Electropolymerization of chiral chromium-salen complexes: New materials for heterogeneous asymmetric catalysis

Article abstract of DOI:10.1039/c2nj21011h

Electrochemical oxidation is described as a very efficient polymerization procedure for the heterogenization of metallic chiral catalysts. From chromium chiral complexes based on salen-thiophene ligands, this methodology provided an efficient access to various polymers. Recovered as insoluble powders, these materials were tested in different enantioselective heterogeneous catalytic reactions. Structural modifications were introduced on the salen core in order to evaluate their influence on redox polymer properties and on the enantioselectivity of the catalysis. Electrochemical experiments showed the particular stability of these deposited materials at the electrode surface and SEM analyses suggested the influence of the electropolymerization conditions on their morphology.

Full text of DOI:10.1039/c2nj21011h

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PAPER
Electropolymerization of chiral chromium–salen complexes:
new materials for heterogeneous asymmetric catalysis
Anaı
Mohamed Mellah*a
¨
s Zulauf,a Xiang Hong,a Francois Brisset,b Emmanuelle Schulz*ab and
¸
Received (in Montpellier, France) 5th December 2011, Accepted 19th March 2012
DOI: 10.1039/c2nj21011h
Electrochemical oxidation is described as a very efficient polymerization procedure for the
heterogenization of metallic chiral catalysts. From chromium chiral complexes based on salen–thiophene
ligands, this methodology provided an efficient access to various polymers. Recovered as insoluble
powders, these materials were tested in different enantioselective heterogeneous catalytic reactions.
Structural modifications were introduced on the salen core in order to evaluate their influence on redox
polymer properties and on the enantioselectivity of the catalysis. Electrochemical experiments showed
the particular stability of these deposited materials at the electrode surface and SEM analyses suggested
the influence of the electropolymerization conditions on their morphology.
are well known to promote various enantioselective reactions
Introduction
such as epoxide ring opening,5 epoxidation,6 hetero Diels–Alder
reaction7 or Nozaki–Hiyama–Kishi reaction,8 we have success-
fully evaluated the electrogenerated new insoluble materials
in asymmetric heterogeneous catalysis based on chromium.9,10
Considering the electrochemical polymerization mechanism, we
have obtained a catalytic heterogeneous material with linear
arrangement in which each monomer unit is a catalytic site. The
targeted structure allowed the asymmetric catalyst to operate
under the best conditions for maintaining its high efficiency for
several cycles.
Chiral metallic complexes are one of the keystones of modern
asymmetric catalysis: they have been applied to numerous
reactions, allowing synthesis of sophisticated molecules with
high enantioselectivities. Even if some homogeneous enantio-
selective catalytic procedures have been successfully developed
at the industrial scale, they remain disputable because of
prohibitive costs, mainly due to the loss of the catalyst at the
end of the reaction. Furthermore, some of these metallic
complexes can be considered as harmful pollutants, especially
in medicinal chemistry. In this context, heterogeneous catalysis
presents many advantages. Many efforts are thus being directed
nowadays towards the efficient recovery and re-use of such
selective catalysts. The main trend consists in rendering catalysts
insoluble and recycling them by simple filtration of the reaction
mixture media.1
Many electrochemical investigations were carried out with salen
complexes, because of their straightforward synthesis, their ability
to incorporate various transition metals and their easy electro-
chemical analysis. Anodic polymerisation of these compounds has
been investigated with varied objectives such as new materials for
energy storage,11 electrochemical sensors12 or catalysis.13 Further-
more, it has been demonstrated that the anodic polymerization of
various salen complexes such as nickel, cobalt, copper, iron,
manganese or palladium allowed for efficient preparation
of metal–salen based modified electrodes.14 In most cases, the
polymerization was realised by cyclic voltammetry or constant-
potential electrolysis in weak donor organic solvents like aceto-
nitrile or dichloromethane.15 The properties of the coated electrodes
have been particularly studied in the reduction of halo- and
dihaloalkanes,16 in the development of electrochemical sensors
for the determination of NO concentration in solution,12a,b for
medicinal products such as dipyrone17 or metal ions in aqueous
media.18,19 Enantioselective electrocatalytic epoxidation of styrene
and stilbene was achieved with chiral manganese–salen complexes
polymerized on a glassy carbon electrode using molecular dioxygen
as oxidant.20 In order to better understand the relationship
between the ligand structure and the properties of the modified
In this field the tetradentate Schiff base (salen) family has
been successfully associated to numerous metals (Mn, Co, V,
Ni, Cr, Fe. . .) and promoted a wide range of enantioselective
transformations.2 Heterogeneous procedures have been applied
to various catalytic reactions with some success but there is
room for improvement in terms of enantioselectivity, activity
and recycling efficiency.3 Identified problems for the limitations
in terms of efficiency are metal leaching, catalysts structure
degradation or accessibility of the catalytic sites. In the last
several years, we have developed our own solution to this
problem based on the heterogenization of chiral salen complexes
by electrochemical polymerization.4 Because chromium complexes
a
´
Equipe de Catalyse Moleculaire, ICMMO, UMR CNRS 8182,
Universite Paris-Sud 11, 91405 Orsay cedex, France.
´
E-mail: mohamed.mellah@u-psud.fr, emmanuelle.schulz@u-psud.fr
b CNRS, Orsay, F-91405, France
c
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electrode, both Reynolds and Swager groups described the
synthesis and the electrochemical behavior of salen transition
metal complexes tagged with thiophene moieties.21,22 The
introduction of thiophene units allowed for electrochemical
polymerization at much lower potentials than the parent salen
complexes. These structural modifications suppressed side
reactions and improved the stability of the resulting polymers.
Of particular note, Kingsborough and Swager prepared
various monomers that were electropolymerized at lower
potentials especially for 3,4-ethylenedioxythiophene modified-
salen and they were able to perform oxygen reduction with very
high efficiency.21c
Scheme 2 Synthesis of chiral salen thiophene chromium complexes.
Electrochemical polymerization
According to the results reported by Swager et al.21 and our
previous work,4 the polymerization could only occur with
salen complexed with the metal. All our attempts for electro-
polymerization of metal-free salen–thiophene ligands were
indeed unsuccessful. Current-potential curves for electro-
polymerization of metal-free salen–thiophene in dichloromethane
solution in the presence of nBu4NBF4 (0.05 M) as electrolyte
support were recorded. The potential started from 0.0 V vs. SCE
and was generally cycled between 0.0 and 1.2 V vs. SCE at a
We report herein the results of our investigations on the
synthesis of stable polymeric materials efficient for hetero-
geneous asymmetric catalytic applications. We focused our
studies on the influence of the ligand structure and electro-
polymerization conditions. To evaluate the influence of these
structural modifications on both electrochemical polymeriza-
tion and catalytic activity, each new salen–thiophene complex
was analyzed by electrochemistry.
potential rate of 0.1 V sꢀ1 1
. A typical example of the cyclic
voltammogram obtained for the attempted electrochemical
growth of poly-salen thiophene (poly-4a) on a platinum electrode
is shown in Fig. 1. A large anodic current wave at +0.77 V/SCE
was observed during the first cycle that probably arose from the
anodic oxidation of the salen thiophene ligand. This peak vanished
after a few scans (5–10) and the current of the second positive peak
at a potential of about 1.00 V quickly reached stable values during
the successive potential scans. The absence of variation indicated
that the oligomeric thiophene–salen ligand formed at the electrode
was insulating. A negative peak appeared at +0.7 V corres-
ponding to the reduction of the oxidized product at the electrode
surface. Finally after about five cycles a perfectly stable redox
system was observed. This cyclic voltammetry can be explained by
the formation of non-conjugated short oligomers such as dimers
or trimers. Electropolymerization of metal-free salen–thiophene
yielded thus species that are not much more conjugated than the
initial monomer. Consequently, the resulting electrodeposited
material is oxidized at the same potential, probably corres-
ponding to the oxidation of the thiophene–phenol ring, which
is independent of the chain length in a non-conjugated polymer.
A preparative electrochemical oxidation of ligand 4a led indeed
to a soluble species in THF. Maldi-TOF analyses confirmed the
formation of only dimers in a low amount.
Results and discussion
Monomers synthesis
We have developed a general strategy for the efficient synthesis
of chiral salen derivatives modified by thiophene groups on the
5,50-position of the phenolic rings (Scheme 1).4 Several varia-
tions were introduced to the general structure of the chiral
salen–thiophene based ligands. The electrochemical behaviour
of these monomers and of the corresponding polymers was
studied according to the structural variations. Using this
synthetic strategy, were prepared various salicylaldehyde
precursors 3. The functionalization introduced on the thiophene
moieties allowed the subsequent evaluation of steric and/or
electronic effects. Finally the aldehydes were condensed with a
chiral diamine to form the corresponding new chiral ligands 4
(Scheme 1).
We then synthesized the corresponding chromium complexes,
as monomers for electrochemical polymerization. All chromium
complexes were obtained according to the same procedure
(Scheme 2) and were isolated after extraction without any
purification as powders with nearly quantitative yields. The
complexation was carried out using chromium chloride (CrCl2)
under inert atmosphere to form chromium(II) complexes,
then a simple air oxidation led to the desired chromium(III)
complexes.7a
In contrast, when the electropolymerization was carried out
from the corresponding complexes, the evolution of the voltammo-
grams was consistent with the formation of conductive organic
polymers at the electrode surface. We have previously reported the
oxidative electropolymerization of various thiophene–salen chiral
Scheme 1 Chiral salen ligands modified by various substituted thiophene
derivatives.
Fig. 1 Electropolymerization of the chiral salen ligand 4a.
c
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complexes and contrarily to their corresponding ligands the
polymerization proceeded smoothly in each case.4 In order to
find the optimal conditions to obtain a more electroactive
redox response, the factors affecting the electroactivity of the
polymers such as the scan rate, potential cyclic limits and
number of cycles were examined. The best conditions to form
the polymer film on platinum or glassy carbon electrode
surfaces were found for a continuous potential sweep between
0 and 1.5 V/SCE for more than 20 cycles at different scan rates.
The continuous current increased in the cyclic voltammetric peaks
during repeated potential scans from 0.0 to 1.5 V indicating film
growth at the electrode surface. Visual inspection of the electrode
also showed the deposition of a dark green-black film.
Fig. 3 Analysis of the polymer poly-5e coated electrode in CH3CN,
nBu4NBF4 (0.1 M), v = 100 mV sꢀ1
.
For every salen–thiophene complex the polymerization led to
the formation of robust films at the platinum electrode surface. As
illustration, Fig. 2 depicts the evolution of consecutive cyclic
voltammograms on a platinum electrode in dichloromethane
solution containing 5 ꢁ 10ꢀ3 M of thiophene–salen complex 5e.
A gradual increase in the amplitude of cyclic voltammetric peaks
was observed as a consequence of consecutive potential cycling,
indicative of the electropolymerization of thiophene–salen
complex 5e and subsequent deposition of a conductive polymeric
film at the electrode.
placed in the electrolytic media free of monomers. The
voltammograms depicted in Fig. 3 resulted from more than
fifty potential scans between 0.0 and 1.3 V. This electro-
chemical behaviour indicated a reversible redox system with-
out variation of the intensity and clearly proved the formation
of a chemically and electrochemically very stable film at the
platinum electrode surface. The modified electrode transferred
to clean electrolyte solution exhibits only one redox couple
with a diffusional profile. Every chromium complex showed
the same cyclic voltammogram profiles for the electrochemical
polymerization and for its electroactivity when it was analyzed. In
Table 1, the characteristic redox potentials Eox and Ered for each
complex are reported according to its structure. The recorded
potentials are in agreement with the electronic effects of each
substituent introduced on the thiophene core. Thus, when
thiophene is substituted by ethylene dioxide at positions
3,4 (EDOT 5f) its oxidation potential was strongly lowered,
0.56 V/SCE instead of 1.10 V for nonsubstituted thiophene.
Alkyl chains such as octyl or cyclopentyl (5b, 5c and 5d)
introduced on positions 3 or 4 have the same influence with
a slight lowering of the potential around 200 mV compared to
non-substituted thiophene.
The coated electrodes were rinsed thoroughly and further tested
under cyclic voltammetry conditions in a fresh acetonitrile solution
containing 10ꢀ1 M nBu4NBF4 as supporting electrolyte without
any monomer. The effect of scan repetition on the electroactivity of
the obtained polymer film was studied between 0.0 and 1.3 V vs.
SCE at a scanning rate of 100 mV sꢀ1. In this potential range, the
polymer exhibited a pair of coupled peaks at 0.82 and 0.7 V as
shown in Fig. 3. The peaks observed correspond to the electro-
chemical behaviour of the polymer formed at the electrode surface,
The modification of the ligand by the introduction of a
bisthiophene unit (5e) emphasized this decrease in the
potential totally in agreement with a more conjugated hetero-
aromatic system. The change of the chiral bridge, using
diphenylethylene diamine (5g) instead of cyclohexane diamine
in 5a, did not apparently lead to any change in the oxidation
potential. In the case of a chemically stable redox species
and an electrochemically reversible system, the voltammetric
behavior of an electrode coated with a monolayer of a redox
couple is characterized by symmetrical anodic and cathodic
waves with a difference in the peak potential values (DE) close
to zero.
Table 1 Structure effect of the complex on the potentials of oxidation
and reduction
Polymer
Eox (V/SCE)
Ered (V/SCE)
DE
Qa/Qc
Poly-5a
Poly-5b
Poly-5c
Poly-5d
Poly-5e
Poly-5f
Poly-5g
1.10
0.89
0.89
0.95
0.82
0.56
1.08
0.56
0.70
0.59
0.76
0.73
0.51
0.52
0.54
0.19
0.30
0.19
0.09
0.05
0.56
1.18
1.26
1.13
0.93
1.15
1.04
1.02
Fig. 2 Anodic polymerization of chromium complex 5e (0.05 M)
with nBu4NBF4 (0.1 M) in CH2Cl2, v = 100 mV sꢀ1
.
c
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The DE values are generally dependent on charge-transfer
kinetics, the scan rate and mass transport for relatively thicker
films. In our case, for all films, the cyclic voltammetry waves
exhibited peak potential separations between 50 and 560 mV
at 100 mV sꢀ1 according to the structure modifications and
diffusional shape, thus indicating a relatively slow charge
transfer rate from the interface between the electrode and
the redox centers through the film.
in the particle size, ranging between 11 and 19 mm. Finally,
after 15 scans of potential (Fig. 4c) a thickening of the deposit
is observed. The pores are filled with growing polymers until
complete obstruction and a smooth surface is formed equivalent
to a conventional film of conducting polymer deposited at the
electrode. Moreover, when this deposit is dried with air to
eliminate solvents from polymerization, the film is cracked and
detached from the electrode (Fig. 4d). Based on these observa-
tions, it seems clear that the conditions of polymerization will
influence the morphology of the polymer. However to obtain an
efficient heterogeneous catalyst, porosity or specific surface area
are important factors. For example, porosity increases the
specific surface area and is therefore a highly valued property
in polymerized catalysts.
Another characteristic value of the stability of the polymers
is the charge Qa/Qc ratio. These Qa/Qc ratios are reported in
Table 1, and for all polymers, Qa/Qc was found to be close to
1, demonstrating the particular stability of those materials able
to exchange reversibly a similar quantity of charge.
We have performed the oxidative electropolymerization of
these new chiral thiophene–salen complexes at a preparative
scale, for the formation of insoluble polymers recovered as
black powders from the electrode surface. The electrosynthesis
was performed at a constant current of 50 mA in an undivided
electrochemical cell fitted with two vitreous carbon (16 cm2)
electrodes in Bu4NBF4 2.0 ꢁ 10ꢀ2 M in CH3CN or CH2Cl2 as
electrolytic medium. The anode potential was monitored by a
saturated calomel electrode (SCE) during the electrolysis. After
more than 2 hours the vitreous carbon anode was recovered and
the deposited polymer removed from the electrode surface. The
residue was washed with various solvents and dried under vacuum
before their use in asymmetric catalysis.
SEM analyses
Scanning electron microscopy (SEM) analyses were performed
to find some information about the morphology and the
strength of the interaction between the electrode and the
polymeric material.
This SEM study of a polychromium–salen complex-coated
electrode was realized by delimiting three areas on a vitreous
carbon electrode, on which the number of potential scans
between 0.0 and 1.3 V/ECS were varied. The electrode surface
modification was carried out using the conditions optimised
for complex 5a electropolymerization.
SEM analysis showed that each electrode was completely
coated with the polymer. The first one obtained after five scans
(Fig. 4a) showed dispersed particles on the electrode surface
with an average particle size of about 500 nm, leading to a
porous structure.
Asymmetric heterogeneous catalysis
These new insoluble polymers, based on chromium complexes,
were tested for their ability to promote successfully hetero-
geneous catalysis in various asymmetric transformations.
As most accessible monomeric chromium complex, 5a was
evaluated in numerous asymmetric reactions and its efficiency
was compared to the results described by the group of Jacobsen
in the presence of the analogous tert-butyl substituted
complex.5,7,23,24 We particularly performed hetero Diels–Alder
reactions,10a nitroaldol transformations10c and epoxides ring
opening with trimethylsilylazide,10b for which high activities and
enantioselectivities were reached, albeit with slightly diminished
values compared to those obtained using the Jacobsen complex.
Replacing the tBu-groups on the salen core by thiophene ones led
thus to a detrimental effect probably due to different electronic
and steric constraints. Correspondingly, the efficiency of poly-5a
was previously evaluated and we were able to report its high
efficiency in those reactions, in terms of both activity and
enantioselectivity. For instance, the asymmetric hetero Diels–Alder
reaction of various aldehydes was performed and poly-5a was
nicely reused in successive catalytic cycles for up to 15 times. This
polymer was then successfully used in a multi-substrate procedure;
with the same batch of poly-5a twenty runs were performed
leading to the synthesis of ten structurally varied pyranones with
stable activities and enantioselectivities. We also described the first
use of a chiral heterogenized salen complex active in the Henry
reaction between various aldehydes and nitromethane. The same
heterogeneous catalystpoly-5a was used five times with a slight loss
of yield and enantioselectivity. We have finally recovered the same
catalyst batch (poly-5a) in different consecutive epoxides ring
After 10 scans (Fig. 4b), the polymer growth is obvious with
the modification of the surface morphology due to an increase
Fig. 4 SEM images of anodic polymerization of chromium complex
5a (0.05 M) with nBu4NBF4 (0.1 M) in CH2Cl2; v = 100 mV sꢀ1
running between 0 and 1.2 V vs ECS. (a) After 5 potential scans, (b)
after 10 potential scans, (c) after 15 potential scans and (d) polymer
recovered from the electrode.
c
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opening reactions with nitrogen-based nucleophiles leading
however to poor results, compared with the values obtained
under homogeneous conditions. This is probably due to the
structure of the polymeric catalyst itself, with the chromium
coordinating sites arranged in a pearl necklace manner. The
essential position of the catalytic sites for an optimal cooperative
bimetallic pathway is possibly, in this case, not feasible.
Analogous poly-5b and poly-5e were therefore also engaged in
those diverse reactions and their ability to promote asymmetric
catalysis is here discussed, in comparison with their homo-
geneous analogues and poly-5a.
Table 3 Henry reaction
Entry
Cat (mol%)
t/h
Solvent
Yielda (%)
eeb (%)
1
2
3
4
5
6
5a (2)
5b (2)
Poly-5a (4)
Poly-5b (4)
(1st reuse)
(2nd reuse)
24
24
24
24
24
24
CH2Cl2
CH2Cl2
MTBE
MTBE
MTBE
MTBE
74
70
499
67
43
34
54
65
47
46
51
To illustrate the hetero Diels–Alder transformation, the synthesis
of 2-cyclohexyl-2,3-dihydro-pyran-4-one was attempted through
reaction between cyclohexanecarboxaldehyde and Danishefsky’s
diene.25 Jacobsen and co-workers described this reaction yielding
the targeted product with an ee of 93% (71% isolated yield) using
the tetrafluoroborate chromium complex of (R,R)-(ꢀ)-N,N0-
bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine at ꢀ20 1C
in MTBE (Table 2).7a
40
a
b
Isolated yields, ee determined by chiral HPLC analysis.
as an atom economic transformation allowing the synthesis of
nitroalcohol building blocks, valuable synthons for the preparation
of biologically active compounds.26 The homogeneous reaction
between o-anisaldehyde and nitromethane was conducted in
dichloromethane at room temperature and both catalysts
5a and 5b delivered the expected nitroalcohol in satisfying yield
but moderate enantioselectivity. In this case again, catalyst 5b
was the most enantioselective promoter. As a comparison, this
Delightfully complexes 5b and 5e were also efficient catalysts
for the synthesis of 2-cyclohexyl-2,3-dihydro-pyran-4-one and
were furthermore more active species than 5a since the targeted
compound was isolated in high yield within the same reaction
time. The highest enantioselectivity value was obtained with 5b as
catalyst and reached 78% ee. The increase in the steric bulk on the
thiophene ring arising from the introduction of the octyl group
was thus beneficial in terms of enantioselectivity. The corres-
ponding polymeric complexes catalyzed also the transformation
and as previously observed in the case of the use of poly-5a this
heterogeneous procedure afforded the desired product albeit with
slightly diminished ee values with regards to the homogeneous
cases. The polymers were nevertheless very active and could be
reused twice with relatively stable efficiencies. As for its homo-
geneous counterpart 5b, poly-5b was the most enantioselective
catalyst of the polymeric species.
_
transformation has been studied by Skarzewski and coworkers
using the classical tBu-salen modified Jacobsen’s catalyst. They
performed the reaction in dichloromethane at ꢀ78 1C and
isolated 1-(2-methoxy-phenyl)-2-nitro-ethanol in 88% yield
and 73% ee (Table 3).24a
The heterogeneous catalysis was performed in MTBE as
best solvent for this procedure as was previously optimized in
the presence of poly-5a.10c The n-octylthiophene-substituted
chromium salen polymer poly-5b catalysed the same reaction
and delivered 1-(2-methoxy-phenyl)-2-nitro-ethanol in 67%
isolated yield and 47% ee. Surprisingly, these values are worse
than those obtained through poly-5a catalysis, contrarily to
the results arising from the homogeneous case. The hetero-
geneous catalyst was nevertheless reused two times showing a
nice stability, at least if one considers the enantioselectivity
values. Only few catalytic systems have been described in the
literature that can promote successively and with a good
stability the asymmetric Henry reaction. Electrogenerated
poly-salen complexes described here showed thus promising
results in this context.
Since poly-5b proved to be more efficient than the unsubstituted
poly-5a in the hetero Diels–Alder transformation, we wondered if
we could demonstrate a broad activity for this catalytic species in
other transformations. The Henry reaction was thus investigated
Table 2 Hetero Diels–Alder reaction
Salen derivatives have furthermore proven their high ability to
catalyze the epoxides ring-opening with a variety of nucleophiles.27
Jacobsen et al. reported indeed that the tBu-substituted chromium
chloride Jacobsen complex promoted the ring opening of
7-oxa-bicyclo[4.1.0]heptane with trimethylsilylazide as nucleo-
phile and delivered the targeted product in 80% yield and with
88% ee when the reaction was conducted in ether.5a The
thiophene-modified derivatives 5a and 5b were analogously
tested, but the reaction was performed at 50 1C in MTBE
leading successfully to (2-azido-cyclohexyloxy)-trimethyl-
silane in high yield and promising enantioselectivities.
Complex 5b led again to the best values. We described earlier
that the use of poly-5a as heterogeneous catalyst under the
same conditions yielded the opened product in high yield
albeit in its nearly racemic form. We assumed in this case that
the arrangement of the catalytic sites in the linear polymeric
Entry
Cat (mol%)
t/h
Temp./1C
Yielda (%)
eeb (%)
1
2
3
4
5
6
7
8
9
10
5a (2)
5b (2)
5e (2)
Poly-5a (4)
Poly-5b (4)
(1st reuse)
(2nd reuse)
Poly-5e (4)
(1st reuse)
(2nd reuse)
22
23
23
21
24
24
24
23
23
23
20
20
20
25
25
25
25
25
25
25
79
92
499
61
89
88
93
71
74
72
78
71
60
72
65
69
62
63
62
80
a
b
Isolated yields, ee determined by chiral HPLC analysis.
c
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Table 4 Ring opening of 7-oxa-bicyclo[4.1.0]heptane with trimethyl-
silylazide as nucleophile
(poly-5a, poly-5b and poly-5e) in various enantioselective
transformations demonstrated the particular stability of these
new chiral polymers with a potential application in very original
procedures i.e. multisubstrates or multireactions heterogeneous
asymmetric catalysis. Finally, this work may open important
perspectives for new enantioselective catalytic processes by
preparing modified surfaces by electrooxidation, to investigate
supported- or fixed bed catalysis.
Entry
Cat (mol%)
t/h
Temp./1C
Yielda (%)
eeb (%)
1
2
3
4
5
6
5a (2)
5b (2)
Poly-5a (4)
Poly-5b (4)
(1st reuse)
(2nd reuse)
24
24
24
72
72
72
50
50
50
20
20
20
70
97
85
60
47
74
59
74
5
34
29
25
Experimental section
Chemicals, instrumentation and analysis
a
b
Isolated yields, ee determined by chiral HPLC analysis.
All reactions were conducted under an argon atmosphere in
oven-dried glassware with magnetic stirring. Solvents were
distilled before use: THF from sodium metal/benzophenone,
CH3CN and CH2Cl2 from calcium hydride. 3-tert-Butyl-5-
bromo-2-hydroxy-benzaldehyde was synthesized according to
ref. 4. 1H NMR and 13C NMR spectra were recorded on either
a Bruker AM 360 (360 MHz), AM 300 (300 MHz) or AM 250
(250 MHz) instrument with samples dissolved in CDCl3 and
data are reported in ppm with the solvent signal as reference
(7.27 ppm for 1H NMR and 77.0 ppm for 13C NMR). Optical
rotations were measured in solution in 10 cm cells at the
sodium D line using a PERKIN ELMER 241 polarimeter. IR
spectra were recorded as KBr disks using a PERKIN-ELMER
spectrometer. Mass spectra were recorded on a micrOTOF-q
Brucker Daltonics spectrometer. Elemental analyses were
performed by the Microanalytical laboratory of the Institut
de Chimie des Substances Naturelles, Gif sur Yvette. Electro-
chemical measurements were performed using a EG&G Princeton
Applied Research (model 362) scanning potentiostat equipped
with an IFELEC (IF 2502) recorder, in an undivided three
electrode cell containing a Pt working electrode, a Pt counter
electrode and a saturated calomel electrode (SCE) as reference.
The solutions were degassed by argon bubbling prior to electro-
polymerization. The cell was stored in a dry atmosphere and
flushed with argon before the electrochemical experiments.
Energy Dispersive Spectroscopy (EDS) was performed on a
Field Emission Gun Scanning Electron Microscope (FEG-SEM)
Zeiss Supra 55 VP. The EDS is a SAMx IDFix analysis package
using a new Silicon Drift detector (SDD) working at 15 kV.
Thiophene–salen chromium complex 5a was synthesized
according to ref. 4. 5b, 5d, 5f and 5g were synthesized as
previously reported.10a The synthesis of complex 5c was also
previously reported.10c
structure was probably not suitable for the necessary cooperative
bimetallic pathway of this demanding reaction. We therefore tested
the use of poly-5b possessing sterically hindering substituents
on the thiophene rings that could thus have led to a different
global structure compared to that of the unsubstituted analogue
poly-5a (Table 4).
The same reaction was tested under milder conditions at
room temperature and the reaction time was raised up to 72 h
to obtain reasonable conversions. The azide-opened epoxide
was in this case nicely obtained with up to 34% ee and the
heterogeneous polymeric catalyst was reused two more times
delivering again the expected product, even if the selectivity
values suffered from a slight decrease.
Preparative electropolymerization afforded therefore insoluble
chiral salen-based complexes that could interestingly promote
asymmetric transformations. They mainly retained the activity of
their homogeneous counterparts but furnished the targeted
products with slightly diminished enantioselectivities. The modifi-
cation of the thiophene structure of the polymeric catalyst has been
shown to have an influence on the enantioselectivity of the tested
asymmetric reaction. Furthermore these catalysts were easily
recovered by simple filtration and could be reused in a subsequent
transformation showing their good stability.28
Conclusions
We have prepared by electrochemical oxidation a range of poly-
mers starting from several structurally different chromium com-
plexes modified by thiophene derivatives. The polymerization took
place only starting from the complexes and not from the ligands,
probably due to the loss of the conjugation in the salen. Under
these conditions the metal center acted as an electronic bridge
rendering the polymerization feasible. Many modifications were
made onto the structure of ligand and all the corresponding
chromium complexes were polymerized efficiently. The electro-
chemical analyses revealed particularly stable polymers with redox
responses in agreement with the electronic effects of the introduced
substituents. SEM analysis enabled to appreciate the evolution of
the material deposited at the electrode surface during the electro-
polymerization by cyclic voltammetry. These SEM analyses
suggested the importance of the electropolymerization conditions
especially to obtain efficient porous materials for heterogeneous
asymmetric catalysis. The application of several polymers
Synthesis and procedures
Synthesis of 3-tert-butyl-2-hydroxy-5-(5-(thiophene-2-yl)thiophene-
2-yl)benzaldehyde (3e). A Schlenk tube was charged with 3-tert-
butyl-5-bromo-2-hydroxybenzaldehyde (416 mg, 1.62 mmol),
5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,20-bithiophene
(945.7 mg, 3.24 mmol), Pd(PPh3)4 (280 mg, 0.24 mmol) and
Na2CO3 (257 mg, 2.43 mmol) and was maintained under argon
by successive vacuum–argon cycles (3 h). Thoroughly degassed
DME (3 mL) and degassed water (1 mL) were introduced with a
cannula in the Schlenk. The mixture was heated at 100 1C for
24 h. Water (20 mL) was added and the aqueous layer was
extracted with CH2Cl2. The organic layer was dried over MgSO4
c
1404 New J. Chem., 2012, 36, 1399–1407
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and the solvents were removed under reduced pressure. The
residue was purified by flash chromatography on silica gel
(heptane/diethylether 98/2) to afford 3-tert-butyl-2-hydroxy-5-
(5-(thiophene-2-yl)thiophene-2-yl)benzaldehyde 3e (537 mg,
quantitative yield) as a yellow solid.
1H NMR (360 MHz, CDCl3, d): 11.87 (s, 1H, OH), 9.91
(s, 1H, H9), 7.77 (s, 1H, H6 ou H4), 7.58 (s, 1H, CH), 7.26–7.21
(m, 2H, CH), 7.15 (s, 2H, H6 ou H4), 7.07–7.05 (m, 1H, CH),
1.52 (s, 9H, H8); 13C NMR (90 MHz, CDCl3, d): 197.0 (C9),
160.7 (C2), 141.9 (Cq), 139.1 (Cq), 137.2 (Cq), 136.3 (Cq),
131.6 (CH), 128.5 (CH), 127.9 (CH), 125.6 (Cq), 124.5 (CH),
124.4 (CH), 123.6 (CH), 123.1 (CH), 120.6 (Cq), 35.0 (C7),
29.1 (C8); EIMS (m/z (%)): 342 (100) [M+], 327 (26)
[M+–CH3], 299 (12), 149 (17); HRMS (ESI, m/z
(%)):[M+–H] calcd for C19H17O2S2 (M–H): 341.0670, found
341.0673; mp: 97–99 1C.
Electropolymerization of the chromium–thiophene–salen
complex (poly-5e). Complex 5e (340 mg, 0.5 mmol) was placed
in an undivided electrochemical cell fitted with a platinum
cathode and a platinum grid (2.25 cm2) as anode. The anode
potential was monitored versus a saturated calomel electrode
(SCE) throughout the electrolysis. nBu4NBF4 (0.03 M in 15 mL
CH2Cl2) was used as a supporting electrolyte and the electrolysis
was performed at a constant current of 50 mA during 2 h. The
platinum grid was recovered with the polymerized complex.
Some insoluble material also settled at the bottom of the cell.
The deposited polymer was used as an insoluble catalyst after
removal from the support and several washings with acetonitrile
and dichloromethane. This residue was dried under vacuum and
used without further purification for the asymmetric catalysis.
Poly-5e was isolated in 59% yield.
Representative procedure for hetero-Diels–Alder reactions
Synthesis of (S,S)-(N,N0-bis(3-tert-butylsalicylidene-5-(thio-
phene-2-yl))-cyclohexane-1,2-diamine (4e). This product was
obtained using 3-tert-butyl-2-hydroxy-5-(5-(thiophene-2-yl)-
thiophene-2-yl)benzaldehyde 3e and (S,S)-cyclohexane-1,2-
diamine. The diamine (0.52 eq.) was added to a solution of
the targeted aldehyde in ethanol (0.07 M) with continuous
stirring, and the mixture was heated at 60 1C for 5 h. The
reaction was cooled to room temperature. The solvents were
removed under reduced pressure and the residue was purified
by flash chromatography on silica gel (heptane/diethylether 9/1) to
afford (S,S)-(N,N0-bis(3-tert-butylsalicylidene-5-(thiophene-2-yl))-
cyclohexane-1,2-diamine 4e (62%) as a yellow solid.
Homogeneous conditions. A Schlenk tube was charged with
catalyst 5a, 5b or 5e (2 mol%) and thoroughly maintained
under argon by three successive vacuum–argon cycles. MTBE
(200 mL), the aldehyde (1 mmol) and Danishefsky’s diene (234 mL,
1.2 mmol) were then introduced through a syringe. The resulting
solution was stirred at room temperature for the specified amount
of time. It was then diluted with CH2Cl2 (2 mL), treated with a
drop of trifluoroacetic acid and further stirred for another 30 min.
The solvents were removed under reduced pressure and the residue
was purified by flash chromatography on silica gel to determine the
yield of the reaction and the enantiomeric excess of the product.
1H NMR (300 MHz, CDCl3, d) 14.06 (bs, 2H, OH), 8.34
(s, 2H, H4), 7.47 (d, J = 1.9 Hz, 2H), 7.20 (dd, J = 1.1 Hz,
5.3 Hz, 2H), 7.15 (dd, J = 1.1 Hz, 3.6 Hz, 2H), 7.09, 7.08, 7.01
(dd, J = 3.8 Hz, 5.3 Hz, 2H), 3.42–3.38 (m, 2H, H1), 2.05–1.65
Heterogeneous conditions. A Schlenk tube was charged with
catalyst poly-5a, poly-5b or poly-5e (4 mol%) and maintained
under argon by three successive vacuum–argon cycles. MTBE
(400 mL), the aldehyde (1 mmol) and Danishefsky’s diene
(1.2 mmol) were then introduced through a syringe. The resulting
suspension was stirred at room temperature for the specified
amount of time. It was then filtered with a filtering syringe and
the residue was washed with MTBE. Then the solvents of the
combined filtrates were partially removed under reduced pressure.
The residue was diluted with CH2Cl2 (2 mL), treated with a drop
of trifluoroacetic acid and stirred for another 30 min. The solvents
were then removed under reduced pressure and the residue was
purified by flash chromatography on silica gel to determine the
yield of the reaction and the enantiomeric excess of the product. In
the Schlenk tube, the polymer catalyst was dried under vacuum for
its reuse and new substrates and solvents were added.
(m, 8H, H2 and H3), 1.47 (s, 18H, H8 ); 13C NMR (62.5 MHz,
0
0
CDCl3, d) 165.4 (C4), 160.3 (C2 ), 143.3 (Cq), 137.9 (Cq), 137.7
(Cq), 135.3 (Cq), 127.8 (CH), 127.2 (CH), 126.8 (CH), 124.4
(CH), 124.1 (Cq), 123.9 (CH), 123.2 (CH), 122.3 (CH), 118.5
0
0
(Cq), 72.2 (C1), 34.9 (C7 ), 32.9 (C2), 29.4 (C8 ), 14.1 (C3);
HRMS (ESI m/z (%)): [M + H]+ calcd for C44H47O2N2S4:
763.2520, found: 763.2524; IR (KBr): n 2931 (w), 2860 (w),
1628 (w), 1474 (s), 1436 (vs), 1271 (vs), 836 (s), 794 (w),
690 (w); mp: 1241C. [a]2D0 ꢀ28.1 (c 1 in CHCl3). UV-vis
(CH3CN): lmax(e) 271 (5000), 351 (12 000).
Preparation of chromium–thiophene–salen complex (5e)
The chiral ligand in dry, degassed THF (0.08 M) was added to
a solution of anhydrous Cr(II)Cl2 (1.15 eq.) in dry, degassed
THF (0.04 M). The resulting brown solution was stirred for
2 h under argon and then exposed to air. Stirring was
continued overnight to give a dark brown solution. It was
diluted with CH2Cl2 and washed with sat. NH4Cl and brine.
The organic layer was dried over MgSO4 and solvents were
removed under reduced pressure to afford the expected
complex as a brown solid in 92% yield.
Representative procedure for Henry reactions
Homogeneous conditions. A Schlenk tube was charged with the
catalyst 5a or 5b (2 mol%) and maintained under an argon
atmosphere by three successive vacuum–argon cycles. DCM
(4 mL), the aldehyde (1 mmol), and nitromethane (2 mL,
37.5 mmol) were introduced. In the case of reactions performed
at low temperature, the mixture was first cooled to the desired
temperature, followed by the addition of a solution of diisopropyl-
ethylamine (1 or 0.5 equiv.) in DCM (4 mL). The resulting solution
was stirred for the specified amount of time. The solvents were then
removed under reduced pressure, and the residue was purified by
flash chromatography on silica gel for the determination of the yield
of the reaction and the enantiomeric excess of the product.
IR (KBr): n 2945, 2857, 2361, 2347, 1621, 1538, 1432, 1411,
1385, 1345, 1323, 1257, 1163, 795, 691; anal. calc. for
C44H44O2N2S4ClCr + THF: C 62.62; H 5.69; N 3.04; S
13.93 found C 63.19; H 5.91; N 2.75; S 14.94%. UV-vis
(CH3CN): lmax(e) 271 (5000), 369 (12 000).
c
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New J. Chem., 2012, 36, 1399–1407 1405

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Heterogeneous conditions. A Schlenk tube was charged with
the catalyst poly-5a or poly-5b (4 mol%), and the procedure is
analogous to that described above. After the resulting suspension
was stirred for the specified amount of time, it was then filtered
with a filtering syringe and the precipitate was thoroughly washed
twice with MTBE. The solvents of the combined filtrates were
removed under reduced pressure, and the residue was purified by
flash chromatography on silica gel for the determination of
the yield of the reaction and the enantiomeric excess of the
product. In the Schlenk tube, the powdered catalyst was washed
with water and MTBE then dried under vacuum and new
substrates and solvents were added for its recycling.
4 A. Voituriez, M. Mellah and E. Schulz, Synth. Met., 2006, 156, 166.
5 (a) L. E. Martinez, J. L. Leighton, D. H. Cartsen and E. N. Jacobsen,
J. Am. Chem. Soc., 1995, 117, 5897; (b) E. N. Jacobsen, Acc. Chem.
Res., 2000, 33, 421.
6 N. J. Kerrigan, H. Mueller-Bunz and D. G. Gilheany, J. Mol.
Catal. A: Chem., 2005, 227, 163.
7 (a) S. E. Schaus, J. Branalt and E. N. Jacobsen, J. Org. Chem.,
1998, 63, 403; (b) K. Aikawa, R. Irie and T. Katsuki, Tetrahedron,
2001, 57, 845; (c) W. Cha"adaj, P. Kwiatkowski and J. Jurczak,
Tetrahedron Lett., 2008, 49, 6810.
8 (a) M. Bandini, P. G. Cozzi, P. Melchiorre and A. Umani-Ronchi,
Angew. Chem., Int. Ed., 1999, 38, 3357; (b) A. Berkessel,
¨
D. Menche, C. A. Sklorz, M. Schroder and I. Paterson, Angew.
Chem., Int. Ed., 2003, 42, 1032.
9 M. Mellah, B. Ansel, F. Patureau, A. Voituriez and E. Schulz,
J. Mol. Catal. A: Chem., 2007, 272, 20.
10 (a) A. Zulauf, M. Mellah, R. Guillot and E. Schulz, Eur. J. Org.
Chem., 2008, 2118; (b) A. Zulauf, M. Mellah and E. Schulz, Chem.
Commun., 2009, 6574; (c) A. Zulauf, M. Mellah and E. Schulz,
J. Org. Chem., 2009, 74, 2242.
11 J.-L. Li, F. Gao, Y.-K. Zhang and X.-D. Wang, Chin. J. Polym.
Sci., 2010, 28, 667.
12 (a) L. Mao, K. Yamamoto, W. Zhou and L. Jin, Electroanalysis
(N. Y.), 2000, 1, 12; (b) L. Mao, Y. Tian, G. Shi, H. Liu, L. Jin,
K. Yamamoto, S. Tao and J. Jin, Anal. Lett., 1998, 31, 1991;
(c) P.-H. Aubert, A. Neudeck, L. Dunsch, P. Audebert,
P. Capdevielle and M. Maumy, J. Electroanal. Chem., 1999,
470, 77; (d) M. F. S. Teixeira and T. R. L. Dadamos, Procedia
Chem., 2009, 1, 297; (e) T. R. L. Dadamos and M. F. S. Teixeira,
Electrochim. Acta, 2009, 54, 4552; (f) J. Tedim, F. Goncalves,
M. F. R. Pereira, J. L. Figueiredo, C. Moura, C. Freire and
A. R. Hillman, Electrochim. Acta, 2008, 53, 6722.
Representative procedure for ring opening epoxides with
trimethylsilylazide
Homogeneous conditions. A Schlenk tube was charged with
the catalyst 5a or 5b (2 mol%) and thoroughly maintained
under an argon atmosphere by three successive vacuum–argon
cycles. MTBE (330 mL) and cyclohexene oxide (1 mmol) were
introduced with a syringe. The resulting solution was stirred at
room temperature and then trimethylsilylazide (197 mL, 1.5 mmol)
was introduced. The mixture was stirred for 72 h. The solvents
were removed under reduced pressure. The residue was purified by
Celite filtration for the determination of the yield of the reaction
and the enantiomeric excess of the product.
13 (a) K. S. Alleman, M. J. Samide, D. G. Peters and M. S. Mubarak,
Curr. Top. Electrochem., 1998, 6, 1; (b) T. Okada, K. Katou,
T. Hirose, M. Yuasa and I. Sekine, Chem. Lett., 1998, 841.
14 (a) L. A. Hoferkamp and K. A. Goldsby, Chem. Mater., 1989,
1, 348; (b) P. Audebert, P. Capdevielle and M. Maumy, New J.
Chem., 1991, 15, 235; (c) P. Audebert, P. Capdevielle and
M. Maumy, New J. Chem., 1992, 16, 697; (d) F. Bedioui,
E. Labbe, S. Gutierrez-Granados and J. Devynck, J. Electroanal.
Chem., 1991, 301, 267; (e) T. R. L. Dadamos and M. F. S. Teixeira,
Electrochim. Acta, 2007, 40, 1852; (f) K. A. Goldsby, J. K. Blaho
and L. A. Hoferkamp, Polyhedron, 1989, 8, 113; (g) P. Audebert,
P. Capdevielle and M. Maumy, Synth. Met., 1991, 43, 3049.
15 (a) K. A. Goldsby, J. K. Blaho and L. A. Hoferkamp, Polyhedron,
1989, 8, 113; (b) C. E. Dahm, D. G. Peters and J. Simonet,
J. Electroanal. Chem., 1996, 410, 163.
16 (a) C. E. Dahm and D. G. Peters, Anal. Chem., 1994, 66, 3117;
(b) C. E. Dahm and D. G. Peters, J. Electroanal. Chem., 1996,
406, 119; (c) K. S. Alleman, S M. J. Samide, D. G. Peters and
M. S. Mubarak, Curr. Top. Electrochem., 1998, 6, 1.
17 (a) M. F. S. Teixeira and T. R. L. Dadamos, Procedia Chem., 2009,
1, 297; (b) T. R. L. Dadamos and M. F. S. Teixeira, Electrochim.
Acta, 2009, 54, 4552.
Heterogeneous conditions. A Schlenk tube was charged with
poly-5a or poly-5b (4 mol%) and placed under an argon
atmosphere by three successive vacuum–argon cycles. MTBE
(330 mL) and the epoxide (1 mmol) were introduced by using a
syringe. The resulting solution was stirred at a given tempera-
ture and then trimethylsilylazide (197 mL, 1.5 mmol) was
introduced. The mixture was stirred for the specified amount
of time and then filtered with a filtering syringe. The solution
was purified by filtration through Celite and the solvents were
removed under reduced pressure. The residue was purified by
flash chromatography on silica gel to give the product, which
was used to determine the yield of the reaction and the
enantiomeric excess. In the Schlenk tube, the catalyst was
dried under vacuum and new substrates and solvents were
added for reuse.
18 J. Tedim, F. Goncalves, M. F. R. Pereira, J. L. Figueiredo,
C. Moura, C. Freire and A. R. Hillman, Electrochim. Acta, 2008,
53, 6722.
Acknowledgements
The Centre National de la Recherche Scientifique, the Ministere de
19 T. Okada, K. Katou, T. Hirose, M. Yuasa and I. Sekine, Chem.
Lett., 1998, 841.
l’Education Nationale de l’Enseignement Supe
´
rieur et de la
Recherche and the program ‘‘Chimie et Developpement Durables’’
´
20 P. Guo and K.-Y. Wong, Electrochem. Commun., 1999, 1, 559.
21 (a) R. P. Kingsborough and T. M. Swager, Adv. Mater., 1998,
10, 1100; (b) R. P. Kingsborough and T. M. Swager, J. Am. Chem.
Soc., 1999, 121, 8825; (c) R. P. Kingsborough and T. M. Swager,
Chem. Mater., 2000, 12, 872; (d) T. Shioya and T. M. Swager,
Chem. Commun., 2002, 1364.
22 (a) J. L. Reddinger and J. R. Reynolds, Synth. Met., 1997, 84, 225;
(b) J. L. Reddinger and J. R. Reynolds, Macromolecules, 1997,
30, 673; (c) J. L. Reddinger and J. R. Reynolds, Chem. Mater.,
1998, 10, 3.
du Centre National de la Recherche Scientifique are kindly
acknowledged for financial support.
Notes and references
1 (a) D. E. Bergbreiter and S. Kobayashi, Chem. Rev., 2009,
109, 257; (b) J. A. Gladysz, Chem. Rev., 2002, 102, 3215. Issue
10 entirely dedicated to recoverable catalysts and reagents;
(c) A. F. Trindade, P. M. P. Gois and C. A. M. Afonso, Chem.
Rev., 2009, 109, 418.
23 P. G. Cozzi and P. Kotrusz, J. Am. Chem. Soc., 2006, 128, 4940.
_
24 (a) R. Kowalczyk, Ł. Sidorowicz and J. Skarzewski, Tetrahedron:
Asymmetry, 2007, 18, 2581; (b) R. Kowalczyk, P. Kwiatkowski,
2 P. G. Cozzi, Chem. Soc. Rev., 2004, 33, 410.
_
3 (a) A. Zulauf, M. Mellah, X. Hong and E. Schulz, Dalton Trans.,
2010, 39, 6911; (b) C. Baleizao and H. Garcia, Chem. Rev., 2006,
106, 3987.
J. Skarzewski and J. Jurczak, J. Org. Chem., 2009, 74, 753.
25 (a) For some rare examples of asymmetric hetero Diels–Alder
reactions promoted by reusable catalysts, see: A. Heckel and
c
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D. Seebach, Helv. Chim. Acta, 2002, 85, 913(b) S. Eno, H. Egami,
T. Uchida and T. Katsuki, Chem. Lett., 2008, 37, 632.
27 For some examples of asymmetric epoxides ring opening reactions with
azides promoted by reusable catalysts, see: (a) S. E. Schaus,
J. F. Larrow and E. N. Jacobsen, J. Org. Chem., 1997, 62, 4197;
(b) C. E. Song, C. R. Oh, E. J. Roh and D. J. Choo, Chem. Commun.,
2000, 1743; (c) B. M. L. Dioos and P. A. Jacobs, J. Catal., 2006,
243, 217; (d) J. Keilitz and R. Haag, Eur. J. Org. Chem., 2009, 3272.
28 We previously proved, in the case of poly-5a, that no leaching
occurred during the recycling procedure (see ref. 10a and b). The
insoluble polymer was indeed added to hetero Diels–Alder or
Henry reaction mixtures, thoroughly stirred and subsequently
removed by filtration. The recovered filtrates showed almost no
catalytic activity when substrates were added, confirming thus that
those transformations were promoted by the catalyst under hetero-
geneous conditions.
26 For recent examples of asymmetric Henry reactions promoted by
reusable catalysts, see: (a) M. Bandini, M. Benaglia, R. Sinisi,
S. Tommasi and A. Umani-Ronchi, Org. Lett., 2007, 9, 2151;
(b) M. Gaab, S. Bellemin-Laponnaz and L. H. Gade, Chem.–Eur. J.,
2009, 15, 5450; (c) T. Nitabaru, A. Nojiri, M. Kobayashi,
N. Kumagai and M. Shibasaki, J. Am. Chem. Soc., 2009,
131, 13860; (d) J.-M. Lee, J. Kim, Y. Shin, C.-E. Yeom, J. E. Lee,
T. Hyeon and B. M. Kim, Tetrahedron: Asymmetry, 2010, 21, 285;
(e) V. J. Mayani, S. H. R. Abdi, R. I. Kureshy, N. H. Khan, A. Das
and H. C. Baja, J. Org. Chem., 2010, 75, 6191; (f) R. I. Kureshy,
A. Das, N. H. Kahn, S. H. R. Abdi and H. C. Bajaj, ACS Catal.,
2011, 1, 1529.
c
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