Recyclable organotungsten Lewis acid and microwave assisted Diels-Alder reactions in water and in ionic liquids

Article abstract of DOI:10.1016/j.tet.2004.09.078

The water-soluble, organotungsten Lewis acid, [OP(2-py)₃W(CO) (NO)₂](BF₄)₂ (1), was synthesized and characterized. A series of 1-catalyzed Diels-Alder reactions were investigated under conventional heating or microwave heating conditions. The cycloaddition reactions were efficiently conducted in either water or in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate. The ionic liquid acts as a powerful medium not only for rate- and selectivity enhancements but also for facilitating catalyst recycling. Dramatic rate acceleration via microwave flash heating as compared to thermal heating was observed. Graphical Abstract

Full text of DOI:10.1016/j.tet.2004.09.078

Tetrahedron 60 (2004) 11903–11909
Recyclable organotungsten Lewis acid and microwave assisted
Diels–Alder reactions in water and in ionic liquids
*
I-Hon Chen, Jun-Nan Young and Shuchun Joyce Yu
Department of Chemistry and Biochemistry, National Chung Cheng University, Min-Hsiung, Chia-Yi 621, Taiwan, ROC
Received 2 September 2004; revised 20 September 2004; accepted 21 September 2004
Available online 28 October 2004
Abstract—The water-soluble, organotungsten Lewis acid, [O]P(2-py)3W(CO)(NO)2](BF4)2 (1), was synthesized and characterized. A
series of 1-catalyzed Diels–Alder reactions were investigated under conventional heating or microwave heating conditions. The
cycloaddition reactions were efficiently conducted in either water or in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate.
The ionic liquid acts as a powerful medium not only for rate- and selectivity enhancements but also for facilitating catalyst recycling.
Dramatic rate acceleration via microwave flash heating as compared to thermal heating was observed.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
and alkylation reactions, aromatic and nucleophilic substi-
tutions, condensations, cycloadditions, protection and
deprotection reactions, esterifications and transesterifica-
tions, heterocyclizations, rearrangements, organometallic
reactions, oxidations and reductions assisted by microwave
heating have been performed and reviewed in articles8,9 or
books.10 In addition, ionic liquids have been demonstrated
to couple very effectively with microwaves through an ionic
conduction mechanism.11b Also, small amounts of an ionic
liquid can be used as additives to increase the dielectric
constant of a molecular solvent.11b Several recent studies in
this area used ionic liquids, or mixtures of ionic liquids and
molecular solvents, as reaction media in a number of
important microwave-heated transformations,11 including
Diels–Alder reactions.12
The Diels–Alder reaction is one of the most powerful
synthetic tools for the construction of six-membered ring
systems. Remarkable success in the rate acceleration and
selectivity enhancement for various Diels–Alder reactions
has been accomplished in the past two decades by the
application of catalyst, high pressure, sonication, and
solvent manipulation, etc.1 Among these the effects
provided by Lewis acid catalysts have been most widely
studied. Special solvent effects of water which are almost
equivalent to catalysis leading to dramatic enhancements of
rate and stereoselectivity have also been demonstrated.2,3
Due to the limited miscibility of water with most organic
substrates, the possibility of using ionic liquids as a solvent
substitutes for water and also as Lewis acid catalysts on
Diels–Alder reactions has been explored.4 A very recent
trend in the development of environmentally benign
processes is to use water5 or room temperature ionic
liquids6 as solvent media for Diels–Alder catalysis. In the
meantime, microwave technology is now a powerful and
innovative tool for improving organic synthesis, because it
is economical, efficient and provides better selectivity,
higher reaction rates, milder reaction conditions, formation
of cleaner products with higher yields and minor wastes.
The successful application of microwave in chemistry dated
since 1975.7 Numerous organic reactions such as acylation
We report herein the synthesis of a novel organotungsten
Lewis acid 1 and a series of 1-catalyzed Diels–Alder
reactions with particular emphasis on the combined effects
of this Lewis acid catalyst, the water or ionic liquid solvent
system, and microwave radiation on the reaction rates and
selectivity. The structures of dienes 2–7 and dienophiles a–f
investigated in the current study are illustrated in Figure 1.
2. Results and discussion
The complex [O]P(2-py)3W(CO)(NO)2](BF4)2 (1) was
easily synthesized in two steps within 30 min from the
commercially available W(CO)6 (Scheme 1). The precursor
O]P(2-py)3W(CO)3 obtained in the first step can be
prepared either through conventional heating or under
Keywords: Organotungsten Lewis acid; Diels–Alder; Microwave; Water;
Ionic liquid.
* Corresponding author. Tel.: C886 5 242 8138; fax: C886 5 272 1040;
e-mail: chejyy@ccu.edu.tw
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.09.078

11904
I.-H. Chen et al. / Tetrahedron 60 (2004) 11903–11909
Figure 1. The numbering scheme for the dienes, dienopliles and the Diels–Alder adducts.
microwave irradiation conditions. The first method took
more than 48 h and gave 80% yield of product after work-up
procedures, while the second method gave 100% yield of
high purity product in just 7 min. The direct reaction of
O]P(2-py)3W(CO)3 and 2 equiv of NOBF4 afforded 1
which can be stored as a crystalline solid in air for months
without significant decomposition. In addition, 1 is very
water-soluble (78 g/L H2O) and possesses strong Lewis
acidity upon loss of the CO ligand. The relative Lewis acid
strength of 1 was found to be comparable to that of AlCl3.13
Scheme 1.

I.-H. Chen et al. / Tetrahedron 60 (2004) 11903–11909
11905
Because of its high solubility and stability in water, we
therefore decided to study the effects of using Lewis acid 1
in water on Diels–Alder reactions.
Due to the usually low substrate solubility and the
possibility of promoting the competing diene polymeriz-
ation, the potential applications of water in the current
1-catalyzed Diels–Alder systems are limited. We therefore
chose the air-stable room-temperature ionic liquid, 1-butyl-
3-methylimidazolium hexafluorophosphate (bmimPF6),15,16
as a solvent alternative. The bmimPF6 is in the same polarity
region as the lower alcohols, such as methanol, ethanol and
1-butanol,17 but it provides miscibility with a much broader
range of organic substrates. Also, due to its neutral and
weakly coordinating nature,18 the bmimPF6 does not
interfere with the behavior of the Lewis acid catalyst
employed. Table 2 shows the results of the 1-catalyzed
Diels–Alder reactions in bmimPF6. As shown in the table, a
much more variety of substrates can now be applied, and
excellent yields and selectivities are obtained in all cases. It
is also found that the enhancement in both the reaction rates
and stereoselectivities in bmimPF6 are not significantly
different from those in water (Table 2, entries 1–3, 5, and
18–21). In contrast to the system in water, the 1-in-bmimPF6
system allowed the reactions of isoprene 5 and dienophiles
a–c to undertake the Diels–Alder path with 78–91% isolated
yields of the cycloaddition adducts (Table 2, entries 15–17).
Similarly, the reaction of diene 6 and dienophile e in
bmimPF6 afforded a 1:1 mixture of products from both
Diels–Alder and the competing diene polymerization path-
ways. On the contrary, the polymerization became dominant
when the reaction of 6 and e was conducted in water
(Table 2, entry 22 and Table 1, entry 8). Since the OTMS
functionality of 1-methoxybutadiene 7 would be deproc-
tected by the PFK6 anion of the ionic liquid to give
crotonaldehyde, diene 7 was not applied in the 1-in-
bmimPF6 system. The cycloadduct from the reaction of 6
and naphthoquinone d was transformed to the more stable
derivative 6d with LiAlH4 in Et2O.
The results of 1-catalyzed Diels–Alder reactions of dienes 2,
6 and 7 with dienophiles a–c and e in water are summarized
in Table 1. As shown in the table, the reactions at both room
temperature and at 50 8C afforded the corresponding
cycloadducts in good to excellent yields and selectivities.
The reaction at room temperature in entry 1 completed in
2 h with a 95:5 of endo-to-exo ratio, while the controlled
reaction in CH3NO2 under similar conditions completed in
5 h with a 97% endo-selectivity (entry 10). In addition, two
other controlled reactions were performed without the
presence of catalyst 1 to give 80% complete in 50 h in
CH3NO2, and totally complete in 8 h in water (entries 11–
12). This clearly demonstrated that both water and 1 can be
applied independently to the reaction system to provide rate
enhancement, and further acceleration can be made by
combining the two in the same system. In addition, a 100%
endo-selectivity was obtained in entries 3, 5, 6, 8 and 9. In
particular, the reaction of 1-methoxybutadiene 6 and methyl
vinyl ketone a in entry 5, as far as we know, has never been
reported previously. It should be noted that the Diels–Alder
adduct of 6 and quinone c was unstable, the aromatized
product 6c was obtained by loss of the methoxy substituent
at C(1) position through elimination.14 Thus, the present
catalytic process provides a potential route to the prep-
aration of substituted dihydroxynaphthalene. The OTMS
functionality of the Diels–Alder adduct 7a in entry 9 was
deprotected by the BFK4 anion of 1 during catalysis.
Reprotection of OTMS can be performed by treating the
Diels–Alder mixture with TMSCl in the presence of
NaHCO3. The diene polymerization became competitive
for the reaction between 6 and methyl acrylate e under
catalytic conditions in water (entry 8). The use of isoprene 5
as the diene source in the Diels–Alder systems, however,
resulted in the diene polymerization only.
Finally, catalyst recycling can be readily accomplished in
the 1-in-bmimPF6 system. The ionic liquid phase containing
Table 1. [O]P(2-py)3W(CO)(NO)2](BF4)2-catalyzed Diels–Alder reactions in H2Oa
Entry
Diene
Dienophile
Diels–Alder adduct
Room temperature/50 8C
Yield (%)c
Time (h)b
endo/exod
1
2
3
4
5
6
2
2
2
2
6
6
6
6
7
a
b
c
e
a
b
c
e
a
2a
2b
2c
2e
6a
6b
6c
6e
7a
2/0.58
2/0.67
2/0.6
8/3.5
2/0.5
1/0.35
1.5/0.42
3/1
93/90
94/92
87/82
99/96
89/80
91/85
85/77
15/100
75/69
95:5/10:1
10:1/19:2
endo only
4:1/3.7:1
endo-syn only
endo-syn only
7e
8f
9g
endo-syn only/polymer
endo-syn only
1/0.35
10h
11i
12j
2
2
2
a
a
a
2a
2a
2a
5
50
8
98
80
92
97:3
95:5
95:5
a Reaction conditions: catalyst loadingZ3 mol% at both room temperature and 50 8C, [substrate]Z0.1 M for entries 1–4 and 10–12; [substrate]Z0.2 M for
entries 5–9.
b Time was set for O99% conversion.
c Isolated yield.
d endo/exo ratios were estimated by 1H NMR spectroscopy.
e Compound 6c was the aromatized product after DA reaction.
Diene polymerization was the only observable reaction path at 50 8C.
f
g The overall yields after OTMS-reprotection.
h The catalysis was carried out in CH3NO2 at room temperature.
i
The reaction was carried out in CH3NO2 at room temperature without catalyst.
The reaction was carried out in water at room temperature without catalyst.
j

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I.-H. Chen et al. / Tetrahedron 60 (2004) 11903–11909
a
Table 2. [O]P(2-py)3W(CO)(NO)2](BF4)2-catalyzed Diels–Alder reactions in bmimPF6
Entry
Diene
Dienophile
Diels–Alder adduct
Room temperature/50 8C
Yield (%)c
Time (h)b
endo:exod
1
2
3
4
2
2
2
2
2
2
3
3
3
3
4
4
4
4
5
5
5
6
6
6
6
6
a
b
c
d
e
2a
2b
2c
2d
2e
2f
0.75/0.35
0.75/0.42
2/0.5
2/0.5
8/3.5
8/4
1.25/0.58
1.25/0.58
8/0.66
8/0.66
4/1.1
97/90
91/88
87/80
87/83
99/97
40/28
92/90
80/76
81/78
93/85
81/80
92/87
83/78
91/85
80/78
91/85
78/70
78/72
79/72
77/73
71/65
40/100
10:1/10:1
10:1/8:1
endo only
endo only
4:1/4:1
4:1/3.5:1
6:1/11:2
6:1/5:1
5
6e
7
f
a
b
c
d
a
b
c
d
a
b
c
a
b
c
3a
3b
3c
3d
4a
4b
4c
4d
5a
5b
5c
6a
6b
6c
6d
6e
8
9
endo only
endo only
10
11
12
13
14
15
16
17
18
19
20f
21g
22h
4/1.1
2/0.58
2/0.58
5/1.7
5/1.7
6/2.5
1/0.35
0.5/0.25
1/0.35
1/0.42
48/3
endo-syn only
endo-syn only
d
e
endo-syn only
4:1/polymers
a Reaction conditions: catalyst loadingZ3 mol% at both room temperature and 50 8C, [substrate]Z1.0 M for entries 11–17; [substrate]Z0.67 M for other
entries.
b Time was set for O99% conversion.
c Isolated yield.
d endo/exo ratios were estimated by 1H NMR spectroscopy.
e Diene polymerization dominated over DA reactions.
f
The yields for the aromatized product 6c.
g Compound 6d was the derivatized product.
h Diene polymerization was the only observable reaction at 50 8C.
bmimPF6 and catalyst 1 was quantitatively recovered after
the removal of the etherated extract of products. Figure 2
showed the results of catalyst recycling experiments. The
recovered 1-in-bmimPF6 can be reused many times without
significant loss of activity even after the tenth application,
while the recovered 1-in-water showed a 20% of decay in
catalytic activity after being recycled six times.19
Diels–Alder reaction with the employment of catalysts in
water or in ionic liquids. We next considered applying the
Lewis acid 1 in water or in bmimPF6 solvent with
microwave heating on a series of Diels–Alder reactions.
Table 3 shows the results of microwave heated 1-catalyzed
Diels–Alder reactions in water. As shown in the table, the
system provided significant rate acceleration as compared to
the data obtained from the system without microwave
heating shown in Table 1. In all cases the reactions were
complete within 1 min with similar selectivity to those
obtained for the thermal reactions in the 1-in-water system.
Table 4 shows the results of microwave heated 1-catalyzed
Diels–Alder reactions in bmimPF6. Interestingly, the 1-in-
bmimPF6 system under microwave irradiation brought
about even more acceleration. As shown in Table 4, most
of the reactions were complete within 30 s (entries 1–4, 6–
13 and 17–20), and even the reaction with a less reactive
dienophile methylacrylate e took only one minute to
complete (entry 5). These results suggested that while
water is an excellent medium for microwave irradiation, the
ionic liquid bmimPF6 coupled with microwave even more
effectively. To compare the efficiency in microwave
conductance between the media water and bmimPF6 the
following experiments were carried out. A maximum
temperature of 50 8C and a maximum power of 20% of
300 W were preselected to afford heating profiles and power
supply curves for both solvents shown in Figure 3. As seen
in the figure, the bmimPF6 reached 50 8C after being heated
for 30 s under sealed vessel conditions, while the water took
48 s to get to 50 8C under the same conditions. Meanwhile,
the preset maximum power of 60 W was inputted to both
Microwave radiation is an alternative to conventional
heating for transforming electromagnetic energy into heat.
There are some documented examples involving the use of
ionic liquids as additives for microwave heated Diels–Alder
reactions in molecular solvents.9b,c However, to our knowl-
edge, there has been no report on the microwave-assisted
Figure 2. Catalyst systems recycling in bmimPF6 (%, upper curve) and in
H2O (B, lower curve). Experiments were conducted in a sealed process
vial containing 1.5 mL of bmimPF6 or 5 mL of H2O. Reaction conditions:
3.4 mmol of 2, 3.4 mmol of a, 3 mol% of 1, 5 mL of bmimPF6 for 45 min or
34 mL of water for 2 h at room temperature.

I.-H. Chen et al. / Tetrahedron 60 (2004) 11903–11909
11907
Table 3. [O]P(2-py)3W(CO)(NO)2](BF4)2-catalyzed Diels–Alder reactions in H2O under microwave irradiationa
Entry
Diene
Dienophile
Diels–Alder adduct
Time (s)
Conversionb (%)
Yieldc (%)
endo/exod
1
2
3
4
5
6
7
2
2
2
2
6
6
6
a
b
c
2a
2b
2c
2e
6a
6b
6c
50
50
50
60
50
50
50
94
92
92
O99
94
94
90
87
84
97
86
88
83
93:4
8:1
endo only
7:2
endo-syn only
endo-syn only
e
a
b
c
92
a Reaction conditions: catalyst loadingZ3 mol%, [reactant]Z0.1 M for entries 1–4; [reactant]Z0.2 M for entries 5–7.
1
b Determined by H NMR.
c Isolated yield.
d endo/exo selectivities were estimated by 1H NMR spectroscopy.
systems in the first 6 s, and the power was found to decrease
gradually to zero at 30 s in bmimPF6 and at 48 s in water to
prevent the temperature from rising over 50 8C. This is
consistent with the results obtained from both Tables 3
and 4.
3. Experimental
3.1. General method
O]P(2-py)3 were synthesized by the oxidation of P(2-py)3
20
with 30% H2O2 in acetone. 1-Butyl-3-methylimidazolium
hexafluorophosphate (bmimPF6) was synthesized by modifi-
cation of a reported procedure.16 All cycloaddition products
have previously been characterized, and the data obtained
corresponded satisfactorily with the reported NMR data, and
comparison with authentic samples.
In conclusion, we have synthesized a water-soluble
organotungsten Lewis acid catalyst, [O]P(2-py)3
W(CO)(NO)2](BF4)2 and have demonstrated a highly
effective methodology for the construction of 6-membered
ring systems via the combined effects of the Lewis acid
catalyst in water or in bmimPF6 under controlled microwave
irradiation. The ionic liquid bmimPF6 acts as a powerful
reaction media not only for rate acceleration and chemos-
electivity enhancement but also for facilitating catalyst
recycling in the [O]P(2-py)3W(CO)(NO)2](BF4)2-
catalyzed Diels–Alder reaction systems. Therefore, this
novel methodology involving the use of catalysts, ionic
liquids, and microwave radiation is expected to have
significant impact in organic synthesis via Diels–Alder
reactions that relied on conventional homogeneous
catalysts. Further applications of this methodology to
other catalytic reactions are under investigations.
3.1.1. Synthesis of O]P(2-py)3W(CO)3 via microwave
process. To a solution of 25 mL CH3CN in Teflon
container, W(CO)6 (300 mg, 0.85 mmol) and O]P(2-py)3
(239 mg, 0.85 mmol) were added. The container was sealed
in the acid digestion vessel and placed into microwave
(a CEM MARS 5e). The microwave was programmed to
give a maximum internal temperature of 190 8C, the
reaction mixture was irradiated at 100% of 300 W power
for 7 min. All volatiles were removed at room temperature
and the crude was dried in vacuo to afford O]P
(2-py)3W(CO)3 (468 mg, 100% yield).
Table 4. [O]P(2-py)3W(CO)(NO)2](BF4)2-catalyzed Diels–Alder reactions in bmimPF6 under microwave irradiationa
Entry
Diene
Dienophile
Diels–Alder adduct
Time (s)
Conversionb (%)
Yieldc (%)
endo/exod
1
2
3
4
5
6
7
8
2
2
2
2
2
3
3
3
3
4
4
4
4
5
5
5
6
6
6
6
a
b
c
d
e
a
b
c
d
a
b
c
d
a
b
c
a
b
c
2a
2b
2c
2d
2e
3a
3b
3c
3d
4a
4b
4c
4d
5a
5b
5c
6a
6b
6c
6d
25
25
30
30
60
25
25
30
30
30
30
30
30
35
35
40
25
25
25
25
95
89
95
92
O99
95
82
95
93
81
93
92
91
86
90
84
82
82
81
81
92
85
81
90
96
90
77
80
90
80
88
80
86
83
88
75
75
78
78
71
8:1
8:1
endo only
endo only
4:1
6:1
6:1
endo only
endo only
9
10
11
12
13
14
15
16
17
18
19e
20f
endo-syn only
endo-syn only
d
endo-syn only
a Reaction conditions: catalyst loadingZ3 mol%, [reactant]Z1.0 M for entries 10–16; [reactant]Z0.67 M for other entries.
1
b Determined by H NMR.
c Isolated yield.
d endo/exo selectivities were estimated by 1H NMR spectroscopy.
e The yields for the aromatized product 6c.
Compound 6d was the derivatized product.
f

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I.-H. Chen et al. / Tetrahedron 60 (2004) 11903–11909
3.1.5. Representative procedure for the 1-catalyzed
Diels–Alder reaction in water under controlled micro-
wave irradiation. A 10 mL sealed microwave process vial
containing an aqueous solution of 1 (22 mg, 0.03 mmol, in
5 mL of H2O) was added compounds 6 (100 mL, 1.0 mmol)
and a (80 mL, 1.0 mmol). The mixture was subjected for
microwave heating (20% of 300 W maximum power, 50 8C
preselected maximum temperature). The process was
1
monitored by H NMR. When the reaction showed O90%
completion, the mixture was extracted with ether (3!
10 mL). MgSO4 was added to the extract for dehydration.
The crude was purified by flash chromatography on SiO2
using hexane/ether (10:1) as eluent to give 6a (132 mg, 86%
yield).
Figure 3. Upper curves: heating profiles for microwave-heated (B)
bmimPF6 and (%) H2O. Lower curves: power supply profiles for (C)
bmimPF6 and ($) H2O. Experiments were conducted in a sealed process
vial containing 1.5 mL of bmimPF6 or 5 mL of H2O.
3.1.6. Representative procedure for the 1-catalyzed
Diels–Alder reaction in bmimPF6 under controlled
microwave irradiation. A 5 mL sealed microwave process
vial containing bmimPF6 solution of 1 (22 mg, 0.03 mmol,
in 1.5 mL of bmimPF6) was added compounds 6 (100 mL,
1.0 mmol) and a (80 mL, 1.0 mmol). The mixture was
subjected for microwave heating (20% of 300 W maximum
power, 50 8C preselected maximum temperature). The
3.1.2. Synthesis of [O]P(2-py)3W(CO)(NO)2](BF4)2 (1).
A CH3NO2 suspension of NOBF4 (43 mg, 0.36 mmol in
3 mL) was cooled to 0 8C before adding O]P(2-py)3
W(CO)3 (100 mg, 0.18 mmol). The mixture was stirred at
0 8C for 20 min before filtration. 20 mL of CH2Cl2 was
transferred into the filtrate, and the mixture was allowed to sit
at 0 8C for 0.5 h to give green crystalline solids. The crude
was washed with CH2Cl2 (3!5 mL) and dried in vacuo
(94 mg, 71% yield). 1H NMR (400 MHz, CD3NO2): dZ9.41
(3H, m), 8.78 (3H, m), 8.63 (3H, m), 8.09 (3H, m). 13C NMR
(100 MHz, CD3NO2): dZ190.5 (CO). 31P NMR (160 MHz,
CD3NO2): dZ7.2. IR (CaF2): nCOZ2156, nNOZ1852,
1763 cmK1. Anal. Calcd for C16H12B2F8N5O4PW: C,
26.44; H, 1.66; N, 9.64. Found: C, 26.74; H, 1.70; N, 9.85%.
1
process was monitored by H NMR. When the reaction
showed O90% completion, the mixture was extracted with
ether (3!10 mL), and MgSO4 was added to the extract. The
mixture was stirred and allowed to sit for 30 min. The ionic
solids containing bmimC, SO42K, Mg2C, and PFK6 were
filtered off. The filtrate was purified by flash chromato-
graphy on SiO2 using hexane/ether (10:1) to give 6a
(116 mg, 75% yield).
3.1.3. Representative procedure for the 1-catalyzed
Diels–Alder reaction in water. Cat.
1 (22 mg,
0.03 mmol) was dissolved in water (5 mL). A mixture of 6
(100 mL, 1.0 mmol) and a (80 mL, 1.0 mmol) was transferred
into the solution. The solution was stirred at room
temperature and monitored by 1H NMR. When the 1H
NMR data showed the reaction to be completed (a total
disappearance of reactants’ signals), the mixture was
extracted with ether (3!15 mL). MgSO4 was added to the
extract for dehydration. The crude was purified by flash
chromatography on SiO2 using hexane/ether (10:1) as eluent
to give 6a(139 mg, 90% yield). 1H NMR (400 MHz, CDCl3):
dZ5.99 (2H, m), 4.05 (1H, m), 3.30 (3H, s), 2.46 (1H, m),
2.22 (1H, m), 2.18 (3H, s), 1.91 (1H, m), 1.83 (2H, m), 13C
NMR (100 MHz, CDCl3): dZ208.96, 132.97, 124.27, 72.59,
56.30, 52.27, 28.01, 25.15, 18.35.; MS (EI) m/e: 154, 153,
139, 122, 107, 84, 79, 78, 43, 42; MC/e calcd: 154.0994,
Acknowledgements
This work was supported by National Science Council of
Taiwan, grant number NSC 91-2113-M-194-015.
References and notes
1. For recent reviews, see: (a) Kagan, H. B.; Riant, O. Chem. Rev.
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found: 154.0992. IR (CaF2): 3206, 2934, 1713, 1650 cmK1
.
3.1.4. Representative procedure for the 1-catalyzed
Diels–Alder reaction in bmimPF6. Cat. 1 (22 mg,
0.03 mmol) was dissolved in bmimPF6 (1.5 mL). A mixture
of 6 (100 mL, 1.0 mmol) and a (80 mL, 1.0 mmol) was
transferred into this ionic solution. The mixture was stirred
at room temperature and monitored by 1H NMR. The
resulting mixture was extracted with ether (3!10 mL).
MgSO4 was added to the extract, and the mixture was stirred
and allowed to sit for 30 min. Trace amount of ionic solids
containing bmimC, SO42K, Mg2C, and PFK6 were filtered
off, and the filtrate was dried and purified by flash
chromatography on SiO2 using hexane/ether (10:1) as
eluent to give 6a (128 mg, 78% yield).
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19. The conversions for 1–10 run in the recycling experiments in
bmimPF6 were 97, 97, 95, 95, 95, 91, 90, 87, 86, and 83%. The
conversions for 1–6 run in water were 96, 93, 89, 85, 81, and
77%.
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