phycene compared to the geometry of the corresponding
porphyrin.8 Therefore, a modified porphycene incorporating
a CF3 or perfluoroalkyl group at the peripheral â-pyrrolic
position will attract considerable interest as a highly electron-
deficient tetrapyrrolic macrocycle, although, to the best of
our knowledge, there is no report describing the preparation
of a fluorine-containing porphycene.9 Recently, we have
synthesized the first trifluoromethylated porphycene, 2,7,-
12,17-tetraethyl-3,6,13,16-tetrakis(trifluoromethyl)por-
phycene, H2Pc(EtioCF3), which seems to be an analogue of
Scheme 1a
etioporphycene, 2,7,12,17-tetraethyl-3,6,13,16-tetrameth-
ylporphycene, H2Pc(EtioCH3).10 As detailed below, the
structure, electrochemical property and unique reactivity of
H2Pc(EtioCF3) suggest that the â-trifluoromethylated por-
phycene falls into the category of the highest electron-
deficient tetrapyrrolic macrocycles.
a Reaction conditions: (a) Br2, SO2Cl2, AcOH, TFA, Ac2O; (b)
H2O; (c) hexylene glycol, p-TsOH, benzene; (d) Pd/C, H2, Et3N,
THF; (e) KI, I2, H2O; (f) Boc2O, DMAP, CH2Cl2; (g) Cu, DMF;
(h) concentrated HCl, AcOH; (i) TiCl4, Zn, CuCl, THF; (j) DDQ,
CHCl3.
As shown in Scheme 1, the synthesis of H2Pc(EtioCF3)
started from the preparation of trifluoromethylpyrrole.11
Although an R-methyl group of the pyrrole ring is generally
oxidized to a carboxyl group through halogenation of the
methyl group by treatment with Br2 and SO2Cl2,12 we
obtained R-formylpyrrole under similar conditions in quan-
titative yield, because the CF3 group might reduce the
reactivity of halogenation at the R-methyl group. After the
formyl group was protected by hexylene glycol, deprotection
of the benzyl ester and decarboxylative iodination led to an
activated R-iodopyrrole. An R,R′-diformylbipyrrole, a pre-
cursor of H2Pc(EtioCF3), was achieved by Ullmann coupling
of the R-iodopyrrole and the following deprotection of acetal
in 45% yield. The cyclization of two 5,5′-diformyl-2,2′-
bipyrrole molecules in the presence of TiCl4 via the reductive
McMurry coupling gave a light-yellow oil as an initial
condensation product that could be a dihydroporphycene
species, although it has never been purified and characterized.
After the addition of 2,3-dichloro-5,6-dicyano-1,4-benzo-
quinone, DDQ, into the solution of the intermediate species,
a greenish blue 18π-electron aromatic porphycene, H2Pc-
(EtioCF3), was obtained in 10% yield. In the case of normal
porphycene synthesis, the aromatization due to the autoxi-
dation occurs spontaneously under ambient atmosphere after
the McMurry cyclization. This finding suggests that an
oxidant such as DDQ is required to prepare H2Pc(EtioCF3),
because the coupling intermediate is not readily oxidized.13
The porphycene H2Pc(EtioCF3) was characterized by UV-
vis, 1H and 19F NMR, and mass spectroscopy. The λmax values
of the Q-band in the UV-vis spectrum for H2Pc(EtioCF3)
appeared at 622, 670, and 719 nm in CH2Cl2, which showed
a 52-63 nm red shift with respect to those observed for H2-
Pc(EtioCH3) as shown in Figure 1. The significant Q-band
shift, which is often seen in a series of porphyrins with bulky
electron-withdrawing groups,14-18 indicates that the HOMO-
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2000; Vol. 2, Chapter 8.
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porphycene where four bromines were introduced at the â-pyrrolic carbons
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H.; Lex, J.; Vogel, E. Angew. Chem., Int. Ed. Engl. 1990, 29, 1390-1393.
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