Dirhodium Coordination Polymers for Asymmetric Cyclopropanation of Diazooxindoles with Olefins: Synthesis and Spectroscopic Analysis

Article abstract of DOI:10.1002/cplu.202000421

A facile approach is reported for the preparation of dirhodium coordination polymers [Rh₂(L1)₂]_n (Rh₂-L1) and [Rh₂(L2)₂]_n (Rh₂-L2; L1=N,N’-(pyromellitoyl)-bis-L-phenyl

Full text of DOI:10.1002/cplu.202000421

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Dirhodium Coordination Polymers for Asymmetric
Cyclopropanation of Diazooxindoles with Olefins: Synthesis
and Spectroscopic Analysis
Zhenzhong Li,[a] Lorenz Rösler,[a] Kevin Herr,[a] Martin Brodrecht,[a] Hergen Breitzke,[a]
Kathrin Hofmann,[a] Hans-Heinrich Limbach,[b] Torsten Gutmann,*[a, c] and
Gerd Buntkowsky*[a]
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A facile approach is reported for the preparation of dirhodium
coordination polymers [Rh2(L1)2]n (Rh2-L1) and [Rh2(L2)2]n (Rh2-
L2; L1=N,N’-(pyromellitoyl)-bis-L-phenylalanine diacid anion,
L2=bis-N,N’-(L-phenylalanyl) naphthalene-1,4,5,8-tetracarboxy-
late diimide) from chiral dicarboxylic acids by ligand exchange.
Multiple techniques including FTIR, XPS, and H! C CP MAS
NMR spectroscopy reveal the formation of the coordination
polymers. 19F MAS NMR was utilized to investigate the
remaining TFA groups in the obtained coordination polymers,
and demonstrated near-quantitative ligand exchange. DR-UV-
vis and XPS confirm the oxidation state of the Rh center and
that the Rh-single bond in the dirhodium node is maintained in
the synthesis of Rh2-L1 and Rh2-L2. Both coordination polymers
exhibit excellent catalytic performance in the asymmetric cyclo-
propanation reaction between styrene and diazooxindole. The
catalysts can be easily recycled and reused without significant
reduction in their catalytic efficiency.
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Introduction
chiral dirhodium catalysts.[38–39] Such spiro cyclopropyloxindoles
constitute an important group of heterocycles with potential
application in medical research, including the use as potent HIV
inhibitor[40–42] and antitumor agent.[43–46]
Homogeneous chiral dirhodium (II) complexes are important
organometallic catalysts that have drawn much attention for
asymmetric catalysis.[1–12] In particular, chiral dirhodium (II)
catalysts are highly efficient catalysts for asymmetric trans-
formations, including CÀ H activation, cyclopropanation, dipolar
cycloaddition, XÀ H insertion, and ylide formation.[13–25] Recently,
they have also emerged as catalysts in CÀ H amination and
olefin aziridination.[12,26–37] This variety of reactions makes chiral
dirhodium catalysts especially attractive for synthesis of
pharmaceuticals or biological active substances. For example,
spiro-cyclopropyloxindole compounds can be synthesized by
cyclopropanation of diazooxindoles with olefins employing
Despite their high activity and selectivity, the unsatisfactory
catalyst recovery and recycling of the costly manufactured
chiral dirhodium catalysts is the primary limiting factor for their
application in chemical industry. Thus, several approaches have
been proposed to handle the catalyst recovery as reviewed by
Gois and co-workers.[47] Heterogenized chiral dirhodium cata-
lysts, which are easily separable from reaction media via
filtration or can be implemented in continuous flow reactors
have the potential to overcome these issues, as they preserve
the expensive catalysts. Accordingly, the synthesis and hetero-
genization of chiral dirhodium (II) complexes is currently a very
active research field. A number of strategies for the heterogeni-
zation of chiral dirhodium catalysts such as forming covalent
bonds via bidentate[48–52] or axial[53–56] binding were proposed.
These heterogeneous catalysts were then successfully used in
various asymmetric reactions. For example, Hashimoto
et al.[57–58] developed a novel approach for immobilizing chiral
dirhodium catalysts such as Rh2(S-PTTL)4 and Rh2(S-TFPTTL)4, by
ligand exchange with bidentate ligands. Subsequent copoly-
merization then led to solid coordination polymers. Davies
et al.[59] described a similar approach for immobilizing Rh2(S-
DOSP)4 with a ligand that can be grafted to a solid support.
They also reported that the immobilization of Rh2(S-MEPY)4 by
anchoring the catalyst via axial binding to cross-linked macro-
porous polystyrene (Argopore) resin via a benzyloxymeth-
ylpyridine linker is feasible.[54] These examples clearly illustrate
that significant progress has been made in immobilizing chiral
dirhodium catalysts on solid carrier materials.
[a] Z. Li, L. Rösler, K. Herr, M. Brodrecht, H. Breitzke, K. Hofmann, T. Gutmann,
G. Buntkowsky
Technical University of Darmstadt
Institute of Inorganic and Physical Chemistry
Alarich-Weiss-Straße 8, 64287 Darmstadt (Germany)
E-mail: gutmann@chemie.tu-darmstadt.de
gerd.buntkowsky@chemie.tu-darmstadt.de
[b] H.-H. Limbach
Freie Universität Berlin
Institute of Chemistry and Biochemistry
Takustraße 3, 14195 Berlin (Germany)
[c] T. Gutmann
University Kassel
Institute of Chemistry and Center for Interdisciplinary Nanostructure Science
and Technology,
Heinrich-Plett-Straße 40, D-34132 Kassel (Germany)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cplu.202000421
© 2020 The Authors. Published by Wiley-VCH GmbH.
This is an open access article under the terms of the Creative Commons
Attribution Non-Commercial NoDerivs License, which permits use and dis-
tribution in any medium, provided the original work is properly cited, the
use is non-commercial and no modifications or adaptations are made.
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Despite these remarkable achievements, however, there still
synthesize chiral dirhodium coordination polymer catalysts
[Rh2(L1)2]n and [Rh2(L2)2]n, abbreviated as Rh2-L1 and Rh2-L2,
using Rh2(TFA)4 and chiral dicarboxylic acids as precursors. The
latter are synthesized from aromatic tetracarboxylic acids with
L-phenylalanine. The dirhodium units in these novel catalysts
are connected via N,N’-(pyromellitoyl)-bis-L-phenylalanine diacid
(H2L1) or bis-N,N’-(L-phenylalanyl) naphthalene-1,4,5,8-tetracar-
boxylic diimide (H2L2) ligands, respectively, depicted in
Scheme 1. The as-prepared chiral dirhodium coordination
polymers are characterized by a combination of different
techniques such as solid-state NMR spectroscopy, FTIR, DR-UV-
vis and XPS. Their catalytic activity, selectivity, stability and
reusability are studied employing the formation of spiro-cyclo-
propyloxindole from diazooxindole and styrene as model
reaction.
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remains a need for development of novel strategies to design
heterogenized chiral dirhodium (II) catalysts that do not require
additional solid carrier materials that may influence the activity
and selectivity of the chiral dirhodium (II) catalyst.
The formation of coordination polymers is a feasible
strategy to heterogenize dirhodium catalysts (II) as demon-
strated by several works.[60–65] Very recently, some of us have
achieved the heterogenization of dirhodium catalysts by ligand
exchange employing ditopic ligands to synthesize dirhodium
coordination polymers.[66–67] The structures of these as-obtained
2D coordination polymers were determined by a combination
of SEM, XRD and 13C and 19F solid-state NMR techniques. These
novel dirhodium coordination polymers exhibited an excellent
catalytic efficiency in the cyclopropanation between styrene
and diazoacetate. Next to these works, a series of chiral metallic
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coordination
polymers,
such
as
Z1M1-110A,[68]
{[Ln2(MnLCl)2(NO3)2(dmf)6(H2O)2] ×H2O}n [Ln=Pr, Nd, Sm, and Results and Discussion
Gd][69] and [Co(cpfa)(bimb)]EtOHH2O,[70] were proposed and
used as catalysts for various organic reactions. This illustrates
that forming a coordination polymer is a suitable way to
heterogenize chiral metal containing catalysts.
However, to the best of our knowledge the synthesis and
application of chiral dirhodium (II) coordination polymers by
ligand substitution has not been described so far. Thus, in the
present work, we report a novel and efficient approach to
Synthesis and basic characterization
The synthesis of Rh2-L1 and Rh2-L2 is schematically depicted in
Scheme 1. The chiral ligands are synthesized starting from
pyromellitic dianhydride or 1,4,5,8-naphthalene dianhydride,
respectively. We chose these two anhydrides owing to their
high electron affinity, excellent thermal stability and feasibility
Scheme 1. (a) Synthesis of chiral dirhodium coordination polymers and possible remaining TFA containing sites. H2L1=N,N’-(pyromellitoyl)-bis-L-phenylalanine
diacid, H2L2=bis-N,N’-(L-phenylalanyl) naphthalene-1,4,5,8-tetracarboxylic diimide. Note: blue circles refer to oxygen atoms. (b) Structure of the homogeneous
Rh2(S-PTPA)4 catalyst.
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to functionalize them through the anhydride position with
arylamino or alkylamino groups.[71–73] Then, the coordination
polymers were obtained by ligand exchange from the precursor
Rh2(TFA)4 and H2L1 or H2L2, respectively (Scheme 1).
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The Rh contents of the coordination polymers Rh2-L1 and
Rh2-L2 were determined by TG analysis following the procedure
of Kaskel et al.[65] (ESI Scheme S1). The nitrogen, hydrogen and
carbon contents were calculated from elemental analyses. As
listed in Table 1, the Rh2 contents of catalysts Rh2-L1 and Rh2-L2
are 0.81 mmol and 0.76 mmol per gram catalyst, respectively.
These experimental Rh2 contents are almost equal to the
predicted theoretical values 0.82 mmol and 0.75 mmol, respec-
tively. The weight percentages (wt%) of C and N (Table 1)
obtained from elemental analysis for Rh2-L1 and Rh2-L2 are
51.70 wt% or 55.66 wt% and 4.09 wt% or 4.33 wt%, respec-
tively. These experimental contents show deviations from the
theoretical contents especially for C (54.83 vs. 51.70 wt% and
55.66 wt% vs. 57.93 wt%). The higher theoretical values may be
explained by the presence of trifluoroacetate groups in Rh2-L1
and Rh2-L2 that have not been substituted by chiral ligands
when the coordination polymers are formed. This assumption is
corroborated by the 19F MAS NMR spectra of the samples
(Figure 4), which display fluorine signals in Rh2-L1 and Rh2-L2.
A detailed analysis of these spectra is given in the section
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Figure 2. FT-IR spectra in the range between 1900–1300 cmÀ 1 of (a)
Rh2(TFA)4, (b) H2L1 and (c) Rh2-L1.
The morphologies of the catalysts were investigated by
SEM. The SEM images in Figure 1 show that both catalysts
exhibit 2D layers that are arranged as little plates/flakes in a
disordered manner. Additionally, the XRD data (Figure S2 in the
Supporting Information) only show very broad background
signals suggesting a disordered arrangement of these layers for
both catalysts.
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on 1H! C CP MAS and 19F MAS NMR. In addition, the hydrogen
contents of the samples are about 0.5 wt% higher than the
theoretical values (Table 1). This is an indication of the presence
of additional hydrogen sources. This hypothesis is underlined
by the TG measurements of Rh2-L1 and Rh2-L2 (Figure S1 in the
Supporting Information) that show a small decrease of mass at
FT-IR characterization
°
100 C. There are two probable candidates for hydrogen
sources, namely the solvent ethyl acetate and water molecules.
The FT-IR spectra of Rh2(TFA)4, H2-L1 and Rh2-L1 are shown in
Figure 2. In the spectra of H2L1 and Rh2-L1 (Figure 2b, c) the
signals at around 1775 and 1718 cmÀ 1 are attributed to the
absorption peaks of C=O asymmetric and symmetric vibration
on imide moiety. The peaks around 1695 and 1605 cmÀ 1 are
assigned to the C=O stretching vibrations of the carboxyl/
carboxylate groups of H2L1 and Rh2-L1.[74–75] In addition, the
absorption peaks around 1495 and 1455 cmÀ 1 are assigned to
the breathing vibrations of C=C bonds in aromatic rings. The
peak around 1378 cmÀ 1 is attributed to the stretching vibrations
of CÀ N bonds.
Table 1. Compositions of Rh2-L1 and Rh2-L2 catalysts.
Sample Content[a,b]
C [wt%] H [wt%] N [wt%] Rh2 [mmol/g]
Rh2-L1
Rh2-L2
Experimental 51.70
Theoretical 54.83
Experimental 55.66
Theoretical 57.93
3.538
2.958
3.573
3.039
4.09
4.57
4.33
4.22
0.81
0.82
0.76
0.75
[a] Experimental contents were determined by elemental analysis (C, H, N)
or TG analysis (Rh2). [b] Theoretical contents were calculated for the ideal
framework of dirhodium units, which are connected with two chiral H2L1 or
H2L2 groups.
In the spectrum of Rh2(TFA)4 (Figure 2a), the absorption
peaks at around 1637 and 1463 cmÀ 1 are assigned to the C=O
Figure 1. SEM images of (a) Rh2-L1, (b) Rh2-L2.
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stretching vibrations of the trifluoroacetate group,[76] which
have disappeared in the obtained Rh2-L1 spectrum (Figure 2c).
This is a strong indication of the success of the ligand exchange
and formation of Rh2-L1. Similar results are also obtained from
the FT-IR spectra (Figure S3 in the Supporting Information) of
the Rh2-L2 catalyst.
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1H! C CP MAS and 19F MAS NMR characterization
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The H! C CP MAS NMR spectra of Rh2(TFA)4, H2L1 and Rh2-L1
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are compared in Figure 3(I). The H! C CP MAS spectrum of
Rh2-L1 (Figure 3(I)c) displays three distinct resonance peaks in
the range between 117 and 138 ppm, which correspond to
aromatic carbons. The two small peaks around 33 and 56 ppm
are assigned to the two carbon atoms C2 and C3 of the
À NÀ CHRÀ CH2Ph (R=À COOH) groups of the ligand system.
These signals are located almost at the same position as for the
ligand H2L1 (Figure 3 Ib).
The signal around 177 ppm in the spectrum of H2L1
(Figure 3 Ib) is characteristic for the carbon atom C1 of
carboxylate groups. This signal has disappeared and a signal at
188 ppm has appeared in the spectrum of Rh2-L1 (Figure 3 Ic). A
similar shift is obtained comparing the spectra of H2L2 and Rh2-
L2 (Figure 3 IIb, c). Here, the peak around 177 ppm (C’1) has
disappeared and a new one at 188 ppm has appeared in the
obtained spectrum of Rh2-L2. Similar spectral changes were also
reported in our previous works.[66–67]
Figure 4. 19F MAS NMR spectrum of (a) Rh2-L1 and (b) Rh2-L2.
respectively, which most probably refer to the carbon atoms C4
and C’4 of imide groups.
For comparison, the Rh2(TFA)4 precursor was investigated. In
Figure 3 Ia and IIa, a significant signal at 23 ppm is observed
next to the signals at 178 and 111 ppm (from trifluoroacetate),
which indicates acetate groups present in the Rh2(TFA)4
precursor leading to a composition of the type Rh2(TFA)4À x
(OAc)x. Finally, the signal at 200 ppm is assigned to COOÀ of the
acetate group in Rh2(TFA)4À x(OAc)x.[66–67] These signals have
disappeared in the spectra of Rh2-L1 (Figure 3 Ic) and Rh2-L2
(Figure 3 IIe).
While the results described above suggest the successful
formation of the coordination polymers, they do not provide
information on the quantity of the formation process. Thus, a
detailed analysis of 19F MAS NMR (Figure 4) spectra was
These spectral changes strongly indicate that the carbox-
ylate groups of H2Ln have displaced the trifluoroacetate groups
of Rh2(TFA)4 and have coordinated with the dirhodium unit in
Rh2-L1 and Rh2-L2.
Finally, two single peaks at around 163 and 165 ppm are
visible in the spectra of H2L1, Rh2-L1 and H2L2, Rh2-L2,
Figure 3. 1H! C CP MAS NMR spectra of (I) (a) Rh2(TFA)4, (b) H2L1 and (c) Rh2-L1, and (II) (a) Rh2(TFA)4, (b) H2L2 and (c) Rh2-L2. Note: Signals marked with * are
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spinning side bands of the signal at 127 ppm referring to carbon atoms in aromatic rings.
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performed to provide information on the TFA containing sites
occurring in the dirhodium coordination polymers. For Rh2-L1
and Rh2-L2, a 19F signal is observed at around À 73.9 ppm. This
signal demonstrates the existence of trifluoroacetate groups in
the dirhodium polymers. The amounts of fluorine in Rh2-L1 and
Rh2-L2 were determined from quantitative 19F MAS NMR to be
0.24 and 0.53 mmol/g, respectively (for details, see the Support-
ing Information). Taking the Rh2 fraction into consideration, the
Rh2/TFA ratio for Rh2-L1 is 10.1, while for Rh2-L2 it is 4.2. These
results demonstrate that 98% respectively 94% of the overall
TFA groups are replaced during the formation process of the
coordination polymers Rh2-L1 and Rh2-L2 (see the Supporting
Information for details) and thus the exchange was almost
quantitative.
Furthermore, the electronic states of rhodium in Rh2-L1 and
Rh2-L2 were inspected by using X-ray photoelectron spectro-
scopy (XPS). As illustrated in Figure 6b and c, the XPS spectra
reveal that all Rh species in Rh2-L1 and Rh2-L2 are present in the
oxidation state +2, according to the binding energies 308.3
respectively 313.2 eV of the Rh 3d5/2 and 3d3/2 levels.[78] In
comparison, the parent Rh2(TFA)4 exhibits significantly different
binding energies of 309.4 and 314.9 eV. Moreover, a signal
around 688 eV is observed in the wide range scan of Rh2(TFA)4,
which is attributed to the binding energy of F 1s. This signal
has almost disappeared and a signal at 399 eV (N 1s) has
appeared in the wide range scan of Rh2-L1 and Rh2-L2
(Figure S3 in the Supporting Information). This change of
binding energies between Rh2(TFA)4 and the coordination
polymers Rh2-L1 and Rh2-L2 further corroborates the ligand
substitution and reveals that the chemical environment of the
dirhodium unit is strongly affected by the chiral ligand in the
dirhodium coordination polymers. Together with the FT-IR, 13C
CP MAS NMR and DR-UV-vis described above, these observa-
tions further confirm the success of the ligand exchange.
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DR-UV-vis and XPS
To further analyze the chemical environment in the obtained
chiral dirhodium coordination polymers, diffuse reflectance
ultraviolet-visible (DR-UV-vis) spectra of Rh2(TFA)4, Rh2-L1 and
Rh2-L2 were recorded. In Figure 5, the DR-UV-vis spectra of
Rh2(TFA)4 and the coordination polymers Rh2-L1 and Rh2-L2 are
compared. All spectra display a broad band which is centered
at around 450 nm (band II). In addition, a second band (band I)
is obtained at different wave lengths at ca. 596, 620 or 626 nm,
respectively. While band I is assigned to RhÀ Rh π*!RhÀ Rh σ*
transitions, band II is attributed to RhÀ Rh π*!RhÀ O σ*
transitions.[77–78] These results clearly indicate that the dirhodium
unit is preserved during the synthesis of the chiral dirhodium
coordination polymers. Importantly, Rh2(TFA)4 shows a signifi-
cantly different position of band I compared to Rh2-L1 and Rh2-
L2. This refers to the electronic structure of the chiral ligand
systems, which is significantly different from the TFA ligand.
This underlines the successful exchange of TFA ligands by the
chiral ligands for both catalyst systems.
Catalytic tests
The catalytic performance of the Rh2-L1 and Rh2-L2 catalysts
was tested in the asymmetric cyclopropanation reaction of
diazooxindole and styrene, a model reaction to prepare spiro-
cyclopropyloxindoles. Typically, in this reaction the four isomers
(see scheme in Table 2) are formed, where the R,R and S,S spiro-
cyclopropyloxindole (cis-cyclopropyloxindole) as well as R,S and
S,R spiro-cyclopropyloxindole (trans-cyclopropyloxindole) are
1
each pairs of enantiomers that cannot be distinguished by H
NMR, while trans and cis-cyclopropyloxindoles are diastereo-
meric to each other.
Both Rh2-L1 and Rh2-L2 exhibit high catalytic performance
in the formation of spiro-cyclopropyloxindoles, with yields of
96% employing Rh2-L1 and 82% for Rh2-L2 under the same
°
reaction conditions (0 C, DCM as solvent, 2.5 h reaction time)
Figure 5. DR-UV-vis spectra of (a) Rh2(TFA)4, (b) Rh2-L1 and (c) Rh2-L2.
Figure 6. XPS spectra of (a) Rh2(TFA)4, (b) Rh2-L1 and (c) Rh2-L2.
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Table 2. Asymmetric Cyclopropanation of Diazooxindole with Styrene.
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°
Entry
Catalyst
X [μmol]
Solvent
Temp. [ C]
[a]Yield [%]
[b]dr
[c]ee [%]
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2
3
4
5
6
Rh2-L2
Rh2-L1
Rh2-L1
Rh2-L1
Rh2-L1
Rh2(S-PTPA)4
3.75
3.75
3.75
3.75
1.50
1.50
DCM
DCM
Toluene
DCE
DCE
DCE
0
0
0
0
0
0
82
96
65
95
86
96
83:17
85:15
60:40
88:12
86:14
96:4
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8
10
13
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1
[a] Overall yields of spiro-cyclopropyloxindoles. [b] The diastereomeric ratio (trans:cis) dr was determined by H NMR of the crude reaction mixture. [c] The
[ee] was analyzed for the dominating trans-enantiomers. It was calculated from data determined by chiral HPLC analysis. Reaction conditions: styrene
(0.75 mmol) and diazooxindole (0.15 mmol) in DCE (3 mL) were added to a two-necked round-bottom flask containing a magnetic stir bar under Ar
°
atmosphere at 0 C, followed by addition of the chiral dirhodium catalyst and then stirred for 3 h.
(entries 1–2). Employing an excess of the substrate styrene, as
major spiro-cyclopropyloxindol products the trans-cyclopropy-
loxindole enantiomers are formed.
The slightly lower yield and selectivity of the coordination
polymer compared to the homogeneous catalyst may refer to
different factors. (i) On the one hand, the accessibility of
catalytic sites in the coordination polymer may be limited due
to mass transport, which leads to a lower yield for the Rh2-L1
coordination polymer. (ii) On the other hand, ca. 2% of
trifluoroacetate groups of Rh2(TFA)4 are not exchanged by chiral
ligands when the Rh2-L1 polymer is synthesized, which yield
defect sites. These defect sites are most probably responsible
for the obtained selectivity of the coordination polymer. A
detailed analysis whether they increase or decrease the
selectivity is beyond the scope of the present work.
The obtained enantioselectivity of the investigated catalyst
systems further refers to the employed ligand system. Espe-
cially, the bulkiness of the functional groups in the ligand
system as well as the complexity of the ligand system itself
strongly act on the enantioselectivity in cyclopropanation
reactions as reviewed by Adly et al.[79] In future, to obtain higher
enantioselectivity phenyl may be replaced by tert-butyl in the
ligand system as shown by the works of Hashimoto and co-
workers who applied the homogenous Rh2(S-PTTL)4 as highly
enantioselective catalyst in asymmetric cyclopropanation.[80–81]
Furthermore, approaches as proposed by Ball and co-workers
using complex peptidic ligand systems could be included in the
synthesis of coordination polymers to obtain higher
selectivities.[82–83]
To identify the parameters that influence the yield and
selectivity, in the next step, the reaction was performed with
Rh2-L1 as catalyst varying the solvent (DCM, toluene, DCE). This
study reveals that 1,2-dichloroethane (DCE) is a more attractive
solvent for this transformation in terms of both spiro-cyclo-
propyloxindole yield and selectivity towards the isomers.
Among the solvents tested, toluene (entry 3) has the
strongest negative effect on the yield, which is significantly
lowered compared to the reaction performed in DCM or DCE
(entries 2, 4). The comparison of DCM with DCE shows similar
yield and diastereomeric ratio, however the enantio-selectivity
is lower in DCM compared to DCE (entries 2, 4).
In the next step, the influence of different amounts of
catalyst is inspected. A slightly higher yield of 95% is obtained
when 3.75 μmol of Rh2-L1 is used compared to 86% when
1.50 μmol is used (entries 4–5). As expected, the diastereoselec-
tivity and the enantioselectivity with respect to the trans-
enantiomers are practically independent on the variation of the
amount of catalyst.
Finally, the efficiencies of Rh2-L1 and the homogeneous
catalyst Rh2(S-PTPA)4 in the asymmetric cyclopropanation of
styrene with diazooxindole were compared. Under similar
reaction conditions, Rh2(S-PTPA)4 catalyzes the reaction to give
the spiro-cyclopropyloxindoles in 96% yield with 96:4 diaster-
eoselectivity and 26% enantioselectivity with respect to the
trans-cyclopropyloxindole enantiomers (entry 6). In comparison
with the homogeneous Rh2(S-PTPA)4, the Rh2-L1 coordination
polymer (entry 5) shows only a slightly lower diastereoselectiv-
ity and enantioselectivity.
Recyclability and Leaching test
To test the stability and reusability of the novel dirhodium
coordination polymers, the Rh2-L1 catalyst was investigated in
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an exemplary way in the asymmetric cyclopropanation reaction
between diazooxindole and styrene. Typically, the catalyst was
separated from the reaction system by centrifugation and
washed with ethyl acetate several times to be reused in another
run under the same conditions. As shown in Figure 7 no
significant decrease in the yield is obtained after Rh2-L1 was
used repetitively five times. This obviously displays the excellent
stability and reusability of this chiral catalyst.
with the homogeneous catalyst, Rh2(S-PTPA)4. The coordination
polymer Rh2-L1 showed excellent stability with negligible
leaching and could be easily recycled and reused at least five
times without significant loss of catalytic activity. After
optimization, this one-pot synthetic approach for preparation of
Rh2-L1 and Rh2-L2 may be extended to develop chiral
dirhodium coordination polymers with different chiral ligands
for chemical transformations with high selectivity in future.
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Meanwhile, the diastereomeric ratio slightly drops down
(ESI Table S3). The enantioselectivity with respect to the trans-
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enantiomers as well gradually drops down with increasing Experimental Section
number of cycles (Figure 7) as has been also observed for other
catalysts in the past.[78] This indicates that the chemical environ-
ment around the Rh2 unit changed. However, the Rh2-L1
catalyst still shows good catalytic activity as figured out by the
calculated TOFs at 1.5 h and 3 h of reaction (see Table S2 in the
Supporting Information), which do not significantly change
with the number of reaction cycles.
Finally, the leaching level of rhodium was examined by ICP-
OES analysis. The result was that any possible amounts of
leached Rhodium are below the detection threshold of ICP-OES
(<0.05 ppm, see Table S4 in the Supporting Information).
Materials
Rhodium trifluoroacetate (Rh2(TFA)4, Acros Organics), pyromel-
litic dianhydride (Alfa Aesar), naphthalene-1,4,5,8-tetracarbox-
ylic acid dianhydride (Alfa Aesar), L-phenylalanine (Carl Roth),
styrene (Sigma-Aldrich), N,N-dimethylformamide (Sigma-Al-
drich), isatin (Acros Organics), p-toluenesulfonyl hydrazide
(TsNHNH2, Acros Organics), acetic acid (Sigma-Aldrich), dichloro-
methane (Sigma-Aldrich), 1,2-dichloroethane (Sigma-Aldrich),
toluene (Sigma-Aldrich) and ethyl acetate 99.5% puriss. (Sigma-
Aldrich) were purchased and used without further purification.
Conclusion
Synthesis of chiral diacid ligands
A facile approach to prepare novel chiral dirhodium coordina-
tion polymers from the precursors Rh2(TFA)4 and chiral
dicarboxylic acids via ligand exchange was developed. The
coordination polymers consist of 2D layers arranged as plates/
flakes in a disordered manner as demonstrated by SEM.
Characterization by FT-IR, 13C CP MAS NMR and XPS revealed
the successful ligand exchange. Quantitative 19F MAS NMR
showed that the ligand exchange was nearly quantitative (>
95%). During the ligand substitution process, the dirhodium
unit stayed intact as proved by DR-UV-vis and XPS.
N,N’-(pyromellitoyl)-bis-L-phenylalanine diacid (H2-L1): H2-L1
was prepared according to the procedure reported
elsewhere.[75,84] A mixture of pyromellitic dianhydride (2.18 g,
10 mmol) and L-phenylalanine (3.30 g, 20 mmol) was dissolved
in 30 mL acetic acid and stirred at room temperature for 5 h,
followed by heating under reflux for 6–8 h. The solvent was
removed under reduced pressure and the residue was dissolved
in 50 mL of cold 1 M hydrochloric acid. The solution was stirred
until a white precipitate was formed. This precipitate was
filtered off, washed with 200 mL of deionized water and dried
under vacuum. (4.40 g, 86% yield), 1H NMR (300 MHz, DMSO-d6)
δ: 13.33 (s, 2H), 8.20 (s, 2H), 7.28–7.08 (m, 10H), 5.18 (dd, J=
11.3, 4.8 Hz, 2H), 3.51-3.55 (dd, J=14.1, 4.8 Hz, 2H), 3.38–3.33
(m, 2H). 13C NMR (75 MHz, DMSO-d6) δ: 169.53, 165.14, 137.09,
136.20, 128.66, 128.32, 126.57, 118.36, 53.63, 33.90.
The catalytic performance, diastereoselectivity and enantio-
selectivity of the coordination polymers depended on the used
chiral ligand system and was only slightly reduced compared
Bis-N,N’-(L-phenylalanyl) naphthalene-1,4,5,8-tetracarbox-
ylic diimide (H2-L2): H2-L2 was synthesized according to the
procedure reported elsewhere.[85–86]
A mixture of 1,4,5,8-
naphthalene dianhydride (0.95 g, 3.56 mmol) and L-phenyl-
alanine (1.18 g, 7.13 mmol) was suspended in 20 mL of N,N-
dimethylformamide (DMF) and refluxed for 12 h, giving a brown
solution. This solution was cooled to room temperature before
it was poured over ice. The product was extracted with ethyl
acetate (2×100 mL), washed with brine, and the solvent was
removed under vacuum. The solid was washed with toluene to
1
give the product. (1.39 g, 70% yield), H NMR (300 MHz, DMSO-
d6) δ: 13.06 (br., 2H), 8.65 (s, 4H), 7.01–7.18 (m, 10H), 5.86 (dd,
J=9.5, 5.6 Hz, 2H), 3.60 (dd, J=13.9, 5.6 Hz, 2H), 3.33 (dd, J=
14.2, 9.3 Hz, 2H), 13C NMR (75 MHz, DMSO-d6) δ: 170.69, 162.46,
Figure 7. Recyclability test of Rh2-L1 in the asymmetric cyclopropanation
between styrene and diazooxindole.
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138.26, 131.70, 129.41, 128.60, 126.80, 126.47, 125.13, 55.06,
34.78.
Synthesis of chiral dirhodium coordination polymers: The
heterogeneous chiral dirhodium polymers were obtained by
ligand exchange according to the literature.[66–67]
enantiomeric excess (ee). 1H NMR of the trans-enantiomers
(300 MHz, CDCl3) δ: 9.09 (br, 1 H), 7.33–7.22 (m, 5 H), 7.11 (t, J=
7.7 Hz, 1 H), 6.97 (d, J=7.5 Hz, 1 H), 6.69 (m, 1 H), 5.96 (d, J=
7.5 Hz, 1 H), 3.37 (t, J=8.6 Hz, 1 H, trans), 2.28–2.23 (m,1 H),
2.08–2.05 (m, 1 H); 13C NMR (75 MHz, CDCl3) δ: 178.89, 141.04,
134.99, 129.95, 128.38, 127.91, 127.44, 126.58, 121.43, 120.97,
109.69, 36.14, 33.74, 22.65. The enantiomeric excess (ee) was
then calculated for the trans-enantiomers from data determined
by chiral HPLC analysis (for details see the Supporting
Information).
1
2
3
4
5
6
7
8
9
Rh2-L1 was synthesized as follows. A solution of Rh2(TFA)4
(0.10 g, 0.15 mmol) and H2L1 (0.23 g, 0.46 mmol) in 75 mL ethyl
acetate (EtOAc) was charged into a 100 mL round-bottomed
flask which was fitted with a Soxhlet extractor containing a
mixture of 1 g K2CO3 and 1 g 4 Å molecular sieve in a cellulose
filter tube. This first Soxhlet approach was used to neutralize
the formed trifluoroacetic acid upon ligand exchange and shift
the equilibrium towards the formation of the coordination
polymer. After 3 days reaction under reflux, the obtained solid
was filtered and washed in a Soxhlet extractor by EtOAc for
another 2 days. This second Soxhlet step, which is performed in
the absence of K2CO3 is a simple washing step, which is not
expected to influence the constitution of the product. Then, the
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Recyclability test
To inspect the recyclability of the coordination polymers,
experiments were exemplary performed for Rh2-L1. The catalyst
was separated from the reaction mixture, washed and dried
after each run of reaction. Then, the Rh2-L1 catalyst was tested
with a fresh mixture of reactants and solvent for subsequent
reaction under the same conditions. Rh2-L1 was recycled 5
times.
For the leaching test, the Rh2-L1 catalyst was removed, the
filtrated solution was collected and the residual rhodium
fraction were studied with ICP-OES.
°
solid was dried at 80 C under vacuum for 12 h yielding Rh2-L1
(72.9 mg, 38% yield).
Rh2-L2 was synthesized similarly, employing a mixture of
Rh2(TFA)4 (0.1 g, 0.15 mmol) and H2L2 (0.26 g, 0.46 mmol) in
°
75 mL EtOAc. The reaction was performed at 120 C for 5 days
yielding Rh2-L2 (51.2 mg, 24% yield).
Catalytic Asymmetric Cyclopropanation
Characterization
3-Diazooxindole was prepared according to the literature.[87]
A
Thermogravimetric (TG) analyses were carried out under oxygen
flow (75 mLminÀ 1) using the simultaneous thermal analyzer TG
209 F3 Tarsus. The Rh loadings were calculated from the TG
analysis data according to the method reported by Kaskel
et al.[65] The C, N and H contents were determined on an
Elemental Analyzer Vario EL III working in CHN mode.
Fourier-transform infrared spectroscopy (FTIR) was con-
ducted using a Perkin Elmer Spectrum spotlight 200 FT-IR
spectrometer with 4 cmÀ 1 resolution.
Scanning electron microscopy (SEM) images were obtained
using a Philips XL30 S-FEG microscope to probe the coordina-
tion polymers employing an electron beam energy of 20 keV.
X-ray powder diffraction (XRD) patterns were measured on
a powder diffractometer (StadiP, Stoe & Cie. GmbH, Darmstadt,
Germany) in transmission geometry using Cu Kα1-radiation (λ=
154.060 pm, Ge[111]-monochromator) and a MYTHEN 1 K (Dectris
Ltd. Baden, Switzerland) detector. The samples were placed
between two X-ray amorphous foils and were measured in the
mixture of isatin (1.47 g, 10 mmol) and TsNHNH2 (2.05 g,
°
11 mmol) in THF (50 mL) was stirred at 65 C for 1 h. After
filtration, the solid was stirred in 50 mL of a 0.2 M NaOH
°
aqueous solution at 65 C for 1 h. The reaction mixture was then
extracted with EtOAc (80 mL, three times). The combined
organic phases were dried over Na2SO4 and the solvent was
removed under vacuum. The residue was recrystallized from
acetone to give the 3-diazooxindole as an orange solid (1.29 g,
81% yield). 1H NMR (300 MHz, CDCl3) δ: 9.46 (s, 1 H), 7.18-93 (m,
4 H), 13C NMR (75 MHz, CDCl3) δ: 169.35, 131.96, 125.56, 122.14,
118.31, 117.25, 110.78.
The catalytic cyclopropanation of styrene with diazooxin-
dole was applied to evaluate the performance of the Rh2-L1 and
Rh2-L2 catalysts. In detail, styrene (0.75 mmol) and diazoox-
indole (0.15 mmol) in 3 mL of an appropriate solvent (here we
focused on dichloromethane (DCM), dichloroethane (DCE) and
toluene according to refs. [38–39]) were added to a two-necked
round-bottom flask equipped with a magnetic stir bar followed
by addition of a specific amount (1.50 or 3.75 μmol Rh2) of the
chiral dirhodium catalyst. The reaction was performed under Ar
°
2θ range of 3–50 .
13
1H! C CP MAS NMR spectra were measured at room
temperature on a 9.4 T Bruker Avance II+ solid state NMR
spectrometer with a 4 mm broadband double-resonance probe
at a frequency of 100.61 MHz for 13C. For all samples cross
polarization experiments were performed with contact times of
1.5 ms at a spinning rate of 12 kHz. 20480 scans were applied
with a repetition delay of 2 s. Spectra were referenced to TMS
using adamantane (δ =38.5 ppm) as external standard.
°
atmosphere at 0 C. After stirring for 3 h, the chiral dirhodium
catalyst was removed by centrifugation and the solvent was
removed under reduced pressure. The diastereomeric ratio (dr)
1
was determined by crude H NMR analysis (for details see the
Supporting Information).
The resulting crude mixture was further purified by silica gel
column chromatography (hexane-EtOAc=2:1 to 1:1) to con-
centrate the trans-enantiomers for determination of the
19F MAS NMR spectra were measured on a 9.4 T Bruker
Avance II+ solid state NMR spectrometer at a frequency of
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Acknowledgements
This work has been supported by the DFG under contract Bu
911/27-1. Open access funding enabled and organized by Projekt
DEAL.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: asymmetric cyclopropanation
·
coordination
polymers · heterogenous catalysis · ligand exchange · rhodium
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