Co-catalytic effects of nitrogen donors on the epoxidation of cyclooctene with tetra-n-butylammonium hydrogen monopersulfate in the presence of manganese(III)tetraarylporphyrins: A comparative study

Article abstract of DOI:10.1016/j.molcata.2007.03.033

The epoxidation of cyclooctene by tetra-n-butylammonium hydrogen monopersulfate was performed in the presence of four different manganese(III)tetraarylporphyrins as catalysts and a number of nitrogen donors as co-catalysts under a fixed co-catalyst/catalyst ratio (100:1). It was observed that the σ-bonding abilities of the nitrogen donors, as related to their pK_a (BH⁺) values, were not a reliable factor for determining their co-catalytic activities, unless there were no π-interactions and/or no significant involvement of steric hindrance. On the other hand, the π-donating ability of the nitrogen donors was found to be of importance in defining their role as co-catalysts. The strong π-donor N-H imidazoles displayed co-catalytic activities greater than strong σ-donor amines and weak π-donor pyridines in the presence of manganese(III)tetraphenylporphyrin acetate (MnTPP(OAc)). In the case of hindered manganese(III)tetramesitylporphyrin acetate and manganese(III)tetrakis(2,6-dichlorophenyl)porphyrin acetate steric hindrance of the nitrogen donors played a dominant role, and the least hindered imidazole (ImH) was the best co-catalyst. When manganese(III)tetrakis(pentafluorophenyl)porphyrin acetate(MnTPFPP(OAc)) was employed as catalyst then pyridines, in general, demonstrated a higher co-catalytic activities than imidazoles. The same trend was also observed under various co-catalyst/catalyst ratios (1-200), and pyridines were always more effective co-catalysts than imidazoles with MnTPFPP(OAc), whereas for MnTPP(OAc) the order for co-catalytic activities of these nitrogen donors was inverted. Also, in the presence of electron deficient manganese(III)tetrakis(4-nitrophenyl)porphyrin acetate imidazoles acted generally as more effective co-catalysts than pyridines, at co-catalyst/catalyst ratios < 100. It is proposed that the varied attractive C-H?F-C interactions between the ortho-C-F σ* orbitals of the pentafluorophenyl groups in MnTPFPP(OAc) complex and C-H σ bonds adjacent to the nitrogen donor sites of pyridines or imidazoles could be responsible for the observed differences. The UV-vis spectra of the catalytic systems containing pyridine and ImH in association with MnTPP(OAc) and MnTPFPP(OAc) were examined for the active oxygen intermediates.

Full text of DOI:10.1016/j.molcata.2007.03.033

Journal of Molecular Catalysis A: Chemical 272 (2007) 191–197
Co-catalytic effects of nitrogen donors on the epoxidation
of cyclooctene with tetra-n-butylammonium hydrogen
monopersulfate in the presence of manganese(III)
tetraarylporphyrins: A comparative study
Daryoush Mohajer∗, Leila Sadeghian
Department of Chemistry, Shiraz University, Shiraz 71454, Iran
Received 24 November 2006; accepted 2 March 2007
Available online 16 March 2007
Abstract
The epoxidation of cyclooctene by tetra-n-butylammonium hydrogen monopersulfate was performed in the presence of four different man-
ganese(III)tetraarylporphyrins as catalysts and a number of nitrogen donors as co-catalysts under a fixed co-catalyst/catalyst ratio (100:1). It was
observed that the ␴-bonding abilities of the nitrogen donors, as related to their pKa (BH+) values, were not a reliable factor for determining their
co-catalytic activities, unless there were no ␲-interactions and/or no significant involvement of steric hindrance. On the other hand, the ␲-donating
ability of the nitrogen donors was found to be of importance in defining their role as co-catalysts. The strong ␲-donor N–H imidazoles displayed co-
catalytic activities greater than strong ␴-donor amines and weak ␲-donor pyridines in the presence of manganese(III)tetraphenylporphyrin acetate
(MnTPP(OAc)). In the case of hindered manganese(III)tetramesitylporphyrin acetate and manganese(III)tetrakis(2,6-dichlorophenyl)porphyrin
acetate steric hindrance of the nitrogen donors played a dominant role, and the least hindered imidazole (ImH) was the best co-catalyst. When
manganese(III)tetrakis(pentafluorophenyl)porphyrin acetate(MnTPFPP(OAc)) was employed as catalyst then pyridines, in general, demonstrated
a higher co-catalytic activities than imidazoles. The same trend was also observed under various co-catalyst/catalyst ratios (1–200), and pyridines
were always more effective co-catalysts than imidazoles with MnTPFPP(OAc), whereas for MnTPP(OAc) the order for co-catalytic activities of
these nitrogen donors was inverted. Also, in the presence of electron deficient manganese(III)tetrakis(4-nitrophenyl)porphyrin acetate imidazoles
acted generally as more effective co-catalysts than pyridines, at co-catalyst/catalyst ratios < 100. It is proposed that the varied attractive C–H· · ·F–C
interactions between the ortho-C–F ␴* orbitals of the pentafluorophenyl groups in MnTPFPP(OAc) complex and C–H ␴ bonds adjacent to the
nitrogen donor sites of pyridines or imidazoles could be responsible for the observed differences. The UV–vis spectra of the catalytic systems
containing pyridine and ImH in association with MnTPP(OAc) and MnTPFPP(OAc) were examined for the active oxygen intermediates.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Manganese porphyrins; Nitrogen donors; Co-catalyst; Epoxidation; Tetra-n-butylammonium hydrogen monopersulfate
1. Introduction
the role of the axial nitrogen donors in oxygenation reactions,
mediated by metalloporphyrins, a detailed analysis of the influ-
ences of the stereoelectronic properties of the nitrogen donors
and the metalloprorphyrins is required.
In this work co-catalytic activities of a selected series of
nitrogen donors, in the presence of five different manganese
porphyrins (MnPor) (Fig. 1) as catalysts, in the epoxidation
of cyclooctene with n-Bu4NHSO5 were studied. The effects of
various co-catalyst/catalyst ratio upon co-catalytic properties of
nitrogen donors were also investigated. Particular attention has
been given to inverted order of co-catalytic activities of pyridines
versus imidazoles in the presence of MnTPP(OAc) as compared
Nitrogen donors have long been used as axial ligands to
mimic the function of axial cysteine thiolate or histidine imi-
dazole in natural metalloproteins [1–8]. Much improvements
have been achieved in catalytic properties of synthetic metal-
loporphyrins for alkene epoxidation from the employment of
pyridines and imidazoles as co-catalyst [9–21]. To understand
∗
Corresponding author. Tel.: +98 711 2284822; fax: +98 711 2280926.
E-mail address: dmohajer@chem.susc.ac.ir (D. Mohajer).
1381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2007.03.033

192
D. Mohajer, L. Sadeghian / Journal of Molecular Catalysis A: Chemical 272 (2007) 191–197
sulfate (0.19 mmol) was then added to the reaction solutions,
at 25 ◦C. The reaction solutions were analyzed immediately by
GLC after stirring for 2 min.
The electronic absorption spectra were recorded in CH2Cl2
solutions utilizing a MultiSpec-1501 Shimadzu spectropho-
tometer.
3. Results and discussion
3.1. Co-catalytic effects of nitrogen donors
The results of epoxidation of cyclooctene by n-Bu4NHSO5
in the presence of four differing MnPor as catalysts, in asso-
ciation with a number of nitrogen donors as co-catalysts,
under similar conditions, are given in Table 1. The nitrogen
donors may be grouped into three classes, the strong pure
␴-donor amines (pKa, 10.75–11.123), the strong ␲-donor imi-
dazoles (pKa, 5.532–7.86), and the weak ␲-donor pyridines
(pKa, 1.86–6.65). The yields of the epoxidation of cyclooctene
presumably reflect the co-catalytic activities of the nitroge-
nous bases, in the presence of the MnPor catalysts. The
co-catalytic activities of the nitrogenous bases depend upon
not only their stereoelectronic properties, but also they relate
to the nature of the MnPor catalysts [31]. The general order of
the co-catalytic activities of the various classes of the nitrogen
donorsforMnTPP(OAc), MnTMP(OAc), and MnTDCPP(OAc)
(imidazoles > amines > pyridines) suggests that the imidazoles
are the most effective co-catalysts, whereas in the case of
MnTPFPP(OAc), the general order of the co-catalytic activ-
ities of the nitrogen donors (pyridines > amines > imidazoles)
indicates that pyridines are the best co-catalysts.
Fig. 1. Manganese(III)tetraarylporphyrins employed in this study.
to MnTPFPP(OAc). Attempts were made to explain the var-
ied co-catalytic activities of nitrogen donors in terms of their
stereoelectronic properties and those of the MnPor complexes.
2. Experimental
2.1. Material
The free base porphyrins TPPH2 [22], T(4-NO2P)PH2
[23], TDCPPH2 [24], TMPH2 [24] and TPFPPH2 [25] were
prepared and purified as reported previously. MnTPP(OAc),
MnT(4-NO2P)P(OAc), MnTDCPP(OAc), MnTMP(OAc) were
obtained using Mn(OAc)2·4H2O according to the procedure of
Adler et al. [26] MnTPFPP(OAc) was synthesized in a manner
similar to that described by Kadish et al. [27]. Nitrogenous bases
were purchased from Merck or Fluka. BzImH was recrystallized
before use [28]. Pyridine, 2-MePy and 2,6-Me2Py were distilled
on KOH and kept over molecular sieves.
n-Bu4NHSO5 was prepared by adding potassium monoper-
sulfate (2.0 g, 6.5 mmol) to a solution of tetra-n-butylammonium
hydrogen sulfate (2.0 g, 5.9 mmol) in water (20 mL) [9,29,30].
The solution was extracted with CH2Cl2 (40 mL) and the organic
phase was dried over sodium sulfate and filtered. After evap-
oration of the solvent, the remaining paste was washed with
hexane (10 mL) and dried under vacuum. The freshly prepared
n-Bu4NHSO5 was a much stronger oxidant than commercially
available samples. Since the oxidizing ability of n-Bu4NHSO5
samples would reduce with time, in order to obtain reproducible
results, the freshly prepared oxidant was refrigerated and used
within 3 days. Caution: n-Bu4NHSO5 should be considered as
a potential explosive.
3.1.1. Amines
The order of the co-catalytic activities of the amines in the
presence of the MnPor catalysts is piperidine > Et2NH > Et3N,
except for MnTPFPP(OAc) (Table 1). This order is in complete
accord with the order of increasing basicities and decreasing
steric hindrances of the amines. The least hindered piperi-
dine, which is also the strongest base (pKa = 11.123), is the
best co-catalyst. Indeed, piperidine is two times stronger than
Et2NH, and four times stronger than Et3N, as co-catalyst, in
the presence of MnTPP(OAc). The order of the co-catalytic
effects of the amines, in combination with MnTMP(OAc), is the
same as that of MnTPP(OAc). In the case of MnTDCPP(OAc),
Et2NH and Et3N show similar activities. On the other hand,
for MnTPFPP(OAc) the co-catalytic activities of piperidine and
Et2NH are similar and much greater than that of Et3N.
3.1.2. Imidazoles
2.2. General oxidation procedure
The imidazoles are generally more efficient co-catalysts than
the amines and the pyridines in the presence of MnTPP(OAc),
MnTMP(OAc), and MnTDCPP(OAc) (Table 1). The order
of the co-catalytic activities of the imidazoles in association
with MnTPP(OAc), ImH > 2-MeImH > BzImH > 2-EtImH ꢀ
1-MeIm, and in combination with MnTMP(OAc) and
MnTDCPP(OAc), ImH ꢀ 2-MeImH > 2-EtImH ꢀ BzImH ꢀ
1-MeIm, are rather closely related, except for the relative order
Stock solutions of MnPor catalysts (0.003 M) and nitro-
gen donors (0.5 M) (except BzImH (0.0625 M)) were prepared
in CH2Cl2. In a 10 mL round-bottom flask the following
were added in the order: MnPor (0.001 mmol, 0.3 mL), nitro-
gen donor (x mmol, as required), cyclooctene (0.1 mmol) and
1.5 mL CH2Cl2. Tetra-n-butylammonium hydrogen monoper-

D. Mohajer, L. Sadeghian / Journal of Molecular Catalysis A: Chemical 272 (2007) 191–197
193
Table 1
Epoxidation yield (%) (co-catalytic activity) for nitrogen donors in the epoxidation of cyclooctene with MnPor catalystsa,b,c
Nitrogen donor
pKa (BH+)d
Epoxidation (%)
MnTPP(OAc)
MnTMP(OAc)
MnTDCPP(OAc)
MnTPFPP(OAc)
Piperidine
Et2NH
Et3N
11.123
11.02
10.75
6.953
7.86e
7.86f
5.532
6.95
40
24
10
87
81
69
75
8
35
47
10
7
4
2
1
4
3
2
4
1
3
2
1
1
1
42
20
6
70
30
20
6
1
4
10
<1
2
3
1
45
4
3
55
42
36
9
5
9
17
2
4
4
1
1
2
4
2
1
1
1
2
0.5
1
0.8
0.7
74
77
48
42
52
35
65
11
90
91
22
93
100
6
2
3
3
2
2
4
2
1
3
3
1
4
0
1
1
ImH
2
3
2
2-MeImH
2-EtImH
BzImH
1-MeIm
Py
4-MePy
4-t-BuPy
2-MePy
2,6-Me2Py
4-CNPy
None
1
0.3
1
1
5.25
6.02
5.99g
5.97
6.65
1.86g
–
0.5
0.3
0.3
9
2
<1
2
1
5
2
2
0.4
Trace
3
Et2NH, diethylamine; Et3N, triethylamine; ImH, imidazole; 2-MeImH, 2-methylimidazole; 2-EtImH, 2-ethylimidazole; BzImH, benzimidazole; 1-MeIm, 1-
methylimidazole; Py, pyridine; 4-MePy, 4-methylpyridine; 4-t-BuPy, 4-tert-butylpyridine; 2-MePy, 2-methylpyridine; 2,6-Me2Py, 2,6-dimethylpyridine; 4-CNPy,
4-cyanopyridine.
a
MnPor/nitrogen donor/cyclooctene/oxidant molar ratio is 1/100/100/190 with [MnPor] = 5 × 10−4 M, in CH2Cl2 at 25 1 ◦C.
b
The reactions proceeded with 100% selectivity.
c
The epoxidation yield (%) were measured relative to the starting cyclooctene. All the reactions were run at least in triplicate, and the data represent an average
of these reactions.
d
pKa values obtained from Ref. [40].
Ref. [41].
Very little change in base strength accompanying the change of a methyl substituent to ethyl was observed (see Ref. [42]).
See Ref. [43].
e
f
g
of the BzImH and 2-EtImH. The least hindered ImH is the
best co-catalyst in the presence of all the MnPor catalysts, with
the exception of MnTPFPP(OAc). In fact, the order of the co-
catalytic activities of the imidazoles primarily reflect the steric
hindrances of both the nitrogen donors and the MnPor catalysts.
In the case of MnTMP(OAc) and MnTDCPP(OAc) the steric
effects are quite pronounced, and the bulky BzImH is the least
effective co-catalyst among the N–H imidazoles. For the most
bulkyMnTMP(OAc)catalyst, theco-catalyticeffectsoftheN–H
imidazoles were widely separated (70 2 to 6 1), whereas for
MnTPP(OAc)theco-catalyticactivitiesofN–Himidazoleswere
much closer to each other (87 4 to 69 2).
The good ␲-donating capability of the N–H imidazoles seems
to play an important role in determining their greater co-catalytic
activities than the strong ␴-donor amines [31]. A particularly
interesting case is the higher co-catalytic activity of BzImH
than the stronger base amines and pyridines, in the presence
of MnTPP(OAc). This is presumably due to the more extended
␲-system of BzImH, and its greater ␲-donating ability toward
the Mn center of the catalyst.
Themuchlowerco-catalyticactivityof1-MeImthantheN–H
imidazoles in the presence of all the MnPor catalysts clearly
demonstrates the significance of the occurrence of N–H· · ·B
(B = nitrogen donor or anionic species) hydrogen bondings in
the case of the N–H imidazoles. Such interactions in the reaction
solution are believed to increase substantially the donor abilities
of the N–H imidazoles toward the MnPor catalysts [32–34].
The order of the co-catalytic activities of the imidazoles in the
presence of MnTPFPP(OAc), BzImH > 2-MeImH > ImH > 2-
EtImH ꢀ 1-MeIm is sharply different from those observed for
the other MnPor catalysts. This “unusual” order seems not to
correspond to the simple stereoelectronic properties of the imi-
dazole and/or MnTPFPP(OAc) complex. It is noteworthy that
both BzImH and 2-MeImH are more effective co-catalysts than
the less hindered ImH.
3.1.3. Pyridines
The co-catalytic behavior of the pyridines, generally, corre-
lates well with their intrinsic basicities, and steric properties,
in association with all the MnPor, except MnTPFPP(OAc). 4-
MePy (pKa = 6.02) is the best co-catalyst followed by the weaker
base Py (pKa = 5.25) in the presence of all the MnPor cata-
lysts, except MnTPFPP(OAc). The bulky 4-t-Bu substituent of
4-t-BuPy (pKa = 5.99) dramatically decreased its co-catalytic
activity, as compared to Py and 4-MePy, in the presence of all the
MnPor, including MnTPFPP(OAc). Also, in the case of 2-MePy
and 2,6-Me2Py the steric hindrance(s) of the methyl group(s),
adjacent to the nitrogen donor sites, significantly decreased their
co-catalytic activities, in comparison with Py, in the presence of
MnTPP(OAc), MnTMP(OAc), and MnTDCPP(OAc). In con-
trast, when MnTPFPP(OAc) was used, 2-MePy and 2,6-Me2Py
acted as excellent co-catalysts. The 4-CNPy, with the lowest
basicity, containing electron withdrawing CN group, was the
weakest co-catalyst.
Pyridines with pKa values similar to or less than those of imi-
dazoles, and with ring sizes larger than imidazoles, are generally
much weaker co-catalysts than imidazoles, in the presence of the
MnPor catalysts, except for MnTPFPP(OAc) (Table 1). In par-

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D. Mohajer, L. Sadeghian / Journal of Molecular Catalysis A: Chemical 272 (2007) 191–197
ities of imidazoles versus pyridines observed in the presence
of MnTPFPP(OAc) as compared to those of the other MnPor
catalysts raises some serious questions concerning their causes.
Here we examine how the co-catalyst/catalyst ratio can possibly
affect the relative order of a number of nitrogen donors as co-
catalysts, in the presence of MnTPP(OAc) and MnTPFPP(OAc)
complexes.
3.2. The effect of co-catalyst/catalyst ratio
The interaction of MnPor catalysts with nitrogen donors
(D) can very rapidly lead to the formation of both catalyti-
cally active 1:1 DMnPor and catalytically inactive 2:1 D2MnPor
species, with the formation constants K1 and K2, respectively
[16]. Both K1 and K2 are related to the nature of MnPor and D
[16]. Large steric hindrance of the species involved would lead
to small K1 and K2, whereas high electrophilicity of MnPor
and also high electron donating ability of D result in large
K1 and K2, as expected. The concentration of the 1:1 DMn-
Por complex, defining the co-catalytic activity of the nitrogen
donors, is presumably related to the nitrogen donor/MnPor ratio.
The co-catalytic activities of six different nitrogen donors in
the presence of MnTPP(OAc) and MnTPFPP(OAc), at various
co-catalyst/catalyst ratios, are presented in Table 2. The gen-
eral order of the co-catalytic activities of the nitrogen donors
are the same, for each of the two MnPor catalysts, at all the
co-catalyst/catalyst ratios (10–200) (Fig. 3). The exceptions cor-
respond to 2-MeImH in combination with MnTPP(OAc) and
2,6-Me2Py with MnTPFPP(OAc). The order of the co-catalytic
activities of some of the nitrogen donors are very close and not
distinguishable at co-catalyst/catalyst ratios ≤ 5 (Fig. 3b).
Fig. 2. Scatter diagrams for the co-catalytic activities of the nitrogen donors in
the presence of (a) MnTPP(OAc) and (b) MnTPFPP(OAc).
ticular, the lower co-catalytic activities of the pyridines than the
imidazoles are more pronounced when the bulky MnTMP(OAc)
and MnTDCPP(OAc) are used.
Fig. 2a and b represents the scatter diagrams for the co-
catalytic properties of the different classes of the nitrogen donors
in the presence of MnTPP(OAc) and MnTPFPP(OAc). They
clearly illustrate that the relative order of the co-catalytic activ-
ities of the pyridines and the imidazoles are reversed in the
presence of these two MnPor catalysts. The scatter diagrams for
MnTMP(OAc) and MnTDCPP(OAc) (not shown) resemble that
of MnTPP(OAc). The inverted order of the co-catalytic activ-
3.2.1. MnTPP(OAc)
Imidazoles were better co-catalysts than pyridines, in asso-
ciation with MnTPP(OAc) at all co-catalyst/catalyst ratios
(10–200) (Fig. 3a). This is primarily due to the greater ␲-
donating ability of the former than the latter ones toward the
Mn center. ImH demonstrates the highest co-catalytic activi-
ties among all the nitrogen donors, at all the co-catalyst/catalyst
ratios. However, its co-catalytic activity decreased at co-
Table 2
Cyclooctene epoxidation yield (%) (co-catalytic activity) for nitrogen donors in association with MnTPP(OAc) and MnTPFPP(OAc) catalysts under different
co-catalyst/catalyst ratiosa,b
Nitrogen donor
Co-catalyst/catalyst ratio
1
5
10
25
50
100
200
ImH
(8)c
(8)
(14)
(12)
(20)
(10)
(12)
(16)
(27)
(44)
(88)
(28)
36(22)
32(30)
20(41)
6(75)
0.7 (96)
0.8 (70)
62(46)
56(61)
39(68)
16(94)
1.3 (98)
3(86)
85(70)
82(74)
67(77)
28(98)
4(98)
88(43)
81(51)
75(65)
38(91)
7(93)
77(29)
58(35)
71(47)
47(76)
8(85)
2-MeImH
BzImH
Py
2-MePy
2,6-Me2MePy
6(94)
10(100)
14(100)
a
The molar ratios for MnPor/nitrogen donor/cyclooctene/oxidant were 1/x/100/190 with x = 10, 25, 50, 100, 200 for MnTPP(OAc) and x = 1, 5, 10, 25, 50, 100,
200 for MnTPFPP(OAc) and [MnPor] = 5 × 10−4 M, in CH2Cl2 at 25 1 ◦C.
b
The epoxidation yield (%) were measured relative to the starting cyclooctene. All the reactions were run at least in triplicate, and the data represent an average
of these reactions, with 10–20%. The largest uncertainty ( 20%) corresponds to some of the lower epoxidation yield (%), i.e., <10.
c
Data inside the parentheses are for MnTPFPP(OAc).

D. Mohajer, L. Sadeghian / Journal of Molecular Catalysis A: Chemical 272 (2007) 191–197
195
displayed co-catalytic activities greater than imidazoles, at all
co-catalyst/catalyst ratios (10–200), contrasting their behavior
in the presence of MnTPP(OAc). Even at the low ratio of 5,
where formation of [D2MnTPFPP]OAc complex is of little sig-
nificance, pyridines seemed to be generally better co-catalysts
than imidazoles (Table 2).
Hindered 2,6-Me2Py showed a greater co-catalytic activity
than the less hindered 2-MePy, and Py at co-catalyst/catalyst
ratios > ∼100. The fixed co-catalytic activity of 2,6-Me2Py
at co-catalyst/catalyst ratios > ∼100 may indicate the inabil-
ity of 2,6-Me2Py to produce the inactive D2MnPor species. At
co-catalyst/catalyst ratios < ∼60 the co-catalytic activity of 2,6-
Me2Py was either close to or less than those of Py and 2-MePy.
On the other hand, Py displayed co-catalytic activities similar
to or less than that of 2-MePy at different co-catalyst/catalyst
ratios (1–200), and its co-catalytic effect never exceeded that
of 2-MePy. In addition, pyridines demonstrated a much sharper
increase in their co-catalytic activities than imidazoles at co-
catalyst/catalyst ratios < ∼20. This suggests that pyridines are
more efficient than imidazoles in forming the catalytically
active DMnTPFPP(OAc), and/or less effective in producing
D2MnTPFPP(OAc).
The order of co-catalytic activities of imidazoles in combi-
nation with MnTPFPP(OAc) showed no changes over the entire
range of the co-catalyst/catalyst ratio. BzImH was the best co-
catalyst followed by hindered 2-MeImH, and the least hindered
ImH acted as the weakest co-catalyst.
Fig. 3. Co-catalytic activities of nitrogen donors in the presence of (a)
MnTPP(OAc) and (b) MnTPFPP(OAc), as a function of different co-
catalyst/catalyst ratio.
The larger size and greater electronegativity of F than H,
should lead to more hindrance and electrophilicity at the Mn cen-
ter in MnTPFPP(OAc) than MnTPP(OAc). These effects should
cause the less hindered and relatively stronger nitrogen donor
ImH to act more efficiently than 2,6-Me2Py as co-catalyst, in the
presence of MnTPFPP(OAc). However, this was not the case at
any co-catalysts/catalyst ratio.
catalyst/catalyst ratios > ∼100. The co-catalytic activity of
2-MeImH, which is slightly less than that of ImH at co-
catalyst/catalyst ratios < ∼50, decreased more rapidly than ImH,
at the ratios > 50. This may indicate that 2-MeImH is either
more effective than ImH in forming the catalytically inactive
2:1 D2MnTPP(OAc) complex, and/or it acts better than ImH, as
a substrate, in a competitive oxidation with cyclooctene [35], at
high concentrations. The co-catalytic activity of BzImH reached
to its maximum value at co-catalyst/catalyst ratio of ∼100,
and then remained practically unchanged at the higher ratios
(Fig. 3a). This suggests that the concentration of the catalyti-
cally active 1:1 DMnTPP(OAc) complex is virtually unchanged
under these conditions, and BzImH is incapable of effectively
forming the catalytically inactive 2:1 D2MnTPP(OAc) species.
Among the pyridines, Py is a better co-catalyst than hindered
2,6-Me2Py and 2-MePy, at almost all the co-catalyst/catalyst
ratios, perhaps reflecting the influence of the steric effects of
the latter ones. However, 2,6-Me2Py is a slightly better co-
catalyst than 2-MePy. In this case, probably the greater basicity
of 2,6-Me2Py has taken precedence over its steric hindrance.
The co-catalytic activities of the pyridines increased continu-
ously as the co-catalyst/catalyst ratios increased, indicating that
the concentration of the catalytically active 1:1 DMnTPP(OAc)
species was always increasing.
3.2.3. MnT(4-NO2P)P(OAc)
To examine how the electrophilicity at the Mn center of
MnPor catalysts can possibly affect the order of the co-catalytic
activities of the nitrogen donors, we used MnT(4-NO2P)P(OAc)
as catalyst in the epoxidation of cyclooctene (Table 3). The
steric hindrance at the Mn center for both MnTPP(OAc) and
MnT(4-NO2P)P(OAc) are very similar. However, the strong ␲-
acceptor 4-NO2 groups in MnT(4-NO2P)P(OAc) can induce a
large electrophilicity at its Mn center. It was observed that, simi-
lar to MnTPP(OAc) catalyst, imidazoles were better co-catalysts
than pyridines in the presence of MnT(4-NO2P)P(OAc), partic-
ularly, at low co-catalyst/catalyst ratios (<50) (Fig. 4). Thus, one
may suspect that the higher electrophilicity of the Mn center in
MnTPFPP(OAc) than MnTPP(OAc) may not be the cause of the
inverted order of the co-catalytic activities of the imidazoles as
compared to the pyridines.
The excellent co-catalytic activity of hindered 2,6-Me2Py
and 2-MePy in the presence of MnTPFPP(OAc) versus their
very poor behavior as co-catalysts in the presence of less hin-
dered MnTPP(OAc) and MnT(4-NO2P)P(OAc) suggests that
some attractive interactions may be operative between the C–H
bonds of their methyl substituents and the ortho-C–F groups
3.2.2. MnTPFPP(OAc)
Fig. 3b illustrates how the co-catalytic properties of the nitro-
gen donors can change by enhancing the co-catalyst/catalyst
ratio, in the presence of MnTPFPP(OAc). Pyridines clearly

196
D. Mohajer, L. Sadeghian / Journal of Molecular Catalysis A: Chemical 272 (2007) 191–197
Table 3
Epoxidation yield (%) (co-catalytic activity) for nitrogen donors in the epox-
idation of cyclooctene with MnT(4-NO2P)P(OAc) catalyst under various
co-catalyst/catalyst ratiosa,b
Nitrogen donor
Co-catalyst/catalyst ratio
10
25
50
100
200
ImH
28
21
50
19
5
54
48
81
31
9
59
55
85
52
16
28
45
40
75
67
25
34
31
27
59
80
31
40
2-MeImH
BzImH
Py
2-MePy
2,6-Me2Py
13
19
a
The molar ratios for MnT(4-NO2P)P(OAc)/nitrogen donor/cyclooctene/
oxidant were 1/x/100/190 with x = 10, 25, 50, 100, 200 and [MnPor] =
5 × 10−4 M, in CH2Cl2 at 25 1 ◦C.
b
Fig. 5. The UV–vis spectra represent the conversion of (I) Mn(TPP)OAc, Soret,
λmax = 478 nm to an Mn-oxo species (II) (Soret, λmax = 406 nm) immediately
after adding both ImH (100 equiv.) and then (n-Bu4N)HSO5 (190 equiv.) to
Mn(TPP)OAc solution (∼10−6 M) in CH2Cl2, at 25 1 ◦C.
The epoxidation yield (%) was measured relative to the starting cyclooctene.
All the reactions were run at least in triplicate, and the data represent an average
of these reactions, with 10–20%. The largest uncertainty ( 20%) corresponds
to some of the lower epoxidation yield (%), i.e., <10.
of MnTPFPP(OAc) [31,36]. This interaction is expected to
lead to effective coordination of 2,6-Me2Py or 2-MePy to
MnTPFPP(OAc), causing their very large co-catalytic activi-
ties. It has been demonstrated that electron delocalization from
C–H ␴ bonds into C–F ␴* orbitals may account for some
unrelated phenomena [37]. Accordingly, it may be that the low-
lying ortho-C–F ␴* orbitals of the pentafluoro phenyl groups in
MnTPFPP(OAc) may indeed act as electron acceptors toward
the C–H ␴ bonds of the methyl substituents of 2,6-Me2Py. The
proposed C–H· · ·F–C interactions may actually occur between
any C–H ␴ bonds of the nitrogen donors adjacent to their
donor sites and the ortho-C–F ␴* orbitals of MnTPFPP(OAc).
However, it seems that C–H (sp3) ␴ bonds of methyl sub-
stituents in the case of 2,6-Me2Py, 2-MePy and 2-MeImH can
have better contacts and stronger donor ability than C–H (sp2)
␴ bonds of Py and ImH rings, toward the empty ortho-C–F
␴* orbitals of MnTPFPP(OAc). Thus, the former may coordi-
nate more effectively than the latter ones to MnTPFPP(OAc)
and act more efficiently as co-catalysts, as it is observed.
Absence of such interactions in the case of MnTPP(OAc) and
MnT(4-NO2P)P(OAc) would make the hindered 2,6-Me2Py an
extremely weak co-catalyst.
3.3. Active oxidant
UV–vis spectra show that addition of n-Bu4NHSO5
(190 equiv.) to a solution of MnTPP(OAc) (∼10−6 M) (Soret,
λmax = 478 nm), in CH2Cl2 (2 mL), containing ImH (100 equiv.),
in the absence or presence of 100 equiv. cyclooctene, yield
an intense Soret band, λmax = 406 nm, presumably due to the
formation of MnTPP(ImH)(O) species [31,38], at room tem-
perature (Fig. 5). On the other hand, when electron deficient
MnTPFPP(OAc) was employed as catalyst, a very unstable
Mn-oxo (Soret, λmax = 428 nm) [39] was formed transiently
and then disappeared rapidly (<15 s), giving a spectrum simi-
lar to that of MnTPFPP(OAc) (Fig. 6). This suggests that, in
the case of MnTPP(OAc), the active oxidizing species may
involve an Mn-oxo compound, whereas for MnTPFPP(OAc),
the active oxidant is perhaps predominately, the six coordi-
nate (ImH)MnTPFPP(HSO5) complex [29]. The employment
of weak ␲-donor Py as co-catalyst, under similar conditions
Fig. 6. The UV–vis spectra display the formation of an Mn-oxo species (II)
(Soret, λmax = 428 nm), withveryshortlifetime(<15 s)immediatelyafteradding
both ImH (100 equiv.) and then (n-Bu4N)HSO5 (190 equiv.) to Mn(TPFPP)OAc
solution (∼10−6 M) in CH2Cl2, Soret, λmax = 474 nm, at 25 1 ◦C.
Fig. 4. Co-catalytic activities of nitrogen donors in the presence of MnT(4-
NO2P)P(OAc) under different co-catalyst/catalyst ratio.

D. Mohajer, L. Sadeghian / Journal of Molecular Catalysis A: Chemical 272 (2007) 191–197
197
as those for ImH, yielded no Mn-oxo species, and the spec-
trum of both MnTPP(OAc) and MnTPFPP(OAc) displayed no
changes, indicating that the active oxygen intermediate is prob-
ably PyMnPor(HSO5) complex. It seems that formation of an
Mn-oxo species requires both a strong ␲-donor axial ligand, and
also a non-electron deficient MnPor.
[8] T.J. McMurry, J.T. Groves, in: P. Ortiz de Montellano (Ed.), Cytochrome
P-450: Mechanism and Biochemistry, Plenum Press, New York, 1986
(Chapter 1).
[9] D. Mohajer, A. Rezaeifard, Tetrahedron Lett. 43 (2002) 1881.
[10] J.P. Collman, A.S. Chien, T.A. Eberspacher, M. Zhong, J.I. Brauman, Inorg.
Chem. 39 (2000) 4625.
[11] E. Baciocchi, T. Boschi, L. Cassioli, C. Galli, L. Jaquinod, A. Lapi, R.
Paolesse, K.M. Smith, P. Tagliatesta, Eur. J. Org. Chem. (1999) 3281.
[12] Z. Gross, S. Ini, J. Org. Chem. 62 (1997) 5514.
[13] D. Mohajer, S. Tangestaninejad, Tetrahedron Lett. 35 (1994) 945.
[14] F. Montanari, Pure Appl. Chem. 66 (1994) 1519.
[15] J.P. Collman, J.I. Brauman, J.P. Fitzgerald, P.D. Hampton, Y. Naruta, T.
Michida, Bull. Chem. Soc. Jpn. 61 (1988) 47.
[16] B. Meunier, M.-E. Carvalho, O. Bortolini, M. Momenteau, Inorg. Chem.
27 (1988) 161.
[17] P.N. Balasubramanian, A. Sinha, T.C. Bruice, J. Am. Chem. Soc. 109(1987)
1456.
[18] L.-C. Yuan, T.C. Bruice, J. Am. Chem. Soc. 108 (1986) 1643.
[19] O. Bortolini, M. Momenteau, B. Meunier, Tetrahedron Lett. 25 (1984)
5773.
[20] J.A.S.J. Razenberg, R.J.M. Nolte, W. Drenth, Tetrahedron Lett. 25 (1984)
789.
[21] E. Guilmet, B. Meunier, Tetrahedron Lett. 23 (1982) 2449.
[22] A.D. Adler, F.R. Longo, J.D. Finarelli, J. Goldmacher, J. Assour, L. Kor-
sakoff, J. Org. Chem. 32 (1967) 476.
[23] A. Bettelheim, B.A. White, S.A. Raybuck, R.W. Murray, Inorg. Chem. 26
(1987) 1009.
[24] P. Hoffmann, A. Robert, B. Meunier, Bull. Soc. Chim. Fr. 129 (1992) 85.
[25] J.S. Lindsey, I.C. Schreiman, H.C. Hsu, P.C. Kearney, A.M. Marguerettaz,
J. Org. Chem. 52 (1987) 827.
[26] A.D. Adler, F.R. Longo, F. Kampas, J. Kim, J. Inorg. Nucl. Chem. 32 (1970)
2443.
4. Conclusions
It was observed that the orders of the co-catalytic effects of
the nitrogen donors were critically dependent upon the steric
hindrances and electronic structures of both the nitrogen donors
and MnPor catalysts. No direct general correspondence was
found between the co-catalytic effects of the nitrogen donors
and their pKa (BH+) values. While the strong ␲-donating N–H
imidazoles displayed higher co-catalytic activities than the weak
␲-donor pyridines, in the presence of MnTPP(OAc), an inverted
order was observed for MnTPFPP(OAc) complex. This behavior
might be related to the differences in C–H· · ·F–C contacts and
interactions between the ortho-C–F bonds of MnTPFPP(OAc)
and the C–H ␴ bonds adjacent to the nitrogen donors sites in
pyridines as compared to imidazoles. Accordingly, it seems that
one has to be very cautions about interpreting the experimen-
tal results concerning the interaction of MnTPFPP(OAc) with
any nitrogen donor containing potential C–H ␴ bond donors.
Finally, it is noteworthy that the order of the co-catalytic activi-
ties of nitrogen donors could also be greatly influenced by their
relative capabilities for the formation of various DMnPor (K1)
and D2MnPor (K2) species.
[27] K.M. Kadish, C. Araullo-McAdams, B.C. Han, M.M. Franzen, J. Am.
Chem. Soc. 112 (1990) 8364.
[28] D.D. Perrin, W.L.F. Armarego, D.R. Perrin, Purification of Laboratory
Chemicals, 2nd ed., Pergamon Press, Elmsford, NY, 1980.
[29] D. Mohajer, Z. Solati, Tetrahedron Lett. 47 (2006) 7007.
[30] S. Compestrini, B. Meunier, Inorg. Chem. 31 (1992) 1999.
[31] D. Mohajer, G. Karimipour, M. Bagherzadeh, New J. Chem. 28 (2004) 740.
[32] L.C. Yuan, T.C. Bruice, J. Am. Chem. Soc. 108 (1986) 1643.
[33] P. Obrien, D.A. Sweigart, Inorg. Chem. 24 (1985) 1405.
[34] F.A. Walker, M.W. Lo, M.T. Ree, J. Am. Chem. Soc. 98 (1976) 5552.
[35] P. Battioni, J.P. Renaud, J.F. Bartoli, M. Reina-Artiles, M. Fort, D. Mansuy,
J. Am. Chem. Soc. 110 (1988) 8462.
Acknowledgement
The authors are thankful for financial support from Shiraz
University Research Council.
References
[36] (a) G.C. Cole, A.C. Legon, Chem. Phys. Lett. 369 (2003) 31;
(b) F. Grepioni, G. Cojazzi, S.M. Draper, N. Scully, D. Braga,
Organometallics 17 (1998) 296.
[1] D. Mansuy, P. Battioni, in: K.M. Kadish, K.M. Smith, R. Guilard (Eds.),
The Porphyrin Handbook, vol. 4, Academic Press, San Diego, 2000, p. 1
(Chapter 26).
[2] J.T. Groves, K. Shalyaev, J. Lee, in: K.M. Kadish, K.M. Smith, R. Guilard
(Eds.), The Porphyrin Handbook, Academic Press, San Diego, 1999, p. 17
(Chapter 27).
[37] W.T. Borden, Chem. Commun. (1998) 1919.
[38] D. Mohajer, R. Tayebee, H. Goudarziafshar, J. Chem. Res. S (1999) 168.
[39] R. Zhang, J.H. Horner, M. Newcomb, J. Am. Chem. Soc. 127 (2005) 6573.
[40] R.L. David, Handbook of Chemistry and Physics, vol. 8, 80th ed., CRC
Press, New York, 1999–2000, p. 8, 46–56.
[3] D. Dolphin, T.G. Traylor, L.Y. Xie, Acc. Chem. Res. 30 (1997)
251.
[41] K. Hoffmann, Imidazole and its Derivatives, Interscience Publishers Inc.,
New York, 1953, Part I, p. 15.
[4] P. Bhyrappa, J.K. Young, J.S. Moore, K.S. Suslick, J. Mol. Catal. A: Chem.
113 (1996) 109.
[42] H.C. Brown, X.R. Mihm, J. Am. Chem. Soc. 77 (1955) 1723.
[43] K. Schofield, Hetero-aromatic Nitrogen Compounds, Pyrroles and
Pyridines, Plenum Press, New York, 1967, p. 146.
[5] D. Mansuy, Coord. Chem. Rev. 125 (1993) 129.
[6] B. Meunier, Chem. Rev. 92 (1992) 1411.
[7] I. Tabushi, Coord. Chem. Rev. 86 (1988) 1.