A High Loading and Recyclable Pentaerythritol Supported Imidazolidin-4-one Catalyst for Enantioselective Diels–Alder Reactions

Article abstract of DOI:10.1007/s10562-016-1724-7

Abstract: The synthesis of high loading and recyclable pentaerythritol supported imidazolidin-4-one catalyst I and its application in enantioselective Diels–Alder reactions of cyclopentadiene and α,β-unsaturated aldehydes with high performance were descri

Full text of DOI:10.1007/s10562-016-1724-7

Catal Lett (2016) 146:1107–1112
DOI 10.1007/s10562-016-1724-7
A High Loading and Recyclable Pentaerythritol Supported
Imidazolidin-4-one Catalyst for Enantioselective Diels–Alder
Reactions
1
1
1
1
1
Kaitao Du
•
Cuifen Lu
•
Zuxing Chen
•
Junqi Nie
•
Guichun Yang
Received: 7 March 2016 / Accepted: 13 March 2016 / Published online: 1 April 2016
Ó Springer Science+Business Media New York 2016
Abstract The synthesis of high loading and recyclable
pentaerythritol supported imidazolidin-4-one catalyst I and
its application in enantioselective Diels–Alder reactions of
cyclopentadiene and a,b-unsaturated aldehydes with high
performance were described. Especially noteworthy, the
pentaerythritol supported imidazolidin-4-one with high
loading capacity can be recovered by simple precipitation
and filtration, and recycled for up to four runs without
observing significant decrease in catalytic activity.
Keywords High loading Á Recyclable Á Pentaerythritol Á
Imidazolidin-4-one Á Diels–Alder reactions
1 Inroduction
During the past decades, much interest has been devoted to
the development of efficient organocatalysts for asymmetric
organic transformations [1–4]. Variety of organocatalysts
have been explored and applied in different organic process,
providing efficient access to valuable chiral compounds.
Among them, chiral imidazolidin-4-one, designed and
developed by Macmillan’s group [5–9], is one of the most
efficient chiral organocatalysts for its high efficiency and
selectivity in asymmetric reactions. However, its practical
use in reactions is still hindered due to the need for high
catalyst loading and difficulties in recovering the catalyst
from the reaction mixture. One obvious way to overcome
these problems is to immobilizing or attaching the catalyst
unit on a recyclable support [10–31], such as PEG [11],
dendrimer [12, 13], siliceous and polymer-coated
mesocellular foam (MCF) [14], fluorous tag [15], poly(-
methyl-hydrosiloxane) (PHMS) [16], ionic liquid [17–20].
In our ongoing research project on the development of
recyclable catalyst, recently we have developed
tetraarylphosphonium supported imidazolidin-4-one and
applied it in asymmetric Diels–Alder reactions [28] and
Graphical Abstract
catalyst (10 mol% catalyst monomer)
O
+
+
Ph
CHO
CH
O
o
Solvent, T( C) HX (10 mol%)
Ph
endo
exo
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-016-1724-7) contains supplementary
material, which is available to authorized users.
&
&
Cuifen Lu
lucf@hubu.edu.cn
1,3-dipolar cycloaddition [29]. Tetraarylphosphonium salt
used as a support successfully overcomes the recycling
issue, but its loading capacity and enantioselectivity still
remains less satisfied.
Guichun Yang
yangguichun@hubu.edu.cn
1
Hubei Collaborative Innovation Center for Advanced
Organochemical Materials & Ministry-of-Education Key
Laboratory for the Synthesis and Application of Organic
Functional Molecules, Hubei University, Wuhan 430062,
China
Hence, we introduce pentaerythritol as a new support to
immobilize chiral imidazolidin-4-one. Pentaerythritol is a
well known, cheap, atoxic, widely used chemical raw
materials, and it is considered to be ideal building blocks in
123

1
108
K. Du et al.
many areas of materials chemistry [32–34] and in the
synthesis of dendrimers [35–37]. Because of its low
molecular weight and its special molecular structure, the
loading capacity of pentaerythritol is higher than many
other kind of supports such as traditional polymer support
Et O and dried under vacuum to obtain I or II as a white
2
solid. Catalyst I (1.12 g, 85 %); Catalyst II (1.0 g, 74 %).
2.3 Synthesis of Pentaerythritol Supported Chiral
Imidazolidin-4-One III
[
10–12, 14, 16, 22], ionic liquid [17–20] and
tetraarylphosphonium salt [28, 29]. Owing to its den-
drimer-like structure, we believe that pentaerythritol used
as a support not only can solve the problem of recovering
which meet the requirement of modern green chemistry for
sustainability, but also may enhance the catalytic perfor-
mance like traditional dendrimer supports do, even without
generation growing. In addition, chiral-functionalized unit
Compound 5 (0.2 g, 0.85 mmol), compound 2 (1.85 g,
6.8 mmol) and dry THF (10 mL) were charged to a 25 mL
three-neck round-bottomed flask equipped with a magnetic
stirrer. The system was degassed and purged with nitrogen,
0
00 000
then CuBr (24 mg, 0.17 mmol) and N,N,N ,N ,N -pen-
tamethyldiethylenetriamine (PMDETA) (35 lL,
0.17 mmol) were added. The reaction mixture was stirred
at room temperature for 20 h and dropped into dilute HCl
(1 wt %, 20 mL). The resultant mixture was extracted with
CH Cl (20 mL 9 3), dried over Na SO , and concen-
immobilized
on
pentaerythritol
for
asymmetric
organocatalysis is still absent in the literature.
Described in this paper is the high loading and recy-
clable pentaerythritol supported chiral imidazolidin-4-ones
organocatalyst for the enantioselective Diels–Alder reac-
tions, which is wished to run the catalyzed reaction under
homogeneous conditions with excellent yields and high
enantioselectivities and recover catalysts by the simple
precipitation method.
2
2
2
4
trated under vacuum to about 2 mL, and slowly poured into
Et O (30 mL). The precipitate was filtered on Celite,
2
washed with 10 mL of Et O and dried under vacuum to
2
obtain catalyst III as a white solid (0.96 g, 86 %).
2.4 General Procedure for the Diels–Alder Reaction
To a solution of catalyst I (50 mg, 0.05 mmol, 0.2 mmol
catalyst monomer) in CH CN/H O (1 mL, 95:5, v/v),
2
Experimental
3
2
0.4 M HClO4 (0.5 mL, 0.2 mmol) was added, and the
2
.1 General
mixture was stirred for 5 min at 25 °C. Freshly distilled
cinnamaldehyde (0.13 mL, 1.0 mmol) and cyclopentadiene
(0.33 mL, 4.0 mmol) were added respectively, and the
resulting mixture was stirred at 25 °C for 12–48 h. MgSO4
was then added, the mixture was filtered, and the organic
solvent evaporated under vacuum. The residue was dis-
solved in the minimum amount of CH Cl (2 mL) and then
Reactions were monitored by TLC using precoated plates of
silica gel HF254 (0.5 mm, Yantai, China). Column chro-
matography was performed with a silica gel column (200–300
mesh, Yantai, China). NMR spectra were recorded on WIPM
1
13
400 spectrometer ( H at 400 MHz, C at 100 MHz). IR
2
2
spectra were recorded on an IR-spectrum one (PE) spectrom-
eter. High-resolution mass spectra (HRMS) were recorded on
Agilent 1260–6224 LC–MS TOF using ESI (electrospray
ionization). Enantioselectivity were determined by HPLC
poured into Et O (30 mL). The precipitate was filtered,
2
washed with Et O (10 mL), dried under vacuum and
2
reused in further reactions as the recycled catalyst. The
filtrate was concentrated under vacuum and purified by
silica gel column chromatography using petroleum ether
and ethyl acetate as eluent to afford the desired product.
(Dionex UltiMate 3000) analysis employing a Daicel Chi-
ralCel OD-H, ChiralPak AS-H columns and Chiralcel OJ-RH.
A UV detector (UVD-3000) was used for the peak detection.
1
The endo/exo ratio was determined by H NMR spec-
troscopy of the crude mixture and the enantiomeric excess
(ee) of the adduct was determined by HPLC on a chiral
phase according to the established procedure [28]. All the
products are known compounds and all the spectroscopic
data matched those reported in the literature [5, 41].
2
.2 General Procedure for Synthesis
of Pentaerythritol Supported Chiral
Imidazolidin-4-Ones I–II
To a solution of compound 3 (1.0 g, 1.3 mmol) or com-
pound 4 (1.0 g, 0.87 mmol) in dry DMF (10 mL) was
added imidazolidin-4-one 1 (1.6 g, 6.8 mmol) and Cs CO3
3 Results and Discussion
2
(
6.1 g, 19.5 mmol) at room temperature. After stirring at
8
0 °C for 30 h, the mixture was evaporated under vacuum,
3.1 Preparation of Catalysts I–III
then the residue was dissolved in the minimum amount of
CH Cl (2 mL) and slowly poured into Et O (30 mL). The
The preparation of pentaerythritol supported chiral imida-
2
2
2
precipitate was filtered on Celite, washed with 10 mL of
zolidin-4-ones I–III is shown in Scheme 1. Chiral
1
23

A High Loading and Recyclable Pentaerythritol Supported Imidazolidin-4-One Catalyst…
1109
TsO
TsO
O
R O
O
1
Cs CO ,DMF
2
3
OTs+ HO
N
R O
O
N
1
60-80 o
C
HN
HN
TsO
1
R O
1
I
3
O
R1O
OR1
O
O
TsO
TsO
TsO
OTs
OTs+HO
OTs
Cs CO ,DMF
2
3
O
N
R1O
O
N
6
0-80 o
C
HN
O
HN
O
4
R1O
OR1
1
II
N3
R2
R2
CuBr,PMDETA
THF, r.t.
N
N
+
N3
N3
O
O
N
N
R2
N
HN
HN
N3
5
2
III
O
O
N
N
R1=
N
R = N
2
O
N
HN
HN
Scheme 1 Preparation of pentaerythritol supported chiral imidazolidin-4-ones I–III
imidazolidin-4-ones 1–2 [28, 40] and pentaerythritol
derivatives 3–5 [38, 39] were prepared by the established
procedure according to the corresponding literatures,
respectively. Catalysts I–II can be obtained with good
yield by the reaction between 3 or 4 with 1 in dry DMF in
the presence of Cs CO . Then a CuAAC reaction between
low to moderate yield and good enantioselectivity, while
only trace amount of product was observed in the solvent
system of THF/H O, CH Cl /H O and 1,4-dioxane/H O.
2
2
2
2
2
Among the solvent system examined, CH CN/H O pro-
3
2
vided the best result (Entry 7, 73 % yield, 55/45 exo/endo,
endo 99 % ee, exo 89 % ee). Therefore, CH CN/H O was
2
3
3
2
5
with 2 was performed to give catalyst III in good yield.
chosen as the solvent to further optimize the reaction
conditions by screening different acids as reaction co-
catalysts.
The loadings of pentaerythritol supported catalysts I–III
are 4.00 mmol/g for catalyst I, 3.87 mmol/g for catalyst II
and 3.02 mmol/g for catalyst III.
As illustrated in Table 1 (Entries 7–14), eight types of
protic acids were screened as the reaction co-catalyst. The
stereoselectivities are comparative with eight I/HX com-
binations, but the chemical yields are very different, with I/
Catalysts I–III are soluble in polar solvents (CH Cl ,
2
2
CH CN, CH OH) and insoluble in less polar solvents
3
3
(
Et O, hexane, toluene), thus the pentaerythritol supported
2
catalysts can be run the catalyzed reaction under homo-
geneous condition in polar solvent and readily isolated by
precipitation from less polar solvents.
HClO securing best chemical yield (Entry 14, 79 % yield).
4
In the further exploration we chose HClO as reaction co-
4
catalysts.
The influence of the catalyst amount (based on the
amount of catalysis monomer) in the Diels–Alder reaction
was also examined (Table 1, entries 14–16). Decreasing
the catalyst amount to 5 mol % preserved the stereoselec-
tivity, but reduced the chemical yield (79 ? 62 % yield).
Increasing the catalyst amount to 20 mol %, the reaction
preformed the best result (Entry 16, 89 % yield, 53/47 exo/
endo, endo 99 % ee, exo 92 % ee).
3
.2 Catalytic Activities of Catalysts I–III in Diels–
Alder Reactions
The catalytic capacity of the prepared pentaerythritol
supported catalysts I–III was examined in the Diels–Alder
reaction between cinnamaldehyde and cyclopentadiene.
And in order to find an optimal conditions for the Diels–
Alder reaction, we first explored the catalytic activities of
the catalyst I (10 mol %, based on the amount of catalysis
monomer) by using HCl (10 mol %) as reaction cocatalyst.
The solvent used in the reaction is often important, so
we investigated the Diels–Alder reaction in different sol-
vent system at 25 °C for 24 h (Table 1, entries 1–7). All of
the mixed solvent system, the volume ratio of the solvent
and water is 95/5. The use of CH NO /H O, CH CN/H O,
Next, the effects of reaction temperature in the Diels–
Alder reaction was investigated (Table 1, entries 16–21).
Among the temperature examined, 25 °C provided the best
yield and comparative stereoselectivity, and it is the best
control temperature, so 25 °C is the best candidate for the
Diels–Alder reaction.
Furthermore, the optimal reaction time of the Diels–
Alder reaction under the condition of CH CN/H O as
3
2
2
3
2
3
2
MeOH/H O and single H O gave the desired product in
2
solvent, HClO as cocatalyst, 25 °C and 5 mol % catalyst
2
4
123

1
110
K. Du et al.
Table 1 Optimization of Diels–Alder reactions between cinnamaldehyde and cyclopentadiene
I /HClO4 (5 mol%)
R
O
+
+
R
CHO
CH CN/H O,25oC
3
2
CHO
R
endo
exo
a
b
c
d
Entry
Catalyst
Solvent
HX
T(°C)
Time (h)
Yield (%)
exo/endo
ee (%)
endo
exo
1
2
3
4
5
6
7
8
9
I
CH
THF/H
1,4-dioxane/H
2
Cl
2
/H
2
O
HCl
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
-20
0
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
12
48
24
24
24
24
24
24
b
trace
trace
trace
65
37
36
73
70
64
40
32
72
76
79
62
89
40
63
75
78
80
52
90
78
81
90
85
82
75
–
–
–
I
2
O
HCl
–
–
–
I
2
O
HCl
–
–
–
I
CH
3
NO
2 2
/H O
HCl
55:45
54:46
55:45
55:45
53:47
55:45
58:42
53:47
53:47
56:44
52:48
54:46
53:47
52:48
52:48
54:46
52:48
52:48
56:44
54:46
54:46
54:46
53:47
52:48
54:46
52:48
99
99
99
99
99
97
99
97
99
99
99
98
99
99
99
99
99
99
99
99
99
99
93
99
99
99
86
89
87
I
MeOH/H
2
O
HCl
I
2
H O
HCl
I
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
O
HCl
I
O
TFA
TfOH
p-TSA
90
92
93
76
91
87
90
91
92
95
95
94
92
92
92
90
90
90
92
91
91
91
I
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24/*
25
26
27
28
29
a
I
I
Cl
HBF
HPF
3
CCOOH
I
4
I
6
I
HClO
HClO
HClO
HClO
HClO
HClO
HClO
HClO
HClO
HClO
HClO
HClO
HClO
HClO
HClO
HClO
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
e
I
f
I
I
I
I
10
15
20
25
25
25
25
25
25
25
25
I
I
I
I
II
III
IV
I
g
h
i
I
I
c
The volume ratio of the solvent and water in the mixed solvent system is 95/5. Isolated yield of a mixture of endo and exo isomers. Exo/
1
endo ratios were determined by H NMR analysis. ee values were determined by HPLC of alcohol after reduction of the formyl group.
d
e
f
mol % catalyst monomer. 20 mol % catalyst monomer.
g
nd
2
h
rd
i
th
3 run. 4 run
5
run.
was examined (Table 1, entry 16, and entries 22–23).
Reducing reaction time to 12 h led to a decrease in the
yield (89 ? 52 % yield). Increasing the reaction time to
with the catalyst I (Table 1, entry 24–25 vs entry16). The
enantioselectivities and chemical yields by the use of the
catalysts II and III were lower than that using catalyst I. It
seems that catalyst II with more branches has crowded
surfaces, so the catalytically active imidazolidin-4-ones
moieties are not freely accessible for enantioselective
reactions, it being likely less available to the reaction
substrate. Hence, the loss of enantioselectivity and chem-
ical yield with catalyst II might be ascribed to steric
4
8 h, the yield was slightly increased to 90 %, while the
stereoselectivity is nearly unaltered or lower, so 24 h is
enough for the reaction.
With the optimized reaction conditions in hand, pen-
taerythritol supported chiral imidazolidin-4-ones II and III
were evaluated for the Diels–Alder reaction to compare
1
23

A High Loading and Recyclable Pentaerythritol Supported Imidazolidin-4-One Catalyst…
1111
hindrance between the imidazolidin-4-ones moieties at the
periphery. Catalyst III with a 1.2.3-triazole linker, which is
designed as a spacer to increase spatial separation between
catalyst moieties and as a block to increase the space
shielding of the catalyst moieties, provides lower yield and
enantioselectivity than catalyst I. These results demonstrate
that the improvement in yield is ascribed to the different
proximity of imidazolidin-4-ones moieties in the terminal
units, while space shielding provided by 1.2.3-triazole has
less impact on the enantioselectivity.
21–23, 28, 30, 31]. A rationale of this finding may be found
in the characteristics of the pentaerythritol employed.
3.3 Recycling and Reuse of Catalyst I
Catalyst recycling and reuse are important issues, so the
recovery and reusability of catalyst I were investigated.
After a reaction was completed, catalyst I which was col-
lected by precipitation, filtration and dried under vacuum,
was then used for the next reaction. Average recovery of
the catalyst ranged from 75 to 85 %. The recovered cata-
For further investigation of the impact of pentaerythritol
structure on catalytic activity, the asymmetric Diels–Alder
reaction of cinnamaldehyde and cyclopentadiene with
methoxylated Macmillian catalyst IV (Fig. 1) under our
optimized conditions was also examined (Table 1, entry
1
lysts were examined by H NMR spectroscopy, which
showed unchanged after each recovery. We have per-
formed four recycling experiments, and the results are
compiled in Table 1 (Entries 16, and 27–29). The data
collected in Table 1 indicates that the stereoselectivity of
the recovered catalyst remained almost intact for at least
four reaction cycles, while the chemical efficiency of the
recovered catalyst slowly eroded upon its iterative reuse.
2
6). It was found that the chemical yield and the ee of exo
was comparative, but pentaerythritol supported catalyst
I exhibited a higher ee of endo than methoxylated
Macmillan catalyst IV. We deduce branched structure of
pentaerythritol had some effect on enantioselectivity but
had little impact on reaction conversion.
3.4 Scope of Catalyst I
To compare the catalytic activity of pentaerythritol
supported catalyst
tetraarylphosphonium supported imidazolidin-4-one [28,
9], it was found that catalyst I exhibited the comparative
I
with our previously reported
Experiments that probe the scope of Diels–Alder reactions
catalyzed by pentaerythritol supported chiral imidazolidin-
4-ones I are summarized in Table 2. Variation of the olefin
substituent (R = Me, n-Pr, Entries 1 and 2) is possible with
higher yield or enantioselectivity ([ 92 % yield,
endo [ 92 % ee, exo [ 90 % ee) and shorter reaction time.
The reaction is also tolerant of aromatic groups on the
dienophile component (Entries 3–5, [ 82 % yield,
endo [ 92 % ee, exo [ 90 % ee). Thus it can be seen that
pentaerythritol supported chiral imidazolidin-4-one I is an
efficient organocatalyst for Diels–Alder reactions.
2
yield and the higher enantioselectivity either for endo
isomer or exo isomer. Especially noteworthy, catalyst
I exhibited the better capacity for stimulating a slightly
higher enantioselectivity for exo isomer as compared to
most of the supported imidazolidin-4-one catalysts [15–18,
4
Conclusions
In conclusion, a high loading and recyclable pentaerythritol
supported chiral imidazolidin-4-one was developed
through the simple synthesis route, and which was applied
Fig. 1 Methoxylated Macmillan catalyst IV
Table 2 Scope of catalyst I in the Diels–Alder reactions
a
b
c
c
Entry
R
Time (h)
Yield (%)
exo/endo
endo ee (%)
exo ee (%)
1
2
3
4
5
a
Me
12
12
24
24
24
96
92
82
80
89
35:65
39:61
52:48
66:34
53:47
95
92
92
92
99
c
93
90
98
93
92
n-Pr
4-MeOPh
4-NO
Ph
2
Ph
b
1
Isolated yield of a mixture of endo and exo isomers. Exo/endo ratios were determined by H NMR analysis. ee values in entries 1–2 were
determined by HPLC after converted into the corresponding N,N-diphenylhydrazone, while ee values in entries 3–5 were determined by HPLC
of alcohol after reduction of the formyl group
123

1
112
K. Du et al.
in enantioselective Diels–Alder reactions of cyclopentadi-
ene and a,b-unsaturated aldehydes providing the desired
products in good yields with excellent enantioselectivities.
It is worth mentioning that the pentaerythritol supported
imidazolidin-4-one catalyst, whose high loading capacity is
comparable with the original Macmillan’s catalyst, can be
readily recovered from the reaction mixture by simple
precipitation and filtration. The enantioselectivity of the
recovered catalyst remained almost intact for at least four
reaction cycles, while the chemical efficiency of the
recovered catalyst slowly eroded upon its iterative reuse.
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Acknowledgments We gratefully acknowledge the National Natu-
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