Meso-β Dibenzo[ a,g ]corannulene-fused porphyrins

Article abstract of DOI:10.1021/ol501115m

FeCl₃-mediated oxidative fusion of meso-linked dibenzo[a,g]corannulene-porphyrin dyads 6M afforded fused porphyrins 7M bearing a five-membered ring connection, but similar oxidation of β-linked dyads 9M provided fused porphyrins 10M bearing a six-membered ring connection, both in a regiospecific manner. While fused dyads 10M exhibit modestly red-shifted absorption and fluorescence profiles, fused dyads 7M display characteristically red-shifted absorption bands reflecting antiaromatic dehydropurpurin electronic networks.

Full text of DOI:10.1021/ol501115m

Letter
pubs.acs.org/OrgLett
meso‑β Dibenzo[a,g]corannulene-Fused Porphyrins
Kensuke Ota, Takayuki Tanaka,* and Atsuhiro Osuka*
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
S
* Supporting Information
ABSTRACT: FeCl3-mediated oxidative fusion of meso-linked
dibenzo[a,g]corannulene−porphyrin dyads 6M afforded fused
porphyrins 7M bearing a five-membered ring connection, but
similar oxidation of β-linked dyads 9M provided fused porphyrins
10M bearing a six-membered ring connection, both in a regiospecific
manner. While fused dyads 10M exhibit modestly red-shifted
absorption and fluorescence profiles, fused dyads 7M display
characteristically red-shifted absorption bands reflecting antiaro-
matic dehydropurpurin electronic networks.
due to facile synthetic access from acenaphthenequinone (see the
Supporting Information). In addition, the bowl depth of
dibenzo[a,g]corannulene 2 (0.83 Å) is more shallow than that
of the parent corannulene 1 (0.87 Å), an attribute that was
envisaged to allow the intramolecular fusion reaction to proceed
more favorably.6,7
After extensive screening of catalysts and reaction conditions,
we found that a Pd(II) precatalyst developed by Buchwald et al.8
was effective for the Suzuki−Miyaura coupling of brominated
porphyrins 4Zn and 4Ni with 8-boryldibenzo[a,g]corannulene 5.
Under the mild conditions shown in Scheme 1, palladium-
catalyzed cross-coupling reactions gave singly linked products
6Zn and 6Ni in 65% and 75% yields, respectively. The coupling
reaction of meso-borylated porphyrins with 8-bromocorannulene
used porphyrins have attracted considerable attention in
Frecent years owing to their attractive properties such as red-
shifted absorption and emission profiles, intriguing nonlinear
optical properties, high electron mobilities, and multicharge
storage capabilities.1 An increasing number of fused porphyrins
have been continuously developed to date,2 including naph-
thalene-fused,2a−c anthracene-fused,2d−f BODIPY-fused,2g and
planar polyaromatic hydrocarbon (PAH)-fused porphyrins.2h−j
In addition, directly fused porphyrin tapes have also been
developed, which exhibit drastically red-shifted absorption
bands.3 In all of these previous reports, the π-conjugated
segment that was fused to the molecular scaffold of interest was
planar. Just a handful of examples exist whereby curved π-
conjugated segments have been successfully employed in fusion
reactions of this type. As a rare and interesting example, Lash et
al. reported a β−β corannulene-fused porphyrin 3 which was
synthesized via a McDonald-type condensation using a
corannulene-fused pyrrole as starting material (Figure 1).4,5 In
this paper, we report the synthesis of meso,β-corannulene-fused
porphyrin dyads which were successfully obtained via cross-
coupling of the appropriate porphyrin and corannulene frag-
ments followed by ferric chloride-mediated oxidative fusion. As a
coupling partner, we chose 8-boryldibenzo[a,g]corannulene 5
Scheme 1. Synthesis of 6M from 4M and 5
Figure 1. Corannulene 1, dibenzocorannulene 2, and Lash’s β,β-
corannulene-fused porphyrin 3.
Received: April 17, 2014
Published: May 12, 2014
© 2014 American Chemical Society
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Letter
afforded the same product albeit with slightly lower efficiency
(ca. 51% for 6Zn). High-resolution atmospheric pressure−
chemical ionization time-of-flight mass-spectroscopy (HR-
APCI-TOF-MS) revealed the parent ion peaks at m/z =
1284.5926 (calcd for C90H84N464Zn: 1284.5982, [M]+) for
6Zn and at m/z = 1278.6000 (calcd for C90H84N458Ni:
1278.6044, [M]+) for 6Ni, respectively. Finally, single-crystal
X-ray diffraction analysis revealed the structure of 6Zn (Figure
2a). The bowl depth of the dibenzocorannulene unit in 6Zn is
Scheme 2. Oxidative Fusion Reaction of 6M
positions in 6Zn (ca. 3.3 Å, Figure S11, Supporting Information),
owing to the bent structure of the dibenzocorannulene segment.
Similarly, the oxidation of 6Ni with FeCl3 gave 7Ni in 88% yield.
The structure of 7Ni has been assigned on the basis of the
1
similarity of its H NMR spectrum to that of 7Zn (Figure S3,
Supporting Information).
We then turned our attention to attaching the dibenzocor-
annulen-8-yl unit at the β-position of the porphyrin scaffold.
Under similar palladium-catalyzed cross-coupling conditions
that worked for the meso-8-linked system, bromoporphyrin 8M10
was coupled with 5 to afford corannulene-appended porphyrins
in 69% yield for 9Zn and 59% yield for 9Ni (Scheme 3).
Figure 2. X-ray crystal structures of (a) 6Zn and (b) 7Zn. Solvent
molecules, hydrogen atoms, and tert-butyl groups were omitted for
clarity. Thermal ellipsoids were scaled to 30% probability level. One of
the two molecules in the asymmetric unit of 7Zn is shown.
Scheme 3. Synthesis of 9M from 8M and 5
0.77 Å, which is more shallow than that of 2 (0.83 Å).7 The
dihedral angle between the mean plane of the Zn(II) porphyrin
unit and the mean plane of the dibenzocorannulene unit
(calculated with respect to C8 and the two neighboring carbon
atoms of the dibenzocorannulene unit) is 77.0°.
Then, we attempted an oxidative fusion reaction of 6M
(Scheme 2). Oxidation of 6Zn with 2,3-dichloro-5,6-dicyano-
1,4-benzoquinone (DDQ) and Sc(OTf)3, a combination known
to be effective for the synthesis of directly fused porphyrin
tapes,3c−e yielded brown solids in 29% yield. The HR-APCI-
TOF-MS of this product revealed the parent ion peaks at m/z =
1282.5889 (calcd for C90H82N464Zn: 1282.5825, [M]+),
suggesting a loss of two hydrogen atoms from 6Zn. The same
product was obtained in a better yield (70%) by oxidation of 6Zn
with FeCl3. Single-crystal X-ray diffraction analysis revealed that
this product was 7Zn, in which an additional bond has been
formed between the 7-position of the dibenzocorannulene unit
and the β-position of the Zn(II) porphyrin framework (Figure
2b). Interestingly, the formation of fused constitutional isomers
was not observed, indicating the exclusive regioselectivity of the
fusion reaction. Oxidative ring-closure reactions that give rise to
fused structures containing a five-membered ring rather than a
six-membered ring are quite rare in the literature.1k,8,9 In the
present case, the observed high regioselectivity of the fusion
reaction may be ascribed to a short distance between the fusing
Corannulene-appended porphyrins 9Zn and 9Ni were then
subjected to oxidative fusion reaction conditions (Scheme 4).
While the DDQ−Sc(OTf)3 couple did not afford any desired
fused products, FeCl3-mediated oxidation was effective to afford
10Zn and 10Ni in 45% and 29% yields, respectively. HR-APCI-
TOF-MS revealed the parent ion peaks at m/z = 1282.5869
(calcd for C90H82N464Zn: 1282.5825, [M]−) for 10Zn, and at m/
z = 1276.5866 (calcd for C90H82N458Ni: 1276.5887, [M]−) for
10Ni, respectively, again suggesting a loss of two hydrogen atoms
from 9M (Scheme 4). The structure of 10Zn has been revealed
by X-ray diffraction analysis to be a fused product, in which the 9-
position of the dibenzocorannulene segment is directly linked to
the meso-position of the porphyrin (Figure 3). In other words,
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Scheme 4. Oxidative Fusion Reaction of 9M
Figure 4. UV/vis absorption (solid line) and fluorescence (dashed line)
spectra of (a) 6Zn, 6Ni, 7Zn, and 7Ni and (b) 9Zn, 9Ni, 10Zn, and
10Ni in CH2Cl2.
Figure 3. X-ray crystal structure of 10Zn. Solvent molecules, hydrogen
atoms, and tert-butyl groups were omitted for clarity. Thermal ellipsoids
were scaled to 30% probability level.
the absorption and fluorescence properties of dyads 7Zn and
10Zn are drastically perturbed, but in different ways. Fused dyad
10Zn displays a Soret band at 494 nm with a shoulder at 465 nm
and Q-bands at 669 and 691 nm. This dyad emits fluorescence at
732 nm with an enhanced quantum yield (ΦF = 0.049) with
respect to 9Zn. Dyad 10Ni exhibits an absorption spectrum that
is similar to that of 10Zn. These spectral features, which are
similar to those of naphthalene-fused porphyrins reported by
Imahori, Cammidge, and Gryko,2a−c can be interpreted in terms
of a simple extension of the π-conjugated network with
preservation of the aromaticity of the segments. In contrast,
the absorption spectrum of 7Zn exhibits a split Soret band at 422
and 520 nm and low energy Q-like bands tailing into near-
infrared (NIR) region around 1050 nm, and 7Zn shows no
fluorescence. The absorption spectrum of 7Ni is similar to that of
7Zn. The spectral features of 7M are clearly different from those
of 10M and are characteristic of antiaromatic porphyrinoids. The
optical properties of 7M may be understood by considering that a
dehydropurpurin-network present in 7M possesses a pseudo 20π
antiaromatic electronic circuit. Such antiaromatic contributions
have been recently recognized in several dehydropurpurin-like
molecules.11
10Zn is a fused product incorporating a six-membered ring as
opposed to a 5-membered ring as seen for the previous fused
dyads. Here again, other fused products were not detected,
indicating high regioselectivity of the fusion reaction. This
regioselectivity may also be explained in terms of the short
distance between the two atoms involved in the fusing bond
formation (Figure S12, Supporting Information). In addition,
possible fused products such as 11M may suffer from steric
congestion between the β-hydrogen atom adjacent to the fused
meso-position and the benzo-segment of the dibenzocorannulene
unit (Figure S13, Supporting Information). The mean-plane
deviations of Zn(II) porphyrin segments are 0.103, 0.075, and
0.085 Å for 6Zn, 7Zn, and 10Zn, respectively, indicating
increased planarity in 7Zn and 10Zn due to the fused structures.
The bowl depths of the dibenzocorannulene unit in 7Zn and
10Zn are 0.71 and 0.83 Å, respectively.
The UV/vis absorption spectra of singly linked dyads 6Zn and
9Zn are very similar, being roughly a superposition of the
individual absorption profiles of the Zn(II) porphyrin and
dibenzocorannulene components (Figure 4). The fluorescence
spectra of 6Zn and 9Zn are similar, with peak maxima at 596 and
651 nm and at 591 and 646 nm, respectively. The fluorescence
quantum yields are 0.031 and 0.028 for 6Zn and 9Zn. These data
indicate only weak electronic interactions between the Zn(II)
porphyrin and dibenzocorannulene units in both 6Zn and 9Zn,
probably due to the twisted conformations. On the other hand,
Cyclic voltammetry (CV) measurements revealed reversible
oxidation and reduction waves at 0.34 and −1.82 V for 6Zn and
0.36 and −1.81 V for 9Zn versus ferrocene/ferrocenium ion
couple, indicating that the directly linked dibenzocorannulene
unit has negligible influence on the electronic properties of the
Zn(II) porphyrin (Table 1). On the other hand, the fused
porphyrins 7Zn and 10Zn exhibited the first oxidation and
reduction potentials at 0.12 and −1.38 V and at 0.23 and −1.46
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Table 1. Electrochemical Properties of 6M, 7M, 9M, and
10M
a
b
compd
E1/2
E1/2
E1/2
E1/2
red.2
ΔE
ox.2
ox.1
red.1
6Zn
6Ni
0.34
−1.82
−1.73
−1.38
−1.34
−1.81
−1.73
−1.46
−1.40
2.16
2.35
1.50
1.68
2.17
2.35
1.69
1.88
0.62
0.12
0.34
0.36
0.62
0.23
0.48
7Zn
7Ni
0.51
0.58
0.69
0.84
0.60
0.65
−1.69
9Zn
9Ni
M.; Skonieczny, K.; Butenschcn
2013, 52, 9900.
̧ , H.; Gryko, D. T. Angew. Chem., Int. Ed.
10Zn
10Ni
−1.76
−1.72
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a
Conditions: Bu4NPF6 electrolyte 0.1 M in PhCN, Ag/AgClO4
́
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HOMO−LUMO gap (= E1/2 − E1/2
[eV]).
red.1
ox.1
V, respectively, indicating stronger electronic perturbation. The
electrochemical HOMO−LUMO gaps are 1.50 eV for 7Zn and
1.69 eV for 10Zn, respectively. The smaller HOMO−LUMO
gap of 7Zn is consistent with the partial antiaromatic character.
In summary, meso-β dibenzocorannulene-fused porphyrins
have been obtained by FeCl3-mediated intramolecular oxidative
ring-closure reactions of dibenzocorannulen-8-yl-porphyrins.
Upon oxidation, meso-8-linked dyads 6M gave fused porphyrins
7M bearing a five-membered ring, and β-8-linked dyads 9M gave
fused porphyrins 10M bearing a six-membered ring both in a
regiospecific manner. Fused porphyrins 10M display red-shifted
absorption spectra and 10Zn shows red-shifted fluorescence
owing to the π-extended conjugation, whereas fused porphyrins
7M show absorption features characteristic of antiaromatic
porphyrinoids and 7Zn is nonfluorescent, reflecting the
contribution of the pseudo 20π electronic circuit of the
dehydropurpurin. Further investigations on the fused porphyr-
inoids with bowl-shaped PAHs are underway in our laboratory.
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedure, complete characterizations (NMR,
UV/vis, fluorescence), DFT calculations, cyclic voltammetry,
and X-ray crystallographic data for 6Zn, 7Zn, and 10Zn. This
material is available free of charge via the Internet at http://pubs.
acs.org.
■
S
(8) Avlasevich, Y.; Kohl, C.; Mullen, K. J. Mater. Chem. 2006, 16, 1053.
(9) Intermolecular cyclodehydrogenation of 2 has been accomplished :
Tsefrikas, V. M. Ph.D. Dissertation, Boston College, 2007.
̈
AUTHOR INFORMATION
Corresponding Authors
■
(10) Fujimoto, K.; Yorimitsu, H.; Osuka, A. Org. Lett. 2014, 16, 972.
(11) (a) Sahoo, A. K.; Mori, S.; Shinokubo, H.; Osuka, A. Angew.
Chem., Int. Ed. 2006, 45, 7972. (b) Lash, T. D.; Smith, B. E.; Melquist, M.
J.; Godfrey, B. A. J. Org. Chem. 2011, 76, 5335. (c) Mitsushige, Y.;
Yamaguchi, S.; Lee, B. S.; Sung, Y. M.; Kuhri, S.; Schierl, C. A.; Guldi, D.
M.; Kim, D.; Matsuo, Y. J. Am. Chem. Soc. 2012, 134, 16540.
(d) Ishizuka, T.; Saegusa, Y.; Shiota, Y.; Ohtake, K.; Yoshizawa, K.;
Kojima, T. Chem. Commun. 2013, 49, 5939. (e) Fukui, N.; Cha, W.-Y.;
Lee, S.; Tokuji, S.; Kim, D.; Yorimitsu, H.; Osuka, A. Angew. Chem., Int.
Ed. 2013, 52, 9728. (f) Fukui, N.; Yorimitsu, H.; Lim, J. M.; Kim, D.;
Osuka, A. Angew. Chem., Int. Ed. 2014, 53, 4395.
*E-mail: taka@kuchem.kyoto-u.ac.jp.
*E-mail: osuka@kuchem.kyoto-u.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by Grant-in-Aid from JSPS (No.
25220802 (Scientific Research (S)).
■
DEDICATION
■
Dedicated to Prof. Lawrence T. Scott (Department of
Chemistry, Boston College) on the occasion of his 70th birthday.
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