High-pressure effects on the Diels-Alder reaction in room temperature ionic liquids

Article abstract of DOI:10.1002/poc.1131

The effect of pressure on the Diels-Alder reaction was examined in room temperature ionic liquids, followed by high-pressure FT-IR spectroscopy using pressures up to ISOMPa. Pressure enhances the kinetic sensitivity of the reaction. The kinetic effect of fluorophobic interactions was examined using ionic liquids with fluorous cations. Ionic liquids in combination with ZnI ₂ as a Lewis acid catalyst were also studied under high pressure. Copyright

Full text of DOI:10.1002/poc.1131

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2007; 20: 109–114
Published online 5 February 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/poc.1131
High-pressure effects on the Diels–Alder reaction in
room temperature ionic liquids
1
1
2
2
1
*
ˇ
´
Ana Vidis, Gabor Laurenczy, Ernst Ku¨ sters, Gottfried Sedelmeier and Paul J. Dyson
1
´
´ ´
Institut des Sciences et Ingenierie Chimiques, Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
2Novartis Pharma AG, WSJ-145.6.54, CH-4002 Basel, Switzerland
Received 19 July 2006; revised 27 September 2006; accepted 19 October 2006
ABSTRACT: The effect of pressure on the Diels–Alder reaction was examined in room temperature ionic liquids,
followed by high-pressure FT-IR spectroscopy using pressures up to 150 MPa. Pressure enhances the kinetic
sensitivity of the reaction. The kinetic effect of fluorophobic interactions was examined using ionic liquids with
fluorous cations. Ionic liquids in combination with ZnI2 as a Lewis acid catalyst were also studied under high pressure.
Copyright # 2007 John Wiley & Sons, Ltd.
KEYWORDS: high pressure; ionic liquids; Diels–Alder reaction; fluorous effect; Lewis acid catalysts.
INTRODUCTION
reactants and the products and is usually determined
experimentally.3
Organic synthesis under high pressure is valid where the
¼
High-pressure kinetics is a useful tool for the
determination and the assessment of the mechanism of
a reaction.1 The volumes of activation deduced by such
kinetic measurements are an indicator of the position of
activation volume, DV , is negative, providing its absolute
¼
magnitude sufficiently large (DV < ꢀ15 cm3 molꢀ1),1
influencing a reaction by (i) accelerating the reaction, (ii)
modifying regioselectivity and diastereoselectivity, and
(iii) by causing changes to the chemical equilibrium.2 The
effect on chemical equilibrium and rates of reaction may
be determined by the relationship between pressure and
the Gibbs’ enthalpy of reaction and activation, Eqn (1).
¼
the transition state. However, DV does not result
exclusively from volume modifications induced by the
bond transformation, rather, it encompasses all the
volume changes that occur during the progression of
the reaction from the initial state, to the transition state,
and within the transition state.4 The activation volume
ꢀ
ꢁ
ꢀ
ꢁ
@ ln kp
@DG
@p
@ ln Kp
@p
¼
DV , Eqn (2), results from two main volume effects: first,
intrinsic part (DV intr) results from the formation or
DV ¼
¼
ꢁ
ꢀ
ꢁRT
¼
T
T
(1)
ꢀ
ꢀ
ꢁ
¼
cleavage of bonds, and second, a solvation contribution
¼
@DG
¼
DV
¼
¼
ꢀ
ꢁRT
T
(DV solv) describing solvent effects on equilibria and
rates under pressure.5
@p
@p
T
¼
where DV, DV are volumes of reaction and activation; Kp
is equilibrium constant at pressure p; kp is rate constant at
p; DG, DG are Gibbs enthalpy and Gibbs enthalpy of
activation.
¼
¼
¼
DV ¼ DVintr þ DVsolv
(2)
¼
The Diels–Alder reaction is one of the most important
carbon-carbon bond forming reactions used to prepare
cyclic structures, usually affording a mixture of isomers
with the selectivity (and reaction rate) being solvent
dependent. It is a prototypical high-pressure reaction, and
has been extensively studied under pressure, because it
shows a strong pressure-induced acceleration.2,6 It is
sometimes inhibited by the low reactivity of reagents,
diene and dienophile, and/or instability of both reagents
and cycloadducts under thermal or Lewis acid catalyzed
conditions. Considerable improvements have often been
made in these cases, but mostly under extreme conditions,
viz. pressures up to 1500 MPa (sometimes in combination
The activation volume of a reaction is determined from
the pressure dependence of the formation rate constant
(kf) and is a measure of the partial molar volume of the
transition state with respect to the partial molar volumes
of the reactants. The volume of reaction corresponds to
the difference between the partial molar volumes of
´
*Correspondence to: P. J. Dyson, Institut des Sciences et Ingenierie
Chimiques, Ecole Polytechnique Federale de Lausanne (EPFL),
´ ´
CH-1015 Lausanne, Switzerland.
E-mail: paul.dyson@epfl.ch
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 109–114

ˇ
A. VIDIS ET AL.
110
with high temperatures and/or the presence of catalysts).2
The pressure effect on cycloaddition reactions has been
widely explored in organic solvents,1,7 aqueous solutions8
and fluorous media,9 and the influence of ionic liquids on
the Diels–Alder reaction under high pressure has been
mentioned in a review.10
Reactions were followed in situ by high-pressure IR
spectroscopy and the reaction profiles were evaluated
using Eqn (3) (further details are provided in supple-
mentary information). Only an overall rate could be
calculated from the spectroscopic data and therefore the
selectivity was determined after reaction by 13C NMR
spectroscopy.
Ionic liquids represent an interesting class of solvent in
which to conduct Diels–Alder reactions, since they are
polar and have no vapor pressure, potentially leading to
high selectivities with the added advantage of facile
product separation. A number of Diels–Alder reactions
have been conducted in ionic liquids under ambient
pressures11–16 as well as reactions in the presence of
Lewis acid catalysts.13,14,17–19 From these studies it
would appear that the selectivity of Diels–Alder reaction
is dependent on the hydrogen bond donor capacity of the
ionic liquids that can stabilize the transition state, with
long substituents on the cation leading to lower
selectivities and strong electrostatic associations between
the ionic liquid ions resulting in a lower interaction
between the ionic liquid and the transition state. In
addition, p-orbital overlap promoted by a low energy
LUMO and decrease in the presence of a low energy
HOMO on the ionic liquid cation appear to promote the
interaction between the transition state and the ionic
liquid indicating that all type of interactions contribute
towards the observed selectivity.
d½Aꢂ
ꢀ
¼ ½Aꢂtꢁ½Bꢂtꢁk
(3)
dt
where [A]t is dienophile conectration; [B]t is cyclopen-
tadiene concentration; k is rate constant.
Preliminary experiments were conducted using the
cation
N-ethyl-N-(2-hydroxyethyl)-N,N-dimethylammonium 1,
chosen because it had a prominent influence on the
rate and the selectivity of the reaction at ambient
pressure, due to the presence of a hydrogen bond donor
moiety.16 The reaction between cyclopentadiene and
methyl acrylate was followed in 1[Tf2N] at different
pressures and at different molar ratios of ionic liquid to
substrate (Table 1).
Irrespective of the conditions used, viz temperature or
concentration, the increase in the rate constant induced by
pressure is similar. Since the reactions at 25 8C were
relatively fast further reactions were monitored at 5 8C. The
effect of pressure was studied at pressures between 0.1 and
150 MPa (Table 2). Pressure-induced acceleration of the
reaction is greatest in 1[Tf2N] and least in 4[Tf2N].
The reaction rate and the pressure effect in dichloromethane
are lower than that observed in the ionic liquids, whereas
ethanol is superior to the ionic liquids in both these respects.
In this paper we investigate the influence of pressure on
the Diels–Alder reaction in ionic liquids. An ionic liquid
with a fluorous cation has been synthesized in order to
study fluorophobic effects on the reaction, and the
influence of the Lewis acid catalyst ZnI2 in ionic liquids
has also been studied under pressure.
Table 2 lists the rate constant ratios, calculated volumes of
activation and
¼
u
values (u ¼ DV25 : DV25, where
DV25 ¼ ꢀ33.5 for the reaction between cyclopentadiene
and methylacrylate, and DV25 ¼ ꢀ42.95 for the reaction
between cyclopentadiene and acrolein).20
RESULTS AND DISCUSSION
¼
The Diels–Alder reaction of cyclopentadiene with two
different dienophiles, methyl acralyte or acrolein
(Scheme 1) in series of [Tf2N]ꢀ based ionic liquids
using pressures up to 150 MPa have been investigated.
Volumes of activation (DV ) were deducted from k
values plotted against pressure (Fig. 1) and extrapolated
using the El’yanov equation at 25 8C (Table 2, footnote c).
ln k shows a linear dependence with pressure up to
IL
+
X
+
0-150 MPa
X
X
endo-products
exo-products
X=COOCH3, CHO
N
N
OH
OH
N
N
2
N
1
3
4
C6H13
C6H13
7
C6F13
N
N
C4F9
N
N
P
C14H29
C6H13
5
6
Scheme 1. Diels–Alder cycloadditions studied and ionic liquid cation structures employed.
Copyright # 2007 John Wiley & Sons, Ltd. J. Phys. Org. Chem. 2007; 20: 109–114
DOI: 10.1002/poc

EFFECT OF PRESSURE ON THE DIELS–ALDER REACTION
111
Table 1. Rate constants for the reaction of cyclopentadiene with methyl acrylate in 1[Tf2N] ionic liquid at 0.1 and 100 MPa
under various conditions (mol ratio of ionic liquid:methyl acrylate ¼ 1:1 or 10:1)
25 8C
Mol ratio 1:1
5 8C
Mol ratio 1:1
5 8C
Mol ratio 10:1
p [MPa]
ka ꢄ 106
88.0
238.1
s
ka ꢄ 106
s
ka ꢄ 106
s
0.1
100
ꢃ0.5
ꢃ0.8
21.9
60.2
ꢃ0.2
ꢃ0.6
18.0
46.2
ꢃ0.3
ꢃ0.4
a In [Mꢀ1 secꢀ1], standard deviation at the 95% confident level.
Table 2. Effect of pressure on rate constants for the reaction between cyclopentadiene and methyl acrylate or acrolein at 5 8C,
rate constant ratios, volumes of activation, and u values
0.1 [MPa]
50 [MPa]
100 [MPa]
150 [MPa]
¼
¼
b
c
Solvent
1[Tf2N]
2[Tf2N]
3[Tf2N]
4[Tf2N]
CH2Cl2
EtOH
Substrate ka ꢄ 106
s
ka ꢄ 106
s
ka ꢄ 106
s
ka ꢄ 10
s
k100/k0.1 DV5
DV25
u
Methyl
acrylate
Methyl
acrylate
Methyl
acrylate
Methyl
acrylate
Methyl
acrylate
Methyl
acrylate
Acrolein
Acrolein
Acrolein
Acrolein
Acrolein
Acrolein
Methyl
acrylate
Methyl
acrylate
21.9
19.6
16.9
15.2
9.3
ꢃ0.2
ꢃ0.1
ꢃ0.2
ꢃ0.6
ꢃ0.7
ꢃ0.6
39.1
31.0
27.3
19.0
ꢃ0.2
ꢃ0.5
ꢃ0.3
ꢃ0.4
60.2
50.5
41.3
28.1
18.2
88.7
ꢃ0.6
ꢃ0.3
ꢃ0.2
ꢃ0.7
ꢃ0.5
ꢃ0.8
88.3 ꢃ0.3
71.0 ꢃ0.5
57.2 ꢃ0.4
44.0 ꢃ0.4
2.7
2.6
2.4
1.9
2.0
4.0
ꢀ23.4 ꢀ25.7 0.8
ꢀ22.5 ꢀ24.7 0.7
ꢀ20.6 ꢀ22.6 0.7
ꢀ15.3 ꢀ16.8 0.5
ꢀ14.6 ꢀ16.1 0.5
ꢀ31.7 ꢀ34.7 1.0
22.6
1[Tf2N]
2[Tf2N]
3[Tf2N]
4[Tf2N]
CH2Cl2
EtOH
209
182
73
71
47
136
ꢃ1
ꢃ1
ꢃ1
ꢃ1
ꢃ1
ꢃ1
ꢃ0.6
432
ꢃ2
ꢃ4
ꢃ3
ꢃ2
604
ꢃ2
ꢃ3
ꢃ3
ꢃ4
ꢃ2
ꢃ3
ꢃ0.4
1190
ꢃ14
ꢃ12
ꢃ4
2.9
2.8
2.7
2.6
2.6
5.0
2.0
ꢀ24.6 ꢀ27.0 0.6
ꢀ23.4 ꢀ25.7 0.6
ꢀ22.6 ꢀ24.8 0.6
ꢀ22.2 ꢀ24.3 0.6
ꢀ23.4 ꢀ25.6 0.6
ꢀ37.4 ꢀ41.0 1.0
ꢀ16.4 ꢀ18.0
393
136
111
501
194
185
131
683
1111
354
326
ꢃ5
5[Tf2N]
23.3
47.2
6[Tf2N]
12.3
19.2
ꢃ0.1
ꢃ0.3
24.7
29.2
ꢃ0.4
ꢃ0.6
1.9
1.5
ꢀ15.4 ꢀ16.1
ꢀ9.7 ꢀ10.7
7[PF3(C2F5)3] Methyl
acrylate
a In [Mꢀ1 secꢀ1] and the standard deviation is at the 95% confident level.
¼
b In [cm3 molꢀ1] determined from the pressure dependence of the rate constant at 5 8C, precision of DV values is estimated to be ꢃ0.5 cm3 molꢀ1
.
¼
¼
¼
c Determined from the extrapolated dependence from DVT using the El’yanov equation DV25 ¼ DVT /[1 þ 4.43 ꢄ 10ꢀ3 Kꢀ1 (T-25 8C)].27,28
-9.2
-9.6
150 MPa allowing the activation volumes to be estimated.
Calculated u values for the ionic liquids are lower than
unity, indicating the transition state is not rigid and is
poorly ordered,4 which most likely is attributable to a
weakening of the concertedness of the reaction, possibly
due to steric hindrance from the ionic liquid components.
Although these data suggest that the transition state is not
well defined, there is some ordering, as indicated by the
difference in the calculated activation volumes, between
1[Tf2N]
2[Tf2N]
3[Tf2N]
4[Tf2N]
-10.0
-10.4
-10.8
-11.2
¼
1[Tf2N] and 4[Tf2N], DDV ¼ 9 ꢃ 0.5 cm3 molꢀ1 for the
reaction of cyclopentadiene and methyl acrylate, and
0
50
100
150
¼
DDV ¼ 3 ꢃ 0.5 cm3 molꢀ1 for the reaction between
p (MPa)
cyclopentadiene and acrolein (lower because the reaction
rates are higher with acrolein).21 Overall, the high-
pressure trends follow those observed at ambient
pressure.
Figure 1. Plotted ln k values against pressure for the reac-
tion between cyclopentadiene and methyl acrylate in differ-
ent ionic liquids (goodness of fit, R > 0.99 was observed in all
cases)
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 109–114
DOI: 10.1002/poc

ˇ
A. VIDIS ET AL.
112
Table 3. Effect of pressure on the endo:exo selectivity of the reaction between cyclopentadiene and methyl acrylate or acrolein
at 5 8C in different ionic liquids
p [MPa]
Substrate
1[Tf2N]
2[Tf2N]
3[Tf2N]
4[Tf2N]
5[Tf2N]
4.6 (4.1)a
5.2
6[Tf2N]
4.5 (3.9)a
5.0
7[PF3(C2F5)3]
0.1
50
100
150
0.1
50
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Acrolein
Acrolein
Acrolein
Acrolein
5.6
5.8
6.0
6.2
6.0
6.2
6.4
6.6
5.2
5.4
5.5
5.7
5.6
5.8
5.9
6.1
4.3
4.5
4.6
4.8
4.6
4.7
4.9
5.1
4.2
4.3
4.5
4.7
4.4
4.5
4.7
4.8
3.5
4.0
100
150
Yields in all the reactions exceed 95%.
a Selectivity obtained when an alkyl chain the same length as the fluorous chain was used.16
¼
Pressure influences selectivity due to secondary orbital
interactions which induce a larger contraction of the
volume of the endo transition state.22,23 The change in
endo:exo selectivity with pressure reveals only a modest
improvement (Table 3).
[DV25 ¼ ꢀ27.1 cm3 molꢀ1] is 4.5 cm3 molꢀ1 more nega-
tive than with no catalyst, and the calculated u value
[0.81] is closer to unity, indicating a progression of the
transition state along the reaction coordinates. Indeed,
both the reaction rate and selectivity of the reaction
increase slightly in the presence of ZnI2.
Highly fluorinated (fluorous) solvents give rise to large
negative volumes of activation for the Diels–Alder
reaction.24 Although fluorophobic interactions decrease
with increasing pressure, many reactions are activated by
pressure in fluorophobic media.9 Thus, the effect of
pressure on the Diels–Alder reaction conducted in the
fluorous ionic liquids 5[Tf2N] and 6[Tf2N] (Scheme 1),
prepared according to a modified literature protocol,25
was investigated. An ionic liquid with a large perfluori-
nated anion was also studied since it has been shown to be
somewhat fluorous-like in nature.26 In 5[Tf2N] the rate
constant at ambient pressure is higher than in the other
ionic liquids, approximately equivalent to ethanol. In
6[Tf2N] and 7[PF3(C2F5)3] the rate constants are similar
to the other ionic liquids. In contrast to fluorous molecular
solvents activation volumes are less negative (Table 2)
and changes in endo:exo selectivity remain small
(Table 3).
CONCLUSIONS
From this study we are able to show how at the high
pressure ionic liquids follow those trends observed at
ambient pressure. Higher reaction rates and selectivities
are obtained in the N-alkyl-N-(2-hydroxyethyl)-N,
N-dimethylammonium liquids where hydrogen bond
donor moieties are separated somewhat from the center
of charge on the cation, although as the length of an alkyl
chain on the ionic liquid cation increases the reaction
rates and selectivities decrease, presumably due to steric
interactions between the transition state and the cation.
Ionic liquids are superior solvents to polar ones such as
dichloromethane, but less effective than polar protic
alcohols, the activation volume in all the ionic liquids
studied was less negative than that of ethanol. Modest
improvements in selectivities and rate constants are
observed in the presence of fluorous groups, either on the
cation or on the anion.
Pressure effects on the Diels–Alder reaction between
cyclopentadiene and methyl acrylate in 3[Tf2N] in the
presence of the Lewis acid catalyst ZnI2 (0.2 mol%)
was investigated (Table 4). The experimentally deter-
mined activation volume in the presence of ZnI2
Table 4. Effect of pressure on rate constants and selectivities for the reaction of cyclopentadiene and methyl acrylate in 3[Tf2N]
containing ZnI2
3[Tf2N] (0.2 mol% ZnI2)
3[Tf2N]
p [MPa]
ka ꢄ 106
s
endo:exo ratio
ka ꢄ 106
s
endo:exo ratio
0.1
50
100
150
23.1
45.0
67.3
101.0
ꢃ0.3
ꢃ0.5
ꢃ0.4
ꢃ0.7
5.4
5.6
5.8
6.0
16.9
27.3
41.3
57.2
ꢃ0.2
4.3
4.5
4.6
4.8
ꢃ0.3
ꢃ0.2
ꢃ0.4
a In [Mꢀ1 secꢀ1], the standard deviation is at the 95% confident level.
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 109–114
DOI: 10.1002/poc

EFFECT OF PRESSURE ON THE DIELS–ALDER REACTION
113
For all the ionic liquids studied u values were below
one, which indicates that the transition state is poorly
ordered, thus implying that design of ionic liquids with
cations that contain specific groups to facilitate alignment
of the substrate to improve selectivities may not lead to
vastly improved systems. A far simpler and more
effective approach involves the incorporation of Lewis
acid anions as a component with the ionic liquid, which
can lead to vastly improved selectivities, as reported
previously for chloroaluminate ionic liquids.13
J ¼ 132 Hz), 38.8 (d, J ¼ 154 Hz), 26.3 (t, J ¼ 132 Hz)
ppm.
Methyl 5-norbornene-2-carboxylate
exo product
13C NMR (neat, 400 MHz) 173.7 (s), 134.4 (d, J ¼
170 Hz), 129.4 (d, J ¼ 170 Hz), 53.5 (t, J ¼ 144) 49.3 (q,
J ¼ 147 Hz), 43.0 (d, J ¼ 148 Hz), 39.1 (d, J ¼ 132 Hz),
39.0 (d, J ¼ 154 Hz), 21.6 (t, J ¼ 131 Hz) ppm.
EXPERIMENTAL SECTION
SUPPORTING INFORMATION
Methyl acrylate was distilled prior to use and cyclopen-
tadiene was obtained by cracking dicyclopentadiene,
distilled under reduced pressure, and stored at ꢀ70 8C.
The ionic liquids illustrated in Scheme 1, that is, 1[Tf2N],
162[Tf2N],163[Tf2N],294[Tf2N],306[I],256[Tf2N],315[I],32
and 5[Tf2N]32 were prepared according to literature
procedures. Trihexyltetradecylphosphonium hexafluoro-
phosphate 7[PF3(C2F5)3] was purchased from Fluka.
Kinetic data are available as supporting information.
Acknowledgements
We thank Novartis, the EPFL and the Swiss National
Science Foundation for financial support.
Diels–Alder reactions
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Typically, cyclopentadiene (0.4 mL, 5.0 mmol) and
methyl acrylate (0.3 mL, 3.3 mmol) were added to ionic
liquid or organic solvent (0.8 mL). For catalyzed reactions
the ionic liquid was doped with ZnI2 (0.2 mol%) and
stirred overnight before use. Alternatively, cyclopenta-
diene (0.4 mL, 5.0 mmol) and acrolein (0.2 mL,
3.3 mmol) were added to ionic liquid or organic solvent
(0.8 mL). In the reactions using fluorous ionic liquids,
cyclopentadiene (0.12 mL, 1.5 mmol) and methyl acrylate
(0.09 mL, 1.0 mmol) were added to ionic liquid (0.4 mL).
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carried out on a Varian Chrompack CP-3380 equipped
with capillary (25 m ꢄ 0.25 mm, using He as carrier gas).
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