Tetrahedron Letters
Convenient peripheral aroyloxylation reactions of porphyrins
and chlorophyll-a-based chlorins with benzoyl peroxide
a
a
a
a
a,b,
⇑
, Young Key Shim c,
⇑
Jiazhu Li , Peng Zhang , Nan-Nan Yao , Li-Li Zhao , Jin-Jun Wang
a
College of Chemistry and Chemical Engineering, Yantai University, Yantai 264005, China
Shandong Applied Research Centre of Gold Nanotechnology (Au-SDARC), Yantai 264005, China
PDT Research Institute, School of Nano System Engineering, Inje University, Gimhae 621-749, Republic of Korea
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
A practical and efficient methodology for the formation of C–O bonds on the porphyrin/chlorin periphery
was developed. The aroyloxy-substituted porphyrins and chlorins related to chlorophyll-a at the b- and
meso-positions, respectively, were conveniently synthesized by the free radical substitution reaction with
benzoyl peroxide and its homologs.
Received 10 October 2013
Revised 11 December 2013
Accepted 25 December 2013
Available online 3 January 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Aroyloxylation
Porphyrin
Chlorin
Benzoyl peroxide
Much effort has been devoted to the synthesis of porphyrins
bearing carbon–heteroatom bonds, such as C–O, C–S, C–N, and
C–P bonds, attached to b- and meso-positions. The introduction
such as introduction of functional groups and metal complexation,
in advance.
1
Although the metal-catalyzed aryloxylation and alkoxylation of
halogenated porphyrins have emerged as a powerful approach for
the formation of C–O bonds on porphyrin periphery, the corre-
sponding acyloxylation, particularly aroyloxylation has not been
systematically explored besides that reported by Iturraspe and
of heteroatom-linked functional groups into the porphyrin periph-
ery can modulate the properties of the porphyrin macrocycle and
provides access to several new, potentially useful porphyrins by
providing reaction sites for further transformations. The porphy-
rins and metalloporphyrins with attached aryloxy and alkoxy
5
Esdaile. To extend the synthetic strategy to C–O bond formation
2
groups have been shown to have interesting properties. Therefore,
on porphyrin/chlorin periphery, herein, we report a new approach
for the synthesis of b- or meso-aroyloxy-substituted porphyrins
and chlorins using benzoyl peroxide (BPO) and its homologs.
In this study, the free radical substitution of tetraphenylporphy-
rin 1 (TPP) was carried out by reacting it with BPO in toluene at
90 °C for 5 h to afford two main products, b-benzoyloxy-TPP 2
the formation of C–O bonds on the tetrapyrrole macrocycle periph-
ery has received much attention, and considerable progress has
3
been made recently. However, in contrast to the large number
of synthetic porphyrins with carbon-based peripheral substituents,
only a few porphyrins with oxygen-based peripheral substituents,
such as aryloxy, alkoxy, hydroxy, and acyloxy moieties, have been
0
and b,b -dibenzoyloxy-TPP 3 in 23% and 15% yields, respectively
2
a,e,g,4
reported.
In addition to the multistep and low-yield synthesis
(Scheme 1). In this reaction, 1 mol equiv of BPO was used as the
aroyloxylation reagent rather than as the initiator. The reaction
temperature was required to be higher than 70 °C that corresponds
to the pyrolysis temperature of BPO; otherwise, the reaction did
not occur, indicating a free radical substitution reaction. The
of b-alkoxyporphyrins from alkoxypyrroles, porphyrins bearing
oxygen-based peripheral substituents have been mainly obtained
by nucleophilic substitution and metal-catalyzed C–O bond forma-
1
b,3a,c
tion reactions.
Although the nucleophilic substitution and
1
metal-catalyzed cross-coupling reactions involve difficult reaction
condition and expensive reagents, it is necessary to develop more
convenient and economical methods for the direct construction
of C–O bonds on the porphyrin periphery without additional steps,
structures of products 2 and 3 were established by their H NMR,
6
UV–visible, and mass spectral analyses. This simple and conve-
nient aroyloxylation method motivated us to extend its application
to other types of porphyrins.
Next, chlorophyll-a derivatives were used to investigate the
C–O bond formation on the chlorin periphery because their
⇑
Corresponding authors. Tel.: +86 535 6903490; fax: +86 535 6902078 (J.-J.W.);
2
0-meso-position possesses special reactivity, such as high
7
regioselectivity, for various electrophilic substitutions. Methyl