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Metamorphosis of Tetrapyrrole Macrocycles.

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Highlights
DOI: 10.1002/anie.200603249
Tetrapyrrole Macrocycles
Metamorphosis of Tetrapyrrole Macrocycles**
Mathias O. Senge* and Natalia N. Sergeeva
Keywords:
corroles · nitrogen heterocycles · oxidation ·
photochemistry · porphyrinoids
Tetrapyrroles are some of the most
ubiquitous pigments and cofactors in
nature with an ever-expanding array of
industrial applications.[1] In chemical
terms all naturally occurring tetrapyrroles are derived from either the porphyrin skeleton porphyrin(1.1.1.1)[2]
with 24 macrocycle atoms (1; four pyrrole rings and four methine bridges; for
example, hemes and chlorophylls) or the
corrin skeleton with 23 macrocycle
atoms (four pyrrole rings and three
bridging meso carbon atoms; for example, vitamin B12).
There has been an immense surge in
the preparation of structural homologues and isomers with different arrangements of the macrocycle atoms of
these systems, and significant advances
have been made to construct so-called
isomeric (for example, N-confused porphyrins or porphycenes),[3] expanded
(texaphyrins or octaphyrin 3),[4] or contracted porphyrins (corrole 2).[5] This
surge was accompanied by significant
progress in the synthetic methodology
for their preparation as well as for the
development of novel hydroporphyrins,
nonaromatic tetrapyrroles, and conformationally designed systems.[1, 6]
Typically all these different classes
of cyclic polypyrrole compounds are
accessible through specific and dedicated synthetic routes, and one cannot
easily be transformed into another
(Scheme 1). A notable exception was
[*] Prof. Dr. M. O. Senge, Dr. N. N. Sergeeva
School of Chemistry
SFI Tetrapyrrole Laboratory
Trinity College Dublin
Dublin 2 (Ireland)
Fax: (+ 353) 1-896-8536
E-mail: sengem@tcd.ie
[**] Writing of this article was made possible
by a Science Foundation Ireland Research
Professorship (SFI 04/RP1/B482).
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Scheme 1. Selected classes of tetrapyrroles and their possible interconversion routes.
the finding that metal complexes of
some octaphyrins[4] can undergo a thermal transformation into spirodicorroles
4.[7] Some of these compounds can be
split into two porphyrins in which the
expanded porphyrins are linked with
“regular” tetrapyrroles.[8, 9]
In a recent paper by Gros et al., an
intriguing observation was made that
further links the different classes of
polypyrrolic classes.[10] Upon standing
under air at room temperature, a solution of the corrole 8 in MeOH/C6D6 was
slowly converted into the porphyrin 9
(Scheme 2). The reaction was accelerated upon direct illumination and produced 9 in 19 % yield within 48 hours.[11]
Indeed, corroles are photosensitive and
less stable in solution compared to their
porphyrin analogues. It was shown earlier that they can easily be transformed
into the corresponding biliverdin species
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
in the presence of oxygen.[12] Intriguingly, porphyrin 9 was not the only product
of this autoconversion, but was accompanied by the formation of the biliverdin
derivative 10, clearly the product of a
similar ring-opening reaction.
Based on these observations and
mechanistic studies, a possible mechanism involves a [2+2] cycloaddition to
form a spiro intermediate, which is
easily oxidized and split into 9 and 10.
Support for this mechanism comes from
the behavior of bimetallic [36]octaphyrin(1.1.1.1.1.1.1.1), which splits into the
two porphyrin macrocycles via the covalently linked spiro intermediates 13
(Scheme 3).[7, 8] Insertion of PdII into the
[34]octaphyrin(1.1.1.0.1.1.1.0) resulted
in the formation of the binuclear complex 11 and its isomer, the bisspirodiporphyrin 12. These two species exist in
reversible thermal equilibrium, with 12
Angew. Chem. Int. Ed. 2006, 45, 7492 – 7495
Angewandte
Chemie
Scheme 2. Room-temperature autoconversion of a free-base corrole into a free-base porphyrin.
While nature and synthetic chemists
typically use linear synthetic strategies
for the construction of individual molecular frameworks, the striking reaction
discovered by Gros et al. indicates the
conceptional possibility for switching
back and forth between different porphyrin macrocycle systems. This becomes even clearer when other macrocycle transformations are taken into
account (Scheme 5). In fact, another
well-known reaction links the corrole
and porphyrin macrocycles. In 1967,
Johnson and co-workers described that
neutral 1-substituted (tetradehydrocorrinato)nickel(II) complexes can undergo a thermal sigmatropic rearrangement
to the respective NiII corroles.[13] Likewise, thermolysis of 1-methyl-19-alkyl-
Scheme 3. Putative intermediates of the octaphyrin conversion.
predominating at room temperature,
and the equilibrium can be shifted
photochemically to 11.[7]
Osuka and co-workers discovered
the ultimate transformation of the copper(II) complex of an octaphyrin into
the corresponding spirocyclobutane intermediate 14, which was split into two
porphyrin molecules 15 through a cycloreversion reaction (Scheme 4).[8, 9] Although the formation of spiro compounds has so far only been experimentally observed for metal complexes, the
close analogy between these reactions
indicates the likelihood of their involvement in the reaction 8!9 as well.
Angew. Chem. Int. Ed. 2006, 45, 7492 – 7495
Scheme 4. Cycloreversion of spirocyclobutane
intermediate 14 into porphyrin 15.
substituted NiII tetradehydrocorrin salts
16 yields the respective porphyrins 17, in
which the “new” meso carbon atom is
derived from the 1-methyl group.[14]
Thus, this reaction is mechanistically
quite different from the ones involving
spirobistetrapyrroles and involves ring
expansion and alkyl migration of the 1substituent. Related examples are the
known
porphyrin-to-homoporphyrin
ring expansions that are observed upon
oxidative cyclizations of a,c-biladiene
salts, where the crucial mechanistic step
is the conversion of a 1,20-disubstituted
20-hydroporphyrin through intermediary ring-opening and ring-closure sequences.[15]
Homoporphyrins, that is, porphyrins
with a two-carbon-atom meso bridge,
are also accessible by stepwise conversion of N-substituted porphyrins. A
classic example is the NiII-catalyzed
rearrangement of 18 (Scheme 5) involving first the formation of a NiII cationic
complex, then cyclization to an aziridine, CC bond migration to a meso
position, and subsequent electrocyclic
ring opening of the cyclopropanoic intermediate.[16]
Conceptionally, a similar case is the
transformation of corroles into hemiporphycenes (formally homocorroles),
which was reported here last year by
Paolesse et al.[17] Reaction of 5,10,15triphenylcorrole 20 with diiodocarbene
gave the corresponding 5-iodo-10,15,20triphenylporphyrin in traces and larger
amounts of the hemiporphycene 21
(Scheme 5) through a ring-expansion
reaction. Such a one-step procedure is
more facile than the original total syntheses of hemiporphycenes.[18] Hemiporphycenes are also accessible by ring
contraction of homoporphyrins.[19] Likewise, a brominated porphycene has been
shown to undergo a thermal ring contraction to isocorrole [porphyrin(2.0.1.0)] in basic medium.[20]
As corroles can undergo ring expansion to porphyrins, porphyrins can undergo ring contraction to corroles. A few
months ago, Callot et al. showed that the
Friedel–Crafts
acylation
of
the
5,10,15,20-tetraarylporphyrinnickel(II)
compound 22 with aryl anhydrides gives
the corrole 23 upon air oxidation in the
presence of pyridine (Scheme 5). Mechanistically this reaction most likely involves a pinacol-type rearrangement
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7493
Highlights
[3]
[4]
[5]
[6]
[7]
Scheme 5. Selected interconversions of different tetrapyrroles; acac = acetylacetonate.
[8]
similar to the one involving the natural
formation of corrinoids,[21] and the reaction is a typical example for the
synthetic possibilities that result from
interactions at the porphyrin periphery.[22]
Thus, porphyrins and related tetrapyrroles not only exist in a wide variety
of different structural isomers and homologues, but there is also an increasing
body of evidence that members of the
individual classes can also be interconverted. Many of these reactions at
present will not find large-scale synthetic applications; often they are restricted
to specific derivatives, substituents, metal complexes, and reaction conditions.
Yet, the observation that some of these
reactions, such as the corrole!porphyrin conversion 8!9, can occur at room
temperature indicates the synthetic po-
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tential of these tetrapyrrole macrocycle
interconversion reactions.
The last 25 years have seen an explosion in the number of new, altered,
and specifically designed tetrapyrroles,
and it might be believed that only
structural, mechanistic, and physical details remain to be studied to optimize
their technical application potential.
Now, links between the different classes
of tetrapyrroles are emerging, and it
appears possible to develop routes for
rapid interconversion.
[9]
[10]
[11]
[12]
[13]
[1] The Porphyrin Handbook, Vol. 1–10
(Eds.: K. M. Kadish, K. M. Smith, R.
Guilard), Academic Press, San Diego,
2000.
[2] The standard nomenclature for the various cyclic tetrapyrrole systems uses
(a.b.c.d) to denote the number of bridg-
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
[14]
ing meso carbon atoms. The “regular”
porphyrin is porphyrin(1.1.1.1), corrole
is porphyrin(1.1.1.0), porphycene is
porphyrin(2.0.2.0), hemiporphycene is
porphyrin(2.1.1.0), etc.
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of 8 and 9 (1:1 ratio). See Figure 1 in
reference [10].
C. Tardieux, C. P. Gros, R. Guilard, J.
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[22] Recent reviews on reactions at the
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