Diels-Alder Reaction in Ionic Liquids
J. Chin. Chem. Soc., Vol. 51, No. 2, 2004 369
R2
commercial suppliers and used as received. NMR spectra
were obtained using a Bruker-200 NMR spectrometer.
R1
R1
R
R
R
R
CH
H
R1
R2
H
H
CH3
H
Preparation of ionic liquids
R
R
R1
1b:
1c:
H
H
H
H
CHO
COCH3
CHO
CN
3
2b:
2c:
2d:
2e:
The 1:1 or 1:2 molar ratio of dialkylimidazolium
halides (EMIC,9 C8MIB,10 and C8C8IB11) to ZnX2 (X = Cl or
Br) were mixed in a flask and heated to 90 °C for 2 days to af-
ford a colorless liquid.9 The resulting ionic liquids (EMIC-
ZnCl2, C8MIB-ZnBr2 and C8C8IB-ZnBr2) were cooled to
room temperature and kept in a N2-filled glove box.
CHO
COCH3
CN
H
H
H
3ab:
3ac:
3ae:
CH3
H
H
H
CHO
CHO
CH3
R1
R
H
Procedure for the Diels-Alder reaction
R
R1
3ad-exo
3ad-endo
Dienophile 2 (5 mmol) was added to 2.5 mmol of ionic
liquid and stirred for 10 minutes at room temperature. Diene
1 (7.5 mmol) was added successively. The resulting mixture
was stirred at room temperature or 50 °C. The reaction times
are given in Table 1. After reaction was complete, the adduct
was extracted with diethyl ether (2 mL ´ 5) followed by chlo-
roform (2 mL ´ 2). This crude product was subjected to GC
analysis and purified by column chromatography over silica
gel eluting with hexane/ethyl acetate (10:1) to yield pure
Diels-Alder products 3aa, 3ab, 3ac, 3ad, 3ae, 3ba, 3bb, 3ca,
3cb, and 3cc. The 1H NMR spectral data were in agreement
with their structures.
CO2C2H5 CH3
3ba:
3bb:
3ca:
3cb:
3cc:
CHO
CO2C2H5
CHO
CH3
H
H
COCH3
H
The high endo selectivity was also observed except for the re-
action of 1a with 2e which only gave a 2:1 ratio of endo/exo.
The reaction with more reactive aldehyde or ketone dieno-
philes was completed in 10 minutes. On the other hand, the
reaction with less reactive acrylonitrile needed 12 hours for
completion. Interestingly, even the less reactive dienes, 2,3-
dimethylbutadiene (1b) and isoprene (1c), reacted with ac-
tive vinyl aldehyde 2b or ketone 2c and gave a good yield at
room temperature (entries 12-18). The formation of an
adduct of 1b or 1c with 2a took place at a considerably lower
rate under the same condition (entries 12 and 15). The yield
increased substantially when the temperature rose to 50 °C
and the reaction time to 8 hours. Surprisingly, the reaction of
isoprene (1c) with 2a-2c supplied exclusively the corre-
sponding para-adduct, 1-methyl-4-substituted-cyclohexenes
3ca-3cc, as single product without contamination with the
other meta regioisomer, 1-methyl-5-substituted-cyclohex-
enes. In all cases, both the yield and the endo/exo ratio de-
creased with extending the reaction time.
ACKNOWLEDGMENT
Financial support from the National Science Council,
Taiwan, Republic of China (NSC 91-2113-M-006-010) is ap-
preciated.
Received June 3, 2003.
In summary, we have shown that the 1:2 molar ratio of
air- and moisture-stable EMIC-ZnCl2 ionic liquid provided
an excellent media and catalyst for performing clean Diels-
Alder reactions. This ionic liquid, indeed, dramatically en-
hanced the Diels-Alder reaction rate and the endo stereo-
selectivity. The activity of recycled ionic liquid remained in-
tact after the fifth use.
REFERENCES
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P.; Keim, W. Angew. Chem. Int. Ed. 2000, 39, 3772. (c)
Welton, T. Chem. Rev. 1999, 99, 2071. (d) Earle, M. J.; Sed-
don, K. R. Pure Appl. Chem. 2000, 72, 1391. (e) Kumar, A.
Chem. Rev. 2001, 101, 1.
EXPERIMENTAL SECTION
3. Lee, C. W. Tetrahedron Lett. 1999, 40, 2461.
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Unless otherwise stated, reagents were obtained from
Sun, I.-Wen
