Effective meso fabrications of subporphyrins

Article abstract of DOI:10.1002/anie.201201853

A meso-free subporphyrin was prepared and quantitatively converted into meso-brominated subporphyrin, which is an effective precursor for the facile installation of substituents at the meso position. The meso-(4-amino) phenylethynyl subporphyrins (see picture; N blue, O red, Br orange, F pale green, B dark green) and a butadiyne-bridged dimer have split Soret-like bands and red-shifted and intensified fluorescence. Copyright

Full text of DOI:10.1002/anie.201201853

Angewandte
Chemie
DOI: 10.1002/anie.201201853
Porphyrinoids
Effective meso Fabrications of Subporphyrins**
Masaaki Kitano, Shin-ya Hayashi, Takayuki Tanaka, Hideki Yorimitsu, Naoki Aratani, and
Atsuhiro Osuka*
Subporphyrins, which are legitimate ring-contracted porphyr-
ins in terms of the regular arrangement of three pyrrolic units
and methine carbons, have emerged as a novel functional
pigment since their first synthesis in 2006.[1] Interest in these
macrocycles initially lay on the influence of symmetry change
from D4h of metalloporphyrins to C3v of boron(III) subpor-
phyrins. In the meantime however, it has turned out that
subporphyrins have attractive attributes, such as a 14p-
electronic aromatic circuit, bowl-shaped bent structure, and
variable electronic properties that are tunable by meso-aryl
substituents.[2,3] Despite these promises, the synthetic chemis-
try of subporphyrins lags far behind the porphyrin counter-
part, since only symmetric meso-aryl-substituted subporphyr-
ins, such as triphenyl subporphyrin 1, can be prepared by the
condensation of tri-N-pyrrolylborane or pyridine-tri-N-pyr-
rolylborane with aryl aldehydes in a practical sense. As has
been extensively demonstrated in the porphyrin chemistry,
rational synthetic routes to nonsymmetrically substituted
subporphyrins are highly desirable for exploration of sub-
porphyrin-based functional molecular systems.[4] So far, how-
ever, such a rational nonsymmetric fabrication method has
been unknown for a subporphyrin except for low-yielding
statistical cross-condensation reactions that entail tedious
separation steps.[2c,d,f] Furthermore, there is no synthetic route
to meso-alkenyl or meso-alkynyl subporphyrins despite their
high promise as functional dyes, as suggested by the very rich
chemistry of porphyrin counterparts that were pioneered by
Therien[5] and Anderson.[6]
Herein, we present the synthesis of meso-free and meso-
bromo subporphyrins 2 and 3, which can be effective synthetic
precursors for nonsymmetrically substituted subporphyrins,
judging from the extensive and successful uses of meso-free[7]
and meso-bromo porphyrins.[8] Tribenzosubporphines 4 were
the first synthesized meso-free subporphyrins, but their meso-
positions are entirely unreactive towards electrophiles such as
bromine and N-bromosuccinimide (NBS), because they
correspond to nodes in their HOMO as revealed by DFT
calculations.[2a] On the other hand, meso-free subporphyrin 2
has been suggested to be a useful precursor for 3 by DFT
calculations (see the Supporting Information), which predicts
favorable HOMO characteristics for meso halogenation.
We thought that meso-free subporphyrin 2 might be
prepared by the condensation of pyridine tripyrromethene
borane salt with trimethyl orthoformate. Along this synthetic
line, we first attempted the condensation of tripyrrane 5[9]
with neat BH3·NEt3 at 1008C for 1 h, and the resulting
solution was evaporated to leave a residue, to which an
equivalent amount of pyridine was added to form pyridine
tripyrromethene borane precursor. This was condensed with
20 equivalents of trimethyl orthoformate in the presence of
trifluoroacetic acid (TFA) to furnish 2 along with its reduced
congener, meso-free subchlorins,[10] as by-products. To make
the separation easier, the resultant mixture was oxidized with
MnO2 for the conversion of the meso-free subchlorins into 2.
Subsequent separation over a simple silica gel column gave 2
in 4.4% yield (method A; Supporting Information). The final
oxidation step is beneficial for product separation but causes
non-negligible oxidative damage to 2. On the basis of
mechanistic consideration that the subchlorin was formed
by the action of acid on subporphyrinogen intermediates,
which was considered for the formation of chlorins in the
Adler porphyrin synthesis,[11] we thus attempted acid-free,
simple thermal reaction of 6 generated in situ from 5 with
BH3·NEt3 and trimethyl orthoformate in 1,2-dichlorobenzene
at various temperatures (method B; Scheme 1). Gratifyingly,
the acid-free condensation reaction at 1008C furnished 2 in
9.7% yield without contamination of subchlorin by-products.
[*] M. Kitano, S. Hayashi, Dr. T. Tanaka, Prof. Dr. H. Yorimitsu,
Dr. N. Aratani, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: osuka@kuchem.kyoto-u.ac.jp
Dr. N. Aratani
1
The H NMR spectrum of 2 in CDCl3 exhibits a singlet at
PRESTO, Japan Science and Technology Agency (Japan)
8.89 ppm owing to the free meso proton. As expected, the
bromination of 2 proceeded with NBS in CHCl3 at 08C to
provide 3 quantitatively. The structures of 2 and 3-OCOCF3
were revealed by a single-crystal X-ray diffraction analysis
[**] This work was supported by Grants-in-Aid (Nos. 22245006 (A) and
20108001 “pi-Space”) for Scientific Research from MEXT.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201853.
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Scheme 1. Synthesis of meso-free and meso-bromo subporphyrins 2
and 3.
(Figure 1).[12] The bowl depths are 1.460 and 1.355 ꢀ for 2 and
3-OCOCF3, respectively. The fluorescence quantum yield FF
of 2 is 0.12, which is similar to that of 1, while that of 3 is
dropped to 0.004 owing to the internal heavy-atom effect.
Scheme 2. Fabrications of 3 through metal-catalyzed cross-coupling
reactions.
Figure 1. X-ray crystal structures of a) 2 and b) 3-OCOCF3. Ellipsoids
are set to 50% probability.
With bromide 3 in hand, we examined its fabrications
through metal-catalyzed cross-coupling reactions (Scheme 2).
The first example was Negishi coupling[8b,13] with benzylzinc
chloride–lithium chloride complex,[14] which allowed the
synthesis of meso-benzyl subporphyrin 7 in 59% yield. The
structure of 7 was confirmed by X-ray analysis of B-(3,5-
dinitrobenzyloxy)subporphyrin 7-DN (Figure 2a). Subpor-
phyrin 7 was found to be slightly unstable in solution under
the air, which is probably due to the presence of benzylic
hydrogen. An alkyl group bearing an ester group was also
installed by a similar coupling reaction with the correspond-
ing functionalized organozinc reagent[15] with the aid of an
NHC-Pd complex, Pd-PEPPSI-IPent,[16] to provide 8 in 50%
yield. These reactions constitute the second synthetic route to
meso-alkyl substituted subporphyrins,[2i] but are much more
useful in terms of rational synthesis and wide functional group
compatibility.
Figure 2. X-ray crystal structures of a) 7-DN, b) 9, and c) 10. Solvent
molecules are omitted for clarity; ellipsoids are set to 50% probability.
Phenylvinyl-substituted subporphyrin 9 was synthesized
by Suzuki–Miyaura coupling with phenylvinylboronate in
71% yield.[17] The structure of 9 was determined by X-ray
analysis; the phenylvinyl group is nearly coplanar to the
subporphyrin core with a dihedral angle of 26.68 (Figure 2b).
This is the first example of subporphyrin bearing a meso-
alkenyl substituent.
In the next step, we also examined meso-alkynylation of 3
by Sonogashira reaction,[2c,18] which is indeed fairly effective
to produce 10–14 in good yields. Apart from phenylacetylene,
both electron-accepting and electron-donating aryl acetylenes
could be used in this coupling reaction. The crystal structures
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were obtained for 10, 11, and 12 (Figure 2c; Supporting
Information).[12]
The meso-substituted subporphyrins that were thus syn-
thesized allowed a comparison of the absorption and fluo-
rescence spectra (Figure 3). The Soret-like band of 2 appears
chromophore. In the arylethynyl-substituted series, both the
electron-donating and electron-accepting groups cause red-
shifts in the Soret-like band as compared with 10. A similar
trend was observed for their fluorescence spectra. The
fluorescence bands and quantum yields are observed as
follows: 9 (552 nm, FF = 0.26), 10 (527 nm, FF = 0.22), 11
(538 nm, FF = 0.23), and 12 (554 nm, FF = 0.039). Remark-
ably, meso-(4-dimethylaminophenyl)ethynyl subporphyrin 13
exhibits a split Soret-like band at 376 and 406 nm and
a
considerably intensified fluorescence quantum yield
(601 nm, FF = 0.59; see the Supporting Information). These
optical properties are reminiscent of those of meso-(4-
dibenzylaminophenyl)subporphyrin 15,[2c] and are similarly
ascribed to a HOMO that is spread over the entire dibenzyl-
aminophenyl group and considerably destabilized relative to
HOMOÀ1. Since subporphyrin 15 displayed the structural
deformations of the meso-(4-aminophenyl) group towards the
quinonoid structure, we prepared meso-(4-dibenzylamino-
phenyl)ethynyl subporphyrin 14 by the same Sonogashira
coupling under the same conditions and succeeded in the X-
ray crystal structural determination of 14-OEt. However, the
meso-alkynyl moiety does not display structural deformation
towards the allene structure (Figure 4).
Figure 3. a) UV/Vis absorption and b) fluorescence spectra of selected
subporphyrins in CH2Cl2.
Figure 4. a) X-ray crystal structure of 14-OEt and b) comparison of
bond lengths to 15. Solvent molecules are omitted for clarity;
ellipsoids are set to 20% probability.
at 362 nm, which is blue-shifted as compared with those of
1 (373 nm) and 8 (367 nm), indicating that the meso-alkyl
substituent causes a slight red-shift in a Soret-like band. The
fluorescence bands and quantum yields are observed for 7
(507 nm, FF = 0.15) and 8 (508 nm, FF = 0.14). The meso-
alkenyl- and meso-alkynyl-substituted subporphyrins display
more red-shifted Soret-like bands; 9 (391 nm), 10 (386 nm),
11 (390 nm), and 12 (396 nm), and Q-like bands: 9 (514 nm),
10 (505 nm), 11 (510 nm), and 12 (516 nm), reflecting the
effective conjugation of meso substituents with subporphyrin
As an extension of this method, meso-trimethylsilyle-
thynyl-substituted subporphyrin 16 was prepared and was
transformed into 1,3-butadiyne-bridged subporphyrin dimer
17 in 89% yield (Scheme 3) by deprotection with tetrabutyl-
ammonium fluoride (TBAF) followed by dimerization under
Glaser coupling conditions. The 1H NMR and high-resolution
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meso-alkyl-, meso-alkenyl-, and meso-alkynyl-substituted
subporphyrins through metal-catalyzed cross-coupling reac-
tions. 1,3-Butadiyne-bridged subporphyrin dimer 17 could be
synthesized though 16. The optical properties of subporphyr-
ins have been modulated by meso substituents. These results
now open a route to functional subporphyrins in a rational
manner. Extensions of this synthetic method to other
nucleophiles and further elaborated oligomeric subporphyr-
ins are actively in progress in our laboratory.
Received: March 8, 2012
Published online: && &&, &&&&
Keywords: bromination · cross-coupling · porphyrinoids ·
subporphyrins · substitution
Scheme 3. Synthesis of 1,3-butadiyne-bridged subporphyrin dimer 17.
.
electrospray time-of-flight mass spectra of 17 are consistent
with its structure, which was confirmed by single-crystal X-ray
structural analysis (Figure 5). Two subporphyrin rings are
linked in an anti manner through a 1,3-butadiyne bridge with
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Angew. Chem. Int. Ed. 2012, 51, 1 – 6
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Communications
Porphyrinoids
A meso-free subporphyrin was prepared
and quantitatively converted into meso-
brominated subporphyrin, which is an
effective precursor for the facile installa-
tion of substituents at the meso position.
The meso-(4-amino)phenylethynyl sub-
porphyrins (see picture; N blue, O red,
Br orange, F pale green, B dark green)
and a butadiyne-bridged dimer have split
Soret-like bands and red-shifted and
intensified fluorescence.
M. Kitano, S. Hayashi, T. Tanaka,
H. Yorimitsu, N. Aratani,
A. Osuka*
&&&& — &&&&
Effective meso Fabrications of
Subporphyrins
6
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