Synthesis of meso, meso-pyrrole-bridged diporphyrins by Cu(I)-mediated annulation

Article abstract of DOI:10.1021/ol100448x

Cu(I)-mediated annulation of meso,meso-1,3-butadiyne-bridged Zn(II) diporphyrin with various amines efficiently provided meso,meso'-pyrrole-bridged Zn(II) diporphyrins. Notable intramolecular electronic interactions in these diporphyrins were observed. A starburst porphyrin pentamer was synthesized via Suzuki-Miyaura coupling of N-bromophenylpyrrole-bridged diporphyrin and meso,meso'-diborylated porphyrin.

Full text of DOI:10.1021/ol100448x

ORGANIC
LETTERS
2010
Vol. 12, No. 8
1820-1823
Synthesis of meso,meso′-Pyrrole-Bridged
Diporphyrins by Cu(I)-Mediated Annulation
Chihiro Maeda,† Hiroshi Shinokubo,*,‡ and Atsuhiro Osuka*,†
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku,
Kyoto 606-8502, Japan, and Department of Applied Chemistry, Graduate School of
Engineering, Nagoya UniVersity, Chikusa-ku, Nagoya 464-8603, Japan
osuka@kuchem.kyoto-u.ac.jp; hshino@apchem.nagoya-u.ac.jp
Received February 22, 2010
ABSTRACT
Cu(I)-mediated annulation of meso,meso′-1,3-butadiyne-bridged Zn(II) diporphyrin with various amines efficiently provided meso,meso′-pyrrole-
bridged Zn(II) diporphyrins. Notable intramolecular electronic interactions in these diporphyrins were observed. A starburst porphyrin pentamer
was synthesized via Suzuki-Miyaura coupling of N-bromophenylpyrrole-bridged diporphyrin and meso,meso′-diborylated porphyrin.
Both cyclic and acyclic porphyrin arrays have been exten-
sively studied in view of mimicry of natural photosynthesis,
applications to molecular wires, and nonlinear optical de-
vices.1 Porphyrin dimers linked by various spacers have been
synthesized as a fundamental structural motif to investigate
the electronic interactions between the porphyrins, which will
influence their electrochemical and photophysical properties.2
Such porphyrin dimers are often prepared by oxidative
coupling reactions or metal-catalyzed coupling reactions.3,4
Acetylene moieties can be selectively activated by transi-
tion metal catalysts, hence allowing site-selective reactions
to provide heterocyclic compounds.5 However, this type of
reaction has been rarely used for meso-ethynylated porphy-
rins, while such species have been actively studied in light
of their highly conjugated electronic properties.6,7 Recently
† Kyoto University.
‡ Nagoya University.
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10.1021/ol100448x  2010 American Chemical Society
Published on Web 03/12/2010

we have explored the transition-metal-catalyzed annulation
reactions of meso-ethynylated porphyrins that effectively
provide 5-azaindole- and 1,2,3-triazole-appended porphy-
rins.7 Here we report Cu(I)-mediated annulation of 1,3-
butadiyne-bridged diporphyrin with various amines, which
provides pyrrole-2,5-bridged diporphyrins in moderate to
good yields.8,9 Although a pyrrole moiety is quite attractive
as a bridging spacer in view of the electron-rich property,
effective conjugation, and potential use as a hydrogen-
bonding donor, handling of pyrroles is not trivial because
of their instability and high reactivity. Hence this annulation
approach, which circumvents functionalization of pyrroles,
is useful from a synthetic viewpoint.
bridge and the porphyrin mean plane is 79°, and that between
the two porphyrin planes is 135° (Figure 1).12
Figure 1. X-ray crystal structure of 2a. meso-Aryl groups and
hydrogen atoms are omitted for clarity. The thermal ellipsoids were
at 50% probability level.
Several synthetic procedures for the transformation of 1,3-
diynes to pyrrole rings have been reported.10,11 We modified
the reaction conditions of the Cu(I)-mediated annulation10
so as to be applicable for the porphyrin system. A mesitylene
solution of 1,3-butadiyne-bridged diporphyrin 1, aniline (50
equiv), and CuCl (4 equiv) was refluxed for 24 h (Scheme
1). After chromatographic separation, N-phenylpyrrole-
This annulation reaction was investigated for various
amines (Table 1). The reaction of p-methoxy- and p-
bromoanilines provided the corresponding diporphyrins 2b
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Scheme 1.
Synthesis of 2aa
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(8) Acid-catalyzed transformation of a 1,3-butadiyne-bridged diporphyrin
to a furan-bridged diporphyrin was reported previously: Arnold, D. P.;
Nitschinsk, L. J. Tetrahedron 1992, 48, 8781–8792.
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Webb, N. W. Aust. J. Chem. 1972, 25, 1991–2001. (b) Aota, H.; Itai, Y.;
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2000, 2, 187–189. (e) Silva, A. M. G.; Tome´, A. C.; Neves, M. G. P. M.
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5228.
a Ar ) 3,5-di-tert-butylphenyl, R ) Ph.
bridged diporphyrin 2a was obtained in 56% yield. The
structure of 2a has been confirmed by the spectroscopic data
and single crystal X-ray diffraction analysis. High-resolution
electro-spray-ionization time-of-flight (HR-ESI-TOF) mass
spectrum detected the parent ion peak of 2a at m/z )
2017.0547 (calcd for C134H150N9Zn2 [M + H]+ ) 2017.0598).
1
The H NMR spectrum of 2a is fully consistent with its
structure, where the pyrrolic ꢀ-protons are observed at δ )
7.76 ppm, and the phenyl protons at the pyrrole bridge are
shifted upfield at δ ) 6.83, 5.59, and 5.57 ppm as a result
of the ring current of the porphyrins. Slow vapor diffusion
of acetonitrile to a dioxane solution of 2a provided its nice
crystals for X-ray diffraction analysis. The crystal structure
of 2a revealed unambiguously a N-phenylpyrrole-bridged
diporphyrin skeleton, in which the Zn-Zn center-to-center
distance is 11.4 Å, the dihedral angle between the pyrrole
(12) Crystallographic data for 2a. Formula: C134H149N9Zn2, Mw
)
2016.36, monoclinic, space groupC2/c, a ) 29.021(5), b ) 10.712(5), c )
49.322(5) Å, ꢀ ) 98.552(5)°, V ) 15162(8) Å3, Z ) 4, Fcalcd ) 0.883 g
cm-3, T ) -183 °C, 6626 measured reflections, 13315 unique reflections
(Rint ) 0.0930), R1 ) 0.0930 (I > 2σ(I)), wR2 ) 0.2347 (all data), GOF )
0.930. The contribution to the scattering arising from the presence of the
disordered solvents in the crystals was removed by use of the utility
SQUEEZE in the PLATON software package: (a) Spek, A. L. PLATON, A
Multipurpose Crystallographic Tool; Utrecht University, Utrecht, The
Netherlands, 2005. (b) Sluis, P.; van der; Spek, A. L. Acta Crystallogr.,
Sect. A 1990, 46, 194–201.
Org. Lett., Vol. 12, No. 8, 2010
1821

first oxidation waves were split into two waves in all cases
(Figure 3), indicating the substantial electronic interaction
Table 1. Synthesis of Pyrrole-Bridged Diporphyrinsa
entry
R
product
yield (%)b
1
2
3
4
5
6
Ph
2a
2b
2c
2d
2e
2f
56
74
79
15
31
25
4-MeO(C6H4)
4-Br(C6H4)
mesityl
benzyl
octyl
a Reaction conditions: 1 (5 µmol), amine (50 equiv), CuCl (4 equiv), in
mesitylene (1 mL), reflux, 24 h. b Isolated yield.
and 2c in good yields, while bulky 2,4,6-trimethylaniline and
alkylamines were found to be less reactive. These synthesized
pyrrole-bridged diporphyrins were fully characterized by their
spectroscopic data (Supporting Information).
The butadiyne-bridged diporphyrin 1 shows the perturbed
and red-shifted UV-vis absorption spectrum.13 Compared with
this, the absorption spectrum of 2a exhibits a split and blue-
shifted Soret band at 426 and 436 nm and Q-bands at 556 and
601 nm (Figure 2), reflecting the decreased electronic interac-
Figure 3. Cyclic voltammograms of (a) 2a, (b) 2c, and (c) 2f.
(solvent, CH2Cl2; supporting electrolyte, Bu4NPF6 (0.10 M); counter
electrode, Pt; reference electrode, Ag/Ag+; scan rate, 0.05 V/s).
Accurate potentials were determined by the differential pulse
voltammetry method (Supporting Information).
between the two porphyrins through the pyrrole spacer. The
splitting potential widths are similar, being 0.08-0.09 V. In
addition, the oxidation potentials are dependent on the
N-substituent on the central pyrrole: 0.27, 0.30, and 0.32 V
for 2f, 2a, and 2c, respectively. This is due to the presence
of the electronic communication between the porphyrin and
the pyrrole spacer, while such dependence is weaker for the
reduction potentials.
Figure 2. UV-vis absorption (left) and fluorescence (right) spectra
in CHCl3 (black line ) 1, red line ) 2a, blue line ) 2e). The
excitation wavelengths are 452 nm for 1, 426 nm for 2a, and 423
nm for 2c.
tions of the meso-bridged pyrrole spacers. The fluorescence
spectrum of 2a is observed at 613 and 653 nm with a clear
vibronic structure. While the absorption and fluorescence spectra
of 2b and 2c are similar to those of 2a, the fluorescence
spectrum of 2d that bears a sterically hindered N-pyrrole aryl
substituent is red-shifted at 630 nm. Interestingly, the fluores-
cence spectra of 2e and 2f that bear N-pyrrole alkyl substituents
are broader and red-shifted, as compared with those of N-aryl-
substituted diporphyrins. The fluorescence quantum yields of
2e and 2f are almost twice as those of 2a-2c (Table 2).
We then anticipated that N-bromophenyl Zn(II) diporphy-
rin 2c would be used as a building block of large and more
elaborated multiporphyrin arrays. Thus, we examined the
Suzuki-Miyaura cross-coupling reaction of 2c with dibo-
rylated porphyrin 3, which furnished starburst Zn(II)-free-
base hybrid porphyrin pentamer 4 in 15% yield (Scheme 2).14
The MALDI-TOF mass measurement detected the parent ion
peak at m/z ) 4715 (calcd for C316H347N22Zn4 [M - H]- )
1
4715). The H NMR spectrum of 4 elucidated its highly
symmetric structure, exhibiting the pyrrolic ꢀ-protons of the
central free base moiety at δ ) 7.33 and 6.38 ppm (Figure
S15 in Supporting Information). This result demonstrates that
meso,meso′-pyrrole-bridged diporphyrins are stable enough
to be used for transition-metal-catalyzed coupling reaction.
Figure 4 shows the UV-vis absorption and fluorescence
spectra of 2a and 4, along with meso-tetraphenyl porphyrin
(TPP). The absorption spectrum of 4 is almost the superposi-
tion of those of 2a and TPP in a 2:1 ratio, allowing a selective
excitation at both Zn(II) porphyrins. The fluorescence
spectrum of 4 upon selective excitation of the Zn(II)
porphyrin parts at 436 nm is practically the same as that of
TPP with a small shoulder at 620 nm, indicating the efficient
Table 2. Photophysical Data
b
compound
λA (nm)a
λF (nm)
ΦF
2a
2b
2c
2d
2e
2f
426, 436
426, 436
429, 437
428, 438
423, 433c
422, 432c
613, 653
613, 655
613, 655
630
645
648
0.052
0.065
0.066
0.073
0.120
0.128
a Soret bands. b Quantum yields were determined with reference to the
value (0.033) of Zn(II)TPP in toluene. c Shoulder.
The electrochemical properties of 2a, 2c, and 2f were
investigated by cyclic voltammetry. Characteristically, the
1822
Org. Lett., Vol. 12, No. 8, 2010

In summary, we have synthesized meso,meso′-pyrrole-
bridged zinc(II) diporphyrins via the Cu(I)-mediated annu-
lation reaction of meso,meso′-1,3-butadiyne-bridged dipor-
phyrin with various amines. In these diporphyrins, notable
electronic interactions are observed in the absorption and
fluorescence spectra as well as the cyclic voltammograms,
which vary upon the pyrrolic N-substituent. In Zn(II)-free-
base hybrid porphyrin pentamer 4, synthesized by the
Suzuki-Miyaura coupling reaction of 2c and 3, the energy
transfer from the Zn(II) porphyrin peripheries to the free-
base porphyrin core takes place efficiently. Extension of this
synthetic strategy to more elaborated pyrrole- or heterocycle-
bridged multiporphyrin arrays is actively pursued in our
laboratory.
Scheme 2. Synthesis of Porphyrin Pentamer 4
Acknowledgment. This work was supported by Grants-
in-Aid for Scientific Research (nos. 19205006 (A) and
20108001 “pi-Space”) from the Ministry of Education,
Culture, Sports, Science, and Technology, Japan. C.M. thanks
the JSPS fellowship for young scientists.
Supporting Information Available: Experimental pro-
cedures, compound data, and crystallographic data for 2a in
CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
intramolecular excitation energy transfer from the zinc(II)
porphyrin peripheries to the free-base porphyrin core. The
fluorescence quantum yield of the pentamer 4 is 7.2%, which
is larger than that of 2a (5.2%). This result again indicates
the efficient intramolecular excitation energy transfer from
the Zn(II) porphyrin peripheries to the free-base TPP core.
OL100448X
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Figure 4
. UV-vis absorption (left) and fluorescence (right) spectra
in CHCl3 (blue line ) 4, red line ) 2a, black line ) TPP).
Org. Lett., Vol. 12, No. 8, 2010
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