Chiral Chromium Salen@rGO as Multipurpose and Recyclable Heterogeneous Catalyst

Article abstract of DOI:10.1002/chem.202101003

The first immobilization of a pyrene-tagged chromium salen complex through π-π noncovalent interactions on reduced graphene oxide (rGO) is described. A very robust supported catalytic system is obtained to promote asymmetric catalysis in repeated cycles, without loss of activity or enantioselectivity. This specific behavior was demonstrated in two different catalytic reactions (up to ten reuses) promoted by chromium salen complexes, the cyclohexene oxide ring-opening reaction and the hetero-Diels-Alder cycloaddition between various aldehydes and Danishefsky's diene. Furthermore, the chiral chromium salen@rGO has been found to be compatible with a multi-substrate type use, in which the structure of the substrate involved is modified each time the catalyst is reused.

Full text of DOI:10.1002/chem.202101003

Accepted Article
Accepted Article
Title: Chiral Chromium Salen@rGO as Multipurpose and Recyclable
Heterogeneous Catalyst
Authors: Emmanuelle Schulz, Mariam Abd El Sater, Mohamed Mellah,
Diana Dragoe, Emilie Kolodziej, and Nada Jaber
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To be cited as: Chem. Eur. J. 10.1002/chem.202101003
Link to VoR: https://doi.org/10.1002/chem.202101003
01/2020

10.1002/chem.202101003
Chemistry - A European Journal
FULL PAPER
Chiral Chromium Salen@rGO as Multipurpose and Recyclable
Heterogeneous Catalyst
Mariam Abd El Sater,[a,b] Mohamed Mellah,[a] Diana Dragoe,[a] Emilie Kolodziej,[a] Nada Jaber,[b]* and
Emmanuelle Schulz[a]*
[a]
[b]
M. Abd El Sater, Dr. M. Mellah, Dr. D. Dragoe, E. Kolodziej, Dr. E. Schulz
Equipe de Catalyse Moléculaire, Université Paris Saclay
Institut de Chimie Moléculaire et des Matériaux d’Orsay
Bâtiment 420, 91405 Orsay cedex, France
E-mail: emmanuelle.schulz@universite-paris-saclay.fr
M. Abd El Sater, Dr. N. Jaber
Laboratoire de Chimie Médicinale et des Produits Naturels
Université Libanaise, Faculté des Sciences (I) et PRASE-EDST
Hadath, Beyrouth, Lebanon
E-mail : nada.jaber@ul.edu.lb
Supporting information for this article is given via a link at the end of the document.
Abstract: The first immobilization of a pyrene-tagged chromium salen
complex through π-π non-covalent interactions on reduced graphene
oxide (rGO) is described. A very robust supported catalytic system is
obtained to promote asymmetric catalysis in repeated cycles, without
loss of activity or enantioselectivity. This specific behavior was
demonstrated in two different catalytic reactions (up to ten reuses)
promoted by chromium salen complexes, the cyclohexene oxide ring-
opening reaction and the hetero-Diels-Alder cycloaddition between
various aldehydes and Danishefsky’s diene. Furthermore, the chiral
chromium salen@rGO has been found to be compatible with a multi-
substrate type use, in which the structure of the substrate involved is
modified each time the catalyst is reused.
used for the Pd-catalyzed heterogeneous hydrogenation of
alkenes and the Ru-catalyzed alcohol oxidation.[6] The diversity of
application is moreover demonstrated by the Fe-catalyzed
ethylene polymerization on multi-walled CNTs,[7] the Rh-promoted
1,4-addition reaction and hydrosilylation of alkynes,[8] and the Cu-
catalyzed CH activation on rGO.[9] In the latter case, the carbon
support not only contributes to make the catalyst insoluble but
also exacerbates its activity by reducing the electron density of
the copper site. Mention should be made, in each case, of the
functionalization of the ligand by
a pyrene group as a
demonstration of its high affinity with the carbon surface by π-π-
interactions. Such non-covalent anchoring on carbon surfaces
proved also its utility by enhancing the cooperativity between two
pyrene-tagged Pd- and Ru-complexes for hydrodefluorination.[10]
In contrast, this immobilization strategy has been much less
developed in the context of asymmetric catalysis, with only four
examples reported, to our knowledge. In the field of Rh-promoted
asymmetric hydrogenation, chiral phosphine-containing catalysts
have been deposited on CNT,[11] or assembled onto rGO.[12] We
have immobilized copper bis(oxazoline) complexes for the
enantioselective formation of C-C bonds,[13] and more recently,
the non-covalent immobilization of Rh complexes was reported
for the asymmetric hydroformylation and implemented in a flow
mode.[14] The use of salen-complexes is ubiquitous in asymmetric
catalysis.[15] These valuable enantiopure catalysts have been
immobilized on various supports or their structures modified to
become insoluble species, in order to be easily recovered by
filtration.[16] We describe here the first immobilization of chromium
salen complexes on carbon supports[17] through π-π interactions
with rGO. The high efficiency of these chromium catalysts[18] is
demonstrated both in the heterogeneous asymmetric ring-
opening (ARO)[19] of a meso epoxide and in the hetero-Diels-Alder
reaction (HDA)[20] of various aldehydes. Their stability in recycling
procedures is also evaluated.[21] These reactions were chosen as
benchmarks for the diversity of the products to which they lead.
While the ARO reaction provides precursors of cyclic 1,2-amino
alcohols, the HDA reaction between aldehydes and the electron-
rich Danishefsky’s diene delivers enantioenriched dihydro-
pyranones, important scaffolds for the preparation of various
natural compounds.[22]
Introduction
Carbon materials have been used extensively as highly stable
supports, ideally porous and mechanically resistant in catalysis,
in particular for the dispersion and stabilization of metal particles
or for the immobilization of organometallic complexes.[1] Their
particular surface chemistry allows either covalent grafting by
complementary functionalization of the support and of the ligand,
or non-covalent bonding via π-π-interactions between
a
polyaromatic functionalized compound and the graphitic sp2
carbon network. In the context of homogeneous supported
catalysis, and precisely when valuable enantiopure chiral
catalysts are engaged, their non-covalent interactions with the
support are based on simple synthetic procedures avoiding its
prior modification. The strength of these supramolecular
interactions can be adjusted as a function of the solvent and the
temperature, allowing both easy recovery of the catalyst and
recycling of the support, in the event of deactivation. On the other
hand, precautions must be taken to suppress metal leaching.[2]
Some successes have already been reported using this
immobilization strategy on different carbon supports, including the
immobilization of Ru-carbene complexes to perform metathesis
reactions.[3] The Au-catalyzed enyne cyclization reaction has
been achieved on multi-walled CNTs[4] and Pd-based catalysts
were non covalently anchored onto magnetic Co/carbon
nanoparticles for Suzuki-Miyaura couplings.[5] rGO was further
1
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Results and Discussion
pyrene-tagged catalysts are all more active than the reference
Jacobsen catalyst. We propose that π-stacking interactions are
strongly probable between the catalysts, allowing thus a favorable
bimetallic interaction for highly active catalytic species. Presence
of non-covalent interactions in salen-based complexes has
already been investigated, to promote their association and study
their influence on the catalysis, particularly for cooperativity.[27]
The pyrene-tagged catalysts nevertheless differed in their
selectivity. Indeed, both Sym-Cr-A and Sym-Cr-B were found to
be poor enantioselective catalysts while the unsymmetric
analogues were much better, with Unsym-Cr-A giving the target
product in 81 % ee. In addition, further experiment showed that
almost complete conversion (96 %) was already achieved in just
6 hours using this catalyst.
We therefore immobilized the salen complexes by π-interactions
with the rGO support by placing the pyrene group far from the
active chromium site (Scheme 1). This strategy aims to preserve
the selectivity of the homogeneous transformation achieved in the
presence of the conventional Jacobsen catalyst. We tackled the
synthesis of salen catalysts tagged with one or two pyrene groups
via links of different lengths, to compare both the accessibility to
the active sites and their stability to recycling. Compounds A and
B, as key intermediates for the synthesis of Sym and Unsym
ligands were easily obtained through click reactions between 1-
[(2-propyn-1-yloxy)methyl]-pyrene[23]
yloxy)butyl)pyrene[24]
and 5-(azidomethyl)-3-(tert-butyl)-2-
hydroxybenzaldehyde[25] in two steps and high isolated yield (see
Scheme and Supporting Information). Then, classical
or
1-(4-(prop-2-yn-1-
1
Table 1. Homogeneous ARO of cyclohexane oxide, catalysts comparison[a]
condensation with 0.5 equivalent of (1S,2S)-cyclohexane-1,2-
diamine directly afforded Sym-A and Sym-B, the latter with a
metal coordinating site remote from pyrene by four methylene
groups. After the preliminary preparation of the diamine
monoammonium salt C,[26] the high yield formation of both Unsym
ligands also occurred by successively reacting 3,5-di-tert-butyl-2-
hydroxybenzaldehyde then A (or B), without isolating the
intermediate ammonium mono-imine. Insertion of chromium in the
coordinating sites was finally performed from CrCl2 in THF under
argon atmosphere and then oxidation in air.[20]
Catalyst
Jacobsen Cat.
Sym-Cr-A
Conv (%) [b]
ee (%) [c]
78
86
88
26
81
8
Unsym-Cr-A
Sym-Cr-B
>99
98
Unsym-Cr-B
>99
70
[a] substrate, 0.5 mmol, [3 M] in Et2O. [b] determined by GC with dodecane as
internal standard. [c] determined by GC (see Supporting Information).
We propose that the absence of two tBu groups in the para
position of the phenol, in the Sym-Cr catalysts, due to the
bifunctionalization by the pyrene groups is the cause for this
significant decrease in enantioselectivity. On the other hand, the
accessibility of the catalytic sites in these symmetrical species can
be constrained by a possible π-π intramolecular stacking between
the two adjacent pyrene groups. In contrast, Unsym-Cr-A
allowed complete conversion and led to the best
enantioselectivity value for the series. This catalyst was therefore
chosen for the rest of the study.
Its immobilization onto the carbon surface was achieved by
impregnation of the rGO suspension with a solution of Unsym-
Cr-A in DCM with a mass ratio of 4/1. After 24 hours of stirring at
room temperature, the solid was filtered off, washed with DCM
and vacuum dried.
Scheme 1. Synthesis of pyrene-tagged salen ligands.
The four corresponding chromium complexes Sym-Cr and
Unsym-Cr (A and B) were prepared in almost quantitative yield
and fully characterized by IR, UV-Vis and HRMS spectroscopy.
They were first tested under homogeneous conditions, to assess
the influence of their structure on their reactivity and selectivity.
Cyclohexene oxide ring opening with TMSN3 was chosen as the
first test reaction carried out in diethyl ether. Different results in
terms of activity and selectivity were obtained depending on the
structure of the catalysts used (see Table 1). In our hands, the
Jacobsen catalyst delivered the ring-opening product in 88 % ee,
but full conversion was not achieved after 24 h of reaction. The
Figure 1. Immobilization of Unsym-Cr-A on rGO.
2
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The efficiency of the immobilization was monitored by UV-Vis
analyses (see Figure 1 and Supporting Information for details).
A calibration line curve for Unsym-Cr-A was indeed obtained by
measuring the absorbance (Abs) of solutions with different
concentrations at the λmax (345 nm, see blue trace in Figure 1).
After 24 h, analysis of the supernatant (orange trace) indicated
that 90% of the complex has been deposited, what was confirmed
by weighing the resulting solid.
Furthermore, X-ray photoelectron spectroscopy was conducted
on powders of both unmodified rGO support and Unsym-Cr-
A@rGO. The wide-scan spectra are shown in Figure 2a. The
presence of signals arising from Cr2p, N1s and Cl2p core-levels
was clearly noticed on the spectrum of Unsym-Cr-A@rGO,
proving that the catalyst had been successfully grafted. The
narrow-scan spectra for Cr2p, N1s and Cl2p were then measured
(Figure 2b), which allowed to determine the position in energy of
the three elements, as well as their atomic ratios. The Cr2p core-
level spectrum consists of two main peaks corresponding to the
spin-orbit components Cr2p3/2 and Cr2p1/2, situated at 577.2 eV
and 586.9 eV, respectively, and separated by 9.7 eV. These
values allowed to assign the Cr3+ according to the literature.[28]
The Cl2p and N1s spectra are centred at 198.7 eV and 400.5 eV
respectively. Expecting information on the catalyst structure,
experimental determination of the Cr/Cl content in Unsym-Cr-
A@rGO was performed and is comparable to the theoretical
value (1), as well as the N/Cl ratio (5), undoubtedly revealing the
presence of intact Unsym-Cr-A on the surface.
with complete conversion after 6 h, indicating no detrimental effect
of the support on the catalytic activity, which is often the case in
heterogeneous reactions. However, the target product was
isolated with a decreased enantioselectivity value than that
observed under homogeneous conditions (see Table 2, entry 1).
After completion, the supernatant from the first run of
heterogeneous catalysis was filtered off and the supported
complex was washed with diethyl ether, the reaction solvent. It
was then dried under vacuum, before its reuse by further addition
of substrate. The second use of the supported catalyst was also
successful, as were subsequent ones, allowing 10 uses of the
same batch of catalyst. The enantiomeric excess reached 79 %,
fortunately almost equal to the high values obtained under
homogeneous conditions (Table 2).
Table 2. Heterogeneous ARO of cyclohexane oxide with Unsym-Cr-A@rGO,
recycling experiments [a]
Catalyst
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
Run 7
Run8
Conv (%) [b]
>99
>99
>99
>99
>99
>99
98
ee (%) [c]
63
75
75
76
79
76
76
98
75
Run 9
Run 10
96
72
96
68
[a] substrate, 0.5 mmol, [3 M] in Et2O. [b] determined by GC with dodecane as
internal standard. [c] determined by GC (see Supporting Information)
The immobilization of Unsym-Cr-A on rGO thus did not alter its
activity and the enantioselectivity values remained also very high
in this procedure. Altogether, an important increase in the
calculated TON of the catalyst is thus obtained by its simple reuse
after filtration.
A second reaction, namely the HDA reaction between aldehydes
and trans-1-methoxy-3-trimethylsilyloxy-buta-1,3-diene, was
tested to evaluate the extent of application of Unsym-Cr-A@rGO
in asymmetric catalysis. To ensure the efficiency of the modified
complexes, the performances of Sym-Cr-A and Unsym-Cr-A
were again compared with the Jacobsen complex in the HDA
reaction involving cyclohexane carboxaldehyde in DCM (Table 3).
In this case again, the unsymmetrical pyrene-derived complex
Unsym-Cr-A provided activity and enantioselectivity values very
close to those obtained by the Jacobsen catalyst, with Sym-Cr-A
far less active. The study therefore continued using Unsym-Cr-A
with rapid screening for solvents, but neither Me-THF nor diethyl
ether outperformed DCM in terms of activity (61 and 67 % yield,
respectively), nor of selectivity (78% ee). A new batch of Unsym-
Cr-A@rGO was prepared and the catalysis was first carried out
in DCM as was the homogeneous reaction. The first Unsym-Cr-
A@rGO catalysis afforded the dihydropyranone with conversion
Figure 2. a) XPS spectra for rGO and Unsym-Cr-A@rGO. b) narrow-scan
spectra for Cr2p, N1s and Cl2p.
The heterogeneous catalysis was then performed under
conditions exactly matching the one used for the tests with the
non-supported catalysts. A black suspension (3M in substrate)
was obtained, corresponding to the introduction of a 2 mol %
catalytic ratio. Pleasingly, the first Unsym-Cr-A@rGO catalysis in
the ARO of cyclohexene oxide afforded the ring-opened product
3
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and enantioselectivity values close to those obtained in
homogeneous conditions.
[a] substrate, 0.5 mmol, [5 M] in DCM, rt, 24 h. [b] determined by GC with
dodecane as internal standard (see Supporting Information). [c] determined by
GC (see Supporting Information).
Table 3. Homogeneous HDA of cyclohexane carboxaldehyde, catalysts
The even more important decrease (14 % conversion) during the
4th use of the supported catalyst led to carrying out the following
washes with cold acetone. Interestingly, this resulted in the
complete recovery of the catalyst activity and enantioselectivity
for its 5th use (entry 5), values which were maintained for the next
four runs. We propose that acetone is both a polar solvent which
promotes π-stacking type interactions, and a favourable medium
for the dispersion of the rGO carbon sheets, which allows a better
accessibility to the catalytic sites.[3a] The recovery procedure was
however not facilitated by the use of acetone as washing solvent
because of settling difficulties. Thus, after the 8th run, this same
catalytic batch was washed with cold diethyl ether, and gratifyingly,
the next run afforded the same results in terms of activity and
enantioselectivity (entries 8-9).The validity of this optimized
washing procedure in diethyl ether was verified by the preparation
of a new batch of Unsym-Cr-A@rGO, which was involved in a
new recycling experiment (entries 10-16); efficient catalysis
occurred for seven reuses of the supported catalyst, with excellent
stability of the enantioselectivity values of the transformation.
The scope of this recovery procedure via non-covalent
interactions was tested for the HDA of various aldehydes.
Consequently, three aromatic aldehydes reacted with
Danishefsky’s diene under homogeneous conditions with
Unsym-Cr-A and the corresponding dihydropyranones were
isolated with good enantioselectivity values (see Supporting
Information). Then, another batch of Unsym-Cr-A@rGO was
prepared and used to catalyse the heterogeneous HDA reaction
of these aldehydes successively, by changing the structure of the
substrate at each reuse of the supported complex (Table 5).
comparison [a]
Catalyst
Conv 1a (%) [b]
Yield 2a (%) [c]
ee 2a (%) [d]
Jacobsen Cat.
Sym-Cr-A
84
23
73
69
-
78
80
78
Unsym-Cr-A
61
[a] substrate, 0.5 mmol, [5 M] in DCM. [b] determined by GC with dodecane as
internal standard. [c] isolated yield. [d] determined by GC (see Supporting
Information).
In this case also and ideally, the presence of the support did not
impact the chiral catalyst efficiency. This was verified by using
unmodified rGO in the reaction conditions, which resulted in no
conversion after 24 h of stirring. After complete conversion, the
heterogeneous catalyst was recovered by filtration and several
solvents were tested to wash the supported complex between
each reuse (see Table 4). The reaction solvent, cold DCM, was
initially used during the first three cycles which led to the
conservation of the enantioselectivity but to a constant decrease
in the conversion with each recycling (entries 1-4).
Table 4. Heterogeneous HDA of cyclohexane-carboxaldehyde with Unsym-
Cr-A@rGO in DCM [a]
Table 5. Heterogeneous HDA of various aldehydes with Unsym-Cr-A@rGO [a]
Conv 1a (%)
ee 2a (%)
Washing
Solvent
DCM
Entry
Run
[b]
[c]
Conv
(%) [b]
ee
Yield
Conv
(%) [b]
ee
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
55
61
49
14
85
82
79
79
79
81
81
81
71
79
81
79
79
80
Run
1
Run
5
(%) [c]
(%) [d]
(%) [c]
DCM
DCM
Acetone
Acetone
Acetone
Acetone
Et2O
68
63
76
75
64
81
63
64
60
30
65
71
83
70
85
84
64
81
60
63
2
3
4
6
7
8
-
10
11
12
13
14
15
16
1
2
3
4
5
6
7
82
85
86
78
74
76
68
80
81
80
80
81
81
81
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
-
4
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10.1002/chem.202101003
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easy synthesis of the catalyst and from the absence of support
modification. As far as we know, this is the first immobilization of
salen derivatives on carbon surfaces by π-π interactions, and its
success opens the way for many extensions, given the wide
applicability of these complexes in asymmetric catalysis. Work is
in progress to further extend this procedure to different
asymmetric catalytic transformations with other pyrene-tagged
salen complexes, both in batch and in flow conditions.
[a] substrate, 0.8 mmol, [5 M] in DCM, rt, 24 h, washing with Et2O. [b] determined
by GC with dodecane as internal standard (see Supporting Information). [c]
determined by GC (see Supporting Information). [d] isolated yield
A
first run was thus devoted to the transformation of
benzaldehyde 1b, resulting in the isolation of the corresponding
dihydropyranone 2b with a good yield and an enantioselectivity
value of 64 % ee, even slightly better than that obtained under
homogeneous conditions (Table 5, run 1 and Supporting
Information for comparison). After washing with diethyl ether, the
recovered catalyst was then engaged in the transformation of
cyclohexane-carboxaldehyde 1a (Table 5, run 2).
Acknowledgements
The subsequent recycling procedure involved consecutively two
other aromatic aldehydes (1c-d, runs 3-4). If the heterogeneous
catalysis turned out to be slightly less effective in terms of activity
compared to the results obtained under homogeneous conditions
(Table 5 and Supporting Information), similar enantioselectivity
values could nevertheless be obtained. The products were easily
recovered without being contaminated by traces of
dihydropyranones prepared in the previous runs.
We acknowledge Université Paris Saclay, CNRS, Ministère de
l’Europe et des Affaires Etrangères (bourse d’excellence “Eiffel”)
and Charm3At LabEx (ANR-11-LABEX-0039) for financial
support. This project has been jointly funded with the support of
the National Council for Scientific Research in Lebanon CNRS-L
and Lebanese University. The authors are also grateful to Drs.
Philippe Dauban and Olivier David for fruitful discussions.
Finally, recovered Unsym-Cr-A@rGO after the 4th run was
reassigned to the HDA of benzaldehyde (run 5), interestingly
offering improved activity and the same enantioselectivity for the
target product 2b. Recycling was continued again with the other
three substrates (1a, 1c and 1d) while maintaining its efficiency
(runs 6-8), as clear evidence of the stability of the procedure.
Spent Unsym-Cr-A@rGO was also subjected to XPS-analyses
and the Cr to C ratio was compared to that determined for the
fresh supported catalyst (see Supporting Information). Within the
limits of precision of the analysis, no leaching of active species
could be determined; with regard to the stability of the catalytic
values, no degradation of the chromium complex has occurred,
obviously. In addition, Unsym-Cr-A@rGO was stirred for 18 h in
DCM and the resulting solution was collected after filtration
without decreasing the temperature. No catalytic activity was
detected after adding substrates to this solution, proving that there
is no release of the active species during the heterogeneous
catalysis reaction.
Conflict of interest
The authors declare no conflict of interest.
Keywords: Asymmetric catalysis • Cr-salen complexes •
Supported catalysts • reduced Graphene Oxide • Recycling
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Entry for the Table of Contents
A pyrene-tagged Cr-salen complex immobilized by π-π interactions at rGO surface delivers a robust, reusable, heterogeneous
catalyst efficient in several asymmetric catalytic reactions
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