Oxidative Cyclodehydrogenation Reactions with Tetraarylporphyrins

Article abstract of DOI:10.1002/ejoc.202001174

The extension of the aromatic π-system of porphyrins is a powerful method to alter their optoelectronic properties. Herein, aryl substituents were fused to porphyrin cores by Scholl oxidation reactions that selectively produced mono- and doubly-fused porphyrins in yields of up to 69 %. Several different aryl substituents attached to the porphyrin were investigated with respect to their reactivity under Scholl conditions. The fused products were fully characterized, i.e., by UV/Vis absorption spectroscopy, which showed drastic changes in the electronic features. Insight into the solid-state behavior was obtained by X-ray crystallography. Our approach represents a novel option for the late-stage functionalization of porphyrin-based compounds.

Full text of DOI:10.1002/ejoc.202001174

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Oxidative Cyclodehydrogenation Reactions with
Tetraarylporphyrins
Authors: Max Martin, Christoph Oleszak, Frank Hampel, and Norbert
Jux
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202001174
Link to VoR: https://doi.org/10.1002/ejoc.202001174
01/2020

10.1002/ejoc.202001174
European Journal of Organic Chemistry
COMMUNICATION
Oxidative Cyclodehydrogenation Reactions with
Tetraarylporphyrins
Max M. Martin,[a] Christoph Oleszak,[a] Frank Hampel[a] and Norbert Jux*[a]
[a]
Dr. M. M. Martin, C. Oleszak, Dr. F. Hampel, Prof. Dr. N. Jux
Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials (ICMM), Chair of Organic Chemistry II
Friedrich-Alexander-University Erlangen-Nuernberg
Nikolaus-Fiebiger-Str. 10, 91058 Erlangen, Germany
e-mai: max.martin@fau.de, norbert.jux@fau.de
http://www.jux.chemie.nat.fau.de
Supporting information for this article is given via a link at the end of the document.
Abstract: The extension of the aromatic π-system of porphyrins is a
powerful method to alter their optoelectronic properties. Herein, aryl
substituents were fused to porphyrin cores by Scholl oxidation
reactions that selectively produced mono- and doubly-fused
porphyrins in yields of up to 69 %. Several different aryl substituents
attached to the porphyrin were investigated with respect to their
reactivity under Scholl conditions. The fused products were fully
characterized, i.e., by UV-vis absorption spectroscopy, which showed
drastic changes in the electronic features. Insight into the solid-state
behavior was obtained by X-ray crystallography. Our approach
represents a novel option for the late-stage functionalization of
porphyrin-based compounds.
were curious whether Scholl oxidations might be a suitable tool
for modifying the porphyrins’ characteristics. This prompted us to
further investigate the reactivity of porphyrins under oxidative
aromatic coupling conditions, which will be presented in the
following.
The first examples of π-extended porphyrins, in which meso-
phenyl substituents were fused to porphyrin cores, were prepared
by an approach based on palladium catalysis.[7-11] For that,
suitable halogenated porphyrins were reacted to the respective π-
extended products with up to four fused phenyl rings per porphyrin
core.[10-11] Later on, Osuka and co-workers prepared similar π-
extended porphyrin motifs, however, via a different route. Their
approach showed that 3,5-di-tert-butylphenyl substituted nickel
porphyrins can be transformed to the respective π-extended
derivatives under Scholl conditions.[31-32] For example, nickel
porphyrins 1 bearing different functional groups at one meso-
position were studied with respect to their reactivity behavior
under Scholl conditions (Scheme 1).[32] In their study, electron
withdrawing substituents R attached to the porphyrin core lead to
a regioselective formation of 10,12- and 18,20-doubly phenylene-
fused nickel porphyrins 2.[32]
The fusion of aromatic units to porphyrin cores has become a
flourishing research area that generated a plethora of π-extended
porphyrins.[1-3] The extension of the chromophores’ π-system
represents an efficient tool to alter and therefore tailor the optical
properties, e.g., the position of the absorption bands. For that,
small polycyclic aromatic hydrocarbons (PAHs) such as
benzene[4-11] or anthracene[12-15] but also larger ones such as
nanographenes[16-17] or graphene itself[18] were fused once or
several times to porphyrins. Typical synthesis procedures either
use already π-extended pyrrole building blocks for porphyrin
synthesis or fuse the desired aromatic units to pre-formed
porphyrins afterwards.[3] Especially the second concept, the post-
fusion of aromatics to porphyrins, established itself as the method
of choice. Due to the versatility of this approach[19-20] that relies on
numerous suitable coupling possibilities, a huge π-extended
porphyrin variety has already been prepared. For example,
oxidative cyclodehydrogenation reactions, commonly known as
Scholl oxidations,[21] are frequently used to fuse aromatic building
blocks to porphyrins.[1-3] During our research, which is based on
carbon-rich porphyrins,[22-30] we regularly expose porphyrins to
oxidative Scholl conditions, yet we did not observe any fusion
reactions up until recently. As long as the porphyrins are in their
free base form, their core structure remains unaffected under
standard Scholl conditions. Recently however, we subjected
nickel porphyrins to Scholl conditions that resulted, instead of
forming a PAH in the porphyrins’ periphery, in a reaction between
the aryl substituents and the porphyrin core.[29] Because the
properties of the obtained product significantly differed to the ones
of common porphyrins, e.g., the UV-vis absorption features, we
Scheme 1. Synthesis of doubly-fused porphyrins 2 that were reported by Osuka
and co-workers in 2016.[32] R = NO2, POPh2, Bpin.
In our work, we approached the question about the reactivity of
nickel porphyrins in Scholl reactions from a different perspective.
Instead of studying the influence of one functional group, we were
interested in the effect of the aryl substituents themselves. For
that, our investigation started with
a
tetrakis(3,5-di-tert-
butylphenyl) substituted nickel porphyrin 3 that was subjected to
oxidative aromatic coupling conditions (Scheme 2). For the Scholl
reaction, nickel porphyrin 3 was reacted with an excess amount
of dry FeCl3 (8 equiv.) in a CH3NO2/CH2Cl2 mixture. Under these
1
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10.1002/ejoc.202001174
European Journal of Organic Chemistry
COMMUNICATION
conditions, the C-C coupling reaction proceeded quickly and
already after 1 h a new intensive olive spot was detected via TLC
analysis (Scheme 2, left TLC). The reaction was stopped after 3 h
and the crude product was purified by silica column
chromatography. An olive green solid that was identified as the
mono-fused porphyrin 4 was obtained in 69 % yield. During the
end of the reaction, we realized that next to 4 another compound
started to accumulate within the reaction mixture. We therefore
tested different reaction conditions, in which more equivalents of
FeCl3 were added and allowed to react for a longer period of time.
And indeed, after a reaction time of 23 h with 16 equiv. of FeCl3 a
new intensive red-brownish spot was detected next to the olive
one of 4 via TLC analysis (Scheme 2, right TLC). After silica
column chromatography, the red-brownish product was isolated
(as well as 4) and identified as the doubly-fused porphyrin 5 (36 %
yield). Although theoretically several different doubly-fused
porphyrins exist, we only detected and isolated a single isomer
(5) under the given reaction conditions. These results are in
contrast to the previous results by the Osuka group,[32] in which
electron-withdrawing groups were accounted for the formation of
doubly-fused isomers like 2 or 5. So far, attempts to synthesize
triply- or quadruply-fused porphyrins with the herein reported
procedure were unsuccessful.
The fused porphyrin products 4 and 5 were unambiguously
identified by NMR and UV-vis spectroscopic as well as mass
spectrometric and X-ray crystallographic techniques. In the NMR
spectra, the fusion of an aryl substituent to the adjacent β-position
is clearly noticeable by an increased number of signals (Figure 1).
For example, nickel porphyrin 3 shows 9 signals in the 13C NMR
spectrum, whereas the mono-fused nickel porphyrin 4, due to its
reduced symmetry, has 48 separate carbon signals (Figure 1b).
The fusion of a second bond to the doubly-fused system 5,
however, increases the symmetry again and therefore less
signals with respect to 4 appear in the NMR spectrum (Figure
S10).
Single crystals, suitable for X-ray crystallography were obtained
for the starting material nickel porphyrin 3 as well as for the mono-
and doubly-fused nickel porphyrins 4 and 5, respectively
(Figure 2). The crystals were grown out of CH2Cl2/MeOH (3, 5) or
CH2Cl2/n-hexane/MeOH (4) solution mixtures. Initial attempts to
crystallize the fused-porphyrins, however, failed because 4 and 5
showed an increased solubility behavior in organic solvents
compared to porphyrin 3. Nevertheless, slow evaporation of the
solvent mixture that is accompanied by a gradually increasing
concentration yielded crystals with a suitable size and quality for
X-ray diffraction experiments.
A comparison between the
structures of 3, 4 and 5 revealed significant differences in the
solid-state characteristics. The molecules of nickel porphyrin 3
are, due to the steric demand of the 3,5-di-tert-butylphenyl
substituents, only loosely packed (Figure 2d). The fusion of aryl
substituents to porphyrin cores does not only extend the
porphyrins’ π-system but also reduces its overall steric demand.
Hence, more dense packing structures are feasible. Therefore,
mono- and doubly-fused porphyrins 4 and 5 feature the formation
of dimers in the crystal with distances between the porphyrin
cores[33] of 4.22 Å and 3.30 Å for 4 and 5, respectively. The most
pronounced interaction within the dimer was observed for the
doubly-fused porphyrin 5, in which the two molecules almost
completely overlap due to π-π interactions (Figure 2h).
Furthermore, a short Ni-Ni distance of 3.51 Å was found for the
dimers of 5. Additionally, a crystal structure of the free-base form
of the doubly-fused porphyrin 5 was obtained (CCDC 2025049),
which showed a similar dimer formation with porphyrin-porphyrin
distances of 3.46 Å. For further information see supporting
information Figure S4.
Scheme 2. Scholl reaction of nickel porphyrin 3. a) FeCl3 (8 equiv.), CH3NO2,
CH2Cl2, 3 h, rt; b) FeCl3 (16 equiv.), CH3NO2, CH2Cl2, 23 h, rt.
Figure 1. Comparison of NMR spectra of starting material porphyrin 3 and mono-fused porphyrin 4. a) 1H NMR (400 MHz); b) 13C NMR (101 MHz).
2
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10.1002/ejoc.202001174
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Figure 2. Single crystal X-ray structures of 3, 4 and 5. a) – c) Structures depicted as ORTEP models with thermal ellipsoids drawn at a 50 % probability. d) – h)
Packing motif of the respective structures. a) – h) Solvent molecules are omitted for clarity. d) – h) Hydrogen atoms are removed for clarity. 3: CCDC 2025048;
4: CCDC 2025050; 5: CCDC 2025051.
The fusion of one or two aryl substituents to the porphyrin core
significantly influences the electronic characteristics. The
changes from 3 to the mono- and doubly-fused porphyrins 4 and
5 are recognizable by distinct color shifts from orange (3) to olive-
green (4) to red-brownish (5). Additionally, fused nickel porphyrins
5 absorb light in such a large window of the visible region their
color intensity appears so strong.
After fusing the 3,5-di-tert-butylphenyl substituents once (4) and
twice (5) to the porphyrin core we started to test other aryl
substituents under Scholl conditions. For that, 4-tert-butylphenyl
and 4-bromophenyl substituents as well as mesityl as a control
group were chosen and reacted (Table 1). For meaningful results,
all reactions were performed under the exact same conditions and
tightly followed by TLC analysis. The only variable was the
reaction time. As a consequence, the chosen reaction conditions
are a compromise that allow suitable reactivity for all porphyrins
but do not represent optimized conditions for the individual
compounds. The isolated products were identified by NMR
spectroscopy and mass spectrometry. Reactivity for all porphyrins
under the given conditions was observed, however, at
significantly different reaction rates (compare Table 1). The
fastest reaction was observed for the 3,5-di-
4
and 5 feature, compared to 3 as well as to other
tetraarylporphyrins, a significantly more intensive color to the
human eye. The changes of the electronic properties were studied
by UV-vis absorption spectroscopy (Figure 3). The fusion of one
aryl ring to the porphyrin core (4) leads to a drastic decrease of
the molar extinction coefficient accompanied by a broadening of
the porphyrins’ B-band with strong absorption features in the
range of 350 – 500 nm. The fusion of a second aryl ring (5) further
broadens the B-band, which is leading almost to an absorption
plateau in the range of 350 – 550 nm. Because molecules 4 and
tert-butylphenyl substituted porphyrin 3,
followed by the 4-tert-butylphenyl substituted
porphyrin 6. The 4-bromophenyl substituted
porphyrin 7 showed, similar to 3 and 6, the
formation of
a fused product that was
observed by TLC analysis (olive green spot).
The reaction of 7, however, seemed to
proceed even slower and less smoothly than
the ones of 3 and 6. As a result, significant
amounts of byproduct were formed during the
reaction of 7, which could not be separated
from the desired fused product. Hence, no
clear characterization of the product and
determination of the yield could be performed.
Finally, the mesityl control group porphyrin 8,
which has no available positions for a fusion
reaction, was tested. As expected, no fusion of
the aryl substituent occurred but instead, a
Figure 3. UV-vis absorption spectra of nickel porphyrin 3 and mono- and doubly-fused nickel porphyrins
4 and 5 in CH2Cl2. The maximum absorption band of each spectrum is labeled.
3
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10.1002/ejoc.202001174
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COMMUNICATION
partial chlorination of the porphyrins was observed by mass
spectrometry (Figure S22).
Keywords: π-extension • porphyrinoids • fusion reaction • Scholl
oxidation • post-functionalization
Table 1. Reactivity comparison of different aryl substituents at the porphyrin. All
reactions were started at the same day under the same conditions: Nickel
porphyrin (50 µmol, 1 equiv.), FeCl3 (800 µmol, 16. equiv.), CH3NO2 (1 mL),
CH2Cl2 (10 mL), rt. For details see supporting information.
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A plane that was defined by the position of the four nitrogen atoms
was calculated for each porphyrin. The distance between the
porphyrin cores was determined by the distance between two
calculated planes.
To conclude, the successful preparation of π-extended porphyrins
by a fusion reaction of the aryl substituents to the porphyrin cores
was presented. Standard FeCl3 mediated Scholl conditions have
proven to be suitable for the fusion of 3,5-di-tert-butylphenyl as
well as 3-tert-butylphenyl substituents to the respective porphyrin
core. Chlorination, which is a typical side reaction, only occurred
for the control group, the mesityl substituted porphyrin 8 that
cannot undergo a fusion reaction.
Finally, we like to emphasize the advantage of the herein
presented concept: With our protocol, simple A4-symmetric
porphyrins, which can be easily prepared in decent yields and
quantities, can be transformed to newly π-extended dyes using
well-established reaction conditions. Due to this simplicity and the
reliance on standard reactions, we think that this concept has a
great potential for the preparation and application of exciting new
dye materials. Additionally, this protocol is suitable for the late
stage modification of many already existing porphyrin arrays. For
example, the absorption characteristics of porphyrin-based light
absorbing arrays might be improved and therefore higher
efficiencies of photovoltaic devices could be achieved.[34-36] The
only limiting factor is that the substituents and functional groups
attached to the porphyrin must be tolerant to Scholl conditions. To
fully reveal the potential of this concept, fusing aryl substituents to
porphyrins by Scholl reactions are currently under further
investigation in our group.
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Funded by the Deutsche Forschungsgemeinschaft (DFG) –
Projektnummer 182849149 – SFB 953. M.M.M. thanks the Fonds
der Chemischen Industrie (FCI) and the Graduate School
Molecular Science (GSMS) for financial support.
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4
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European Journal of Organic Chemistry
COMMUNICATION
Entry for the Table of Contents
The transformation of A4-symmetric nickel porphyrins to the respective π-extended derivatives via a Scholl oxidative route is
presented. Different meso-aryl substituents were successfully fused to the porphyrin core using standard Scholl conditions. The
photophysical characteristics of fused-porphyrins were significantly different to the non-fused derivatives. Insight into the solid-state
properties was obtained by X-ray crystallography.
Key-Topic
π-extended porphyrins
Institute and/or researcher Twitter usernames: FAU_Germany, MaxMJCM
5
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