Angewandte
Chemie
[
OMIM][PF ] < [BMIM]I. Our preliminary investigation sug-
Kinetic analysis: In a standard kinetic run the dienophile was
6
added to the ionic liquid (1 mmol in 1 mL of ionic liquid), and the
reaction mixture was allowed to equilibrate at the desired temper-
ature. The temperature was controlled using a Julabo constant-
temperature bath with an accuracy of Æ 0.01 K. The reaction was
initiated by addition of 1 (1 mmol in 1 mL). The reaction progress was
monitored at appropriate time intervals by extraction of aliquots with
ether followed by appropriate dilution and GC analysis. (Varian CP-
3800 gas chromatograph; for details, see the Supporting Information).
The rate constants thus determined were reproducible to within 6%.
gested that the k values for these reactions decrease with the
increase in the viscosity of RTILs. To provide tentative
support for this observation, the reaction of 1 with 2a was
2
carried out at 298.15 K in a mixture of [BMIM][BF ] with
4
dichloromethane (DCM) (45 mol% of [BMIM][BF ] in
4
[
11]
5
5 mol% of DCM). Here, DCM (h = ꢀ 18 cP) was used
as a “viscosity reducer” for [BMIM][BF ] (h = 233 cP). The
resulting rate constant, k = 5.79 10 dm mol s , is about
4
À5
3
À1 À1
2
2
0% higher than that measured in pure [BMIM][BF ]. It is,
4
Received: February 1, 2006
Revised: April 25, 2006
Published online: June 27, 2006
however, not possible to state at this stage with confidence
that the viscosity of a RTIL is an important parameter to
correlate kinetic data of Diels–Alder reactions. In a recent
study with a series of RTILs, it was shown that the Diels–
Alder reaction was fastest in the RTIL of highest viscosity.
Inadequate experimental data do not allow us to draw any
conclusion at this stage.
Keywords: Diels–Alder reaction · ionic liquids · kinetics ·
[
5d]
.
solvent effects · viscosity
The results of preliminary temperature-dependent kinetic
[1] D. C. Rideout, R. Breslow, J. Am. Chem. Soc. 1980, 102, 7816.
[2] For examples of water-promoted Diels–Alder reactions, see:
a) R. Breslow, Acc. Chem. Res. 1991, 24,159; b) W. Blokzijl, M. J.
Blandamer, J. B. F. N. Engberts, J. Am. Chem. Soc. 1991, 113,
+
investigations are shown in Figure 2. The DH values for the
reaction of 1 with 2a, obtained from the transition-state
À1
À1
theory plots (Figure 2), are 55.3 kJmol and 60.9 kJmol for
EMIM][BF ] and [BMIM][PF ], respectively. Any change in
4
1
241; c) R. Breslow, T. Guo, Proc. Natl. Acad. Sci. USA 1990, 87,
67.
[
4
6
temperature is bound to alter both the H-bonding ability and
the viscosity of RTILs. The observed temperature effect may
result from a change in either or both these parameters. A
detailed study of the theories of condensed-phase kinetics to
explain the results is being carried out in our laboratory and
will be reported in the future.
[
[
[
3] a) Ionic Liquids in Synthesis (Eds.: P. Wassercheid, T. Welton),
Wiley-VCH, Weinheim, 2003; b) R. D. Rogers, K. R. Seddon in
Ionic Liquids: Industrial Applications to Green Chemistry, ACS
Symposium Series 818, American Chemical Society, Washing-
ton, DC, 2002; c) C. F. Poole, J. Chromatogr. A 2004, 1037, 49.
4] a) T. Welton, Chem. Rev. 1999, 99, 2071; b) M. J. Earle, K. R.
Seddon, Pure Appl. Chem. 2000, 72, 1391; c) P. Wassercheid, M.
Keim, Angew. Chem. Int. Ed. 2000, 39, 3772; d) R. Sheldon,
Chem. Commun. 2001, 23, 2399; e) C. Chiappe, D. J. Pieracinni,
J. Phys. Org. Chem. 2005, 18, 275.
5] For reports on Diels–Alder reactions in ionic liquids see:
a) D. A. Jaeger, C. E. Tucker, Tetrahedron Lett. 1989, 30, 1785;
b) M. J. Earle, P. B. McCormac, K. R. Seddon, Green Chem.
1999, 1, 23; c) C. Lee, Tetrahedron Lett. 1999, 40, 2461; d) A.
Aggarwal, N. L. Lancaster, A. R. Sethi, T. Welton, Green Chem.
2002, 4, 517; e) A. Kumar, S. S. Pawar, J. Org. Chem. 2004, 69,
1419.
Figure 2. Eyring plots of Diels–Alder reaction of 1 + 2a in [EMIM][BF ]
[6] C. Reichardt, Solvent Effects in Organic Chemistry, Verlag
Chemie, Weinheim, 1979.
4
2
2
(
&
) (r =0.991) and [BMIM][PF ] (~) (r =0.996).
6
[
7] a) W. Blokzijl, J. B. F. N. Engberts, M. J. Blandamer, J. Am.
Chem. Soc. 1990, 112, 1197; b) W. Blokzijl, J. B. F. N. Engberts, J.
Am. Chem. Soc. 1991, 113, 5440; c) W. Blokzijl, J. B. F. N.
Engberts, Angew. Chem. 1993, 32, 1610; Angew. Chem. Int. Ed.
Engl. 1993, 32, 1545; d) T. Rispens, J. B. F. N. Engberts, J. Org.
Chem. 2002, 67, 7369; e) S. Otto, J. B. F. N. Engberts, Org.
Biomol. Chem. 2003, 1, 2809.
The present results indicate that water, and not a RTIL, is
definitely the solvent of choice for carrying out Diels–Alder
reactions. The results merit further investigation to correlate
the rates of these reactions with other properties of RTILs.
Also designing new RTILs or using RTIL mixtures with better
properties is highly desirable in order to encourage their use
as “green solvents”.
[8] J. F. Blake, W. L. Jorgensen, J. Am. Chem. Soc. 1991, 113, 7430.
[
9] a) M. R. J. Dack, Chem. Soc. Rev. 1975, 4, 211; b) M. C. Pirrung,
Chem. Eur. J. 2006, 12, 1312.
[
10] The h values of the RTILs were obtained from the following
sources: a) P. Bonhote, A. P. Dias, K. Kalyansundaram, M.
Gratzel, Inorg. Chem. 1996, 35, 1168; b) P. A. Z. Suarez, S.
Einloft, J. E. Dudlis, R. F. deSouza, J. Dupont, J. Chim. Phys.
1998, 95, 1626; c) K. R. Seddon, A. Stark, J. Torres, Pure Appl.
Chem. 2000, 72, 2275; d) J. G. Huddleston, A. E. Visser, W. M.
Reichert, H. D. G. A. Brokers, R. D. Rogers, Green Chem. 2001,
3, 156; e) A. Noda, K. Hayamizu, M. Watanabe, J. Phys. Chem. B
2001, 105, 4603; f) S. N. Baker, G. A. Baker, M. A. Kane, F. V.
Bright, J. Phys. Chem. B 2001, 105, 9663; g) L. C. Bronco, J. N.
Rosa, C. A. M. Afonso, Chem. Eur. J. 2002, 8, 3671.
Experimental Section
Cyclopentadiene (1) was freshly distilled from dicyclopentadiene
prior to use. Acrylates 2a, 2b (low-pressure distillation), and 2c were
distilled prior to use. 1-Butyl-3-methylimidazolium tetrafluoroborate
[
BMIM][BF ], 1-butyl-3-methylimidazolium hexafluorophosphate
4
[
BMIM][PF ], 1-butyl-3-methylimidazolium iodide [BMIM]I, 1-
6
octyl-3-methylimidazolium tetrafluoroborate [OMIM][BF ] and 1-
4
ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF4] were
[
8a]
synthesized by the reported procedure.
The RTILs were thor-
oughly dried by heating at 708C under high vacuum for several hours
before each kinetic run. All manipulations were carried out under an
atmosphere of dry nitrogen to exclude moisture.
[11] J. Wang, Y. Tian, Y. Zhao, K. Zhuo, Green Chem. 2003, 5, 618.
Angew. Chem. Int. Ed. 2006, 45, 4824 –4825
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4825