Enhanced reactivity and selectivity of asymmetric oxa-Michael addition of 2′-hydroxychalcones in carbon confined spaces

Article abstract of DOI:10.1039/c7cc01096f

Carbon nanotubes (CNTs) are employed as nanoscale reaction vessels for the asymmetric oxa-Michael addition of 2′-hydroxychalcones. A systematic comparison of the catalytic activities of chiral phosphoric acid treated ferrite nanoparticles (chiral ferrites) has been studied for the synthesis of flavanones. Higher reactivity and selectivity with switching of enantiomers were observed when these chiral ferrites are inside the CNT channel.

Full text of DOI:10.1039/c7cc01096f

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Enhanced Reactivity and Selectivity of Asymmetric oxa-Michael
Addition of 2’-Hydroxychalcone in Carbon Confined Space
Received 00th January 20xx,
Accepted 00th January 20xx
Melad Shaikha , Kiran Kumar Atyamb, Mahendra Sahuaand Kalluri V. S. Ranganatha*
DOI: 10.1039/x0xx00000x
www.rsc.org/
resultant products have important properties of anti-tumor
and anti-inflammatory properties. The isomerization of 2’-
hydroxychalcones to flavanones belongs to oxa-Michael
addition reaction, which produces chromane core that is
characteristic of many flavanoids.21 A significant number of
asymmetric catalysts have been reported for the chiral
flavanones through the direct asymmetric isomerization of 2’-
hydroxychalcone. For instance, the asymmetric isomerization
of 2’-hydroxychalcone has been reported by chiral acid and
base catalysts in high yields and excellent ee’s.22 In that
Hintermann clearly explained that the racemization and
reversible reactions of flavanone to 2’-hydroxychalcones under
particular conditions are major risks to get the enantioriched
flavanones. Recently, Scheidt and Feng groups independently
developed a new protocol for the enantioselective synthesis of
flavanones using a quinine-derived thiourea catalyst and chiral
N,N’-dioxide nickel (II) complex respectively.23,24 The catalytic
asymmetric reactions promoted by chiral phosphoric acids
(CPA) and their metal complexes have become powerful
methodology to synthesize structurally diverse organic
compounds.25,26
Carbon nanotubes (CNTs) are employed as nanoscale reaction
vessels for the asymmetric oxa-Michael addition of 2’-
hydroxychalcones.
A systematic comparasion of catalytic
activities of chiral phosphoric acid treated ferrite nanoparticles
(chiral ferrite) has been studied for the synthesis of flavanones.
Higher reactivity, selectivity with switching of enantiomer was
observed when these chiral ferrites are inside the CNT channel.
Confinement of molecules inside nanoscale containers
provides a powerful strategy for achieving high activity and
selectivity.1,2 The growing demand for enantiopure compounds
in the daily life has stimulated an increasing interest in
asymmetric catalysis.3 Homogeneous asymmetric catalysis has
proven its usefulness in a number of organic reactions with
high enantioselectivity, however only limited number of them
have been used in industry because of the intrinsic problems
such as recovery of the expensive catalyst.4-5 Fortunately,
asymmetric heterogeneous catalysis has many advantages
over homogeneous catalysis, in terms of recyclability.6 The
orientation of the substrate on the catalyst support is crucial in
generating the asymmetric heterogeneous catalyst, where
choice of the support plays a major role. The change of the
support may drastically change the (increase or decrease)
enantioselectivity and in some cases reversal of enantiomers
can also be observed.7-8 The inversion of enantioselectivity is
due to change of solvent, temperature, heterogeneous
support and also by addition of co-solvent.9 In addition, the
switching of isomer is more pronounced on moving from
homogeneous to heterogeneous catalytic system.10
Herein, we report the highly enantioselective oxa-Michael
addition of 2’-hydroxychalcone to flavanones in good yields
and ee’s using chiral ferrites either inside or outside CNTs
(Scheme 1).
The utilization of nanomaterials in various directions is
exploring rapidly due to the change of properties after surface
treatment with various functional groups.11-12 Recently, we
have also contributed to the effect of surface functionalization
of ferrite and nano magnesium oxides with dicarboxylic acids
and imidazolium salts in the catalytic oxidation and
dehydration reactions.13-16 By introducing the suitable
functionality, the properties of CNTs have been explored in
various applications, including catalysis.17-20 The intramolecular
oxa-Michael addition has been a significant challenge since the
Scheme 1 Asymmetric oxa-Michael addition of 2’-hydroxychalcone to flavanone
catalyzed by CPA modified MNPs inside and outside CNTs.
The heterogeneous catalyst developed here is the surface
modified ferrites, i.e magnetite nanoparticles (MNP),27 and
cobalt ferrite nanoparticles (CNP) with a versatile chiral
ligands, (R)-(−)-1,1′-Binaphthyl-2,2′-diyl hydrogen phosphate
a.Department Of Chemistry, Guru Ghasidas Central University, Bilaspur, India-
495009
E-mail: rangakvs@gmail.com
b Dept. of Materials Science and Engineering, RGUKT, Basar India-50401
Electronic Supplementary Information (ESI) available
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(CPA) (L1) and (R)-3,3’-Bis(9-anthracenyl)-1,1′-Binaphthyl-2,2′- enantiomer of flavanone was observed when MNP-L1 is inside
diyl hydrogen phosphate (L2). This new class of chiral catalyst CNT (Fig. 2).
is readily formed through the sturdy interaction of MNPs with
CPA, and generates a chiral solid. Enhanced enantioselectivity
has been observed when these chiral solids are encapsulated
either inside CNT or decorated on outside CNT as shown in
1 in the direct oxa-Michael addition of 2’-
scheme
hydroxychalcone to flavanone (upto 90%). It is well known that
CNTs are having strong affinity towards aromatic compounds
due to
selected because of expected strong affinity (
of naphthyl and anthracenyl groups with CNTs.
π
-
π
stacking. Therefore chiral ligands L1 and L2 are
π-π
interactions)
Fig. 2 HPLC traces of flavanones catalyzed by (a) MNP-L1 and (b) MNP-L1/CNT
Asymmetric oxa-Michael addition of 2’-hydroxychalcone to
flavanone was initially evaluated using homogeneous L1
complexes of Fe(II) and Fe(III), which are prepared by reacting
iron (II)/iron (III) chlorides in the presence of base (Fig. 1).
Notably, the reaction proceeds quite well and gave the
product 64, 68% yields with 24% and 28% ee respectively after
This may be due to the confinement effect of CNTs inside the
channels of nanoreactor and having π- interactions of aromatic
groups with CNTs.30-33 Moreover, the enhanced reactivity and
selectivity was observed due to the confinement of CNTs in
asymmetric hydrogenations using cinchonidine and in
nitroaldol reactions using hetrobimetallic catalyst .34-36
24 h in chlorobenzene at 100°C using both these complexes.
Notably, in our present reaction, lowering the temperature did
not show the improvement of enantioselectivity (Table 1,
entry 8 & 9). Surprisingly, no enantioselectivity was observed
when the oxa-Michael addition of 2’-hydroxychalcone was
carried out in DMF (Table 1, entry 10). There is a little
improvement in the ee (upto 94%) and yield (upto 70%) of
Fig. 1 Fe(II) and Fe(III) complexes of L1.
flavanone in the presence of MNP-L2/CNT at 100°C in
In continuation of our study for the development of surface
modified metal oxides, we synthesized L1 treated ferrites.
chlorobenzene, however the major enantiomer is same as with
that of MNP-L2 catalyzed reaction. This may be due to the bulk
anthracenyl groups could not oriented properly inside the
channels of CNT and the reaction could takes place on the
surface of CNT and thus giving the enantio enriched product in
94%. The higher selectivity of MNP-L2/CNT is due to the steric
and electronic effect of the ligand and thus leads to give the
product in higher selectivity compared to MNP-L1/CNT. In
order to understand better the effect of MNP and CNT, two
separate experiments were conducted in the presence of
chlorobenzene using CNT and L2. It was observed that there
was no reaction in the presence of only CNTs and 15% of
flavanone was isolated with 11% ee using L2 (only L2) after 15
h (Table 1, entry 14). An electromagnetic material like CNPs
decorated on CNTs showed high catalytic activity in the
dehydration of fructose,19 were used in our asymmetric
cyclization reaction, did show the ee after incorporating them
into CNTs as like MNPs (Table 1, entry 15-19). A systematic
study of asymmetric cyclization of 2’-hydroxychalcone
Since
paramagnetic
material
(MNP)
and
highly
electromagnetic material (CNP) could be easily recovered after
the reaction, we selected these two inverse spinels in our
present study. As a preliminary experiment, we studied oxa-
Michael addition of 2’-hydroxychalcone to flavanone using
MNPs, CNPs and the results are shown in the Table S1. Later,
L1 treated MNP and CNPs have been evaluated for the
synthesis of flavanones at different temperatures. It was found
that modified MNPs performed as a versatile nanocatalyst
furnishing the chiral flavanone in 79% yield with 20% ee in
chlorobenzene at 100°C. The rate of reaction is significantly
increased with L1 treated MNPs than naked MNPs and as the
concentration of ligand increases, the enantioselectivity
increases (Table 1, entry 1 to 4) (up to 24% ee). When L2
treated MNPs used as a catalyst in the cyclization of 2’-
hydroxychalcone in chlorobenzene, flavanone was isolated in
52% yield with 38% ee (Table 1, entry 5, 6). Since using of CNTs
is enhancing the catalytic activity through confinement in
hydrogenation and other reactions, we incorporated surface
anchored ferrites into CNTs through a capillary phenomenon.
Since the chlorobenzene had turned out to induce high
enantioselectivity in the cyclization using MNP-L1, catalyses
with all catalysts (MNP-L1/CNT, MNP-L2/CNT, CNP-L1/CNT,
CNP-L2/CNT) in that solvent. Indeed, 2’-hydroxychalcone
underwent catalytic cyclization to produce chiral flavanones in
the presence of ferrite encapsulated CNTs are shown in the
Table 1. Surprisingly, the chiral flavanones were isolated in
68% yield with 90% enantioselectivity using carbon confined
chiral ferrite (Table 1, entry 7). The enhanced
enantioselectivity may be due to the confinement effect of
chiral ferries inside CNTs. This result shows the importance of
asymmetric catalysis within the nanotubes and channels of
CNTs act as highly efficient nano reactors for the asymmetric
oxa-Michael addition reactions. Surprisingly, opposite
catalyzed by Br∅nsted acids including camphorsulphonic acid,
CPA has also been reported earlier with lower yields and
ee’s.37 To evaluate the effect of confinement of MNPs on their
catalytic performance, we prepared chiral MNPs on outside
CNTs and tested in oxa-Michael addition reaction. Notably,
flavanones were isolated in 63% yield and 65% ee, which is
having the same configuration as that of MNP-L1. This result
clearly reveals that the effect of MNPs inside the CNTs and
produce opposite enantiomer. Moreover, the turnover
frequency (TOF) of M-L1/CNT, 34.3 h-1 is much higher than
MNP-L1, (2.5 h-1), MNP-L2 (2.7 h-1) and even higher than MNP-
L1/CNTs (outside) (27.3 h-1) (Fig. S7). Further, various ratios of
MNP-L1 were prepared and encapsulated inside CNT are used
in oxa-Michael addition. In all these cases inversion of isomer
was observed to that of MNP-L1 (HPLC spectras, SI). The
crystallinity and structure of modified ferrites are confirmed by
2 | J. Name., 2012, 00, 1-3
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powder XRD. The Fig. 3 shows the XRD pattern of the before conversion of 70% yield with 93% ee was obtained even after
and after treatment of MNPs with L1. The broadening of the five consecutive cycles.
characteristic peak after reacting with L1, indicates that
Table 1 Asymmetric oxa-Michael addition of 2’-hydroxychalcone using various
catalysts
interaction of iron with modifier is preferred than iron-iron
interactions. The analysis of the diffraction pattern showed the
Entry
Catalyst
Solvent
Temp.
(°C)
Yield
(%)
Selectivity
(% of ee)
formation of simple cubic spinel structure due to the strongest
reflection from (311) plane at 35.69 using CuKα. In addition,
the presence of (220), (311), (422), (511) revealed the cubic
spinel phase of L1 modified ferrites either inside or outside
CNTs (Fig. 3). The crystallite size was found to be 16.2nm, 12.0
nm, 14.3 nm, 11.4 nms for MNP, MNP-L1, CNP and CNP-L1
respectively. In addition 2θ at 26.2° correspond to reflections
of (002) planes of CNTs were observed for both inside and
outside MNPs.
°
1
2
3
4
5
MNP-L1
MNP-L1
MNP-L1
MNP-L1
MNP-L2
Chlorobenzene
Chlorobenzene
Chlorobenzene
Chlorobenzene
Chlorobenzene
100
100
100
100
100
60
79
84
82
52
15 (S)a
20 (S)b
22 (S)c
24 (S)d
38 (S)
6
7
CNP-L2
Chlorobenzene
100
100
80
47
68
65
52
90
70
55
90
15
47
40
70
72
80
63
30 (S)
90 (R)
88 (R)
90 (R)
- -
M-L1/CNT Chlorobenzene
M-L1/CNT Chlorobenzene
M-L1/CNT Chlorobenzene
8
9
40
10
11
12
13
14
15
16
17
18
19
20
M-L1/CNT
DMF
100
100
80
M-L2/CNT Chlorobenzene
M-L2/CNT Chlorobenzene
94 (S)
65 (S)
- -
M-L1/CNT
L2
DMF
100
100
100
80
Chlorobenzene
Chlorobenzene
Chlorobenzene
Chlorobenzene
Chlorobenzene
DMF
11(S)
30 (S)
20 (S)
93 (R)
84 (R)
--
Fig. 3 PXRD pattern of (a) CNT (b) MNP-L1 (10 mol %) (c) CNP-L1 (d) MNP-L1/CNT
(e) CNP-L1/CNT (f) MNP-L1 outside of CNT
CNP-L2
CNP-L2
C-L2/CNT
C-L1/CNT
C-L2/CNT
In addition, MNP-L1/CNT has also been characterized by XPS,
which confirm the presence of MNPs decorated in CNTs. The
doublet photoelectron profile of Fe2p spectra spin-orbital split
into distinct Fe2p3/2 and Fe2p1/2 orbit at binding energies at
710.75 eV and 724.23 eV respectively (Fig. S1, Supporting
Information). The binding energy corresponding to 531.6 eV is
attributed for O 1S of Fe3O4 (Fig. S1) and it is identified to be
the bridge oxygen (O2-) between the octahedral iron (Fe3+) and
the tetrahedral (Fe2+) lattice. Along with iron and oxygen, the
binding energies at 284.35, 285.12 and 286.55 eV
corresponding to C 1s of different carbon atoms of Sp2 and Sp3
100
100
100
100
M-L1/CNT Chlorobenzene
65(S)e
Reaction conditions: 0.5 mmol of reactant, 3.0 mL of solvent, 20.0 mg of the
a
c
catalyst. 5 mol% of L1 on MNP. b 7.5 mol% of L1 on MNP. 10 mol% of L1 on
MNP. d 15 mol% of L1 on MNP. eMNP-L1(10 mol%) outside CNTs
respectively (Fig. S1). The binding energy at 134.1 and 133.5 This result indicates that catalyst is stable and active without
eV attributed for phosphorus 2p1/2 and 2p3/2 orbitals. The
effective decoration of MNPs in CNTs has also been verified by
IR spectroscopy (Fig. S2). The mechanism of asymmetric
catalysis on chirally anchored materials is complicated and is
not yet been well understood. It is generally believed that
chiral moiety, (in our case CPA) is adsorbed onto MNP
surfaces, and induces enantioselectivity due to the metal-CPA
interactions. However, in case of MNP-L1/CNT, MNPs were
encapsulated inside the channels of CNTs and thus the
asymmetric oxa-Michael addition reaction could take place in
a restricted space hence gave the product with inversion of
selectivity. Under the optimized reaction conditions, a variety
of representative 2’-hydroxychalcones reacted smoothly in the
presence of either MNP/L1 or MNP-L1/CNT in moderate to
good yields with good enantioselectivity (Table 2). Remarkably,
no reaction was observed with aliphatic substituted
unsaturated keontes (Table 2, entries 19 & 20). To evaluate
the stability of the catalyst, the recycling of MNP-L2/CNT was
performed for the asymmetric oxa-Michael addition reaction
deterioration in activity or enantioselectivity (Fig. S5). The
recovered catalyst was analysed through inductively coupled
plasma atomic emission spectroscopy (ICP-AES) and observed
that there is no leaching of iron from the catalyst. Apart from
other methods, the MNP-L1/CNT catalysts were also
characterized by HR-TEM, which indicates the crystallite size, is
around 8 nm (Fig. 4). It shows the morphology and size
distribution of MNPs inside CNTs.
Fig. 4 HR-TEM of (A) MNP-L1/CNT (B) TEM of MNP-L1/CNT
of 2’-hydroxychalcone in chlorobenzene at 100°C. The chiral
solid catalyst was separated by external magnet, washed
thoroughly with ethanol and reused for next cycle. A constant
It can see that size of NPs is distributed from 7.8 nm to 16.4
nm, and the mean particle size is around 13.8 nm (Fig. 4).
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MNP-L1
88
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76
86
82
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4MeO-Ph
4Me-Ph
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3Cl-Ph
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-45
73
MNP-L1/CNT
MNP-L1
-31
64
3NO2-Ph
MNP-L1/CNT
MNP-L1
-63
--
,
CH2-Ph
C2H5
MNP-L1
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MNP-L1
---
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addition of 2’-hydroxychalcones. No additives or additional
substitution in the reactant are required to get the
enantioriched products. Catalytic activity and selectivity was
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