Effect of hydrogen bonding on catalytic activity of some manganese porphyrins in epoxidation reactions

Article abstract of DOI:10.1142/S1088424611003094

Mn(III)-tetra phenyl porphyrin-acetate (MnTPPOAc) and some kinds of meso-phenyl substituted porphyrins by hydroxyl groups and their Mn(III) complexes were synthesized. These Mn-porphyrins were used as catalyst in the epoxidation of various alkenes with tetra-n-butylammonium hydrogen monopersulfate (n-Bu₄NHSO₅) as oxidant and tetra-n-butylammonium acetate (n-Bu₄NOAc) as the axial ligand. The following order of catalytic activity was observed for cyclooctene: T(2,3-OHP)PMnOAc ? T(2,4,6-OHP)PMnOAc < T(4-OHP)PMnOAc < T(2,6-OHP)PMnOAc < TPPMnOAc and T(2,3-OHP)PMnOAc ? TPPMnOAc > T(4-OHP)PMnOAc > T(2,4,6-OHP)PMnOAc > T(2,6-OHP)PMnOAc for other alkenes. Different activity and stability of the catalysts were interpreted based on the hydrogen bonding between hydroxyl groups with appropriate orientation on the meso-position of the phenyl groups and axial bases or oxidant. T(2,3-OHP)PMnOAc catalyst has shown optimal condition for effective hydrogen bonding. In the case of other catalysts, electronic and steric factors overcome the hydrogen bonding effect.

Full text of DOI:10.1142/S1088424611003094

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2011; 15: 181–187
DOI: 10.1142/S1088424611003094
Published at http://www.worldscinet.com/jpp/
Effect of hydrogen bonding on catalytic activity of some
manganese porphyrins in epoxidation reactions
Tahereh Alemohammad, Nasser Safari* and Samira Osati
Chemistry Department, Shahid Beheshti University, G.C, Evin, Tehran, 19839-63113, Iran
Received 12 January 2011
Accepted 5 March 2011
ABSTRACT: Mn(III)-tetra phenyl porphyrin-acetate (MnTPPOAc) and some kinds of meso-phenyl
substituted porphyrins by hydroxyl groups and their Mn(III) complexes were synthesized. These Mn-
porphyrins were used as catalyst in the epoxidation of various alkenes with tetra-n-butylammonium
hydrogen monopersulfate (n-Bu4NHSO5) as oxidant and tetra-n-butylammonium acetate (n-Bu4NOAc)
as the axial ligand. The following order of catalytic activity was observed for cyclooctene: T(2,3-OHP)
PMnOAc >> T(2,4,6-OHP)PMnOAc ≥ T(4-OHP)PMnOAc ≥ T(2,6-OHP)PMnOAc ≥ TPPMnOAc and
T(2,3-OHP)PMnOAc >> TPPMnOAc > T(4-OHP)PMnOAc > T(2,4,6-OHP)PMnOAc > T(2,6-OHP)
PMnOAc for other alkenes. Different activity and stability of the catalysts were interpreted based on the
hydrogen bonding between hydroxyl groups with appropriate orientation on the meso-position of the
phenyl groups and axial bases or oxidant. T(2,3-OHP)PMnOAc catalyst has shown optimal condition
for effective hydrogen bonding. In the case of other catalysts, electronic and steric factors overcome the
hydrogen bonding effect.
KEYWORDS: hydroxy porphyrins, olefin epoxidation, oxone, axial ligand, hydrogen bonding.
substituents on the porphyrins rings results in different
INTRODUCTION
electronic and steric properties. The first-generation cata-
Catalytic epoxidation of olefins have attracted much
lysts such as tetra phenyl and tetra mesityl metallopor-
attention, both in industry and in the laboratory scale,
phyrins were found to suffer from rapid auto-oxidation
because epoxides are intermediate of many synthetic
degradation of the porphyrins ring.
processes [1, 2]. Complex of transition metals with many
To overcome this problem, second generation porphy-
kind of ligands such as porphyrins [3–9], oxazolines [10,
rins catalysts containing electron withdrawing substitu-
11] and salens [12, 13] have been used as catalyst for
ents on the phenyl rings attached to the meso-position of
olefin epoxidation reactions in the presence of monoox-
the porphyrins were developed [19–30]. In addition, the
ygen donors such as PhIO [14], Oxon [15], tetra-n-
substituents near the metal center had the advantage of
butylammonium monopersulfate (TBAM) [16], O2 [17].
inhibiting the bimolecular self-destruction of the porphy-
Groves et al. were among the first researchers who
significantly mimic cytochrome-P450 using synthetic
rins catalysts. Besides the substitution of bulky groups
in ortho positions result in the more hindered rotation of
iron porphyrins in alkenes oxidation using single oxy-
axial base around the metal-nitrogen bond, and also Van
gen donor reagents [18]. Since then, many factors that
der Waals interactions between the ortho groups and the
can affect selectivity and activity of the synthetic mod-
axial ligand. The second generation was more resistant to
els were studied; such as electronic and steric properties
oxidative destruction but the electron deficiency on the
of substituents, solvent and axial ligands. Changing the
phenyl ring could reduce the rate of the catalytic reac-
tion. Therefore, there has been much research on catalyst
modifications in order to optimize the selectivity, product
SPP full member in good standing
yields and distributions, catalyst stability and the rate of
the oxidation processes [31–39]. However, less attention
*Correspondence to: Nasser Safari, email: n-safari@cc.sbu.ac.ir,
tel: +98 21-22401765, fax: +98 21-22403041
Copyright © 2011 World Scientific Publishing Company

182
T. AlemOhAmmAD et al.
has been paid to the weak interactions such as hydrogen
bonding. Mohajer et al. have shown unusual co-catalytic
activity of 2,6-dimethyl pyridine in the presence of Mn-
(tetrakis(pentafluorophenyl)porphyrin) acetate resulting
from hydrogen bonding between ortho-C-F groups of
Mn(Por) with 2,6-substituents of the pyridine [40]. We
have also shown that in the methoxy substituted manga-
nese porphyrins, substitution at 2 plus 3 position of the
phenyl increased reactivity owning to the inter-molecular
and intra-molecular hydrogen bonding [16]. In this work,
to study in particular effect of hydrogen bonding on selec-
tivity and catalytic activity of synthetic systems, some
kinds of manganese tetra phenyl porphyrins with hydroxyl
group on phenyl ring (Fig. 1) were synthesized and used
in the epoxidation of various olefins UV-visible was used
to find a clue to the kind and effects of hydrogen bonding
and to the reactivity and stability of these catalysts.
Epoxidation reactions conditions
Stock solutions of MnPor (3 × 10-3 M) in CH3OH,
TBAA (0.1 M) and alkenes (0.05 M) in CH2Cl2 were pre-
pared. In a 5 mL round bottom flask the following were
added in the order of: alkenes (0.5 mL, 0.025 mmol),
porphyrin catalyst (3 × 10-4 mmol), TBAA (x mmol as
required) and tetra-n-butylammonium hydrogen monop-
ersulfate (0.05 mmol, 0.0202 g). The mixture was stirred
for 10 h and then analyzed by GC.
Notice: n-octane was used as internal standard in all of
the epoxidation reactions.
Cis- and trans-stilbene epoxidation. Stock solutions
were prepared as mentioned above. Mixture of these solu-
tions Mn(Por) (0.5 mL,15 × 10-4 mmol), alkene (2.5 mL,
0.125 mmol) and TBAA (150 µl, 15 × 10-3 mmol) were
introduced to a 5 mL round bottom flask and then 0.1 g
(0.25 mmol) TBAM was added to initiate the reaction.
After the required time, the solvent was removed under
reduced pressure and 0.5 mL of n-pentane was added to
precipitate Mn(por), TBAM and TBAA. The resulting
mixture was filtered, and n-pentane was removed under
vacuum. The mixture was analyzed by 1H NMR spectros-
copy UV-vis spectra.
Catalyst stability and state were estimated by study-
ing of visible spectra of three solutions as follows: (a)
3 × 10-5 M catalyst in methanol, (b) 3 × 10-5 M catalyst
+ TBAM with catalyst/TBAM molar ratio was 1:164 in
a mixture of CH3OH/CH2Cl2 with volumetric ratio of
1:5, and (c) 3 × 10-5 M catalyst + TBAM + TBAA with
catalyst:TBAA:TBAM/1:10:164 molar ratio in a mix-
ture of CH3OH/CH2Cl2 with volumetric ratio of 1:5. All
spectra were recorded after 5 min.
EXPERIMENTAL
Material
All of the reagents were supplied by Merck or Fluka
Co. and employed without further purification. Free-
base porphyrins; meso-tetraphenylporphyrin (H2TPP),
5,10,15,20-tetrakis(2,3-dimethoxyphenyl)porphyrin,
5,10,15,20-tetrakis(2,6-dimethoxyphenyl)porphyrin,
5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin, 5,10,-
15,20-tetrakis(2,4,6-trimethoxyphenyl)porphyrin,
5,10,15,20-tetrakis(3,4-dimethoxyphenyl)porphyrin,
5,10,15,20-tetrakis(2-methoxyphenyl)porphyrin were pre-
pared by the Lindsey method [41]. These free-base porphy-
rinsweremetalatedbycorrespondingsalts;Mn(OAc)2·4H2O
according to the Adler method [42]. James method with
some modification in its purification was used for demethy-
lation of these porphyrins to corresponding hydroxy por-
phyrins [43].
Tetra-n-butylammonium acetate (TBAA) was pre-
pared using the literature method [44] and was used
as axial ligand. Oxon is the trademark of triple salt,
2KHSO5·KHSO4·K2SO4. The active oxidant in this mix-
ture is HSO5-. Oxon is not soluble in organic solvents,
thus to overcome this problem corresponding soluble
salt, tetra-n-butylammonium monopersulfate (TBAM),
was prepared using the procedure in the literature [45].
To obtain reproducible results, only freshly produced
oxidant was used and kept in a refrigerator.
RESULTS AND DISCUSSION
The oxidation of various alkenes was studied in the
presence of manganese porphyrin derivatives shown in
Fig. 1 and n-Bu4HSO5 as oxidant. The reaction solvent
was CH2Cl2:MeOH in the ratio of 5:1. All of the catalysts
are soluble in MeOH and therefore, in the aforemen-
tioned solvent mixture homogeneity of reaction medium
was preserved. The reaction time, concentration of reac-
tants and catalysts and axial ligand bases were optimized
by standard procedures [16]. The highest yields were
obtained in 10 h after starting the reaction.
Manganese porphyrins are very sensitive to the kind
and amount of axial bases [46]. Effects of different axial
bases used in this study are indicated in bar graph of
Fig. 2 which shows that TBAA has the highest co-catalyst
activity among the axial bases used. Our results also seem
to support previous studies in the field on the axial ligand
effect in iron-porphyrins, where it is shown that anionic
axial ligands give enhanced reactivity over oxidants with
neutral ligands such as imidazole [47–48].
Instrumentation
Gas Chromatography (GC) analyses were performed
on the Agilent Technology 7890A which was equipped
with HP-1 capilary column. Cis and trans-stilbene
1
epoxidation progress followed by H NMR BRUKER
AVANCE-300 MHz. Electronic spectra for formation,
purity and stability of synthesized porphyrins were
recorded on a Shimidazo 2100 spectrophotometer.
Figure3indicatespercentageconversionofcyclooctene
to cyclooctene oxide in the presence of different molar
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effeCT Of hyDrOgen bOnDIng On CATAlyTIC ACTIvITy Of SOme mAngAneSe POrPhyrInS
183
substitution on the phenyl attached to the porphyrin ring.
Trends of the catalyst reactivity is in the order of: T(2,3-
OHP)PMnOAc >> TPPMnOAc > T(4-OHP)PMnOAc
> T(2,4,6-OHP)PMnOAc > T(2,6-OHP)PMnOAc. This
trend holds for all of the alkenes used in this study except
for cyclooctene in which TPPMnOAc, with no OH on the
phenyls, shows the least activity.
Table 2 indicates percentage conversion of cyclo-
octene and the yield of cyclootene epoxide formation
in the presence of OH substituted TPPMnOAc cata-
lysts. To note, for all of the substrates used in this study,
T(2,3OHP)PMnOAc catalyst displayed the highest sub-
strate conversions and oxidation yields. In our previous
work when we employed methoxy substituted manga-
Fig. 1. Structure of porphyrins used in this work
nese porphyrins, exceptionally higher activity of T(2,3-
OMeP)PMnOAc was also observed [16]. We presumed
that this variation in the alkene conversion was related
to the hydrogen bonding ability between the active part
-
of the oxidant (HSO5 ) and the methoxy groups. To sub-
stantiate this claim, we used hydroxy substituted por-
phyrins which are more accessible to hydrogen bonding.
The effective parameters for porphyrins derivatives
reactivity are as follows: (a) hydrogen bonding ability
of the substituted OH with oxidant and axial base; (b)
electron donation on the porphyrin by OH substituted
on the phenyl rings; (c) steric hindrance of the phenyl
substituted OH that has a diverse effect on the porphy-
rin rings. Substitution at ortho position of the phenyl is
more effective than para posi-
Fig. 2. Bar graph for percentage conversion of cyclooctene vs.
kind of axial ligand
tion, entry 2 and 5 of Table 2.
Since electron donating abili-
ties for OH at the ortho posi-
tion is almost similar to that of
para substituted one, different
reactivity seen is related to the
hydrogen bonding at 2 posi-
tions. When substitution took
place at 2 and 6 position of the
phenyl, the reactivity of the cat-
alyst is considerably blocked.
This can be related to the steric
hindrance preventing proximal
effects of the oxidant to the
metals, or by the ability for
Fig. 3. Percentage conversion of cyclooctene vs. axial base/T(2,3-OHP)PMnOAc molar
ratio
hydrogen bonding that result
in blockage of catalytic site as
(B)2Mn(por). When two differ-
ratios of imidazole and tetra-n-butyl ammonium acetate,
ent hydroxyl groups are attached on the 2 and 3 posi-
tions of the phenyls, electron donation to the porphyrin
π system is relatively high and the catalyst has hydrogen
bonding ability with axial bases, as well as oxidant. In
such cases catalysts show highest activity. Comparison
between reactivity of T(2,3-OHP)PMnOAc with T(3,4-
OHP)PMnOAc confirms the above conclusion. Finally,
we observed enhanced catalytic activity with hydroxyl
donor group in the R2 or R3 positions of meso-phenyl
Mn(III) porphyrins, as combination of steric effect and
TBAA, as axial bases. The highest conversion percent-
age was obtained in 10 fold excess of axial bases to the
T(2,3-OHP)PMnOAc catalyst. The optimum molar ratio
of catalyst/cocatalyst/substrate/oxidant was 1/10/83/164.
The catalyst employed in this study have different
electron donating, steric and hydrogen bonding abili-
ties. Table 1 shows the conversion and epoxy formation
of alkenes with these catalysts. As it is evident from
Table 1, the alkenes conversion percentages are very sen-
sitive to the catalyst structure particularly to the hydroxyl
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184
T. AlemOhAmmAD et al.
Table 1. Conversion (epoxy yield) for different kinds of alkenes in the presence of TBAA and all of applied catalystsa
Substrate
Catalyst
OH
OH
HO
OH
OH
OH
) PMnOAc
T(HO
) PMnOAc
T(
) PMnOAc
) PMnOAc
T(HO
T(
) PMnOAc
T(
75 (68)
45 (42)
38 (35)
35 (30)
33 (20)
79.4 (49.5)
[2.5]b
21.5 (10.5)
[0]b
33.5 (21)
[0]b
30.1 (19.7)
[0]b
40.1 (10.1)
[3.3]b
80.9 (68)
[2.8]c
30.1 (21.6)
[3.1]c
30.7 (21.1)
[3.8]c
22.1 (11.7)
[3.3]c
47.1 (17.3)
[8.3]c
78 (66.4)
37.2 (24.8)
36.8 (18.6)
29.4 (20.1)
56.3 (23.3)
Cl
98.1 (45)
62.3 (18.9)
64.3 (14.7)
52 (15.9)
77.7 (17.4)
MeO
88 (77.7)
[5.6]d
42.2 (37.9)
[3.5]d
43.3 (42.8)
[5.4]d
33.8 (31)
[4.5]d
70.7 (48.8)
[16.1]d
e
e
e
e
76.5 (57.3)
[19.1]f
-
-
-
-
e
e
e
e
Ph
50 (44)
-
-
-
-
Ph
24.9 (11.3)
23.5 (10.9)
11.1 (3.7)
13.4 (4.3)
10.2 (3.1)
10.9 (3.2)
9.3 (2.9)
8.5 (2.3)
19.2 (2.5)
17.5 (3.4)
aAll reactions were carried out in optimum time and reagents concentration. Reactions were carried out in triplicate and
the averages are reported with 5%. GC measurements were done by internal standard method. Substare was deficient reagents.
b % 2-cyclohexene 1-one, c % benzaldehyde, d % acetophenone, d % trans-stilbene oxide, e epoxidation not done.
Table 2. Cycloctene epoxidation in the presence of TBAA
as axial base and all of the applied catalysts under optimum
reaction condition
caused in red shift of the Soret band [50]. Table 3 shows
max Soret bands of the manganese porphyrin used in this
λ
study. From the data in Table 3 the following facts are
evident: (a) OH substitution on the phenyl increase π
interaction and donation from phenyl π system to the π
system of the porphyrin (entry 4 and 5); (b) OH substitu-
tion on the ortho position of the phenyls results in more
hindrance interaction between phenyls and the porphyrin
rings and makes the phenyl rings more toward perpendic-
ular to the porphyrin plane which has inverse effects to
the π donation. This reduction in the red shift can also be
related to the effect of hydrogen bond between the OAc
axial base with OH of the phenyls which is effective only
when OH is placed on the ortho position.
Entry
Catalyst
Conversion, Epoxy yield,
%
%
1
2
3
4
5
6
7
T(2,3-OHP)PMnOAc
T(2-OHP)PMnOAc
T(2,4,6-OHP)PMnOAc
T(3,4-OHP)PMnOAc
T(4-OHP)PMnOAc
T(2,6-OHP)PMnOAc
TPPMnOAc
75
68
58.1
45
52.1
42
42
37.3
35
38
35
30
33
20
-
Addition of the oxidant HSO5 to the reaction medium
containing the catalysts caused blue shift for T(2-OHP)
PMOAc and T(2,3-OHP)PMOAc but red shifts for the
other substituted porphyrin. Figure 4 shows the effect
of the oxidant and axial bases to the OH substituted
porphyrins.
Three categories were observed as it is evident from
Fig. 4 and Table 3. Addition of oxidant to the T(2-OHP)
PMnOAc and T(2,3-OHP)PMOAc results in blue shifts
in the Soret band, trace A of Fig. 4. As it is clear from
trace B of Fig. 4, addition of oxidant to the T(4-OHP)
hydrogen bonding ability that is in line with computa-
tionalstudiesonsubstrateepoxidationbyiron-porphyrins
with different meso-substituents [49].
UV-visible spectra and catalyst stability
Soret band of porphyrins and metalloporphyrins
are sensitive to the π electron density of the porphyrin
ring. Electron donation by substituents on the porphyrin
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effeCT Of hyDrOgen bOnDIng On CATAlyTIC ACTIvITy Of SOme mAngAneSe POrPhyrInS
185
PMnOAc and T(3,4-OHP)PMnOAc cause red shifts in
hydrogen bonding at active site of the catalyst cannot
form and addition of the axial base or oxidant results in
occupation of 6 position of the catalyst. These results
in more electron donation to the porphyrin and caused
red shift, and also makes the catalyst less stable and less
active (compare dashed lines in trace A and B which are
for active catalysts). For third categories in trace C, when
two ortho positions of the phenyls on the catalyst are
occupied by OH, steric hindrance exists between oxidant
and catalysts that makes effective interaction between
the Soret bands. In the third category shown in trace C,
T(2,6-OHP)PMnOAc and T(2,4,6-OHP)PMnOAc, addi-
tion of the oxidant and axial base did not change the Soret
bands. Our explanation for trace A in Fig. 4 is hydro-
gen bonding between axial base and also oxidants with
catalyst which results in the electron reduction to the π
system of the porphyrins. This reduction also results in
stabilization of the porphyrins in the catalytic cycles due
to less degradation of the porphyrins. For trace B effective
Table 3. λmax of the Soret bands of catalysts used in this study and its changes in the presence of oxidant
and TBAA
Entry
Catalyst
λmax(cat.)
λmax(cat. + TBAM)
λmax(cat. + TBAM + TBAA)
1
2
3
4
5
6
7
T(2,3-OHP)PMnOAc
T(3,4-OHP)PMnOAc
T(2,6-OHP)PMnOAc
T(4-OHP)PMnOAc
T(2-OHP)PMnOAc
T(2,4,6-OHP)PMnOAc
TPPMnOAc
469.8
472.8
470.8
471.6
469.1
475.2
467.4
464.6
474.6
471.4
474.2
466
467.6
474.2
471.6
477.4
467.8
475.6
468.96
475.4
468.9
Fig. 4. The UV-vis spectra of some catalysts. In each specta (a) represents catalyst solution (3.5 × 10-5 M), (b) relates to solution of
catalyst (3.5 × 10-5 M) and oxidant and (c) is solution of three components; catalyst (3.5 × 10-5 M), oxidant and TBAA
O
S
O
S
O
O
O
O
H
O
O
H
O
O
O
H
H
O
O
O
OH
O
O
OH
OH
TBAO
TBAO
Mn
Mn
Mn
HO
HO
HO
O
HO
O
H
O
Fig. 5. Steric hindrance between oxidant and catalyst because of exsitance of OH groups on ortho positions of phenyl rings on
T(2,4,6-OHP)PMnOAc and T(2,6-OHP)PMnOAc catalysts
Copyright © 2011 World Scientific Publishing Company
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186
T. AlemOhAmmAD et al.
them difficult. In this case, the Soret bond did not change
considerably. These interactions are shown in cartoons
of Fig. 5.
16. Aghabali A and Safari N. J. Porphyrins Phthalocya-
nines 2010; 14: 335–341.
17. Hajimohammadi M and Safari N. J. Porphyrins
Phthalocyanines 2010; 14: 639–645.
18. Groves JT, Nemo TE and Meyers RS. J. Am. Chem.
Soc. 1979; 101: 1032–1033.
CONCLUSION
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4625–4629.
In conclusion, hydrogen bonding is a very effective fac-
tor in catalytic activity of the biomimetic models of cyto-
chrome P-450. T(2,3-OHP)PMnOAc is a very effective
catalyst in the oxidation of alkenes using n-Bu4NHSO5,
which has the ability for hydrogen bonding with catalyst.
This hydrogen bonding can affect the stability of the cata-
lyst as well as its reactivity. This study may suggest that in
the cytochrome P-450 oxidation process in biology, hydro-
gen bonding is also an important factor to be considered.
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