Aryl nitroporphycenes and derivatives: First regioselective synthesis of dinitroporphycenes

Article abstract of DOI:10.1142/S1088424611003744

The nitration reaction of 2,7,12,17-tetraphenylporphycene has been studied. The use of AgNO₃ and a mixture of acetic acid and 1,2-dichloroethane as a mild nitrating system provides an optimized preparation of 9-nitro-2,7,12,17-tetraphenylporphy

Full text of DOI:10.1142/S1088424611003744

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2011; 15: 865–870
DOI: 10.1142/S1088424611003744
Published at http://www.worldscinet.com/jpp/
Aryl nitroporphycenes and derivatives: first regioselective
synthesis of dinitroporphycenes
Gonzalo Anguera, Maria C. Llinàs, Xavier Batllori and David Sánchez-García*
Grup d’Enginyeria Molecular, Institut Químic de Sarrià, Universitat Ramon Llull, Via Augusta 390,
08017-Barcelona, Spain
Dedicated to Professor Karl M. Kadish on the occasion of his 65th birthday
Received 29 April 2011
Accepted 19 May 2011
ABSTRACT: The nitration reaction of 2,7,12,17-tetraphenylporphycene has been studied. The use of
AgNO3 and a mixture of acetic acid and 1,2-dichloroethane as a mild nitrating system provides an
optimized preparation of 9-nitro-2,7,12,17-tetraphenylporphycene and a regioselective synthesis of
9,20-dinitro-2,7,12,17-tetraphenylporphycene. While 25 min of reaction are needed to obtain the
mononitrated compound, 4 h are necessary to yield a mixture of 9,20-dinitro and 9,19-dinitro 2,7,12,17-
tetraphenylporphycene in a proportion of 3 to 1. From this mixture, the geometric isomers can be
isolated by fractional crystallization. 9-Nitro-2,7,12,17-tetraphenylporphycene can be reduced to the
corresponding amino derivative, which is the starting material to obtain 9-(glutaric methylesteramide)-
2,7,12,17-tetraphenylporphycene, a versatile derivative useful for conjugation.
KEYWORDS: porphycenes, porphyrinoids, photosensitizers, aromaticity, nitration.
From a practical point of view, the latter possibility
INTRODUCTION
emerges as the most straightforward route to prepare
Porphycene is the first and most stable constitutional
derivatives for conjugation to biological vectors. With
isomer of porphyrin, prepared in 1986 by Vogel and
this aim, the 9-amino and 9-hydroxy derivatives of
coworkers [1]. These planar and aromatic macrocycles
some 2,7,12,17-alkyl substituted porphycenes have been
[2] are endowed with a unique set of physical and chemi-
prepared and coupled with carotenoids [12] and poly-
cal properties that have attracted interest in many fields
mers [13]. It is noteworthy that 9-amino derivatives are
ranging from theoretical chemistry, material chemistry
prepared with higher yields than the 9-hydroxy counter-
or biochemistry, and more specifically in photodynamic
parts, although their synthesis implies two steps: nitra-
therapy (PDT).
tion and Zinin reduction to the amine. In general, this
In order to prepare porphycene derivatives amenable
strategy combines experimental simplicity and economy
to be used in specific applications, two main strategies
without significantly eroding porphycene photophysical
of derivatization have been devised: synthesis de novo
properties [8].
(2,7,12,17-substituted porphycenes [3, 4], etioporphy-
The incorporation of aromatic groups at positions
cenes [5], alkyl and aryl meso substituted porphycenes
2,7,12,17 [14, 15] affects the optical properties of por-
[6] and benzoporphycenes [7]) and direct functionaliza-
phycene, leading to a dramatic bathochromic shift of the
tion of the macrocycle (hydroxylation, halogenation,
Q-bands. This enhanced absorption in the red region of
nitration [8, 9] and sulfonylation [10, 11]).
the spectrum is of paramount importance in applications
such as PDT, since the optimal optical window in biolog-
ical tissue is located between 660–750 nm [16]. There-
SPP full member in good standing
fore, the combination of aromatic substitution with the
possibility of preparation of 9-amino conjugates would
open the door for the design of new photosensitizers with
*Correspondence to: David Sánchez-García, tel: +34 932-67-
20-00, fax: +34 932-05-62-66, email: david.sanchez@iqs.url.
edu
Copyright © 2011 World Scientific Publishing Company

866
G. AnGuerA et al.
3
6
2
7
4
5
1, TPPo, R, R', R'' = -H
2, 9-NTPPo, R = -NO2 , R', R'' = -H
3, 9,20-DNTPPo, R = R' = -NO2, R'' = -H
4, 9,19-DNTPPo, R = R'' = -NO2, R' = -H
5, 9-ATPPo, R = NH2, R' = R'' = H
1
8
R'
R
22
N
H
N
20
19
9
H
N
10
24
23
N
R''
18
11
15 14
6, 9-EGlamTPPo, R = -NHCO(CH2)3CO2Me R'' = R' = -H
17
12
16
13
Chart 1. Chemical structure of 2,7,12,17-tetraphenylporphycene (TPPo), 9-nitro-, 9,20-dinitro, 9,19-dinitro-, 9-amino and
9-(glutaric methylesteramide)- 2,7,12,17-tetraphenylporphycenes
improved photophysical properties and selectivity. The
main drawback of this scheme is the low yield that ham-
pers the nitration of 2,7,12,17-tetraphenylporphycene (1)
[17]. Considering this fact, we were prompted to study and
optimize the preparation of 9-nitro-2,7,12,17-tetraphe-
nylporphycene (2) with the goal of using this compound
as a starting material to prepare amido conjugates [12].
(1:1) as eluent. Compound 2 was obtained in 90% yield
(88 mg) as a dark green powder, mp > 300 °C. 1H NMR
(400 MHz, CDCl3): δH, ppm 10.34 (s, 1H, C(10)-H), 9.82
(d, 1H, J = 11 Hz, C(19)-H), 9.74 (d, 1H, J = 11 Hz,
C(20)-H), 9.57–9.46 (4s, 4H, C(3,6,13,16)-H), 8.32–7.61
(m, 20H, 4 Ph), 3.45 (brs, 2H, NH).
3
3
Preparation of 9,20-dinitro-2,7,12,17-tetraphenyl-
porphycene (9,20-DNTPPo, 3) and of 9,19-dinitro-2,
7,12,17-tetraphenylporphycene (9,19-DNTPPo, 4). To
a solution of 50 mg (0.08 mmol) of 2,7,12,17-tetraphe-
nylporphycene (1) in 15 mL of acetic acid and 15 mL
of 1,2-dichloroethane, 1.3 g (7.7 mmol) of AgNO3 were
added. The resulting mixture was heated at 80 °C with
stirring for 4 h. Then 50 mL of water were added and the
mixture was extracted with dichloromethane. The com-
bined organic phases were dried with MgSO4 and the
solvent was evaporated. The residue was purified using
silica gel column chromatography with cyclohexane and
dichloromethane (1:1) as eluent. The second green frac-
tion was recrystallized twice from a mixture of cyclo-
hexane and dichloromethane yielding 3 (44%, 40 mg) as
a dark green powder, mp > 300 °C. The resulting solu-
tion was evaporated and the solid wasrecrystallized from
cyclohexane to render 4 as a dark green powder (18%,
13 mg), mp > 300 °C. 3. 1H NMR (400 MHz, CDCl3): δH,
ppm 10.30 (s, 2H), 9.50 (s, 2H), 9.35 (s, 2H), 8.26 (d, 4H,
J = 8 Hz), 7.97 (d, 4H, J = 8 Hz), 7.86 (t, 4H, J = 8 Hz),
7.76–7.61 (m, 8H), 4.31 (brs, 2H, NH). 13C NMR (75
MHz, CDCl3): δC, ppm 147.6, 144.6, 150.5, 139.9, 138.7,
137.3, 135.7, 135.6, 134.5, 131.3, 129.6, 129.4, 129.1,
128.9, 128.8, 128.5, 124.1, 112.4. IR (film, CHCl3):
ν, cm-1 3400–3200 (st N–H), 3057, 3027 (as C–NO2),
2924, 1531 (as C–NO2), 1326 (st C–NO2), 945, 843,
762, 699 (Ph). HRMS (HPLC-ESI-TOF): m/z calcd. for
C44H29N6O4 705.2245, found 705.2248. UV-vis: λmax, nm
(ε, M-1.cm-1) 679 (2.1 × 104), 629 (3.5 × 104), 600 (2.8 ×
104), 413 (9.2 × 104). 4. 1H NMR (400 MHz, CDCl3): δH,
ppm 10.23 (s, 2H), 9.43 (s, 2H), 9.30 (s, 2H), 8.26 (d, 4H,
J = 8 Hz), 7.93 (d, 4H, J = 8 Hz), 7.86 (t, 4H, J = 8 Hz),
7.74 (t, 4H, J = 8 Hz), 7.76–7.61 (m, 8H), 4.31 (brs, 2H,
NH). 13C NMR (75 MHz, CDCl3): δC, ppm 147.4, 144.4,
140.4, 139.7, 138.5, 137.1, 135.7, 135.4, 134.5, 131.4,
129.6, 129.3, 129.0, 128.9, 128.8, 128.4, 123.4, 112.3.
EXPERIMENTAL
Chemicals
All solvents (1,2-dichloroethane (DCE), tetrahydro-
furan, dichloromethane and cyclohexane) and starting
materials for synthesis were purchased fromAldrich and
were used as received. Spectroscopic grade chloroform
was used as received. 2,7,12,17-Tetraphenylporphycene
(TPPo) was synthesized as reported in the literature
[14]. Purity of all compounds was checked by HPLC
or tlc.
NMR spectroscopy
NMR spectra were recorded using a 400 MR spec-
13
trometer (1H at 400 MHz and C at 100.5 MHz). NMR
spectra were recorded at 25 °C in deuterochloroform
using tetramethylsilane as internal reference. NOESY1D
experiments were recorded using a mixing time of 500 ms
and a selective bandwidth of 10 Hz.
Synthesis
Preparation of 9-nitro-2,7,12,17-tetraphenylporphy-
cene (2). To a solution of 92 mg (0.15 mmol) of
2,7,12,17-tetraphenylporphycene (1) in 35 mL of acetic
acid and 35 mL of 1,2-dichloroethane, 2.4 g (14 mmol)
of AgNO3 were added. The resulting mixture was heated
at 80 °C with stirring for 25 min. Then 50 mL of water
were added and the mixture was extracted with dichlo-
romethane. The combined organic phases were dried
with MgSO4 and the solvent was evaporated. The resi-
due was purified using silica gel column chromatogra-
phy with a mixture of cyclohexane and dichloromethane
Copyright © 2011 World Scientific Publishing Company
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Aryl nItrOPOrPhyCeneS AnD DerIvAtIveS
867
IR (KBr): ν, cm-1 3400–3200 (st N–H), 3056, 3025, 1530
(as C–NO2), 1327 (st C–NO2), 945, 762, 697 (Ph). HRMS
(ESI–TOF): m/z calcd. for C44H29N6O4 705.2245, found
705.2230. UV-vis: λmax, nm (ε, M-1.cm-1) 679 (2.1 × 104),
629 (3.2 × 104), 600 (2.5 × 104), 413 (8.3 × 104).
popularity in order to introduce substituents into the
peripheral positions for instance in porphyrins and chlo-
rins [20]. However, electrophilic substitution still plays
a central role since halogenated and nitrated derivatives
provide the starting materials for other functionaliza-
tion and conjugation reactions. Particularly, electrophilic
substitutions on porphyrins are well known whether the
electrophile attacks on a meso-position [21–23] or in the
pyrrole position [24]. Recently, the group of Paolesse has
studied in detail the regioselective nitration of β-pyrrolic
positions of corroles [25–27].
It is noteworthy that electrophilic substitution in por-
phycenes has been closely examined. The first examples
were reported in the early patents by Vogel, where bromi-
nation and nitration of porphycenes were disclosed along
with the mono hydroxylation of the meso-position 9 [8].
According to the regioselectivity of the substitution, it is
possible to distinguish two kind of electrophiles: electro-
philes that attack on pyrroles (positions 3,6,13,16) and
electrophiles that attack on the meso-positions (positions
9,10,19,20). Typical reagents that react with the beta
position of pyrroles are sulfonyl chloride and bromine
[8, 10]. These reactions, usually, give mixtures of poly-
substituted isomers that must be purified. Interestingly,
while halogens and sulfonyl groups substitute pyrrolic
positions 3,6,13,16, the nitration of porphycenes renders
only the mono 9-nitro derivative [8, 17].
Preparation of 9-(glutaric methylesteramide)-2,7,
12,17-tetraphenylporphycene (9-EGlamTPPo, 6). A
solution of 109 mg (0.17 mmol) of 9-amino-2,7,12,17-
tetraphenylporphycene (prepared as described in the
literature [17] and used without chromatographic puri-
fication) in 15 mL dry tetrahydrofuran and 15 mL dry
pyridine was added at room temperature dropwise in 10
min by stirring a solution of 0.4 mL (2.89 mmol) glu-
taric methylester acid chloride in 10 mL of dry tetra-
hydrofuran. The solution was stirred for an additional
hour at room temperature, diluted with tetrahydrofuran,
cooled to 0 °C and treated with ice chilled water. The
mixture was washed twice with 10% sulfuric acid, twice
with water and once with 5% aqueous sodium hydrogen
carbonate. After evaporation of the solvent of the sepa-
rated organic layer, the residue was chromatographed
with dichloromethane/ethyl acetate (20:1) on silica gel.
Following evaporation of the solvent under vacuum and
washing of the resulting solid with pentanes, 111 mg of
the title compound were obtained in 93% yield as a dark
blue powder, mp > 300 °C. 1H NMR (400 MHz, CDCl3):
δH, ppm 10.24 (s, 1H), 9.77 (d, J = 12, 1H), 9.73 (d, J =
12, 1H), 9.59 (s, 1H), 9.55 (s, 1H), 9.50 (s, 1H), 9.39 (s,
1H), 8.68 (s, 1H), 8.41 (d, J = 8, 2H), 8.28 (t, J = 8, 4H),
7.96 (d, J = 8, 2H), 7.80 (m, 6H), 7.74 (m, 2H), 7.67
(m, 4H), 4.46 (brs, 1H), 4.04 (brs, 1H), 3.74 (s, 3H), 2.44
(s, 2H), 1.97 (s, 4H). 13C NMR (100 MHz, CDCl3): δC,
ppm 173.6, 170.7, 144.9, 144.8, 144.16, 143.4, 141.8,
140.5, 140.4, 138.7, 137.6, 136.4, 136.3, 135.8, 134.4,
133.5, 133.4, 131.6, 131.4, 131.3, 130.1, 129.2, 129.1,
129.0, 127.9, 127.8, 126.5, 125.1, 123.8, 123.5, 123.3,
51.6, 35.6, 33.4, 20.3. HRMS (ESI–TOF): m/z calcd. for
C50H40N5O3 758.3126, found 758.3128.
The nitration reaction of the 2,7,12,17-tetraphenylpor-
phycene with AgNO3 as mild nitrating reagent yields
a mixture of the 9-nitroTPPo in a modest yield (20%)
along with different polynitrated porphycenes, in a very
low rate [17]. In order to optimize the formation of 9-NT-
PPo (2) different solvents and temperature conditions
have been tested. Gratifyingly, when 1,2-dichloroethane
was used as a solvent at 80 °C the nitration was found
to proceed smoothly. The reaction rate of nitration is
quite slow, however, the addition of an excess of AgNO3
completes the transformation in 25 min. It was observed
that longer reaction times produced increasing quantities
of the polynitrated mixture of porphycenes (Scheme 1).
1
RESULTS AND DISCUSSION
Surprisingly, H NMR analysis of this mixture revealed
the presence of only two sets of signals with a ratio of 3
to 1, tentatively ascribed to two different dinitrated por-
phycenes 3 and 4. With the aim to gain insight into the
composition of this mixture, the reaction conditions of the
nitration of 1 were adjusted to maximize the amount of
dinitroporphycenes produced. It was found that 4 h were
necessary to yield the mixture of the geometric isomers
Synthesis
Among other methods, functionalization of porphyri-
noids can be conveniently accomplished by electrophilic
or nucleophilic substitutions [18, 19]. Alternatively, the
use of palladium cross-coupling reactions has gained
Ph
Ph
Ph
NO2
Ph
NO2
Ph
Ph
Ph
Ph
NO2
O2N
N
H
N
N
H
N
N
H
N
N
H
N
AgNO3
+
+
H
N
H
N
H
N
H
N
DCE/AcOH
N
N
N
N
O2N
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
1
2
3
4
Scheme 1. Synthesis of 9-nitro-, 9,20-dinitro and 9,19-dinitro-TPPo
Copyright © 2011 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2011; 15: 867–870

868
G. AnGuerA et al.
Ph
NH2
Ph
Ph
Ph
Ph
Ph
Ph
NO2
Ph
Cl
H
CO2Me
N
N
H
N
N
H
N
N
H
N
Na2S2O4
CH2Cl2
O
CO2Me
H
N
H
N
H
N
O
pyridine/THF
N
N
N
Ph
Ph
Ph
Ph
2
5
6
Scheme 2. Synthesis of 9-(glutaric methylesteramide)-2,7,12,17-tetraphenylporphycene
almost quantitatively along with traces of the 9-NTPPo
and little amounts of a complex mixture of uninvesti-
gated green colored porphycenes. NMR analysis of the
mixture allowed confirming the prima facie evidence
that the compounds present in the mixture were two
unprecedented meso dinitrated porphycenes, namely the
9,20-dinitro-2,7,12,17-tetraphenylporphycene (3, 9,20-
DNTPPo) and the 9,19-dinitro-2,7,12,17-tetraphenylpor-
phycene (4, 9,19 DNTPPo) (cf. NMR spectroscopy).
Column chromatography was unsuccessful to separate
the isomers. Fortunately, recrystallization of the mixture
from cyclohexane/dichloromethane provided pure iso-
mer 3. Evaporation of the solvents and recrystallization
of the residue with cyclohexane allowed the isolation of
the counterpart isomer 4.
It is not clear whether the nitration takes place via
a polar or a radical complex mechanism. However, we
postulate that Ag+ plays an important role in the process.
According to Olah [28, 29], the nitration with AgNO3
would start with the formation of a AgNO3 complex with
the non-bonding pairs of electrons of one of the pyrrolic
nitrogens. This complexation would enable the preferen-
tial nitration at the proximal meso-position.Another ques-
tion is the origin of the regioselectivity observed between
positions 19 and 20 in porphycenes 3 and 4. We hypothe-
size that taking into account our mechanism proposal it is
plausible that tautomerism of 2 could explain the ratio of
geometric isomers. However, at this stage more research
is still needed since other rationales cannot be ruled out.
Thanks to the optimization of the synthesis of 9-ni-
tro-2,7,12,17-tetraphenylporphycene (2) it is possible
to synthetize this porphycene in a preparative scale to
be transformed into 9-amino-2,7,12,17-tetraphenylpor-
phycene (5). The amino derivative 5 is an ideal starting
material for conjugation since it allows taking advantage
of the well-known chemistry of the amides. As a proof
of concept, the synthesis of the versatile 9-(glutaric
methylesteramide)-2,7,12,17-tetraphenylporphycene
(9-EGlamTPPo, 6) was proposed (Scheme 2).
The synthesis of 6 consisted of the reduction of the
nitro group to the amine with Na2S2O4 as reductant under
basic conditions (Zinin reduction) and amide coupling
with the corresponding acid chloride. The Zinin reduc-
tion was performed as previously described without any
further purification in order to avoid losses of the product
during chromatography [17]. It must be pointed out that
the progress of the reaction must be followed by tlc to
minimize overreduction. The crude amino derivative 5
is obtained nearly quantitatively and pure enough to be
coupled with acid chloride in the presence of pyridine
to yield the 9-(glutaric methylesteramide)-2,7,12,17-
tetraphenylporphycene in 93% yield.
Ph
O2N
Ph
NO2
Ph
Ph
NO2
N
H
N
N
H
N
+
H
N
H
N
N
N
O2N
Ph
Ph
Ph
Ph
3
4
H
H
Ph
Ph
NO2
N
N
H
N
H
N
O2N
Ph
Ph
4
Ph
O2N
Ph
NO2
N
H
N
H
N
N
Ph
Ph
3
Fig. 1. 1H NMR of the mixture and NOESY1D of 9,20-nitro-, and 9,19-nitro-TPPo
Copyright © 2011 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2011; 15: 868–870

Aryl nItrOPOrPhyCeneS AnD DerIvAtIveS
869
NMR spectroscopy
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Identification of both isomers from each other was
carried out by means of monodimensional transient NOE
experiments (NOESY1D). Due to the different geometry
of these compounds, signals at δH = 9.3–9.5 ppm (CH of
pyrrole rings) come from fairly different pairs of protons.
Thus, both methynes of vicinal pyrrole rings of isomer
3 contribute to the same signal, while in isomer 4 each
of these protons contributes to a different signal, so a
NOE between the corresponding pair of signals is only
expected in the second case.
Two NOE experiments were recorded on the original
mixture of both isomers, selecting signals at δH = 9.50
(major isomer) and at δH = 9.43 (minor isomer) (Fig. 1).
No enhancement of the signal at δH = 9.35 was observed
in the first experiment, while the signal at δH = 9.30 was
enhanced in the second. This result unambiguously iden-
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isomer must be isomer 3.
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CONCLUSION
Optimization of the synthesis of 9-nitro-2,7,12,17-
tetraphenylporphycene (2) by using an excess of AgNO3
and 1,2-dichloroethane as solvent has been accom-
plished. This new procedure allows obtaining 9-(glutaric
methylesteramide)-2,7,12,17-tetraphenylporphycene (6)
in two steps in a preparative scale. An increase in the time
of the nitration reaction of 2,7,12,17-tetraphenylporphy-
cene (1) proceeds with the regioselective formation of two
dinitro isomers: 20,9-nitro-2,7,12,17-tetraphenylporphy-
cene (3) and 19,9-nitro-2,7,12,17-tetraphenylporphycene
(4) in a ratio of 3 to 1. These geometric isomers were
isolated by fractional crystallization and their structure
confirmed by NMR. Dinitro porphycenes 3 and 4 could
be synthetically useful allowing, in principle, the attach-
ment of two different groups in order to provide specific
properties to the macrocycle such as water solubility for
potential PDT studies.
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Fellowships by the IQS grants (GA and MCL) are
acknowledged. We would like to thank Dr. Magda Faiges
for her assistance in UV-vis spectroscopy.
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