Robust and Recyclable Self-Supported Chiral Nickel Catalyst for the Enantioselective Michael Addition

Article abstract of DOI:10.1002/adsc.201600083

A simple chemical modification of a chiral diamine ligand may produce a robust and recyclable enantioselective catalyst. Metallopolymers based on chiral cyclohexyldiamine-containing ditopic ligands and nickel(II) complexes have been readily prepared and a

Full text of DOI:10.1002/adsc.201600083

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DOI: 10.1002/adsc.201600083
Robust and Recyclable Self-Supported Chiral Nickel Catalyst for
the Enantioselective Michael Addition
Damien Bissessar,a Thierry Achard,a and StØphane Bellemin-Laponnaza,b,*
a
Institut de Physique et Chimie des MatØriaux de Strasbourg, UniversitØ de Strasbourg, CNRS UMR 7504, 23 rue du
Loess, BP 43, 67034 Strasbourg Cedex 2, France
Fax: (+33)-3-8810-7246; e-mail: bellemin@unistra.fr
University of Strasbourg Institute for Advanced Study (USIAS), 5 allØe du GØnØral Rouvillois, 67083 Strasbourg, France
b
Received: January 20, 2016; Revised: March 2, 2016; Published online: June 2, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600083.
Abstract: A simple chemical modification of a chiral bled this chemistry to be carried out on the bench
diamine ligand may produce a robust and recyclable without the use of any air-free techniques and with
enantioselective catalyst. Metallopolymers based on non-degassed solvents. Finally, the nature of the cata-
chiral cyclohexyldiamine-containing ditopic ligands lyst was studied by non-linear effect experiments,
and nickel(II) complexes have been readily prepared giving a negative non-linear effect (NLE) as a conse-
and applied in catalytic enantioselective Michael ad- quence of an in situ decrease in the ee of the active
ditions of 1,3-dicarbonyl compounds to nitroalkenes. species consistent with the trapping of homochiral
High yields and good enantioselectivities have been aggregates.
obtained and the catalytic systems have been recy-
cled up to 11 times without loss of either activity or
enantioselectivity at
a low catalyst loading of Keywords: catalyst recycling; diamine ligand; enan-
0.75 mol%. Moreover, the nickel metallopolymers tioselective catalysis; Michael addition; multitopic li-
were found to be air- and moisture-stable, which ena- gands
Introduction
egies have been investigated; however none of them
has been completely satisfactory. For example, the im-
The preparation of enantiomerically pure products is mobilization of the catalyst on a solid support or the
a continuous demand in particular in the fragrance use of non-conventional media have demonstrated
and pharmaceutical industry. Although the asymmet- good performances but very few are really competi-
ric synthesis or the synthesis of racemic compounds tive with their homogeneous analogues.[3]
followed by chiral separation remain predominant in
In this context, the dynamic self-supported catalysts
the production of these compounds, the enantioselec- which aim to combine the advantages of both tech-
tive catalysis offers significant potential advantages niques (homogeneous and heterogeneous) are attrac-
over these classical procedures. A chiral catalyst, tive.[4] The system remains aggregated – and possibly
being continually regenerated, can produce many heterogeneous – at its resting state and is dissolved in
molecules of chiral product. Indeed, enantiopure mol- the reaction medium at its catalytically active state
ecules are produced in nature by such a chirality (release-and-capture strategy, Figure 1). Such dynamic
transfer from enzymatic catalysts and today some self-supported catalysts may be achieved by the com-
chiral catalytic processes have been applied on an in- bination of a multitopic ligand and a metal salt pre-
dustrial scale. However, most of the chiral homogene-
ous systems frequently show activities that are not
sufficient for a valuable scale-up application and they
suffer from two main drawbacks which are the possi-
ble product contamination and the inability to reuse
the chiral catalyst.
For all these reasons, the development of recovera-
ble enantioselective catalytic systems is nowadays
considered as an important objective.[1,2] Various strat- Figure 1. Principle of dynamic self-supported catalysts.
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Figure 2. Structures of chiral nickel complexes: Evans cata-
lyst (R=Bn: 1) and self-supported catalyst (R=Bn: 4; R=
p-OMe-Bn: 4OMe).
cursor. In this area, highly symmetrical bis(oxazoline)
ligands have proved to be interesting tools to develop
such self-supported chiral catalysts.[5,6] They have
been applied with success in several copper-based cat-
alytic reactions such as cyclopropanation,[7] allylic oxi-
dation,[8] a-hydrazination,[9] benzoylation[10] or Henry
reaction.[10,11]
In this work, we demonstrate that the dynamic self-
supported catalyst approach in not limited to copper
salts and oxazoline-based ligands. In 2007, chiral bis-
(cyclohexyldiamine)-based Ni(II) [NiL2] complexes
have been reported by Evans and co-workers as
highly efficient catalysts for the enantioselective Mi-
chael addition of 1,3-dicarbonyl substrates to nitroal-
kenes.[12] Mechanistic studies suggested that the sub-
strate displaces a diamine ligand thus generating an
active catalyst [NiL] that contains only one diamine
ligand. With these observations, we successfully de-
Scheme 1. Preparation of the chiral ditopic ligands and their
corresponding complexes. Reagents and conditions: (i) ter-
ephthaldehyde (0.5 equiv.), MeOH, 4 h, room temperature
(87%); (ii) NaBH4, MeOH, 4 h, 08C to room temperature
(93%); (iii) TFA, CH2Cl2, 1 h (quant.); (iv) benzaldehyde,
MeOH, 4 h, room tremperature, followed by NaBH4,
MeOH, 4 h, 08C to room temperature (70–92%); (v) NiBr2,
CH3CN reflux, 5 h (79–80%).
signed
a
chiral ditopic cyclohexyldiamine-based
ligand that generates a dynamic self-supported cata- dride to obtain the 1,4-phenylenebis(methylene) de-
lyst with nickel(II) (Figure 2). Our investigations rivative (2) in 93% yield. TFA deprotection followed
showed that the self-supported catalyst is robust and by benzaldehyde, p-anisaldehyde or p-nitrobenzalde-
air stable and could be recycled up to 11 times with- hyde condensation and reduction with sodium boro-
out any loss of activity and enantioselectivity.[13]
hydride gave the expected ditopic ligands 3, 3OMe, 3NO2
in 70–92% yield. The overall yields for the sequence
range from 57% yield for 3OMe and 74% for 3NO2
.
Results and Discussion
The preparation of nickel(II) complexes was done
by reaction of the desired ditopic ligand 3 and 3OMe
with one equiv. of NiBr2 in refluxing acetronitrile.
After cooling to room temperature, an off-white solid
Synthesis and Characterization of the Catalyst
A very easy and straightforward procedure has been could be filtered, washed with diethyl ether and dried
developed for the synthesis of the chiral ditopic cyclo- which correspond to metallopolymer 4 and 4OMe, re-
hexyldiamine-based ligands (Scheme 1). (1R,2R)-cis- spectively.[14] The 1:1 ligand-to-metal stoichiometry
N-Boc-1,2-cyclohexanediamine was treated with consistent for a metallopolymer was confirmed by ele-
0.5 equiv. of terephthalaldehyde in dry methanol mental analysis. Note that the mononuclear nickel
giving the corresponding bis-imine product (87% complex Ni(II)-bis[(R,R)-N,N’-dibenzylcyclohexane-
yield) which was then treated with sodium borohy- 1,2-diamine]Br2 (1) has been synthesized and used for
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comparison purpose, since it is structurally similar to ed in an increase in enantiomeric excess up to 92%
the polytopic ligands developed in our study.
(entries 7 and 8); we thus fixed these experimental
conditions for further studies: 0.75 mol% Ni, CH2Cl2
as solvent at 48C.
Catalytic Properties of the Catalyst
We next studied the possibility of recycling the cat-
alyst (4). The results are summarized in Table 2. For
As a test reaction we examined the activity of our cat- the catalyst recycling, diethyl ether was added to the
alysts in the enantioselective Michael addition of die- reaction media at the end of each run, leading to the
ster malonates to nitroalkenes. From the literature, instantaneous precipitation of the catalytic system.
the nickel mononuclear catalyst (1) displays a good The solid was then recovered, washed with diethyl
activity and enantioselectivity in THF, EtOAc, di- ether, dried and then used again for another catalytic
chloromethane or toluene as solvent, the latter being run. First, only one catalytic run could be conducted
the best choice.[12]
with the monomeric Evans catalyst (1), since the cata-
The reaction of nitrostyrene with dimethyl malo- lytic system could not be recovered at the end of the
nate was used as reference reaction and some repre- reaction. Notably using the metallopolymer (4), we
sentative results are presented in Table 1. All catalytic could perform up to 6 cycles for the reaction of nitro-
tests were conducted in air with non-degassed re- styrene with dimethyl malonate with no significant al-
agent-grade solvents. The metallopolymer (4) was teration in enantioselectivity and yield (92% ee, 99%
found to be completely insoluble in toluene or ethyl conversion for each run; entry 1). The seventh and
acetate and accordingly no conversion was observed eighth sequences displayed a slight decrease of enan-
when carrying the reaction in such a solvent (entries 1 tioselectivity (89% ee and 76% ee, respectively). In-
and 2). On the other hand, good enantiomeric excess- terestingly, the number of cycles was increased (up to
es and activities were observed in THF or dichloro- 11 cycles) while running the reaction at room temper-
methane (83% ee and 85% ee, respectively; entries 3 ature rather than at 48C albeit with a lower enantio-
and 4), which is consistent with a good solubility of selectivity (83–85% ee and conversion >96% for all
the pre-catalyst. Under the same experimental condi- catalytic runs; entry 2). Finally, good results were
tions in CH2Cl2, the Evans catalyst (1) afforded 89% demonstrated for a series of 5 substrates (entries 3–7).
ee with a poorer activity [8 h to go to completion vs. Quantitative conversions and good enantiomeric ex-
3 h with (4)]. The use of the p-methoxy-containing cesses (90–92% ee on average) could be obtained and
metallopolymer (4OMe) did not improve the activity a minimum of 5 consecutive runs could be performed
and enantioselectivity, as displayed in entry 5. Finally,
a decrease of the reaction temperature to 48C result-
Table 1. Enantioselective Michael addition of dimethyl mal-
onate to nitrostyrene.[a]
Entry Cat.
Solvent Temp. Time
[h]
Conv. [%] ee
[%]
1
2
(4)
(4)
toluene r.t.
EtOAc r.t.
no reac-
tion
no reac-
tion
95
98
97
99
99
3
4
5
6
7
8
(4)
(4)
THF
r.t.
5
3
5
8
16
16
83
85
71
89
91
92
CH2Cl2 r.t.
(4OMe) CH2Cl2 r.t.
(1)
(4)
(4)
CH2Cl2 r.t.
THF
48C
CH2Cl2 48C
99
[a]
Experimental conditions: 0.75 mol% based on Ni. Con-
version determined by H NMR. Enantiomeric excesses
were determined by HPLC using a Chiracel AD-H
column.
Figure 3. Pictures of a) metallopolymer (4); b) Evans cata-
lyst (1) and after exposition to air; c) metallopolymer (4)
after 2 weeks; d) (1) Evans catalyst after 2 days (scale bar:
5 mm).
1
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Table 2. Enantioselective Michael addition catalyzed by (4) and evaluation of the recycling.[a]
[a]
Experimental conditions: 0.75 mol% cat. (based on Ni), 16 h, 48C in CH2Cl2. Conversion determined by
1H NMR. Enantiomeric excesses were determined by HPLC using a Chiracel AD-H column.
[b]
Room temperature.
[c]
C6HBr: para-bromophenyl.
[d]
[e]
1
dr=1.05:1 (determined by H NMR), ee of the minor diastereomer given in the Supporting Information.
1
dr=3.5:1(determined by H NMR), ee of the minor diastereomer given in the Supporting Information.
in all cases without any loss of activity and enantiose- served for the compound (4) (images a and c) where-
lectivity. as exposition to air of the mononuclear nickel com-
An important feature of the nickel metallopolymer plex (1) lead to an oily green compound after only
(4) is its high air and moisture stability, which enable two days (images b and d). Moreover, the exposed
this chemistry to be carried out on the bench without catalyst (1) displays very poor catalytic activity (38%
use of any air-free techniques and with non-degassed conversion, 0% ee) whereas no change of activity and
solvents. Indeed, the metallopolymer (4) is air-stable enantioselectivity was observed for exposed catalyst
and can be stored exposed to air at room temperature (4) (91% conversion, 91% ee) and also one month
indefinitely. Figure 3 shows images of the metallopo- later.
lymer (4) and the Evans catalyst (1) when exposed to
Evans and co-workers studied the non-linear effects
air. After two weeks exposure, no change was ob- (NLE) in the reaction of diethyl malonate and nitro-
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styrene catalyzed by the nickel catalyst (1) and they ascribed to a decrease of complexed substrate by bias-
found a linear relationship between the enantiomeric ing the equilibrium in favor of diamine complexes.
excess of the product and the enantiomeric excess of
the catalyst.[12] The authors concluded that the active
species might be a monomeric species, a suggestion Conclusions
that is also consistent with further kinetic studies. We
anticipated that this behavior could be different while In summary, we have described that a simple chemical
using our dynamic self-supported catalyst (4) since modification of a chiral diamine ligand may produce
the active species might be in equilibrium with poly- a highly efficient, robust and recyclable enantioselec-
meric inactive entities (i.e., reservoir effect).[15] tive catalyst. The reaction of the chiral ditopic dia-
Indeed, a significant negative non-linear relationship mine ligand with NiBr2 precursor generated a metallo-
between the ee of the product and that of the ligand polymer which could be stored in air at room temper-
was found as shown in Figure 4. The above asymmet- ature indefinitely. This system was able to act as an
ric depletion is a consequence of an in situ decrease efficient self-supported catalyst in the Michael addi-
in the ee of the active catalyst consistent with a trap- tions of 1,3-dicarbonyl compounds to nitroalkenes
ping of a homochiral aggregate. The shape of the with very easy operational simplicity on the bench
curve may be the result of complex mixtures of mono- with non-degassed reagent-grade solvents and a low
nuclear species, oligomers and polymers, which could catalyst loading. Easy recovery of the catalyst has
not be modelled by the simple mathematical models been demonstrated in up to 11 cycles without loss of
developed by Kagan.[16] As another evidence for the activity or enantioselectivity. Finally, the nature of the
presence of equilibrium between an active monomeric active catalyst has been studied by non-linear experi-
species and catalytically inactive metallopolymer, ments revealing that the mechanism most likely in-
a slower reaction rate (and unchanged enantioselec- volves equilibrium between an active monomeric spe-
tivity) was observed when an excess of ditopic ligand cies and a catalytically inactive homochiral metallopo-
was added in the reaction media [1 equiv. of (3) based lymer. Future efforts will be directed to extend the
on Ni, see the Supporting Information]. This can be scope of the metallopolymer to related nickel-based
catalytic asymmetric reactions.[17]
Experimental Section
General Considerations
All reactions (except catalytic runs) were performed under
an inert atmosphere of argon or nitrogen using standard
Schlenk line techniques. Solvents were purified and de-
gassed by standard procedures. All reagents were used with-
out further purification. (1R,2R)-trans-N-Boc-1,2-cyclohexa-
nediamine was synthesized according a reported proce-
dure.[18] 1H and 13C nuclear magnetic resonance (NMR)
spectra were recorded on a Bruker AVANCE 300 spectrom-
eter using the residual solvent peak as reference (CDCl3:
dH =7.26 ppm; dC =77.16 ppm) at 298 K. HR-MS ESI analy-
ses were performed on a microTOF, Bruker Daltonics.
HPLC analyses were performed on a Gilson apparatus (UV-
VIS156/321 PUMP) with Chiralcel Daicel columns using n-
hexane/i-PrOH eluents.
Synthesis of Di-tert-butyl [(1R,1’R,2R,2’R)-{[1,4-
Phenylene-bis(methylene)]-bis(azanediyl)}-bis(cy-
clohexane-2,1-diyl)]dicarbamate (2)
(1R,2R)-trans-N-Boc-1,2-cyclohexanediamine
(300 mg,
1.4 mmol, 2 equiv.) was dissolved in dry methanol (10 mL).
Terephthaldehyde (0.7 mmol, 94 mg) was added and the re-
action mixture was stirred at room temperature for 4 h. The
resulting precipitate was collected by filtration. The solid
was then dissolved in methanol (20 mL) and the mixture
was cooled to 08C. Sodium borohydride (100 mg) was added
Figure 4. Enantiomeric excess of the product as function of
the enantiomeric excess of the chiral auxiliary (4) showing
a negative nonlinear effect.
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portion wise and the mixture was stirred at 258C for 4 h.
After evaporation of the solvents, water was added and the
mixture was extracted with CHCl3 (3”15 mL). The com-
bined organic layers were dried over sodium sulfate and the
solvent was removed under vacuum to give a brown solid;
yield: 345 mg (93%); [a]2D0: À117.3 (c 1.14, CHCl3). MS
m/z=571.40 [M+H]+; 1H NMR (400 MHz, CDCl3): d=
7.19–7.13 (m, 8H, Harom), 6.78–6.74 (m, 4H, Harom), 3.80 [d,
1J=13 Hz, 2H, -NH(CH2)], 3.73 [d, 1J=13 Hz, 2H,
-NH(CH2)], 3.71 (s, 6H, -OCH3), 3.57 [d, 1J=13 Hz, 2H,
-NH(CH2)], 3.51 [d, 1J=13 Hz, 2H, -NH(CH2)], 2.20–2.14
[m, 4H, -NH(CH)], 2.10–2.04 (m, 4H, HCy), 1.73 (br, 4H,
-NH), 1.67–1.61 (m, 4H, HCy), 1.18–1.10 (m, 4H, HCy), 1.01–
0.89 (m, 4H, HCy); 13C NMR (100 MHz, CDCl3): d=158.46
(Carom), 139.5 (Carom), 133.3 (Carom), 129.2 (Carom), 128.1
(Carom), 113.7 (Carom), 60.9 [-NH(CH)], 60.8 [-NH(CH)], 55.3
(-OCH3), 50.7 [-NH(CH2)], 50.3 [-NH(CH2)], 31.6 (CCy),
25.1 (CCy).
1
(ES+): m/z=531.13 [M+H]+; H NMR (400 MHz, CDCl3):
d=7.19 (s, 4H, Harom), 4.42 (br, 2H, -NH), 3.81 [d, 1J=
1
13 Hz, 2H, -NH(CH2)], 3.59 [d, J=13 Hz, 2H, -NH(CH2)],
3.25 [m, 2H, -NHboc(CH)], 2.18 [m, 2H, -NH(CH)], 2.08–
2.02 (m, 2H, HCy), 2.01–1.97 (m, 2H, HCy), 1.68–1.57 (m,
4H, HCy), 1.37 (s, 18H, Ht-Bu), 1.28–1.12 (m, 4H, HCy), 1.10–
1.06 (m, 2H, HCy), 1.05–0.98 (m, 2H, HCy); 13C NMR
(100 MHz, CDCl3): d=156.0 (C=O), 139.4 (-Carom),128.1
(-Carom), 79.2 (Ct-Bu), 60.4 [-NH(CH)], 55.3 [-NHboc(CH)],
50.2 [-NH(CH2)], 32.9 (CCy), 31.6 (CCy), 28.4 (Ct-Bu), 25.6
(CCy), 24.8 (CCy).
(1R,1’R,2R,2’R)-N1,N1’-[1,4-Phenylenebis(methylene)]-
bis[N2-(4-nitrobenzyl)cyclohexane-1,2-diamine] (3NO2): The
compound 3NO2 was obtained from p-nitrobenzaldehyde ac-
cording to the general procedure as a yellow solid; yield:
168 mg (92%); [a]2D0: À106.8 (c 1.14, CHCl3). MS (ES+):
m/z=601.34 [M+H]+; 1H NMR (400 MHz, CDCl3): d=
8.11–8.03 (m, 4H, Harom), 7.43–7.36 (m, 4H, Harom), 7.20–7.17
1
(m, 4H, Harom), 3.90 [d, J=14 Hz, 2H, -NH(CH2)], 3.84 [d,
General Procedure for (3)
1J=14 Hz, 2H, -NH(CH2)], 3.70 [d, 1J=14 Hz, 2H,
-NH(CH2)], 3.57 [d, 1J=14 Hz, 2H, -NH(CH2)], 2.25–2.15
[m, 4H, -NH(CH)], 2.09–2.00 (m, 4H, HCy), 1.93 (br, 4H,
-NH), 1.70–1.62 (m, 4H, HCy), 1.21–1.12 (m, 4H, HCy), 1.03–
0.91 (m, 4H, HCy); 13C NMR (100 MHz, CDCl3): d=149.0
(Carom), 146.9 (Carom), 139.5 (Carom), 128.5 (Carom), 128.2
(Carom), 123.5 (Carom), 61.3 [-NH(CH)], 61.0 [-NH(CH)], 50.7
[-NH(CH2)], 50.3 [-NH(CH2)], 31.7 (CCy), 31.5 (CCy), 25.0
(CCy).
2 (300 mg, 0.56 mmol) was dissolved in dichloromethane
(5 mL) and trifluoroacetic acid (5 mL) was added. The reac-
tion mixture was stirred for 1 h at room temperature. Then
the excess of reagent and solvent were removed under
vacuum. The resulting oil was neutralized by potassium hy-
droxide (2N), extracted with dichloromethane (5”10 mL),
and dried over sodium sulfate. After evaporation, the oil
was dissolved in anhydrous methanol (5 mL), the corre-
sponding aromatic aldehyde (in excess) was added and the
reaction mixture was stirred for 4 h at room temperature. To
the reaction mixture, sodium borohydride (8.0 equiv.) was
added. After 4 h, the solvent was evaporated and water was
added and the mixture extracted with chloroform (3
”15 mL). The combined organic layers were dried over
sodium sulfate and the solvent was removed under vacuum.
The crude mixture was dissolved in a small amount of DCM
and the dropwise addition of HCl (36% in diethyl ether so-
lution) provided a white precipitate. This solid was filtered
and washed with diethyl ether to provide the pure hydro-
chloride amine salt derivatives as a white solid, while the
benzylic alcohol (by-product) remained in solution.
Synthesis of Metallopolymer (4)
A mixture of NiBr2 (64 mg, 0.3 mmol) and 3 (150 mg,
0.3 mmol) in acetonitrile (5 mL) was refluxed for 5 h. After
solvent removal, the residue was washed with ether (3”
10 mL) to afford the expected compound as a microcrystal-
À
line pale grey powder; yield: 80%. IR: n=3256 (N H),
2932, 2854, 1450, 1440, 1089, 913, 874, 736, 695 cmÀ1; ele-
mental analysis calcd. (%) for C34H46Br2N4Ni: C 56.00, H
6.36, N 7.68; found: C 56.00, H 6.44, N, 7.47.
(1R,1’R,2R,2’R)-N1,N1’-[1,4-Phenylenebis(methylene)]-
bis(N2-benzylcyclohexane-1,2-diamine) (3): The compound 3
was obtained from benzaldehyde according to the general
procedure as a white solid; yield: 131 mg (85%); [a]2D0:
Synthesis of (4OMe
)
A mixture of NiBr2 (64 mg, 0.3 mmol) and 3OMe (150 mg,
0.3 mmol) in acetonitrile (5 mL) was refluxed for 5 h. After
solvent removal, the residue was washed with ether (3”
10 mL) to afford the expected compound as a microcrystal-
À
À164.6 (c 1.14, CHCl3). IR (CH2Cl2): n=3297 (N H), 3022,
2918, 2852, 1496, 1450, 1112, 730 cmÀ1; MS (ES+): m/z=
À
1
511.37 [M+H]+; H NMR (400 MHz, CDCl3): d=7.31–7.12
line pale yellow powder; yield: 79%. IR: n=3256 (N H),
2932, 2854, 1450, 1440, 1089, 913, 874, 736, 695 cmÀ1; ele-
mental analysis calcd. (%) for C36H50Br2N4NiO2: C 54.78, H
6.38, N 7.10; found: C 54.82, H 6.49, N, 7.04.
1
(m, 14H, Harom), 3.84 [d, J=7 Hz, 2H, -NH(CH2)], 3.80 [d,
1J=7 Hz, 2H, -NH(CH2)], 3.59 [d, 1J=8 Hz, 2H,
-NH(CH2)], 3.56 [d, 1J=8 Hz, 2H, -NH(CH2)], 2.16–2.23
[m, 4H, -NH(CH)], 2.12–2.05 (m, 4H, HCy), 2.01 (br, 4H,
-NH), 1.70–1.62 (m, 4H, HCy), 1.20–1.09 (m, 4H, HCy), 1.04–
0.92 (m, 4H, HCy); 13C NMR (100 MHz, CDCl3): d=140.9
(Carom), 139.3 (Carom), 128.3 (Carom), 128.12 (Carom), 128.10
(Carom), 126.8 (Carom), 60.85 [-NH(CH)], 60.81 [-NH(CH)],
50.8 [-NH(CH2)], 50.5 [-NH(CH2)], 31.4 (CCy), 25.0 (CCy).
(1R,1’R,2R,2’R)-N1,N1’-[1,4-Phenylenebis(methylene)]-
bis[N2-(4-methoxybenzyl)cyclohexane-1,2-diamine] (3OMe):
The compound 3OMe was obtained from anisaldehyde ac-
cording to the general procedure as a red solid; yield:
120 mg (70%); [a]2D0: À91.3 (c 1.14, CHCl3). MS (ES+):
General Procedure for the Enantioselective
Reactions and Recycling (Table 1, entry 1)
The metallopolymer (4) (2.3 mg, 0.003 mmol), dimethyl mal-
onate (69 mg, 0.52 mmol) and (E)-nitrostyrene (65 mg,
0.43 mmol) were added in a vial with dichloromethane
(1.0 mL) under air. The mixture then was stirred at 48C.
After 16 h, diethyl ether was added and a precipitate was in-
stantly formed. The organic phase was isolated, concentrat-
ed and the product was purified by flash chromatography
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(AcEt/cyclohexane 1:4) to give a white solid; yield: 99%;
92% ee).
The catalyst recovered by centrifugation was washed with
diethyl ether, dried and reused in a renewed catalytic run.
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Acknowledgements
The authors gratefully acknowledge the CNRS and the Uni-
versity of Strasbourg.
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