Linear fully conjugated meso-aryl pentapyrrins

Article abstract of DOI:10.1016/j.tetlet.2009.09.145

A meso-2,6-dichlorophenyl pentapyrrin was prepared using a modified method for dipyrrane synthesis followed by simple DDQ oxidation. The fully conjugated structures formed via redox inter-conversions were studied using crystallographic as well as observed

Full text of DOI:10.1016/j.tetlet.2009.09.145

Tetrahedron Letters 50 (2009) 6909–6912
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Tetrahedron Letters
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Linear fully conjugated meso-aryl pentapyrrins
*
Ji-Young Shin, Steven S. Hepperle, David Dolphin
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada V6T 1Z1
a r t i c l e i n f o
a b s t r a c t
Article history:
A meso-2,6-dichlorophenyl pentapyrrin was prepared using a modified method for dipyrrane synthesis
followed by simple DDQ oxidation. The fully conjugated structures formed via redox inter-conversions
were studied using crystallographic as well as observed and calculated spectral data.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 27 August 2009
Revised 23 September 2009
Accepted 24 September 2009
Available online 29 September 2009
meso-Substituted polypyrranes have become important precur-
sors for the synthesis of meso-substituted expanded, contracted,
and isomeric porphyrins.1–3 Synthetic pathways to these com-
pounds have been well established.4 A one-flask synthesis of dipyr-
romethanes was introduced, primarily, by Lindsey’s group4 and
developed to achieve the synthesis of multigram quantities of
dipyrromethanes via simple purification methods.5 Utilizing a
modified synthetic method,6 Lee’s group have prepared meso-
substituted tripyrromethanes, as the major component, along with
other products such as dipyrromethanes and larger polypyrrome-
thanes. A previously reported synthetic method for tripyrrome-
thanes7 was employed in our recent research to prepare
polypyrromethanes
ranging
from
tripyrromethanes
to
heptapyrromethanes.8
Since the oxidation of meso-aryl tripyrromethanes can result in
the formation of macrocyclic compounds4 and tripyrrinone iso-
mers,8 we were also interested in the oxidations of relatively high-
er polypyrromethanes. Gross and his colleagues reported the
formation of a meso-aryl pentapyrrin from a solvent-free conden-
sation reaction of pyrrole and pentafluorobenzaldehyde.9 The aro-
maticity of macrocyclic compounds such as porphyrins and
expanded porphyrins have been extensively studied during the last
Figure 1. Crystal structure of 1;16 intramolecular hydrogen-bonds are denoted by
dashed lines.
decade,10–15 while linear oligopyrroles, having a continuous
p-con-
jugated network, have been less well understood due to their flex-
ibility and consequent lack of control of their configurations. The
aromaticity of conjugated macrocyclic compounds has been widely
studied and here we discuss the aromaticity of open-chain
compounds.
Fortunately, single crystals of a 2,6-dichlorophenyl-substituted
pentapyrrin having three amino-Ns in its molecular skeleton were
successfully grown and the crystal structure of 1 (Fig. 1) represents
one of three possible oxidation states that retains a conjugated net-
work along the main skeleton as shown in Scheme 1. Using the ini-
tial crystallographic data, the other oxidation states containing
N
H
N
H
N
H
ox
ox
N
N
N
HN
N
NH
HH
NN
HH
red
red
NN
N
N
1-2H
1
1+2H
Scheme 1. Three possible forms of a pentapyrrin showing various oxidation levels.
different conjugation networks were geometrically-optimized by
DFT calculation.
A series of meso-2,6-dichlorophenyl oligopyrroles was prepared
from pyrrole, dipyrromethane, and arylaldehydes by acid-
* Corresponding author. Tel.: +1 604 822 4571; fax: +1 604 822 9678.
E-mail address: david.dolphin@ubc.ca (D. Dolphin).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.145

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J.-Y. Shin et al. / Tetrahedron Letters 50 (2009) 6909–6912
Cl
Cl
ox
N
H
N
H
Cl
Cl
n
HN
N
n
Scheme 2. meso-2,6-Dichlorophenyl polypyrromethanes and polypyrrins.
catalyzed condensation using TFA (see the Supplementary data and
Scheme 2), and oxidized to from fully conjugated linear oligomers.
Based on the NMR data for tetrapyrrin, pentapyrrin, and hexapyr-
rin, the molecular structures are delocalized symmetric com-
Figure 3. Optical spectral dependence of 1 relative to the oxidation level; these
theoretical spectra were obtained by DFT calculations.
pounds stabilized by intramolecular hydrogen-bonding. As
a
result, only half the expected number of hydrogen signals was ob-
served in the spectra. Here, we focus on the structural and spectral
properties of the pentapyrrins.
A single crystal of 2,6-dichlorophenyl-substituted pentapyrrin
(1) was obtained by vapor diffusion of hexane into a CH2Cl2 solu-
tion. The structure was determined by X-ray diffraction analysis
as shown in Figure 1. Similar to the crystal structure of meso-per-
fluorophenyl pentapyrrin reported by Gross and co-workers the
crystal structure of meso-1,6-dichlorophenylpentapyrrin exhibits
a helical structure made more rigid by intramolecular hydrogen-
bonds between the imino-Ns and amino-NHs (Fig. 1). The NH peak
was observed at 12.59 nm which is considerably downfield due to
the intramolecular hydrogen-bond formation. The NMR data con-
firms the symmetry of the molecule. As noted above, the pentapyr-
rin can exist in three different p-conjugated networks as shown in
Scheme 1. The most stable form of the pentapyrrin can be expected
as 1 due to the nature of the intramolecular hydrogen-bonds as re-
vealed by the single crystal analysis.
Figure 4. Optical spectral changes observed upon the reduction of 1 using NaBH4:
before (dark blue line) and after (purple line) addition. The color of the solution
changed to the original orange after removal of excess borohydride by filtration and
standing open to the air.
The optical spectrum of a CH2Cl2 solution gradually changed
upon the addition of DDQ (Fig. 2). B-like bands were hypsochromi-
cally-shifted while the Q-like bands appeared over a broad range
(700–1000 nm) with bathochromic shifts. This B-like band shift
in the observed optical spectrum was also confirmed in the calcu-
lated optical spectrum17,18 (Fig. 3). Reduction of 1, in acetone, using
NaBH4 gave the changes shown in Figure 4. The desired product
1+2H is unstable due, presumably, to the lack of any internal
hydrogen-bonding. As shown in the photograph in Figure 4, the
reddish-pink solution (1+2H) after reduction of 1 returned to the
original orange color of 1 once the excess borohydride was
removed.
The geometrically-optimized structures shown in Figure 5
were obtained by Gaussian calculations.17,19 Compared to the
structure of 1+2H where all Ns are protonated, the structures
of 1 and 1-2H having imino-Ns showed a more compact config-
uration due to the intramolecular hydrogen-bonding, with 1
being the most compact. As shown in Figure 6, the dihedral an-
gles for each compound were determined as 82.9°, 43.1°, and
48.5° for 1+2H, 1, and 1-2H, respectively, and 1 was thus con-
sidered to be the most stable form. Also, relatively, 1 exhibits
the smallest dihedral angle between the mean planes for both
of the dipyrrins attached to the middle pyrrole ring (Fig. 6).
Typical N–HÁ Á ÁN hydrogen-bond strengths are 13 kJ/mol. Mole-
cule 1 is expected to have an additional stability of roughly
26 kJ/mol as it possesses two more NH–N intramolecular hydro-
gen-bonds than 1-2H. Compound 1+2H has no such additional
stability.
NMR data of 1 and 1+2H were obtained by DFT calculation.17,20
The results for 1 are very consistent with the observed NMR data
(Fig. 7, Table 1). On the other hand, it was thought that the increase
in flexibility for 1+2H, due to the absence of intramolecular hydro-
gen-bonds, would alter the chemical shift magnitude in NMR.
While the values of the observed and calculated data for 1 are
56.8 and 52.0 ppm for 13C and 6.7 and 6.6 ppm for 1H, respectively,
the values on the calculated data for 1+2H are 41.1 ppm and
2.4 ppm for 13C and 1H, respectively.
Macrocyclic compounds containing continuous
p-conjugated
networks have been widely studied with respect to their magnetic
properties. We have here reported how changes in conjugation
networks in open-chain fully conjugated molecules influence their
overall geometry and molecular properties.
Figure 2. Optical spectral changes of 1 in following progressive oxidation; the dark
blue line is the final spectrum.

J.-Y. Shin et al. / Tetrahedron Letters 50 (2009) 6909–6912
6911
Figure 5. Geometry of the optimized structures of pentapyrrins (a) 1+2H, (b) 1 and (c) 1-2H.
Table 1
NMR data comparison for compound 1
Label
Observed
Calculated19
1-2H
1
1-2H
1
1+2H
C1, C24
C2, C23
C3, C22
C4, C21
C5, C20
C6, C19
C7, C18
C8, C17
C9, C16
C10, C15
C11, C14
C12, C13
H1, H12
H2, H11
H3, H10
H4, H9
—
—
—
—
—
—
—
—
—
—
—
—
6.51
6.08
5.90
6.47
6.23
6.36
—
131.67
165.10
127.24
133.34
157.52
136.61
165.50
128.14
131.17
154.92
139.14
142.13
122.01
7.79
6.63
6.78
6.62
6.85
6.76
—
—
12.97
127.81
110.69
118.32
134.04
133.17
153.20
131.11
123.47
162.65
112.69
145.39
125.91
7.06
6.55
6.22
6.82
6.29
6.55
12.84
—
117.36
109.25
108.39
131.85
104.95
122.65
124.15
122.65
142.24
101.10
130.23
111.60
7.19
6.69
6.08
6.10
6.05
6.21
8.40
7.85
8.45
112.47
119.41
133.69
130.33
152.23
134.97
126.51
166.60
109.83
147.75
129.01
6.50
6.08
5.89
6.48
6.22
6.36
12.59
—
Figure 6. Dihedral angles between two dipyrromethene mean planes attached to
the middle pyrrole ring of , and
,
.
H5, H8
H6, H7
H25, H29
H26, H28
H27
—
12.9
12.59
11.79
Research into further understanding the structural interconver-
sion of pentapyrrins and their diverse -conjugated forms as well
p
as investigation into other polypyrrins is underway.
Acknowledgments
This work was supported by the Natural Sciences and Engineer-
ing Research Council (NSERC) of Canada. We thank Dr. Brian Pat-
rick of the UBC Department of Chemistry for the single crystal
diffraction analysis.
Figure 7. Atomic labels for the NMR data.

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J.-Y. Shin et al. / Tetrahedron Letters 50 (2009) 6909–6912
16. CCDC 745166; see the Supplementary data.
Supplementary data
17. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.;
Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.;
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A.
J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G.
A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I..; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
GAUSSIAN 03, Revision E.01; Gaussian: Wallingford, CT, 2007.
Supplementary data (NMR spectra of the compounds and crys-
tallographic (cif) and DFT calculation data) associated with this
article can be found, in the online version, at doi:10.1016/
j.tetlet.2009.09.145.
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