etate (PIDA) and phenyliodine bis(trifluoroacetate) (PIFA),
have been widely recognized as safe and useful oxidants
having reactivities similar to metal oxidizers.7 Particularly,
PIFA, with a highly electrophilic iodine center, can promote
many coupling reactions,8 including the synthesis of
meso-meso directly linked diporphyrins with 0.6 equiv of
PIFA.9 We have also used this coupling reaction in the
synthesis of singly linked chiral diporphyrins,10 which should
be terminated by addition of NaBH4 and CH3OH when
excessive PIFA was added. Without termination, triply linked
diporphyrins would be obtained as the main byproduct.
In this paper, a facile and potent method was described to
prepare triply fused diporphyrins via the PIFA oxidative
reaction in high yield, and this turned out to be more
convenient and economical than previous methods. Since
PIFA could selectively promote the oxidation reaction of
Zn(II) porphyrin, triply-singly interlacedly linked tetrapor-
phyrin, a novel covalently linked mode for molecular wire
and functional material, was prepared from Zn(II)-Ni(II)
hybrid diporphyrin.
A general procedure for PIFA oxidation has been opti-
mized. To a solution of triaryl-Zn(II)-porphyrins in freshly
distilled CH2Cl2 at -78 °C was added PIFA, and the resulting
mixture was stirred at room temperature and then terminated
by NaBH4 and CH3OH. With this procedure, the amount of
PIFA added had great influence on the product distributions
(Scheme 1 and Table 1). Treating 1a or 1b with 0.5 equiv
of PIFA resulted in meso-meso singly linked dimer 2a or
2b. As the amount of PIFA added up to 1.5 equiv, a mixture
was obtained with triply linked porphyrin dimer 3a (55%)
and singly linked porphyrin dimer 2a (29%). Only triply
linked porphyrins were produced with highest yields (90%
of 3a and 83% of 3b) when 2.5 equiv was used. Increasing
the amount of PIFA (up to 5 equiv) could depress the yield
(67% of 3a). Additionally, singly linked diporphyrin 2a also
could turn into triply linked diporphyrin 3a with the same
procedure in similar yield (84%).
Scheme 1. PIFA-Promoted Coupling Reactions
and remove excess PIFA was essential. After terminating
the reaction mixture with NaBH4 and CH3OH, the pure
product (90% of 3a) was obtained by filtration with a short
pad of silica gel. Aqueous NaHCO3, aqueous Na2S2O3, and
triethylamine were also utilized in the reaction of 1a but with
lower yields (45%, 76%, and 80%, respectively).
Table 1. Yields of Oxidative Coupling Reaction of 1a and 1b
with PIFA
yielda
entry
reactant
solvent
CH2Cl2
equiv
time
2
3
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
0.5
1.5
2.5
5
0.5
1.5
2.5
2.5
2.5
2.5
2.5
5 min
2 h
2 h
2 h
5 min
2 h
2 h
2 h
2 h
5 h
86
29
0
0
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
ClCH2CH2Cl
toluene
THF
55
90
67
0
0
93
23
0
53
83
80
78
68
0
0
0
0
It is noteworthy that feeding and postprocessing could
affect the yields of triply linked diporphyrins. Feeding PIFA
at -78 °C could reduce the polymeric residue. In contrast,
the triply linked porphyrin 3a was isolated in a lower yield
(77%) when PIFA was added at room temperature. A refined
workup procedure to reduce one electron oxidation product
10
11
24 h
36b
a Isolated yield of 2a or 2b and 3a or 3b. b Together with the starting
porphyrin and iodic porphyrin.
Similar results of the solvent effects in the PIFA coupling
reactions were also reported by Chen and Guo.9 The reaction
of 1b could be promoted in noncoordinating solvents with
68-83% yields of 3b. However, in THF, a trace amount of
triply linked diporphyrin was obtained. Moreover, when
PIDA was used instead of PIFA, singly linked diporphyrin
2b was obtained as the main product after 24 h reaction.
It is well-known that the center metal of porphyrin can
affect the reactivity and reaction sites in some oxidative
reactions.6,11 The Cu(II), Ni(II), and Pd(II) porphyrins were
also used instead of Zn(II) porphyrin in this reaction (Scheme
2). After stirring for 3 days, meso-meso singly linked
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