Kinetic study of meso-tetraphenylporphyrin synthesis under microwave irradiation

Article abstract of DOI:10.1002/jhet.5570450224

(Chemical Equation Presented) A typical synthesis of meso- tetraphenylporphyrin was carried out from condensation of pyrrole with benzaldehyde in acidic medium under microwave irradiation. A batch of tests were carried out and a systematic evaluation of o

Full text of DOI:10.1002/jhet.5570450224

Mar-Apr 2008
453
Kinetic Study of meso-Tetraphenylporphyrin Synthesis Under
Microwave Irradiation
Rodrigo De Paula, Maria A.F. Faustino, Diana C.G.A. Pinto, Maria G.P.M.S. Neves,
José A.S. Cavaleiro*
University of Aveiro, Department of Chemistry, Campus Santiago. 3810-193, Aveiro-Portugal,
Tel.: +351 234 370 717 ; Fax: +351 234 370 084 ; e-mail : jcavaleiro@ua.pt
Received June 5, 2007
CHO
CH3CH2CO2H
C6H5NO2
NH
N
N
4
4
+
N
MW (650 W; 200 ˚C - 3 bar)
5 minutes
HN
H
35%
A typical synthesis of meso-tetraphenylporphyrin was carried out from condensation of pyrrole with
benzaldehyde in acidic medium under microwave irradiation. A batch of tests were carried out and a
systematic evaluation of organic acid, reactant concentration, microwave power, time of reaction, presence
of added oxidant at normal pressure conditions and in closed vessels system, allowed us to find a good set
of conditions within the range of study. The best result was achieved in only 5 minutes of irradiation
showing a great reduction in reaction time and reaching yields higher than those mentioned in literature for
the synthesis under classical synthetic heating method.
J. Heterocyclic Chem., 45, 453 (2008).
studies about reproducibility have shown that the results
obtained in domestic MW devices are not reliable due to
the lack of reproducibility [1,2]. Indeed, such domestic
devices have on-off switches that do not allow keeping a
constant energy power. For reliable and reproducible
procedures it has been established that MW synthesis in
dedicated devices should be considered.
INTRODUCTION
Several reports on the use of microwave (MW)
heating to accelerate organic chemical transformations
have appeared two decades ago involving the use of a
domestic microwave oven; since then, the use of MW
irradiation in organic synthesis has been increasing
annually, offering
a
promising alternative for
It is well known that the role of porphyrins in biological
functions such as photosynthesis, catalytic oxidative
transformations, transportation and storage of molecular
oxygen has been greatly assisted by studies using
synthetic models like meso-tetraarylporphyrins. The
potentiality of this type of porphyrins in a great number of
scientific fields (e.g., medicine, catalysis and in the
production of new electronic materials) is also well
established. The lack of functionalization of such type of
macrocycles is being overcome by intensive work
developed by several research groups and reported in
excellent reviews [11-15].
Macrocycle reactivity studies in addition to all those
potential applications point out that an easy and efficient
access to the starting porphyrin is required. The most
common synthetic methods leading to meso-tetraphenyl-
porphyrin (TPP) being used worldwide are summarized in
Table 1 [16]. Others methods derived from those shown
conventional heating sources [1-3]. It seems that the
reason for the observed rate enhancements is a purely
thermal/kinetic effect which is a consequence of the
high reaction temperatures that can be rapidly attained
when irradiating materials in a microwave field [4].
MW irradiation produces efficient internal heating by
direct coupling of MW energy with molecules being
present in the reaction mixture. Such procedure usually
reduces reaction times, avoids side reactions, increases
the yields and improves reproducibility [5,6].
As far as porphyrin synthesis is concerned, certain
publications describing the use of MW irradiations can be
found [7,8]; in some cases, domestic MW ovens have
been used. However uncontrolled MW heating in such
ovens involves non-continuous pulsed irradiation without
proper control of reaction temperature or pressure. If the
rate-enhancements are due to thermal effects, then one
needs to accurately control the temperature. Other

454
R. De Paula, M. A. F. Faustino, D. C. G. A. Pinto, M. G. P. M. S. Neves, J. A. S. Cavaleiro
Vol 45
Table 1
Experimental conditions for the most common methods in meso-tetraphenylporphyrin synthesis*.
Porphyrin Synthetic Methods
Rothemund
Adler
Lindsey
(I) Propionic acid
(II) Acetic acid
(I) 141 °C
(II) 120 °C
Dichloromethane
Chloroform
Solvent
Pyridine
Temperature
220 °C
-
25 °C
TFA
Catalyst
Same as solvent
BF3.etherate
BF3.etherate/ethanol
DDQ or p-chloranil
0.001 – 0.1 mol.dm-3
1 h
Oxidant
-
O2
Reactant concentration
Time of reaction
Procedure
3.6 mol.dm-3
48 h
0.3 – 1.0 mol.dm-3
0.5 – 1.0 h
One step
One step
Two steps
Purification
(work up)
Yield
Crystal separation
Filtration
Chromatography**
< 10%
Restrict
~ 20%
Medium
Up to 40%
Large
Method range
*Adapted from [16]; ** The yield is calculated by either UV-Visible spectrum or HPLC.
medium, thus making the work up less tedious.
[16,31-33].
in Table 1 can be found in literature but without great
advantage [17,18].
Following our interest [19-22] on the use of Microwave
Assisted Organic Synthesis we decided to study the
synthesis of TPP, using the Adler et al. conditions, but
under MW irradiation. It was aimed to look for how the
acid used, the power applied, the reaction time, the
reactants concentration and lastly, how the closed vessels
system can affect the yield in the synthesis of TPP. This
porphyrin was chosen because it is the most simple one
and it is being used in a wide range of chemical studies
mainly related to the reactivity and functionalization of
the macrocycle [23-30].
RESULTS AND DISCUSSION
MW heating at normal pressure. As it is mentioned
in Table 1, Adler and co-workers have found that the best
conditions for the condensation of pyrrole with benzalde-
hyde requires the use of boiling acetic or propionic acids;
in propionic acid TPP usually precipitates in the reaction
medium and the crystals can be collected by filtration.
As it was mentioned above we have selected the
reaction conditions developed by Adler and coworkers
(Table 1) to carry out the condensation reaction between
pyrrole and benzaldehyde. Their studies showed that the
best results were obtained when the condensation reaction
is performed in the presence of acetic acid or propionic
acid. Much poorer results were reported when longer
chain organic acids like butyric acid or mixtures of those
acids with toluene or benzene were used [16]. Under
those conditions the organic acid being used acts as the
solvent and the catalyst.
Figure 1. Effect of acid on the TPP yield under 650 W irradiation during
5 and 10 minutes at reflux.
These facts prompted us to look for the effect of these
two acids on the yield of TPP under microwave
irradiation at normal pressure. These reactions were
carried out using both reagents in a concentration of 0.20
mol.dm-3, which is close to the one reported by Adler, and
an irradiation power of 650 W. The yields were
determined after 5 and 10 minutes of irradiation and the
results obtained are shown in Fig. 1. It can be observed
that in acetic acid the yields are extremely low when
compared with those obtained in propionic acid, even
when the reaction is extended for 10 minutes.
Those authors in their original method observed that in
propionic acid TPP usually precipitates in the reaction
Presumably, due to the bigger pKa and dielectric
constants of acetic acid [34], there is more polymerization

Mar-Apr 2008
Kinetic Study of meso-Tetraphenylporphyrin Synthesis Under Microwave Irradiation
455
when using this acid in comparison with the case using
propionic acid. As a result we have selected propionic
acid as the solvent for further optimization studies.
The obtained results indicate that the best yields of TPP
(15-18%) can be achieved with a 650 W power irradiation
during 5 or 10 minutes, having a concentration of 0.20
mol.dm-3 for both reagents.
To check the power value selected (650 W) and so the
energy amount applied to the reaction, it was decided to
perform other experiments using the conditions reported
above, but irradiating with different MW powers. As
depicted in Figure 2, the power of 650 W provides the best
yield in such synthesis independently of how long the
reaction is irradiated (5 or 10 minutes). Nevertheless, in the
10 minutes irradiation case, irradiations with 650 W or 700
W gave rise to similar yields. Also with 650 W irradiations,
at longer times, no improvement was reached (Figure 2).
The presence of chlorin as a by-product is known for a
long time [35,36]. It usually occurs in 5% amounts [37]
and can be oxidized by DDQ as it is mentioned in Table
1. A good alternative for such oxidation relies on the use
of nitrobenzene in the reaction mixture [38]. Due to its
dielectric constant and high boiling point, nitrobenzene
has good features for MW irradiation [34]. Fig. 4 shows
the heating profile obtained in a typical experiment in
MW under reflux. Temperatures above the boiling point
of the solution are obtained faster in the presence of
nitrobenzene.
Figure 2. Effect of reaction time and energy applied on the TPP yield. *The
experiment for 10 minutes at 750 W was not run due to abnormal heating.
The concentration effect was also a parameter to be
evaluated. These studies were carried out using the two,
0.1 and 0.4 mol.dm-3 reagents concentrations. The vessels
were irradiated during 5 and 10 min at 650 W. Figure 3
summarizes the results obtained under the reaction
conditions selected.
Figure 4. Heating profile (MW 650 W) obtained in reaction carried out
with and without nitrobenzene under normal pressure glass reactor.
Conditions: 20 mL solvent (only propionic acid or the mixture propionic
acid/nitrobenzene 13:7), with pyrrole and benzaldehyde at
concentration of 0.20 mol.dm-3.
a
An experiment under these “oxidative” conditions
allowed us to obtain in a slightly better yield (20%) a
chlorin-free TPP after 5 minutes of MW irradiation. This
result led us to evaluate the nitrobenzene oxidizing effect
under pressurized system, using the mixture propionic
acid/nitrobenzene (13:7).
Closed vessel systems. In previous optimization studies
all experiments were run at reflux temperature (155 – 160
ºC) and normal pressure (Figure 4). The closed vessels
system can bring temperatures above those from the
solution boiling point, thus originating pressure values
higher than the normal one. The potential effect on the
TPP yield was then evaluated under these new conditions,
mainly considering the effects of reactants’ concentration,
irradiation time (2.5, 5.0 and 7.5 minutes) and
temperature.
Figure 3. Relationship between reactants concentration and time of
reaction on the yield of TPP, under 650 W power irradiation.

456
R. De Paula, M. A. F. Faustino, D. C. G. A. Pinto, M. G. P. M. S. Neves, J. A. S. Cavaleiro
Vol 45
It should be mentioned that it was assumed that the
reaction follows a pseudo-zero-order kinetics (equation
1), as it was determined before by Longo and co-workers
[32].
[TPP] = [TPP]0 + kt
equation 1
where [TPP] represents the molar concentration at a
given time; k is the pseudo-zero-order rate constant and t,
the time, given in minutes. For t=0, we can consider
[TPP]0=0.
The rate constant estimation can be seen in Figure 5.
The diluted condition (0.10 mol.dm-3) is not attractive
since a low rate constant was found and a longer reaction
time is then required for synthesis. With the 0.40 mol.dm-3
concentration the rate constant value is slightly smaller
than the value obtained when a concentration of 0.20
mol.dm-3 was used; the latter brings a higher TPP yield
and a higher rate constant [39].
(A)
(B)
Figure 6. (A)-TPP yields and selected temperature values (at 650 W
initial power). The standard deviation is 2.7%; (B)-profile obtained for
TPP synthesis under closed vessel system. Conditions: 20 mL propionic
acid/nitrobenzene (13:7), 0.20 mol.dm-3 of reactants concentration
during 5 min. in closed vessels.
The best TPP yield found was 35% (±2.7%) at 200 °C
(as shown in Figure 6-A). When reaching this temperature
value, the internal pressure also increased, achieving a
maximum value of 3 bar. Based on these results the
general view for chlorin-free TPP synthesis could be
rewritten as shown in Scheme I.
Figure 5. Results obtained for kinetic analysis of TPP synthesis under
MW irradiation in closed vessels showing the pseudo-zero order rate
constant obtained. Conditions: 20 mL of
a mixture propionic
acid/nitrobenzene (13:7), 0.20 mol.dm-3 reactants concentration and 650
W of MW power.
These results prompted us to find some physical-
chemistry parameters for TPP synthesis under MW
irradiation and to explain the effect attributed to the MW
acceleration in the synthesis when compared with the oil-
bath heating method.
To evaluate how higher temperatures than those due to
reflux can affect the TPP yield, a batch of reactions were
carried out at the same time and under the same
conditions.
Microwave effect. As postulated by several authors
[6,10,40], the effect provided by MW irradiation goes
beyond achieving the temperature in less time. The
different way for heating the system can affect the
Figure 6-A shows the yield obtained under the chosen
temperature values while Fig. 6-B presents the heating
profile for TPP synthesis in the studied temperature range.

Mar-Apr 2008
Kinetic Study of meso-Tetraphenylporphyrin Synthesis Under Microwave Irradiation
457
Scheme I
Schematic view to TPP synthesis under our synthetic conditions.
CHO
CH3CH2CO2H, 13 mL
NH
N
N
C6H5NO2, 7 mL
4
4
+
N
H
MW (650 W; 200 ˚C - 3 bar)
5 minutes
HN
35%
physical-chemistry parameters. Based on the results
shown in Fig. 6, we could estimate the rate constant and
activation energy for porphyrin synthesis.
Our tests for estimating the rate constant for TPP
synthesis (Figure 5) took into consideration what was
previously described by Longo [32] (reaction proceeding
in a pseudo-zero order kinetics).
The increment observed in our studies under
microwave irradiation can be explained through an
increment on the rate constant for the reaction; this can be
illustrated by the Arrhenius theory [10,4] (equation 2)
From Figure 7 the activation energy found for TPP
synthesis under MW is 14.9 kJ. This value is smaller than
58.5 kJ, the one obtained by Longo et al. [32], for the
same porphyrin.
These differences occur due to thermodynamic factors;
when the solvent and the reactants absorb energy, the
molecules can be aligned with the electromagnetic field
produced by MW. When this takes place, the reactant
molecules are best organized within the solvent cage. That
phenomenon does the activation energy of the transition
state to be reduced and consequently the entropy [4]. In
Table 2, we show the data reported by Longo et al. [32]
with our entropy value, obtained by using the same
approach as described in [41]
ln k = ln A – Ea/RT
equation 2
where k is the pseudo-zero order rate constant given in
mol.dm-3.min-1; A is the collision frequency in mol per
unit time; Ea is the activation energy given in Joule and R
is the ideal gas constant.
Using the results shown in Figure 6-A the activation
energy for such reaction under microwave irradiation can
be obtained.
Table 2
Entropy change in TPP synthesis under MW irradiation [42].
Condition
-T ΔS, kJ
4.18
Oil-bath (Longo condition)
MW (experimental result)
3.93
CONCLUSION
We describe here the microwave effect on the TPP
synthesis. The time saving and a good yield for TPP
indicates that MW can be used as an efficient alternative
energy source. Our best results were obtained using a
reactant concentration equals 0.20 mol.dm-3, propionic
acid (13 mL) and nitrobenzene (7 mL) mixture in only 5
minutes under a power of 650 W at pressurized system
(up to 3 bar; 200 ºC). An extension of these conditions to
the synthesis of several other meso-tetraarylporphyrins is
being carried out.
EXPERIMENTAL
The microwave device (Milestone Inc.) operates at fixed
frequency (2450 MHz) in multimode (run up to 12 flask
simultaneously). The equipment can control the temperature (up
to 250 ºC) and the pressure (up to 55 bar) in a range of power
values from 0 to 1000 Watts (steps in 10 W).
Figure 7. Correlation between rate constant and the inverse of
temperature obtained by kinetic measurements from 160 to 200 ºC. The
experimental Arrhenius function found was ln k = -1798.7.T-1 – 1.7987;
R2 = 0.91.

458
R. De Paula, M. A. F. Faustino, D. C. G. A. Pinto, M. G. P. M. S. Neves, J. A. S. Cavaleiro
Vol 45
Commun., 2001, 31, 33.
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Pyrrole was purchased from Aldrich and was distilled before
use. Benzaldehyde was used as received (Riedel-de-Häen).
Acetic and propionic acids were purchased from Sigma-Aldrich
and nitrobenzene was from Acros.
Unless other conditions are specified, a typical TPP synthesis
was carried out by putting into a normal pressure glass reactor,
pyrrole (0.280 mL, 4 mmoles), benzaldehyde (0.410 mL, 4
mmoles) and propionic acid (20 mL) as solvent. In the cases
where acetic acid was tested, the same amount (20 mL) was
employed as solvent. In experiments where nitrobenzene was
used, the volumes of acid and nitrobenzene were, respectively,
13 mL and 7 mL. The reactions were carried out usually for 5 or
10 minutes (sometimes in 7.5 min) under a specified MW
power. Once the reaction was completed, the mixture was
cooled down and methanol was added to promote the porphyrin
crystallization. The TPP crystals were collected by filtration.
Recrystallization was made in chloroform/methanol. The
experiments for the established conditions were carried out at
least, three times each one in order to verify the reproducibility;
the yields were the mean values obtained in those experiments.
The structure of the porphyrinic material was confirmed by
comparing the UV-Vis and 1H-NMR spectra with those
described in literature. UV-vis (CHCl3): λ, nm (log ε) 418
1
(5.65), 515 (4.25), 550 (3.87), 589 (3.74), 646 nm (3.58). H
NMR (300 MHz; deuteriochloroform): δH, ppm -2.78 (2H, s,
pyrrole NH), 7.71-7.78 (12H, m, Hm-, Hp-Ph), 8.18-8.23 (8H,
m, Ho-Ph), 8.84 (8H, s, pyrrole-H).
The kinetic measurements were performed under 650 W
irradiation power and the higher temperature achieved was 200
ºC. The reaction was stopped at different intervals (2.5, 5.0 and
7.5 min). The yield for each time was calculated after work up
and crystallization as mentioned above. For rate constant
estimation, the zero-order approximation was considered from
the TPP formation. The activation energy values were found by
following the Arrhenius theory; experiments were done by
fixing a given temperature value and the heating was achieved
using 650 W as initial power. The energy was cut off when
matching the set up temperature. For these experiments the
temperature values chosen were 160, 180 and 200 ºC.
[23] Cavaleiro, J.A.S., Neves, M.G.P.M.S., Tomé, A.C., Silva,
A.M.S., Faustino, M.A.F., Lacerda, P.S., Silva, A.M.G., J. Heterocycl.
Chem., 2000, 37, 527.
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ARKIVOC, 2003 (xiv), 107.
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[29] Lacerda, P.S.S., Silva, A.M.G., Tomé, A.C., Neves,
M.G.P.M.S., Silva, M.A.S., Cavaleiro, J.A.S., Llamas-Saiz, A.L.,
Angew. Chem., Int. Ed., 2006, 45, 5487.
[30] Giuntini, F., Faustino, M.A.F., Neves, M.G.P.M.S., Tomé,
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Acknowledgement. Thanks are due to University of Aveiro,
Fundação para Ciência e a Tecnologia (FCT) and FEDER for
funding the Organic Chemistry Research Unit. One of us (R. De
Paula) also thanks FCT for his PhD grant (SFRH/BD/
25666/2005) and Dr. Ana Margarida G. Silva for her helpful
contribution and comments.
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[39] In the experiments for the rate constant estimations, six
vessels were irradiated at the same time. The irradiation was stopped
every 2.5 minutes and then 2 vessels were withdrawn from the MW
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Mar-Apr 2008
Kinetic Study of meso-Tetraphenylporphyrin Synthesis Under Microwave Irradiation
459
the final slope of the MW heating curve was achieved, but only a small
amount of porphyrin was formed; at 5.0 minutes the reaction continues
to be heated; at this time the reaction is complete because almost no
difference in TPP yield can be observed between the two 5.0 and 7.5
minute heating times.
[41] Murzin, D., Salmi, T., In Catalytic Kinetics, Elsevier,
Amsterdam, 2005, pp. 75-109; Gibbs Energy of Activation, IUPAC
Golden Book, 2nd. Edition, 1997.
[42] In the transition state, we have a state function given by
!
!
!
, where G is the Gibbs energy, H the enthalpy and S
" G = " H #T." S
the entropy in the transition state. At this point, we can consider
[40] Langa, F., De La Cruz, P., De La Hoz, A., Díaz-Ortiz, A.,
Díez-Barra, E., Contemp. Org. Synth., 1997, 4, 373.
Ea # "!G
#
and
, according the Eyring theory.
" H = Ea ! R.T