Cupreine grafted onto silica as an enantioselective and recyclable catalyst for the 1,4-addition of malonate to trans-β-nitrostyrene

Article abstract of DOI:10.1039/c5ra02292d

Preparation of silica supported cupreine (CPN) and its high catalytic performance for the asymmetric 1,4-addition of dimethyl malonate to trans-β-nitrostyrene in some biomass-derived solvents (2-methyl THF, EtOAc and EtOH) are reported for the first time.

Full text of DOI:10.1039/c5ra02292d

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Cupreine grafted onto silica as an enantioselective
and recyclable catalyst for the 1,4-addition of
malonate to trans-b-nitrostyrene†
Cite this: RSC Adv., 2015, 5, 29386
Received 5th February 2015
Accepted 19th March 2015
*
I. Billault, R. Launez and M.-C. Scherrmann
DOI: 10.1039/c5ra02292d
www.rsc.org/advances
Preparation of silica supported cupreine (CPN) and its high catalytic denitively the interest given to this signicant class of
performance for the asymmetric 1,4-addition of dimethyl malonate to catalysts.8
trans-b-nitrostyrene in some biomass-derived solvents (2-methyl THF,
However, all the catalysts reported above have been used
only under homogeneous conditions. Unfortunately, such
conditions suffer from tedious work-up procedure with tricky
separation of catalyst from nal product leading to catalyst loss
and high cost process. Organocatalyst immobilisation may
overcome these limitations by allowing easier recovering of the
catalyst and it's recycling. This strategy can lead to a practical
and economic catalytic system since higher turnover number
(TON) could be reached. Furthermore, graed catalyst offers the
opportunity to implement the catalysis under continuous ow
conditions.9
Several Cinchona-based catalysts have been already graed
onto different homemade10 or commercially available poly-
mers.11 Only some examples were reported concerning the
graing of Cinchona alkaloids as organocatalysts onto different
porous inorganic supports such as silica (silica gel 60,12
SBA-15,13 MCM-41,12a,13a and Aerosil 200 14), Fe3O4 magnetic
nanoparticles,15 gold nanoparticles,16 and porous zirconium
phosphonate.17
EtOAc and EtOH) are reported for the first time.
Ten years ago, the pioneering work of Deng and co-workers on
Cinchona alkaloids bearing a 60-hydroxyquinoleine such as
cupreine (CPN) or cupreidine (CPD) and their C9–OR derivatives
revealed their great efficiency for the 1,4-addition of malonate to
nitroalkenes with higher enantioselectivity than their parents
quinine or quinidine, respectively (Fig. 1).1,2
Aerward cupreine, cupreidine and their C9–OR derivatives
were included in the bifunctional catalyst class and proved their
powerful efficiency by catalysing with high enantioselectivity
and/or diastereoelectivity different conjugate additions between
a large panel of partners3 and other addition reactions (e.g.
Friedel–Cra,4 Henry5 and nitroaldol reactions6) together with
Kornblum DelaMare rearrangement.7 Recently, Chauhan and
Chimni have reviewed all the asymmetric transformations cat-
alysed by the bifunctional catalysts bearing an aromatic
hydroxyl group including CPN and its C9 derivatives and proved
Depending on the Cinchona alkaloid catalyst and the support
used, the enantioselectivity performance are mostly maintained
or slightly lower compared to their homogeneous counterpart
that pointed out the good behaviour of those catalysts for the
immobilisation strategy. However, in general the turnover
frequency (TOF) decreases for graed catalyst due to mass
transfer resistance and in addition, recycling remains oen a
challenge. To the best of our knowledge any Cinchona alkaloid
bearing a 60-hydroxyquinoline function has not been anchored
onto insoluble support yet. Among all the insoluble supports
already used, those based on silica have retained our attention
especially for the versatility that their offer to anchor a catalyst:
graing onto preformed silica support ordered or not, as well as
by using a co-condensation process. Furthermore, as residual
silanol groups (Si–OH) on the silica surface could be not inert
during the catalysis, a possible end-capping of those residuals
Si–OH is still possible to modify the surface of this support.
Fig. 1 Structure of Cinchona alkaloids CPN and CPD.
´
ˆ
Universite Paris Sud, ICMMO, UMR CNRS 8182, Batiment 420, 91405 Orsay Cedex,
France. E-mail: isabelle.billault@u-psud.fr
† Electronic supplementary information (ESI) available: Experimental details,
synthesis procedures and characterisation of Si-CPN: elemental analysis,
thermogravimetry, BET and FT-IR analysis. See DOI: 10.1039/c5ra02292d
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Herein we report (i) the rst preparation of supported CPN
The asymmetric Michael addition to nitroalkene is a useful
onto the simplest and cheapest amorphous silica support, the tool for the preparation of versatile optically active building
˚
silica gel Si 60 A (70–200 mm), (ii) the catalytic performances of blocks as intermediate in the synthesis of some biological active
the anchored CPN bifunctional catalyst for the asymmetric compounds.1,22 The catalytic performances of supported cata-
1,4-addition of dimethyl malonate to the trans-b-nitrostyrene lysts Si-CPN A to C were thus evaluated for the asymmetric 1,4
especially in solvents derived from biomass as our interest is the addition of dimethyl malonate to trans-b-nitrostyrene as a
development of new process using alternative solvents,18 and model reaction and compared with those obtained with the
(iii) the recycling of the supported catalysts.
homogeneous counterpart. Reactions were carried out with
CPN was readily prepared in one step from commercially trans-b-nitrostyrene 1 (0.5 M) and dimethyl malonate 2 (1.5 eq.)
available quinine1 then graed to the silica support through a at room temperature in the presence of supported catalysts
thioether bond involving the 3-mercaptopropyl triethoxysilane Si-CPN or CPN under agitation in biomass-derived solvents
(MPTES) as a linker between the catalyst and silica.19 Based on (Me-THF, AcOEt, EtOH) and in THF for comparison. All results
this strategy, three alternative routes were tested to prepare are summarised in Table 1.
different batches of Si-CPN (Scheme 1); route A: the silica
We were pleased to observe that the Michael addition took
surface was rst functionalized by mercapto groups (Si–SH), place with Si-CPN A (12 mol%) in THF while the reaction did not
and then CPN was bound through a thiol–ene reaction per- proceed during the same time in the presence of unmodied
formed under heterogeneous conditions using 2,20-azobis(2- silica alone. Furthermore, the ee% of the Michael adduct 3
methylpropionitrile) (AIBN) as radical initiator; route
B
(S congured) was almost preserved (entries 1 and 2).1 To the
involved the end-capping of the residual silanols remaining free best of our knowledge, no conformational analysis of CPN has
at the surface of the silica by using N,N-dimethyl trimethylsi- been investigated yet. Nevertheless, extended conformational
lylamine (TMS-DMA)20 before the immobilisation of CPN; studies of the Cinchona alkaloids such as quinine (Fig. 1) have
nally, route C: the thiol–ene reaction was rst carried out been well explored by NMR in different solvents and calculation
under homogeneous conditions to link MPTES to CPN,21 then approaches. All these analysis have proved that quinine adopts
the adduct (MPTES–CPN) was tethered onto the silica surface preferentially the so-called anti-open conformation in solu-
aer a washing step to remove unreacted MPTES.
tion.23 Based on the hypothesis that CPN adopts the same open
All batches of Si-CPN A, B and C were characterized by clas- conformation; three possible models for the arrangement of
sical techniques: elemental analysis (C, N and S%), thermog- both reagents 1 and 2 with the catalyst CPN are shown in Fig. 2.
ravimetry, BET and FT-IR analysis (See ESI†). Elemental analysis
In model A we postulated that the p-stacking interaction
proved a CPN loading of 0.364, 0.268 and 0.225 mmol gꢀ1 of between the p-donor quinoline ring and the p-acceptor trans-b-
silica for Si-CPN A, B and C, respectively. Thermogravimetric nitrostyrene determines the absolute conguration S of product
analysis performed on all Si-CPN, patently displayed two 3 since only the Si face of the double bond of 1 remains acces-
different species graed onto silica surface: a rst weight loss sible for the nucleophile attack of 2. In contrast, both models B
was observed at 300 ꢁC for the linker with SH group remaining and C show a loss of H-bonding with loss of p-stacking inter-
free, while a second weight loss, consequent to the degradation action or a severe steric interaction, respectively.
ꢁ
of the graed catalyst, was seen at 500 C. By comparing the N
As expected, the anchored CPN displayed same stereo-
and S% from the elemental analysis, the residual SH groups selectivity and a TOF only 2.5 times lower compared to its
remaining onto the silica surface aer graing CPN could be homogeneous counterpart. As the TOF was lower with graed
estimated too. The residual SH groups were most abundant for catalyst Si-CPN A, the 1,4 addition using catalysts Si-CPN B and
Si-CPN A (60%) and B (69%) compare to Si-CPN C (18%). C were performed with 27 and 22 mol% catalyst, respectively.
Thereby, the route C offers the possibility to remove most of the Unfortunately, with the two latter supported catalysts the ee%
residual SH groups onto the silica surface.
were slightly lower in THF, especially when the capped Si-CPN B
was used (entries 3 and 4). This last observation was in accor-
dance with results already reported for quinidine graed onto
capped Kieselgel 60.12a,d Moreover, while the TOF of Si-CPN A
and B were identical, that of Si-CPN C was halved.
Next, we investigated the Si-CPN catalytic activity by per-
forming the Michael addition in different biomass-derived
solvents. MeTHF and AcOEt are not classically used for
Michael addition however, as shown in Table 1, both solvents
are suitable for this reaction since 92 and 89 ee% were obtained
respectively for 3 under homogeneous conditions (entries 5 and
9). Furthermore, we were delighted to observe a better TOF in
MeTHF than in THF. Surprisingly the reaction was twice as long
as in THF when AcOEt was used as the solvent, leading to a
quite poor TOF even under homogeneous conditions (entry 9).
By using Si-CPN A or C in MeTHF, the ee% were still comparable
to the ones obtained with homogeneous counterpart CPN, while
Scheme 1 Preparation of the supported Si-CPN A, B and C.
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Table 1 Catalytic performances comparison between CPN and Si-CPN A to C
Entry
Catalyst (mol%)
Solvent
Time (h)
Yielda (%) (conv.b (%))
eec (%) 3
TOFd
1
2
3
4
5
6
7
8
CPN (10)
THF
THF
THF
THF
24
62
28
72
16
41
20
20
48
62
48
72
24
30
21
48
88 (100)
90 (95)
97 (99)
95 (99)
89 (100)
89 (97)
90 (94)
95 (99)
84 (100)
90 (95)
93 (97)
93 (97)
97 (100)
93 (96)
93 (96)
92 (98)
90
87
73
80
92
87
77
88
89
82
76
83
57
60
42
54
0.42
0.13
0.13
0.06
0.62
0.20
0.17
0.22
0.21
0.13
0.07
0.06
0.42
0.26
0.17
0.09
Si-CPN A (12)
Si-CPN B (27)
Si-CPN C (22)
CPN (10)
Si-CPN A (12)
Si-CPN B (27)
Si-CPN C (22)
CPN (10)
Si-CPN A (12)
Si-CPN B (27)
Si-CPN C (22)
CPN (10)
Si-CPN A (12)
Si-CPN B (27)
Si-CPN C (22)
MeTHF
MeTHF
MeTHF
MeTHF
EtOAc
EtOAc
EtOAc
EtOAc
EtOH
EtOH
EtOH
EtOH
9
10
11
12
13
14
15
16
a
Isolated yield. b Conv. (%) determined on crude reaction mixture by HPLC with Si-CPN or RMN-1H with CPN. c Determined by HPLC analysis on a
Kromasil 3-CelluCoat RP column. d Turn over frequency (mol of 3 per mol per cat per h) calculated using conversion.
the ee% was always lower with the capped Si-CPN B (entries 5–8). be the best biomass-derived solvent (ee% and TOF) giving better
In the presence of AcOEt, all batches of Si-CPN showed a lower results than THF for the asymmetric 1,4-addition of dimethyl
catalytic efficiency compared to CPN (entries 9–12). EtOH proved malonate to the trans-b-nitrostyrene using silica supported CPN.
to be not suitable for this reaction since the ee% of 3 decreased THF and MeTHF displayed quite similar proprieties excepting
drastically whatever the conditions (entries 13–16). In this that unlike THF, MeTHF is not miscible to water,25 a strong
solvent we noticed that the Michael addition somewhat occurred hydrogen bond donor solvent. Thus, MeTHF could probably not
at 10% conversion without catalyst. Furthermore, we assumed desorb the water molecules always adsorbed onto the silica
that in a solvent with a strong hydrogen bond donor property surface leading to water content lower in reaction mixture. As
such as EtOH, the arrangement between the catalyst and the MeTHF offered the best catalytic activities, it was selected to
reactants, based on p-stacking and hydrogen bonding,24 is not as examine the recyclability of the supported Si-CPN (Table 2). The
favoured than in an aprotic solvent giving rise to a lower enan- catalysts Si-CPN were recovered by ltration aer the reaction,
tioselectivity of the addition (Fig. 2). MeTHF was thus found to washed then dried under vacuum before the next run.
All recovered catalysts Si-CPN A to C were able to be recycled
at least three and even up to ve runs without any loss of
catalytic activities (Table 2). As the number of recycling could be
dependent of the initial catalyst mol% engaged in the rst run,
the turn over number (TON) have to be considered to compare
them. Thereby, thanks to recycling, all the catalysts Si-CPN
nearly exhibited similar TON values around 14.
In conclusion, we have developed a novel heterogeneous
chiral catalyst by graing CPN onto amorphous silica. Our
results attested for the rst time that silica is a suitable support
to immobilise CPN while retaining the asymmetric catalytic
performance of this bifuctional catalyst almost unchanged.
Among all supported catalysts Si-CPN prepared, the Si-CPN A
displayed the best catalytic efficiency despite the presence of
residual SH groups onto the silica surface. We are now inves-
tigating the use of Si-CPN A for various applications under
Fig. 2 Postulated models for the 1,4-addition of dimethyl malonate 2
continuous ow conditions using a packed bed reactor.
to trans-b-nitrostyrene 1 in the presence of CPN.
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Table 2 Recycling experiments for Si-CPN A, B and Ca
Entry
Run
Catalyst (mol%) Time (h)
Si-CPN A (36%) 16 h
Conv.b (%)
Yieldc (%)
eed (%)
TONe
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
1
2
3
4
1
2
3
4
100
99
99
99
97
93
65
93
94
82
64
99
97
95
43
95
92
96
96
94
90
nd
89
93
77
nd
95
95
91
nd
86
86
87
84
85
83
83
83
86
85
84
88
87
87
79
2.8
5.6
8.4
11.2
13.9
16.2
—
4.4
8.9
12.8
—
4.4
Si-CPN B (21%) 32 h
Si-CPN C (22%) 20 h
9
10
11
12
13
14
15
8.7
12.9
—
a
All reactions were carried out in same conditions than for results in Table 1. b Conversion determined by HPLC analysis. c Isolated yield, nd: not
determined. d Determined by HPLC analysis on a Kromasil 3-CelluCoat RP column. e Turn over number (mol of 3 per mol per cat) calculated using
conversion.
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