Ring-opening with one dioxygen molecule in the coupled oxidation of iron tetraarylporphyrins

Article abstract of DOI:10.1016/j.tetlet.2014.01.055

A dioxygen molecule was activated on the iron complex of tetraarylporphyrins and both oxygen atoms of the O₂ molecule were added to cleave the macrocycle to yield a linear tetrapyrrole intermediate, from which two linear tetrapyrrole products, biladienone and bilindione, are formed, as revealed by isotope labeling experiments with oxygen-18.

Full text of DOI:10.1016/j.tetlet.2014.01.055

Tetrahedron Letters 55 (2014) 1532–1535
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Ring-opening with one dioxygen molecule in the coupled oxidation
of iron tetraarylporphyrins
⇑
Jonas P. L. Sandell, Kazuhisa Kakeya, Tadashi Mizutani
Department of Molecular Chemistry and Biochemistry, Faculty of Science and Engineering, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A dioxygen molecule was activated on the iron complex of tetraarylporphyrins and both oxygen atoms of
the O2 molecule were added to cleave the macrocycle to yield a linear tetrapyrrole intermediate, from
which two linear tetrapyrrole products, biladienone and bilindione, are formed, as revealed by isotope
labeling experiments with oxygen-18.
Received 25 November 2013
Revised 8 January 2014
Accepted 14 January 2014
Available online 18 January 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Oxidation
Porphyrin
Chlorophyll
Bilin
Biochemical oxidation of heme or chlorophyll occurs by the
action of heme oxygenase (HO)1 or pheophorbide a oxygenase,2
respectively, to yield linear tetrapyrroles with important biological
functions and characteristic optical properties. As a model reaction
of biological oxidation of cyclic tetrapyrroles, coupled oxidation3 of
heme or the iron complex of octaethylporphyrin (OEP) has been
thoroughly studied, where the iron complexes of cyclic tetrapyr-
roles react with dioxygen and a reducing agent such as ascorbic
acid or hydrazine to give the iron complexes of 5-oxaporphyrin,
which are subsequently cleaved non-hydrolytically to yield linear
tetrapyrroles.4,5 The substrates for the coupled oxidation have
been limited to porphyrins carrying substituents in the pyrrolic
b-positions but none on the bridging methine carbons (meso-
positions). meso-Substituted and b-unsubstituted porphyrins are
easily prepared from one-pot condensations of pyrrole and alde-
hyde.6,7 This type of porphyrins has not been found to occur in nat-
ure and they are typically inactive toward oxidation by HO,
although meso-substituted monomethyl or monoformyl porphy-
rins have been reported active as substrates.8,9 It is only quite re-
cently that reaction conditions have been found that allow
coupled oxidation of meso-tetrasubstituted, b-unsubstituted por-
phyrins, specifically meso-tetraphenylporphyrin (TPP) and its
derivatives with functional groups on the phenyl substituents.10,11
Consequently, the mechanisms involved in the latter reactions are
not yet well understood. The aim of this research is to better
understand the coupled oxidation of meso-substituted porphyrins,
outlining the differences and similarities with the coupled
oxidation of meso-unsubstituted porphyrins.
With the introduction of aryl substituents at the meso-position
of the substrate, a second linear tetrapyrrole product besides
bilindione 3 is formed, namely biladienone 2, where the meso-
carbon is retained and the neighboring meso-position has been
hydroxylated.12 A similar acyl substituted linear tetrapyrrole has
been isolated as an oxidation product of chlorophylls catalyzed
by pheophorbide a oxygenase. Biladienone is the major product
formed, with yields typically ranging between 20% and 60% while
bilindione is rarely found in yields above 6%.13 We now report on
18O labeling studies on the production of these linear tetrapyrroles.
The coupled oxidation was carried out in 18O2 atmosphere
(Scheme 1).14 After acidic work-up the products were separated
and then analyzed by MALDI-TOF MS, thus the ratio of unlabeled,
singly labeled, and doubly labeled products could be determined.
Compound 3 was observed as the molecular ion while 2 could only
be observed as the dehydroxylated fragment in MALDI-TOF MS
spectra. Results showed that (1,20-18O2)2 and (1,19-18O2)3 formed
almost exclusively (see Table 1, entry 2), leading to the conclusion
that both oxygen atoms at the site of ring-opening in both linear
tetrapyrroles are derived from dioxygen.
Fast atom bombardment mass spectrometry (FAB-MS) was em-
ployed for the analysis of crude reaction mixtures after coupled
oxidation, the advantage being that the molecular ion of 2 is
observed. In the mass spectrum of the 16O2-reaction mixture, this
molecular ion [M+H]+ was observed at m/z 897. In the 18O2-
reaction mixture, that peak disappeared, and a strong peak at m/
z 903 appeared, corresponding with 18O3-[M+H]+ (Figs. S11-12,
⇑
Corresponding author. Tel.: +81 774 65 6623; fax: +81 774 65 6794.
E-mail address: tmizutan@mail.doshisha.ac.jp (T. Mizutani).
http://dx.doi.org/10.1016/j.tetlet.2014.01.055
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

J. P. L. Sandell et al. / Tetrahedron Letters 55 (2014) 1532–1535
1533
Ã
Scheme 1. Isotope labeling coupled oxidation of FeCl[tetraarylporphyrin]. Sites in which labeling occurs are marked by ‘ ’.
Table 1
Results from labeling coupled oxidations, expressed in units of relative intensity of the three mass spectrum peaks corresponding with products carrying no, one, or two oxygen-
18 labels
Entry
Experiment
Bilindione
Rel. int. (%)
Biladienone
Rel. int. (%)
Bilindione
Biladienone
m/z
m/z
1
Oxygen-16
733
100.0
879
100.0
735
13.6
881
17.7
737
733
1.0
0.1
883
879
0.1
0.6
2
Oxygen-18
735
10.7
881
3.9
737
733
100.0
62.5
883
879
100.0
100.0
3
1:1 Mixture
735
100.0
881
52.6
737
733
46.2
100.0
883
879
95.3
100.0
4a
Oxygen-16
H218O
735
8.6
881
14.2
737
733
0.03
45.3
883
879
0.3
100.0
5
Labeling
oxidation of 4
735
737
100.0
16.2
881
883
20.2
0.2
Bars represent relative intensity.
a
Reaction was carried out in 16O2 atmosphere with 0.05 mL H218O added to reaction mixture.
Supporting Information). This showed that the third oxygen atom
of biladienone 2 was also derived from dioxygen, contrary to our
previous assumption that hydration occurs during aqueous work-
up. The FAB-MS spectrum of isolated labeled 2 did not show the
signal at m/z 903, which is probably explained by a rapid exchange
of the hydroxyl group during acidic work-up.
To determine whether the oxygen atoms incorporated into the
product were derived from two different oxygen molecules
(two-molecule mechanism) or from a single oxygen molecule
(one-molecule mechanism), an experiment was carried out with
an atmosphere consisting of 18O2 and 16O2 in a 1:1 ratio. Results
are given in Table 1, entry 3. Biladienones 2, (18O)2, and (18O2)2

1534
J. P. L. Sandell et al. / Tetrahedron Letters 55 (2014) 1532–1535
Ar
Ar
(the dehydroxylated fragments thereof) were found in a ratio of
roughly 2:1:2. The expected ratio from a one-molecule mechanism
would be 1:0:1 (excluding contributions from natural abundance
of heavier isotopes) while from a two-molecule mechanism one
would expect a ratio of 1:2:1. The results are consistent with a
one-molecule mechanism, although the intensity of the peak at
m/z 881 in entry 3 is higher than that of the reference, unlabeled
2 (entry 1). This peak at two m/z-units above that of unlabeled 2
is clearly much higher than expected from natural abundance of
heavier isotopes alone. Incorporation of 16O during acidic work-
up can be excluded since the peak at m/z 881 was negligibly small
in entry 2. We suggest that the two-molecule mechanism may be
involved in side-reactions.
O
O
Ar
O
O
O
N
N
N
O
FeIII
FeIII
FeIII
O
O
OH
py
py
py
H+
OH
O
N
N
H
Scheme 3. Proposed reaction mechanism of coupled oxidation of 19-benzoylbili-
none iron complex.
For the coupled oxidation of a b-substituted and meso-unsubsti-
tuted porphyrin iron complex, the 5-oxaporphyrin iron complex
(verdoheme) forms as an intermediate. Verdoheme can be con-
verted to bilindione via either a hydrolytic step or redox reactions
using O2.1c For the conversion of [meso-tetraarylporphyrinato]iron
to bilindione, 5-oxaporphyrin could be an intermediate so it would
be interesting to see if some of the oxygen atoms of bilindione are
derived from water. A reaction was carried out where a small
amount of H218O was added to a reaction mixture otherwise pre-
pared under dry conditions. This was subsequently oxidized in
an 16O2 atmosphere. The results, presented in Table 1, entry 4,
show no labeled products, leading to the conclusion that no hydro-
lytic step is present in the reaction mechanism. Further studies are
required to determine whether or not a 5-oxaporphyrin intermedi-
ate is present in the reaction mechanism, as is widely believed to
be the case in the coupled oxidation of heme.15
the yield of 2 was less than half. The remaining 80% of 4 was
converted to other pigments. We expect that the active species
in the oxidation is iron-hydroperoxide. When the hydroperoxide
attacks at the benzoyl carbonyl carbon, 3 is formed (Scheme 3).
The reaction mechanism of coupled oxidation of 4 to 3 likely
follows that of Baeyer–Villiger oxidation (see Scheme 3). In
contrast, when the hydroperoxide attacks at meso-position 15, 2
is formed. Attack at other positions results in the mixture of
unidentified other pigments.
Previous studies have shown that electron-withdrawing
functional groups in the para-position of the aryl substituents of
1 result in higher yields of 3, while electron donating groups favor
formation of 2.13 The proposed Baeyer–Villiger mechanism would
be consistent with these substituent effects.
A labeling coupled oxidation of unlabeled 4 was carried out,
using an atmosphere of 18O2 (entry 5 in Table 1). As would be
expected, the resulting bilindione was predominantly singly
labeled while biladienone remained unlabeled. Thus, one of the
lactam oxygen atoms of bilindione was derived from a second
molecule of dioxygen.
The one-molecular mechanism of biladienone formation sug-
gests that the first oxidation occurs via a hydroperoxide intermedi-
ate which undergoes a Criegee-type rearrangement or via a
dioxetane intermediate which decomposes to form biladienone.
These reaction mechanisms have been also discussed in the mech-
anistic studies of heme dioxygenase.16
To summarize the implications of these results, it appears
that to a large extent, the initial oxidation occurs through a one-
molecule mechanism, giving ring-opened 19-benzoylbilinone.
Coordination of a second molecule of dioxygen results in an iron-
bound hydroperoxide, which may attack the carbonyl at position
20 leading to formation of 3, the competitive reaction being attack
at position 15, forming 2. Intermediate 4 retains the meso-carbon,
in contrast to the 5-oxaporphyrin iron complex which is an inter-
mediate of oxidation of meso-unsubstituted porphyrin and where
the meso-carbon was expelled as carbon monoxide.
In conclusion, we have shown that coupled oxidation of 1
occurs exclusively with dioxygen as the oxygen source, with no
evidence of hydrolytic mechanisms. The two oxygen atoms at the
site of ring-opening in 2 are derived from one dioxygen molecule,
while a second molecule of O2 delivers the third oxygen, which
eventually becomes the 15-OH group. The two lactam oxygen
atoms of 3 are derived from two dioxygen molecules, implying that
there is at least one intermediate involved. Furthermore, coupled
oxidation of the iron complex of 19-benzoylbilinone 4 has been
carried out, resulting in the same products (2 and 3) as the coupled
oxidation of 1, indicating that 4 may be a key intermediate in the
reaction.
A two-molecule mechanism of bilindione formation is identical
with the reaction of heme oxygenase,17 and it suggests that oxygen
atoms are introduced in a stepwise manner. Thus, there is at least
one intermediate in the formation of bilindione. A plausible
candidate for the intermediate is the 19-benzoylbilinone iron
complex 4. 19-Benzoylbilinone is a linear tetrapyrrole in which
the
p–electron conjugation is extended throughout the pyrroles
and meso-carbons. The free base of 19-benzoylbilinone and some
of its metal complexes excluding the iron complex have been
reported.12,18 The iron complex 4 was synthesized by dehydration
of biladienone 2 obtained from coupled oxidation to form free base
19-benzoylbilinone and subsequent metal insertion with iron(II)
acetate. Reaction of this iron complex under coupled oxidation
conditions resulted in the formation of 2, 3 and other pigments
(Scheme 2). The isolated yields of 2 and 3 from coupled oxidation
of 4 were 15.9% and 5.2%, respectively. Compared to coupled oxida-
tion of iron porphyrin 1,13 the yield of 3 was almost the same, but
R
O
O
1) O2, ascorbic acid,
N
N
N
N
pyridine, CHCl3
Fe
Acknowledgments
R
R
2 + 3
5
15
2) 2 M HCl
+ other chromophores
10
R = COOCH3
This work was supported by ‘Creating Research Center for
Advanced Molecular Biochemistry’, Strategic Development of
Research Infrastructure for Private Universities, the Ministry of
Education, Culture, Sports, Science and Technology (MEXT), Japan.
J.P.L.S. thanks the ITO Foundation for International Education
Exchange for the provided scholarship.
R
4
Scheme 2. Coupled oxidation of the iron complex of 19-benzoylbilinone (4).

J. P. L. Sandell et al. / Tetrahedron Letters 55 (2014) 1532–1535
1535
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