Unexpected one-pot synthesis of A3-type unsymmetrical porphyrin

Article abstract of DOI:10.1016/j.tetlet.2013.08.086

The first direct synthesis of A₃-type unsymmetrical porphyrin is achieved via conventional pyrrole-aldehyde condensation in a one-pot procedure in an appreciable yield. Generalization of this approach to a variety of other aldehydes revealed th

Full text of DOI:10.1016/j.tetlet.2013.08.086

Accepted Manuscript
Unexpected one-pot synthesis of A -Type Unsymmetrical Porphyrin
3
Mian H.R. Mahmood, Hai-Yang Liu, Hua-Hua Wang, Yi-Yu Jiang, C.-K.
Chang
PII:
S0040-4039(13)01472-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.08.086
TETL 43449
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
4 June 2013
10 August 2013
20 August 2013
Please cite this article as: Mahmood, M.H.R., Liu, H-Y., Wang, H-H., Jiang, Y-Y., Chang, C.-K., Unexpected one-
pot synthesis of A -Type Unsymmetrical Porphyrin, Tetrahedron Letters (2013), doi: http://dx.doi.org/10.1016/
3
j.tetlet.2013.08.086
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1
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Unexpected one-pot synthesis of A -Type Unsymmetrical Porphyrin
3
a,b
a,*
a
a
c*
Mian HR Mahmood, Hai-Yang Liu, Hua-Hua Wang, Yi-Yu Jiang, C.-K. Chang
Corresponding author: Tel. & Fax. +86-020-22236805; E-mail: chhyliu@scut.edu.cn (H.-Y. Liu)

Tetrahedron Letters
journal homepage: www.elsevier.com
Unexpected one-pot synthesis of A -Type Unsymmetrical Porphyrin
3
a,b
a,*
a
a
c*
Mian HR Mahmood, Hai-Yang Liu, Hua-Hua Wang, Yi-Yu Jiang, C.-K. Chang
a
b
c
Department of Chemistry, South China University of Technology, Guangzhou 510641, China
Department of Chemistry, University of Education, Lahore 54770, Pakistan
Department of Chemistry, Michigan State university, E. Lansing, MI 48824, USA
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
The first direct synthesis of A
3
-Type unsymmetrical porphyrin is achieved via conventional
pyrrole-aldehyde condensation in one-pot procedure in an appreciable yield. Generalization of
this approach to variety of other aldehydes revealed that it is advantageous for highly electron-
rich aldehydes giving unsymmetrical porphyrin, while electron-poor aldehydes even failed to
cyclize under the same experimental conditions.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Unsymmetrical porphyrin
Corrole
Pyrrole-aldehyde condensation
Synthesis
1
The chemistry of porphyrin and related compounds have
undergone a renaissance in the past few years owing to their
electron-rich aldehydes, we have found a very convenient one-
pot approach towards unsymmetrical porphyrins, based on acid-
catalyzed pyrrole/aldehyde condensation that largely depend on
the type of aldehyde used.
2
perspective applications in photodynamic therapy, mimicking
enzymes, catalytic reactions, molecular electronic devices,
solar energy transduction and sensing. The diversity in
functions has a great impetus on the extensive development of
novel synthetic methodologies for porphyrins, as well as
3
4
5
6
7
8
designing9 strategies for facile modification of preformed
porphyrin in order to tune the desired properties.
Among
meso-substituted
porphyrins,
symmetrical
porphyrins are more easily accessible than unsymmetrical
10
porphyrins (Scheme 1). The nature and propensity of meso-
substituents is mainly responsible for modulating the electronic,
11
chemical and thermal properties of porphyrin derivatives. The
term “unsymmetrical” generally refers to either the presence of
unidentical meso-substituents on the porphyrin periphery, as in
A
3
B-type porphyrins, or unevenly decorated porphyrin periphery.
This latter case represents A -type porphyrins, in which three out
of four available meso-positions are occupied by aryl groups, and
C-20 serves as methine-bridge only. A -type porphyrins are
typically applied as building blocks in porphyrin
3
Scheme 1. Structures of porphyrin and corrole
3
The macrocyclic compounds derived from electron-rich
aldehydes have been invoked as suitable candidates for
12,9a,e
functionalization,
including porphyrin dyads, arrays and
16
molecular organic frameworks, and as guest molecules in
mechanistic investigations.
Our choice for 4-(N,N-
13
porphyrin-based optical memory media . More recently, it has
been demonstrated that such electron-rich porphyrin derivatives
have potential for enhancing the photoelectrical performance of
dye-sensitized solar cells simply by modulating their HOMO-
dimethylamino)phenyl aldehyde as starting material towards
corrole, however, did not lead to success by literature methods
17-
19
. While the original solvent-free method was found suitable
18
only for electron-deficient aldehydes, it was reported to be
applicable for relatively less electron-deficient aldehydes as
14
LUMO energy gaps. Despite their importance, the development
of novel synthetic protocols for -type porphyrins have
remained a considerable challenge and the methods reported till
20
A
3
well. In our hands, this method did not turn out to be useful
towards any macrocycle starting from 4-(N,N-
dimethylamino)phenyl aldehyde, and no any indication was
observed even for the precursor that leads to ring closure after
treating with an oxidant. We further tried to synthesize corrole
under modified conditions, such as heating the reaction mixture
12d,14,15
to date offers complexity of multistep procedures,
most often resulted in a limited yield. Unfortunately, no report
exists for one-pot synthesis of -type unsymmetrical
porphyrins. In our efforts to synthesize corroles starting from
which
A
3
Corresponding author: Tel. & Fax. +86-020-22236805; E-mail: chhyliu@scut.edu.cn (H.-Y. Liu)

1
2
1
22
for a long time or the higher dilution conditions. The higher
dilution under acid catalysis was observed to be useful for the
synthesis of porphyrins from sensitive aldehydes, to establish an
equilibrium between porphyrinogen and polypyrrylmethanes.
This was the turning point that directed towards the current
tested under the same conditions, and a few typical results are
summarized in Table 1. We found that highly electron-rich
aldehyde 1a repeatedly gives unsymmetrical porphyrin 1, while
less electron-rich aldehydes such as 2a and 3a gave symmetrical
porphyrins. This implies that electronic features of the
substituents attached on the peripheral positions play a key role
for stabilizing the macrocycle in either case. In contrast, electron-
poor aldehydes such as pentafluoro benzaldehyde did not yield
29
unexpected findings of one-pot synthesis of
3
A -type
unsymmetrical porphyrins. Thus, refluxing equimolar
dichloromethane solution of pyrrole and aldehyde in the presence
of trifluoroacetic acid, followed by oxidation with DDQ and
simple chromatographic work-up gave purple colored product.
Its absorption spectrum (Fig. 1) gave the characteristic Soret- and
Q-band of porphyrinic compounds. Poprhyrins generally have
four Q-bands in contrast to three Q-bands in corroles. The
observed three Q-bands suspected it to be corrole.
Nevertheless, the appearance of mass ion peak at 668.3854 in the
mass spectrum was more surprising (Fig. S5), because it neither
30
any macrocycle, instead a linear polypyrrole was identified as
the sole isolable fraction as indicated by mass spectra (Fig. S9).
Exception to this behavior was para-nitro benzaldehyde, where
23
1
7b,31
the only isolated product was the corresponding corrole.
24
While electron-rich corroles are found less stable, and prolong
stirring of their methanol or benzene solution at room
temperature may cause interconversion into the corresponding
unsymmetrical porphyrin, it is not surprising that highly
electron-rich aldehydes may yield relatively more stable electron-
rich unsymmetrical porphyrins. It must be noted, however, that
the current method is not much suitable for the synthesis of
a
25
4
corresponds to A -type porphyrin nor corrole, yet the difference
between the observed mass and that of expected corrole (if it
would be) was only a mass equivalent to carbon. This implies the
presence of an additional meso-carbon (a methine bridge (-CH))
in the compound. Apparently, in the absence of any external
source for an additional meso-carbon in our reaction system,
this observation was truly unexpected. H NMR spectra often
provides decisive informations about the presence or absence of
symmetrical porphyrins (A -Type), particularly because of their
4
8
limited yield (Table 1) compared with the literature procedures.
25
1
1
2d,e,14,15
free meso-carbon in the porphyrins,
and may hence
differentiate between symmetrical and unsymmetrical
1
porphyrins. The H NMR spectrum indicated a peak at 10.14
ppm (Fig. S10) corresponding to the presence of one meso C-H
bond in the observed compound, while its symmetrical analogue
26
did not give this peak. The same phenomenon was depicted in
1
3
C NMR, where the presence of one free meso-carbon was
15c,27
unambiguously identified by a peak at 103.93 ppm,
while the
other meso-carbons appeared at 119.95 (C10) and 120.07 (C5,
C15) (Fig. S11) Further indication came from infra-red spectrum
of 1 (Fig. 1 inset) which showed a characteristic =C-H stretching
-
1
14
vibration near 3000 cm due to free meso-carbon . From these
observations, it is safe to conclude the formation of an
28
3
unsymmetrical (A -type) porphyrin 1 (Scheme 2) via acid-
catalyzed condensation of 4-(N,N-dimethylamino)phenyl
aldehyde and pyrrole.
Scheme 2. One-pot synthesis of porphyrins and corrole.
While unsymmetrical porphyrins are generally obtained
1
2d,14
from meso-unsubstituted dipyrromethane,
we found that
subphthalocyanines may undergo ring-expansion to give
32
unsymmetrical phthalocyanines.
The existence of
a
subporphyrin-type intermediate to give a similar reaction in the
present case may not be expected in the absence of borane
33
34
precursor and formaldehyde source. In contrast, ring expanded
porphyrinoids were isolated in the condensation reaction of
electron-poor aldehyde and pyrrole, whose metal complex
derivatives may give two identical symmetrical porphyrins via
35
thermal splitting. This phenomenon was suggested to be metal-
assisted and generated symmetrical porphyrins only, which limit
its application in the current observation.
Previously, a 2π-2π cycloaddition reaction with subsequent
formation of a spirocyclobutane intermediate had been proposed
to occur in case of auto-conversion of a corrole into an
Figure 1. UV-Vis spectrum of unsymmetrical porphyrin 1. Inset shows IR
-1
stretching vibration for free meso =C-H near 3000 cm
25
unsymmetrical porphyrin. For the current observation, it seems
more likely that a corrole is formed at some intermediate stage,
which being relatively unstable due to very rich π-electronic
cloud, undergoes conversion to its relatively stable
unsymmetrical porphyrin via similar mechanism (Scheme 3). As
this phenomenon is favored only in electron-rich aldehydes, any
The encouraging results in case of 1 were sufficiently
motivating to observe other aldehydes under the same
experimental conditions in order to rationalize the findings. A
wide range of aldehydes differing in electronic properties were

1
Table 1. The products obtained from various aldehydes in the current study.
Entry
Aldehyde
Product
Isolated yield (%)
1
2
3
4
5
4-(N,N-dimethylamino)phenyl aldehyde 1a
4-Tolylaldehyde 2a
Unsymmetrical porphyrin 1
Symmetrical porphyrin 2
Symmetrical porphyrin 3
Corrole 4
15
8
4-Methoxy benzaldehyde 3a
4-Nitro benzaldehyde 4a
9
6
Pentafluoro benzaldehyde 5a
Tetrapyrropentamethene 5
-
In short, we report the first direct synthesis of
unsymmetrical porphyrin via conventional acid-catalyzed
condensation of aldehyde and pyrrole in one-pot two-step
approach. The current investigation reveals that electronic
properties of aldehydes may be predominant in such processes,
and may direct towards unexpected findings. As porphyrins have
become the molecules of interest owing to their diverse
applications, the design of novel synthetic routes towards
porphyrins under various reaction conditions is still demanding.
Further work is under progress on various electron-rich and
electron-poor aldehydes in order to optimize the reaction
conditions, and to draw a clear demarcation between their
behaviors leading towards unsymmetrical porphyrins with the
confirmation of the plausible mechanism.
Acknowledgments
We thankfully acknowledge financial support from
National Natural Science Foundation China under grant
20971046 and 21171057
Supporting Information
The spectroscopic characterization data (UV-Vis, MS and NMR)
of the studied compounds is given in the supporting information.
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Scheme 3. Plausible mechanism for one-pot synthesis of
unsymmetrical porphyrins
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are formed in such cases. The identification of corrole in one case
also directs its formation at intermediate stage in the other cases.

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was cooled down to room temperature and was quenched by
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benzoquinone (DDQ) was added to the mixture and stirred for
another 1 hour. The purification procedure for 1 is given below:
After the completion of reaction, the solvent was dried in vacuum
and was purified by flash chromatography on neutral alumina
using dichloromethane/triethylamine (V/V 20:1) eluent followed
by basic alumina using dichloromethane. For analytical purpose,
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007, 63, 2455-2465.
Vis λmax(CH
2
Cl
2
)/nm 432, 518, 566, 653. H NMR (CDCl3, 400
MHz) δ (ppm) 10.14 (s, 1H), 9.31 (d, J = 4.6 Hz, 2H), 9.11 (d, J =
4.4 Hz, 2H), 8.98 (d, J = 5.5 Hz, 4H), 8.06-8.19 (m, 6H), 7.15 (dd,
J = 14.5, 8.5 Hz, 6H), 3.28 (s, 18H), -2.8 (s, 2H) TOF-MS: Found
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+
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MH 668.3854 Cald 667.34
2
010, 5, 904-909. (b) Xu, H. J.; Gross, C. P.; Brandes, S.; Ge, P.
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2
2 2
methylphenyl) porphyrin 2. UV-Vis λmax(CH Cl )/nm 408, 514,
1
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550, 589, 646. H NMR (CDCl3, 400 MHz) δ (ppm) 8.88 (s, 8H),
8.12 (d, J = 7.7 Hz, 8H), 7.58 (d, J = 7.7 Hz, 8H), 2.73 (s, 12H), -
+
2.75 (s, 2H) ESI-MS: Found MH 671.6 Cald 670.31. 5,10,15,20-
1
2. (a) Yedukondalu, M.; Maity, D. K.; Ravikanth, M. J. Porphyrins
Phthalocyanines 2011, 15, 83-98. (b) Senge, M. O. Acc. Chem.
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K. Y.; Lysenko, A. B.; Taniguchi, M.; Lindsey, J. S. Tetrahedron
Tetrakis(4-methoxyphenyl)
porphyrin
3.
UV-Vis
1
λ
max(CH
2
Cl )/nm 420, 518, 566, 653. H NMR (CDCl3, 400 MHz)
2
δ (ppm) 8.89 (s, 8H), 8.15 (d, J = 8.3 Hz, 8H), 7.31 (d, J = 8.4 Hz,
8H), 4.13 (s, 12H), -2.73 (s, 2H) TOF-MS: Found MH 735.163
Cald 734.29
+
2
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