Novel one-pot synthesis of 5-alkenyl-15-alkynylporphyrins and their derivatisation to a butadiyne-linked benzoporphyrin dimer

Article abstract of DOI:10.1039/b513848e

5-Alkenyl-15-alkynylporphyrins have been obtained unexpectedly by [2 + 2] acid-catalyzed condensation of dipyrrylmethane and TMS propynal in addition to 5,15-dialkynylporphyrin, and the unsymmetrical porphyrin can be converted to a butadiyne-linked dimer

Full text of DOI:10.1039/b513848e

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Novel one-pot synthesis of 5-alkenyl-15-alkynylporphyrins and their
derivatisation to a butadiyne-linked benzoporphyrin dimer{
Hiroko Yamada,*a Kayo Kushibe,a Tetsuo Okujima,a Hidemitsu Unob and Noboru Ono*a
Received (in Cambridge, UK) 29th September 2005, Accepted 19th November 2005
First published as an Advance Article on the web 6th December 2005
DOI: 10.1039/b513848e
5-Alkenyl-15-alkynylporphyrins have been obtained unexpect-
edly by [2 + 2] acid-catalyzed condensation of dipyrrylmethane
and TMS propynal in addition to 5,15-dialkynylporphyrin, and
the unsymmetrical porphyrin can be converted to a butadiyne-
linked dimer by selective desilylation of the alkynyl TMS.
Highly conjugated porphyrins and metalloporphyrins have
attracted strong attention in connection with unusual electro-
optical and non-linear optical properties such as two-photon
absorption.1–3 Alkynyl substituents are the most effective way of
making conjugated connections to the meso positions of
porphyrins and 5,15-dialkynylporphyrins are useful building
blocks for making highly conjugated porphyrin oligomers.4–7
Additionally, tetrabenzoporphyrins (TBPs) and related
p-expanded complexes show intriguing optical properties in the
near-IR region and are very attractive for applications such as
non-linear optical materials, optical memories, and opto-electronic
materials.8 Considering these properties, p-expanded oligomers of
TBPs connected by alkynyl substituents at the meso positions
might be promising highly conjugated materials. However, to our
knowledge, such materials have rarely been reported because of
the low solubility of TBPs in common organic solvents.9 Recently
we10 and others11 have reported soluble precursors of TBPs which,
after purification, could easily be converted to TBPs. Using our
method, we have tried to prepare 5,15-dialkynyl-TBP 4a from
bicyclo[2.2.2]octadiene (BCOD) ring-fused 5,15-dialkynylpor-
phyrin 1a via the retro-Diels–Alder reaction (Fig. 1).10 During
the synthesis of 1a by [2 + 2] acid-catalyzed condensation of a
dipyrrylmethane, an unsymmetrical 5-alkenyl-15-alkynylporphyrin
2a was obtained unexpectedly in addition to the targeted
porphyrin 1a. As far as we know, a few 5-alkenyl-15-alkynylpor-
phyrins, which have been prepared by a partial nucleophilic
addition to dialkynylporphyrin4a or a long multistep synthesis,12
have been reported so far. With the [2 + 2] acid-catalyzed synthesis
using substituted propynal, only 5,15-dialkynyl porphyrin has been
reported.4a,b,d,8d This 5-alkenyl-15-alkynylporphyrin 2a is a very
attractive building block, especially for the synthesis of butadiyne-
linked porphyrin oligomers and dimers, since selective cleavage of
Fig. 1 Porphyrin monomers 1–5.
the trimethylsilyl (TMS) group on either the vinyl or alkynyl end is
possible by exploiting the different reactivities of the two TMS
groups.13 We also report here the preparation of a butadiyne-
linked TBP dimer by selective cleavage of the TMS group at the
alkynyl end of porphyrin 2a.
A typical synthetic route for porphyrins 1a–5a is shown in
Scheme 1. An acid-catalyzed condensation of a-free dipyrryl-
methane 7, which was prepared from pyrrole 610a in two steps, and
TMS-propynal (9), which was prepared from TMS-propyne (8) in
two steps, was performed in methanol using p-toluenesulfonic acid
at 0 uC, followed by oxidation with 2,3-dichloro-5,6-dicyano-1,
4-benzoquinone (DDQ).4a The two types of porphyrins, 1a and 2a,
aDepartment of Chemistry, Faculty of Science, Ehime University,
Bunkyo-cho 2-5, Matsuyama 790-8577, Japan.
E-mail: yamada@chem.sci.ehime-u.ac.jp; ononbr@dpc.ehime-u.ac.jp;
Fax: +81 89 927 9615; Tel: +81 89 927 9613
Scheme 1 Synthesis of porphyrins, 1a–5a. Reagents and conditions: (i)
methylal, acetic acid, H2SO4, CH2Cl2, rt, 1 h, quant.; (ii) NaOH, ethylene
glycol, 175 uC, 3 h, 92%; (iii) n-BuLi, THF, 278 uC, 30 min, then
ClCO2Me, rt, 2.5 h, 87%; (iv) DIBAL, 278 uC, 15 min, then MeOH, 2 h,
rt, 78%; (v) p-TsOH, MeOH, 0 uC, 3 h, then DDQ, rt, 30 min, 1a: 30%, 2a:
10%; (vi) in the dark otherwise in same condition with step (v); 1a: 33%,
3a: 19%; (vii) hn, 84%; (viii) 200 uC, 10 min, quant.
bDivision of Synthesis and Analysis, Department of Molecular Science,
Integrated Center for Sciences, Ehime University, Bunkyo-cho 2-5,
Matsuyama, 790-8577, and CREST, Japan Science Technology
Corporation (JST), Japan. Fax: +81 89 927; Tel: +81 89 927 9660
{ Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b513848e
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Chem. Commun., 2006, 383–385 | 383

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0.60 ppm. This result indicated that the TMS moiety at the end of
the cis-alkenyl group was located above the porphyrin ring and
their ring currents influenced its chemical shift.
The UV-vis absorption spectra of BCOD porphyrins 1a, 2a and
3a are shown in Fig. 3. Porphyrins 2a and 3a showed similar
spectra and their Soret bands (peaks at 417 nm) were blue-shifted
by 5 nm compared to that of porphyrin 1a (422 nm), which
indicated that 1a was more conjugated than porphyrins 2a and 3a.
When porphyrins 1a and 2a were heated to 200 uC under vacuum,
benzoporphyrins 4a and 5a were obtained quantitatively
(Scheme 1). Their UV-vis absorption spectra are shown in
Fig. 3(a). The sharp Soret bands of 4a and 5a were observed at
460 nm (shoulder at 449 nm) and 450 nm (shoulder at 441 nm),
respectively, which were red-shifted by more than 30 nm compared
to those of the corresponding precursor BCOD porphyrins. The
absorbance of Q bands of porphyrin 4a and 5a were stronger than
those of BCOD porphyrins and reached to 710 and 689 nm,
respectively. In THF containing 5% pyridine, porphyrins 4a and
5a showed Soret peaks at 459 and 448 nm, respectively, and Q
bands reached to 710 and 690 nm, respectively (see ESI,{ Fig. S2).
The reported spectrum of free-base TBP without any substituents
at meso positions has shown the Soret peak at 427 nm and a longer
Q band peak at 662 nm in THF containing 5% pyridine.8d,14 These
results suggested that p-expansion by alkenyl and alkynyl
substituents also caused a red shift of more than 20 nm.
Fig. 2 1H NMR spectra of porphyrin (a) 1a, (b) 2a and (c) 3a in CDCl3.
were obtained with Rf values of 0.75 and 0.49, respectively, on
silica-gel TLC using CHCl3 as the eluent. Purification by silica-gel
column chromatography gave the expected 5,15-dialkynylpor-
phyrin 1a as the first green band in 30% isolated yield and the
unexpected porphyrin 2a as the second red band in 10% yield. The
second porphyrin was identified by TOF MS and by H and 13
1
C
NMR measurements. The TOF MS spectrum of this fraction
showed a peak at m/z = 817 (M+ + 1), which lay two mass units
In order to investigate the generality of the unsymmetrical
reaction described above, various dipyrrylmethanes and
alkynylaldehydes were reacted under conditions similar to the
preparation of porphyrins 1a and 2a. When triisopropylsilyl-
(TIPS)propynal was used instead of TMS propynal, 5-alkenyl-15-
alkynylporphyin was obtained in 10% yield in addition to
5,15-dialkynylporphyrin in 14% yield (ESI,{ Table S1). However,
1
higher than porphyrin 1a (m/z = 815 (M+ + 1)). The H NMR
spectrum of 2a is shown in Fig. 2 with that of porphyrin 1a for
comparison. Because of the stereoisomers of the BCOD groups,
the vinyl hydrogen atoms (He and Hf) of porphyrins 2a showed
two pairs of hydrogen peaks at 9.68 and 6.66; and at 9.67 and
6.67 ppm, respectively, with coupling constants of 19 Hz, typical
for trans vicinal coupling of olefinic hydrogen atoms. Although the
hydrogen peak at the methyne position of 1a (Ha) appeared at
6.53 ppm, those peaks of 2a appeared at 6.56 (Ha) and 5.78 (Hd)
ppm because of the different anisotropic effects of alkene versus
alkyne groups. The peaks of the other methyne hydrogens, Hband
Hc, were almost the same for 1a and 2a. The peaks of TMS
hydrogens were split into two signals at 0.69 and 0.60 ppm for 2a.
When all stages from the condensation reaction to the
purification on silica-gel column chromatography were performed
in the dark, only cis-type porphyrin 3a (Rf = 0.39) was obtained in
19% yield besides porphyrin 1a (33%). Porphyrin 3a could be
converted to the trans form by photoirradiation, which was
monitored by NMR. A solution of 3a in CDCl3 in a NMR tube
was irradiated with the light (lex = 420 nm) under an argon
atmosphere. After an irradiation for 12 h, the ratio of trans 2a to
cis 3a was increased to 0.84 (see ESI, Fig. S1{). This result
suggested that for the [2 + 2] porphyrin synthesis using BCOD
ring-fused dipyrrylmethane and TMS-propynal, dialkynylpor-
phyrin 1a and cis-type porphyrin 3a were formed at first, after
which the cis porphyrin was converted to trans by room light. The
1H NMR spectra of 3a is shown in Fig. 2(c). For porphyrin 3a,
four types of hydrogen peaks were observed for Hg at 10.04, 9.92,
9.91 and 9.79 ppm with coupling constants of 15 Hz, typical for cis
vicinal coupling.{ Interestingly, the alkynyl and vinyl TMS
hydrogen peaks of 3a were observed at 0.64 ppm as a singlet
and at 21.06 to 20.94 ppm as multiple singlets, respectively, while
both TMS hydrogens of 2a were observed as singlets at 0.69 and
Fig. 3 UV-vis absorption spectra of (a) porphyrins 1a (red solid line), 2a
(blue solid line), 3a (green solid line), 4a (red dotted line) and 5a (blue
dotted line) in CH2Cl2 and (b) zinc porphyrin monomers 4b (red dotted
line) and 5b (blue dotted line), and dimers 11 (red solid line) and 12 (blue
solid line) in DMF.
384 | Chem. Commun., 2006, 383–385
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with phenylpropynal under the same experimental conditions, an
analogous unsymmetrical porphyrin was not obtained, but only
the corresponding 5,15-dialkynylporphyrin in 3% isolated yield
(ESI,{ Table S1).§ Furthermore, when b-unsubstituted dipyrryl-
methane or all-b-ethyldipyrrylmethane was used instead of BCOD
ring-fused dipyrrylmethane, the corresponding 5,15-dialkynylpor-
phyrins were obtained in 3 and 34% yields, respectively, without
formation of unsymmetrical porphyrins, consistent with previous
reports (ESI,{ Table S1).4a,b,d,8d These results suggested that the
combination of BCOD ring-fused dipyrrylmethane and trialk-
ylsilylpropynal was critical for the one-pot synthesis of 5-alkenyl-
15-alkynyl unsymmetrical porphyrins. The mechanism of the
partial hydrogenation of alkynes are not clear and is under
investigation with a speculation as follows; a protonation of the
alkynyl carbon at a position of TMS was followed by a 1,2-
hydride transfer from the methyne position of a porphodimethene
intermediate to the b-carbon of TMS.
partial financial support H. Y. thanks Nissan Science Foundation
and the Hayashi Memorial Foundation for Female Natural
Scientists and N. O. thanks the Asian Office of Aerospace
Research and Development (AOARD-05-4051).
Notes and references
{ The coupling constants for the other vinyl proton, Hh, could not be
estimated accurately, because of overlapping signals.
§ The same experimental condition for TMS propynal was performed but
not optimized for phenylpropynal. This would be the reason for the low
yield.
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Selective cleavage of the TMS group of the unsymmetrical
porphyrin 2b gave the porphyrin 10 and Glaser–Hay coupling of
10 gave porphyrin dimer 11 in 33% yield (Scheme 2).4b It was
possible to convert dimer 11 quantitatively to benzoporphyrin
dimer 12 by heating it to 200 uC under vacuum. Since Zn
complexes of TBP monomers and dimers were poorly soluble in
common organic solvents, such as CH2Cl2 and THF, their
absorption spectra were measured in DMF (7 6 1027 M) as
shown in Fig. 3(b). The absorption spectrum of dimer 11 showed
broadening and splitting of the Soret band, as has been reported
for butadiyne-linked porphyrin dimers in CH2Cl2.2,4b This splitting
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Scheme 2 Synthesis of porphyrin dimers, 11 and 12. Reagents and
conditions: (i) Zn(OAc)2?2H2O, CHCl3, 75 uC, 2 h, 85%; (ii) K2CO3, THF,
70 uC, 5 h, 99%; (iii) CuCl, TMEDA, O2, CH2Cl2, rt, 20 min, 33%; (iv)
200 uC, 10 min, quant.
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In conclusion, we have found a novel one-pot synthesis of
5-alkenyl-15-alkynylporphyrin 2, and have succeeded in preparing
the TBP dimer in moderate yield by selective desilylation of
porphyrin 2b. The further development of these unsymmetrical
porphyrins to the higher oligomers by selective cleavage of the
TMS or TIPS group is under investigation.
The authors thank Venture Business Laboratory, Ehime
University, for its help for using TOF-MS spectrometer. For
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Chem. Commun., 2006, 383–385 | 385