Photooxidation of N-confused porphyrin: A route to N-confused biliverdin analogues

Article abstract of DOI:10.1002/chem.200903182

Clarity through confusion: The photo-oxidation of an N-confused porphyrin dianion provides a unique N-confused biliverdin analogue with two types of coordination surroundings: (NNNO) and (CNOO), as demonstrated by a palladium(II) complex (see figure). (Figure Presented)

Full text of DOI:10.1002/chem.200903182

COMMUNICATION
DOI: 10.1002/chem.200903182
Photooxidation of N-Confused Porphyrin:
A Route to N-Confused Biliverdin Analogues
Jacek Wojaczyn´ski and Lechosław Latos-Grazyn´ski*[a]
˙
Carbaporphyrinoids have been recognized as a family of
As compared to regular porphyrins, relative little study
was devoted to the degradation of their N-confused iso-
mers.[9] Initially, the frequently observed decomposition of
these macrocycles during metallation process led Furuta
et al. to investigate the nature of the degradation product.[10]
They found that in the course of copper(II) insertion in the
presence of air N-confused tetraphenylporphyrin 3 under-
went an oxidative transformation. From the reaction mix-
ture, copper(II) complex of a linear tripyrrole 4 was isolated
in 34% yield, and no other products were identified
(Scheme 1).
macrocyclic compounds used as a versatile platform for or-
ganometallic chemistry.[1] However, no linear analogue of
À
these compounds capable of C M bond formation has been
reported so far.
Degradation of tetrapyrrolic macrocycles was widely in-
vestigated in the context of heme breakdown and the oxida-
tive chlorophyll ring opening observed in vivo.[2] Model
studies involved chemical oxidation with various reagents
(e.g., CeIV or TlIII salts),[3] as well as coupled oxidation of
porphyrin complexes of redox-active metals.[4] The photooxi-
dation process of metalloporphyrins leading to linear oligo-
pyrrolic products was also studied.[5] Fuhrhop et al. estab-
lished the structure of the final product 1 of magnesium oc-
taethylporphyrin degradation.[6]
The mechanism of photooxidation of tetraphenylporphyr-
in complexes (TPP)M was investigated by several groups.[7]
A proper structure (2) of the final product of this process
was deduced from detailed NMR spectroscopic and mass
spectrometric studies by Cavaleiro et al.[8]
Scheme 1. Degradation of the N-confused tetraphenylporphyrin. a) Cu-
AHCTUNGTRENNUNG
(OAc)2, O2, toluene; b) H+.
Further studies on the regioselectivity of the process
showed that the N-confused pyrrole was cleaved together
with the 5-aryl substituent, which proved the primary attack
[10]
À
of dioxygen at the C(1) C(20) bond. This was in contrast
to the findings of Pawlicki et al. who observed that the cop-
per(II) complex of pyrrole-appended O-confused tetraary-
loxaporphyrin reacted with dioxygen, yielding both possible
tripyrrolic degradation products (resulting from attack on
´
´
[a] Dr. J. Wojaczynski, Prof. Dr. L. Latos-Graz˙ynski
Department of Chemistry, University of Wrocław
14 F. Joliot-Curie St., 50 383 Wrocław (Poland)
Fax : (+48)71-328-2348
À
À
either the C(1) C(20) or C(4) C(5) bond) in 7:3 ratio,
along with the product of oxygen atom insertion into a
copper–carbon bond.[11]
E-mail: llg@wchuwr.pl
Linear tetrapyrroles were also obtained through a rational
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903182.
synthetic approach.[12] Some of them were used as ligands,
Chem. Eur. J. 2010, 16, 2679 – 2682
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exhibiting interesting, and sometimes quite unpredictable,
coordination chemistry.[13]
Herein, we present the first example of a linear tetrapyr-
rolic product obtained in the course of photoxidation of the
N-confused tetraphenylporphyrin anion and its ability to ac-
commodate two metal cations into the coordination core.
When a solution of 2-aza-21-carba-5,10,15,20-tetraphenyl-
porphyrin (3) in chloroform was photooxygenated for 5 h,
only trace amounts of decomposition products could be de-
tected in the 1H NMR spectrum. Under these conditions,
however, 5,10,15,20-tetraphenylporphyrin remains complete-
ly intact. It was previously shown that porphyrin dianions
are more prone for dioxygen attack than porphyrins them-
selves due to increased electron density in the macrocy-
cle.[6,8] We therefore decided to perform the photooxidation
of N-confused porphyrin under basic conditions. Sodium
methoxide dissolved in methanol was added to a solution of
3 in tetrahydrofuran and a stream of air was passed through
the solution, which was irradiated with visible light. Its deep
Figure 1. 1H NMR spectra (500 MHz, CDCl3, 300 K) of 5 (A) and palladi-
um complex 7 (B). Assignments: pyrrole-H=pyrrole resonances; 20-
Ph=signals of phenyl substituent on C(20); Ph=remaining phenyl sig-
nals; s=residual solvent peak. Inset in A shows the NH peaks of 5.
1
green color gradually turned brown and the H NMR spec-
ed additional structural information. In particular, the H(13)
signal was found at d=7.25 ppm through its correlation with
H(23) proton at d=6.44 ppm (J=1.1 Hz), confirming the
presence of inverted pyrrole in compound 5. Interestingly,
this crucial fragment did not appear to be a terminal unit, as
expected from the previous investigations on the oxidative
degradation of confused porphyrins.[10,11] The C(15) reso-
nance was found at d=79.4 ppm in the 13C NMR spectrum,
as expected for an sp3 carbon atom linked to the methoxy
group. A specific contact found in the NOE map of H(23)
and the methoxy substituent further confirmed the inversion
of the N-confused pyrrole.
trum of reaction mixture taken after 1 h showed a large
quantity of degradation products. After aqueous workup,
chromatographic separation on a basic alumina column was
performed. Elution with dichloromethane yielded the un-
reacted substrate, which was followed by fractions contain-
ing tripyrrolic products (tripyrrinone 4 and its isomer), and
a red-violet band that was separated and evaporated to dry-
ness, yielding compound 5.
In a full analogy to the photooxidation of the TPP dia-
nion,[8] a mechanism can be proposed, including the cleav-
À
age of the C(a) CACTHNUTRGNEUNG(meso) bond by a dioxygen molecule, fol-
lowed by an addition of methanol to the primary product 6
(Scheme 2). Although compound 6 has not been directly iso-
lated, we found that it can be stabilized through coordina-
tion with palladium(II) (see below). Consequently, a struc-
ture similar to compound 2 was deduced for the final prod-
uct 5, albeit exhibiting the unique pyrrole with the N(12)
and C(13) atoms located in the inner part of the molecule.
Such an orientation of the N-confused ring was proposed by
Smith et al. for one of the observed two isomers of b-octa-
substituted N-confused biliverdin.[12a]
The structure of product 5 was established by using NMR
1
(1H, 13C) spectroscopy and mass spectrometry. The H NMR
spectrum of 5 is shown in Figure 1A. Seven resonances of
equal intensity in the d=6–7 ppm region can be attributed
to 7 inequivalent pyrrolic protons of a linear N-confused tet-
rapyrrole. These peaks are accompanied by three broad sig-
nals in the low-field part of the spectrum (d=9.53, 9.85, and
11.92 ppm) assigned to the inner NH protons. The resonance
at d=3.19 ppm, which is 3H in intensity, confirms the pres-
ence of the methoxy substituent in 5. The remaining reso-
nances can be found in the d=7–8 ppm region. The overall
picture resembles the spectrum obtained for compound 2
formed in the course of photooxidation of TPP dianion in
the presence of methoxide.
The analysis of 2D NMR spectra (COSY, NOESY,
HMQC, HMBC) allowed complete assignment and provid-
Scheme 2. Photooxidation of the N-confused tetraphenylporphyrin dia-
nion. a) O2, hn; b) MeOH.
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Chem. Eur. J. 2010, 16, 2679 – 2682

Photooxidation of N-Confused Porphyrin
COMMUNICATION
The mass spectrum of 5 (ESI) exhibited a peak at m/z
647.24463 that corresponds to the proposed structure from
which the meso-methoxy substituent was removed
([MÀOMe]+).
Single crystals of bimetallic complex were obtained by a
slow diffusion of n-hexane into a solution of 7 in chloro-
form, and its structure was determined by X-ray crystallog-
raphy (Figure 2). The complex 7, which crystallized as the
The symmetry of the starting macrocycle (Cs) is lower
than in the case of the TPP dianion. In principle, each one
À
of eight inequivalent C(a) CACTHNUTRGNEUGN(meso) bonds can be broken,
leading to eight possible open-chain isomeric products.
À
Compound 5 results from cleavage of the C(10) C(11) bond
in the original macrocycle. At present we have determined
that 5 is the principal tetrapyrrolic product of the photooxy-
genation of the N-confused porphyrin dianion. In fact the al-
leged regioselectivity could be explained by a limited stabili-
ty of another cleavage products, as demonstrated by a tetra-
pyrrolic precursor(s) of compound 4. Apparently these tet-
rapyrroles formed by bond cleavage at C(5) or C(20) meso
positions are easily converted to a tripyrrinone 4 by one
À
more molecule of dioxygen, which attacks another Ca Cmeso
bond leading to the loss of an inverted pyrrole.[10] On the
other hand, we found that when compound 5 was subjected
to photooxidation conditions, decomposition was observed
within 15 min. Therefore, the short reaction time is a prereq-
uisite to isolate 5 among the photodegradation products of
N-confused porphyrin. Control experiments showed that
both light and oxygen are necessary for the observed reac-
tivity.
Compound 5, a biliverdin analogue with an inverted pyr-
role, can be considered as a potential tetrapyrrolic ligand
prearranged to place a carbon donor in the coordination
core. Its quick reaction with palladium(II) acetate in metha-
nol (10 min at room temperature) resulted in a blue solu-
tion, from which bimetallic complex 7 was isolated.
Figure 2. An ORTEP plot of the crystal structure of dipalladium complex
7·CHCl3 (thermal elipsoids are drawn at the 30% probability level).
Bottom: side view of the molecule (CHCl3 and phenyl substituents omit-
ted).
CHCl3 solvate, contains two palladium(II) ions coordinated
by a trianion derived from compound 6 (formed from 5 by
the loss of methanol). Six donating atoms are provided by
the linear oligopyrrole: three nitrogen atoms of regular pyr-
roles, N(12) and C(13) of the inverted one, which serves as a
bridging unit, and, additionally, the carbonyl oxygen atom
from C(20). Two metal ions are also bridged by a hydroxyl
anion. Their coordination geometry can be described as
square planar, albeit with a marked distortion seen for
Pd(2). This effect can be attributed to weak coordination of
1
The H NMR spectrum of compound 7 is shown in Fig-
À
ure 1B. Seven pyrrolic resonances (d=5.88–6.83 ppm) are
accompanied by phenyl signals (d=7.25–8.00 ppm) and an
exchangeable peak at d=5.64 ppm, which was attributed to
O(20), which results in a long Pd O(20) bond (2.174(8) ꢁ)
and O(25)-Pd(2)-O(20) bond angle equal to 103.9(3)8. The
overall structure of the coordinated tetrapyrrolic fragment is
nearly planar, albeit with significant out-of-plane displace-
ment of the terminal pyrrolone unit (Figure 2). Palladium–
À
the OH bridging group. Low-field NH signals, and H(13)
are absent from this spectrum as a result of metal coordina-
tion. The disappearance of methoxy resonance, characteris-
tic for compound 5, indicates the ligand rearrangement
upon coordination of palladium(II) ions. In the mass spec-
trum of 7, the apparent molecular peak [M+H]+ was found
at m/z 875.0325. This value and the characteristic isotopic
pattern prove the presence of two palladium ions in com-
pound 7.
À
À
nitrogen bond lengths (Pd(1) N(21) 2.020(10), Pd(1) N(22)
À
À
2.010(10), Pd(1) N(12) 2.012(9), Pd(2) N(24) 1.967(9) ꢁ)
are in the range found in PdII complexes with linear oligo-
pyrroles,[13f,14]
whereas
the
Pd(2) C(14)
distance
À
(1.903(12) ꢁ) is among the shortest observed for coordinat-
ed pyrrole unit of N-confused porphyrins and expanded por-
phyrins.[15]
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´
´
L. Latos-Graz˙ynski and J. Wojaczynski
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Acknowledgements
The work was supported by the Ministry of Science and Higher Educa-
tion (grant no. N N204 013536).
Keywords: biliverdins
·
palladium
porphyrinoids
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Received: November 19, 2009
Published online: January 29, 2010
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