Switchable regioselectivity in the PIFA-BF3·Et 2O mediated oxidative coupling of meso-brominated Ni(ii) porphyrin

Article abstract of DOI:10.1039/c4ob01517g

A simple and efficient method has been developed for the switchable synthesis of directly linked meso-brominated Ni(ii) porphyrin dimers through PIFA-BF₃·Et₂O mediated oxidative coupling. The respective syntheses of meso-meso or meso-β singly, doubly, and triply linked porphyrin dimers can be easily realized with the same reagent system. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob01517g

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Switchable regioselectivity in the PIFA–BF3·Et2O
mediated oxidative coupling of meso-brominated
Ni(II) porphyrin†
Cite this: Org. Biomol. Chem., 2014,
12, 6990
Received 18th July 2014,
Accepted 23rd July 2014
Chuan-Mi Feng, Yi-Zhou Zhu,* Yun Zang, Yu-Zhang Tong and Jian-Yu Zheng*
DOI: 10.1039/c4ob01517g
www.rsc.org/obc
A simple and efficient method has been developed for the switch-
Iodine(III) reagents have been proved to be powerful in
able synthesis of directly linked meso-brominated Ni(II) porphyrin oxidative coupling, and successfully used to synthesize directly
dimers through PIFA–BF3·Et2O mediated oxidative coupling. The linked porphyrin dimers.7 However, they were found to be
respective syntheses of meso–meso or meso–β singly, doubly, and inert to substrates with high oxidation potential. In view of the
triply linked porphyrin dimers can be easily realized with the same effective improvement of the oxidation potential of phenyl-
reagent system.
iodine(III) bis(trifluoroacetate) (PIFA) caused by BF3·Et2O,8 PIFA–
BF3·Et2O was rationally expected to achieve more powerful oxi-
dative coupling of porphyrin monomers. Herein, we report a
PIFA–BF3·Et2O mediated oxidative coupling reaction for the
highly effective synthesis of five directly linked meso-bromi-
nated Ni(II) porphyrin dimers from the same brominated
monomer. The switchable regioselectivity of this metal-free
reaction is found to be strongly dependent on the amount of
PIFA and the feeding order of PIFA vs. BF3·Et2O.
Directly linked porphyrin arrays have exhibited distinctive elec-
trochemical, photochemical, and photophysical properties due
to the strong electronic or excitonic interactions between two
closely adjacent porphyrin moieties.1 They have thus been
used extensively in molecular wires,2 photoelectric conversion
devices,3 functional supramolecular systems,4 and nonlinear
optical materials.5
Great efforts have been devoted to finding convenient syn-
thetic methods for directly linked porphyrin arrays, and one-
electron oxidative coupling was regarded as the most attractive
one due to its high efficiency and simplicity.6 With the pio-
neering and successful work by Osuka and co-workers, several
reagent systems such as AgPF6, BAHA, DDQ–Sc(OTf)3, and
AuCl3–AgOTf have been successively developed and applied to
synthesize different types of directly linked porphyrin
dimers.6a–f However, a conventional Suzuki coupling reaction
is still needed to synthesize meso–β singly linked porphyrin
dimers with prefunctionalized porphyrin monomers. Mean-
while, a method is generally wanted to achieve the switchable
synthesis of different types of porphyrin dimers. Since the pro-
perties of such porphyrin dimers are closely related to the
linking type and number, there remains a need to develop a
convenient and efficient way to prepare various directly linked
porphyrin dimers economically.
Halogenated porphyrin is one of the most frequently used
building blocks to achieve porphyrin-based functional mole-
cules.9 And though the meso-brominated porphyrin substrate
is reported to be sensitive to oxidizing conditions, a reliable
and effective method for achieving the oxidative coupling of
meso-brominated porphyrin is still lacking. As shown in
Table S1,† in the absence of BF3·Et2O, treating 1 with 0.5
equiv. of PIFA resulted in meso–meso singly linked porphyrin
dimer 4 in 90% yield (Scheme 1), while meso–β singly linked
porphyrin dimer 2 was obtained by using 0.5 equiv. of PIFA in
the presence of BF3·Et2O (Scheme 1). The molecular structure
of 2 was identified using NMR and MS data, and further con-
firmed by X-ray crystal structure analysis (Fig. 1). The Ni1–Ni2
distance is 8.70 Å which is longer than that of the meso–β and
meso–β doubly linked Ni(II) porphyrin dimer (8.61 Å).1b,6b The
newly formed meso–β bond (C30–C50) length is evaluated to be
1.48 Å, and equal to the C–C single bond length (1.48 Å) of
1,3-butadiene. The mean distance of Ni–N is 1.93 Å, equal to
that reported for Ni(II)–TPP.6b
State Key Laboratory and Institute of Elemento-Organic Chemistry, Collaborative
Innovation Center of Chemical Science and Engineering (Tianjin) Nankai University,
Increasing the amount of PIFA from 0.5 to 0.75 equiv.
resulted in a decreased yield of 2, and the occurrence of fused
porphyrin dimer 3 (Table S1,† entry 4). When the feeding
amount of PIFA was increased up to 1 equiv., the yield of com-
pound 3 reached the maximum 75% (Scheme 1). A slight
decrease in yield was observed with a further increase in the
Tianjin, China. E-mail: zhuyizhou@nankai.edu.cn, jyzheng@nankai.edu.cn;
Fax: +86-022-2350-5572; Tel: +86-022-2350-5572
†Electronic supplementary information (ESI) available: Synthetic procedures,
NMR data and CIF files for the single crystal X-ray structure of 2. CCDC 978038.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4ob01517g
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reaction.10 It also needs no tedious chromatographic sepa-
ration, as reported with electrochemical oxidation. What’s
more, the existence of the bromine atom in the product will
grant access to a variety of subsequent functionalizations.
In consideration of the obvious assisting effect of BF3·Et2O
on the improvement of the oxidation potential of PIFA, we sub-
sequently treated the meso–meso singly linked porphyrin 4
with PIFA–BF3·Et2O to attempt to synthesize other types of
fused porphyrin. Fortunately, treating compound 4 with 0.5
and 2.0 equiv. of PIFA in the presence of BF3·Et2O gave doubly
and triply linked porphyrin dimers (5 and 6) in 93% and 90%
yields, respectively (Table S2†). Since the meso–meso singly
linked porphyrin 4 can be prepared only with PIFA (Scheme 1),
a one-pot two-step method was then established to synthesize
the fused dimers containing meso–meso linkages. As shown in
Scheme 2, treating 1 with 1.0 equiv. of PIFA first and sub-
sequently with 0.5 equiv. of BF3·Et2O could give β–β and meso–
meso doubly fused diporphyrin 5 in 84% yield. Adding
BF3·Et2O can activate the PIFA which remains unreacted and
further form β–β bonds. Increasing the amount of PIFA will
benefit the formation of triply fused porphyrin dimers. When
2.5 equiv. of PIFA was added, porphyrin dimer 6 was finally
obtained in 80% yield (Scheme 2).
Scheme 1 Regioselective oxidative coupling of 1.
As mentioned above, the PIFA–BF3·Et2O mediated oxidative
coupling reaction is of great advantage in the synthesis of
directly linked porphyrin dimers from easily accessible por-
phyrin monomers. No transition metal reagent is needed for
the transformation. Only adjustment of the amount of PIFA
and feeding order of PIFA vs. BF3·Et2O is needed to synthesize
different types of porphyrin dimers. The amount of PIFA corres-
ponds to the number of bonds formed. And BF3·Et2O is used
to improve the oxidation potential of PIFA, whether added
before or after PIFA feeding. When BF3·Et2O is introduced into
the reaction mixture prior to the real oxidant PIFA, a large
excess environment of BF3·Et2O could be maintained as the
PIFA is dropped in. That will ensure the PIFA added is efficien-
tly and quickly activated, and then oxidizes the porphyrin to
form its radical cation at the β–carbon.6d,11 Thereafter, attack-
ing the neutral porphyrin at the meso position produces the
meso–β linked porphyrin dimer. When PIFA is introduced first,
it oxidizes the porphyrin monomer to form meso–meso singly
linked diporphyrin and that state is retained,7,8 then the
Fig. 1 X-ray crystal structure of 2 (CCDC 978038).
amount of PIFA (Table S1,† entry 6). As illustrated above, the
oxidant PIFA and auxiliary reagent BF3·Et2O both have a great
influence on the regioselective oxidative coupling. The pres-
ence of BF3·Et2O undoubtedly improves the oxidation potential
of PIFA, and finally leads to a practical route to synthesize
meso–β linked porphyrin dimers. This method needs no
further derivatization of the porphyrin substrate, as is required
of the conventional transition metal catalyzed coupling Scheme 2 PIFA–BF3·Et2O mediated fusion reaction of 1.
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unreacted PIFA is activated by the BF3·Et2O added later to porphyrin dimers). In contrast to the sharp Soret band of 1,
further oxidize the singly linked intermediate to a fused one. the singly linked dimers 2, 7 and 8 exhibit perturbed Soret
The demetalation of metalloporphyrins was further investi- bands because of excitonic coupling. The Soret bands of the
gated. Meso–meso singly linked Ni(II) porphyrin arrays were meso–β singly linked dimers 2 and 8 are observed as broad
reported to be easily transformed into the corresponding free- bands at 417 and 419 nm with shoulders at 431 and 432 nm,
bases using concentrated sulfuric acid.7d,12 The fused por- respectively. That of freebase 7 is obviously split and appears
phyrin dimers 3 and 6 were smoothly demetalated with the at 423 and 438 nm. The Q bands at 534 and 530 nm are
same method and the freebases were obtained in 76% and observed for both compounds 2 and 8. Similar to the freebase
81% yields, respectively (Scheme S1†). With regard to meso–β porphyrin monomer, the Q bands of compound 7 consist of
singly linked compound 2, the mixture of TFA and H2SO4 four peaks, and the maximum absorption occurs at 522, 557,
could be used to give the freebase porphyrin dimer 7 597 and 653 nm, respectively. Owing to more extensive conju-
(Scheme 3). The meso-brominated porphyrin dimers could be gation, the spectra of the fused dimers reach into the infrared
useful precursors for further functionalization using the region and include three broadened and red-shifted absorp-
simple SNAr reaction we recently reported.9a A typical experi- tion zones (Fig. S1†).
ment was carried out in DMF with compound 2 and phenol
In conclusion, we have developed a simple and efficient
in the presence of Cs2CO3 at 100 °C, and the corresponding method to synthesize various meso-brominated Ni(II) por-
meso-derivatized 8 was obtained in 81% yield. This simple phyrin dimers. This PIFA–BF3·Et2O mediated oxidative coup-
methodology will grant access to a variety of meso-functionali- ling reaction shows excellent regioselectivity. By simply
zation possibilities for directly linked porphyrins.
changing the amount of PIFA and the feeding order of PIFA vs.
The interesting absorption of the porphyrin derivatives is of BF3·Et2O, the respective syntheses of meso–meso singly, meso–β
great significance to the development of photoelectric conver- singly, meso–β and meso–β doubly, meso–meso and β–β doubly,
sion materials and sensors.3,13 UV-vis absorption spectra of and β–β, meso–meso and β′–β′ triply linked porphyrin arrays
meso–β singly linked dimers 2, 7, 8, and the monomer 1 are can be easily realized. The facile demetalation and SNAr reac-
shown in Fig. 2 (see Fig. S1† for spectra of other directly linked tion of brominated porphyrin dimers make this method more
valuable and general.
We thank the 973 Program (2011CB932502), NSFC (no.
21172126 and 21272123), and Asia Research Center in Nankai
University (no. AS1217) for their generous financial support.
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