Reductive demetallation of Cu-corroles - A new protective strategy towards functional free-base corroles

Article abstract of DOI:10.1039/b819185a

A novel procedure for the reductive demetallation of Cu-meso- triarylcorroles has been disclosed. The reversible sequence copper metallation/demetallation was proven to be an effective protection/deprotection strategy towards sophisticated functionalized

Full text of DOI:10.1039/b819185a

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Reductive demetallation of Cu-corroles—a new protective strategy towards
functional free-base corroles†
Thien Huynh Ngo, Wim Van Rossom, Wim Dehaen* and Wouter Maes
Received 29th October 2008, Accepted 18th November 2008
First published as an Advance Article on the web 16th December 2008
DOI: 10.1039/b819185a
A novel procedure for the reductive demetallation of Cu-
meso-triarylcorroles has been disclosed. The reversible se-
quence copper metallation/demetallation was proven to be
an effective protection/deprotection strategy towards sophis-
ticated functionalized free-base corroles.
meso-pyrimidinyl-substitutedA2B-corroles has recently been
optimized and various functionalization reactions have been
performed on the Cu-metallated pyrimidinylcorroles in high
yields.6b However, when the same reactions were performed
on the Fb analogues, a significant drop in the yield of the
substituted pyrimidinylcorrole derivatives was observed, due to
decomposition of the less stable Fb corroles. Hence, it might
be advantageous to perform all functionalization steps on the
Cu-metallated pyrimidinylcorroles and remove the copper metal,
after its protective action, from the corrolato ligand in the last step.
Copper is often used as the metal of choice for the synthesis
and functionalization of metallocorroles, due to its easy insertion,
the absence of a (labile) axial ligand and its diamagnetic ground
state.6b,7 Cu-metallation of corroles has usually been performed
with copper(II) acetate as the metal source in pyridine at room
temperature,6b,7a,d–h,j–l and Cu-corroles are often oversimplified to
Cu(III)-species with a thermally accessible paramagnetic triplet
excited state.7 The electronic ground state of Cu-corroles has been
revisited recently.8
During the course of our continuing studies on pyrimidinylcor-
roles, it was discovered that copper insertion can also be performed
in tetrahydrofuran in an even more efficient way. The procedure
involves the use of 3 equivalents of anhydrous copper(II) acetate,
which is simply stirred with the Fb corrole in THF for 10 minutes
under an inert atmosphere, and this method could easily be
extended to other meso-triarylcorroles. The desired Cu-corroles
were obtained in excellent yields ranging from 85–95% after flash
column chromatographic purification (Scheme 1).†
Corroles, ring contracted porphyrin analogues lacking one meso-
carbon atom, have recently evolved from rare macrocycles to
widely studied and attractive porphyrinoids with particular
applications.1 The renewed interest in corroles and their prop-
erties has been initiated and facilitated by the vast amount of
progress that has been achieved for the synthesis and func-
tionalization of meso-triarylcorroles during the last decade.2
Post-macrocyclization functionalizations of the corrole skeleton,
either elaborations of the meso-groups or the introduction of
substituents on the b-pyrrolic positions, are often performed on the
generally more stable metallocorroles rather than their free-base
(Fb) counterparts, to protect the trianionic corrole cavity from
(de)protonation or to avoid extensive (oxidative) fragmentation of
the rather fragile corrole framework.2 For certain applications, it
might however be required to obtain a functionalized Fb corrole,
e.g., to benefit from its (enhanced) fluorescence. Functionalized Fb
corroles would also enable the introduction of particular desirable
metal ions, resulting in paramagnetic metallocorrole complexes,
at the final stage of the synthetic protocol only, allowing NMR
verification at every step throughout the reaction sequence. In
porphyrin chemistry, this problem is commonly solved by the
reversible coordination of metal ions in the porphyrinato ligand.
Unfortunately, removal of a metal from the corrole cavity after
functionalization is often problematic, and no general corrole
demetallation strategies have been established yet. To date, only
demetallations of Mn-, Ag-, and, very recently, Cu-corroles have
been reported under acidic conditions, but the general scope of
these procedures remains very limited.3,4
meso-4,6-Dichloropyrimidin-5-yl-substituted
porphyrinoids
are versatile building blocks for the preparation of sophisticated
functionalized porphyrinoids and macrostructures, due to
the ease and broad scope of post-macrocyclization synthetic
modifications, e.g., nucleophilic aromatic substitution (SNAr) and
Pd-catalyzed cross-coupling reactions, that can be performed
on the dichloropyrimidinyl moieties.5,6 The synthesis of
Scheme 1 Cu-metallation and demetallation of meso-triarylcorroles 1–4.
Inspired by the straightforward demetallation of metallopor-
phyrins, several attempts were undertaken to demetallate Cu-
pyrimidinylcorroles with different (mixtures of) acids. After using
TFA/H2SO4, HCl (aq) or BBr3 in dichloromethane, slow partial
demetallation was observed by electrospray mass spectrometry
(ESI-MS), but, unfortunately, no satisfying method resulting in
Molecular Design and Synthesis, Department of Chemistry, Katholieke
Universiteit Leuven, Celestijnenlaan 200F, 3001, Leuven, Belgium.
E-mail: wim.dehaen@chem.kuleuven.be; Fax: +32 16327990; Tel: +32
16327439
† Electronic supplementary information (ESI) available: Experimental
procedures, characterization data, and 1H NMR spectra for all novel Cu-
and Fb corroles. See DOI: 10.1039/b819185a
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the successful isolation of the Fb analogues in an acceptable
yield could be established. Very recently, Paolesse and co-workers
reported the (almost complete) removal of Cu (and also Mn and, to
some extent, Fe) from 5,10,15-triphenyl- and b-octaalkylcorroles
under rather harsh acidic conditions (H2SO4 in CHCl3).3c When
their procedure was, however, applied to Cu-pyrimidinylcorrole
3b, in our hands, no smooth demetallation was obtained. Although
partial copper removal was observed by ESI-MS, TLC analysis
showed a mixture of (non-fluorescent) products.
A solution to our quest for appropriate demetallation conditions
was provided in a serendipitous way. Upon reduction of Cu-10-
(4-nitrophenyl)-5,15-bis(2,4,6-trimethylphenyl)corrole (5b), again
easily obtained from the Fb analogue 5a9,10 by our Cu(OAc)2/THF
protocol (95% yield), to the p-aminophenyl-functionalized ana-
logue, according to a standard method (SnCl2·2H2O and concen-
trated aqueous HCl in ethanol at 60 ◦C) described for the Fb
nitrocorrole 5a,11 simultaneous demetallation and reduction of
the nitro group was observed and Fb aminocorrole 1a11,12 was
obtained in 90% yield (Scheme 2).
observations, a Cu(III)–Cu(II) transition is likely to be involved
in the initial step of the demetallation reaction. Demetallation of
Cu-corroles has, however, never been observed during chemical or
electrochemical reduction studies.
The demetallation progress was monitored by UV-vis spec-
troscopy (Fig. 1). No changes in the absorption spectrum were
observed on addition of SnCl2·2H2O, while 10 minutes after the
addition of HCl a shift in the Soret-band from 404 to 419 nm
was noticed, in accordance with the protonated Fb corrole. The
reaction seems to occur completely and no traces of the Cu-corrole
were detected.
Fig. 1 UV-vis absorption spectra taken during demetallation of 3b.
The demetallation method could easily be extended to other,
more generally applied Cu-triarylcorroles, e.g., 1b, 2b, 4b
(Scheme 1).†‡ The obtained yields were very high to nearly
quantitative (85–96%). Completion of the demetallation reaction
can be verified by ESI-MS or TLC, since the Fb corroles show a
(slightly) lower Rf value, and a colour change from dark brown to
green, illustrative for the protonated Fb corrole, can be observed.§
For none of the conducted demetallation reactions, isocorrole
species, as reported by Paolesse et al. for the removal of copper
under acidic conditions,3c could be detected (by ESI-MS).
A particular case is formed by 5,10,15-tris(pentafluoro-
phenyl)corrole (tpfc),15 the most intensively studied corrole to
date. According to a previous study by Gross and co-workers,
metallation of H3(tpfc) 6a with Cu(OAc)2·H2O (3 equiv) in
pyridine at reflux temperature for 10 minutes affords 30% of Cu-
corrole 6b, while 40% of the corresponding dinuclear 3,3¢-corrole
dimer 7b is obtained.7g The Cu(II) p-cation radical character
of Cu-corroles is likely to be responsible for this dimerization
process.16 The same reaction carried out at room temperature in
pyridine for 30 minutes produces only traces of the bis(corrole) 7b
and Cu(tpfc) 6b is isolated in 73% yield. When the metallation
of H3(tpfc) 6a was carried out in THF at room temperature,
6b was obtained in 82–91% yield, together with small amounts
of dimer 7b, varying from 1–7%. Demetallation of Cu(tpfc) 6b
under the general conditions afforded H3(tpfc) 6a in 54% yield.
The lower demetallation yield compared to all other investigated
corroles seems to reflect the modest stability of Cu(tpfc).7g Care
has to be taken to perform both the metallation and demetallation
reaction for tpfc as quickly as possible. As for pyrimidinylcorroles,
application of the demetallation conditions reported by Paolesse
and co-workers did not give satisfying results for Cu(tpfc) either.3c
On applying the demetallation conditions to bis(Cu-corrole)
7b, the demetallated derivative 7a was obtained in 43% yield after
flash chromatographic purification (Scheme 3). The modest yield
Scheme 2 Reduction and simultaneous demetallation of meso-p-nitro-
phenyl-substituted A2B-corrole 5b.
After this initial result, it was tried to extend the Cu-
demetallation method to other meso-triarylcorroles. First of all,
the demetallation conditions were applied to Cu-5-(4,6-dichloro-
2-thiomethylpyrimidin-5-yl)-A2B-corrole 3b,6b which was easily
synthesized from the Fb derivative 3a in 94% yield (Scheme 1).
When the demetallation procedure was performed in ethanol or
acetonitrile, a sluggish reaction and incomplete copper removal
were observed. To ensure sufficient solubility of both the tin
salt and the Cu-corrole, the solvent system was changed to
an acetonitrile/dichloromethane mixture (2:1). On stirring 3b
with 10 equivalents of SnCl2·2H2O in the presence of HCl at
room temperature, complete demetallation was observed after
10 minutes and, after work-up and flash column purification,
Fb analogue 3a was obtained in an excellent 94% yield. No
demetallated corrole could be obtained on omitting the reductant
or HCl from the reaction mixture, both components are essential
for a successful demetallation. The reductive conditions seem
to destabilize the corrolato copper complex resulting in the
(protonated) Fb corrole. Similar reductive demetallation of Cu(II)-
porphyrins has been known for quite some time.13 On the other
hand, various electrochemical studies have indicated that Cu-
corroles undergo a reversible metal-centered one-electron (first)
reduction, formally assigned as a Cu(III)–Cu(II) process, while
the (first) oxidation is corrole-centered.7b,d,f,g,j–l,14 Cu(III)–Cu(II)
reduction could also be achieved chemically.7g Based on these
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Scheme 4 Nucleophilic aromatic substitution followed by demetallation.
Scheme 3 Demetallation of bis(Cu-corrole) 7b.
corroles is rather limited due to their inherent lower stability and
Fb corroles often show a different and distinct reactivity. In some
cases, however, Fb corroles were still preferred since demetallation
was hard to achieve.
could again be attributable to the limited stability of the (Cu-)
corrole dimer. It has to be noticed, however, that this metalla-
tion/demetallation procedure is still superior to the previously
reported method affording the same b,b¢-dimer 7a (H3(tpfc) in
1,2,4-trichlorobenzene at 200 ◦C for 6 h, 2% yield of the 3,3¢-
dimer).†16c
To prove the usefulness of our demetallation strategy for the
synthesis of b-substituted Fb corroles, corrole bromination was
initially chosen as an example. The bromination of corroles has
been found to provide higher yields for metallocorrole derivatives
rather than their Fb analogues.7d,17b,d,f,i,j,l Moreover, a corrole-
isocorrole equilibrium has been observed for bromination of Fb
corroles.17b Cu-tris(2,6-dichlorophenyl)corrole (Cu(tdcc) 4b) was
converted to the octabrominated derivative 9b according to a
procedure reported by Ghosh et al. (44% yield, Scheme 5).7d In
the UV-vis spectrum, both absorption bands show a bathochromic
shift of 23 nm upon full bromination.7d Demetallation of 9b under
the general conditions resulted, however, in simultaneous reductive
demetallation and partial debromination, affording a mixture
of polybrominated (mostly tetra- and pentabromo) Fb corroles.
Similar partially brominated metallocorroles have recently been
obtained by bromination of Ge-triphenylcorrole.17l Sequential
reductive debromination of octabrominated Cu-corroles has also
been observed electrochemically, albeit at much more negative
potentials than the Cu(III)-Cu(II) reduction process.14b
We then turned our focus to octachlorinated Cu-corroles,
hoping that the b-C–Cl bonds would be more resistant towards
the applied reductive conditions. Chlorination of corroles has, to
our knowledge, not been reported yet for aromatic corroles.16d
On applying standard chlorination conditions previously used for
metalloporphyrins (N-chlorosuccinimide in o-dichlorobenzene at
high temperature),18 fully chlorinated Cu-A2B-pyrimidinylcorrole
11b was prepared in 46% yield. Running the reaction overnight
at 140 ◦C with a higher excess of N-chlorosuccinimide resulted
in partial overhalogenation, probably due to chlorination of
the mesityl moieties. The Soret band of 11b is red-shifted by
14 nm (to 417 nm) upon octachlorination. The polychlorinated
As mentioned above, most of the post-macrocyclization func-
tionalizations of corroles have been performed on metallocor-
roles due to their improved stability compared to their Fb
analogues. Moreover, a number of Cu-catalyzed reactions, e.g.,
the Liebeskind-Srogl reaction on pyrimidinylcorroles,6b inevitably
result in copper insertion when performed on Fb corroles, limiting
the synthetic accessibility of functionalized Fb corroles. However,
since demetallation of Cu-corroles can now smoothly be achieved,
the copper metallation/demetallation sequence can be used as
a protection/deprotection strategy towards previously difficultly
accessible or even inaccessible Fb corroles. As a proof-of-principle,
nucleophilic and electrophilic aromatic substitution reactions were
carried out on Cu-metallated meso-triarylcorroles, followed by
demetallation to obtain novel functionalized Fb corroles.
After SNAr on the meso-dichloropyrimidinyl moiety of Cu-
corrole 3b with 4-tert-butylphenol (81% yield), according to our
previously optimized conditions,6b the general demetallation pro-
cedure was used to obtain substituted Fb corrole 8a in 90% yield
(Scheme 4).† The overall yield for the three-step procedure
involving subsequent metallation, SNAr and demetallation is as
high as 68%. When the SNAr reaction was carried out directly on
Fb corrole 3a, only 29% of the pursued Fb corrole 8a could be
isolated.
The introduction of substituents on the b-pyrrolic peripheral
positions of corroles has already been performed on both metal-
free and metallocorroles, and often shows a high degree of
regioselectivity not seen for analogous porphyrins.7d,m,17 The scope
of electrophilic aromatic substitution (SEAr) reactions on Fb
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Scheme 5 Octahalogenation and subsequent demetallation of Cu-corroles 4b, 10b.
pyrimidinylcorrole was subjected to the demetallation conditions
and again partial dehalogenation was observed. The reaction time
and (the amount of) the reducing agent (e.g., CrCl2) were varied,
but at every trial a mixture of Cu-corrole 11b, Fb corrole 11a
and partially dechlorinated corroles was observed. The desired
Fb octachlorocorrole could be isolated from this mixture in
varying amounts by preparative TLC and shows a broad Soret-
like band at 430 nm. When only one equivalent of SnCl2·2H2O
was used, dechlorination could essentially be avoided and 20%
of Fb corrole 11a could be isolated after purification by column
chromatography, while 70% of the starting corrole 11b could be
recovered.†
In summary, a new reductive demetallation strategy for Cu-
meso-triarylcorroles has been established, that is more generally
applicable and affords higher yields than other recently avail-
able procedures, and its beneficial use in synthetic strategies
towards various complex functionalized corrole structures has
been demonstrated.
§ In a control experiment, it was verified that residual amounts of Cu-
corrole 10b can be detected in the presence of an excess of the analogous
Fb corrole 10a in ratios up to at least 1:20 by ESI-MS ( ) (limit of detection
2–5 mol%).
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The authors thank the IWT (Institute for the Promotion of
Innovation through Science and Technology in Flanders) for a
doctoral fellowship to T. H. Ngo, the FWO (Fund for Scientific
Research-Flanders) for financial support and a postdoctoral
fellowship to W. Maes, and the KU Leuven and the Ministerie
voor Wetenschapsbeleid for continuing financial support.
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‡ General procedure for the demetallation of Cu-meso-triarylcorroles:
To a solution of the respective Cu-corrole (0.05 mmol) in acetoni-
trile/dichloromethane (2:1; 15 mL), SnCl2·2H2O (113 mg, 0.5 mmol, 10
equiv) was added and the resulting mixture was stirred for 10 min at rt
under Ar. Subsequently, concentrated aqueous HCl (1 mL) was added and
stirring was continued for 10 min at rt under Ar. The completion of the
demetallation process was monitored by ESI-MS and TLC.§ The mixture
was diluted with diethyl ether, washed with water till neutral, dried over
Na2SO4 and the drying agent was filtered off. The solvent was evaporated
under reduced pressure and the pure Fb corroles were obtained as purple
solids after flash column chromatography (silica, eluent CH2Cl2/heptane
mixtures).
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