LETTER
77
Efficient Synthesis of meso-meso-Linked Diporphyrins by Nickel(0)-Mediated
Ullmann Homocoupling
Synthesis of
m
eso
-
i
meso-L
u
inked
D
ipor
-
phyrinsQiang Lu, Yong Guo, Qing-Yun Chen*
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Road, Shanghai 200032, P. R. of China
Fax +86(21)64166128; E-mail: Chenqy@mail.sioc.ac.cn
Received 5 September 2010
combination of starting porphyrin (44%), porphyrin dimer
Abstract: meso-meso-Linked diporphyrins have been prepared by
nickel(0)-mediated Ullmann homocoupling under mild conditions.
(20.7%), porphyrin trimer (2.9%), and porphyrin tetramer
(0.9%) was obtained, when [5,15-bis(3,5-di-tert-butyl-
phenyl)porphyrinato] zinc(II) was used.6b Hence, the de-
velopment of a new general approach to nickel(II)
diporphyrins, especially without oligomers, is demand-
ing.
Key words: diporphyrins, bromoporphyrins, dimerization, Ull-
mann homocoupling, debromination
Porphyrin arrays are of particular usefulness in the field of
molecular electronic device and artificial photosynthetic
system.1 Among them, meso-meso directly linked dipor-
phyrins are important precursors for fully conjugated tri-
ply fused porphyrins.2 Meanwhile, meso-meso linkages
are widely found in the construction of the multiporphyrin
arrays.3 Thus, a lot of methods for the formation of meso-
meso covalent bond are known. Among several
approaches4–6 to the meso-meso diporphyrins, the oxida-
tive dimerizations of monomeric porphorins are preva-
lent. The oxidants for the homocouplings are AgPF6,5a
DDQ,3d,5b bis(trifluoroacetoxy)iodobenzene (PIFA),5c and
PhIX2 (X = Cl or F).5d Under oxidative conditions, an im-
portant discovery is the dependence of the coupling prod-
ucts on the metals incorporated in the porphyrins. Usually,
the products were mainly the coupling products when zin-
cated porphyrins were used, whereas chlorination5d or
meso-b-dimerization2d,e,f,6a together with homocoupling,
also took place in some cases of nickel(II) porphyrin com-
plexes, though the nickel(II) porphyrins could be oxidated
by PIFA to form meso-meso directly linked diporphyrins.
Furthermore, all the oxidative couplings of the 5,15-dis-
ubstituted porphyrins which have two free meso positions
resulted in a mixture of oligomers except for Senge’s
case.5a–5c For example, Ag(I)-promoted oxidative cou-
pling of a zinc(II) 5,15-diarylporphyrin provides a por-
phyrin dimer only in 25% isolated yield and a porphyrin
trimer in 4% yield, based on the amount of the starting
porphyrin used.5a When zincated 5,15-diphenylporphyrin
was treated with PIFA, several porphyrin oligomers
(dimer, 17%; trimer, 16%; tetramer, 11%; pentamer, 7%)
were obtained.5c When PhIX2 was employed as an oxi-
dant, the coupling of zinc(II)–5,15-diphenylporphyrin
gave dimer product in 15% yield, together with chlorinat-
ed products, unidentifiable oligomers, and unreacted
starting porphyrin.5d Under electrochemical conditions, a
Ullmann reaction is the reaction of aryl halides with
stoichiometric copper to produce aryl–aryl bonds. The
similarity of the aromatic properties between benzene and
porphyrin encouraged us to utilize Ullmann reaction in the
coupling of meso-bromoporphyrin. In the first several at-
tempts to use copper to promote the coupling of nick-
el(II)–5,10,15-triphenylbromoporphyrin complex (Ni1a)
in DMF at 100 °C, we found that all the starting porphyrin
was debrominated. The debromination was also found in
the case of 5,10,15-triphenylbromoporphyrin (1a) or its
zinc(II) complex (Zn1a) under the classic Ullmann reac-
tion conditions, to give a debrominated copper(II) com-
plex as the final product. After that, we turned our interest
into the NiCl2(PPh3)2/Zn system for nickel(0)-mediated
Ullmann homocoupling. When the reaction of porphyrin
1a was carried out at 60 °C in the presence of 0.2 equiva-
lents of NiCl2(PPh3)2/2 equivalents of zinc in DMA for
one hour, a mixture of reduction product Zn3a (76%
yield) and coupling product Zn24a (21% yield) was
formed (Table 1, entry 1). In the same conditions, zinc(II)
and nickel(II) metalloporphyrin complexes (Zn1a and
Ni1a) provided a similar result (Table 1, entries 2 and 3).
Interestingly, Ni1a gave more coupling product. The sim-
ilar coupling reactivities of nickel(II) porphyrins to that of
zinc(II) porphyrins have been found also under a few pre-
vious oxidative dimerization conditions.5c,d According to
those data, PIFA and PhIX2 were successful reagents used
in the coupling of 5,10,15-trisubstituted nickel(II) por-
phyrin.5c,d Meso-meso-linked nickelated diporphyrins pre-
pared by nickel(0)-mediated Ullmann homocoupling
became another successful method used in the coupling of
nickel(II) porphyrin. Unexpectedly, no coupling products
were observed when iodoporphyrins (2a, Zn2a, Ni2a)
were used under various conditions (Table 1, entries 4–6).
The screening of the solvents showed that the reaction
went smoothly also in DMF and pyridine and slowly in
THF (Table 1, entries 7–9). The temperature played an
important role in the reaction. A decrease of the tempera-
ture favored the formation of the coupling product
(Table 1, entries 3, 10–12). The dimerization afforded di-
SYNLETT 2011, No. 1, pp 0077–0080
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Advanced online publication: 10.12.2010
DOI: 10.1055/s-0030-1259101; Art ID: W13910ST
© Georg Thieme Verlag Stuttgart · New York
Lu, Xiu-Qiang
