Synthesis and a Catalytic Study of Diastereomeric Cationic Chiral-at-Cobalt Complexes Based on (R, R)-1,2-Diphenylethylenediamine

Article abstract of DOI:10.1021/acs.inorgchem.1c00855

Here we report the first synthesis of two diastereomeric cationic octahedral Co(III) complexes based on commercially available (R,R)-1,2-diphenylethylenediamine and salicylaldehyde. Both diastereoisomers with opposite chiralities at the metal center (Λ and Δconfigurations) were prepared. The new Co(III) complexes possessed both acidic hydrogen-bond donating (HBD) NH moieties and nucleophilic counteranions and operate as bifunctional chiral catalysts for the challenging kinetic resolution of terminal and disubstituted epoxides by the reaction with CO2 under mild conditions. The highest selectivity factor (s) of 2.8 for the trans-chalcone epoxide was achieved at low catalyst loading (2 mol %) in chlorobenzene, which is the best achieved result currently for this type of substrate.

Full text of DOI:10.1021/acs.inorgchem.1c00855

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Synthesis and a Catalytic Study of Diastereomeric Cationic Chiral-at-
Cobalt Complexes Based on (R,R)‑1,2-Diphenylethylenediamine
Mikhail A. Emelyanov,§ Nadezhda V. Stoletova,§ Alexander F. Smol’yakov, Mikhail M. Il’in,
Victor I. Maleev, and Vladimir A. Larionov*
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ABSTRACT: Here we report the first synthesis of two diastereomeric cationic
octahedral Co(III) complexes based on commercially available (R,R)-1,2-diphenylethy-
lenediamine and salicylaldehyde. Both diastereoisomers with opposite chiralities at the
metal center (Λ and Δ configurations) were prepared. The new Co(III) complexes
possessed both acidic hydrogen-bond donating (HBD) NH moieties and nucleophilic
counteranions and operate as bifunctional chiral catalysts for the challenging kinetic
resolution of terminal and disubstituted epoxides by the reaction with CO2 under mild
conditions. The highest selectivity factor (s) of 2.8 for the trans-chalcone epoxide was
achieved at low catalyst loading (2 mol %) in chlorobenzene, which is the best achieved
result currently for this type of substrate.
stereoselectively as single Λ isomers (Λ(R,R)-1 X−, where X
= F, Cl, or I) starting from (R,R)-1,2-cyclohexanediamine, and,
accordingly, the Δ complexes (Δ(S,S)-1 X−) were obtained
from the (S,S) form of the diamine (Scheme 1a). As expected,
Λ(R,R)-1 X−and Δ(S,S)-1 X− operated as enantiomeric
catalysts providing the opposite enantiomers of the target
products.7c−e Recently, we also showed that the complex
Δ(S,S)-1b I− with an iodide counteranion was an efficient
bifunctional HBD/nucleophilic catalyst for the synthesis of
cyclic carbonates from racemic epoxides and carbon dioxide
under relatively mild conditions.13 Unfortunately, no kinetic
resolution was observed for monosubstituted epoxides in this
case.13
Here, we report the first synthesis of both (in contrast to our
previous results7c) Λ and Δ diastereomers of the chiral-at-
metal Co(III) complexes Λ(R,R)-2 I− andΔ(R,R)-2 I− based
on the single enantiomer of the commercially available (R,R)-
1,2-diphenylethylenediamine and salicylaldehyde (Scheme 1b).
As a proof of principle, we demonstrated that both obtained
diastereomers 2 catalyze the kinetic resolution of different
epoxides 3 with CO2 with a higher enantiocontrol than the
previously reported Co(III) complexes 1 (Scheme 1c).
Notably, the diastereomeric complexes 2 act as “pseudo-
INTRODUCTION
■
Chirality is a unique property of molecules that plays an
important role in living organisms, biology, and chemistry.
Generally, the chiral centers in molecules are presented by
carbon, nitrogen, silicon, phosphorus, and sulfur atoms,1 but
there is also a plethora of examples with a chirality at metal
centers (metal centrochirality) in the literature.2,3 Metal
complexes may exhibit a chirality owing to the different
arrangement of (a)chiral ligands around the metal center, and
these are called “chiral-at-metal” or “stereogenic-at-metal”
complexes.3,4 In case the metal complexes have an octahedral
geometry, they are designated as Λ (a left-handed propeller)
and Δ (a right-handed propeller) configurations.3,4 It was
demonstrated that the chiral-at-metal complexes (mostly based
on cobalt, iridium, rhodium, ruthenium, iron, etc.) are an
efficient and promising class of chiral catalysts for different
enantioselective reactions.3c,4−11
For example, in 2004, some of us introduced the chiral-at-
metal anionic cobalt(III) complexes featuring chiral ligands as
a novel class of metal-templated asymmetric catalysts for the
enantioselective cyanosilylation of benzaldehyde.6a These
complexes were obtained from the enantiopure α-amino
acids as a mixture of both Λ and Δ diastereomers (the
diastereomeric ratio (dr) depended on the structure of the
amino acid).6,12 We and others elaborated the positively
charged chiral-at-metal cobalt(III) complexes featuring chiral
ligands as an efficient class of metal-templated hydrogen-bond
donor (HBD) catalysts for different asymmetric trans-
formations operating as “organocatalysts”.5f,g,7 In particular, it
was found that the cobalt(III) complexes were formed
Received: March 19, 2021
https://doi.org/10.1021/acs.inorgchem.1c00855
© XXXX American Chemical Society
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enantiomers” providing both diastereomers of the desired
cyclic carbonates 4 with s-value up to 2.8.
Scheme 1. Design of the Octahedral Chiral-at-Metal
Cobalt(III) Complexes
a
However, although many efficient catalysts are known for
the kinetic resolution of racemic monosubstituted epox-
ides,14−16 the resolution of disubstituted epoxides poses
significant challenges.17 To best of our knowledge, only one
example of the kinetic resolution of disubstituted epoxides with
CO2 has been reported.16b In 2017, Ema et al. reported the
kinetic resolution of trans-stilbene oxide derivatives catalyzed
by the chiral complex macrocycle achieving an s-factor of
13.16b
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the Complexes 2.
The diastereomeric complexes 2 were prepared according to
the procedure described by us earlier as shown in Scheme 2.7c
The reaction of the in situ obtained Schiff base of (R,R)-1,2-
diphenylethylenediamine hydrochloride and salicylaldehyde
with the corresponding Co(III) salt followed by a treatment
with a saturated aqueous solution of sodium chloride gave a
mixture of the diastereomeric Co(III) complexes 2 with a
chloride anion in a ratio of 1:1 (Λ/Δ) and a 56% combined
yield.
The Co(III) complex 2 of the Λ configuration with a
chloride anion was isolated in a pure form by a regular flash
silica gel chromatography; however, the second Δ diaster-
eomer contained some amount of the Λ form. Fortunately, the
Δ-2 complex was easily separated by chromatography from the
Λ isomer as the iodide form (see the Supporting Information
for details). The obtained enantiomerically pure Co(III)
complexes Λ(R,R)-2 I− andΔ(R,R)-2 I−were fully charac-
1
terized by H and 13C NMR spectroscopy, circular dichroism
(CD), UV−vis spectroscopy, and elemental analysis. The
structures of both diastereomers of 2 were unambiguously
confirmed by a single-crystal X-ray diffraction analysis (Figure
1). In both complexes, the central Co(III) ion is six-
Figure 1. X-ray structures of the Co(III) complexes Λ(R,R)-2 Cl−
and Δ(R,R)-2 I− (hydrogen atoms at carbons are omitted for clarity).
a
(a) From (R,R)- or (S,S)-1,2-cyclohexanediamine (previous work);
(b) from (R,R)-1,2-diphenylethylenediamine (this work); (c) kinetic
resolution of different epoxides 3 with CO2.
Scheme 2. Protocol for the Synthesis of the Diastereomeric Co(III) Complexes 2
B
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Figure 2. CD spectra of the complexes Λ(R,R)-2 I− (red curve) and Δ(R,R)-2 I− (blue dashed curve) recorded in CH3OH (c = 0.86 × 10−3 M).
coordinated, and, as a result, they have octahedral geometries
and C2 symmetry. The halide counteranions are connected
with the complex-cation via a set of hydrogen bonds with NH2
groups5d as indicated by the distances of NH···CI− equal to
2.279 and 2.460 Å in the case of Λ(R,R)-2 Cl− and the
distances of NH···I− equal to 2.725 and 2.882 Å in the case of
Δ(R,R)-2 l− (see Figure 1).
Table 1. Reaction Conditions Screening for the Kinetic
Resolution of the Chalcone Epoxide 3a with CO2 Catalyzed
by the Co(III) Complexes 1 and 2
a
The CD spectra of the Co(III) complexes Λ(R,R)-2 I− and
Δ(R,R)-2 I− are shown in Figure 2, and they confirm the
diastereomeric structures of these complexes. Next, the UV−
vis spectra of Λ(R,R)-2 I− and Δ(R,R)-2 I− were recorded in
CH2Cl2 (see Figure S16 in the Supporting Information). The
spectra showed intense bands in the region of 380−390 nm for
both diastereomers, which, probably, were associated with the
ligand-to-metal charge-transfer transition and confirmed the
coordination of nitrogen atoms of the tridentate ligands to the
central Co(III) atom. The less-intense bands at 486 and 600
nm for Λ(R,R)-2 I− and the only one at 588 nm for Δ(R,R)-2
I− could be assigned to the d → d transitions of Co(III) ions
(see Figure S16 in the Supporting Information). Furthermore,
the differences in the position of d → d transitions of the
Co(III) diastereomers, most likely, explain the difference in
colors: Λ(R,R)-2 I− is dark brown and Δ(R,R)-2 I− is dark
green.
Catalytic Study. As a proof of principle, we investigated
the catalytic activity of the HBD complexes 1 and 2 with
iodide anions in the challenging kinetic resolution of the trans-
chalcone epoxide 3a with CO2 (Table 1). On the one hand,
the resulting enantiomerically enriched epoxides and cyclic
carbonates are versatile building blocks for the synthesis of
valuable biologically active compounds and pharmaceuticals.17
On the other hand, the hydrolysis of the cyclic carbonate will
lead to the corresponding chiral diols.18 All the reactions
shown in Table 1 were conducted at 50 °C under a CO2
pressure of 20 bar in the presence of 2 mol % of the catalysts.
The reaction in toluene in the presence of Λ(R,R)-2 I−
provided the trans-cyclic carbonate 4a with 49% conversion
ee of
b
c
d
entry
catalyst
solvent
convn.,
%
e
4a, %
s
1
2
3
4
5
6
7
8
Λ(R,R)-2 I− toluene
49 (32%)
32
−33
19
−13
17
−28
18
−20
40 (24)
−38
−15
−18
2.6
2.1
1.6
1.3
1.5
1.8
1.7
1.5
Δ(R,R)-2 I−
16
38
15
28
9
Λ(R,R)-2 I− CH2Cl2
Δ(R,R)-2 I−
h
Λ(R,R)-2 I− DCE
Δ(R,R)-2 I−
Λ(R,R)-2 I− 1,4-dioxane 52
Δ(R,R)-2 I−
13
33
9
35
29
h
f
g
9
Λ(R,R)-2 I− CB
Δ(R,R)-2 I−
2.8 (2.9)
2.3
10
11
12
Λ(R,R)-1a I−
Λ(R,R)-1b I−
1.5
1.5
a
Reaction conditions: epoxide 3a (0.35 mmol, 1 equiv) and catalyst
(2 mol %) were stirred in a 10 mL volume autoclave with 20 bar of
CO2 at 50 °C for 24 h. Each test was conducted at least twice.
b
Conversion determined by a 1H NMR analysis of the crude reaction
c
d
mixture. Determined by a chiral HPLC analysis. Calculated as (ln[1
− c(1 + ee4)])/(ln[1 − c(1 − ee4)]), where c is the conversion, and
ee4 is the enantiomeric excess of the cyclic carbonate product 4a.
e
f
Isolated yield of the product 4a. The ee of the remaining epoxide 3a
g
is shown in parentheses. Calculated as (ln[(1 − c)(1 − ee3)])/(ln[(1
− c)(1 + ee3)]), where c is the conversion calculated as (ee3/(ee3 +
ee4), ee4 is the enantiomeric excess of the cyclic carbonate 4a, and ee3
h
is the enantiomeric excess of the epoxide 3a.19 DCE = 1,2-
dichloroethane, CB = chlorobenzene.
C
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and 32 ee, which corresponded to an s-factor of 2.6 (entry 1).
The related diastereoisomer Δ(R,R)-2 I− displayed a lower
catalytic activity with only 16% conversion and, importantly,
gave the opposite enantiomer of 4a with 33% ee and an s-factor
of 2.1 (entry 2). The testing of some other solvents (entries 3−
10) revealed that Λ(R,R)-2 I− is more active than Δ(R,R)-2 I−
in terms of conversion. It was found that chlorobenzene is the
solvent of choice to get the best s-factors (2.8 and 2.3,
correspondingly), where ee values were 40% and −38%
(entries 9 and 10). The remaining epoxide was partially
enantiomerically enriched 24% ee (entry 9). Expectedly, the
selectivity factor s determined by using Kagan’s equation19 was
the same (2.9) as for the s-factor calculated without taking into
consideration the remaining epoxide ee (see Table 1). In
contrast, the previously reported catalysts Λ(R,R)-1a I− and
Λ(R,R)-1b I− provided the cyclic carbonate 4a with a lower s-
factor (1.5) (compare entries 9−12), showing the superiority
of the complexes 2 obtained in this work. Although the s-values
are moderate, they are the best currently for this type of
substrate. Note that the absolute configuration of the obtained
product 4a was not assigned.
metal Co(III) complexes 2 based on commercially available
(R,R)-1,2-diphenylethylenediamine and salicylaldehyde. The
obtained metal-templated Λ and Δ complexes 2 with an iodide
anion act as bifunctional hydrogen-bond donor/nucleophilic
catalysts for the kinetic resolution of several epoxides with
carbon dioxide providing both diastereomers of the target
cyclic carbonates. The highest s-factor of 2.8 was achieved at a
33% conversion and low catalyst loading (2 mol %) in
chlorobenzene for the trans-chalcone epoxide 3a. It was
observed that the Λ diastereomer of the complex 2 is superior
to the Δ form in terms of catalytic activity. In summary, the
attractive key features of the chiral-at-cobalt complexes
presented here are (1) the formation of both diastereomers
of the Co(III) complex in contrast to those of our previous
results7 is observed for the first time; (2) as a proof of
c
principle, it was demonstrated that the obtained complexes act
as pseudo-enantiomeric catalysts in the challenging kinetic
resolution of epoxides with CO2; (3) the modification of the
core of the Co(III) complex by changing the chiral diamine led
to the formation of a more efficient catalyst for the reaction in
terms of enantioselectivity. Work is ongoing to further explore
the application of the obtained diastereomeric Co(III)
complexes in other organic transformations.
Next, in order to demonstrate the general applicability of the
reaction, the terminal epoxides 3b−3d were examined (Figure
3). Because the reactivity of the terminal epoxides is higher, we
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00855.
Procedure for the preparation of the complexes 2, the
general procedure for the coupling of epoxides 3 with
CO2, X-ray diffraction study of the Co(III) complexes 2,
copies of UV−vis and 1H and 13C NMR spectra, HPLC
traces (PDF)
Accession Codes
CCDC 2063910−2063911 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Figure 3. Kinetic resolution of different epoxides. Reaction
conditions: 2 mol % catalyst, 1 bar of CO2, 25 °C, 6−8 h.
1
Conversions were determined by a H NMR analysis. The ee was
AUTHOR INFORMATION
determined by a chiral HPLC analysis.
■
Corresponding Author
could perform the reactions with 1 bar of CO2 at 25 °C. The
coupling of styrene oxide 3b with CO2 provided the desired
carbonate 4b in 13% conversion with 6% ee (s = 1.1). Glycidyl
phenylether 3c gave the product 4c with 30% conversion and
18% ee (s = 1.5). N,N-Diphenylaminomethyloxirane 3d
afforded the carbonate 4d with 31% conversion and 23% ee
(s = 1.8). Notably, no kinetic resolution was observed for
monosubstituted epoxides in the case of the catalyst Δ(S,S)-1b
I−.13 We assume that the HBD ability of the Co(III)
complexes is responsible for the epoxide activation via weak
hydrogen-bonding interactions20 and that the reaction
mechanism is analogous to the mechanism described in our
previous reports.13,21
Vladimir A. Larionov − A.N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
119991 Moscow, Russian Federation; Peoples’ Friendship
University of Russia (RUDN University), 117198 Moscow,
Russian Federation; orcid.org/0000-0003-3535-1292;
Email: larionov@ineos.ac.ru
Authors
Mikhail A. Emelyanov − A.N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
119991 Moscow, Russian Federation
Nadezhda V. Stoletova − A.N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
119991 Moscow, Russian Federation
Alexander F. Smol’yakov − A.N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
119991 Moscow, Russian Federation
CONCLUSIONS
■
In conclusion, we here reported the first synthesis and
characterization of the diastereomeric octahedral chiral-at-
D
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Article
(b) Zhang, L.; Meggers, E. Stereogenic-only-at-metal asymmetric
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Larionov, V. A. Asymmetric catalysis with octahedral stereogenic-at-
metal complexes featuring chiral ligands. Coord. Chem. Rev. 2018, 376,
95−113.
Mikhail M. Il’in − A.N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
119991 Moscow, Russian Federation
Victor I. Maleev − A.N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
119991 Moscow, Russian Federation; orcid.org/0000-
0002-8096-4197
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00855
Author Contributions
§(M.A.E. and N.V.S.) Both authors contributed equally to this
work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work has been supported by Russian Science Foundation
(RSF Grant No. 20-13-00155). X-ray studies were performed
with financial support from the Ministry of Science and Higher
Education of the Russian Federation using the equipment of
the Center for Molecular Composition Studies of INEOS RAS.
The publication has been supported by RUDN University
Strategic Academic Leadership Program (UV−vis and optical
rotation measurements).
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