Selective and Additive-Free Hydrogenation of Nitroarenes Mediated by a DMSO-Tagged Molecular Cobalt Corrole Catalyst

Article abstract of DOI:10.1002/ejoc.202100073

We report on the first cobalt corrole that effectively mediates the homogeneous hydrogenation of structurally diverse nitroarenes to afford the corresponding amines. The given catalyst is easily assembled prior to use from 4-tert-butylbenzaldehyde and pyrrole followed by metalation of the resulting corrole macrocycle with cobalt(II) acetate. The thus-prepared complex is self-contained in that the hydrogenation protocol is free from the requirement for adding any auxiliary reagent to elicit the catalytic activity of the applied metal complex. Moreover, a containment system is not required for the assembly of the hydrogenation reaction set-up as both the autoclave and the reaction vessels are readily charged under a regular laboratory atmosphere.

Full text of DOI:10.1002/ejoc.202100073

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Selective and Additive-Free Hydrogenation of Nitroarenes
Mediated by a DMSO-Tagged Molecular Cobalt Corrole
Catalyst
Daniel Timelthaler,[a] Wolfgang Schöfberger,[b] and Christoph Topf*[a]
We report on the first cobalt corrole that effectively mediates
the homogeneous hydrogenation of structurally diverse nitro-
arenes to afford the corresponding amines. The given catalyst is
easily assembled prior to use from 4-tert-butylbenzaldehyde
and pyrrole followed by metalation of the resulting corrole
macrocycle with cobalt(II) acetate. The thus-prepared complex
is self-contained in that the hydrogenation protocol is free from
the requirement for adding any auxiliary reagent to elicit the
catalytic activity of the applied metal complex. Moreover, a
containment system is not required for the assembly of the
hydrogenation reaction set-up as both the autoclave and the
reaction vessels are readily charged under a regular laboratory
atmosphere.
Introduction
was shown to catalyze the hydrogenation of olefins, imines, and
ketones.[14] Quite recently, Zhang and Wen with coworkers
communicated a cobalt/tetraphosphine-based homogeneous
The design of modern hydrogenation catalysts based on earth-
abundant metals relies, to a large extent, on the incorporation
of pincer motifs into the catalyst architecture to guarantee a
well-balanced stability-reactivity relationship of the active
species. Especially the reports on pertinent Mn[1,2]- and Co-
complexes[3,4] have ignited a new major wave of research in the
field of base-metal-centered redox catalysis.
catalytic protocol that relies on the formidable P2CÀ PPh22NPh
2
ligand which fixes the Co-center in a square planar environment
(Figure 1).[15] However, the respective full-fledged catalyst that
eventually facilitates the hydrogenation of polar double bonds
and N-heteroarenes is only obtained upon the addition of high
amounts of KOH (10 mol%).
Effective donor atom sets in the ligand framework of potent
cobalt hydrogenation catalysts are PNP,[5] NNN,[6] CNC,[7] NNP,[8]
and NNS[9] constellations. By contrast, tridentate ligands that
spread out a tripodal geometry around the metal center might
also be capable of catalyzing transformation with gaseous
hydrogen.[10] However, cobalt complexes with bidentate ligands
containing N, P, and NHC donors have also been successfully
implemented in various transformations.[11] Strikingly, certain
cobalt-bisphosphine combinations enable challenging and
highly sought-after enantioselective hydrogenation reactions of
enamides and α,β-unsaturated carboxylic acids.[12] Homoleptic
cobalt complexes that allow for organic transformations with H2
gas are rare, yet the early K3[Co(CN)5] was demonstrated to
effect the low-pressure-hydrogenation of conjugated C=C
double bonds,[13] whereas an anthracene-tethered cobaltate
With respect to the precedence of catalytically active cobalt
complexes in which the active Co-ions are firmly anchored in
the cavity of
a macrocyclic ring, certain cobalt corrole
assemblies have been reported to function as efficient electro-
catalysts for water splitting[16] and redox-transformations involv-
ing molecular oxygen[17] as well as mediators that bring about
the highly relevant electroreduction of CO2.[18,19] Furthermore,
metallocorroles found application as components for gas
sensors.[20] However, quite remarkably, the application of these
macrocyclic metal complexes in reduction processes using
cheap and widely abundant molecular hydrogen has escaped
attention so far. This circumstance together with the observa-
tion of a PPh3-tagged Co-corrole to function as selective sensor
for nitrite and nitrate ions[21] inspired us to test congeneric
complex 1 (Figure 1) for its ability to catalyze the hydrogenation
of structurally analogous nitroarenes. Traditionally, this atom-
efficient catalytic transformation is well achieved through 3d-
metal-based heterogeneous catalysis whereas related homoge-
neous protocols significantly lag behind in development.[22] For
the first time, we herein demonstrate that Co-corrole 1
represents a decent homogeneous catalyst for the reduction of
nitroarenes to the corresponding aniline derivatives using
gaseous H2.
[a] D. Timelthaler, Dr. C. Topf
Institute of Catalysis (INCA),
Johannes Kepler University (JKU), 4040 Linz, Austria
E-mail: christoph.topf@jku.at
https://www.jku.at/en/institute-of-catalysis/
[b] Dr. W. Schöfberger
Institute of Organic Chemistry,
Johannes Kepler University (JKU),
4040 Linz, Austria
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Figure 1. (A) Molecular structure of a well-defined Co-P2CÀ PPh22NPh2 coordination compound with a labile apical acetonitrile (ACN) ligand.15 The catalytic activity
of this cobalt complex hinges upon the addition of a substoichiometric amount of KOH (10 mol%). (B) Structural drawing of the macrocyclic cobalt corrole
complex 1 that is a catalyst in its own right and which was successfully employed in the rare homogeneous hydrogenation of nitro compounds to afford
amines without the addition of any auxiliary reagents; the appendant dimethylsulfoxide (DMSO) serves as a dummy ligand for an incoming substrate.
Results and Discussion
We then started a systematic condition evaluation in order
to establish an optimized reaction parameter set for the given
nitrobenzene hydrogenation. A solvent variation study soon
revealed that the use of a protic reaction medium is mandatory
for the catalytic activity of complex 1 since all tested common
non-protic solvents including THF, MTBE, 1,4-dioxane, DMSO,
toluene, and n-heptane resulted in full collapse of the catalytic
activity. Interestingly, when the reaction was performed under
neat conditions, higher substrate conversions were achieved
compared to the non-protic solvents case. Yet, the overall yields
could not compete with those obtained for reactions that were
run in protic reaction media and amongst the latter, EtOH
clearly outperformed its congeners MeOH, i-PrOH, n-BuOH, and
H2O (Table S1). Expectedly, the use of alcohols bearing longer
alkyl chains such as n-BuOH gave rise to higher portions of the
above-mentioned N-alkylaniline. The use of EtOH as solvent was
therefore encouraged considering the fact that (industrial)
chemical processes should generally strive for replacing petro-
leum-based chemicals with analogues that are also accessible
through biomass-valorization in order to comply with the
principles of green chemistry.[26]
Our studies commenced with the synthesis of an A3-tris(4-tert-
butylphenyl)corrole according to
a
standard literature
procedure[23] whereby we chose the bulky tert-butyl substituent
by virtue of its ability to confer considerable stability on the
full-fledged catalyst. The thus-obtained macrocycle was then
metalated with cobalt(II) acetate[24] to afford the DMSO-tagged
cobalt corrole complex 1. Both syntheses and in-depth charac-
terization of this metal complex class have been performed and
reported recently by the groups of Paolesse, Gros, and Kadish.[25]
Therein, the authors stress the aptitude of the ligated dimeth-
ylsulfoxide moiety to readily dissociate from the Co-center
already at room temperature and this fact clearly renders the
appendant DMSO an ideal placeholder for the incoming nitro-
arene substrates.
Initially, we probed the catalytic activity of Co-corrole 1 in
the homogeneous hydrogenation of nitrobenzene 2a (40 bar
°
H2, 120 C) in a mixture of THF and MeOH (1/1 by volume) using
a catalyst loading of 5 mol% (Scheme 1). We chose this
particular solvent mix in order to provide maximum solubility
for the reaction components. To our delight, complete con-
version of the model substrate was observed after a period of
15 h. GC-MS analysis of the reaction mixture then revealed
almost quantitative formation of projected aniline 3a (99%)
albeit with concomitant generation of minute quantities of N-
methylaniline as by-product (1%). The latter is presumably
formed through single methylation of 3a by the solvent MeOH
which obviously functioned as an alkylation agent in this case.
In order to substantiate the homogeneous nature of the
introduced Co-corrole-based nitroarene hydrogenation, we
eventually performed a Maitlis’ filtration experiment.[27] For this
purpose, the standard nitrobenzene hydrogenation reaction
°
(120 C, 40 bar H2) was interrupted after 2 h whereas the
reaction mixture was then filtrated through a PTFE filter (pore
size 0.2 μm) under inert conditions. After that, the clear filtrate
was again pressurized with H2 (40 bar) while the catalytic
transformation was hereafter re-enacted and allowed to run for
Scheme 1. Hydrogenation of nitrobenzene as benchmark reaction for catalyst 1. Compound 3a was isolated in almost quantitative yield as its hydrochloride
salt.
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another 13 h until completion. In accordance with our expect-
ations, the filtration did not affect the catalyst performance in
any way.
as cyano-tagged 2m were selectively hydrogenated at the NO2
group with excellent to good yields without competing
reductive transformation of the other reducible of the sub-
strates. The presence of hydroxy groups in compounds 2n, p is
also readily accommodated by the given protocol. However,
while phenolic substrate 2o gave rise to almost quantitative
yield of the aniline hydrochloride 4o (93%), the hydroxyalkyl-
congeners furnished products 4n and 4p in medium 72% and
78% yield, respectively.
Ethyl ester 2r was exclusively reduced at the nitro group
albeit with concomitant transesterification if an alcohol other
than EtOH was used as the solvent. Delightfully, our metal-
locorrole-based system is also well-compatible with acidic CO2H
motifs that usually hamper related hydrogenation protocols.[10]
In this context, benzoic acid derivative 2q was smoothly
transformed into 4q when the catalyst amount was slightly
increased to 3 mol% instead of the standard 2 mol% loading.
As a further notable asset, we did not observe formation of the
corresponding ethyl ester although EtOH is present in large
excess during the catalytic process. In order to curb the extent
of 1,4-hydrogenation in cinnamic acid derivatives like 2t, the
catalyst loading had to be decreased to 1.5 mol% but still, it
was not possible to fully suppress the Michael reduction of this
substrate.
In cases with two NO2 groups having to be reduced within
the same molecule, such as 2,2’-dinitrobiphenyl 2u, the catalyst
amount must be increased accordingly (5 mol%) to achieve
quantitative yield of salt 4u.
The broad applicability of complex 1 as nitro-hydrogenation
catalyst was further underpinned by the fact that the complex
even copes with N-heterocyclic compounds featuring the
pyridine or the quinoline core; products 4v, x were all obtained
in good yields exceeding 80%. Obviously, the catalytic Co
center in 1 is not blocked by the sp2-N donors of these
heterocycles.
Furthermore, we also conducted the nitrobenzene hydro-
genation in the presence of a related soluble cobalt tetrakis(4-
tert-butyl)porphyrine complex which was synthesized according
to a published procedure[28] and strikingly, this compound was
completely inactive under the standard reaction conditions.
Hence, we conclude that the ligating macrocycle effectively
prohibits the reduction of the metal cation to cobalt (nano)
particles that are effective hydrogenation catalysts in their own
right.[29]
Deterred by the rather high catalyst loading (5 mol%) we
then aimed for reducing the latter whilst still maintaining full
substrate conversion. We soon found out that most of the nitro
substrates were fully converted into the desired anilines at
°
120 C and 40 bar H2 after a period of 15 h using a loading of
only 2 mol% of catalyst 1 (vide infra). As a notable positive side
effect of the applied lower catalyst amount, we observed
reduced formation of the aforementioned detrimental alkylated
anilines.
Further variations of the physical reaction parameters
resulted in a marked decrease of the catalytic activity of the
°
°
given system. Application of lower temperatures (80 C, 100 C,
°
and 110 C, respectively) led to incomplete nitrobenzene
conversion and accordingly the yield of aniline was significantly
diminished. Similar adverse effects were observed upon reduc-
tion of the H2 pressure from 40 bar to 20 bar (Table S2) and on
stopping the reaction after 2 h, 6 h, and 10 h, respectively
(Table S3).
With the intention to provide the cobalt central atom of
complex 1 in a low oxidation state to foster the hydrogenation
process,[12a] we conducted the nitrobenzene reduction in the
presence of solid reducing agents such as Zn, Mn, and Mg.
Rewardingly, the pertinent reaction also proceeded well with-
out external reductants and moreover, the catalytic system did
not benefit in any way from the addition of acids or bases
(Table S4). If ammonia was added to a solution of 1 in ethanol,
formation of the known bis(ammine) cobalt corrole complex
was observed[25b] which, however, proved to be inactive in the
target transformation. These experimental findings provided
compelling reasons that the given Co-catalyzed nitroarene
hydrogenation is best performed without any additives.
Subsequently, a variety of functionalized substrates 2a, y
was subjected to pressure hydrogenation in the presence of
catalyst 1 (Table 1) and H2 gas. On completion of the catalytic
transformation, the initially formed anilines 3a, y were isolated
as their hydrochloride salts 4a, y through precipitation from the
reaction mixture upon addition of etheric HCl solution.
Halogenated nitrobenzenes 2b, i were neatly converted
into the anilines and quite remarkably, hydrodehalogenation
products were not observed in the GC-MS spectrum. In
addition, nitroarene 2e served as test substrate for performing
the catalytic hydrogenation applying a higher substrate loading
(1 mmol). In this particular case 88% isolated yield 4e were
obtained versus 97% when applying the standard 0.25 mmol
substrate amount. The three nitro acetophenones 2j, l as well
Substrate 2y with a para ethynyl group was fully hydro-
genated to yield the ethyl-tagged salt 4y with good yield.
Interestingly, if the parent nitrobenzene was equipped with a
vinyl group in the meta position, the C=C bond remained intact
during the hydrogenation process (vide infra).
We further included challenging 3-nitrostyrene 2z and
heterocycle 2aa to the substrate panoply. Although catalyst 1
sharply discriminated between the NO2 group and the exocyclic
C=C double bond in 2y, i.e. no 3-ethylaniline was detected in
the GC-MS chromatogram, we were not able to isolate and
characterize a pristine portion of 3-aminostyrene 3z, neither in
the free amine form nor as its HCl salt 4z since compound 3z is
notoriously prone to rapid polymerization (Scheme 2A).
Amine 3aa is an immediate precursor to the anticancer
drug Imatinib and its laboratory synthesis usually entails the
use of pyrophoric Raney nickel[30] or waste-producing stoichio-
metric reagents such as alkaline Na2S2O431 or SnCl2/HCl.[32] On
the contrary, cobalt corrole 1 permits access to an atom-
efficient way for the manufacture of 3aa using cheap H2 gas
(Scheme 2B).
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Table 1. Products obtained upon homogeneous hydrogenation of nitrobenzenes effected by complex 1.[a]
°
[a] Reaction conditions: nitro compound 2a, x (0.25 mmol), loading of catalyst 1 as indicated, ethanol (1.5 mL), 120 C, 40 bar H2, and reaction time of 15 h. [b]
Additionally, upscaling to 1 mmol substrate was performed with 2 mol% of 1 (88% isolated yield 4e was obtained). [c] Formation of the corresponding
saturated amino-tagged cinnamic acid was observed (4% as determined by 1H NMR), i.e. 1,4-hydrogenation occurred to a minor extent. [d] 4-Methyl-3-
nitropyridin-2-amine was used as the substrate. [e] 1-Ethynyl-4-nitrobenzene was used as substrate.
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Scheme 2. (A) Selective homogeneous hydrogenation of nitrostyrene 2z and attempted isolation of 3z as the HCl salt 4z. (B) Synthesis of the Imatinib
precursor 3aa using molecular H2 and complex 1 as catalyst.
Thermo Fisher Scientific LTQ Orbitrap XL. NMR data were collected
on a Bruker Avance III 300 MHz or 500 MHz spectrometer. Spectra
Conclusion
for the various cores were recorded as follows: 300 MHz for 1H NMR
and 75.5 MHz for 13C{1H} NMR on the 300 MHz spectrometer or
We herein introduced a simple and robust catalytic protocol for
1
500 MHz for H NMR, 125.8 MHz for 13C{1H} NMR and 470.5 MHz for
19F NMR on the 500 MHz device. Chemical shifts are listed in parts
the rare homogeneous hydrogenation of nitroarenes effected
by a well-accessible Co-corrole complex. This macrocyclic
compound tolerates a broad array of functional groups and
does not require any additives such as hydrides or bases to
develop catalytic activity. Since the pertinent catalyst is readily
handled under open-flask conditions, the tedious and time-
consuming in- and outfeed of the autoclave between an inert-
gas-filled glovebox and the ordinary laboratory atmosphere is
advantageously omitted. The utility of the given metallocorrole
as hydrogenation catalyst was further demonstrated in the
synthesis of an anticancer drug precursor.
per million (ppm) on the delta scale (δ). Axis calibration was
1
performed using the residual nondeuterated solvent for H NMR as
a reference.
Safety Statement Concerning High Pressure Hydrogenations. The
hydrogen container (200 bar, 50 L) is placed in a tapping-unit-
equipped safety storage cabinet. The vessel is attached to a control
panel that allows for fine adjustment of the desired H2 pressure
used for the hydrogenation reaction. The charging procedure of
the autoclave is performed under an efficient fume hood with a
built-in hydrogen sensor which is electronically connected to a
magnetic valve that instantaneously stops the gas supply in case of
any H2 leakage that might occur during the filling procedure.
Furthermore, both an optical and an acoustical alarm signal is
triggered as soon as free H2 is detected inside the hood.
Experimental Section
Standard Protocol for the Catalytic Hydrogenation of Nitroarenes.
General Information. All chemicals were purchased from Merck
(including Sigma-Aldrich), Fluorochem, Acros Organics, Alfa Aesar,
VWR, Roth, TCI, or Chem Lab and used as received without further
purification. The catalytic hydrogenation reactions were carried out
in a 300 mL Parr autoclave whereas the used H2 gas (5.0 purity) was
purchased from Linde Gas GmbH. Practical work under inert
conditions was performed in a LABmaster Pro glovebox from Braun
filled with argon (6.0 purity from Linde Gas GmbH). GC-MS analyses
were carried out on a Shimadzu GC-MS QP-2020 using helium
(5.0 purity from Linde Gas GmbH) as carrier gas. UV-Vis absorption
spectra were recorded on a Shimadzu UV-1800 spectrophotometer
A 4 mL glass vial was initially charged with a magnetic stirring bar,
complex
1
(2.5–12.5 μmol), and the respective nitroarene
(0.25 mmol). After that, the solvent (1.5 mL) was added and the
reaction vessel was sealed with a needle-penetrated septum cap.
The glass vials were then placed in a drilled Al plate with a capacity
to accommodate seven vessels. Hereafter, the Al inlet was trans-
ferred into the autoclave which was then tightly sealed and flushed
three times with hydrogen (30 bar) before being pressurized to the
definite value. The reaction vessel was then placed on a stirring
plate and heated to the required temperature. On completion of
the catalytic transformation, the autoclave was cooled to room
and high-resolution mass spectrometry was performed on
a
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thus-diluted reactions solutions were stirred on air for a period of
30 min in order to drive off excess hydrogen. Finally, an aliquot of
50 μL was taken from each vial, mixed with 0.5 mL of methanol
(0.5 mL), and analyzed by GC-MS.
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Standard Procedure for the Isolation of the Hydrogenation
Products. The ethanolic reaction solution as obtained from the
catalytic hydrogenation experiment was transferred into a 50 mL
round-bottom flask whereupon the solvent was removed in vacuo.
The residuals were taken up in dichloromethane and an excess of
2 M HCl solution in diethyl ether was added to initiate prompt
formation of a precipitate. The suspension was then filtered and
the collected solid was washed with dichloromethane until the
draining filtrate appeared colorless. Eventually, the thus-obtained
hydrochloride salt was dried in a desiccator over silica gel.
Alternative Procedure for the Isolation of the Salts 4c, 4p, 4s,
and 4y. Since these products proved to be partially or completely
soluble in dichloromethane, a different isolation method was
elaborated. First, the ethanolic reaction solution from the hydro-
genation experiment was evaporated to dryness and after that, 3 M
aqueous HCl solution (2 mL) was added. The thus-obtained
suspension was filtrated and the clear aqueous filtrate was then
evaporated to dryness leaving behind the hydrochloride salt.
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Acknowledgments
Financial support was provided by the Austrian Science Fund
(FWF), Standalone Project P 32045 ‘Metallocorrole-Based Catalysts
for Biomass Valorization’. Furthermore, we gratefully thank Univ.-
Prof. Dr. Marko Hapke from the INCA at JKU for pleasant and
fruitful discussions and the generous support. Moreover, we are
much obliged to DI Thomas Bögl from the department of
Analytical Chemistry at the JKU for performing the HR-MS
measurements of the cobalt corrole catalyst and the various
hydrogenation products.
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Conflict of Interest
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The authors declare no conflict of interest.
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Keywords: Catalysis · Cobalt · Corroles · Hydrogenation ·
Nitroarenes
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Full Papers
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Manuscript received: January 22, 2021
Revised manuscript received: March 16, 2021
Accepted manuscript online: March 16, 2021
Eur. J. Org. Chem. 2021, 2114–2120
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© 2021 The Authors. European Journal of Organic Chemistry published
by Wiley-VCH GmbH