An alternative synthesis of β-pyrrolic acetylene-substituted porphyrins

Article abstract of DOI:10.1016/j.tetlet.2008.07.059

A modified Horner-Emmons condensation reaction has been employed in the synthesis of acetylene-substituted porphyrins at the β-pyrrolic position. This technique was shown to have many advantages over the typically employed Sonogashira coupling method, including negating the requirement for a brominated porphyrin starting material. The electronic spectra of 2-(4′-carboxyphenyl)ethynyl-5,10,15,20-tetraphenylporphyrinato zinc(II) showed a red shift compared to the double bond equivalent, 4-(trans-2′-(2″-(5″,10″,15″,20″-tetraphenylporphyrinato zinc(II)yl))ethen-1′-yl)-1-benzoic acid. Comparison of the X-ray structures of 2-((4′-formyl)phenyl)ethynyl-5,10,15,20-tetraphenylporphyrinato zinc(II) and its analogue 4-(trans-2′-(2″-(5″,10″,15″,20″-tetraphenylporphyrinato copper(II)yl))ethen-1′-yl)-1-benzaldehyde showed an unexpected decrease in planarity in the analogue with the triple bond as opposed to that with the double bond.

Full text of DOI:10.1016/j.tetlet.2008.07.059

Tetrahedron Letters 49 (2008) 5632–5635
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
An alternative synthesis of b-pyrrolic acetylene-substituted porphyrins
a
Adam W. I. Stephenson a, Pawel Wagner b, Ashton C. Partridge a, , Kenneth W. Jolley ,
*
Vyacheslav V. Filichev a, David L. Officer b
a IFS MacDiarmid Centre, Massey University, Private Bag 11222, Palmerston North, New Zealand
b Intelligent Polymer Research Institute and Department of Chemistry, ARC Centre of Excellence for Electromaterials Science, University of Wollongong,
Northfields Avenue, Wollongong, NSW 2522, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
A modified Horner–Emmons condensation reaction has been employed in the synthesis of acetylene-
substituted porphyrins at the b-pyrrolic position. This technique was shown to have many advantages
over the typically employed Sonogashira coupling method, including negating the requirement for a bro-
minated porphyrin starting material. The electronic spectra of 2-(40-carboxyphenyl)ethynyl-5,10,15,20-
tetraphenylporphyrinato zinc(II) showed a red shift compared to the double bond equivalent, 4-(trans-
20-(200-(500,1000,1500,2000-tetraphenylporphyrinato zinc(II)yl))ethen-10-yl)-1-benzoic acid. Comparison of
the X-ray structures of 2-((40-formyl)phenyl)ethynyl-5,10,15,20-tetraphenylporphyrinato zinc(II) and
its analogue 4-(trans-20-(200-(500,1000,1500,2000-tetraphenylporphyrinato copper(II)yl))ethen-10-yl)-1-benzal-
dehyde showed an unexpected decrease in planarity in the analogue with the triple bond as opposed to
that with the double bond.
Received 6 June 2008
Revised 30 June 2008
Accepted 9 July 2008
Available online 12 July 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Plants and bacteria capture solar energy using porphyrin-based
chromophores, and convert it into chemical energy. The ability to
modify and tune the photophysical properties of synthetic porphy-
rins via the introduction of specific substituents has led to the
understanding and design of numerous porphyrin photonic assem-
blies, which mimic photosynthetic solar energy transduction by
converting solar energy to chemical potential in the form of
long-lived charge separated species.1–7 We have shown that the
efficiency of porphyrin-based dye sensitized solar cells (DSSCs) is
dependent on the extent of conjugation between the porphyrin
ring and the binding group, and a combination of a diethenyl linker
in the b-pyrrolic position and a malonic acid binding group
resulted in the highest cell efficiency of 7.1%.8 The degree of conju-
gation is dependent upon the degree of co-planarity between the
porphyrin ring and the linker.9 X-ray crystallographic results10,11
indicated that an ethene-linked bridge between the porphyrin core
and binding group at the b-pyrrolic position gave a dihedral angle
of 17°. In this work we report the synthesis of several ethyne
substituted 5,10,15,20-tetraphenylporphyrins (TPPs), the expecta-
tion being that the ethynyl functionality would induce greater
planarity and hence a greater photovoltaic efficiency.
2-bromo-5,10,15,20-tetraphenylporphyrin are restricted to small
scale,21 relatively low yielding reactions,15,12,22–25 and the target
compounds are difficult to purify. Sonogashira coupling also re-
quires the use of a copper catalyst, which has the potential to met-
alate/transmetalate the porphyrin.26 Alternative copper-free
coupling has been performed using Pd catalysts15 and ligands such
as AsPh3.27,15,12 However, the restriction imposed by the expense
and toxicity of the reagents, together with the harsh reaction con-
ditions (high reaction temperatures for extended times), restricts
the application of this approach and possible future scale-up.
In this work b-pyrrolic substituted porphyrins were derived
from a modified Horner–Emmons reaction. This method has been
shown to have generic applicability in the synthesis of a range of
symmetrical and non-symmetrical substituted diaryl acetyl-
enes,28–31 and negates the requirement for both a brominated
starting material and a metal catalyst. The reaction conditions
were also amenable to scale-up, and in the current application
the target compounds could be produced in high yield from the
readily synthesized and high yielding, 2-formylporphyrin
1
(Scheme 1).32 To the best of our knowledge, the Horner–Emmons
reaction has not been used previously for the synthesis of acetyl-
ene modified porphyrins.
Acetylene substitution of porphyrins at the b-pyrrolic position
is typically achieved via
a
Sonogashira12–14 or other metal-
A typical Horner–Emmons reaction uses equimolar amounts of
reactants and 2 equiv of base. However, in the synthesis of
acetylene-functionalized porphyrins 4a–d these conditions
resulted in the isolation of starting material 1 as well as an
inseparable mixture of the halovinyl intermediates 3 and the
desired products 4. Moderate to high yields of the desired products
catalyzed coupling reaction from the corresponding 2-bromo-
porphyrins.15–20 However, Sonogashira coupling reactions on
* Corresponding author. Tel.: +64 6 356 9099; fax: +64 6 350 5612.
E-mail address: A.Partridge@massey.ac.nz (A. C. Partridge).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.07.059

A. W. I. Stephenson et al. / Tetrahedron Letters 49 (2008) 5632–5635
5633
Ph
Ph
X
NH
N
N
NH
N
N
i
PhO
PhO
Ph
Ph
Ph
Ph
P
O
Ar
HN
HN
CHO
Ar
2a-e
Ph
Ph
X
3a-e
1
a)
Ar = C6H5, X = Cl
Ar = 4-MeOC6H5, X = Cl
b)
Ph
c)
Ar = 4-Benzonitrile, X = Br
Ar = 5,5-Dimethyl-2-phenyl-[1,3]dioxane, X = Br
e) Ar = 4-pyridyl, X = Cl
f) Ar = 4-Benzamide
d)
NH
N
N
Ph
Ph
HN
Ar
Ph
4a-f
Scheme 1. Synthesis of TPP derivates with an acetylene linker at the b-pyrrolic position. Reagents and conditions: (i) THF, t-BuOK (10% soln.), rt.
Table 1
Product yields
nates were prepared as the chloro analogues could not be synthe-
sized via the above method. Compounds 2c and 2d were obtained
using milder conditions (tetrabutylammonium bromide, 2,3-
dichloro-5,6-dicyanobenzoquinone and triphenylphosphine34),
which also prevented hydrolysis of the 1,3-dioxane ring in 2d.
The modified Horner–Emmons reaction provided a means of
attaching a range of acetylene-linked aromatic substituents, but
was restricted to substituents on the aryl ring that did not react
with the t-BuOK. As such, the reaction of 1 with diphenyl
chloro(4-methoxycarbonylphenyl)methylphosphonate or diphenyl
chloro(4-nitrophenyl)methylphosphonate failed to produce the
desired product but instead gave an intractable mixture of multiple
products. However, in the case of 4c, the reaction afforded a readily
separated mixture of the desired product and the unexpected
2-(40-benzamide)ethynyl-5,10,15,20-tetraphenylporphyrin (4f, cf.
Table 1). Although not a common method, the use of base for the
reduction of nitriles to amides has been previously reported.35–37
In order to demonstrate the versatility of the synthetic method
even for base-sensitive acetylene-substituted porphyrins, the
synthesis of the zinc porphyrin carboxylic acid Zn-7 (Scheme 2)
was carried out from the protected aldehyde 4d. The chelation of
zinc by these porphyrins provided the opportunity to compare
the spectroscopic properties of the acetylene-linked porphyrins
directly with their ethene analogues.
Substituent (Ar)
Product
Yield (%)
Phenyl
4-Methoxyphenyl
4-Benzonitrile
5,5-Dimethyl-2-phenyl-[1,3]dioxane
4-Pyridyl
4-Benzamide
4a
4b
4c
4d
4e
4f
54
84
26
88
70
36
(Table 1) were obtained by significantly reducing the reaction time
and dramatically increasing the concentration of base to a 10%
solution of t-BuOK (ca. 80 equiv). This resulted in only trace
amounts of the halovinyl intermediate 3, as confirmed by the NH
signal at ꢀ2.6 ppm in the 1H NMR spectra. Further treatment of
the isolated products with t-BuOK resulted in spectroscopically
pure materials, although in significantly lower yield. The pure form
of the aldehyde 5 was obtained directly via hydrolysis of the crude
4d (Scheme 2).
Phosphonates 2a, 2b and 2e were synthesized from the corres-
ponding aldehydes using diphenyl phosphite29 followed by conver-
sion to the chlorophosphonates with phosphorus oxychloride and
N,N-diethylaniline.33 In the cases of 2c and 2d, the bromophospho-
Scheme 2. Reagents and conditions: (i) DCM, TFA, H2O, rt, 1 h, 81% (ii) DCM, MeOH, Zn(OAc)2ꢁ2H2O, rt, 1 h, 99% (iii) THF, MeOH, NaCN, MnO2, reflux overnight, 88% (iv) THF,
MeOH, KOH, H2O, reflux overnight, 73%.

5634
A. W. I. Stephenson et al. / Tetrahedron Letters 49 (2008) 5632–5635
All products were characterized using 1-D and 2-D 1H and 13C
NMR spectroscopy, mass spectrometry and UV–visible spectro-
scopy. All the data collected (cf. Supplementary data) were
consistent with the proposed structures.
the starting material. The X-ray crystal data showed that the dihe-
dral angle between the plane of the porphyrin core and the plane of
the aryl substituent was greater in the acetylene-linked molecule
compared to that in an ethylene analogue. Also, the electronic
properties indicated only a slight bathochromic shift of the acety-
lene analogue compared to the corresponding ethylene analogue.
Both these observations are the focus of an on-going DFT
investigation.
The electronic spectra (Fig. 1) showed that both the Soret and
the two Q bands of the carboxylic acid derivative Zn-7 were red
shifted to 1.5, 2.5 and 5.5 nm, respectively, relative to the equiva-
lent ethene compound.6 Although the red shift is less than in
previous examples, this is consistent with observations in other
b-alkyne porphyrin derivatives.38,12 Single crystals of Zn-5 suitable
for X-ray diffraction were obtained by slow diffusion of methanol
into a solution of Zn-5 in dichloromethane (Fig. 2).39 Against
expectations, the dihedral angle between the plane of the benzene
ring and the plane of best fit made by the porphyrin core was
31.98(16)°, and almost twice that of the value of the corresponding
angle in 4-(trans-20-(200-(500,1000,1500,2000-tetraphenylporphyrinato
copper(II)yl))ethen-10-yl)-1-benzaldehyde of 17(2)°.10 A density
functional theory (DFT) study is currently being undertaken in an
attempt to account for these observations.
Acknowledgements
The authors wish to acknowledge Dr. Wayne Campbell for his
assistance and the financial support from the MacDiarmid Institute
for Advanced Materials and Nanotechnology and the Royal Society
of NZ Marsden Fund.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2008.07.059.
In conclusion, the use of modified Horner–Emmons conditions
has provided a versatile method for synthesizing acetylene-linked
substituents on the b-pyrrolic position of TPP. This resulted in a
high yielding, scalable methodology that negated the need for a
metal catalyst and 2-bromo-5,10,15,20-tetraphenylporphyrin as
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