4-CNPy, 4-cyanopyridine; 4-MePy, 4-methylpyridine; 4-t-BuPy, 4-tert-butylpyridine; 2-MePy, 2-methylpyridine; 2,6-Me2Py,
2,6-dimethylpyridine; 4-NH2Py, 4-aminopyridine; 4-NMe2Py, 4-(dimethylamino)pyridine; 3,4-(NH2)2Py, 3,4-diaminopyridine; 2,3-(NH2)2Py,
2,3-diaminopyridine; 2,6-(NH2)2Py, 2,6-diaminopyridine; ImH, imidazole; 4(5)-MeImH, 4- or 5-methylimidazole (tautomeric); 2-MeImH, 2-methyl-
imidazole; 2-EtImH, 2-ethylimidazole; BzImH, benzimidazole; 1-MeIm, 1-methylimidazole; DMF, N,N-dimethylformamide. c pKa values obtained
d
e
f
g
from: ref. 33. ref. 34. ref. 35. ref. 36. Very little change in base strength accompanying the change of a methyl substituent to ethyl was
h
observed (see ref. 11). See ref. 17.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 4 0 – 7 4 7
742
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4

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Table 4 Effect of 2,6-Me2Py on cyclooctene epoxidation by the
Mn(TMP)OAc/ImH/n-Bu4NIO4 system
active one in association with all Mn(Por) except for
Mn(TPFPP)OAc. In line with the arguments presented above
for 2,6-Me2Py, this is presumably related to the formation of
N–Hꢀ ꢀ ꢀF–C hydrogen bonds13b between N–H groups of an
axially coordinated 2,6-(NH2)2Py and the ortho-fluorines of
two nearby meso-pentafluorophenyl groups of the bonded
Mn(TPFPP)OAc. Such hydrogen bonding is expected to lead
to:(i) a closer approach of the nitrogen donor site of 2,6-
(NH2)2Py to the Mn center and (ii) a shift of some electron
density to the hydrogen-bonded N–H group, leading to an
increase in the donor ability of the nitrogen donor atom.
2,6-Me2Py/
Mn(TMP)
OAc ratio
ImH/Mn(TMP)
OAc ratio
Run
% Epoxidation,a
1
2
3
4
0
10
0
1
25
29
32
0.8
0.8
0.8
2
10
a
Epoxide yields after 6 h are based on the starting cyclooctene in
CH2Cl2 with a fixed Mn(TMP)OAc:cyclooctene:n-Bu4NIO4 ratio of
1:83:83 under air at 25 ꢄ 3 ꢅC with [Mn(Por)] ¼ 3 ꢃ 10ꢂ3 M.
Imidazoles. Strong p-donors ImH10,14 and 4(5)-MeImH are
generally far better co-catalysts than all the nitrogen donors
presented in Table 3. The much lower co-catalytic activities
of 2-MeImH and 2-EtImH than ImH and 4(5)-MeImH are
presumably due to the steric properties of the 2-substituents,2c
which is more clearly shown in the presence of bulky
Mn(TMP)OAc. Rather bulky and flat BzImH displays similar
or even greater co-catalytic activity than 2-MeImH and 2-
EtImH, whereas they have greater pKa values (Table 3). This
may primarily reflect the effects of the more extended p-system
of BzImH, its better p-donor ability and also a comfortable
coordination to the Mn center. 1-MeIm with a pKa value very
close to that of ImH and a size approaching that of 4(5)-
MeImH is the weakest co-catalyst in this class. 1-MeIm is
the only one in this class that lacks an N–H bond and, conse-
quently, it cannot form N–Hꢀ ꢀ ꢀB hydrogen bonds when it is
coordinated to the Mn(Por)15 (Scheme 1).
Mn(TPP)OAc with DMF and strong s-donor amines provides
evidence for the importance of p-donation by DMF.17
Fig. 2 shows the yield of cyclooctene epoxidation for
Mn(TPP)OAc in the presence of various donors. The four
classes of nitrogen donors are generally grouped in separate
regions, with less steric strong p-donors manifesting the best
co-catalytic activities. Similar diagrams for Mn(TMP)OAc
and Mn(TDCPP)OAc (not shown) are more scattered, reflect-
ing greater steric effects within each class of donors with these
catalysts.
Active oxidant
Formation of such hydrogen bonds leads to a shift of elec-
tron density from B to the N–H group, making its nitrogen
donor site a stronger donor. To examine the importance of this
hydrogen bonding in enhancing the donor ability of ImH, we
added different amounts of an essentially non-coordinating
2,6-Me2Py into the Mn(TMP)OAc/ImH/n-Bu4NIO4 catalytic
system and monitored the epoxidation of cyclooctene (Table
4). With no 2,6-Me2Py in the system, and at an ImH/
Mn(TMP)OAc ratio of 0.8, the epoxidation yield was 25%
(Run 2). Under similar conditions but with a 2,6-Me2Py/
Mn(TMP)OAc ratio of 2 (Run 3), the epoxidation yield
reached 29%. When the ratio of 2,6-Me2Py was further
increased to 10 (Run 4) an epoxidation yield of 32% was
achieved. Similar experiments using 1-MeIm instead of ImH
showed that the presence or absence of 2,6-Me2Py in the sys-
tem has no effect on the epoxidation yield. These results
strongly suggest that ImH can form hydrogen bonds with
2,6-Me2Py through its N–H group, thus increasing its donor
ability, whereas 1-MeIm is structurally incapable of such
bonding. Possible distal hydrogen bonding between the coordi-
nated periodate and one or more ImH should also be consid-
ered16 (Scheme 1). Such interacꢂtions are expected to contribute
to a facile dissociation of IO3 prior to the oxygen transfer
step.
The nature of the active oxidant in the Mn(Por)/ImH/n-
Bu4NIO4 system is of interest. Interaction of the Mn(TPP)OAc
catalyst wꢂith an excess of n-Bu4NIO4 and exchange of OAcꢂ
with IO4
, leading to the formation of six-coordinate
Mn(TPP)(IO4)(ImH), under catalytic conditions, essentially
caused no changes in the position of its Soret band.18 While
the positions of the Soret bands for Mn(TPP)X (X ¼ OAcꢂ,
IO4ꢂ) are very close to each other (ꢁ478 nm) that of
Mn(TPP)F (456 nm) is quite different.19 Employment of the
latter as the starting manganese porphyrin catalyst clearly indi-
cated the formation of Mn(TPP)(IO4)(ImH) as the active oxi-
dant, with a drastic red shift in the Soret position [Fig. 3(A)].
It is notable that addition of H2O (2 ml) into this one-phase
catalytic system immediately led to the formation of a high-
valent Mn-oxo porphyrin species (Soret lmax ¼ 406 nm),
which had been observed in the two-phase NaIO4/
Mn(TPP)OAc system2a [Fig. 3(B)]. Parallel results were
obtained with Mn(TMP)OAc in terms of formation of
Mn(TMP)(IO4)(ImH) (Soret lmax ¼ 479 nm) as the active oxi-
dant, or formation of the corresponding high-valent Mn-oxo
porphyrin species (Soret lmax ¼ 408 nm), by adding H2O.
The co-catalytic effect of DMF, as a very weak base, is nota-
ble (Table 3). The comparable activation achieved for
Fig. 2 Scatter diagram showing co-catalytic activities versus pKa
values of four classes of nitrogen donors for Mn(TPP)OAc.
Scheme 1
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 4 0 – 7 4 7
743

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contrast, CH2Cl2 lacks these capabilities and consequently
the coordinated periodate apparently remains almost intact
prior to the epoxidation step in the one-phase system.
Interestingly, the competitive epoxidation of cis- and trans-
stilbenes with the one-phase Mn(TMP)OAc/ImH/PhIO
system in CH2Cl2 resulted in a cis : trans ratio of 3.7 (3.65,
alkenes/PhIO ¼ 50) for the products, which is very different
from that of the one-phase Mn(TMP)OAc/ImH/n-Bu4NIO4
system. Knowing that the former catalytic system would also
=
give a O Mn(Por) species as the active intermediate com-
plex,20 and based on the arguments presented above, this
finding is again suggestive of a bulky Mn(TMP)(IO4)(ImH)
as the functioning oxidant in the one-phase periodate system.
Axial p-bonding
Fig. 3 (A) The spectra represent complete conversion of Mn(TPP)F
(peak I) into Mn(TPP)(IO4)(ImH) (peak II) in the presence of n-
Bu4NIO4 and ImH in CH2Cl2 under catalytic molar ratios. The inset
B demonstrates formation of a high-valent Mn-oxo species (peak I)
(Soret lmax ¼ 406 nm) by adding some H2O into a CH2Cl2 solution
of Mn(TPP)(IO4)(ImH) (peak II).
The key role of p-bonding interactions of nitrogenous axial
ligands in activating periodate is clearly shown by our experi-
mental data. To explain this effect a simple qualitative d orbital
splitting scheme for the possible p-interactions along the z
molecular axis of Mn(Por)(IO4)(ImH), is presented in Fig. 4.
In the middle (A) no p-interactions between (dxz , dyz) orbitals
and axial ligands are assumed, whereas Por ! metal p-bonds
are considered to be present.21 For a perfect matching of the
p orbitals of the axial ligands with the metal p(dxz , dyz) orbi-
tals, and on the basis of the relative orientations of the ImH
and Mn–O–I planes, two idealized forms (B and C) may be
envisioned. Actually, these are the preferred orientations of
ImH since metal px and py orbitals, which are fixed by the
strong s-bond with the porphyrin nitrogens, can also make
an important contribution to the p-bonding interactions with
axial ligands.22 In the B configuration the ImH and Mn–O–I
planes are co-planar and only the dxz orbital of Mn(III) over-
laps with both the p-donor orbital of ImH and the empty
px* orbital of O–IO3ꢂ. In the C form ImH and Mn–O–I planes
are perpendicular and dyz accepts p-electron density from ImH
To confirm that the active oxidizing species in the one-
phase system is indeed the six-coordinate Mn(Por)(IO4)(ImH)
and not a transiently formed high-valent Mn-oxo porphyrin
species, we performed three different competitive epoxidation
reactions of cis- and trans-stilbene with n-Bu4NIO4 , NaIO4
(two-phase) and PhIO oxidants in the presence of
Mn(TMP)OAc, using similar molar ratios of the reagents
(Table 5). With this bulky catalyst and periodate as the starting
oxidant, each isomer only yields its corresponding epoxide,
with no formation of isomerized products. While oxidation
of trans-stilbene with PhIO gave only the trans-stilbene oxide,
oxidation of cis-stilbene produced > 98% cis-stilbene oxide
and <2% of trans-stilbene oxide. Thus, simply from the cis-
to trans-stilbene oxide ratios direct information may be
obtained about the relative reactivities of the related alkenes
in these catalytic systems.
and dxz donates electron density into the px* orbital of the
ꢂ
bound O–IO3
. Clearly such unsymmetrical p-bonding
The higher ratio of cis- to trans-stilbene oxide products
obtained for the one-phase (ꢁ8) as compared to the two-phase
(3.26) system clearly suggests that a larger steric hindrance is
operative at the active site of the intermediate oxidant in the
one-phase system than in the two-phase one, hence favoring
better oxidation of the less hindered cis-stilbene. The forma-
between Mn(III) dp orbitals and the axial ligand p-systems will
remove the degeneracy of the dxz , dyz orbitals. Shift of electron
density from the dp orbital into px* will facilitate eventual
cleavage of the metal bound O–I and the transfer of a single
oxygen to alkene. Strong p-donor nitrogenous ligands such
as ImH and 4-NH2Py are expected to be more effective in ele-
vating the energy of the dp-donor orbital at the Mn(III) center.
The p-acceptor 4-CNPy would lower the energy of the inter-
acting dp orbital and hence reducing the electron transfer into
the px* orbital. In the absence of a description for the spin
state of Mn(Por)(ImH)(IO4), particularly when it is involved
in the oxygenation of alkenes, possible occurrence of a
low-spin $ high-spin dynamic equilibrium is of concern. In
accordance with Fig. 4, it might be anticipated that a
=
tion of a high-valent O Mn(TMP)(ImH) (Soret lmax ¼ 407
nm)2a species and the relatively bulky Mn(TMP)(IO4)(ImH)
as the reactive oxidants in the two-phase and one-phase sys-
tems, respectively, seem to be quite consistent with the results
of these competition reactions. It appears that the presence of
H2O in the two-phase catalytic system plays a key role in gen-
=
erating the high-valent O Mn(Por)(ImH) species. The strong
hydrogen bonding ability of H2O as the solvent with the coor-
dinated periodate and the effective solvation and stabilization
of the dissociated IO3ꢂ anion seems to provide a strong driving
high-spin Mn(Por)(IO4)(ImH) with the
B configuration,
(dxy)1 < (dyz)1 < (pxz*)1 < (d2z)1, would be more favorable
than the C form, (dxy)1 < (dxz)1 < (pyz*)1 < (d2z )1, for electron
=
force for the formation of an O Mn(TMP)(ImH) species. In
Table 5 Competitive catalytic epoxidation of cis- and trans-stilbenes with Mn(TMP)OAc/ImH/n-Bu4NIO4 , Mn(TMP)OAc/ImH/NaIO4 and
Mn(TMP)OAc/ImH/PhIO systemsa
% Epoxidation
Mn(TMP)OAc/ImH/n-Bu4NIO4 (4 h)
Mn(TMP)OAc/ImH/NaIO4 (4 h)
Mn(TMP)OAc/ImH/PhIO (5 min)
cis
trans
cis/trans ratio
cis
trans
cis/trans ratio
cis
trans
cis/trans ratio
8.3
1.04
ꢁ8
9.01
2.76
3.26
12 (4.5)b
3.24 (1.23)b
3.7 (3.65)b
a
The molar ratios for Mn(TMP)OAc:ImH:(cis- and trans- stilbene):n-Bu4NIO4 are 1:10:(501 and 501):167 in CH2Cl2 , for Mn(TMP)OAc:
ImH:(cis- and trans- stilbenes):NaIO4:n-Bu4NBr (phase transfer catalyst) are 1:10:(501 and 501):167:8 in CH2Cl2/H2O (1:5 v/v) and for
Mn(TMP)OAc:ImH:(cis- and trans- stilbenes):PhIO are 1:10:(501 and 501):167. b 1:10:(2075 and 2075):83 in CH2Cl2 under air at 25 ꢄ 2 ꢅC.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 4 0 – 7 4 7
744
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4

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in which the planes of the coordinated axial ligands eclipse the
equatorial Mn–Npor bonds, maximum steric repulsions are
expected to occur with the porphyrin core.24 Pyridine deriva-
tives with lower p-donor abilities than imidazoles and presum-
ably with larger non-bonding interactions with the porphyrin
core are expected to deviate more from the ideal geometries.22
It seems reasonable to suppose that the observed differences in
co-catalytic activities of axial nitrogen donors are partly a con-
sequence of their orientation caused by a combination of steric
and electronic factors.
Postulated mechanism
Based on the available data in this study concerning the nature
of the active oxidant, the higher reactivities of electron-rich
alkenes and electron-rich Mn(Por), and also the high co-cata-
lytic activities of strong p-donor nitrogenous axial ligands, as
well as the excellent selectivities of the epoxidations, a con-
certed mechanism is proposed. Scheme 2 represents a possible
catalytic cycle for alkene epoxidation, in which formation of
the proposed intermediate 3 involves interaction of the alkene
with the bound oxygen atom of the active oxidant 2, resem-
bling the active intermediate in alkene epoxidation by
Mn(Por)X/KHSO5/4-t-BuPy.7b A similar epoxidizing inter-
mediate in iron porphyrin complex catalyzed epoxidation reac-
tions has also been proposed.25 Possible orbital interactions
for the idealized stereochemistry in the intermediate 3, with
alkene C–C bond located in the yz plane, are given in Fig. 5.
These consist of a nucleophilic attack of the alkene p-orbital
Fig. 4 A simple idealized molecular orbital diagram iꢂllustrating the
maximum possible p-interactions of axial ImH and IO4 ligands with
(dxz , dyz) metal orbitals in low- and high-spin [Mn(Por)(ImH)(IO4)]:
(A) no p-bonding along the z axis, (B) p-bonding with a single dxz
orbital; (C) p-bonding with both dxz and dyz orbitals.
donation to px* because the p-donor orbital (pxz*) in the for-
mer is apparently higher in energy than the p-donor (dxz) in the
latter. In contrast, for the low-spin case the filled dxz in the C
form is closer to px* than pxz is in the B form, hence the C con-
figuration is preferred.
An effective p-interaction between porphyrin p-orbitals and
(dxz , dyz) is also expected to occur in Mn(Por)(ImH)(IO4).
Comparison of the results of styrene epoxidation by various
para-substituted Mn(Por) showed the importance of
Por ! Mn p-donation21 in activating periodate (vide supra).
Axial s-donation by nitrogenous ligands, which raises the
metal d2z energy, is expected to be of less importance to the
energies of the dp orbitals of the complexes than are the p-
bonds of the ligand donor atom. However, the observed
co-catalytic activities of s-donor axial ligands may be
explained in terms of their ability to bring Mn(III) closer
toward the center of the porphyrin core plane, making
p-donation from porphyrin into (dxz , dyz) more feasible.
When Fe(TPP)OAc and Cr(TPP)Cl were utilized instead of
Mn(TPP)OAc, very little (5% epoxide yield) or no catalytic
activity, respectively, was observed under similar conditions.
While formation of a six-coordinate [Fe(TPP)(ImH)2]þ3c,23
seems to be the primary reason for the inactivity of the iron
complex, the absence of activity for Cr(TPP)Cl may be related
to p-bonding effects. Assuming the simple diagram to be
appropriate for chromium, then six-coordinate low-spin
Cr(TPP)(ImH)(IO4), (dxy)2 < (dxz , dyz)1, having a single elec-
tron available for p-interactions is expected to be less active
than the corresponding low-spin Mn(Por) with two such elec-
trons. This suggests that the number of electrons in (dxz , dyz)
orbitals is critically important in activating the periodate.
However, it should be noted that if both Cr(TPP)(ImH)(IO4)
and Mn(TPP)(ImH)(IO4) were high-spin with similar (dp)1
electron populations, the simple diagram seems to provide no
clue for their different catalytic behavior. Thus, from all con-
siderations above, it may be concluded that if a low-spin $
high-spin equilibrium existed at all for Mn(Por)(ImH)(IO4),
then the low-spin C form would probably be the most effec-
tive catalytic center for epoxidation.
ꢂ
on the s* of O–IO3 [Fig. 5(a)] and a back-bonding of the
metal-bound oxygen sp2 lone pair to the alkene p* ([Fig.
5(b)]. Both of these interactions can reduce the alkene’s C–C
bond order, facilitating free rotation around this bond, leading
to the cis ! trans isomerization. It is reasonable to presume
that the free rotation occurs prior to the formation of inter-
mediate 3. The epoxide ring closure, in the course of formation
of intermediate 3, requires some major structural changes in
the alkene and also involves strong steric interactions with
the porphyrin ligand. Thus, it is expected to have a high energy
barrier and is presumably the rate-limiting step. This view is
apparently in agreement with our experimental results, the
proposed p-bonding model and the orbital interactions
assumed for ring closure. The observed higher epoxidation rate
of electron-rich alkenes, relative to electron-poor ones, may be
explained in terms of their better p-donation toward s* of the
metal-bound O–I. The influence of strong p-donor nitrogenous
ligands and Mn(Por) with p-donor substituents (Table 3) upon
the rate-limiting step is probably due to their ability to popu-
late the dyz donor orbital. Interaction of dyz as a p-donor with
the sp2 lone pair would destabilize this orbital and make it a
stronger donor toward alkene p*, hence enhancing the rate
of ring closure.
The alternative approach of alkene through the xz plane
toward coordinated periodate, and its interaction with the
While for all axial ligands, s-interaction with the Mn center
is independent of ligand orientation, significant variation in
bonding parameters is found for the metal-imidazole p-inter-
action as a function of rotation.22 Possible orientations of
various nitrogenous axial ligands are a result of competition
between their p-bonding propensity and their steric inter-
actions with the porphyrin core. For the idealized orientations
Scheme 2 Proposed catalytic cycle for epoxidation of alkenes by the
n-Bu4NIO4/Mn(Por)X/ImH system.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 4 0 – 7 4 7
745

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distilled on NaOH and kept over molecular sieves. Tetrabuty-
lammonium periodate (n-Bu4NIO4 ; Merck) was dried under
vacuum and P2O5 . All the alkenes were purchased from com-
mercial vendors and passed through a short column of neutral
alumina immediately prior to use. Dichloromethane (Merck)
was further purified by repeated washing with concentrated
H2SO4 until the aqueous layer was colorless, followed by
washing with water and 5% Na2CO3 solution. Then, it was
dried by CaCl2 and distilled over CaH2 .
Fig. 5 Possible orbital interactions for the idealized stereochemistry
in the intermediate 3: (a) nucleophilic attack of the alkene p-orbital
on the s* orbital of the O–IOꢂ3 bond, (b) back-bonding of the
metal-bound oxygen sp2 lone pair to the alkene p* orbital.
General oxidation procedure
Stock solutions of Mn(Por) catalysts (0.003 M) and nitrogen-
ous bases (0.5 M) in CH2Cl2 were prepared. In a 10 mL round-
bottom flask were added in order: alkene (0.25 mmol),
Mn(Por) (0.003 mmol, 1.0 mL), nitrogenous base (0.03 mmol,
60 mL) and tetrabutylammonium periodate (required amount)
to achieve the desired ratio (see footnotes in Table 1 and 3). In
the case of Mn(TDCPP)OAc, 0.0045 mmol (9.0 mL), of nitro-
genous base was used. The mixture was stirred thoroughly for
the required time at ambient temperature in air. The resulting
solution was directly analyzed by GLC. Similar results were
obtained under Ar. Epoxidations of cis- and trans-stilbenes
were determined by 1H NMR (Bruker Avance DPX 250
MHz spectrometer). All the reactions were run at least in
duplicate. The electronic absorption spectra were recorded in
px* orbital of O–IO3ꢂ, may also be considered. However, this
side attack might seriously be hampered by steric effects due to
the close proximity of alkene and the coordinated periodate,
and also fꢂrom mutual interactions of both alkene and the
bound IO4 with the single phenyl group located in between
them. Also, the electron density in px* from the dp ! px*
bonding interaction (Fig. 4) may inhibit the alkene’s nucleo-
philic attack.
It is notable that cis ! trans isomerization in the epoxida-
tion of cis-stilbene has been taken as evidence for a radical
intermediate.2b,4a,20a However, in accord with our proposed
mechanism and the possible orbital interactions in the forma-
tion of intermediate 3 (Fig. 5), it appears that cis ! trans
isomerization does not necessarily require formation of a
radical species. This seems to be consistent with the very high
selectivities of epoxidation reactions observed with the
Mn(Por)/ImH/n-Bu4NIO4 catalytic system.
CH2Cl2 solutions utilizing
spectrophotometer.
a
MultiSpect-150-Shimadzu
Competition reactions
Competitive catalytic epoxidations of cis- and trans-stilbenes
were performed at ambient temperature. In the one-phase sys-
tem, cis- and trans-stilbenes (1.5 mmol each) were reacted with
n-Bu4NIO4 (0.5 mmol) in the presence of Mn(TMP)OAc
(0.003 mmol) and imidazole (0.03 mmol) in CH2Cl2 (1.0
mL). The solution was stirred for 4 h and the cis/trans epoxide
Conclusions
1
While the steric properties of Mn(Por), alkenes and axial nitro-
genous bases used in this study are of paramount importance
in determining the overall catalytic reaction times and epoxi-
dation yields, clear evidence is provided for: (i) Mn(Por) with
electron-withdrawing substituents on their phenyl groups are
much less effective catalysts than those with electron-donor
substituents; (ii) electron-rich alkenes display higher reactivity
than electron-poor ones; (iii) nitrogen donor axial ligands with
p-donor capability are much more effective co-catalysts than
pure s-donor amines and no definite general correlation exists
between co-catalytic activities and pKa values of nitrogenous
bases; (iv) pyridines with methyl or amino groups in their
2 or 2,6 positions demonstrate exceptionally high co-catalytic
activities in the presence of Mn(TPFPP)OAc, suggesting the
presence of C–Fꢀ ꢀ ꢀH–C and C–Fꢀ ꢀ ꢀH–N hydrogen bonding
in these systems.
ratio was measured by H NMR. Similarly, in the two-phase
system equal amounts of cis- and trans-stilbenes (1.5 mmol
each) were mixed with Mn(TMP)OAc (0.003 mmol), imidazole
(0.03 mmol) and the phase-transfer catalyst n-Bu4NBr (0.024
mmol) in CH2Cl2 (1.0 mL), and reacted with 0.5 mmol of
NaIO4 (in 5 mL H2O). The solvent mixture was stirred for
4 h and the cis/trans epoxide ratio was analyzed by 1H NMR.
For the PhIO system cis- and trans-stilbenes (1.5 mmol each)
were reacted with PhIO (0.5 mmol) in the presence of
Mn(TMP)OAc (0.003 mmol) and imidazole (0.03 mmol) in
CH2Cl2 (1.0 mL). The reaction mixture was stirred for 5 min
at ambient temperature and then quenched by addition of an
aqueous solution of sodium metabisulfite (10%) with stirring.
The analysis was performed by 1H NMR. The reaction was
repeated using a large excess (12.45 mmol) of cis- and trans-
stilbenes (6.225 mmol each) with 0.25 mmol of PhIO without
changing the amounts of the other components.
Experimental
Acknowledgements
Materials
The free base porphyrins: TPPH2 ,26 T(4-OMeP)PH2 ,26 T(4-
We thank the Shiraz University Research Council for support
of this work under project 78-SC-1290-672. The support of
the National Research Council of Iran (NRCI) under grant
number 844 is gratefully acknowledged.
NO2P)PH2 ,27 TDCPPH2 ,28 TMPH2 and TPFPPH2 were
prepared and purified by methods reported previously.
Mn(TPP)OAc, Mn[T(4-OMeP)P]OAc, Mn[T(4-NO2P)P]OAc,
28
,29
Mn(TDCPP)OAc,
Mn(TMP)OAc,
Fe(TPP)OAc
and
Cr(TPP)Cl were obtained using the corresponding
Mn(OAc)2ꢀ4H2O, Fe(OAc)2 or CrCl2 according to the proce-
dure of Adler et al..30 Mn(TPFPP)OAc was synthesized in a
manner similar to that described by Kadish and coworkers.31
Nitrogenous bases were purchased from Merck or Fluka.
BzImH, 4-NH2Py, 2,3-(NH2)2Py, 3,4-(NH2)2Py and 2,6-
(NH2)2Py were recrystallized before use.32 Pyridine was
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T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 4 0 – 7 4 7
747