A magnetoclick imidazolidinone nanocatalyst for asymmetric 1,3-dipolar cycloadditions

Article abstract of DOI:10.1002/adsc.201300624

A 1,3-dipolar azide-alkyne cycloaddition has been used to prepare a magnetic nanoparticle immobilized MacMillan catalyst that catalyzes the enantioselective 1,3-dipolar cycloaddition between nitrones and α,β-unsaturated aldehydes. The catalyst can be recovered and recycled for five consecutive runs without any significant loss in yields and diastereo- and enantioselectivities of the isoxazolidines. Copyright

Full text of DOI:10.1002/adsc.201300624

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DOI: 10.1002/adsc.201300624
A Magnetoclick Imidazolidinone Nanocatalyst for Asymmetric
1,3-Dipolar Cycloadditions
Sreenivasarao Pagoti,a Debasish Dutta,a and Jyotirmayee Dasha,*
a
Department of Organic Chemistry, Indian Association for the Cultivation of Science Jadavpur, Kolkata – 700032, India
E-mail: ocjd@iacs.res.in
Received: July 17, 2013; Revised: October 15, 2013; Published online: November 27, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300624.
Abstract: A 1,3-dipolar azide–alkyne cycloaddition
has been used to prepare a magnetic nanoparticle
immobilized MacMillan catalyst that catalyzes the
enantioselective 1,3-dipolar cycloaddition between
nitrones and a,b-unsaturated aldehydes. The cata-
lyst can be recovered and recycled for five consecu-
tive runs without any significant loss in yields and
diastereo- and enantioselectivities of the isoxazoli-
dines.
catalysts on magnetic nanosupports may facilitate the
use of organocatalytic processes for industrial applica-
tions.
The chiral MacMillan imidazolidinone is an effi-
cient and versatile organocatalyst for enantioselective
reactions involving a,b-unsaturated aldehydes such as
the Diels–Alder reaction,[6] 1,3-dipolar cycloaddi-
tion,[7] Friedel–Crafts-type alkylation,[8,9] and other or-
ganocatalytic processes.[10] Given its importance and
efficiency, it has been heterogenized in some ways
and used for Diels–Alder reactions[5a,11] and Friedel–
Crafts alkylations.[5b,12] We envisaged that the develop-
ment of an easily synthesizable, inexpensive, recycla-
ble and stereochemically efficient imidazolidinone
catalyst still has to be realized for the 1,3-dipolar cy-
cloaddition[13] which is a very useful method in organ-
ic synthesis.[14] Herein we report the design and syn-
Keywords: asymmetric catalysis; click chemistry;
1,3-dipolar cycloaddition; enantioselectivity; green
chemistry; magnetic separation; nano catalyst
Heterogeneous organocatalysts are emerging as pow- thesis of a new magnetic nanoparticle functionalized
erful tools for asymmetric catalysis because of their chiral imidazolidinone catalyst using 1,3-dipolar
potential advantages over homogeneous catalysts, azide–alkyne cycloaddition which catalyzes the 1,3-di-
such as efficient activity, ease in recovery and poten- polar cycloaddition of nitrones 1 and 2 with a,b-unsa-
tial reusability.[1,2] The immobilization of organocata- turated aldehydes 3 to provide isoxazolidines 4 and 5
lysts is important for the large-scale production of in a highly stereoselective manner (Scheme 1).
chiral compounds, which are abundant as biologically
In the course of our investigations into applications
active molecules and drugs. The key challenges for of MacMillanꢀs imidazolidinone-based catalyst,[15] the
heterogeneous organocatalysts are to develop cost-ef- design of a multifunctional prolinamide using “click
fective strategies for immobilization and to use easily chemistry”[16] and a recyclable magnetic nanocata-
recoverable supports which can help to overcome the lyst,[17] we became interested to design a novel mag-
problems of mechanical loss of the support during re- netically recoverable imidazolidinone catalyst using
cycling experiments. Furthermore, the supported cata- the azide–alkyne cycloaddition. The MNPs supported
lyst should be catalytically comparable or superior to imidazolidinone catalyst has been prepared via two
its homogeneous counterpart and easily recovered strategies using “click chemistry”[18] as shown in
and reused many times with unchanged activity and Scheme 2. In our synthesis we have efficiently ac-
selectivity.[2]
cessed an imidazolidinone alkyne building block 8 in
Magnetic nanoparticles (MNPs) have recently four steps in 72% overall yield from l-phenylalanine
emerged as sustainable catalyst supports[3–5] owing to 6. The Boc protection of l-phenylalanine 6 followed
their easy functionalization and synthesis, low toxicity, by coupling with propargylamine, and subsequent re-
large surface area, facile separation via magnetic moval of the Boc group afforded the amide derivative
force with high dispersion property in organic sol- 7. The amide 7 readily underwent microwave-promot-
vents. Therefore, the development of an easy and ed cyACHTNUGTRENcNUNG lization with acetone in DMF using a catalytic
straightforward strategy for immobilization of organo- amount of p-toluenesulfonic acid (p-TsOH) to afford
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A Magnetoclick Imidazolidinone Nanocatalyst for Asymmetric 1,3-Dipolar Cycloadditions
Scheme 1. Heterogeneous organocatalytic 1,3-dipolar nitrone cycloaddition.
Scheme 2. Synthesis of functional iron nanoparticles.
the imidazolidinone 8 in 81% yield. The dopamine supported dopamine azide 10-MNP was treated with
azide 10 was prepared from dopamine 9, which was the imidazolidinone-containing alkyne 8 in the pres-
then grafted onto Fe3O4 MNPs by ultrasonication. ence of Na-ascorbate and CuSO4·5H2O in t-BuOH/
Magnetite nanoparticles (MNPs) used for this work H2O (1:1) as solvent to afford MNP-A with
were easily prepared via the co-precipitation tech- 0.10 mmolgÀ1 loading. The loading was determined
nique (see the Supporting Information). The MNP from elemental analysis. The catalyst MNP-A was iso-
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Sreenivasarao Pagoti et al.
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Figure 1. (a) TEM image of Fe3O4 MNPs; (b) TEM image of MNP-A; (c) TEM image of recovered MNP-A after 5 cycles;
scale bar 20 nm. (d) Magnetization curves for MNP-A and MNP-B measured at 300 K.
lated by magnetic decantation and washed with meth- spherical nanometer dimensions of Fe3O4 MNPs. The
anol and dicholoromethane several times. The incor- average diameter is 12 nm for MNP-A and MNP-B,
poration of azide functional groups onto the MNP while the average diameter is 11 nm for Fe3O4 MNPs
was confirmed by FT-IR analysis (Supporting Infor- (Figure 1a–c, Supporting Information, Figure S4 and
mation, Figure S1). For a comparison, the homogene- Figure S5). Magnetization curves measured at room
ous imidazolidinone catalyst 11 was prepared from temperature showed that MNPs functionalized with
imidazolidinone 8 and dopamine azide 10 in 67% imidazolidinone are superparamagnetic (Figure 1d).
yield. Immobilization of 11 on Fe3O4 MNPs was fur- MNP-A and MNP-B possessed a saturation magneti-
ther carried out by ultrasonicating a mixture of 11 zation (ss) of 65.5 and 60.5 emugÀ1, respectively (T=
and Fe3O4 MNPs in methanol at room temperature 300 K), which are a little lower than that of the syn-
for 2 h. The MNP supported catalyst MNP-B was ob- thesized Fe3O4-NPs (81.8 emugÀ1). The functionalized
tained with 0.14 mmolgÀ1 loading of the chiral imida- MNPs are highly stable and easily dispersible in water
1
zolidinone 11 determined from H NMR analysis and and a range of organic solvents.
elemental analysis.
We next examined the 1,3-dipolar cycloaddition be-
The average sizes of heterogeneous organocatalysts tween N-benzylidenebenzylamine N-oxide (1a) with
MNP-A and MNP-B were determined using transmis- crotonaldehyde (3a) using 20 mol% functional mag-
sion electron microscopy (TEM) analysis. The TEM netic nanocatalyst MNP-A as shown in Table 1. The
images revealed that both A and B preserved the cycloaddition was performed in nitromethane-water
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A Magnetoclick Imidazolidinone Nanocatalyst for Asymmetric 1,3-Dipolar Cycloadditions
Table 1. Optimization of reaction conditions for 1,3-dipolar cycloaddition using different Brønsted acids.[a]
Entry
HX
Temperature
Time [h]
Conversion[b] [%]
dr 4aa endo:exo[b]
% ee (endo)[c]
1
2
3
4
5
6
7
8
9
TFA
HCl
HCl
HCl
HCl
HBr
HClO4
p-TsOH
EtSO3H
ClAcOH
r.t.
r.t.
r.t.
-208C
-208C
r.t.
r.t.
r.t.
70
70
90
90
120
70
70
70
70
70
>99
>99
>99
50
>99
95
>99
90
50
95:05
70:30
86:14
92:7
92:7
82:18
90:10
81:19
nd
50
90
90
93
93
65
55
70
nd
nd
r.t.
r.t.
10
45
nd
[a]
The reactions were carried out using catalyst A (0.06 mmol), HX (0.06 mmol), nitrone (0.3 mmol), (E)-crotonaldehyde
(1.2 mmol), CH3NO2 (3.8 mL), and H2O (0.2 mL).
Conversions and endo/exo ratios were determined by H NMR analysis of the crude reaction mixtures.
Product enantiomeric ratios were determined by HPLC using a Chiralcel ADV column after reduction of the aldehyde
to alcohol with NaBH4.
[b]
[c]
1
varying Brønsted acid component of the catalyst mogeneous counterparts. The heterogeneous imidazo-
MNP-A. Using TFA as the salt, an excellent conver- lidinone MNP-B (entry 4, Table 2) exhibited compa-
sion (>99%) was achieved (entry 1, Table 1) and the rable yield, diastereoselectivity and enantioselectivity
desired isoxazolidine 4aa was obtained with a high as MNP-A (entry 3, Table 2). Since MNP-A was
diastereoselectivity (dr, 95:05 endo/exo) at room tem- easily accessible using a convergent synthetic proce-
perature for 70 h. However, the enantioselectivity was dure with comparable loading as MNP-B, the scope
in the non-satisfactory range of only 50%. Similarly, of the reaction using various nitrones was surveyed
the cycloaddition proceeded smoothly using HClO4, using MNP-A at À208C.
HBr, and p-TsOH derived catalysts of MNP-A to
As illustrated in Table 2, an array of functionalized
give high conversion and high dr, although a low N-methyl- and N-benzyl-substituted nitrones under-
enantiomeric excess of the major endo adduct was ob- went effective cycloaddition to afford the desired iso-
tained (Table 1, entries 6–8). When the cycloaddition xazolidines 4 and 5 in high yields and good to excel-
was carried out employing 20 mol% of HCl as a coca- lent diastereoselectivities and enantioselectivities
talyst, the catalytic efficiency of MNP-A was signifi- (Table 2). The reaction took place smoothly with cro-
cantly improved. The reaction proceeded with high tonaldehyde 3a (entries 1–9 and entries 18–20) and
conversion (>99%) under similar conditions at room acrolein 3b (entries 10–12) as dipolarophiles to afford
temperature for 70 h and a high diastereoselectivity the desired isoxazolidines in high yields with good to
(70:30) and enantioselectivity (90%) were observed excellent diastereoselectivity and enantioselectivity
for the cycloadduct 4aa (Table 1, entry 2).
(62–96% yield, 80:20 to 93:7 endo:exo, 82–95% ee).
The dr value was further improved with a longer re- As anticipated, we observed some limitations in this
action time (Table 1, entry 3). Superior levels of asym- asymmetric 1,3-dipolar cycloaddition. The reaction of
metric induction and diastereocontrol was obtained methacrolein (3e) with N-benzylidenebenzylamine N-
using the HCl salt of MNP-A at À208C which afford- oxide (1a) was found to be sluggish at À208C
ed the isoxazolidine 4aa in 93% ee, 92:7 dr and in (entry 15). However, when the reaction mixture was
greater than 99% conversion (Table 1, entry 5). stirred at room temperature, the desired product 4ae
Under the optimized reaction conditions, the homoge- was obtained in moderate yield (51%) and selectivity
neous imidazolidinone 11 and the catalyst precursor 8 (70:30 endo:exo, 56% ee). No reaction was observed
exhibited relatively low diastereo- and enantioselec- with sterically crowded aldehydes such as (E)-4-phe-
tivities of the desired isoxazolidines (entries 1 and 2, nylbut-2-enal (3c), cinnamaldehyde (3d) and (E)-2-
Table 2). This may be attributed to the high surface methylpent-2-enal (3f) at room temperature or at
areas and small size of magnetite nanoparticles, which 808C (entries 13, 14 and 17).
are more accessible to the reactants compared to ho-
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Sreenivasarao Pagoti et al.
Table 2. Application of the catalyst for 1,3-dipolar cycloadditions between different nirones and a,b-unstaurated aldehydes.[a]
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Entry
Nitrones
Aldehydes
Cat.
Time
[h]
Yield[b]
[%]
dr
% ee
ACHTUNGTRENNUNG
(endo)[d]
endo:exo[c]
1
2
3
4
5
6
7
8
1a, R1 =Bn, R2 =Ph
3a, R3 =Me, R4 =H
3a, R3 =Me, R4 =H
3a, R3 =Me, R4 =H
3a, R3 =Me, R4 =H
3a, R3 =Me, R4 =H
3a, R3 =Me, R4 =H
3a, R3 =Me, R4 =H
3a, R3 =Me, R4 =H
3a, R3 =Me, R4 =H
3b, R3 =H, R4 =H
3b, R3 =H, R4 =H
3b, R3 =H, R4 =H
3c, R3 =PhCH2, R4 =H
3d, R3 =Ph, R4 =H
3e, R3 =H, R4 =Me
3e, R3 =H, R4 =Me
3f, R3 =Et, R4 =Me
3a, R3 =Me, R4 =H
3a, R3 =Me, R4 =H
3a, R3 =Me, R4 =H
8
11
101
111
120
120
118
110
120
120
111
120
112
110
120
120
120
96[e]
120
120
120
115
4aa, 93
4aa, 91
4aa, 96
4aa, 95
4ba, 86
4ba, 85
4ca, 80
4da, 89
4ea,90
4ab,75
4bb, 65
4bc, 73
–
84:16
84:16
92:7
90:10
86:14
85:15
95:5
93:7
90:10
86:14
85:15
80:20
–
87
80
93
90
93
86
94
94
92
85
90
82
–
1a, R1 =Bn, R2 =Ph
1a, R1 =Bn, R2 =Ph
MNP-A
MNP-B
MNP-A
MNP-B
MNP-A
MNP-A
MNP-A
MNP-A
MNP-A
MNP-A
MNP-A
MNP-A
MNP-A
MNP-A
MNP-A
MNP-A
MNP-A
MNP-A
1a, R1 =Bn, R2 =Ph
1b, R1 =Bn, R2 =4-BrC6H4
1b, R1 =Bn, R2 =4-BrC6H4
1c, R1 =Bn, R2 =4-ClC6H4
1d, R1 =Bn, R2 =4-MeC6H4
1e, R1 =Bn, R2 =4-MeOC6H4
1a, R1 =Bn, R2 =Ph
9
10
11
12
13
14
15
16
17
18
19
20
1b, R1 =Bn, R2 =4-BrC6H4
1c, R1 =Bn, R2 =4-ClC6H4
1a, R1 =Bn, R2 =Ph
1a, R1 =Bn, R2 =Ph
–
–
–
–
–
1a, R1 =Bn, R2 =Ph
4ae, 10
4ae, 51
–
5aa, 62
5ba, 73
5ca, 75
1a, R1 =Bn, R2 =Ph
70:30
–
92:8
86:14
87:13
56
–
95
94
94
1a, R1 =Bn, R2 =Ph
2a, R1 =Me, R2 =Ph
2b, R1 =Me, R2 =4-Cl C6H4
2c, R1 =Me, R2 =4-MeC6H4
[a]
See experimental procedures.
Isolated yield.
[b]
[c]
[d]
[e]
The endo/exo ratio was determined by 1H NMR analysis of crude reaction mixture.
The enantiomeric excess was determined by chiral HPLC analysis.
The reaction was performed at room temperature.
Next we have evaluated the reusability of MNP-A with those reported for imidazolidinones as reaction
in the 1,3-dipolar cycloaddition of N- benzylideneben- catalyst. Moreover the catalyst could be magnetically
zylamine N-oxide (1a) with (E)-crotonaldehyde (3a) recovered and recycled for four times without any sig-
(see the Supporting Information).The recovery of nificant loss in yields and enantioselectivities of the
MNP-A catalyst is achieved quantitatively by magnet- endo adducts. The catalyst should find versatile appli-
ic separation and it was successfully reused for 4 cations in several catalytic reactions.
times with no substantial decrease in yield (yield from
94% to 89%) and with no appreciable decrease of di-
astereo- and enantiocontrol, affording endo isomers
with enantioselectivity always higher than 89% (Sup-
Experimental Section
porting Information, Table S1). The TEM image of
the recovered catalyst after 5 cycles reveals that the
catalyst maintains its nanospheric dimensions with an
average diameter of 13 nm (Figure 1c and the Sup-
General Procedure for the 1,3-Dipolar Cycloaddition
using MNP-A
To a solution of the MNP-A (0.06 mmol) in CH3NO2
(3.8 mL) was added 0.3N HCl (0.2 mL, 0.06 mmol). The
porting Information).
mixture was stirred at room temperature for 10 min and
In conclusion, we have developed a new MNP sup-
then cooled to À208C. Then nitrone 1 or 2 (0.3 mmol) and
ported imidazolidinone catalyst which was used for
aldehyde 3 (1.2 mmol) were added to the flask with stirring.
Additional aldehyde 3 (0.9 mmolꢂ3) was added to the reac-
tion mixture at 24 h intervals until the specified reaction
time was reached. The catalyst MNP-A was removed from
the first time for an asymmetric 1,3 dipolar cycloaddi-
tion of various nitrones with a,b-unsaturated alde-
hydes. The desired isoxazolidines were obtained in
high yields with excellent endo diastereoselectivity
the reaction mixture using an external magnet. The resulting
and enantioselectivity. The results are comparable solution was evaporated under vacuum and purified by
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A Magnetoclick Imidazolidinone Nanocatalyst for Asymmetric 1,3-Dipolar Cycloadditions
silica gel column chromatography to afford the desired
product 4 and 5. The recovered catalyst was washed three
times with MeOH, followed by CH2Cl2 and dried under
vacuum, and was then reused for recycling experiments
(Supporting Information, Table S1).
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Supporting Information
Experimental procedures, characterization data of com-
pounds, FT-IR, PXRD data, TEM images, recycling of the
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1
catalyst, the copies of H NMR and 13C NMR spectra and
HPLC analysis spectra are available in the Supporting Infor-
mation.
Acknowledgements
We thank DST-Nanomission and DST-Green Chemistry
India for research funding. SRP and DD thank CSIR for re-
search fellowships.
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