Synthesis and characterization of copper complexes with tripodal ligands bearing amino acid groups

Article abstract of DOI:10.1002/zaac.202000320

The tripodal ligand (2-aminoethyl)bis(2-pyridylmethyl)amine (uns-penp), known for its Cu/O₂ intermediates, was modified at one side arm by a selection of amino acids. With L-Tyrosine (Tyr), L-Histidine (His) and L-Lysine (Lys) it was possible to introduce chirality into the tripodal ligand system and to investigate the corresponding copper(I) complexes [Cu{L-His(BPh₃)uns-penp}], [Cu(L-Lys)uns-penp]OTf and [Cu(L-Tyr)uns-penp]OTf. [Cu{L-His(BPh₃)uns-penp}] could be structurally characterized and represents the first example of a copper(I) complex with a coordinated imidazole ring of the histidine ligand. Furthermore, these complexes demonstrated catalytic activity for the oxygenation of thioanisole with hydrogen peroxide as an oxidant.

Full text of DOI:10.1002/zaac.202000320

Title: Synthesis and characterization of copper complexes with tripodal
ligands bearing amino acid groups
Authors: Christopher Gawlig, Jannis Jung, Doreen Mollenhauer, and
Siegfried Schindler
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To be cited as: Z. anorg. allg. Chem. 10.1002/zaac.202000320
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10.1002/zaac.202000320
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ARTICLE
Synthesis and characterization of copper complexes with tripodal
ligands bearing amino acid groups
Christopher Gawlig,[a] Jannis Jung[b,c], Doreen Mollenhauer,[b,c] and Siegfried Schindler*[a]
Dedicated to the 70th birthday of Prof. Dr. Peter Klüfers.
Abstract:
The
tripodal
ligand
(2-aminoethyl)bis(2-
Examining these compact model systems and their spectral
features makes it possible to gain information about the
geometric/electronic structures of the active species of the
catalytic metal centers. Especially the ligand uns-penp is
interesting because of the increased stability towards copper(I)
ions. Furthermore, it obtains a primary amino group that allows
easy modifications of the ligand itself. Those modifications can
include the attachment of a side arm that introduces stereo
chemical information to the ligand system, which in turn is of great
importance in the catalytic synthesis of products with an
enantiomeric excess in pharmaceutical industry.[9] For this
purpose, the use of amino acids is a promising approach to create
chirality and at the same time to obtain realistic models close to
the natural copper complexes in proteins. This furthermore can
provide an opportunity to connect oxygen sensitive copper
complexes to peptide chains. Related Approaches have been
reported in the past where amino acids have been attached to the
original ligand by amide bonds.[10–14] However, the complexes
described therein showed coordination of the amide oxygen to the
respective metal centers of Cu(II), Zn(II) or Ni(II), which is not
representative of the framework of metal-containing proteins in
nature. We wanted to avoid this type of coordination and instead
create a direct bond between the metal center and the chiral side
arm of the attached amino acid of the chelate ligand. In recent
research, the group of Shinobu Itoh et al. developed a model for
the active-site of the lytic polysaccharide monooxygenase. In this
example imidazole rings of the used ligand could form a direct
coordination with the central copper ion.[15] We wanted to pursue
this approach but through the direct use of amino acids.
pyridylmethyl)amine (uns-penp), known for its Cu/O2 intermediates,
was modified at one side arm by a selection of amino acids. With L-
Tyrosine (Tyr), L-Histidine (His) and L-Lysine (Lys) it was possible to
introduce chirality into the tripodal ligand system and to investigate
the corresponding copper(I) complexes [Cu{L-His(BPh3)uns-penp}],
[Cu(L-Lys)uns-penp]OTf
and
[Cu(L-Tyr)uns-penp]OTf.
[Cu{L-His(BPh3)uns-penp}] could be structurally characterized and
represents the first example of a copper(I) complex with a coordinated
imidazole ring of the histidine ligand. Furthermore, these complexes
demonstrated catalytic activity for the oxygenation of thioanisole with
hydrogen peroxide as an oxidant.
Introduction
Copper/dioxygen intermediates play
a significant role in
oxygenation reactions in nature and in industrial applications.[1,2]
Complexes of this type are observed for example in the active site
of the oxygen carrier copper protein hemocyanin (Hc) in
arthropods and mollusks. Copper enzymes such as tyrosinase or
superoxide dismutase, are responsible for the ortho hydroxylation
of phenol or the reduction of superoxide and are common in
almost every lifeform.[1] Copper complexes with tripodal ligands
based
aminoethyl)amine
pyridylmethyl)amine (uns-penp) proved to be quite useful in the
study of reversible dioxygen binding (Fig. 1).[3–8]
on
tris(2-pyridylmethyl)-amine
(tren) and
(tmpa),
tris(2-
(2-aminoethyl)bis(2-
Figure 1. Tripodal ligands tmpa (left), tren (middle) and uns-penp (right).
[a]
Prof. Dr. S. Schindler, C. Gawlig
E-mail: Siegfried.Schindler@anorg.chemie.uni-giessen.de
Justus-Liebig-Universität Gießen
Institut für Anorganische und Analytische Chemie
Heinrich-Buff-Ring 17
35392 Gießen, Germany
[b]
[c]
J. Jung, Prof. Dr. D. Mollenhauer
Justus-Liebig-Universität Gießen
Institut für Physikalische Chemie
Heinrich-Buff-Ring 17
Figure 2. Simplified drawing of uns-penp with attached amino acid groups
forming corresponding copper(I) complexes. Also pictured are the common
amino acids that function as ligands in copper proteins (histidine, lysine and
tyrosine).
35392 Gießen, Germany
J.Jung, Prof. Dr. D. Mollenhauer
Center for Materials Research (ZfM/LaMa)
Justus-Liebig University Giessen
Heinrich-Buff-Ring 16, 35392 Giessen (Germany)
:
Supporting information for this article is available on the WWW
under XXX or from the author.
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For the selection of the amino acids, histidine, lysine and tyrosine
were chosen for the attachment to the ligand uns-penp with
regard to their natural occurrence of the complexing amino acid
ligands in copper-containing proteins. A simplified view of the
model complexes described herein is presented in Figure 2. The
coupling of the chiral side arm with uns-penp was performed via
Liquid Phase Peptide Coupling (LPPC), which relies on the
protection of the amine function of the amino acid with a
tert-butyloxycarbonyl (Boc) group to prevent the coupling of two
equal substrates or the formation of polymer side products.
Common coupling reagents are the combination of a carbodiimide
(e.g. DCC, DIC, EDC) and 1-hydroxybenzotriazol (HOBt).[16,17]
most of the conversion had caused the formation of unidentified
side products while only a very small amount of the desired
product was obtained. This is mainly caused by the participation
of the imidazole ring of histidine in the coupling process, forming
polymeric side products with the carboxylic groups of other
histidine molecules. To prevent this, it is necessary to introduce a
protection group for the NIm of histidine. It turned out that a trityl
group (-CPh3) was best suited for its protection, because it is
stable in neutral and alkaline media and furthermore, towards
nucleophiles, which is required for the conditions of a LPPC. This
approach allowed obtaining compound 2 in
a good yield
(Scheme. 2)
Results and Discussion
Synthesis and characterization of the ligands
The tripodal ligand uns-penp (1) was first synthesized and
characterized in 1987 by Mandel et al.[18] Copper(I) and copper(II)
complexes of this ligand as well as of the methyl derivative
(Me2-uns-penp) have been described previously.[19] There are
different synthetic approaches to obtain this ligand, however, so
far we obtained the best yields by the reaction of
2-pyridinecarboxaldehyde and 1-acetylethlyene diamine followed
by a reduction of the tertiary amine with Na(AcO)3BH. In the last
step, the acetyl protection group was cleaved under acetic
conditions with 5 M HCl solution (Scheme 1).
Scheme 2. Synthesis of Boc-L-His(trt)-uns-penp (2) and Boc-His-uns-penp (3).
The trityl protective group, like the Boc group, is labile in acidic
conditions and therefore, a selective cleavage is necessary to
remove the trityl group while leaving the Boc group intact. By
applying a common method for the cleavage[20], stirring 2 in a 90%
acidic acid solution at 60 °C for two hours, Boc-L-His-uns-penp (3,
Scheme 2) was obtained in excellent yields and no cleavage of
the Boc protection group was observed.
Boc-L-Lys-uns-penp
The coupling of 1 with L-Lysine was performed under the same
reaction conditions as with L-Histidine (Scheme 3)
Scheme 1. Synthesis of acetyl uns-penp and uns-penp (1).
Boc-L-His-uns-penp
Histidine is one of the most common amino acids found in the
active sit of Type I, II and III copper enzymes such as e. g.
tyrosinase. This natural occurrence as a ligand offers a promising
candidate for a successful complexation of copper combined with
uns-penp (1). The synthesis was carried out according to a
general procedure for the preparation of peptides via LPPC with
1
and
Boc-L-His-OH.
As
coupling
and
reagents,
1-Ethyl-3-(3-
1-
hydroxybenzotriazol
(HOBt)
dimethylaminopropyl)carbodiimid (EDC) were replaced by a
combined reagent O-(Benzotriazoyl)tetramethyluronium-
Scheme 3. Synthesis of Boc-L-Lys-uns-penp (4).
tetrafluoroborat (TBTU), as a higher yield could be obtained with
these. The mixture was stirred at room temperature in DCM for
24 hours. However, analysis via mass spectrometry showed that
As with Histidine, it is necessary to use an additional protection
group with Lysine, which has an additional primary amino group
at its side arm that can interfere in the coupling process with 1.
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The benzyl carbamate group (Cbz) is providing a good option for
a selective cleavage via hydrogenation, while leaving the Boc
group intact. By using 5 mol% Pd/C with hydrogen gas at room
temperature successful cleavage was achieved and 4 could be
obtained in good yields (74 %).
bond with the imidazole nitrogen Nδ was observed leading to the
new ligand 6. This was furthermore confirmed by obtaining
crystals of the corresponding copper(I) complex, [Cu(6)]
(Scheme 5)
that
were
suitable
for
crystallographic
characterization. The molecular structure of [Cu(6)] is presented
in Figure 3, crystallographic data are reported in the Supporting
Information. The copper(I) ion is coordinated in a distorted
tetrahedral manner. Besides coordination of the two pyridyl
nitrogen atoms and the amine nitrogen atom of the uns-penp
Boc-L-Tyr-uns-penp
The synthesis of Boc-L-Tyr-uns-penp (5) with 1 and L-Tyrosine
was also performed with TBTU as a coupling reagent in DCM. An
additional protective group for the hydroxyl group is not necessary. molecule the copper(I) ion is furthermore bonded to the nitrogen
No major side products could be observed by ESI-MS (Scheme-4). atom of the imidazole ring. Due to the fact that histidine – with
coordination through the imidazole unit – is quite common in
copper proteins it could be expected that a large number of
structurally characterized copper complexes with this amino acid
should be known. However, the opposite is true and with only a
few exceptions of copper(II) histidine complexes[13,14,21–23] most of
the few crystal structures reported (according to a search in the
Cambridge Crystallographic Data Base) show no coordination to
Scheme 4. Synthesis of Boc-L-Tyr-uns-penp (5).
the imidazole ring. To the best of our knowledge, complex
[Cu{Boc-L-His(BPh3)-uns-penp}] is the first example of
a
structurally characterized copper(I) complex with a coordinated
imidazole ring of L-histidine. This compound was the only
copper(I) complex of our ligand series of amino acids for which it
Synthesis and characterization of copper(I) complexes
The copper(I) complexes of all ligands were prepared under inert
conditions in an argon atmosphere. Copper(I) salts [Cu(MeCN4)]X
(X= OTf, ClO4) were used for complexation. By using 3 it was not
possible to synthesize a stable copper(I)-complex. After an initial
formation of the complex, indicated by a color change of the
solution to yellow, a green/brown color developed after about
20-30 minutes and furthermore, a reddish precipitate (elemental
copper) formed. All attempts to avoid this disproportionation
reaction failed. However, by using [Cu(MeCN)4]PF6 followed by
an anion exchange with NaBPh4, it was possible to isolate a stable
copper(I)-complex (Scheme 5).
was possible to obtain a crystalline sample suitable for
crystallographic characterization. Another positive side effect of
the triphenyl boron group attached to the ligand is the increased
stability of the copper(I)-complex itself. In contrast to our attempts
(that failed as described above) to obtain a complex with ligand 3,
[Cu(6)] did not show signs of decomposition for a longer period of
time. Most likely, this is caused by the substituted proton of the
imidazole ring in 3. Trials to remove protons by adding
triethylamine or diisopropylethylamine prior to adding the copper
salt were not successful either and again only enforced
disproportionation/decomposition. Our hypothesis is also
supported by a test that showed when using ligand 2 with
[Cu(MeCN)4]OTf, a stable copper(I) complex could be obtained.
Due to the trityl group on the imidazole ring, analogous to the
BPh3 group from compound 6, there is no longer a proton that can
impair the stability of the complex.
Scheme 5. Formation of Boc-L-His(BPh3)-uns-penp (6) and copper complex
[Cu{Boc-L-His(BPh3)-uns-penp}].
Figure 3. Molecular structure of [Cu{Boc-L-His(BPh3)-uns-penp}] ([Cu(6)]) with
a distorted tetrahedral coordination geometry and coordinated imidazole ring of
L-histidine. Ellipsoids are set to 50% probability level.
In contrast to our expectations, analytics via mass spectrometry
and infrared spectroscopy revealed that our ligand underwent a
chemical transformation during the anion exchange. Cleavage of
one of the phenyl rings of the BPh4 anion and the formation of a
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Syntheses of the copper(I)-complexes with ligands 4 and 5 were
carried out under similar conditions as described above. For the
preparation of the copper(I) complex of 5 a stoichiometric amount
of triethylamine was added additionally due to the acidic nature of
the phenol side arm. It was possible to obtain yellow solids of both
complexes. No crystals were obtained for structural analysis via
SC-XRD. A conformational search applying the Conformer-
Rotamer Ensemble Sampling Tool (CREST) at semiempirical
level of theory based on the analytical data and subsequent DFT
(PBE, cc-pVDZ) calculations allow to suggest structural models
for both complexes, [Cu(4)(MeCN)]OTf and [Cu(5)(MeCN)]OTf
(Figure 4). The calculations within this study serve to obtain a
structural guess of the complexes, rather than representing a
theoretical consideration.
It is interesting to note that in both cases calculations suggest the
amide oxygen is coordinated to the copper(I) ion. We are not
aware that this has been reported previously for copper(I)
complexes, however, the amide oxygen atom coordination has
been observed for two very similar copper(II) complexes.[24,25] The
structural assignment was furthermore supported by IR
measurements and elemental analysis. Unfortunately, we did not
succeed to record decent NMR spectra for the complexes with
ligands 4, 5 and 6 due to the instability of the complexes
(paramagnetic Cu(II) formed in the samples).
Cyclic voltammetry
The electrochemical properties of [Cu(Me2-uns-penp)]OTf,
[Cu(acetyl-uns-penp]OTf, [Cu(4)MeCN]OTf, [Cu(5)(MeCN)]OTf,
[Cu(6)] were examined by cyclic voltammetry in acetonitrile. With
the exception of [Cu(6)] all complexes showed a reversible redox
reaction. As an example, the cyclic voltammogram of
[Cu(4)(MeCN)]OTf is presented in Figure 5 (ferrocene has been
used as an internal standard). The E1/2 values do not differ too
much for all complexes (except for [Cu(6)]) investigated and all
electrochemical data are reported in the Supporting Information.
Figure 5 Cyclic voltammogram of [Cu(4)(MeCN)]OTf, left without and right with
ferrocene added (298 K, scan rate: 100 mVs-1, c[Cu(4)(MeCN)]OTf = 1 mM,
electrolyte (NBu4)BF4: 0.1 M).
Figure 4 Molecular structures obtained by CREST//DFT calculations based on
the analytical data. The cations of [Cu(4)(MeCN)]OTf and [Cu(5)(MeCN)]OTf
have been calculated at DFT level of theory. For clarity, the hydrogen atoms
have been removed. CPK coloring scheme: black – carbon, red – oxygen, blue
– nitrogen, orange – copper.
Copper(II) complexes
Efforts to prepare copper(II) complexes were carried out with 3
and 4. Cu(ClO4)2 · 6 H2O as well as Cu(OTf)2 were used as copper
salts for the complexation. Unfortunately, it was not possible to
obtain crystalline samples for molecular structure determination.
Solid samples furthermore showed impurities that hampered
further characterization. However, mixing stoichiometric amounts
of Cu(ClO4)2 · 6 H2O with ligands 3 and 5 allowed UV-vis
measurements in methanol. Maxima of λmax= 690 nm for both
complexes support a complexation of copper(II) in a distorted
square planar geometry in solution (Figure 6).[26]
Table 1. Bond length of the coordination complex of the cations of
[Cu(4)(MeCN)]OTf and [Cu(5)(MeCN)]OTf.
Bond length [Å]
Cu – N (Amine)
Cu – N (Pyridine-1)
Cu – N (Pyridine-2)
Cu – N (Nitrile)
Cu – O
[Cu(4)(MeCN)]+
2.533
[Cu(5)(MeCN)]+
2.644
1.996
2.031
Reactivity of copper(I) complexes towards dioxygen and
hydrogen peroxide
1.984
2.039
2.091
1.941
As described in the introduction it is well known that a large
number of copper(I) complexes with tripodal ligands including as
[Cu(Me2-uns-penp]X react with dioxygen to form dinuclear end-
on-peroxido complexes. However, reacting all our copper(I)
complexes [Cu(4)(MeCN)]OTf, [Cu(5)(MeCN)]OTf and [Cu(6)] as
2.214
2.267
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well as [Cu(acetyl-uns-penp]OTf and [Cu{L-His(trt)uns-penp}]OTf
in a benchtop test with dioxygen in solution at low temperatures
(-80 °C) only showed a slow color change to a green colored
solutions indicating oxidation to copper(II) without a detectable
intermediate. This was furthermore confirmed by low temperature
stopped-flow measurements as described previously.[19]
Reactions of all complexes including [Cu(Me2-uns-penp)]OTf with
hydrogen peroxide only led to green solutions with more or less
gas bubbles (dioxygen from decomposition of hydrogen peroxide).
hours. After several attempts applying molecular oxygen as the
sole oxidant in a temperature range between -80 °C and room
temperature, it became clear that no conversion had taken place.
For that reason, hydrogen peroxide was used for further
experiments. All reactions were analyzed and quantified via
GC/MS. The results are presented in Table 2.
Table 2. Oxygenation of thioanisole with copper(I) catalysts and H2O2.
Total
#
Catalyst
TON
Sulfoxide
[%]
Sulfone
[%]
conversion
[%]
a
b
c
-
0
2
4
0
0
3
5
0
0.6
1
CuCl
traces
traces
[Cu(uns-
penp)]OTf
d
e
f
[Cu(Me2-uns-
penp)]OTf
40
12
84
6
8
3
48
15
97
8
3
11
19
2
[Cu(Ac-uns-
penp)]OTf
[Cu(4)MeCN)]
13
2
OTf
Figure 6. UV/Vis spectrum of [Cu(4)](OTf)2 with a λmax= 690 nm.
g
h
i
[Cu(5)(MeCN)]
OTf
[Cu(6)]
23
8
10
2
34
10
13
3
Catalytic oxygenation of thioanisole
[Cu{Boc-
LHis(trt)uns-
penp}]OTf
Despite the fact that no "dioxygen adduct" complexes were
formed as intermediates it seemed worthwhile to test their
properties with regard to catalytic oxidations. As a substrate
thioanisole was used and its oxygenation to sulfoxide and/or
sulfone was investigated using the copper complexes with the
ligands 4, 5, 6 and [Cu(Boc-L-His(trt)uns-penp}] in comparison
with uns-penp, [Cu{acetyl-uns-penp]OTf, [Cu(Me2-uns-penp)]OTf
in combination with dioxygen or hydrogen peroxide according to
Scheme 6.
0.5 mol% catalyst, 2 h, 1 eq. H2O2, MeCN, room temperature.
Using just hydrogen peroxide without adding a copper complex
no oxidation was observed (Table 2, entry a). Furthermore,
applying only CuCl as a copper source negligible conversion was
observed (Table 2, entry b). Furthermore, [Cu(uns-penp)]OTf
showed nearly no reactivity at all. In contrast, using [Cu(Me2-uns-
penp)]OTf conversion to 40% sulfoxide and 8% sulfone was
observed (entry d). However, when [Cu(acetyl-uns-penp]OTf was
applied conversion went down to only 12% for the sulfoxide and
3% sulfone (entry e). All copper(I) complexes with the ligands 4,
5, 6 and [Cu{Boc-L-His(trt)uns-penp}]OTf applied as suspensions
did lead to the oxygenation of thioanisole (Table 1, entries f, g, h
and i). However, it was not possible to gain an enantiomeric
excess by any of the three catalysts. In all cases, the sulfoxide is
the main product, with the highest conversion of 84% with catalyst
[Cu(Boc-L-Lys-uns-penp)(MeCN)]OTf (4) (entry f).
oxidation to the sulfone distinguishes between the reactions, with
the highest conversion of 13% with [Cu{Boc-
L-His(BPh3)-uns-penp}] (6). Compared to CuCl (entry b) with only
2% of the sulfoxide and traces of the sulfone and, the amino acid
uns-penp catalysts show a significant higher catalytic activity with
maximum conversions of 97% (entry f). Besides, in case of the
oxygenation with catalyst [Cu(4)(MeCN)]OTf, the sulfoxide was
synthesized and isolated. After purification by column
Scheme 6. Oxygenation of thioanisole with hydrogen peroxide.
Especially the sulfoxide as a product is of interest, because of its
widespread application in pharmaceutical chemistry. This and the
simple analysis of the two oxidation products methyl phenyl
sulfoxide and methyl phenyl sulfone make thioanisole a popular
substrate for catalytic oxygenation of sulfides.[27,28] Another
interesting property of the sulfoxide is its chirality. By examining
the reaction with a chiral gas chromatography column, it is
possible to determine if a catalyst can form an enantiomeric
excess of a specific enantiomer. The reaction shown in Scheme 6
was carried out at room temperature with a reaction time of 2
A further
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solution were added and the organic layer was extracted with 2x50 mL
DCM. Furthermore, the combined organic solution was washed with brine
(2x50 mL), dried over anhydrous Na2SO4, and the solvent evaporated
in vacuo. The crude product is a yellowish oil (4.11 g, 14.4 mmol, 85%)
and was used without further purification in the next step to remove the
acetyl protecting group.
chromatography a yield of 69% was determined. From the results
it is obvious that we do not see an effect of redox potential or the
ability to form a peroxido complex as an intermediate species. We
suspect a Fenton-like mechanism due to the gas development
that occurs after adding hydrogen peroxide to the reaction
solution.[29]
Synthesis of uns-penp (1).[19] The crude N-Acetyl-uns-penp (4.11 g,
14.4 mmol) was dissolved in 40 mL of a 5 M hydrochloric acid solution and
refluxed for 24 hours. The pH of the cooled solution was increased to 10
by the addition of NaOH. The crude product was extracted with 3x50 mL
DCM, dried over Na2SO4 and the solvent was evaporated. The obtained
oil was purified by Kugelrohr distillation in vacuo (0.2 mbar) and at 150 °C.
The yellowish-colorless oil (2.7 g, 11 mmol, 77%) was stored under argon.
1H-NMR (400 MHz, CDCl3): δ = 8.53 (ddd, 2H), 7.65 (td, 2H), 7.50 (dt, 2H),
7.14 (dd, 2H), 3.85 (s, 4H), 2.85 – 2.72 (m, 2H), 2.66 (t, 2H). 13C-NMR
(101 MHz, CDCl3): δ = 159.6, 149.0, 136.3, 122.9, 122.8, 121.9, 77.5, 77.1,
76.8, 60.7, 57.4, 39.6.
Conclusions
Our results showed that it is possible to synthesize amino acid
derivatives based on the tripodal ligand uns-penp. These modified
flexible ligand frameworks are able to adapt to the coordination
sphere geometry of copper(I) metal ions. Furthermore, they
provide an opportunity to connect these complex units to peptide
chains. In the case of L-Lysine, L-Histidine and L-Tyrosine,
copper(I) complexes were isolated, however, it was not possible
to obtain the corresponding copper(II) complexes. We could
determine the molecular structure of [Cu{L-His(BPh3)uns-penp}]
by using single crystal X-ray diffraction. This is the first example
of a structurally characterized copper(I) complex with coordination
to the nitrogen atom of the imidazole ring of histidine. However, in
contrast to our previous findings (applying copper(I) complexes
with related ligands) it was not possible to detect a "dioxygen
adduct" intermediate in benchtop experiments (or stopped-flow
measurements) when dioxygen was reacted with the described
copper(I) complexes herein. In addition, the influence of the
chirality introduced with the amino acid groups in the uns-penp
ligand system was investigated for the catalytic oxygenation of
thioanisole to 1-methyl phenyl sulfoxide, which bears a stereo
center at its sulfur atom. While no enantiomeric excess could be
detected all the catalysts could oxidize thioanisole to the sulfoxide,
with the highest conversion of 97%.
Synthesis of Boc-L-His(trt)-uns-penp (2). In a 100 mL Schlenk flask and
under inert conditions, uns-penp (0.24 g, 1.0 mmol), Boc-L-His(trt)-OH
(0.50 g, 1.0 mmol), TBTU (0.35 g, 1.1 mmol) and TEA (0.12 g, 1.2 mmol)
was mixed with 30 mL of dry DCM. The mixture was stirred for 24 hours at
room temperature. After that, the reaction was quenched with 250 mL of
EtOAc and washed with 3x50 mL 0.5 M citric acid, 3x50 mL sat. NaHCO3
solution and 3x50 mL brine. The combined organic layers again were
acidified to pH 2 with conc. HCl. The aqueous phase was separated and
NaOH was added till pH 12 was reached, followed by an extraction with
DCM. The solution was dried with MgSO4 and the solvent was evaporated.
A greenish colored product (0.58 g, 0.80 mmol, 80%) was obtained
(ESI-MS: [M+H]+=722.36, [M+Na]+=744.36)
1H-NMR (400 MHz, CDCl3): δ 8.58 –8.43 (m, 2H), 7.76 (s, 1H), 7.63 (d,
2H), 7.40 (d, 2H), 7.37 –7.21 (m, 11H), 7.14 –7.00 (m, 9H), 6.60 (d, 1H),
4.48 (s, 1H), 4.12 (q, 1H), 3.85 (d, 2H), 3.77 (d, 2H), 3.34 (dt, 1H), 3.26 –
3.03 (m, 2H), 2.92 (dd, 1H), 2.74 –2.58 (m, 2H), 1.41 (s, 9H). 13C-NMR
(101 MHz, CDCl3) δ 171.1, 159.2, 155.7, 149.1, 142.4, 138.3, 137.0, 136.7,
129.7, 128.0, 123.0, 122.1, 119.6, 79.5, 75.2, 60.1, 53.4, 52.4, 38.6, 28.4,
21.1.
Experimental Section
Synthesis of Boc-L-His-uns-penp (3). Boc-L-His(trt)-uns-penp (0.58 g,
0.80 mmol) was mixed with 90% acetic acid and stirred for two hours at
60 °C. After the reaction was allowed to cool, the acetic acid / water was
evaporated. The oily remains were solved in 40 mL DCM and washed with
3x40 mL sat. NaHCO3 solution and brine. The combined organic layers
were acidified to pH 2 with conc. HCl. The aqueous phase was separated
and again basified to pH 12 with NaOH, followed by an extraction with
DCM. The solution was dried with MgSO4 and the solvent was evaporated.
The yellowish-colorless oil was obtained in a yield of 90% (0.35g,
0.72 mmol). ESI-MS: [M+H]+= 480.26, [M+Na]+= 502.25; 1H-NMR (400
MHz, CDCl3): δ 8.43 (dt, 2H), 7.77 (s, 1H), 7.56 (td, 2H), 7.31 (d, 1H), 7.29
– 7.23 (m, 5H), 7.08 (dd, 2H), 6.72 (s, 1H), 5.81 (d, 1H), 3.71 (s, 4H), 3.24
(s, 1H), 3.07 (d, 1H), 2.94 (dd, 1H), 2.64 – 2.55 (m, 2H), 1.35 (s, 9H). 13C-
NMR (101 MHz, CDCl3): δ 170.6, 157.8, 148.1, 145.9, 135.7, 133.9, 126.9,
126.2, 122.3, 121.3, 81.0, 76.3, 76.0, 75.7, 58.8, 52.4, 51.5, 37.6, 36.4,
27.3.
Materials and Methods. All chemicals used were of p.a. quality and were
purchased from either Acros Organics, ACS or Sigma Aldrich. Dry
purchased solvents for air sensitive synthesis were redistilled under argon.
The preparation and handling of air sensitive compounds were performed
in a glovebox or under standard Schlenk techniques. Electrospray-
ionization MS (ESI-MS) measurements were performed on a Bruker
microTOF mass spectrometer. The conversions of the catalytic reactions
were determined with a Hewlett Packard 5890 gas chromatograph (GC)
and an Agilent Technologies 7820A GC-System coupled with an Agilent
Technologies 5977B MSD EI mass spectrometer (GC/MS). For NMR
measurements a Bruker Avance II 400 MHz (AV II 400) was used for all
samples. IR spectroscopy was performed on a Jasco FT/IR 4100 and all
samples were measured as KBR-pellets. UV-vis spectra were obtained
using an Agilent 8453 spectrophotometer. Diffraction data were collected
on a BRUKER D8 Venture system. Elemental Analysis was performed by
a Thermo Scientific FlashEA 1112. Due to the extreme sensitivity of the
copper(I) complexes towards dioxygen only elemental analyses could be
obtained for complexes [Cu(4)(MeCN)]OTf, [Cu(5)(MeCN)]OTf.
Synthesis of Boc-L-Lys(Z)-uns-penp. In an 100 mL flask, uns-penp
(0.50 g, 1.9 mmol), Boc-L-Lys(Z)-OH (0.72 g, 1.9 mmol), TBTU (0.67 g,
2.1 mmol) and TEA (0.12 g, 2.1 mmol) was mixed with 30 mL of dry DCM.
The mixture was stirred for 24 hours at room temperature. 250 mL of
EtOAc was added. The solution was washed with (3x50 mL) 0.5 M citric
acid, (3x50 mL) sat. NaHCO3 and (3x50 mL) brine. After that, the organic
layer was dried over MgSO4 and the solvent evaporated. A reddish oil
Synthesis of N-acetyl-uns-penp.[19] Under Schlenk conditions,
2-pyridinecarbaldehyde (4.3 g, 40 mmol) and N-acetylethylenediamine
(2.0 g, 20 mmol) were dissolved in 100 mL 1,2-dichoroethane.
Na(AcO)3BH (12.1 g, 57.1 mmol) was added and the solution was stirred
for 4 hours at room temperature. After that, 100 mL of 2 M aqueous NaOH
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10.1002/zaac.202000320
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ARTICLE
(73%, 0.84 g, 1.4 mmol) was obtained. The following cleavage of the cbz
protective group was carried out without further purification.
Synthesis of [Cu(Boc-L-Lys-uns-penp)(MeCN)]OTf. Under inert
conditions, 50 mg (0.10 mmol) of 4 was dissolved in 2 mL of acetone with
the addition of 15 µL (0.11 mmol) triethylamine. In a second vial, 37 mg
(0.10 mmol) of [Cu(MeCN)4]OTf was dissolved in 1 mL acetone which was
added dropwise to the Boc-L-Tyr-uns-penp solution. The solvent of the
solution was evaporated and a yellow solid was obtained in a quantitative
yield. IR-(KBr disc)/cm-1: 3325 (m), 3073 (m), 2939 (m), 2864 (m), 1711
(s), 1668 (s), 1516 (m), 1366 (m), 1268 (s), 1160 (s), 1036 (s), 762 (m),
638 (s), 570 (m), 518 (m). Elemental analysis: found: C, 48.4%; H, 6.4%;
N, 11.4%; C28H41CuF3N7O8S x 2 acetone requires: C, 48.0%; H, 6.2%; N,
11.9%.
Synthesis of Boc-L-Lys-uns-penp (4). 0.84 g (1.4 mmol) Boc-L-Lys(Z)-
uns-penp was dissolved in methanol in a 100 mL flask. One spate tip of
the Pd/C was added to the solution and a balloon filled with hydrogen gas
was attached to the flask. The suspension was vigorously stirred at room
temperature for 24 hours. The solution was filtered and the solvent
evaporated. The remaining oil was dissolved in 100 mL EtOAc and
extracted three times with 2 M HCl solution. The pH of the aqueous phase
then was adjusted to 12 by a slow addition of NaOH solution. After that,
the solution was again extracted with (3x50 mL) EtOAc. The combined
organic phases were dried over MgSO4 and the solvent was evaporated.
The yield of the grass green solid is 78% (0.514 g, 1.5 mmol).
ESI-MS: [M+H]+=471.29, [M+Na]+= 493.30;
1H-NMR (400 MHz, CDCl3): δ 8.58 (dd, 2H), 7.33 (dt, 2H), 7.17 (dd, 2H),
5.47 (d, 1H), 4.25 (d, 1H), 3.87 (s, 4H), 3.38 (d, 1H), 3.01 (s, 1H), 2.77 (t,
2H), 2.71 (t, 2H), 1.44 (m, 13H). 13C-NMR (101 MHz, CDCl3): δ 171.8,
159.0, 155.6, 149.1, 136.6, 123.2, 122.2, 79.5, 77.3, 77.0, 76.7, 59.9, 54.3,
52.5, 41.6, 38.6, 33.4, 32.6, 28.4, 22.6.
Synthesis of [Cu(Boc-L-Tyr-uns-penp)(MeCN)]OTf. Under inert
conditions, 50 mg (0.10 mmol) of 5 was dissolved in 2 mL of acetone with
the addition of 15 µL (0.11 mmol) triethylamine. In a second vial, 37 mg
(0.10 mmol) of [Cu(MeCN)4]OTf was dissolved in 1 mL acetone which was
added dropwise to the Boc-L-Tyr-uns-penp solution. The solvent of the
solution was evaporated and a yellow solid was obtained in a quantitative
yield. IR-(KBr disc)/cm-1: 3340 (m), 2977 (m), 2928 (m), 2851 (w), 1712
(s), 1602 (m), 1518 (s), 1442 (m), 1367 (m), 1253 (s), 1159 (s), 1029 (s),
827 (w), 760 (m), 637 (s), 570 (w), 515 (w). Elemental analysis: found: C,
51.0%; H, 5.9%; N, 9.8%; C31H38CuF3N6O8S x 2 acetone requires: C,
50.8%; H, 5.8%; N, 9.6%.
Synthesis of Boc-L-Tyr-uns-penp (5). 0.464
g
(1.64 mmol)
Boc-L-Tyrosine, 0.436 g uns-penp, 0.372 g (1.80 mmol) DCC, 0.276 g
(1.80 mmol) HOBt and 0.250 mL (1.80 mmol) TEA were dissolved in
50 mL DCM. The reaction was stirred for 24 hours at room temperature.
200 mL EtOAc was added and the solution washed with 3x50 mL sat.
NaHCO3, 3x50 mL 0.05 M citric acid solution and 3x50 mL brine. The
solution was dried over Na2SO4 and the solvent evaporated. The light-
yellow solid was cleaned via column chromatography. The yield was 32%
(0,291 g, 0.576 mmol). ESI-MS: [M+H]+ = 506.27, [M+Na]+ = 528.25
1H-NMR (400 MHz, CDCl3): δ 8.50 – 8.43 (m, 2H), 7.62 (s, 1H), 7.55 (td,
2H), 7.22 (d, 3H), 7.09 (dd, 2H), 6.96 – 6.89 (m, 2H), 6.64 – 6.55 (m, 2H),
5.34 (d, 1H), 4.33 (d, 1H), 3.67 (s, 4H), 3.19 (q, 2H), 2.58 – 2.53 (m, 2H),
1.39 (d, 12H), 1.20 (d, 1H). 13C-NMR (101 MHz, CDCl3): δ 170.2, 157.8,
154.5, 154.3, 148.0, 135.8, 129.4, 127.1, 122.4, 121.4, 114.9, 76.3, 76.0,
75.7, 58.6, 55.1, 52.4, 51.6, 37.7, 36.4, 27.3.
Synthesis
of
[Cu(uns-penp)]OTf,
[Cu(Me2-uns-penp)]OTf
[Cu(Ac-uns-penp)]OTf and complexes.[19] Under inert conditions, 1 eq.
of the ligand was dissolved in 2 mL of acetone with the addition of 1.1 eq.
of triethylamine. In a second vial, 1 eq. of [Cu(MeCN)4]OTf was dissolved
in 1 mL acetone which was added dropwise to the ligand solution. The
complexes were precipitated in n-pentane and yellow solids were obtained
in a quantitative yield.
Catalytic oxygenation of thioanisole. 1.1 mL thioanisole (1.24 g, 10
mmol) and 5 mol% of catalyst [Cu(4)(MeCN)]OTf, [Cu(5)(MeCN)]OTf or
[Cu(6)] were suspended in 10 mL acetonitrile. After that, a hydrogen
peroxide solution (50%, 560 µL, 10 mmol) was added dropwise in the
stirred solution. To prevent the evaporation of the substrate the flask was
sealed and the cap was equipped with a cannula which was plugged
through a septum for pressure equalizing. The solution was stirred until the
gas formation subsided and was then examined via GC/MS.
Synthesis of [Cu{Boc-L-His(trt)-uns-penp}] (6). Under inert conditions,
100 mg (0.139 mmol) Boc-L-His(trt)-uns-penp were dissolved in 4 mL
acetone. In another vial, 51.8 mg (0.139 mmol) of [Cu(MeCN)4]PF6 was
dissolved in 2 mL acetone and was added dropwise to the stirred solution
of Boc-L-His(BPh3)-uns-penp. A yellow suspension forms. For anion
exchange, 47.6 mg NaBPh4 (0.139 mmol) were added and the suspension
became a clear yellow solution. For precipitation, the solution was added
dropwise to an excess of diethyl ether and the resulting yellow solid
complex (132 mg, 86%) was filtered and washed again with ether.
IR-(KBr-disc)/cm-1: 3411 (w), 3057 (m), 2988 (m), 1708 (m), 1588 (m),
1478 (m),1427 (m), 1153 (m), 1031 (m), 853 (s), 745 (s), 711 (s), 605 (m),
561 (w), 510 (w).
Isolation of 1-Methylphenylsulfoxide. The reaction solution was filtered
and the solvent evaporated in vacuo. The resulting brownish crude solid
was purified via column chromatography (1:1 EtOAc/n-Hex, Rf: 0.32). A
pure white solid was obtained with a yield of 950 mg (69%).
1H-NMR (400 MHz, Chloroform-d): δ 7.71 – 7.60 (m, 2H), 7.60 – 7.46
(m, 3H), 2.73 (s, 3H).
13C-NMR (101 MHz, CDCl3): δ 145.7, 131.0, 129.4, 123.5, 77.4, 77.1, 76.8,
44.0.
Synthesis of [Cu{Boc-L-His(BPh3)-uns-penp}] (6). Under inert
conditions, 200 mg (0.417 mmol) Boc-L-His-uns-penp was dissolved in
4 mL acetone. In another vial, 155 mg (0.417 mmol) of [Cu(MeCN)4]PF6
was dissolved in 2 mL acetone and was added dropwise to the stirred
solution of Boc-L-His(BPh3)-uns-penp. A yellow suspension forms. For
anion exchange, 143 mg NaBPh4 (0.417 mmol) was added and the
suspension became a clear yellow solution. For precipitation, the solution
was added dropwise to an excess of diethyl ether and the resulting yellow
solid complex (130 mg, 40%) was filtered and washed again with ether.
Crystals were obtained by evaporation of the complex solution in acetone.
(CCDC deposition number 2025303) IR-(KBr disc)/cm-1: 3401 (m), 3311
(m), 3060 (m), 1715 (m), 1570 (m), 1361 (s), 1321 (m), 1277 (s), 1153 (s),
1029 (s), 1134 (s), 1088 (m), 1005 (m), 704 (m), 636 (m), 570 (w), 514 (m).
Computational Details
The composition of the complexes [Cu(4)MeCN)]+ and [Cu(5)MeCN)]+ was
derived from the IR and elemental analysis. The creation and analysation
of the geometric structures have been performed using the program
CREST[30] with the iMTD-GC algorithm and the semiempirical method
GFN2-xTB[31] to obtain the energetical most favourable conformation. For
the five conformers with the lowest total energy at GFN2-xTB level of
theory DFT calculations have been conducted employing the program
Turbomole[32–34], version 6.6. The PBE[35] exchange-correlation functional
with RI approximation[36–38] and D3 (BJ)[39,40] dispersion correction have
been chosen for the structural optimization and the calculation of the
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10.1002/zaac.202000320
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ARTICLE
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used. Unrestricted KS-DFT calculations considering a singlet state have
been employed for the single positively charged complexes. The electronic
convergence criterium has been set to 10-7 EH, the convergence of the
structural relaxation to 10-6 EH. Frequency calculations confirm the
calculated structures as minima. The complex structures with the lowest
total energy according to the semiempirical and DFT methods are
presented within this study and the structural data is given in the SI.
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Layout 1:
FULL PAPER
The tripodal ligand 2-aminoethyl)bis(2-
pyridylmethyl)amine was modified by
an attachment of the amino acids L-
histidine, L-tyrosine and L-lysine.
Copper(I) complexes of those ligands
were synthesized and catalytically
tested by the oxidation of thioanisole
with H2O2.
Christopher Gawlig, Jannis Jung,
Doreen Mollenhauer and Siegfried
Schindler*
Page No. – Page No.
Synthesis and characterization of
copper complexes with tripodal
ligands bearing amino acid groups
Additional Author information for the electronic version of the article.
Author:
Siegfried Schindler
ORCID identifier: http://orcid.org/0000-0002-9991-9513
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