Non-Covalently Immobilized Chiral Imidazolidinone on Sulfated-Chitin: Reusable Heterogeneous Organocatalysts for Asymmetric Diels-Alder Reaction

Article abstract of DOI:10.1002/adsc.201901036

A heterogeneous chiral imidazolodinone catalyst was synthesized by immobilization on a sulfated chitin through non-covalent ionic interactions. The chitin-based organocatalyst promoted the asymmetric Diels-Alder reaction with high enantioselectivity under heterogeneous conditions and was successfully reused multiple times without apparent loss of catalytic activity and enantioselectivity.

Full text of DOI:10.1002/adsc.201901036

Title: Non-Covalently Immobilized Chiral Imidazolidinone on Sulfated-
Chitin: Reusable Heterogeneous Organocatalysts for Asymmetric
Diels-Alder Reaction
Authors: Mirai Watanabe, Takuya Sakai, Marina Oka, Yuki Makinose,
Hidetoshi Miyazaki, and Hiroki Iida
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201901036
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10.1002/adsc.201901036
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Non-Covalently Immobilized Chiral Imidazolidinone on
Sulfated-Chitin: Reusable Heterogeneous Organocatalysts for
Asymmetric Diels-Alder Reaction
Mirai Watanabe,a Takuya Sakai,a Marina Oka,a Yuki Makinose,a Hidetoshi Miyazakia
and Hiroki Iidaa*
a
Department of Chemistry, Graduate School of Natural Science and Technology, Shimane University, 1060
Nishikawatsu, Matsue 690-8504, Japan
E-mail: iida@riko.shimane-u.ac.jp
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
regular helical conformation in cristalin[3] and
Abstract.
A heterogeneous chiral imidazolodinone
solution phases.[4] If these linear helical polymers are
employed as solid supports for organocatalysts, their
controlled regular structure is expected to provide a
uniform stereocontrolled reaction field around the
supported catalyst, which may minimalize the loss of
catalytic activity due to immobilization. Indeed, the
well-controlled helical conformation of the polymers
has recently been proved effective for enhancing both
the enantioselectivity and catalytic activity of the
supported catalysts.[5]
catalyst was synthesized by immobilization on a sulfated
chitin through non-covalent ionic interactions. The
chitin-based organocatalyst promoted the asymmetric
Diels-Alder reaction with high enantioselectivity under
heterogeneous conditions and was successfully reused
multiple times without apparent loss of catalytic activity
and enantioselectivity.
Keywords: Organocatalysis; heterogeneous catalysis;
asymmetric Diels-Alder reaction; imidazolidinone; chitin
With the anticipation that the use of the easily
available chitin would further open up opportunities
for the preparation of diverse and efficient polymer-
based catalysts, we recently developed a novel chitin-
based achiral supramolecular catalyst.[6] In that study,
we took advantage of the ionic immobilization
strategy for cationic catalysts,[7,8] where ionic
interactions between organocatalysts and the
synthetic polymer supports were used for catalyst
preparation instead of the conventional covalent
bonding linkages. As a result, we demonstrated the
use of a reusable supramolecular catalyst in which
flavinium and alkali metal cations were non-
covalently assembled on anionic sulfated-chitin via
The development of polymer-immobilized chiral
organocatalysts has attracted increasing attention
owing to the advantages such as ready product
separation, and catalyst reusability, due to which
these catalysts are major contributors to green and
sustainable chemistry.[1] The immobilization of
organocatalysts has been typically performed by
covalent attachment onto artificial polymer scaffolds,
which generally requires time- and cost-intensive
polymeric catalyst synthesis. In addition, the random
conformation of the polymers and the chemical
modifications for attaching the covalent linkages
around the catalytically active sites often results in
decreased catalytic activity and selectivity of the
immobilized catalyst in comparison with the
corresponding small molecule catalysts. Therefore,
development of a simple immobilization method
which uses readily accessible stereocontrolled
polymer scaffolds and avoids chemical modification
of the catalyst is necessary.
Chitin is the world’s second most abundant
biopolymer with global reserves of 1011 tons and is
routinely extracted from crustacean shells, insect
exoskeletons, and fungi.[2] It is chemically stable but
is also degradable under specific conditions. It has
poor solubility in common solvents and contains
many functionalities which offer possibilities for
modification. Furthermore, chitins tend to form a
ionic
interactions
without
the
chemical
modification.[6] Due to the cooperative catalysis of
the flavinium, alkali metal, and sulfated chitin, the
heterogeneous supramolecular catalyst afforded
superior catalytic activity for Baeyer-Villiger
oxidation of ketones in comparison with the
corresponding homogeneous flavinium catalyst. To
apply this strategy to asymmetric organocatalysis, we
report herein, the synthesis of the first set of chitin-
immobilized chiral organocatalysts (sc-1•Nas) which
use an anionic sulfated-chitin (sc-Na) as a scaffold
and
the
commercially
available
chiral
imidazolidinone (1) as a cationic organocatalyst
module. Furthermore, we investigated the catalytic
activity, enantioselectivity, and reusability of the
prepared catalyst for catalyzing an asymmetric Diels-
Alder reaction.
1
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10.1002/adsc.201901036
Advanced Synthesis & Catalysis
Chiral imidazolidinones (1) developed by
11% D.I. (sc-1S0.11•Na) and 50% D.I. (sc-1S0.50•Na)
were obtained, respectively (entries 1 and 2). To
achieve complete exchange the Na+ ion of sc-Na with
1, we prepared sc-2Na by complexation of sc-Na
with 15-crown-5 (2) and attempted the ion-exchange
reaction between (S)-1•Cl and sc-2Na in H2O under
homogeneous conditions (method B in Scheme 1).
After stirring an aqueous solution of (S)-1•Cl and sc-
2Na for 15 min, the reaction mixture was washed
with Et2O and the residue was reprecipitated in EtOH
to afford sc-1S0.97•Na with 97% D.I. (entry 3 in Table
1). Importantly, this immobilization experiment was
reproducible (entries 3 and 4). Similarly, the use of
MacMillan are versatile cationic organocatalysts for
diverse asymmetric transformations including
asymmetric Diels-Alder reactions, 1,3-dipolar
cycloadditions, Friedel-Crafts alkylations and so
on,[9] and a series of these catalysts are commercially
available.[10] Owing to the typical challenges faced in
the recovery and reuse of these catalysts, several
approaches for immobilizing imidazolidinone
derivatives onto solid supports have been
developed.[7d,11] However, these immobilization
processes often suffer from tedious synthetic routes,
low selectivities, and efficiencies. Despite the many
advances in this area, the development of a facile
approach which provides eco-friendly immobilized
heterogeneous chiral imidazolidinone catalysts with
high enantioselectivity and catalytic activity remains
a challenge.
(R)-1•Cl afforded poly-1•2R
without a significant
0.96
1
difference of D.I. (entry 5). The H NMR spectra of
(S)-2•Cl, poly-1•Na, poly-1•2S0.98, and sc-1R0.96•Na in
D2O revealed that the immobilization occurred
without any decomposition of 1 or the sulfated chitin
(Figure 1A (a-d)). Figure 1B shows the photographs
and SEM images of sc-Na (f and h) and sc-1S0.97·Na
(g and i). After immobilization of 1, the original
microfibers of the lyophilized sulfated chitin grew
slightly, but the three-dimensional sponge-like
scaffold with relatively large surface area remained
without any significant change (Figure 1B (h and i)).
Interestingly, the diastereomeric pair, sc-1S0.97•Na,
and sc-1R0.96•Na showed nearly identical signals,
O
O
O
Na
O
O
O
OSO3
OSO3 Na
15-crown-5
O
O
O
(2)
HO
HO
n
n
NHAc
NHAc
method B
sc-Na
sc-2 Na
O
Me
N
Me
Me
1•Cl
Bn
Cl–
N
method A
H2
Table 1. Immobilization of cationic 1 on anionic sc-Na
2
NaCl
NaCl
1•Cl
D.I.[a]
Sulfated
chitin
Catalyst
(equiv.)
Yield
(%)
O
Me
Entry
Product
(Y,
N
Me
Me
equiv.)
Bn
N
(S)-1•Cl
(0.16)
(S)-1•Cl
(10)
OSO3
H2
OSO3 Na
1[b]
2[b]
3[c]
4[c]
5[c]
sc-Na
sc-Na
sc-1S0.11•Na
sc-1S0.50•Na
sc-1S0.97•Na
sc-1S0.98•Na
sc-1R0.96•Na
0.11
0.50
0.97
0.98
0.96
0.15
96
97
90
90
78
95
O
O
O
O
HO
HO
NHAc
nY
NHAc
n(1-Y)
sc-1•Na
(S)-1•Cl
(20)
sc-2Na
sc-2Na
sc-2Na
sc0.55-Na
Scheme 1. Synthesis of sc-1•Na via ion-exchange reaction
between 1•Cl and sc-Na.
(S)-1•Cl
(20)
(R)-1•Cl
(20)
The chitin-immobilized imidazolidinone catalysts sc-
1•Nas were readily prepared by an ion-exchange
reaction between 1•Cl and 6-O-sulfated chitin (sc-
Na), which was synthesized by the simple sulfation
of commercially available chitin (Scheme 1)[12] and
the results for the synthesis are summarized in Table
1. The ion-exchange reaction between 1•Cl and
lyophilized sc-Na (degree of sulfation, D.S. = 0.99
equiv., Supporting Information) was first carried out
under heterogeneous conditions by soaking sc-Na in
a MeOH solution of 1•Cl for 30 min, according to the
procedure used for the preparation of chitin-
immobilized flavinium catalyst (entries 1 and 2 in
Table 1, method A in Scheme 1).[6] Since the
obtained supramolecular sc-1•Na showed moderate
solubility in H2O, the amount of immobilized 1
(degree of immobilization, D.I.) was estimated by
NMR analysis in D2O. When 0.16 and 10 equivs. of
(S)-1•Cl were used in MeOH (0.08 M), sc-1•Nas with
(S)-1•Cl
(5.5)
sc0.55
-
6[b,d]
1S0.15•Na
b)
a)
1
Determined by H NMR. Immobilization was carried
out heterogeneously by soaking sc-Na in a solution of 1•Cl
c)
in MeOH (0.08 M) for 30 min at ca. 25 ˚C.
The
immobilization was carried out homogeneously by mixing
sc-2Na (0.08 M) and 1•Cl in H2O for 15 min at ca. 25 ˚C.
d) sc0.55-Na was used.
2
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10.1002/adsc.201901036
Advanced Synthesis & Catalysis
the highest level of enantioselectivity (exo 91% ee,
endo 92% ee, entry 4). While immobilization of
homogeneous chiral catalysts onto solid supports is
often known to result in a decrease of their inherent
enantioselectivity, it is noteworthy that the catalytic
activity and enantioselectivity of heterogeneous sc-
1S0.97•Na was almost comparable to that of the
unsupported homogeneous catalyst 1•Cl (entry 1).
Interestingly, Sc0.55-1S0.15•Na (D.S. = 0.55 equiv)
showed a relatively lower enantioselectivity than that
of the corresponding sc-1S0.11•Na (D.S. = 0.99 equiv,
entries 2 and 5). The observed D.I. and D.S. effects
suggested that the high catalytic activity and
enantioselectivity of sc-1S0.97•Na originates from its
high degree of the structural uniformity. Sc-1R0.96•Na
which immobilizes (R)-1 gave the exo- and endo-
(2R)-5 with almost comparable enantioselectivity
(exo 88% ee, endo 90% ee) to its diastereomeric
isomer sc-1S0.97•Na without significant match-
mismatch effect (entry 6). The chirality of chitin did
not seriously affect the chiral reaction field of 1, as
1
revealed by the H NMR measurement (Figure 1A (c
and d)). Therefore, sc-Na can be used as a universal
support for (R)- and (S)-1 without consideration of
the match-mismatch effects. The weak and loose
ionic interactions of the organocatalyst with the chiral
sulfated chitin likely prevents a significant decrease
of the inherent enantioselectivity of the well-designed
organocatalyst.
1
Figure 1. (A) H NMR spectra (500 MHz, D2O, 80 ˚C) of
1•Cl (a), sc-Na (b), sc-1S0.98•Na (c), sc-1R0.96•Na (d), and
recovered sc-1S0.98•Na collected after the asymmetric
Diels-Alder reaction (Entry 10 in Table 2). (B)
Photographs (f,g) and SEC images (h,i) of sc-Na and sc-
1S0.97•Na.
which suggested that the chirality of chitin may not
significantly affect the inherent chirality of 1. The
supramolecular catalyst, sc0.55-1S0.15•Na was also
prepared from partially sulfated sc0.55-Na (D.S. = 0.55
equiv, entry 6).
Having prepared a series of chitin-immobilized
catalysts, we turned attention to studying their
catalytic activity. We began with the evaluation of the
catalysis of the Diels-Alder reaction between 3a and
4 in MeOH/H2O at 23 ˚C, which is known to proceed
in an asymmetric fashion with the use of 1•Cl (entry
1, Table 2),[9a] using sc-1•Nas, and were pleased to
see that the reaction proceeded in an asymmetric
fashion. A series of sc-1•Nas with different D.I.
successfully promoted the catalytic asymmetric
Diels-Alder reaction under heterogeneous conditions,
and furnished the corresponding exo- and endo-(2S)-
5a in high enantioselectivity (entries 2-4 in Table 2).
Both catalytic activity and enantioselectivity
appeared to be dependent on D.I. and increased with
increasing D.I., and as a result, sc-1S0.97•Na provided
3
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10.1002/adsc.201901036
Advanced Synthesis & Catalysis
[a]
Table 2. Asymmetric Diels-Alder reaction catalyzed by sc-1•Na
Entry
1
Catalyst (mol%)
(S)-1•Cl (5)
Product
5a
Time (h)
72
Yield[b] (%)
Exo:endo[c]
1.3:1
1.1:1
1.2:1
1.2:1
1.1:1
1.2:1
1.2:1[e]
1.2:1
1.2:1
1.2:1
1.2:1
1.2:1
1.3:1
1.3:1
1.2:1
1.2:1
1.2:1
1:1.7
Ee[d] (exo, %)
Ee[d] (endo, %)
91 (2S)
86 (2S)
90 (2S)
92 (2S)
79 (2S)
90 (2R)
93 (2S)[e]
93 (2S)
90 ( 2S)
91 (2S)
93 (2S)
91 (2S)
94[f]
74
47
50
66
59
60
82[e]
91
81
81
80
79
80
77
73
75
63
61
90 (2S)
88 (2S)
90 (2S)
91 (2S)
84 (2S)
88 (2R)
91 (2S)[e]
91 (2S)
90 ( 2S)
91 (2S)
91 (2S)
90 (2S)
93[f]
2
sc-1S0.11•Na (5)
sc-1S0.50•Na (5)
sc-1S0.97•Na (5)
sc0.55-1S0.15•Na (5)
sc-1R0.96•Na (5)
sc-1S0.98•Na (20)
sc-1S0.98•Na (20), 1st
2nd reuse
5a
72
3
5a
72
4
5a
72
5
5a
72
6
5a
72
7
5a
24
8
5a
24
9
5a
24
10
11
12
13
14
15
16
17[g]
3rd reuse
4th reuse
5th reuse
5a
24
5a
24
5a
24
sc-1S0.97•Na (20)
sc-1S0.97•Na (20)
sc-1S0.97•Na (20)
sc-1S0.97•Na (20)
sc-1S0.98•Na (20)
sc-1S0.97•Na (20)
5b
5c
72
48
90[f]
93[f]
88[f]
89[f]
92[f]
95[f]
96[f]
91[f]
5d
5e
24
24
5f
24
18[g]
5g
48
89[f]
95[f]
a)
Conditions: 3 (0.5 M), 4 (3 equiv.), catalyst (5–20 mol%), MeOH/H2O (19:1, v/v), 23 ˚C, 24–72 h. Catalyst loading of
the polymeric catalysts was calculated based on the amount of the immobilized 1. b) Isolated yield. Determined by H
c)
1
d)
1
NMR. Determined by H NMR after reaction with (1R,2R)-N-(4-toluenesulfonyl)-1,2-diphenylethylene-1,2-diamine
(TsDPEN). Average of two runs. f) Determined by chiral HPLC measurement using Daicel Chiralpak IA-3 or IB-3
e)
g)
column after the products were converted to the corresponding alcohols with NaBH4. Conditions: 3 (0.42 M), 4 (3
equiv.), catalyst (20 mol%), MeOH/DMF/H2O (19:4:1, v/v), 23 ˚C, 24–48 h.
With the use of 20 mol% of sc-1S0.98• the
asymmetric reaction proceeded efficiently to afford
the product in 81% yield along with the highest ee
(exo 91% ee, endo 93% ee, entry 7). After reaction
completion, sc-1S0.98•Na was easily separable by
simple filtration of the reaction mixture, due to the
insolubility of sc-1•Nas in common organic solvents
such as MeOH, EtOH, DMSO, CH3CN, 1,4-dioxane,
THF, EtOAc, Et2O, hexane, CH2Cl2, and CHCl3. The
1H NMR spectrum of the recovered sc-1S0.98•Na
indicated that 92% of 1 remained intact without
major dissociation (Fig. 1(e)). Since the cationic 1 is
tightly attached onto anionic sulfated chitin through
ionic interactions, the recovered sc-1S0.98•Na could be
reused at least 5 times without significant loss of the
catalytic activity and enantioselectivity (entries 8-12
in Table 2). Using substituted cinnamaldehydes
bearing electron-donating and withdrawing, OMe,
Me, Cl, Br, and NO2 groups (3b-g), the catalytic
products 5b-g in good yields and high
enantioselectivities (entries 13-18 in Table 2).
In conclusion, we developed a facile approach for
preparation of a recyclable heterogeneous asymmetric
organocatalyst from commercially available chiral
imidazolidinone 1 and the abundantly available chitin,
which could perform asymmetric Diels-Alder
reactions under the heterogeneous conditions without
decreasing the inherent catalytic activity and
enantioselectivity of the parent homogeneous catalyst
1. This is the first example of a non-covalently
immobilized asymmetric organocatalyst on a chitin
derivative. Although the catalytic activity and
selectivity may not reach a satisfactory level for
practical applications at this stage, these findings not
only demonstrates the possibility for use of the
naturally occurring chitin, but also provides a novel
strategy for heterogeneous catalyst design, which will
enable access to novel recyclable and practical
asymmetric organocatalysts, which fulfill the
requirements of green and sustainable chemistry.
asymmetric
Diels-Alder
reaction
proceeded
successfully and delivered the corresponding
4
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10.1002/adsc.201901036
Advanced Synthesis & Catalysis
Experimental Section
[3] a) K. H. Gardner, J. Blackwell, Biopolymers 1975, 14,
1581-1595; b) R. Minke, J. Blackwell, J. Mol. Biol.
1978, 120, 167-181; c) T. Yui, K. Imada, K. Okuyama,
Y. Obata, K. Suzuki, K. Ogawa, Macromolecules 1994,
27, 7601-7605.
Typical procedure for synthesis of sc-1•Na (method A).
Sc-Na (206 mg, 0.686 mmol) was soaked in a solution of
1•Cl in MeOH (80 mM, 43.3 mL, 3.5 mmol) for 30 min at
room temperature. The insoluble polymer was collected by
filtration, and soaked again in a solution of 1•Cl in MeOH
(80 mM, 42.4 mL, 3.4 mmol) for 30 min at room
temperature. The resulted white polymer was then
collected by filtration, and washed with MeOH (14 mL),
and dried in vacuo at room temperature, giving sc-
1S0.50•Na (264 mg, 97%) as a white powder. These results
[4] a) E. F. Franca, R. D. Lins, L. C. G. Freitas, T. P.
Straatsma, J. Chem. Theory Comput. 2008, 4, 2141-
2149; b) E. F. Franca, L. C. G. Freitas, R. D. Lins,
Biopolymers 2011, 95, 448-460.
1
are summarised in Table 1 and the H NMR spectra are
shown in Figure S1.
[5] For examples of helical polymer-based catalysts, see: a)
M. Reggelin, S. Doerr, M. Klussmann, M. Schultz, M.
Holbach, Proc. Natl. Acad. Sci. U. S. A. 2004, 101,
5461-5466; b) K. Maeda, K. Tanaka, K. Morino, E.
Yashima, Macromolecules 2007, 40, 6783-6785; c) T.
Yamamoto, M. Suginome, Angew. Chem. 2009, 121,
547-550; Angew. Chem. Int. Ed. 2009, 48, 539-542; d)
Z. Tang, H. Iida, H.-Y. Hu, E. Yashima, ACS Macro
Lett. 2012, 1, 261-265; e) Y. Akai, T. Yamamoto, Y.
Nagata, T. Ohmura, M. Suginome, J. Am. Chem. Soc.
2012, 134, 11092-11095; f) D. Zhang, C. Ren, W.
Yang, J. Deng, Macromol. Rapid Commun. 2012, 33,
652-657; g) H. Iida, S. Iwahana, T. Mizoguchi, E.
Yashima, J. Am. Chem. Soc. 2012, 134, 15103-15113;
h) T. Yamamoto, R. Murakami, M. Suginome, J. Am.
Chem. Soc. 2017, 139, 2557-2560; i) E. Yashima, N.
Ousaka, D. Taura, K. Shimomura, T. Ikai, K. Maeda,
Chem. Rev. 2016, 116, 13752-13990.
Spectroscopic data of sc-1S0.50•Na: White powder. IR (KBr,
cm-1): 3448, 1714, 1655, 1559, 1382, 1216, 1004, 811, 756.
1H NMR (500 MHz, D2O, 80 ˚C): δ 7.55-7.33 (m, 2.5H,
ArH), 4.62 (br, 1H, -CH(O-)2), 4.58-4.53 (m, 0.5H,
PhCH2CH-), 4.38-4.01 (2H, CH2, overlapped with HOD),
3.95–3.52 (br, m, 4H, CH), 3.45 (dd, J = 12, 4.6 Hz, 0.5H,
PhCH2-), 3.18 (dd, J = 15, 8.7 Hz, 0.5H, PhCH2-), 2.89 (s,
1.5H, -NCH3), 2.07 (s, 2.73H, -COCH3), 1.62 (s, 1.5H, -
NC(N)CH3), 1.60 (s, 1.5H, -NC(N)CH3). Anal. Calcd (%)
for C14.32H21.33N2Na0.49O8.38S0.99: C, 43.15; H, 5.39; N, 7.03.
Found: C, 43.39; H, 5.16; N, 7.07.
Typical procedure for synthesis of sc-1•Na (method B).
15-Crown-5 (1.09 g, 4.95 mmol) was added to an aqueous
solution of sc-Na (83 mM, 20.0 mL, 1.7 mmol), and the
mixture was stirred for 2 h at room temperature. The
resulted aqueous solution was washed with Et2O (100 mL),
and lyophilized to give sc-2⊃Na (1.29 g, 83%) as a white
powder. An aqueous solution of sc-2⊃Na (78 mM, 4.30
mL, 0.34 mmol) was prepared, and to this was added 1•Cl
(1.71 g, 6.7 mmol). After the mixture was stirred for 15
min at room temperature, the obtained polymer complex
was precipitated in EtOH (216 mL), collected by filtration,
washed with EtOH (1.9 mL), and dried in vacuo to give sc-
1S0.98•Na (149 mg, 90%) as a white powder. These results
[6] T. Sakai, M. Watanabe, R. Ohkado, Y. Arakawa, Y.
Imada, H. Iida, ChemSusChem 2019, 12, 1640-1645.
[7] a) S. Luo, J. Li, L. Zhang, H. Xu, J.-P. Cheng, Chem.
Eur. J. 2008, 14, 1273-1281; b) Y. Arakawa, N.
Haraguchi, S. Itsuno, Angew. Chem. 2008, 120, 8356-
8359; Angew. Chem. Int. Ed. Engl. 2008, 47, 8232-
8235; c) J. S. Gao, J. Liu, S. Y. Bai, P. Y. Wang, H.
Zhong, Q. H. Yang, C. Li, J. Mater. Chem. 2009, 19,
8580-8588; d) N. Haraguchi, Y. Takemura, S. Itsuno,
Tetrahedron Lett. 2010, 51, 1205-1208; e) R. C. Zhang,
G. H. Yin, Y. Li, X. L. Yan, L. G. Chen, RSC Adv.
2015, 5, 3461-3464.
1
are summarised in Table 1 and the H NMR spectra are
shown in Figure S1.
Spectroscopic data of sc-1S0.98•Na: White powder. IR (KBr,
cm-1): 3446, 1715, 1653, 1558, 1396, 1218, 1008, 809, 753.
1H NMR (500 MHz, D2O, 80 ˚C): δ 7.66-7.19 (m, 4.9H,
ArH), 4.78-4.52 (m, 1.98H, -CH(O-)2, PhCH2CH-), 4.51-
4.03 (2H, CH2, overlapped with HOD), 3.90–3.53 (m, 4H,
ArH), 3.47 (br, 0.98H, PhCH2-), 3.20 (br, 0.98H, PhCH2-),
2.89 (s, 2.94H, -NCH3), 2.07 (s, 2.73H, -COCH3), 1.63 (s,
2.94H, -NC(N)CH3), 1.60 (s, 2.94H, -NC(N)CH3). Anal.
Calcd (%) for C20.56H30.45N2.96Na0.01O8.86S0.99: C, 50.11; H,
6.23; N, 8.41. Found: C, 49.82; H, 6.41; N, 8.35.
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M. Hassan, RSC Adv. 2014, 4, 52023-52043 and ref 1b.
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This work was supported in part by JSPS KAKENHI (Grant-in-
Aid for Scientific Research (C), no. 16K05797 and 19K05617)
and the Shorai Foundation For Science and Technology.
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UPDATE
Non-Covalently Immobilized Chiral
Imidazolidinone on Sulfated-Chitin: Reusable
Heterogeneous Organocatalysts for Asymmetric
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Adv. Synth. Catal. Year, Volume, Page – Page
Mirai Watanabe, Takuya Sakai, Marina Oka, Yuki
Makinose, Hidetoshi Miyazaki and Hiroki Iida*
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