Chemistry Letters Vol.33, No.1 (2004)
41
metalation, bromination, and introduction of a dioxaborolan
group. The synthesis of a series of pyrenes 6a–f with or without
alkoxy group(s), which enabled changing of the oxidation poten-
tials of 7 systematically (vide infra), was carried out by a com-
bination of bromination, substitution, and trans-halogenation re-
actions.9 Important intermediates 7a–f (Zn) were prepared by
the Suzuki–Miyaura coupling of 4 with 6a–f in acceptable yields
(ca. 70%). The corresponding free bases 7a–f (2H) were pre-
pared by treatments of 7a–f (Zn) with mineral acid. Nickel(II)
acetylacetonate processing of the free bases 7a–f (2H) produced
the nickel complexes.
There are two well-known representative reaction condi-
tions for the intramolecular ring closure reactions that produce
polyaromatic compounds: the reductive condition via an anion
radical intermediate and the oxidative condition via a cation rad-
ical one.6 Treatment of 7a with potassium–sodium alloy afford-
ed a green solution suggesting the formation of an anion radical
of 7a; however, quenching of the reaction mixture led to the re-
covery of the starting material.
the corresponding pyrene-fused porphyrin, 2f (Zn).
The oxidative intramolecular coupling reaction via a cation
biradical is the most plausible reaction mechanism, i.e., the for-
mation of a biradical located on both porphyrin and pyrene moi-
eties and the subsequent ring closure reaction.10 To obtain basic
information regarding this reaction mechanism, systematic stud-
ies were carried out, changing the oxidation potential of the pyr-
ene moiety. Not only trialkoxy derivatives (7f (Zn)) but also di-
and mono-alkoxy derivatives 7b–e (Zn) afforded the corre-
sponding pyrene-fused porphyrins. However, the precursor hav-
ing no alkoxy group (7a (Zn)) did not promote the ring closure
reaction.
In conclusion, novel and highly conjugated pyrene-fused
porphyrins were prepared and spectroscopically characterized.
The reactions discussed above enable the opening of a new field
of ꢀ-expanded porphyrin chemistry that has been hitherto re-
stricted to porphyrin-fused porphyrins. We are currently investi-
gating the detailed properties of the molecules, the reaction
mechanism and other ring closure reactions of meso-arylpor-
phyrins (aryl = polyaromatics such as perylene, coronene, and
so on).
An oxidative ring closure reaction using combination of an
oxidizing agent and a Lewis acid is known as the Scholl reaction
and many revised methods have been reported. Employment of
the widely used reaction condition, i.e., the combination of
PhI(OTf)2 and BF3ꢁOEt2 developed by Kita et al.,10 afforded a
brown product in 65% yield. Matrix-assisted laser desorption
ionization time-of-flight (MALDI–TOF) mass spectroscopy de-
tected the disappearance of two hydrogen atoms, Mþ = 1234.3
(calcd. 1234.6 for C82H88N4NiO3), thereby supporting the for-
mation of 2f (Ni). Reliable evidence for the structure elucidation
of 2f (Ni) was provided by 1H NMR analysis including rotation
framework nuclear Overhauser effect measurements.11 Charac-
teristic resonance was observed in the low frequency region,
namely, two singlet peaks and a set of doublet peaks attributable
to protons of bay-region 2f (Ni) (see Formula 1). These shifts
were due to the ring current effect of the adjacent ꢀ-system
and the compression effect from the sterically crowded protons.
Reflecting the expansion of the ꢀ-system, two notable fea-
tures were found for 2f (Ni): a negative shift of the oxidation po-
tential and a bathochromic shift of the absorption spectrum. The
electrochemical oxidation potential of 2f (Ni) is negatively shift-
ed compared with that of 7f (Ni); it is 0.14 V for 2f (Ni) and
0.33 V for 7f (Ni) (vs AgNO3 in PhCN). Reflecting the drastic
change in color of 7f (Ni), the absorption spectrum of 2f (Ni)
is extremely different from that of 7f (Ni), which is a simple su-
perimposition of porphyrin and pyrene absorptions.11 The spec-
trum of 2f (Ni) is composed of mainly three bands: Band I
This work was supported in part by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan and Japan Science and
Technology Corporation.
References and Notes
1
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¨
2
J. L. Sessler, A. Gebauer, and S. J. Weghorn, in ‘‘The Porphyrin
Handbook Vol. 2,’’ ed. by K. M. Kadish, K. M. Smith, and R.
Guilard, Academic Press, San Diego (2000), Vol. 1.
K.-i. Sugiura, H. Tanaka, T. Matsumoto, T. Kawai, and Y. Sakata,
Chem. Lett., 1999, 1193.
K.-i. Sugiura, T. Matsumoto, S. Ohkouchi, Y. Naitoh, T. Kawai, Y.
Takai, K. Ushiroda, and Y. Sakata, Chem. Commun., 1999, 1957.
a) A. Tsuda, A. Nakano, H. Furuta, H. Yamochi, and A. Osuka,
Angew. Chem., Int. Ed. Engl., 39, 558 (2000). b) A. Tsuda and A.
Osuka, Science, 293, 79 (2001). c) A. Tsuda, H. Furuta, and A.
Osuka, J. Am. Chem. Soc., 123, 10304 (2001).
3
4
5
6
7
8
9
P. Michel and A. Moradpour, Synthesis, 1988, 894, and references
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A. G. Hyslop, M. A. Kellett, P. M. Iovine, and M. J. Therien, J. Am.
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a) H. Vollmann, H. Becker, M. Corell, H. Streeck, and G. Langbein,
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(13800 cmꢂ1, 724 nm), Q-band-like Band II (19300 cmꢂ1
519 nm), and Soret-band-like split Band III (23300 cmꢂ1
,
,
430 nm and 23800 cmꢂ1, 420 nm). Reflecting the highly conju-
gated structure, Band I appeared in the low energy region. This
wavelength is similar to that of nickel(II) [2]porphyracene 1,
13460 cmꢂ1 (743 nm).4 The decrease of the intensities of Band
III (log " ¼ 4:38 for 23300 cmꢂ1 and 4.41 for 23800 cmꢂ1) is at-
tributable to the lowering of the symmetry and the new ꢀ-elec-
tron system of pyrene-fused porphyrin.
A similar reaction of the corresponding zinc complex 7f
(Zn) induced the demetalation to afford the free base, 7f (2H).
The employment of a reaction condition using a weak Lewis
acid, i.e., the combination of 2,3-dichloro-4,5-dicyano-p-benzo-
quinone (DDQ) and Sc(OTf)3,5 solved the problem and afforded
10 Y. Kita, M. Gyoten, M. Ohtsubo, H. Tohma, and T. Takada, Chem.
Commun., 1996, 1481.
11 Selected spectroscopic data of 7f (Ni): 1H NMR (CDCl3) ꢁ ¼ 9:66
(s, 1H), 9.48 (s, 1H), 9.34 (d, J ¼ 4:9 Hz, 1H), 8.91 (d, J ¼ 4:9 Hz,
1H), 8.63 (d, J ¼ 4:9 Hz, 1H), 8.58–8.54 (m, 5H), 8.51 (d,
J ¼ 8:9 Hz, 1H), 8.03-7.95 (m, 4H), 7.87 (br t, 2H), 7.78 (br d,
1H), 7.70 (br d, 1H), 7.66–7.62 (m, 3H), 7.19 (s, 1H), 4.58 (t,
J ¼ 7:0 Hz, 2H), 4.47 (t, J ¼ 7:0 Hz, 2H), 4.42 (t, J ¼ 7:0 Hz,
2H), 2.20–2.05 (m, 6H), 1.80–1.70 (m, 6H), 1.55 (s, 9H), 1.49 (s,
9H), 1.16 (t, J ¼ 7:0 Hz, 3H), 1.12 (t, J ¼ 7:0 Hz, 3H), and 1.11
(t, J ¼ 7:0 Hz, 3H) ppm. UV–vis. (CHCl3) ꢂ ¼ 13800 (724 nm,
log " ¼ 4:57), 14970 (sh., 668 nm, log " ¼ 4:16), 19300 (519 nm,
log " ¼ 4:52), 23300 (430 nm, log " ¼ 4:38), 23800 (420 nm,
log " ¼ 4:41), 26400 (379 nm, log " ¼ 4:29) cmꢂ1
.
Published on the web (Advance View) December 15, 2003; DOI 10.1246/cl.2004.40