Recyclable tetraarylphosphonium supported chiral imidazolidin-4-one organocatalyst for Diels-Alder reactions

Article abstract of DOI:10.1016/j.catcom.2013.01.014

The tetraarylphosphonium supported chiral imidazolidin-4-ones were synthesized and used to catalyze the Diels-Alder reactions of cyclopentadiene and α,β-unsaturated aldehydes. High enantiomeric excesses and good yields were obtained, and catalyst recyclin

Full text of DOI:10.1016/j.catcom.2013.01.014

Catalysis Communications 35 (2013) 1–5
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Recyclable tetraarylphosphonium supported chiral imidazolidin-4-one
organocatalyst for Diels–Alder reactions
⁎
Zihan Lin, Zuxing Chen, Guichun Yang, Cuifen Lu
Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei University, Wuhan 430062, PR China
School of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The tetraarylphosphonium supported chiral imidazolidin-4-ones were synthesized and used to catalyze the
Diels–Alder reactions of cyclopentadiene and α,β-unsaturated aldehydes. High enantiomeric excesses and
good yields were obtained, and catalyst recycling (up to five cycles) was accompanied by almost intact
enantioselectivity and some loss of the chemical efficiency.
Received 27 October 2012
Received in revised form 11 December 2012
Accepted 4 January 2013
Available online 27 January 2013
© 2013 Elsevier B.V. All rights reserved.
Keywords:
Organocatalyst
Imidazolidin-4-one
Diels–Alder reaction
Tetraarylphosphonium
Recyclable
1. Introduction
Chiral imidazolidin-4-one
1 (Fig. 1), one of the key chiral
organocatalysts developed by MacMillan's group [19–23], is
a
In the recent years, interest has increasingly taken in the develop-
ment of organocatalysts for organic reactions due to the fact that they
are environmentally benign, easy to handle, and highly efficient
[1–5]. However, the need for high loading and separation of the
organocatalyst from the product are still the issues that need to be
addressed in this area. One obvious way is to immobilize the catalytic
residue on a support [6–14], such as ionic liquid [6], polystyrene [7],
fluorous tag [8], PEG [9], silicagel [10], and dendrimer [11]. The im-
mobilization of chiral catalysts on a support allows easy recovery
and recycling of often expensive catalysts in the process of facilitating
product isolation and purification.
Tetraarylphosphonium salts as new soluble supports are of high
interest due to their lower molecular weight (≈450) when compared
to polymeric materials (≧3000), which results in the loading, at least
ten times higher than those with other traditional polymer supports
[15–18]. Being soluble in many organic solvents and insoluble in a
few other solvents, these supports allow us to run a catalyzed reac-
tion under homogeneous conditions and to isolate and recover the
catalyst as if it was immobilized to a polymer. Moreover, having a
melting point over 100 °C, tetraarylphosphonium salts make purifica-
tion simpler than traditional ionic liquids.
well-known enantioselective catalyst for many different chemical
transformations, such as Diels–Alder [19,20], Michael addition [21],
1,3-dipolaraddition [22], Friedel–Craft alkylation reactions [23], and
so on. Although the immobilization of MacMillan's catalyst onto poly-
meric or inorganic materials has been developed by several groups
[24–30], there is no reported tetraarylphosphonium salts variant to
date.
Described in this paper is the tetraarylphosphonium supported chi-
ral imidazolidin-4-ones organocatalyst for the enantioselective Diels–
Alder reactions which have comparable yields and enantioselectivities
to the non-supported chiral imidazolidin-4-one. It can be readily
recovered and reused several times with slight loss of catalytic activity
and stereoselectivity.
2. Experimental
2.1. General
Reactions were monitored by TLC using precoated plates of silica
gel HF254 (0.5 mm, Yantai, China). Column chromatography was
performed with a silica-gel column (200–300 mesh, Yantai, China).
NMR spectra were recorded on Varian Unity Inova 600 spectrometer
(1H at 600 MHz and 13C at 150 MHz). IR spectra were recorded on an
IR-spectrum one (PE) spectrometer. HPLC was performed with Dionex
UltiMate 3000 and equipped with a chiral column (Chiralcel OJ-RH,
Daicel) using CH3CN/H2O as an eluent. A UV detector (UVD-3000) was
used for the peak detection.
⁎
Corresponding author at: Ministry-of-Education Key Laboratory for the Synthesis and
Application of Organic Functional Molecules, Hubei University, Wuhan 430062, PR China.
Tel.: +86 27 5086 5322.
E-mail address: lucf@hubu.edu.cn (C. Lu).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.01.014

2
Z. Lin et al. / Catalysis Communications 35 (2013) 1–5
O
80%. The filtrate was concentrated under vacuum, and the formed
R3
dimethyl acetal was hydrolyzed by stirring the crude product mixture
in TFA/H2O/CHCl3 (1:1:2) for 2 h at 25 °C, followed by neutralization
with saturated aqueous NaHCO3, extraction with Et2O and purification
by silica gel chromatography (n-hexane/EtOAc, 100:1, v/v) to give the
Diels–Alder adducts 6 and 7 (0.174 g, 88%), in which the endo/exo
ratio was determined by 1H NMR. All the Diels–Alder adducts in the
paper are known compounds that exhibited spectroscopic data identi-
cal to those reported in the literature [19,31].
N
a: R1=R2=R3=Me;
b: R1=R2=Me, R3=n-Bu;
c: R1=H, R2=C(CH3), R3=Me.
R2
HN
R1
1
Fig. 1. Structure of chiral imidazolidin-4-one 1.
2.4. General procedure for determination of enantioselectivity of the
2.2. General procedure for preparation of the tetraarylphosphonium
Diels–Alder products
supported chiral imidazolidin-4-ones 5a–c
Sodium borohydride (80 mg, 2.1 mmol) was added to a stirred solu-
tion of endo/exo mixtures of aldehydes 6 and 7 (48 mg, 0.7 mmol) in
methanol (5.0 mL) at 0 °C. After the solution was stirred at 25 °C for
2 h, the reaction was quenched by the addition of aqueous ammonium
chloride. The product was extracted with CH2Cl2 and the combined
organic layer was washed with water and brine, dried over MgSO4
and concentrated to give a mixture of the endo-and exo-alcohols
which were analyzed by HPLC using Daicel Chiralcel OJ-RH column
(CH3CN/H2O: 40/60, flow rate 0.6 mL/min, λ=225 nm; for the exo
isomer: TR1 =22.59 min (minor), TR2 =26.40 min (major); for the
endo isomer: TR1 =29.06 min (major), TR2 =32.05 min (minor)).
To a stirred solution of the Diels–Alder adduct (204 mg, 1.5 mmol)
derived from crotonaldehyde and cyclopentadiene (Entry 1 in Table 3)
in ethanol (10 mL) at room temperature, 2,4-dinitrophenylhydrazine
(296 mg, 1.5 mmol) was added, and the resulting orange solution was
stirred at room temperature for 30 min. The crude reaction mixture
was poured into water (10 mL) and the aqueous phase was extracted
with ether (4×10 mL). The organic solution was washed with brine
(20 mL), dried over MgSO4 and concentrated to give the corresponding
N,N-diphenylhydrazone that could be analyzed directly by HPLC using
Daicel Chiralcel OD-H column ((hexane/2-propanol:: 95/5, flow rate
1.0 mL/min, λ=254 nm; for the endo isomer: TR1 =14.07 min (minor),
To a solution of compound 4 (0.514 g, 1.0 mmol) and imidazolidin-
4-one 2a (0.351 g, 1.5 mmol) in CH2Cl2 (10 mL), PPh3 (0.393 g,
1.5 mmol) was added at 0 °C. After stirring for 15 min, the slurry
was treated dropwise with DEAD (0.3 mL, 1.5 mmol) in CH2Cl2
(10 mL). Within 15 h of stirring at 0 °C, the reaction mixture was
concentrated in vacuum to about 5 mL, then the residue was slowly
poured in Et2O (30 mL). The precipitate was filtered on Celite and
washed with 20 mL of diethyl ether, the residue was purified by
flash chromatography (CH2Cl2/MeOH, 50:1) to obtain 5a (0.651 g,
89%). IR (NaCl): υ 3418, 2972, 1683, 1109, 838 (PF6) cm−1 1H NMR
;
(600 MHz, CDCl3): δ 7.88–7.84 (m, 3H, ArH), 7.83–7.82 (m, 2H, ArH),
7.76–7.73 (m, 6H, ArH), 7.63–7.60 (m, 8H, ArH), 7.17 (d, J=7.8 Hz,
2H, ArH), 6.92 (d, J=7.8 Hz, 2H, ArH), 5.21 (s, 2H, CH2O), 3.75 (t, J=
5.4 Hz, 1H, COCH), 3.08 (dd, J=4.8, 14.4 Hz, 1H, PhCH), 2.96 (dd, J=
6.6, 14.4 Hz, 1H, PhCH), 2.74 (s, 3H, NCH3), 1.26 (s, 3H, CCH3), 1.19 (s,
3H, CCH3); 13C NMR (150 MHz, CDCl3): δ 173.10, 156.95, 146.09,
135.58, 134.47, 134.40, 134.25, 134.18, 130.57, 128.84, 117.77, 117.18,
114.98, 68.50, 59.22, 59.18, 36.37, 27.13, 25.03.
2.3. General procedure for the Diels–Alder reaction
To a solution of 5a (0.073 g, 0.1 mmol) in CH3OH/H2O (2 mL, 95:5,
v/v), 0.1 M HCl (1.0 mL, 0.1 mmol) was added, and the 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
24–48 h. MgSO4 was then added, after 2 h the mixture was filtered,
and the organic solvent evaporated under vacuum. The residue was
dissolved in the minimum amount of CH2Cl2 (5 mL) and then poured
in Et2O (30 mL). The precipitate was filtered, washed with Et2O
(5 mL) and dried under vacuum to afford the tetraarylphosphonium
supported catalyst. Average recovery of the catalyst ranged from 70 to
TR2 =18.17 min (major); for the exo isomer: TR1 =21.19 min (major),
TR2 =30.07 min (minor)).
3. Results and discussion
3.1. Preparation of catalysts 5a–c
The synthesis for the tetraarylphosphonium supported chiral
imidazolidin-4-ones 5a–c is shown in Scheme 1. Starting from
L-tyrosine methyl ester hydrochloride, imidazolidin-4-ones 2a–c
O
O
1) R3NH2, EtOH,
R3
OMe
30 oC
N
HO
HO
a: R1=R2=R3=Me; 65%.
NH2HCl 2) R1COR2, PTSA,
MeOH, 60 oC
R2
b: R1=R2=Me, R3=n-Bu; 62%.
HN
R1
c: R1=H, R2=C(CH3), R3=Me. 63%.
2
OH
OH
OH
PPh3, NiBr2
Ph3P+
KPF6
Ph3P+
Br
Br-
MeCN, 25 oC, 99%
-
(CH2OH)2, 180 oC, 91%
PF6
3
4
O
R3
R2
2, DEAD, Ph3P
a: R1=R2=R3=Me; 89%.
Ph3P+
N
O
CH2Cl2, 0 oC
b: R1=R2=Me, R3=n-Bu; 85%.
-
HN
c: R1=H, R2=C(CH3), R3=Me. 82%.
PF6
R1
5
Scheme 1. Synthesis of the tetraarylphosphonium supported chiral imidazolidin-4- ones 5a–c.

Z. Lin et al. / Catalysis Communications 35 (2013) 1–5
3
Table 1
Asymmetric Diels–Alder reaction of cinnamaldehyde with cyclopentadiene.
Entry
Catalyst
Time (h)
Yield (%)a
Endo/exob
ee (%)c
Endo
Exo
1
2
3
4
5
6
7
8
5a/HCl
5a/TFA
5a/HClO4
5a/HCld
5a/HCle
5a/HClf
5a/HClg
5a/HClh
5a/HCli
5b/HCl
5c/HCl
1a/HCl
1b/HCl
1c/HCl
24
48
48
24
48
24
24
48
48
48
48
24
24
24
88
55
58
65
30
57
90
50
85
55
53
98
89
75
57/43
60/40
61/39
57/43
55/45
57/43
57/43
54/46
55/45
60/40
57/43
43/57
56/44
63/37
92
93
91
92
90
92
92
90
90
92
93
93
91
89
71
72
72
71
71
71
71
83
85
71
73
93
89
89
9
10
11
12
13
14
a
Isolated yield of a mixture of endo and exo isomers.
b
c
d
e
f
Exo/endo ratios were determined by 1HNMR analysis.
ee values were determined by HPLC of alcohol after reduction of the formyl group.
Reaction was run in MeCN/H2O (95:5, v/v).
Reaction was run in THF/H2O (95:5, v/v).
0.05 mol equiv of catalyst.
g
h
i
0.2 mol equiv of catalyst.
Reaction was run at 0 °C.
Reaction was run at 40 °C.
3.2. Catalytic activities of catalysts 5a–c in Diels–Alder reactions
were easily obtained in good yields [15]. On the other hand, the
functionalized tetraarylphosphonium salt 3 was obtained by Ni-
catalyzed reaction, which is conducted in ethylene glycol using a
readily available Ni (II) and triphenylphosphine [14]. Subjecting 3
to potassium hexafluorophosphate in aqueous solution furnished
the corresponding hexafluorophosphate 4 in quantitative yield.
The solubility of tetraarylphosphonium salts is counterion-dependent;
therefore, to simplify the purification procedure, the bromide counter-
ion was exchanged to generate less soluble phosphonium salts. Then,
by the Mitsunobu reaction of imidazolidin-4-ones 2a–c with tetraaryl-
phosphonium salt 4, we can get the tetraarylphosphonium supported
chiral imidazolidin-4-ones 5a–c. The tetraarylphosphonium supported
chiral imidazolidin-4-ones 5a–c are stable white solid, and they were
kept on bench for weeks without any sign of decomposition. They
were soluble in polar solvent systems (CH2Cl2, CH3CN, DMSO, DMF),
and insoluble in less polar solvents (hexane, Et2O, toluene), thus the
tetraarylphosphonium supported catalysts could be readily isolated by
precipitation from hexane, Et2O or toluene.
In order to demonstrate the efficiency of the tetraarylphosphonium
supported chiral imidazolidin-4-ones 5a–c, the asymmetric Diels–Alder
reaction of cinnamaldehyde and cyclopentadiene was chosen as a
model reaction. In situ conversion of the tetraarylphosphonium
supported chiral imidazolidin-4-ones 5a–c to the catalytically ac-
tive species involved the addition of an equimolar amount of differ-
ent acids, and the resulting ammonium salts (0.1 mol equiv) were
employed, under different conditions, in the Diels–Alder reaction
between cinnamaldehyde (1.0 mol equiv) and cyclopentadiene
(3.0 mol equiv) to afford mixtures of endo-cycloadduct 6 and exo-
cycloadducts 7. The results are reported in Table 1, together with
those obtained from non-supported organocatalyst 1. Yields were
determined on the isolated products, diastereoisomeric ratios were
determined by 600 MHz 1H NMR analysis of the crude products,
and confirmed on isolated compounds; enantiomeric excesses (ee)
were obtained by HPLC analysis employing Daicel Chiralcel OJ-RH
column after quantitative reduction of aldehyde to the correspond-
ing alcohol with NaBH4.
As reported in Table 1, effects of acid cocatalyst on reactivity and selec-
tivity in the present Diels–Alder reaction were examined (Entries 1–3).
The reactions were carried out in the presence of 0.1 mol equiv of differ-
ent 5a/HX combinations in MeOH/H2O (95:5, v/v) at 25 °C. The
stereoselectivities are comparative with three 5a/HX combinations, but
the chemical yields are very different, with 5a/HCl securing best chemical
yield. In the further exploration we chose HCl as acid cocalalyst.
To check the effect of the solvent, reactions were performed with
HCl as acid cocatalyst in different mixed solvents (Entries 1, 4 and 5).
Among the solvents examined, MeOH/H2O provided a better result
(Entry 1, 88% yield, 57/43 endo/exo, endo 92% ee, exo 71% ee) than that
in CH3CN/H2O (Entry 4), while the use of THF/H2O led to a substantial
decrease in chemical yield (Entry 5).
Table 2
Recycling of catalyst 5a for asymmetric Diels–Alder reaction between cinnamaldehyde
and cyclopentadiene.
Run
Catalyst
Time (h)
Yield (%)a
Endo/exob
ee (%)c
Endo
Exo
1
2
3
4
5
5a/HCl
5a/HCl
5a/HCl
5a/HCl
5a/HCl
24
24
24
48
48
88
78
65
60
60
57/43
57/43
57/43
55/45
54/46
92
92
91
89
88
71
73
73
71
70
a
b
c
Isolated yield of a mixture of endo and exo isomers.
Exo/endo ratios were determined by 1H NMR analysis.
ee values were determined by HPLC of alcohol after reduction of the formyl group.

4
Z. Lin et al. / Catalysis Communications 35 (2013) 1–5
Table 3
Scope of catalyst 5a in the asymmetric Diels–Alder reaction of cyclopentadiene and representative dienophiles.
Entry
R
Time (h)
Yield (%)a
Endo/exob
ee (%)c
Endo
Exo
1
2
3
4
5
Me
n-Pr
4-MeOPh
4-NO2Ph
Ph
12
12
24
24
24
95
92
78
75
88
59/41
65/35
71/29
41/59
57/43
92
85
91
90
92
91
93
70
92
71
a
Isolated yield of a mixture of endo and exo isomers.
Exo/endo ratios were determined by 1H 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
b
c
alcohol after reduction of the formyl group.
The influence of the catalyst amount in MeOH/H2O on the Diels–
Alder reaction was also examined (Entries 1, 6 and 7). Decreasing the
catalyst amount to 0.05 mol equiv slowed down the reaction, but pre-
served the stereoselectivity. Increasing the catalyst amount to 0.2 mol
equiv, the chemical yield and stereoselectivity were comparable to the
reaction with 0.1 mol equiv catalyst.
Furthermore, the effect of temperature on the Diels–Alder reac-
tion was investigated (Entries 1, 8 and 9). Independent of the catalyst
counterion, lower or comparative yields were obtained when the re-
action was carried out at 0 °C and 40 °C respectively. Thus, the tem-
perature of 25 °C is more suitable for this reaction.
With the optimized reaction condition in hand, the tetraaryl-
phosphonium supported chiral imidazolidin-4-ones 5b and 5c were
evaluated for the Diels-Alder reaction for a comparison to the use of
5a (Entries 10 and 11). The diastereoselectivity and enantioselectivity
by the use of the catalysts 5b and 5c were higher or comparative, but
the chemical yields were lower than that using catalyst 5a. We propose
that maybe the steric hindrance influenced the catalyst activity.
To compare the catalytic activity of the tetraarylphos-phonium
supported catalysts 5a–c with the non-supported catalysts 1a–c, the
asymmetric Diels–Alder reaction of cinnamaldehyde and cyclopentadiene
with the non-supported catalysts 1a–c under our optimized conditions
was also examined (Entry 12). It was found that the chemical yield by
the use of the non-supported catalysts 1a–c was higher than the
tetraarylphosphonium supported catalysts 5a–c, the ee of endo and exo
was comparative. We deduce that the structure of tetraarylphosphonium
salt had some effect on reaction conversion but had little impact on
enantioselectivity.
3.4. Scope of catalyst 5a
To probe the scope of Diels–Alder reactions catalyzed by the
tetraarylphosphonium supported chiral imidazolidin-4-one 5a, four
other Diels–Alder reactions of cyclopentadiene and representative
dienophiles (α,β-unsaturated aldehydes) were conducted (Table 3,
Entries 1–4). Variation of the olefin substituent (R=Me, n-Pr, Entries
1 and 2) is possible with higher yield or enantioselectivity (>92%
yield, endo >85% ee, exo >91% ee). The reaction is also tolerant of ar-
omatic groups on the dienophile component (Entries 3–5, >75%
yield, endo >90% ee, exo >70% ee). Thus, the generality of the
tetraarylphosphonium supported chiral imidazolidin-4-one 5a as an
efficient organocatalyst for Diels–Alder reactions has been clearly
demonstrated.
4. Conclusions
In conclusion, a simple procedure for preparation of the tetraaryl-
phosphonium supported chiral imidazolidin-4-ones 5a–c was developed.
The tetraarylphosphonium supported organocatalyst 5a provides
consistently high enantioselectivities in Diels–Alder reactions of
cyclopentadiene and α,β-unsaturated aldehydes, and it can readily
be recovered from the reaction mixture by simple precipitation, fil-
tration and drying. The enantioselectivity of the recovered catalyst
remained almost intact for at least five reaction cycles, while the
chemical efficiency of the recovered catalyst slowly eroded upon
its iterative re-use.
Acknowledgments
3.3. Recycling and reuse of catalyst 5a
We gratefully acknowledge the National Science Foundation of
China (No. 21042005) and the Natural Sciences Foundation of Hubei
province in China (No. 2010CDA019) for providing financial support.
We are also grateful to the referees for their valuable suggestions.
In addition to facilitating the separation of the catalyst from the
reaction products, catalyst immobilization on a support allows simple
recovery and recycling. The recycling experiments were also readily
performed. After a reaction was completed, the catalyst 5a which
was collected by precipitation, filtration and dried, was then used
for the next reaction. We have performed five recycling experiments
for the Diels–Alder reaction of cinnamaldehyde and cyclopentadiene
with the tetraarylphosphonium supported chiral imidazolidin-4-one
5a, and the results are compiled in Table 2. The data collected in
Table 2 indicate that the stereoselectivity of the recovered catalyst
remained almost intact for at least five reaction cycles, while the
chemical efficiency of the recovered catalyst slowly eroded upon its
iterative re-use.
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