Meso-tetracinnamylporphyrin: Synthesis, characterization and the catalytic activity of its Mn(III) complex in olefin epoxidation with tetra-n-butylammonium hydrogen monopersulfate

Article abstract of DOI:10.1016/j.poly.2010.01.009

Meso-tetracinnamylporphyrin (H₂tcp) has been synthesized and characterized by UV-Vis, ¹H NMR and IR spectroscopies. Protonation of H₂tcp with HCl is accompanied by a blue shift (77 cm^-1) of the Q(0,0) band, probably indicating a decrease in the efficiency of the π-electron donation from the meso-substituents to the a_2u orbital in comparison with the corresponding dication of meso-tetraphenylporphyrin (H₂tpp). The unusual NH signal multiplicity observed in ¹H NMR spectrum of H₂tcp shows evidence of a long distance coupling between the vinyl protons and NH ones. This long range coupling can occur between the remote centers using the a_2u orbital as a connector. Mntcp(OAc) shows higher catalytic activity (ca. 1.2-fold higher) than Mntpp(OAc) in olefin epoxidation with tetra-n-butylammonium hydrogen monopersulfate (TBAO). The low ratio (～1.5) of cis- to trans-stilbene oxide, in competitive oxidation of cis- and trans-stilbene with TBAO, strongly suggests the involvement a common high-valent manganese oxo complex as the reactive intermediate responsible for oxygen atom transfer.

Full text of DOI:10.1016/j.poly.2010.01.009

Polyhedron 29 (2010) 1492–1496
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Meso-tetracinnamylporphyrin: Synthesis, characterization
and the catalytic activity of its Mn(III) complex in olefin epoxidation
with tetra-n-butylammonium hydrogen monopersulfate
b
c
Saeed Zakavi a, , Amir Salami Ashtiani , Saeed Rayati
*
a Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), P.O. Box 45195-1159, Gava Zang, Zanjan, Iran
b Department of Chemistry, Damghan University of Basic Sciences, 36716-41167 Damghan, Iran
c Department of Chemistry, K.N. Toosi University of Technology, Tehran 15418, Iran
a r t i c l e i n f o
a b s t r a c t
Meso-tetracinnamylporphyrin (H2tcp) has been synthesized and characterized by UV–Vis, 1H NMR and IR
Article history:
spectroscopies. Protonation of H2tcp with HCl is accompanied by a blue shift (77 cmÀ1) of the Q(0,0)
Received 30 September 2009
Accepted 8 January 2010
Available online 14 January 2010
band, probably indicating a decrease in the efficiency of the p-electron donation from the meso-substit-
uents to the a2u orbital in comparison with the corresponding dication of meso-tetraphenylporphyrin
(H2tpp). The unusual NH signal multiplicity observed in 1H NMR spectrum of H2tcp shows evidence of
a long distance coupling between the vinyl protons and NH ones. This long range coupling can occur
between the remote centers using the a2u orbital as a connector. Mntcp(OAc) shows higher catalytic
activity (ca. 1.2-fold higher) than Mntpp(OAc) in olefin epoxidation with tetra-n-butylammonium hydro-
gen monopersulfate (TBAO). The low ratio (ꢀ1.5) of cis- to trans-stilbene oxide, in competitive oxidation
of cis- and trans-stilbene with TBAO, strongly suggests the involvement a common high-valent manga-
nese oxo complex as the reactive intermediate responsible for oxygen atom transfer.
Keywords:
Meso-tetracinnamylporphyrin
Epoxidation
Tetra-n-butylammonium hydrogen
monopersulfate
Dication
Meso-
tetracinnamylporphyrinatomanganese(III)
acetate
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
the porphyrin mean plane such as meso-tetramesitylporphyrin
(H2tmp) and meso-tetra(2,6-dichlorophenyl)porphyrin (H2tdcpp)
Metal complexes of porphyrins bearing different meso- and/or
b-substituents have been extensively used as models for cyto-
chrome P450 enzymes. Peripheral functionalization of porphyrins
with the aim of obtaining more stable ‘‘second- and third-genera-
tion” porphyrins with halogen, nitro or sulfone substituents is an
effective strategy for the development of novel porphyrin-based
catalysts for oxidation reactions [1]. The stabilization of HOMO le-
vel of the so called electron-deficient porphyrins, leads to signifi-
cantly improved catalytic activity and stability of their
corresponding metalloporphyrins towards oxidative degradation
[2–4]. For the a2u orbitals, the electron densities are largest on
meso-carbons and the central nitrogen atoms [5]. Accordingly,
the a2u orbital can be destabilized through resonance interactions
[9,10]. [Mntmp]OAc has been found to show higher catalytic activ-
ity relative to Mn(tpp)OAc in the oxidation of sterically unhindered
cycloalkanes and aryalkanes with periodate as oxidant [11]. In a re-
cent work, similar catalytic activities for Ru(IV)(tmp)Cl2 and
Ru(IV)(tdcpp)Cl2 have been reported [12]. Herein, we report the
synthesis and characterization of H2tcp (Fig. 1) as a porphyrin with
weak electron-donating unsaturated substituents at meso posi-
tions. In addition, the catalytic activity of Mntcp(OAc) in olefins
epoxidation with tetra-n-butylammonium hydrogen monopersul-
fate (TBAO) is presented and compared with that of Mntpp(OAc)
in the same conditions.
2. Experimental
between the meso-aryl and porphyrin
p-systems [6]. Large dihe-
dral angles between the porphyrin macrocyclic plane and the aryl
substituents hinder the effective resonance interactions [7,8]. The
introduction of bulky substituents at ortho-positions of meso-aryl
substituents or/and pyrrole b-positions gives porphyrins with in-
creased dihedral angles between the meso-substituted aryls and
2.1. Instruments and materials
1H spectra were obtained in CDCl3 solutions with a Bruker
FT-NMR 250 (250 MHz) spectrometer. The residual CHCl3 in con-
ventional 99.8% CDCl3 gives a signal at d = 7.26 ppm, which was
used for calibration of the chemical shift scale. The absorption
spectra were recorded on a double beam GBC-916 UV–Vis spectro-
photometer. The reaction products of oxidation were analyzed by a
* Corresponding author. Tel.: +98 232 524 47 87; fax: +98 241 4153232.
E-mail address: zakavi@iasbs.ac.ir (S. Zakavi).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.01.009

S. Zakavi et al. / Polyhedron 29 (2010) 1492–1496
1493
2.4. General oxidation procedure
H
R
R =
C
Stock solution of Mntcp(OAc) (0.0006 M) and imidazole (0.5 M)
were prepared in CH2Cl2. In a 10 ml round-bottom flask were
added successively: alkene (0.06 mmol), catalyst (0.0006 mmol,
C
H
NH
N
N
H
1.0 ml), imidazole (0.012 mmol, 24 ll). Tetrabutylammonium ox-
(1)
one (0.15 mmol, 0.060 g) was then added to the reaction mixture
at room temperature. The solution was stirred thoroughly for
2 min at ambient temperature. Formation of epoxide was detected
by GC analysis. In the case of cis- and trans-stilbene, the products
were analyzed by 1H NMR.
C
R
(2)
C
HN
H
R
3. Results and discussion
Fig. 1. Meso-tetracinnamylporphyrin, H2tcp.
3.1. UV–Vis spectra
Varian-3800 gas chromatograph equipped with a HP-5 capillary
m) and
UV–Vis spectrum of H2tcp is shown in Fig. 2. Although the Soret
band of H2tcp shows no observable shift with respect to that of
H2tpp, the Q(0,0) band is red-shifted (165 cmÀ1) relative to the lat-
ter (Fig. 2, Table 1). According to the general belief that only a large
red shift in the UV–Vis bands is indicative of a very nonplanar por-
phyrin macrocycle, an essentially planar conformation is expected
for H2tcp in solution [16]. Due to the distribution of the a2u elec-
tron densities on the meso positions and the central nitrogen
atoms, the Q(0,0) band is more sensitive to the electronic proper-
ties of the meso-substituents than the a1u orbital [5,17]. Diprotona-
tion of H2tcp with HCl leads to the red shift (1414 cmÀ1) of the
column (phenylmethyl siloxane 30 m  320
lm  0.25
l
flame-ionization detector. IR spectra were taken with KBr pellets
on a Perkin Elmer RXI Spectrophotometer. Pyrrole, imidazole, cin-
namaldehyde and benzaldehyde were obtained from Fluka and
Merck. Pyrrole was distilled before use and the others were used
without further purification. Alkenes were obtained from Fluka
and Merck. Styrene, a-methylstyrene, and cyclohexene were puri-
fied with a short alumina column before use. Tetrabutylammo-
nium hydrogen sulfate and Oxone were purchased from Merck
and Acros, respectively.
2.2. Synthesis and metallation of H2tcp
H2tcp was synthesized and purified according to the method re-
ported by Neya et al. for the synthesis of meso-tetraalkylporphy-
rins, with some modifications in the procedure as detailed
hereinafter [13]. Pyrrole (4.2 ml, 0.0605 mole) and cinnamalde-
hyde (3 ml, 0.0238 mole) were added at 2.5: 1 molar ratios to an
Erlenmeyer flask containing 300 ml of propionic acid, 12 ml of
water and 1 ml of pyridine. The mixture stirred at reflux tempera-
ture of the solvent (ca. 140 °C) for 30 min. Then, a supplementary
amount of cinnamaldehyde (1.2 ml, 0.0095 mole) was added and
the solution was stirred for another 2 h under reflux conditions.
Chloroform (300 ml) was added to the cooled solution, and the
mixture was washed with water (300 ml  2), 50 mM sodium
hydroxide (300 ml  2), and water (300 ml) to remove propionic
acid. The chloroform solution was evaporated to dryness. Purifica-
tion with soxhlet extraction (methanol) was carried out before
washing the residue with methanol (5 ml  about 10) on a centri-
fuge (5000 rpm, 3 min). The solid was chromatographed (three
times) on a neutral alumina column with dichloromethane. H2tcp
(275 mg, 4.5%) was obtained after recrystallization from dichloro-
Fig. 2. UV–Vis spectra of H2tcp (full line) and H2tcp(HCl)2 (dot line) in CH2Cl2.
Table 1
UV–Vis spectral data of H2tcp, H2tpp and the diacids in dichloromethane.a
methane/methanol. UV–Vis in CH2Cl2, kmax (log e): 417 (5.50) (Sor-
et), 516 (4.25), 552 (4.14), 590 (4.01), 654 (3.99). 1H NMR (Bruker
FT-NMR 250) (d ppm): À2.43 to À2.73 (2H, br, sextet, NH); 9.49
(8H, b, br); 8.85 (quintet, 4H, CH(1)); 8.19 (4H, br d, CH(2)); 7.78
(8H, br s, Ho); 7.39–7.60 (12H, multiplet, Hm,p). IR spectrum shows
N–H stretching band at 3311 cmÀ1. Pattern in the CH stretching re-
gion is similar to that of vinyl benzene and observed in the range of
2852–2956 cmÀ1. Mntcp(OAc) (kmax = 479 nm in dichloromethane)
has been prepared and purified according to the the method of Ad-
ler et al. [14].
Porphyrin
Soret (k/nm)
Q (k/nm)
IV
III
II
Ib
H2tcp
416
516
552
590
654
H2tcp(HCl)2
442
596.7
651
D
m
(cmÀ1
)
À1414
77.5c
647
H2tpp
H2tpp(HCl)2
(cmÀ1
417
514
520
548
557
590
602
444
656
D
m
)
À1458
À212c
H2t(n-Pr)p
416
661
H2t(n-Pr)p(HCl)
434
À997
643
D
m
(cmÀ1
)
423.5c
a
Acid treatment of H2tcp gave the green porphyrin dication. Concentrated HCl
2.3. Preparation and metallation of H2tpp
was added to a solution of porphyrin in CH2Cl2 and the mixture was stirred for
10 min.
b
Q(0,0).
The preparation and metallation of H2tpp was carried out fol-
lowing the literature methods [14,15].
c
Shift of the Q(0,0) band.

1494
S. Zakavi et al. / Polyhedron 29 (2010) 1492–1496
meso-carbon atoms and consequently to the unsaturated bonds
conjugated to the porphyrin ring [5]. Therefore, long range cou-
plings between the NH protons and those of the peripheral substit-
uents can be created using the a2u orbital as a connector. It should
be noted that non-bulky unsaturated chains or rings are able to lie
coplanar with the porphyrin nucleus and form extended delocal-
ized
p systems [17,20]. Very broad signals observed for NH protons
(also for b-protons) may be possibly due to the existence of various
isomers resulting from different orientations of cinnamyl groups. It
should be noted that the common broadening of NH signal of free
base porphyrins has been rationalized by the intramolecular N–
HÁÁÁN hydrogen bonds between the adjacent pyrrole rings [21].
The b protons appear as an unusual broad signal at d 9.49 ppm.
The broadening of signal indicates that b protons are subject to a
range of different chemical environments which is probably the re-
sult of different interactions of cinnamyl groups with the protons
of pyrrole rings (see Fig. 1). A broad quintet at 8.85 ppm and a
broad signal at 8.19 ppm with very similar integrations (consider-
ing 8 for b protons give ca. 4 for the two signals) were assigned to
the CH(1) and CH(2) protons, respectively. The proton adjacent to
the porphyrin macrocycle, CH(1), is more deshielded than CH(2)
one by the porphyrin ring current. 1H NMR spectrum of cinnamal-
dehyde, shows a quintet at d 6.7 ppm for CH(1) and a broad doublet
signal at 7.55 for CH(2) protons [22,23].
Fig. 3. 1H NMR spectra of H2tcp in NH region.
Soret band and the blue shift (77.5 cmÀ1) of the Q(0,0) band. The
observed blue shift of the Q(0,0) band may be attributed to the de-
creased
p-donation from the meso-cinnamyl substitution to the
porphyrin core in comparison with that of meso-phenyl groups of
H2tpp and other meso-tetraarylporphyrins [6–8,18]. It should be
noted that the adduct formation of meso-tetraalkylporphyrins
(with primary and secondary alkyl residues) with Lewis or protic
acids also causes the blue shift of the Q(0,0) band [18,19]. Accord-
ingly, the title porphyrin may be considered as an electron-defi-
cient one with respect to meso-tetraphenylporphyrin.
3.2. 1H NMR spectrum
3.3. Catalytic studies
In 1H NMR spectrum of H2tcp, the NH protons appear as a broad
multiplet between 2.43 and À2.73 ppm which suggests a long dis-
tance coupling between the vinyl protons and NH ones (Fig. 3). The
a2u electron densities which are largest on the meso-carbon atoms
and pyrrole nitrogen atoms connect the nitrogen atoms to the
3.3.1. Oxidation of alkenes
Mntcp(OAc) has been used as a catalyst for oxidation of differ-
ent alkenes (Table 2). It is observed that the conjugated double
bonds and unstrained cyclic alkenes are more reactive than simple
ones. It has been shown by quantum–chemical calculations that
Table 2
Oxidation of olefins with TBAO in the presence of Mntcp(OAc) and imidazole at room temperature in dichloromethane (for 20 min).a
Entry
1
Alkene
Conversionb (%)
84
Epoxide
Selectivity (%)
100
o
2
80
65c
CH3
CH3
o
3
4
99
70
100
70
o
o
5
98
100
o
6
7
27
100
o
59d
96.6
o
3.4
o
o
8
44d
100
a
The molar ratios for alkene:oxidant:ImH:catalyst are 100:250:20:1, all were used in concentrations 0.2 times lower than that have been used for catalytic oxidation of
alkenes with TBAO in the presence of other metalloporphyrins, for controlling the reaction rate [26,29–32].
b
Analyzed by GC. Authentic samples were prepared according to the literature for entries 2, 5, 6, 7 and 8 [27].
c
Acetophenone is the by-product.
d
Analyzed by 1H NMR.

S. Zakavi et al. / Polyhedron 29 (2010) 1492–1496
1495
the strained double bonds, which already deviate from planarity,
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
have a destabilized HOMO with respect to the planar ones and
should form particularly strong bonds to transition metals [24].
This may explain the higher reactivity of the strained double bond
of indene relative to that of styrene, although the nearly planar
structure of indene which can diminish the steric hindrance about
the reaction center may be also involved. Contrary to other reports,
a significant difference between the reactivity of cyclohexene and
cyclooctene has been observed [25–27]. The observed pattern of
reactivity agrees with that reported for metalloporphyrin catalyzed
epoxidation of alkenes with tetrabutylammonium periodate [28].
This observation may possibly reveal the nonplanar orientation
of cinnamyl groups with respect to the porphyrin mean plane in
the active oxidant (vide infra) which in turn leads to a decrease
in the reactivity of more bulky alkenes such as cyclooctene.
0
50
100
150
Time (s)
200
250
Fig. 5. Absorbance versus time for the degradation of Mntcp(OAc).
3.3.2. Comparison of catalytic activity of Mntcp(OAc) and Mntpp(OAc)
Comparative oxidation of indene and
a-methyl styrene have
been carried out in the presence of Mntcp(OAc) and Mntpp(OAc)
(Table 3). It is observed that Mntcp(OAc) shows higher catalytic
activity (ca. 1.2 fold higher) than Mntpp(OAc). This may again
show the enhanced electron-deficient character of porphyrin core
due to the substitution of cinnamyl groups for phenyl ones.
Table 4
Competitive oxidation of cis- and trans-stilbene with TBAO in the presence of
Mntcp(OAc) and imidazole in dichloromethane.a,b
Cis-oxide (%)
12.8 0.4
Trans-oxide (%)
Cis/trans ratio
Time (min)
20
8.7 0.5
1.47 0.2
a
3.3.3. Stability of Mntcp(OAc)
The molar ratios for alkene:oxidant:ImH:catalyst are: (500,500):250:20:1.
Analyzed by 1H NMR.
b
The title porphyrin possesses extended
p-conjugation with four
cinnamyl substituents at its periphery. This extended
p-conjuga-
tion is expected to enhance the stability of Mntcp(OAc) towards
oxidation with different intermediates such as radicals or/and high
valent oxo-species through the delocalization of radical or carbo-
cation centers which may form in oxidative conditions. The UV–
Vis spectrum of a solution containing catalyst, alkene, ImH and
TBAO has been recorded at 30 s intervals (Fig. 4). It is observed that
Mntcp(OAc) has been degraded ca. 36% (compare curve a and curve
e) after 2 min. After 2.5 min (curve e), the rate of degradation de-
creases so that the reaction seems to proceed in the presence of
a nearly constant catalyst concentration (Fig. 5).
3.3.4. The nature of active oxidant
Epoxidation of cis- and trans-stilbene, in a competitive reaction,
has been shown to be a useful probe to elucidate the nature of ac-
tive oxidant in the metalloporphyrin catalyzed epoxidation of ole-
fins in the absence of nonbulky ortho-substituents on the phenyl
groups of meso-tetraphenylporphyrins [28,29]. Competitive oxida-
tion of cis- and trans-stilbene with TBAO in the presence of
Mntcp(OAc) (Table 4) has been performed and yielded a ratio of
1.47 for cis- to trans-stilbene oxide. This ratio suggests a high va-
lent Mn–Oxo species as the sole active oxidant.
Table 3
Comparative oxidation of indene and
a-methyl styrene with TBAO in the presence of
imidazole, catalyzed by Mntcp(OAc)(1) and Mntpp(OAc)(2) in dichloromethane.a
Alkene
Conversion
(%, for 1)
Conversion
(%, for 2)
Time
(min)
a
-Methyl styrene
80
98
3
4
59
80
2
3
20
20
4. Conclusions
Indene
a
See the footnote of Table 2 for reaction conditions; all experiments were
In summary, the synthesis, characterization and metalation of
H2tcp are reported. UV–Vis studies imply that H2tcp may be con-
sidered as an electron-deficient porphyrin in comparison with
meso-tetraphenylporphyrin. The presence of CH'CH spacer in
repeated three times.
H2tcp seems to cause a decrease in p-donation from phenyl groups
to the porphyrin core. Mntcp(OAc) shows higher catalytic activity
than Mntpp(OAc) in olefin epoxidation with TBAO. Competitive
oxidation of cis- and trans-stilbene indicates the involvement of a
high valent manganese oxo intermediate in the catalytic cycle.
a
b
c
0.6
d
e
f
Acknowledgements
0.35
Financial Support of this work by the Institute for Advanced
Studies in Basic Sciences (IASBS) and Damghan University of Basic
Sciences Research Councils is acknowledged.
0.1
400
450
500
550
600
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