Diels-Alder reaction of cyclopentadiene and alkyl acrylates in the presence of pyrrolidinium ionic liquids with various anions

Article abstract of DOI:10.1007/s10562-011-0558-6

The conversion and stereoselectivity of transformation to endo and exo norbornene derivatives was determined in the Diels-Alder reaction of cyclopentadiene with alkyl acrylates. The reactions were carried out in the pyrrolidinium ionic liquids in the presence of metal chlorides and trifluoromethanesulfonates as the catalysts. Shorter reaction times and higher conversions of dienophile were observed in a comparison with analogous cycloadditions carried out in the presence of conventional organic solvents. A higher stereoselectivity to the endo isomer was found in the majority of cases. The ionic liquids composed of 1-butyl-1-methylpyrrolidinium cation (Pyrr _1.4) and various anions were used. The influence of ionic liquid anion and several metal chlorides and metal triflates used as the catalysts on the conversion was determined. Graphical Abstract: The conversion and stereoselectivity of transformation to endo and exo norbornene derivatives was determined in the Diels-Alder reaction of cyclopentadiene with alkyl acrylates. The reactions were carried out in the pyrrolidinium ionic liquids in the presence of metal chlorides and trifluoromethanesulfonates as the catalysts. Shorter reaction times and higher conversions of dienophile were observed in a comparison with analogous cycloadditions carried out in the presence of conventional organic solvents. A higher stereoselectivity to the endo isomer was found in the majority of cases. The ionic liquids composed of 1-butyl-1-methylpyrrolidinium cation (Pyrr_1.4) and various anions were used. The influence of ionic liquid anion and several metal chlorides and metal triflates used as the catalysts on the conversion was determined.

Full text of DOI:10.1007/s10562-011-0558-6

Catal Lett (2011) 141:742–747
DOI 10.1007/s10562-011-0558-6
Diels–Alder Reaction of Cyclopentadiene and Alkyl Acrylates
in the Presence of Pyrrolidinium Ionic Liquids with Various
Anions
•
Waldemar Stefaniak Ewa Janus Eugeniusz Milchert
•
Received: 20 October 2010 / Accepted: 26 January 2011 / Published online: 10 February 2011
Ó The Author(s) 2011. This article is published with open access at Springerlink.com
Abstract The conversion and stereoselectivity of trans-
formation to endo and exo norbornene derivatives was
determined in the Diels–Alder reaction of cyclopentadiene
with alkyl acrylates. The reactions were carried out in the
pyrrolidinium ionic liquids in the presence of metal chlorides
and trifluoromethanesulfonates as the catalysts. Shorter
reaction times and higher conversions of dienophile were
observed in a comparison with analogous cycloadditions
carried out in the presence of conventional organic solvents.
A higher stereoselectivity to the endo isomer was found in
the majority of cases. The ionic liquids composed of 1-butyl-
1-methylpyrrolidinium cation (Pyrr1.4) and various anions
were used. The influence of ionic liquid anion and several
metal chlorides and metal triflates used as the catalysts on the
conversion was determined.
medium of water [2], surfactants [3], lithium perchlo-
rate(VII) [4], alkyl ammonium nitrate [5], tetrahydrofuran–
hydrogen bromide (HBr–THF) complex [6], and Lewis
acid. In recent years, the ionic liquids [7–13] have been
used as the Diels–Alder reaction medium instead of con-
ventional organic solvents. A good solubility of the cata-
lysts has been of crucial importance in their applications.
The catalysts of these cycloaddition reactions are fre-
quently trifluoromethanesulfonates and chlorides of the
rare-earth elements such as: M(OTf)3, MCl3 (M = Sc, La,
ꢁ
Ce, Y, OTf ¼ CF3SO3 ) [10–19]. The ionic liquids used in
this type of reaction include the salts containing imidazo-
lium [20], pyridinium [21], phosphonium [22] cation and
bis(trifluoromethylsulfonyl)imide as the anion. The reac-
tion of cyclopentadiene with acroleine is an exemplary of
such application of the ionic liquids. In the medium of
Keywords Diels–Alder reaction ꢀ Ionic liquids ꢀ
1-butyl-3-methylimidazolium
bis(trifluoromethylsulfo-
Lewis acids
nyl)imide-bmim[NTf2] and InCl3 as the catalyst, the ste-
reoselectivity of endo isomer amounted to 16, whereas it
was 8.8 in the EtOH/InCl3 medium [12].
1 Introduction
In this work the pyrrolidinium ionic liquids were used in
the Diels–Alder reaction of cyclopentadiene with esters of
acrylic acids. The catalysts selected for this study were
trifluoromethanesulfonates and chlorides of alkali metals,
alkaline earth metals, transition metals and rare-earth ele-
ments as the well as chlorides of aluminum, tin and indium.
The course of reaction was presented on Scheme 1.
The Diels–Alder reaction is a particularly useful method
for the synthesis of compounds with cyclic structure [1].
The majority of syntheses proceed with the formation of a
mixture of isomers and the yield and reaction stereoselec-
tivity depends to a large degree on the nature of solvent and
catalyst. An enhancement in the reaction stereoselectivity
can be achieved through the introduction into the reaction
2 Experimental
W. Stefaniak ꢀ E. Janus (&) ꢀ E. Milchert
Department of Organic Chemical Technology, West Pomeranian
University of Technology, Szczecin, Pulaski Str. 10, 70-322
Szczecin, Poland
2.1 Raw Materials
The ionic liquids included: 1-butyl-1-methylpyrrolidinium
bis(trifluoromethylsulfonyl)imide Pyrr1.4[NTf2] (98% pure
e-mail: ejanus@zut.edu.pl
123

Diels–Alder Reaction in Pyrrolidinium Ionic Liquids
743
addition of 0.0996 g (1.5 mmol) of cyclopentadiene, the
vial was located in the thermostat bath at temperature of
25 °C and was stirred. The content of dienophile in the
reaction mixture was determined by GC method every
30 min over 4 h, or until the conversion of dienophile was
over 90% (the reaction time was calculated from a moment
of cyclopentadiene addition). The peak area ratio of die-
nophile to the internal standard was again determined and
the dienophile conversion was calculated. The conversion
determined in such a way is the same as the yield of nor-
bornene derivative. The dienophile undergoes the reaction
only with the formation of endo and exo derivative of
norbornene. The endo/exo ratio was calculated based on the
GC analysis, whereas in the case of 2-hydroxypropyl
acrylate based on the analysis of H1 NMR and C13 NMR
spectra. In the case of reaction carried out in the presence
of catalyst, the catalyst was first weighed in the amount
corresponding to 1 or 3 mol% in relation to the dienophile
and a constant amount of ionic liquids—250 lL was
added. The catalyst concentration in the ionic liquid
amounted to 0.04 or 0.12 mol/dm3. The catalyst was stirred
in the ionic liquid until the complete dissolution.
Scheme 1 The reaction of cyclopentadiene with dienophiles
Solvent Innovation), 1-butyl-1-methylpyrrolidinium tri-
fluoromethanesulfonates Pyrr1.4[OTf] (98% pure Solvent
Innovation), 1-butyl-1-methylpyrrolidinium tris(pentafluo-
roethyl)trifluorophosphates Pyrr1.4[FAP] ([99% Merck).
The liquids were dried in a vacuum drier, at temperature of
95 °C, under pressure of 5 mbar over 24 h. The water con-
tent in the ionic liquids was determined by coulometric Karl–
Fischer titration using Metrohm 831 KF Coulometers.
Cyclopentadiene was obtained as a results of thermal
cracking of dicyclopentadiene ([95% pure) purchased from
Fluka. As dienophiles were used the following acrylates:
methyl (MeAkr) (99% Aldrich), ethyl (EtAkr) (99%
Aldrich), butyl (BuAkr) (99% Aldrich) and 2-hydroxypropyl
(HpAkr) (98% Aldrich). Gas chromatographic (GC) mea-
surements were performed using cyclohexanone (CH)
([99% Merck) as an internal standard. As the catalysts were
the following used chlorides: scandium ScCl3 (99.9%
Aldrich), ytterbium YbCl3 (99.9% Aldrich), yttrium YCl3
(99.99% Aldrich), lanthanum LaCl3 (99.9% Aldrich), mag-
nesium MgCl2 (99.9% Aldrich), trifluoromethanesulfonates:
bismuth Bi(OTf)3 (99.9% Aldrich), cerium Ce(OTf)3 (99%
Acros Organics), zinc Zn(OTf)2 (98% Aldrich), tin Sn(OTf)2
(98% Acros Organics), aluminum Al(OTf)3 (99.9%
Aldrich), indium In(OTf)3 (99.9% Aldrich), ytterbium
hydrate Yb(OTf)3ꢀH2O (25–28% Yb Aldrich), ytterbium
Yb(OTf)3 (99.99% Aldrich), yttrium Y(OTf)3 (98%
Aldrich), lanthanum hydrate La(OTf)3ꢀH2O (99.9%
Aldrich), lanthanum La(OTf)3 (99% Acros Organics),
magnesium Mg(OTf)2 (97% Aldrich), cuprum Cu(OTf)2
(98% Aldrich), neodynium Nd(OTf)3 (99% Acros Organ-
ics), scandium Sc(OTf)3 ([97% Chemica), sodium NaOTf
(98% Aldrich), calcium Ca(OTf)2 (99.9% Aldrich).
3 Results and Discussion
The results of reaction of cyclopentadiene with selected
alkyl acrylates (Scheme 1) in ionic liquids as the reaction
medium without the catalyst were presented in Table 1. The
reaction of cyclopentadiene with methyl acrylate and ethyl
acrylate proceeded very fast in the medium of 1-butyl-1-
methylpyrrolidinium trifluoromethanesulfonates Pyrr1.4[OTf],
whereas at much lower rate in 1-butyl-1-methylpyrrolidi-
nium tris(pentafluoroethyl)trifluorophosphate Pyrr1.4[FAP].
The Diels–Alder reaction of cyclopentadiene with methyl
acrylate gave the largest fraction of endo isomer (84%) in the
liquid with [OTf]- anion, whereas lower (81%) in the case
of remaining liquids. In the reaction of cyclopentadiene with
ethyl acrylate the highest stereoselectivity of the endo iso-
mer was achieved in the ionic liquids with [OTf]- anion
(84%), whereas a lower stereoselectivity was obtained with
[NTf2]- (82%) and [FAP]- (81%). The stereoselectivity did
not change during the course of process. The reaction of
cyclopentadiene with butyl acrylate proceeded very fast and
with the highest conversion of dienophile in the presence of
liquid with [OTf]- anion, whereas at the lowest rate in the
liquid with [NTf2]- anion. The highest selectivity of endo
isomer in this reaction amounted to 84% when the ionic
liquid was with [NTf2]- anion, in the remaining liquids was
82%. Among all considered dienophiles the highest con-
version equal to 70% was achieved after 2 h for the reaction
of cyclopentadiene with 2-hydroxypropyl acrylate, and
that carried out in the presence of liquid with [NTf2]- anion.
2.2 Procedure for the Reaction
The reactions were carried out in 4 mL screwed vials
equipped with a magnetic stirrer. The vial was charged
with the ionic liquid in the amount of 250 lL, cyclohex-
anone—20 lL (0.0198 g) and with dienophile in the
amount corresponding to 1 mmol. A sample was collected
from such prepared mixture and was subjected to GC
analysis. The peak area ratio (dienophile/internal standard)
was calculated from obtained chromatograms. After the
123

744
W. Stefaniak et al.
Table 1 Conversion and stereoselectivity of the reaction of cyclopentadiene with selected alkyl esters of acrylic acid in ionic liquids as the
reaction medium without the catalyst
Ionic liquid CP ? MeAcr
CP ? EtAcr
CP ? BuAcr
CP ? 2HpAcr
Conversion Stereoselectivity Conversion Stereoselectivity Conversion Stereoselectivity Conversion Stereoselectivity
[mol %] endo:exo [mol %] endo:exo [mol %] endo:exo [mol %] endo:exo
2 h 4 h (%) 2 h 4 h (%) 2 h 4 h (%) 2 h 4 h (%)
Pyrr1.4[NTf2] 45
63
Pyrr1.4[FAP] 39 66
Pyrr1.4[OTf] 48 67
81:19
36
35
57
52
82:18
81:19
84:16
26
35 52
73 83
48
84:16
82:18
82:18
70
56
55
82
88
82
100:0
100:0
100:0
81:19
84:16
59 73
Products
2-methoxycarbonyl-5-
norbornene
2-ethoxycarbonyl-5-
norbornene
2-butoxycarbonyl-5-
norbornene
2-(2-hydroxypropyl)oxy-
carbonyl-5-norbornene
[NTf2] = (CF3SO2)2N, [FAP] = (C2F5)3PF3, [OTf] = CF3SO3
In this cycloaddition but also carried out in the presence of
[FAP]- anion was obtained the highest conversion (88%)
after 4 h. In the ionic liquid with [OTf]- anion a high con-
version (80%) was also obtained after 4 h. The endo ste-
reoisomer is formed exclusively in all ionic liquids tested
during the synthesis of norbornene derivative. Its presence
the stereoselectivity of the endo isomer up to a value of 93%
in a comparison with the reaction without the catalyst (81%)
was obtained in Pyrr1.4[NTf2] liquid in the presence of the
catalysts: Sc(OTf)3 and Y(OTf)3. However, a competitive
reaction of cyclopentadiene oligomerization proceeded in
the presence of these catalysts in Pyrr1.4[NTf2] liquid and
after about 30 min the changes of yield of the main reaction
product practically were not observed. In the case of the
catalysts: Cu(OTf)2, Al(OTf)3, Bi(OTf)3, Ce(OTf)3,
In(OTf)3, and Sn(OTf)2, the oligomerization of cyclo-
pentadiene was prevailing and 2-methoxycarbonyl-5-nor-
bornene was formed with a low yield (below 1%). It was
found that the addition of cyclopentadiene caused turbidity
of the reaction mixture and subsequently produced a dark-
ening effect and solidification. The reaction was accompa-
nied by the release of heat. The reaction, when conducted in
the Pyrr1.4[NTf2] liquid with scandium and yttrium tri-
fluoromethanesulfonates used in the amount of 3 mol%
gave higher stereoselectivities of endo isomer (93–94%).
However, these catalysts caused the formation of 2-meth-
oxycarbonyl-5-norbornene with lower yields due to accel-
erated the oligomerization of cyclopentadiene. The other
catalysts e.g. Nd(OTf)3, La(OTf)3, used at higher concen-
trations (3 mol%), slightly increased the conversion and
stereoselectivity of the endo isomer in relation to the values
obtained at concentration of 1%. An increase in the con-
centration of Mg(OTf)3 from 1 to 3 mol% did not cause any
changes of the conversion and reaction stereoselectivity.
The chloride catalyst used in the amount of 1 mol% in
relation to methyl acrylate in the ionic liquid with [NTf2]-
anion exhibited the highest activity in the case of ytter-
bium. The largest increase in the stereoselectivity of endo
isomer in the ionic liquid Pyrr1.4[NTf2] in relation to the
reaction without the catalyst was obtained in the presence
of ytterbium and scandium chlorides. An increase of the
catalyst amount to 3 mol%, in the case of liquid
Pyrr1.4[NTf2] demonstrated a high activity of yttrium,
ytterbium and scandium chlorides. The conversion of 96%
1
was confirmed on the basis of H NMR and 13C NMR
spectrum and by the literature date [23]. A comparison of
acrylates used in these syntheses, reveals that their reactivity
is similar in the liquids with [NTf2]- and [FAP]- anions. In a
series of methyl, ethyl, butyl acrylate, the reactivity in the
presence of [NTf2]- and [FAP]- anions decreases along
with an extension of the chain length of the ester groups and
is always lower than that in the presence of [OTf]- anion. In
the reaction of cyclopentadiene with butyl acrylate in the
presence of [NTf2]- after 4 h, the product was obtained with
48% yield. In the ionic liquid with [OTf]- anion, the con-
version of butyl acrylate was the highest in the considered
series of acrylates and the ionic liquids. The conversion
amounted to 73% after 2 h, whereas 83% after 4 h. The
reaction of 2-hydroxypropyl acrylate proceeded in a differ-
ent manner. Its conversion was similar after 4 h, and has the
highest value among the used acrylates irrespective of the
ionic liquid.
The studies of catalysts activity in the form of metal
chlorides and trifluoromethanesulfonates were performed
for the reaction of cyclopentadiene with methyl acrylate in
the presence of ionic liquid Pyrr1.4[NTf2] (Table 2). It was
found that not all the compounds caused an increase in the
rate and the stereoselectivity of transformation in the direc-
tion of endo isomer in a comparison with the reaction carried
out without the catalyst. The conversion of methyl acrylate in
the liquid Pyrr1.4[NTf2] without the catalyst amounted to
63% after 4 h. In a group of triflates used in the amount of
1%mol in relation to methyl acrylates, the largest increase of
conversion was obtained for trifluoromethanesulfonates
of lanthanum La(OTf)3, magnesium Mg(OTf)3, calcium
Ca(OTf)3, and neodymiumNd(OTf)3. The largest increase in
123

Diels–Alder Reaction in Pyrrolidinium Ionic Liquids
745
in the Diels–Alder reaction exhibited the catalysts: ScCl3
and YbCl3. High conversions of butyl acrylate occurred in
the presence of YCl3 (86%) and YbCl3 (85%) in the
medium of Pyrr1.4[NTf2]. The highest stereoselectivity
expressed by a percentage fraction of the endo isomer
in the product was obtained in the reactions catalyzed
by scandium and yttrium trifluoromethanesulfonates
(94%), in the presence of ionic liquid Pyrr1.4[NTf2]. A
high stereoselectivity of the endo isomer (93%) was also
achieved in the reaction catalyzed by YCl3 in the liquid
Pyrr1.4[FAP].
Table 2 Conversion and stereoselectivity of reaction of cyclopenta-
diene with methyl acrylate in the ionic liquid Pyrr1.4[NTf2]
Entry Catalyst
Conversion [mol %]
Stereoselectiviy
endo:exo %
2 h
4 h
1
Y(OTf)3
Y(OTf)3
44a
74a
46
91:9
c
2
93:7
3
Yb(OTf)3
Yb(OTf)3
Sc(OTf)3
61
90
86:14
91:9
c
4
73
5
31a
54a
50
93:7
c
6
Sc(OTf)3
La(OTf)3
La(OTf)3
94:6
The reaction of cyclopentadiene with ethyl (2b), butyl
(2c) and 2-hydroxypropyl acrylates (2d) in the liquid
Pyrr1.4[NTf2] (Table 4) in the presence of the most active
catalysts: ScCl3, YbCl3 and YCl3 also exhibits a significant
increase of the conversion of acrylates and the stereose-
lectivity towards the endo isomer. In the case of scandium
(III) chloride the product was obtained with the quantita-
tive yield after 60 min whereas with yttrium (III) chloride
after 120 min. In the presence of triflates Y, Yb and Sc
after a period of 2–3 h, a significant conversion of dieno-
phile was not observed. The stereoselectivity of transfor-
mation to the endo isomer in the presence of each of the
catalysts was significantly larger than that without the
catalyst. The highest stereoselectivity in the reaction of
cyclopentadiene with ethyl acrylate (2b) was obtained in
the presence of the catalysts Sc(OTf)3 and Y(OTf)3 (93%).
In the reaction of cyclopentadiene with butyl acrylate (2c)
the highest conversion was observed in the case of ScCl3
(99% after 15 min) and the highest stereoselectivities of
endo 2-butoxycarbonyl-5-norbornene (92–94%) occurred
in the presence of triflates Y, Yb, and Sc. The reaction of
cyclopentadiene with 2-hydroxypropyl acrylate has con-
firmed the regularity previously found. The conversion of
dienophile amounted to 92% in the presence of ScCl3
already after 15 min. Similar conversions after longer
reaction times were obtained with the remaining catalysts:
YbCl3-93% after 1 h, YCl3-94% after 2 h. In the presence
of triflates: scandium yttrium and ytterbium, the dienophile
conversion at a level of about 90% occurred after 4 h. In
the reaction of 2-hydroxypropyl acrylate was formed
exclusively endo 2-(2-hydroxypropyl)oxycarbonyl-5-nor-
bornene. During the reaction of selected alkyl acrylates
with cyclopentadiene in the medium of ionic liquid
Pyrr1.4[NTf2] the most active catalysts are: ScCl3, YbCl3
and YCl3. The formation of norbornene derivatives pro-
ceeds very fast in their presence with a high stereoselec-
tivity of the endo isomer (over 89%). However, in the
presence of triflates Sc, Yb and Y the conversions of
acrylates were found to be lower and the reactions proceed
at a slower rate. A side reaction of cyclopentadiene olig-
omerization proceeds in this case.
7
80
84
62
48
71
92
54
71
71
72
62
86:14
87:13
84:16
84:16
86:16
89:11
85:15
82:18
83:17
82:18
82:18
91:9
c
8
63
9
La(OTf)3ꢀH2O
Yb(OTf)3ꢀH2O 34
Nd(OTf)3
Nd(OTf)3
Zn(OTf)2
Ca(OTf)2
42
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
a
56
84
34
54
52
52
48
99
73
96a
49
96b
43
48
c
Mg(OTf)2
Mg(OTf)2
YCl3
c
c
YCl3
YbCl3
82
63
88:12
91:9
c
YbCl3
ScCl3
88:12
91:9
c
ScCl3
MgCl2
LaCl3
64
53
81:19
75:25
Conversion after 1 h,
b
c
Conversion after 15 min
Catalyst 3 mol% vs methyl acrylate
was achieved just after 15 min in the presence of scandium
chloride. The same conversion was obtained after 1 h for
ytterbium chloride, whereas for yttrium chloride the con-
version was 99% after 2 h. The stereoselectivity to the
endo isomer amounted to 91% in the presence of each of
these chlorides.
The reaction of cyclopentadiene with butyl acrylate and in
the presence of several most active catalysts was carried out
in three pyrrolidinium ionic liquids: Pyrr1.4[NTf2], Pyr-
r
1.4[OTf], liquid Pyrr1.4[FAP] (Table 3). All the reactions
proceeded with higher yields in the liquid with [NTf2]-
anion. In the presence of the trifluoromethanesulfonate
catalysts: scandium, yttrium, ytterbium and the ionic
liquid Pyrr1.4[FAP] proceeded the oligomerization of
cyclopentadiene. Similarly as in the case of the reaction of
cyclopentadiene with methyl acrylate, the highest activity
123

746
W. Stefaniak et al.
Table 3 Conversion and stereoselectivity reaction of cyclopentadiene with butyl acrylate in pyrrolidinium ionic liquids with various anions
Entry
Catalysta
Pyrr1.4[NTf2]
Pyrr1.4[OTf]
Pyrr1.4[FAP]
Conversion
[mol %]
0.25 h
Stereoselectivity
endo:exo (%)
Conversion
[mol %]
0.25 h
Stereoselectivity
endo:exo (%)
Conversion
[mol %]
0.25 h
Stereoselectivity
endo:exo (%)
1
2
3
4
5
6
a
ScCl3
99
73
88
51
96
60
89:11
89:11
94:6
82
61
63
47
61
46
91:9
46
–
82:12
–
Sc(OTf)3
YCl3
90:10
88:12
90:10
88:12
89:11
63
–
93:7
–
Y(OTf)3
YbCl3
91:9
94:6
61
–
91:9
–
Yb(OTf)3
90:10
Catalyst 3 mol% vs dienophile
Table 4 Conversion and stereoselectivity reaction of cyclopentadiene with alkyl acrylates in ionic liquid Pyrr1.4[NTf2]
Entry
Catalyst
Cyclopentadiene ? 2b
Cyclopentadiene ? 2c
Cyclopentadiene ? 2d
Conversion
[mol %]
Stereoselectivity
endo:exo %
Conversion
[mol %]
Stereoselectivity
endo:exo %
Conversion
[mol %]
2 h
4 h
2 h
4 h
2 h
4 h
1
2
3
4
5
6
a
ScCl3
96b
70
92:8
93:7
91:9
93:7
92:8
91:9
99a
87
95b
89:11
94:6
92a
85
Sc(OTf)3
YCl3
89
93
82
91
86
90
95
90:10
94:6
94
Y(OTf)3
YbCl3
86
93c
91
83
87
80
93c
96a
80
90:10
92:8
Yb(OTf)3
80
83
Conversion after 15 min
Conversion after 30 min
Conversion after 1 h
b
c
Cyclopentadiene concentration 6 mol/dm3, alkyl acrylate concentration 4 mol/dm3, catalyst 3 mol% versus alkyl acrylate
4 Conclusions
derivatives are obtained with the yield over 90% at tem-
perature 25 °C in the presence 3 mol% of catalyst in relation
to the dienophile (the catalyst concentration in the ionic
liquid amounts 0.012 mol/dm3). The reaction times required
to achieve such yields amount from 30 to 120 min. The
stereoselectivity of the endo isomer amounts over 89% in
this case and varies widely depending on the kind of metal
atom in the chloride or triflate and the type of dienophile.
The ionic liquids composed of 1-butyl-1-methylpyrrolidi-
nium cation and anions: bis(trifluoromethylsulfonyl)imide
[NTf2]-, trifluoromethanesulfonate [OTf]-, tris(pentafluo-
roethyl)trifluorophosphate [FAP]- are convenient media for
the Diels–Alder reaction between cyclopentadiene and alkyl
esters of acrylic acid. The reaction already proceeds at room
temperature without the catalyst and in the presence of ionic
liquids (Table 1), and the dienophile conversion and stere-
oselectivity depends on the kind of dienophile and anion in
the ionic liquid. The above mentioned pyrrolidinium ionic
liquids in several case form the effective catalytic systems
with chlorides and trifluoromethanesulfonates of metals.
These catalysts enhance the conversion of acrylates, the
yield of norbonene derivatives and the stereoselectivity of
the endo isomer. However, some of them show significant
increase in the oligomerization rate of cyclopentadiene. The
most active catalysts of the reaction of cyclopentadiene with
alkyl acrylates are ScCl3, YbCl3 and YCl3. The norbornene
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