Effect of chiral ionic liquids on palladium-catalyzed Heck arylation of 2,3-dihydrofuran

Article abstract of DOI:10.1016/j.apcata.2011.09.038

It is demonstrated that a relatively small amount of IL (IL = ionic liquid) can dramatically affect conversion in the Heck arylation of 2,3-dihydrofuran with iodobenzene, catalyzed by Pd(OAc)₂ in DMF as a solvent. In all reactions, 2-phenyl-2,3

Full text of DOI:10.1016/j.apcata.2011.09.038

Applied Catalysis A: General 409–410 (2011) 148–155
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Effect of chiral ionic liquids on palladium-catalyzed Heck arylation of
2,3-dihydrofuran
Rafał Roszaka,b, Anna M. Trzeciaka,∗, Juliusz Pernakc, Nina Boruckac
a University of Wrocław, Faculty of Chemistry, 14 F. Joliot-Curie St., 50-383 Wrocław, Poland
b Wrocław University of Technology, Institute of Physical and Theoretical Chemistry, Wybrzez˙ e Wyspian´skiego 27, 50-370 Wrocław, Poland
c Poznan´ University of Technology, Department of Chemical Tehnology, pl. M. Skłodowskiej-Curie 2, 60-965 Poznan´, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
It is demonstrated that a relatively small amount of IL (IL = ionic liquid) can dramatically affect conversion
in the Heck arylation of 2,3-dihydrofuran with iodobenzene, catalyzed by Pd(OAc)2 in DMF as a solvent. In
all reactions, 2-phenyl-2,3-dihydrofuran (3) was obtained as the main product, and conversion increased
even up to 10 times when pyridinium salts with 1-butyl-4-methylpyridinium cation were applied. In
a 1:1 mixture of DMF and H2O as solvent, the addition of ILs led to a remarkable deactivation of the
catalyst, and this effect was most visible in the presence of imidazolium salts containing a 1-butyl-3-
methylimidazolium cation. The influence of the anionic part of ILs on the reaction course was tested
using a series of morpholinium salts and, depending on the anion, conversion varied from 0.4% to even
100%. When morpholinium salts with chiral anions were used, e.e. values of up to 10% were obtained,
which is the highest value for the Heck reaction involving IL as a chirality source.
Received 30 July 2011
Received in revised form
25 September 2011
Accepted 29 September 2011
Available online 6 October 2011
Keywords:
Homogeneous catalysis
Palladium
Heck reaction
Asymmetric catalysis
Ionic liquids
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
migration resulted in the formation of carbonyl products [5]; (ii)
arylation of other linear olefins [6]; (iii) arylation of cyclic olefins
The Heck coupling is one of the most important C–C bond
forming reactions [1] with many applications such as the syn-
thesis of heterocyclic compounds [2] or total synthesis of natural
products [3]. Typically, this reaction involves monosubstituted
olefin and aryl halide in the presence of a base and a palladium
compound as a catalyst precursor. In the case of olefins bear-
ing electron-withdrawing substituents (EWG, e.g. –COOR, –CN),
reaction selectivity is controlled by two factors: electronic, which
causes arylation in the ␤-position to the EWG, and steric, which
facilitates the formation of the trans product. Consequently, the for-
mation of 1,2-trans-disubstituted olefin is preferred. The arylation
of an electron-reach olefin, such as an olefin with the –OR group,
can produce 1,2-trans or 1,1-branched products. In the latter case,
selectivity can be controlled by the solvent or other factors [4].
The Heck coupling involving an olefin bearing a hydrogen atom
in the allylic position can be accompanied by migration of the
carbon–carbon double bond leading to the formation of a prod-
uct with a chiral carbon atom. Reactions of this type have been
intensively investigated in four variants with respect to the kind of
substrate used: (i) arylation of allylic alcohols where double bond
including ring-closing Heck coupling [7]; (iv) arylation of hetero-
cyclic olefins.
As a model Heck reaction of cyclic olefin containing a het-
eroatom, primarily 2,3-dihydrofuran (DHF) was tested. Arylation
of DHF occurred exclusively at the C2 position (Fig. 1); how-
ever, due to carbon–carbon double bond migration, three products,
namely 2, 3 and 4, can be formed. Arylation of DHF performed
in the presence of a palladium catalyst containing a phosphorous
ligand can lead selectively to 2-aryl-2,5-dihydrofuran (2) or 2-
aryl-2,3-dihydrofuran (3), depending on the kind of ligand used.
Monodentate phosphorous ligands [8] and bidentate ligands with
P–N [9], P–As [10], P–S [11], or P–O [12] donor atoms gave pre-
dominantly product 2. In contrast, when palladium complexes with
bidentate P–P ligands [13] were used, 3 was formed as the main
reaction product [14]. However reversed selectivity was also noted
[14].
Compounds 2 and 3 possess chiral centers on tertiary carbon
atoms, and, consequently, they can be obtained as nonracemic
mixtures. The stereoselective version of the Heck reaction was
intensively tested with various phosphorous ligands [15].
Due to economical and environmental factors, P-free catalytic
systems have been attracting increasing interest. Several P-free cat-
alysts for the Heck reaction of DHF have been reported [16], and
the most promising one was the application of “Jeffery conditions”
∗
Corresponding author. Tel.: +48 71 3757253; fax: +48 71 3282348.
E-mail address: anna.trzeciak@chem.uni.wroc.pl (A.M. Trzeciak).
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.09.038

R. Roszak et al. / Applied Catalysis A: General 409–410 (2011) 148–155
149
which made it possible to selectively obtain product 2 or 3. When
catalyst
base
+
+ ArX
+
performed in the presence of [Bu4N]OAc, the reaction led mainly
to 2 with the ratio of 3 to 2 amounting to up to 3:97, whereas when
[Bu4N]Cl was applied, product 3 dominated with the ratio of 3 to 2
amounting to up to 92:8 [16d].
Ar
Ar
Ar
O
O
O
O
2
3
4
Fig. 1. Heck cross-coupling of 2,3-dihydrofuran with iodobenzene.
To the best of our knowledge only one example of a Heck
reaction of DHF designed to obtain chiral products under P-free
conditions has been reported [17]. In that reaction, involving
a chiral 2-[4-alkyl-4,5-dihydro-1,3-oxazol-2-yl]pyridine bidentate
ligand, a mixture of both products 2 and 3 was obtained, (the ratio of
3 to 2 was in the range 0.76–2.56) with e.e. up to 29% for compound
2 and up to 60% for product 3.
X-
+
Imidazolium IL (5)
N
N
R3
R1
R2
Another approach to performing the Heck coupling in a stereos-
elective fashion involves the application of chiral ionic liquids (CIL).
A recent review [18] surveys the latest most representative devel-
opments and the progress concerning ILs from their fundamental
properties to applications in catalytic processes.
Chiral ionic liquids represent one of the most interesting groups
of ionic liquids, and the number of publications dealing with CILs
has been growing rapidly [19–23]. Recently, the synthesis of CILs
directly from a natural (1R,2S,5R)-(−)-menthol-based chiral pool
has been described [24].
The source of chirality can be provided either on the anion or
on the cation or on both the anion and the cation. However, the
most common chiral anions used are natural aminoacids [25,26].
CILs can be applied in asymmetric catalysis (e.g. aldol reaction,
Baylis–Hillman reaction or the Michael addition reaction) [27]. In
a case of metal mediated catalysis, CILs are used mainly in double
bond addition reactions as a chirality source [28] or as a “chiral
booster” for classic chiral ligands [29].
There are only three reports about application of CILs in the
Heck coupling. Gayet et al. [30] reported the arylation of DHF with
iodobenzene catalyzed by a chiral pyridinium IL with [PdCl4]2−
as a counterion in [BMIM]PF6 medium; however, no enantiomeric
excess was observed. Two other examples of Heck-type arylation
involving CILs are the oxyarylation of 7-benzyloxy-2H-chromene
[31], which gave a desirable product with 5% e.e., and the aryla-
tion of aza-endocyclic acrylate [32] in various imidazolium and
pyridinium CILs, which produce only racemic mixtures.
In this paper, we present the first stereoselective Heck cou-
pling performed in the presence of CILs with stereogenic centers
present in the anionic part of the CIL. In particular, morpholin-
ium based CILs were studied. The increasing interest in studies
of ILs containing morpholinium cations is related to their char-
acteristic structure and, especially, the presence of nitrogen and
oxygen atoms, which are responsible for their unique proper-
ties. Morpholinium ILs have found application as electrolytes [33]
and gel electrolytes [34], ionic liquid crystals (ILC), a class of
liquid-crystalline compounds [35], and reaction medium [36]. In
particular, ILs based on the morpholinium cation are favored
because of their electrochemical stability [37]. Morpholinium
ILs are, in fact, less toxic than commonly used imidazolium-,
pyridinium-, or tetraalkylammonium-based ILs [38].
IL
5a
5b
5c
5d
5e
5f
R1
Bu
Bu
Bu
Bu
Et
R2 R3
X
H
H
H
H
H
H
H
Me
Me
Me
Me
Me
Me
Me
BF4
PF6
Cl
Br
EtSO3
Et
Me(OEt)2OSO3
5g
5h
octyl
Me
BF4
Cl
Me C2H4OH
+
Me
N
Bu
Pyridinium IL (6)
X-
6a
6b
6c
6d
BF4
PF6
Cl
Br
Fig. 2. Imidazolium and pyridinium ILs.
In a reference reaction performed in a DMF/H2O mixture with-
out an IL, 27% of cross-coupling products were obtained.
The effect of the IL amount on the course of the Heck arylation
in this solvent was tested with two imidazolium ILs, namely 1-
butyl-3-methylimidazolium salts, 5a and 5c, with different anions
(Fig. 3). It could be expected that 5c, which contained a coordinating
Cl− counterion, could easily form a Pd-carbene complex, whereas
5a could exhibit a much weaker tendency to form a Pd-carbene
complex [39]. However, actually both of the imidazolium salts acted
40
35
5a
30
5c
25
20
15
10
5
2. Results and discussion
The arylation of DHF with iodobenzene was performed with
Pd(OAc)2 as a catalyst precursor, and first the effect of the solvent
and the base on the reaction course was studied. The preliminary
results showed that DMF and a DMF/H2O mixture (1:1) were the
most suitable for our system, and the highest conversions were
obtained using cesium and potassium carbonates as bases. Next,
the effect of ILs was investigated, and imidazolium and pyridinium
ILs used in these studies are presented in Fig. 2.
0
0
20
40
60
80
IL/Pd ratio
Fig. 3. Effect of the IL/Pd ratio on conversion in the Heck cross-coupling of DHF with
PhI.

150
R. Roszak et al. / Applied Catalysis A: General 409–410 (2011) 148–155
Table 1
9
Heck arylation of DHF in DMF/H2O with various ILs.
IL
Conversion (%)
Selectivity 3/(2 + 4)
8
–
27.0
34.0
21.4
25.1
22.5
8.0
5.4
5.0
5a
5b
5c
5d
5e
5h
5f
5g
6d
6a
6b
6c
7l
7
6
5
4
3
2
1
0
5.3
4.7
7.7
1.9
4.1
0.9
3.5
6.5
4.1
3.9
4.6
3.8
4.6
27.2
2.5
10.7
12.8
20.4
18.4
19.5
21.2
30.1
7f
without IL
5d, 20 eq
5a, 20 eq
Reaction conditions: 8 mg Pd(OAc)2; 0.4 mL PhI; 0.7 mL DHF; 0.6 g K2CO3; 2 h; 60 ◦C;
IL/Pd = 20, solvent: DMF:H2O 3 mL:3 mL; 0.15 mL mesitylene (GC standard).
very similarly; namely, in both cases the addition of a small amount
of an IL slightly increased reaction conversion compared with the
reaction performed without an IL. However, further increase of the
IL amount, up to IL-to-Pd of ca. 30, caused a dramatic deactivation
of the system (Fig. 3). As the highest conversion was obtained with
5a at an IL/Pd ratio of 20, we tested a broad range of different ILs
under these conditions, and the results are presented in Table 1
(Table 1).
0
1
2
3
4
time [h]
Fig. 5. Regioselectivity [(3/(2 + 4)] of the Heck reaction over time.
In the best case, conversion of up to 34% was achieved, and the
highest selectivity was 7.7. In most cases, the conversion to cross-
coupling products was lower than in the absence of ILs.
(5) caused catalyst deactivation. It can also be concluded that the
effect of the anion seems to be less important in this case.
It is worth to note that in the Heck reaction performed in
DMF/H2O only a very small effect of ILs on the DHF conversion
was observed, in contrast to reactions performed in pure DMF.
Conversion of DHF in DMF/H2O remainded on the level of 30%, sim-
ilarly as in the absence of ILs (Table 1). Most probably, the presence
of water facilitated deactivation of the catalyst with formation of
inactive “palladium black”. Reduction of Pd(II) to Pd(0) is generally
expected under basic conditions [40]. There is an important step of
the catalytic reaction, however in the absence of efficient stabiliza-
tion “palladium black” can be formed easily. In the absence of water,
DMF, strong donor solvent, stabilizes better a coordinatively unsat-
urated Pd(0) species preventing their agglomeration. The higher
activity of palladium in the presence of pyridinium salts can be
related to the formation of more active ionic species of the type
Pd(0)L2(OAc)− [43b], while imidazolium salts can form Pd-carbene
complexes, less active under these conditions.
To obtain more information about the kinetics of the reactions
studied, additional experiments were carried out for two selected
systems containing 5a and 5d, respectively. As can be concluded
from the data presented in Fig. 4, a reaction started without any
induction period with the highest rate noted in the first 30 min.
Interestingly, extension of the reaction time to 4 h made it possi-
ble to obtain higher conversions, 31% and 40% in the presence of 5d
and 5a, respectively. At the same time, regioselectivity expressed
as [3/(2 + 4)] ratio, decreased in reaction with 5a and increased only
slightly in reaction with 5d (Fig. 5).
In further studies, DMF was used as solvent instead of a
DMF/H2O mixture. Under these conditions, the conversion of DHF
in the absence of an IL was only 23.5%. The effect of imidazolium (5),
pyridinium (6), and morpholinium (7) salts on the Heck arylation
of DHF shown in Fig. 6 is much more pronounced than in DMF/H2O.
Thus, a remarkable increase in conversion, up to 83%, was obtained
when pyridinium salts (6) were used. In contrast, imidazolium salts
100
80
60
40
20
0
50
40
30
20
without IL
10
0
5d, 20 eq
5a, 20 eq
0
10
20
30
40
IL/Pd ratio
0
1
2
3
4
6d
6a
7d
5d
5a
time [h]
Fig. 6. Effect of the IL/Pd ratio on conversion in the Heck cross-coupling of DHF with
PhI.
Fig. 4. Heck reaction over time.

R. Roszak et al. / Applied Catalysis A: General 409–410 (2011) 148–155
151
Table 3
100
80
60
40
20
0
Heck arylation of DHF in DMF with various [Bmorf][A] salts.
A =
Conversion [%]
Selectivity [3/(2 + 4)]
e.e. of 3 (%)
e.e. of 2 (%)
7a
7b
7c
7d
7e
7n
7m
7h
7i
7k
7o
7l
7f
8o
19.6
20.4
29.2
35.8
35.6
0.4
16.3
63.2
67.3
70.8
71.7
99.3
100.0
80.2
5.7
4.2
5.7
5.1
4.1
–
6.0
3.3
3.1
3.7
2.7
1.9
2.4
2.7
–
–
–
–
–
–
–
–
–
–
<4.0
9.9
5.8
<4.0
6.0
<4.0
<4.0
4.0
<4.0
<4.0
<4.0
<4.0
<4.0
<4.0
<4.0
<4.0
4.2
17.3
Reaction conditions: 8 mg Pd(OAc)2; 0.4 mL PhI; 0.15 mL mesitylene; 0.7 mL DHF;
0.6 g K2CO3; 2 h; 70 ◦C; IL/Pd = 20, solvent: DMF 6 mL; 8o – dimethyldidodecylam-
monium (S)-lactate.
0
20
40
60
80
[6a]/[Pd] ([6a]/[Pd] x10 for 0.1 % Pd)
1 % Pd, DMF/H2O
1 % Pd, DMF
glass transition was found almost for all compounds, with the only
exception of 7m. For all studied CILs any phase transitions (crys-
tallization temperature and melting point) were noticed, however
a wide liquid range (difference between glass transition −Tg and
thermal degradation −Tonset(5%)) was observed, even 300 ◦C or more
as for CILs 7j and 7i, respectively.
0.1 % Pd, DMF
Fig. 7. Effect of the 6a/Pd ratio on conversion in the Heck cross-coupling of DHF
with PhI.
Therefore, we chose 6a for further studies of the influence of
IL concentration on the Heck arylation (Fig. 7). Conversion to the
Heck product with 1 mol% of Pd(OAc)2 in DMF increased from 23.5%
(without an IL) to 91% at IL/Pd = 80 which corresponded to a con-
centration of 6a equal to 0.39 mol/dm3. The effect of the IL amount
is even more visible at lower catalyst loading, and in reactions per-
formed with 0.1 mol% of Pd, conversion increased from 6% (reaction
without an IL) to 73% at IL/Pd = 800 (reaction with 6a concentration
of 0.39 mol/dm3). A reverse tendency, namely a decrease in con-
version with increasing [6a]:[Pd] ratio, was observed in DMF/H2O.
This shows again that DMF/H2O is not an appropriate solvent for
the Heck reaction with imidazolium and pyridinium ILs. However,
when pure 6a, without DMF, was used as solvent, conversion was
only 30% (for 1 mol% of Pd).
It has been suspected [41] that the anionic part of the IL can play
an important role in the Heck arylation, so to study such an effect,
a series of morpholinium salts was prepared (Fig. 8).
As shown in Table 2, thermal stability of morpholinium ILs
depends on chiral anion. The most stable CILs contained anion 7k
and 7j and their decomposition started about 200 ◦C. The less stable
was compound with anion 7f decomposed at 104 ◦C. For four mor-
pholinium CILs the second step of decomposition was observed, for
7g at 232 ◦C (39% mass loss), 7h at 238 ◦C (43.4% mass loss), 7i at
239 ◦C (41.9% mass loss) and for 7l at 214 ◦C (31.2% mass loss). A
The yield of the Heck reaction performed in DMF with mor-
pholinium salts containing inorganic anions varied from 19.6% for
the Cl− anion (7a) to 35.8% for the BF4 anion (7d), whereas selec-
−
tivity ranged from 4.2 (for 7b) to 5.7 (for 7a and 7c) (Table 3). In
the case of morpholinium ILs with organic anions, the anion effect
was even more clearly seen. When 4-hydroxyproline anion, 7n,
was applied, only traces of the product were obtained, whereas
with benzyloxypropionate, 7l, or methylnitromethyloxocyclopen-
taneacetate, 7f, anion conversion was almost quantitative. In
reaction with 7m (morpholinium quinate), the products were
obtained with significantly lower yields than in reactions with
inorganic morpholinium salts, whereas all other ILs with organic
anions gave products with yields that were approximately three
times higher. The selectivity of reactions with morpholinium salts
containing organic anions, equal to 1.9–3.7, was generally lower
than with those containing inorganic anions. However, exception-
ally, the reaction proceeding with 7m gave product 3 with higher
selectivity than all the other morpholinium ILs tested.
The stereoselective Heck coupling of DHF with iodobenzene has
to the best of our knowledge not been reported in the literature. In
the typical procedure applied for the Heck arylation of DHF, phenyl
triflate was used. However, the main disadvantage of this proce-
dure is the easy hydrolysis of triflate, limiting its application in
the presence of water. Thus, iodobenzene is a cheaper and more
easily available substrate. Table 3 summarizes the data, illustrat-
ing the effect of anions present in morpholinium salts on the yield
and selectivity of the coupling reaction. The obtained e.e. values
were up to 9.9% for 7m; lower e.e. values were obtained for 7h
and 7k (nearly 6.0%), whereas for 7f, e.e. was equal 4.0%. For all
other CILs, e.e. values below 4.0% were noted. These values are
lower compared with results obtained with phosphorus ligands;
however, they are higher than those obtained in reactions pro-
ceeding in CILs with chiral cations. Interestingly, the highest e.e.
value, 17.3, for side-product 2, was obtained with the application
of tetraalkylammonium salt 8o with a (S)-lactate anion. It is worth
noting the influence of the type of cation on reaction stereoselec-
tivity, which is clearly seen when the results obtained with 7o and
8o are compared.
Table 2
4-Benzyl-4-methylmorpholinium ILs ([Bmorf][A]), thermal stability and phase
transition.
a
b
c
d
A =
Tg (◦C)
Tonset(5%) (◦C)
Tonset (◦C)
T1 (◦C)
7f
−31.42
−27.85
−40.53
−38.20
−25.50
−24.77
−17.73
–
104
170
132
142
196
200
114
156
107
210
249
250
268
272
276
274
232
250
–
7g
7h
7i
7j
7k
7l
232(39)
238(43.4)
239(41.9)
–
–
214(31.2)
–
–
7m
7n
−40.43
a
Glass transition temp determined by DSC.
Decomposition temp determined from onset to 5 wt% mass loss.
Decomposition temp determined from onset to 50% mass loss.
The proposed mechanism of the Heck reaction, inspired by the
literature [42], is presented in Fig. 9. After the oxidative addition of
aryl halidate to palladium, there are two possible routes, starting
b
c
d
Second step decomposition (in parenthesis decomposition in percent).

152
R. Roszak et al. / Applied Catalysis A: General 409–410 (2011) 148–155
morpholinium salts:
[BMmorf][A]
A = Cl
7a
7b
O
PF6
N+
NO3
7c
7d
7e
BF4
CF3CO2
[BMmorf]
O
OH
O-
OH
OH
O-
H
H
Br
O
O
O
O
H3C
O-
O
O-
O-
NO2
CH3
7j
Cl
7i
CH3
7f
7g
7h
O
O-
HO
Br
HO
OH
H
O-
O
O-
H3C
O
O
O-
H3C
O-
N
H
OH
CH3
O
HO
O
CH3
OH
7k
7l
7m
7n
7o
Fig. 8. Morpholinium ILs used in the Heck reaction.
from the B1 or the B2 intermediate respectively. The anionic part
of ILs can play an important role in the B1–B2 equilibrium and, in
particular, carboxylate anions can compete with halide X− (origi-
nating from ArX or from IL) for a place in the coordination sphere of
palladium. Consequently, a halide-free pathway can be considered,
as shown in the left cycle in Fig. 9.
The reactivity of B-type intermediates depends on the type
of ligand rather than on the charge of the complex, and the
relative activity of these complexes, as described by Amatore
[43], decreased in the order ArPdL2OAc > ArPdL2+ > ArPdL2I. There-
fore, promotion of the halide-free pathway should cause higher
conversion.
O
O
Ar
Ar
O
Pd
Pd
L
X
L
RCO2
O
O
L
L
Ar
Pd
Ar
Ar
X
X
Ar
L
L
L
Pd
Pd
Pd
RCO2-
L
X
RCO2
L
L
L
B2
B1
ArX
O
O
"Pd0"
A
L
Ar
L
Ar
Pd
Pd
H
X
H
RCO2
Ar
O
Ar
O
Ar
Ar
O
O
3
2
3
2
Fig. 9. The proposed mechanism of Heck reaction.

R. Roszak et al. / Applied Catalysis A: General 409–410 (2011) 148–155
153
In fact, the yields of reactions involving morpholinium salts with
carboxylate anions were around twice as high as those with non-
coordinating BF4 or PF6 anions. An exceptionally low yield was
observed for 7m and 7n containing a carboxylic group directly
linked to a bulky substituent. In this case, the ability of the anion to
coordinate to palladium can be limited by the steric hindrance.
In the case of phosphorus-free Heck arylation, as in the system
under study, the mechanism involving Pd(0) nanoparticles is com-
monly accepted [44]. Consequently, the role of the IL as a stabilizing
agent for nanoparticles preventing their agglomeration should be
considered. The surface of palladium(0) nanoparticles can be nega-
tively charged [45], and therefore the cationic part of the IL can play
a decisive role in electrostatic interaction. Accordingly, a strong
influence of the cation on the reaction course was observed in a
series of imidazolium, pyridinium, and morpholinium salts.
acid were obtained from Aldrich and used without further
purification. (R)-(+)-2-(Benzyloxy)propionic acid and (2S, 4R)-4-
hydroxypyrrolidine-2-carboxylic acid were obtained from Fluka.
All solvents were used as obtained without further purification.
Synthesis of 4-benzyl-4-methylmorpholinium nitrate(V),
tetrafluoroborate and hexafluorophosphate have been recently
described [49].
4.2. Synthesis of 4-benzyl-4-methylmorpholinium ionic liquids
4-Benzyl-4-methylmorpholinium chloride was synthesized by
treating 4-methylmorpholine with benzyl chloride. Morpholin-
ium ionic liquids with anions 7f – 7l were synthesized by
adding appropriate chiral carboxylic acid (0.015 mol) to portion of
4-benzyl-4-methylmorpholinium chloride (0.015 mol) and a stoi-
chiometric amount of potassium hydroxide dissolved in methanol.
The solution was stirred at room temperature for 1 h and then
filtrated to remove the precipitated potassium chloride. After evap-
oration of methanol, the product was dissolved in acetone and the
rest of inorganic salt was removed. The final product was dried in
vacuum at 45 ◦C for 2 h.
Two ILs, with anions 7m and 7n, were obtained in the simi-
lar procedure, only after evaporation of methanol the product was
dissolved in acetone and methanol (1:1) mixture and the rest of
inorganic salt was removed. The final product was dried in vacuum
at 45 ◦C for 2 h.
3. Conclusions
The Heck cross-coupling reaction of iodobenzene with 2,3-
dihydrofuran, catalyzed by Pd(OAc)2 in the absence of ionic liquids,
gives the highest conversion in DMF/water mixture as a solvent. The
addition of a small amount of ILs leads to a significant increase in
conversion, but only in absence of water.
ILs have a very strong effect on the catalytic reaction conversion,
depending on their composition. The strongest promoting effect
was observed when pyridinium salts were applied, whereas imi-
dazolium salts caused deactivation of the catalyst. The effect of the
anionic part of ILs on the reaction conversion seems to be even
more significant. We suggest that reaction pathway leading to the
higher DHF conversion proceeds according to the left part of the
scheme presented on Fig. 9, with participation of Pd-carboxylate
intermediates. With only two exceptions, namely 7n and 7m, in
the presence of ILs containing carboxylate functionalized anion, the
DHF conversion was higher than with non-carboxylate type anions.
An important effect of anions on the reactivity of anionic Pd(0) com-
plexes in Heck reaction was also demonstrated by Amatore et al.
[43,46].
In order to explain an observed effect of the cations on the
reaction course we propose to consider interactions of the in situ
formed Pd(0) nanoparticles with cationic part of IL, according
to the model described in the literature [45]. Important role of
such interactions in the formation of new palladium species on
the surface of Pd(0) nanoparticles was also discussed in [47].
1H NMR spectra were recorded on the Mercury Gemini 300
spectrometer operating at 300 MHz with TMS as the internal stan-
dard. 13C NMR spectra were obtained with the same instrument
at 75 MHz. CHN elemental analyses were performed at the Adam
Mickiewicz University, Poznan (Poland).
4-Benzyl-4-methylmorpholinium
((1S-(1ˇ,2␣,3ˇ)-(+)-3-
methyl-2-(nitromethyl)-5-oxocyclopentaneacetate,
[BMmorf][7f] 1H NMR (DMSO-d6) ı ppm 1.03 (d, J = 3.11 Hz,
3H); 2.00 (m, 3H); 2.35 (m, 2H); 2.61 (d, J = 7.88 Hz, 2H); 3.10
(s, 3H); 3.39 (t, J = 4.21 Hz, 2H); 3.55 (qw, J = 4.94 Hz, 2H); 3.97
(t, J = 1.59 Hz, 4H); 4.71 (d, J = 8.42 Hz, 2H); 4.79 (s, 2H); 7.52 (m,
3H); 7.61 (m, 2H); 13C NMR (DMSO-d6) ı ppm 18.1; 33.1; 36.6;
44.8; 45.8; 46.5; 50.4; 58.4; 59.9; 67.5; 78.1; 127.4; 128.8; 130.3;
133.4; 172.9; 216.8.
Elemental analysis calc. (%) for C21H30N2O6 (406.47): C 62.05, H
7.44, N 6.89. Found: C 62.55, H 7.79, N 6.11.
Alternatively, the formation of discrete supramolecular species of
4-Benzyl-4-methylmorpholinium
(S)-(+)-mandelate,
n+
the type [(cation)x(X)x−n
]
[(cation)x−n(X)x]n− on the surface of
[BMmorf][7g], 1H NMR (DMSO-d6) ı ppm 3.02 (s, 3H); 3.30
(t, J = 4.29 Hz, 2H); 3.51 (qw, J = 5.02 Hz, 2H); 3.93 (t, J = 2.57 Hz,
4H); 4.10 (s, 1H); 4.46 (s, 1H); 4.70 (s, 2H); 7.20 (m, 3H); 7.35 (m,
2H); 7.51 (m, 3H); 7.54 (m, 2H); 13C NMR (DMSO-d6) ı ppm 44.7;
58.4; 59.8; 67.5; 73.6; 125.8; 126.2; 127.22; 127.25 128.8; 130.3;
133.3; 144.3; 173.6.
Pd(0) nanoparticles can be considered [48]. Most probably, both
forms, namely monomolecular palladium complexes and palla-
dium nanoparticles, participate in the catalytic process; however,
details of the mechanism are not clear yet
Reactions with morpholinium salts containing chiral anions
exhibited asymmetric induction, which confirmed an efficient
interaction of the anionic part of CILs with palladium intermediate.
The obtained e.e. values are up to 10% which is the highest value
for Heck arylation in CILs. On the basis of the obtained results, we
believe that tuning the anionic part of the CIL, in contrast to tuning
the cationic part, is a much more efficient way to receive a chiral
product in Heck arylation.
Elemental analysis calc. (%) for C20H25NO4 (343.41): C 69.95, H
7.34, N 4.08. Found: C 70.46, H 6.78, N 4.64.
4-Benzyl-4-methylmorpholinium
(R)-(−)-mandelate,
[BMmorf][7h], 1H NMR (DMSO-d6) ı ppm 2.07 (s, 1H); 3.02
(s, 3H); 3.31 (t, J = 4.27 Hz, 2H); 3.51 (qw, J = 5.05 Hz, 2H); 3.93 (t,
J = 1.71 Hz, 4H); 4.54 (s, 1H); 4.70 (s, 2H); 7.22 (m, 3H); 7.37 (m,
2H); 7.52 (m, 3H); 7.55 (m, 2H); 13C NMR (DMSO-d6) ı ppm 44.8;
58.4; 59.9; 67.6; 73.5; 126.0; 126.3; 127.2; 127.3; 128.9; 130.3;
133.3; 143.9; 173.9.
4. Experimental
Elemental analysis calc. (%) for C20H25NO4 (343.41): C 69.95, H
7.34, N 4.08. Found: C 69.51, H 7.84, N 3.69.
4.1. Materials
4-Benzyl-4-methylmorpholinium (R)-(−)-3-chloromande-
late, [BMmorf][7i], 1H NMR (DMSO-d6) ı ppm 3.05 (s, 3H); 3.33 (t,
J = 4.27 Hz, 2H); 3.53 (qw, J = 5.13 Hz, 2H); 3.95 (t, J = 1.77 Hz, 4H);
4.51 (s, 2H); 4.72 (s, 2H); 7.20 (t, J = 1.16 Hz, 1H); 7.25 (s, 1H) 7.34
(d, J = 0.64 Hz, 1H); 7.40 (d, J = 0.82 Hz, 1H); 7.52 (m, 3H); 7.55 (m,
2H); 13C NMR (DMSO-d6) ı ppm 44.7; 58.4; 59.8; 67.5; 72.8; 124.8;
4-Methylmorpholine, benzyl chloride, potassium hydroxide,
((1S-(1ˇ,2␣,3ˇ)-(+)-3-methyl-2-(nitromethyl)-5-oxocyclopenta-
neacetic acid, (S)-(+)-mandelic acid, (R)-(−)-mandelic acid, (R)-
(−)-3-chloromandelic acid, (S)-(−)-2-bromo-3-methylbutyric
acid, (R)-(+)-2-bromo-3-methylbutyric acid and (D)-(−)-quinic

154
R. Roszak et al. / Applied Catalysis A: General 409–410 (2011) 148–155
125.7; 125.8; 127.2; 128.9; 129.1; 130.3; 132.1; 133.2; 146.6;
172.8.
temperatures (Tonset5% and Tonset) were determined from onset 5
mass loss and 50% mass loss respectively, under nitrogen.
Elemental analysis calc. (%) for C20H24ClNO4 (377.86): C 63.57,
H 6.40, N 3.71. Found: C 63.93, H 6.02, N 3.99.
4.4. Heck reaction
4-Benzyl-4-methylmorpholinium
(S)-(−)-2-bromo-3-
methylbutyrate, [BMmorf][7j], 1H NMR (DMSO-d6) ı ppm 1.02
(d, J = 1.28 Hz, 6H); 2.19 (m, 1H); 3.13 (s, 3H); 3.40 (t, J = 4.21 Hz,
2H); 3.59 (qw, J = 5.20 Hz, 2H); 4.00 (t, J = 1.65 Hz, 4H); 4.68 (d,
J = 1.92 Hz, 1H); 4.85 (s, 2H); 7.54 (m, 3H); 7.62 (m, 2H); 13C NMR
(DMSO-d6) ı ppm 19.0; 31.7; 44.9; 55.4; 58.4; 59.8; 67.2; 127.3;
128.8; 130.3; 133.2; 170.2.
The Heck arylation of DHF with PhI was carried out under
N2 atmosphere using standard Schlenk technique. Reagents were
introduced to the Schlenk tube in a following order: Pd(OAc)2
(8.0 mg, 0.0356 mmol, 1 mol.%), solvent DMF (6 mL) or DMF/H2O
(3 mL + 3 mL), IL (appropriate amount), K2CO3 (0.6 g, 4.34 mmol),
PhI (0.4 mL, 3.57 mmol) mesitylene (internal standard, 0.15 mL)
and DHF (0.7 mL, 8.59 mmol). If not mentioned otherwise the reac-
tion was carried out at 70 ◦C for 2 h. Afterwards, reaction mixture
performed in DMF was quenched with H2O (3 mL) and organic
products were separated by extraction with diethyl ether (3 times:
10 mL, 7 mL and 7 mL). Products were analysed by GC-FID (Hewlett
Packard 8454A). Products 2, 3, 4 were identified by comparison
of MS spectra and retention times with literature data. Products
of reactions proceeding in DMF:H2O mixture were extracted as
mentioned above but without quenching.
Elemental analysis calc. (%) for C17H26BrNO3 (372.29): C 54.84,
H 7.06, N 3.76. Found: C 54.33, H 6.75, N 4.18.
4-Benzyl-4-methylmorpholinium
(R)-(+)-2-bromo-3-
methylbutyrate, [BMmorf][7k] 1H NMR (DMSO-d6) ı ppm 1.02
(d, J = 1.10 Hz, 6H); 2.19 (m, 1H); 3.14 (s, 3H); 3.40 (t, J = 4.27 Hz,
2H); 3.60 (qw, J = 5.20 Hz, 2H); 4.00 (t, J = 1.71 Hz, 4H); 4.65 (d,
J = 1.83 Hz, 1H); 4.86 (s, 2H); 7.54 (m, 3H); 7.63 (m, 2H); 13C NMR
(DMSO-d6) ı ppm 19.2; 31.7; 44.8; 55.7; 58.4; 59.8; 67.2; 127.3;
128.8; 130.3; 133.2; 170.3.
Elemental analysis calc. (%) for C17H26BrNO3 (372.29): C 54.84,
H 7.04, N 3.76. Found: C 55.27, H 7.52, N 3.07.
Enantiomeric excess (e.e.) values were determined by GC-FID
(Hewlett Packard 8454A) with chiral ␤-cyclodextrine column.
4-Benzyl-4-methylmorpholinium
(R)-(+)-2-
(benzyloxy)propionate, [BMmorf][7l] 1H NMR (DMSO-d6)
ı
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59.9; 67.4; 69.8; 76.6; 126.8; 127.3; 127.4; 128.0; 128.8; 130.2;
133.3; 139.7; 174.8.
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