New synthetic strategy for diporphyrins: Pinacol coupling-rearrangement

Article abstract of DOI:10.1002/chem.201100872

A new peak for porphyrins! A new approach to methylene-bridged diporphyrins has been explored, which consists of SmI₂-mediated pinacol coupling followed by BF₃-assisted pinacol-to-pinacolone rearrangement (see scheme). Diporphyrin 1

Full text of DOI:10.1002/chem.201100872

DOI: 10.1002/chem.201100872
New Synthetic Strategy for Diporphyrins: Pinacol Coupling–Rearrangement
Sumito Tokuji, Chihiro Maeda, Hideki Yorimitsu,* and Atsuhiro Osuka*[a]
Numerous kinds of porphyrin oligomers have been syn-
thesized and explored as biomimetic models for photosyn-
thetic systems and oxygen reducing enzymes, photonic mate-
rials, and functional molecular devices.[1–3] In particular, por-
phyrin dimers linked by various spacers have been synthe-
sized as very attractive motifs to understand the dipole and
electronic exchange interactions between the adjacent por-
phyrin subunits or to create building blocks for larger por-
phyrin arrays.[1,4] Recently, we have reported the synthesis
of b,b’-doubly 2,6-pyridylene-bridged NiII porphyrin belts by
using the Suzuki–Miyaura coupling reaction.[4a] Their re-
markably bent structures have encouraged the synthetic ex-
tension to a cyclic porphyrin nanobarrel.[4b]
In creating new porphyrin dimers of interest, it is impor-
tant to develop a novel strategy for connecting two porphy-
rin monomer units. We envisioned that SmI2-mediated pina-
Scheme 1. Synthesis of acetyl porphyrins 1Ni, 2Ni, and 3Ni.
col coupling of acyl porphyrin would provide a reliable and
unique tool.[5] This was indeed the case for a b-acetyl NiII
porphyrin, which was subjected to pinacol coupling to pro-
vide a diol. The diol underwent a rearrangement under
acidic conditions to yield a pinacol-type diporphyrin, which
underwent further dehydrative rearrangement to provide a
conformationally constrained diporphyrin, which is doubly
bridged by methylene groups and represents a rare case in
the literature.[6,7]
The X-ray crystallographic analyses unambiguously eluci-
dated the structures of 2Ni and 3Ni (see the Supporting In-
formation).[10] Interestingly, the acetyl group at the meso po-
sition in 3Ni is almost perpendicular (81.58) to the porphyrin
framework, whereas the acetyl groups at the b positions in
2Ni are nearly coplanar (2.0 and 10.58), which reflects differ-
ent steric environments at the meso- and b positions.
The starting acetylated porphyrins were prepared as illus-
trated in Scheme 1. Although a few synthetic approaches to
acetyl porphyrins have already been reported,[8] we have
employed different methods for the efficient synthesis of b-
and meso-acetyl porphyrins. For the synthesis of b-acetyl
porphyrins, b,b’-diboryl NiII porphyrin was treated with
acetic anhydride in the presence of catalytic amounts of pal-
ladium acetate and tris(4-methoxyphenyl)phosphine in THF
containing a small amount of water, which provided b-acetyl
NiII porphyrins 1Ni and 2Ni in 24 and 56% yields, respec-
tively (Scheme 1).[9] For a meso-acetyl porphyrin, a meso-tri-
methylsilylethynyl NiII porphyrin was exposed to sulfuric
acid in chloroform, which resulted in the formation of 3Ni
in 56% yield (Scheme 1).
Reaction of 1Ni with SmI2 in THF quantitatively provided
diporphyrinated pinacol analogue 4Ni. However, 4Ni was
unstable and gradually oxidized back into 1Ni in the air. We
therefore subjected 4Ni immediately to pinacol rearrange-
ment conditions without further purification, which was
done through treatment of crude 4Ni with BF3·OEt2 to give
5Ni in high yield (Scheme 2). It is worth noting that the mi-
gration of a bulkier porphyrin subunit was preferred over
that of methyl group and that no rearrangement proceeded
in the reaction of freebase porphyrin with BF3·OEt2, in
which the protonation of the inner nitrogen atoms would
occur to prohibit the rearrangement. Curiously, no conver-
sion was observed in the reaction of meso-acetyl porphyrin
3Ni with SmI2 under similar pinacol coupling conditions.
The parent mass ion peak of 5Ni was observed at m/z=
1952.0371 (calcd for C128H148N8Ni2ONa: 1952.0375 [M+
Na]+) in its high-resolution electrospray-ionization time-of-
flight (HR-ESI-TOF) mass spectrum, indicating the forma-
tion of a dimeric product. The 1H NMR spectrum of 5Ni
showed one singlet signal derived from the meso proton at
d=9.84 ppm and two singlet signals from the methyl groups
at d=3.28 and 2.61 ppm. The UV/Vis absorption spectrum
[a] S. Tokuji, Dr. C. Maeda, Prof. Dr. H. Yorimitsu, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan)
Fax : (+81)75-753-3970
E-mail: yori@kuchem.kyoto-u.ac.jp
osuka@kuchem.kyoto-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100872.
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A plausible reaction mechanism is shown in Scheme 4.
Since 5H did not rearrange under the same acidic condi-
tions, we thought that the intramolecular Friedel–Crafts re-
action preceded the demetalation; the protonation of the
Scheme 2. Synthesis of 5Ni.
of 5Ni displays a non-split but perturbed Soret band proba-
bly due to exciton coupling of the two porphyrins held in a
skewed arrangement (see the Supporting Information).
Treatment of 5Ni with concentrated sulfuric acid and tri-
fluoroacetic acid for demetalation gave two compounds;
one was the expected pinacolone diporphyrin, 5H, and the
other was surprisingly a tetrahydropentalene-fused dipor-
phyrin, 6H (Scheme 3). These two dimers were easily sepa-
Scheme 4. A plausible reaction mechanism.
carbonyl oxygen atom followed by nucleophilic attack of the
NiII porphyrin subunit toward the activated acetyl group
(Scheme 4, A) would form a fused cyclopentene skeleton.
The subsequent nucleophilic attack along with the elimina-
tion of water (Scheme 4, B) should construct a doubly fused
tetrahydropentalene structure.
Zinc metalations of 5H and 6H were conducted to obtain
the single crystals of these dimers for X-ray analyses
(Scheme 3). The X-ray crystallographic analysis of 5Zn un-
ambiguously elucidated its dimeric structure, which has a
skewed arrangement of the two porphyrin rings
(Figure 1).[11] The two planes through the four nitrogen
atoms of the porphyrins form a dihedral angle of 758. This
structural feature is similar to that of the hydroxymethy-
lene-bridged porphyrin dimer reported by Senge.[6a] As for
6Zn, the final structural proof was obtained from its single-
crystal X-ray diffraction analysis (Figure 2),[11] which re-
vealed a tetrahydropentalene-fused gable structure with a
dihedral angle of 998.
Scheme 3. Synthesis of 5H, 5Zn, 6H, and 6Zn. TFA=trifluoroacetic acid.
Ac=acetyl.
The UV/Vis absorption spectra of 5 and 6 can be ex-
plained in terms of the excitonic coupling theory;[12,13] the
changes in the absorption spectra would result from the dif-
ference of the spacial arrangement of porphyrin subunits in
5 and 6. The Soret band of 5H is split to a red-shifted band
at 429 nm and the original band at 419 nm along with a
blue-shifted shoulder around 400 nm (Figure 3). The red-
and blue-shifted absorption bands of 5H are due to J- and
H-type dipole–dipole interactions, respectively. On the other
hand, the UV/Vis absorption spectrum of 6H displays a
Soret band at 412 nm and a considerably broader Soret
band at 436 nm, which have been assigned due to H- and J-
type dipole–dipole interactions, respectively. The fluores-
rated by silica gel column chromatography. The HR-ESI-
TOF mass spectrum of 6H displayed the parent ion peaks at
m/z=1800.2116 (calcd for C128H151N8: 1800.2062 [M+H]+),
whereas that of 5H displayed a peak at m/z=1818.2232
(calcd for C128H153N8O: 1818.2162 [M+H]+). This result in-
dicates the occurrence of dehydration reaction and demeta-
1
lation in the transformation from 5Ni to 6H. The H NMR
spectrum of 6H revealed a disappearance of the meso
proton. The 13C NMR spectrum displayed distinct two sig-
nals (d=82.85 and 57.78 ppm) derived from two carbon
atoms at the benzyl-like positions between the two porphy-
rin subunits.
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A. Osuka, H. Yorimitsu et al.
Figure 3. UV/Vis absorption (solid lines) and fluorescence (dashed lines)
spectra in CH2Cl2 (black line=5H, red line=5Zn, blue line=6H, green
line=6Zn). The excitation wavelengths are 428 (for 5H), 419 (for 5Zn),
412 (for 6H), and 415 nm (for 6Zn). The fluorescence quantum yields
(FF) are 6.3 (for 5H), 2.7 (for 5Zn), 7.7 (for 6H), and 3.9% (for 6Zn).
cence spectra of 5H and 6H are shown in Figure 3. The fluo-
rescence quantum yield of 6H (FF =7.7%) is slightly higher
than that of 5H (FF =6.3%), probably because of its rigid
structure. A similar tendency was observed in the spectra of
5Zn and 6Zn.
In summary, we have accomplished the palladium-cata-
lyzed b-acetylation of b-boryl NiII porphyrins, the pinacol
coupling of b-acetyl NiII porphyrin with SmI2, and the acid-
catalyzed pinacol–pinacolone rearrangement and further
acid-catalyzed dehydrative rearrangement of a pinacolone-
type diporphyrin to a tetrahydropentalene-fused diporphyr-
in. In the final diporphyrin system, the two porphyrins are
held close in a conformationally restricted manner but elec-
tronically insulated, which will be useful for studies on intra-
molecular electron transfer and energy transfer reactions.
Figure 1. X-ray crystal structure of 5Zn. a) Top and b) side view. tert-
Butyl groups, solvent molecules, and hydrogen atoms are omitted for
clarity. Thermal ellipsoids are at the 30% probability level.
Acknowledgements
This work was supported by Grants-in-Aid (nos. 22245006 (A) and
20108006 “pi-Space”, and 22106523 “Integrated Organic Synthesis”)
from MEXT. S.T. and C.M. acknowledge a JSPS Fellowship for Young
Scientists.
Keywords: acetylation
·
nickel
·
pinacol coupling–
rearrangement · porphyrin · samarium
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Chem. Eur. J. 2011, 17, 7154 – 7157

New Synthetic Strategy for Diporphyrins
COMMUNICATION
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Received: March 22, 2011
Published online: May 11, 2011
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