Palladium-catalyzed asymmetric Heck arylation of 2,3-dihydrofuran-effect of prolinate salts

Article abstract of DOI:10.1039/c2dt31672b

Chiral ionic liquids (CILs) containing l-prolinate and l-lactate anions and non-chiral quaternary ammonium cations were employed in the palladium catalyzed enantioselective Heck arylation of 2,3-dihydrofuran with aryl iodides (iodobenzene, 4-iodotoluene, 2-iodoanisole, 4-iodoanisole, 4-iodoacetophenone). In all the reactions 2-aryl-2,3-dihydrofuran (3) was obtained as the main product with the yield up to 52% at the total conversion reaching 83%. Product 3, 2-phenyl-2,3-dihydrofuran, was obtained with excellent enantioselectivity (>99% ee) in a 6 h reaction with tetrabutylammonium l-prolinate. In the proposed homogeneous reaction Pd(0) nanoparticles are considered as a resting state of the catalyst and a source of soluble palladium species catalyzing the Heck reaction. The yield and stereoselectivity of the Heck reaction are strongly influenced by the kind of non-chiral cations present in CILs. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2dt31672b

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Palladium-catalyzed asymmetric Heck arylation of
2,3-dihydrofuran – effect of prolinate salts†‡
Cite this: DOI: 10.1039/c2dt31672b
Adam Morel,a Ewelina Silarska,a Anna M. Trzeciak*a and Juliusz Pernakb
Chiral ionic liquids (CILs) containing L-prolinate and L-lactate anions and non-chiral quaternary
ammonium cations were employed in the palladium catalyzed enantioselective Heck arylation of
2,3-dihydrofuran with aryl iodides (iodobenzene, 4-iodotoluene, 2-iodoanisole, 4-iodoanisole, 4-iodoace-
tophenone). In all the reactions 2-aryl-2,3-dihydrofuran (3) was obtained as the main product with the
yield up to 52% at the total conversion reaching 83%. Product 3, 2-phenyl-2,3-dihydrofuran, was
obtained with excellent enantioselectivity (>99% ee) in a 6 h reaction with tetrabutylammonium L-proli-
nate. In the proposed homogeneous reaction Pd(0) nanoparticles are considered as a resting state of the
catalyst and a source of soluble palladium species catalyzing the Heck reaction. The yield and stereoselec-
tivity of the Heck reaction are strongly influenced by the kind of non-chiral cations present in CILs.
Received 25th July 2012,
Accepted 18th October 2012
DOI: 10.1039/c2dt31672b
www.rsc.org/dalton
such as monomolecular palladium complexes or palladium
nanoparticles.4 Spectacular examples of highly stereoselective
Introduction
The Heck cross-coupling is one of the most efficient new Heck coupling of cyclopentene were reported recently.5 Suc-
carbon–carbon bond formation methods leading to arylated cessful enantioselective arylation of unactivated cyclic olefins
olefins as the reaction products.1 The broad applicability of was also carried out with an arenediazonium salt using chiral
the Heck reaction is due to the fact that it can be used for the bisoxazolines as the ligands.6
arylation of olefins bearing electron-withdrawing substituents
(EWG, e.g. –COOR, –CN), as well as for electron-rich olefins, triflates is probably the most explored Heck reaction of a cyclic
such as olefins with the –OR group.1 olefin containing a heteroatom.4a,7 Arylation of DHF takes
The coupling reaction of 2,3-dihydrofuran (DHF) with aryl
In particular, the Heck coupling involving an olefin bearing place exclusively at the C2 position (Fig. 1); however, due to
a hydrogen atom in the allylic position may be accompanied carbon–carbon double bond migration, three products,
by migration of the carbon–carbon double bond resulting in namely 2, 3 and 4, might be formed. According to the mechan-
the formation of a product with a chiral carbon atom. Reac- istic considerations 2-aryl-2,5-dihydrofuran (2) is the kinetic
tions of this type can be distinguished in respect to the kind of product whereas 2-aryl-2,3-dihydrofuran (3) is the thermodyn-
the substrate used: (i) arylation of allylic alcohols where amic one.4a,7b,8 Consequently, it should be expected that 3 will
double bond migration results in the formation of carbonyl be formed as the major product and such selectivity was in
products;2 (ii) arylation of linear olefins;3 (iii) arylation of fact observed when palladium complexes with bidentate P–P
cyclic olefins including ring-closing Heck coupling;4 (iv) aryla- ligands9 were used. However, in reactions with monodentate
tion of heterocyclic olefins.5
phosphorous ligands10 and bidentate ligands with P–N,5
Arylation of cyclic olefins (e.g. cyclohexene, cyclopentene) P–As,11 P–S,12 or P–O13 donor atoms predominantly product 2
was studied mainly in the presence of non-chiral catalysts, was formed. Several phosphane-free catalysts for the Heck
reaction of DHF have also been reported.14
The stereoselective version of the Heck arylation of DHF
aFaculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie St., 50-383 Wrocław,
was intensively tested with various phosphorous ligands.4a,7,15
Poland. E-mail: anna.trzeciak@chem.uni.wroc.pl; Fax: +48 71 3282348;
The high ee value for product 3 was achieved with BINAP phos-
Tel: +48 71 3757253
phine as the chiral ligand added in situ to the palladium
bDepartment of Chemical Technology, Poznań University of Technology,
pl. M. Skłodowskiej-Curie 2, 60-965 Poznań, Poland.
E-mail: juliusz.pernak@put.poznan.pl; Fax: +48 61 6653649; Tel: +48 61 6653682
†This paper is dedicated to Professor David Cole-Hamilton, good friend and
great chemist, on the occasion of his retirement and for his outstanding contri-
bution to transition metal catalysis.
‡Electronic supplementary information (ESI)
available. See DOI:
10.1039/c2dt31672b
Fig. 1 Heck cross-coupling of 2,3-dihydrofuran with iodobenzene.
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precursor.4a The excellent asymmetric induction was also
noted when the isolated palladium complex with coordinated
BINAP was applied. Comparably good results were reported for
the systems containing [Pd2(dba)3]·dba (dba = dibenzylidene-
acetone) and different biaryl phosphite-oxazoline ligands.
Interestingly, when BINAP(O) (monooxidized BINAP) was
applied, product 2 was formed as the major one with a high
yield and ee up to 94%.16 The phosphine-free system based on
chiral oxazoline derivatives used in the Heck arylation of DHF
enabled us to obtain up to 29% ee for compound 2 and up to
60% for product 3.17
Alternatively to the chiral ligands, ionic liquids containing
stereogenic centers (CILs) can be used to introduce asym-
metric induction in the Heck reaction.18 There are many
reports about successful application of CILs in metal-catalyzed
organic reactions, however only a few of them refer to the Heck
coupling. Gayet et al.19 reported the arylation of DHF with
iodobenzene catalyzed by a pyridinium CIL with [PdCl4]2− as a
counterion in [BMIM]PF6 medium, nevertheless no enantio-
meric excess was observed. Two other examples of the Heck-
type arylation involving CILs are the oxyarylation of 7-benzyl-
oxy-2H-chromene,20 which gave a desirable product with 5%
ee, and the arylation of aza-endocyclic acrylate21 in various
imidazolium and pyridinium CILs, which produced only
racemic mixtures.
Recently, we have shown that CILs with stereogenic centers
present in the anions are promising co-catalysts in the Heck
coupling of DHF with iodobenzene.22 The best result, yet still
rather moderate (ee up to 9.9%), has been obtained for
quinate and mandelate morpholinium salts. It should be also
pointed out that 2 was formed with ee 17.3% in the presence
of dimethyldidodecylammonium (S)-lactate.
Considering the promoting effect of the chiral anion
present in CIL on the asymmetric induction, one may expect
that the ammonium salts bearing chiral anions can also be
attractive co-catalysts in the asymmetric Heck reaction leading
to the stereoselective formation of the coupling products. The
expected advantage of that type of CIL is a reasonable cost and
simplicity of the synthesis, especially when salts of amino
acids are used as the substrates. The amino acids represent a
very attractive class of the substrates for preparation of CILs,
being often available in both enantiomeric forms which might
be used as the chiral anions. CILs of that type have been
recently employed in the asymmetric Michael addition.23
Moreover, their anti-bacterial and anti-fungal activities were
confirmed.23 Herein, we present studies of the model asym-
metric Heck reaction with a palladium catalyst modified by
four CILs containing (L)-lactate and (L)-prolinate anions and
non-chiral quaternary cations.
Fig. 2 Effect of the [CIL] : [Pd] ratio on conversion in Heck cross-coupling of
DHF with PhI.
conditions optimized in our previous studies (temperature
70 °C, K2CO3 as a base).22 Four CILs were applied as the co-
catalysts in the catalytic systems: [DDA][L-PRO] (DDA = didodecyl-
dimethylammonium cation, L-PRO = (L)-prolinate), [BA][L-PRO]
(BA = cation with C12H25 and C14H29 alkyl groups in pro-
portions equal to 60 and 40%, respectively), [Bu4N][L-PRO] and
[Bu4N][L-Lact] (L-Lact = L-lactate anion).
Fig. 2 presents the dependence of iodobenzene conversions
on the amount of CILs at their different concentrations versus
palladium. In all the cases 3 was the major reaction product,
whereas the amounts of 2 and 4 were at the level of 15 and
8%, respectively. The most productive was [Bu4N][L-lact], which
provided up to 50% of the arylated products after 2 h. Interest-
ingly, only in the reactions with this co-catalyst the conversion
increased with the increase of the CIL amount. For the remain-
ing three CILs the best results were obtained at [CIL] : [Pd]
ratios equal to 4 or 6, whereas the application of higher
amounts of CILs resulted in a decrease of conversion. A poss-
ible reason for the observed trend might be a stronger
bonding to palladium of the prolinate ligand than the lactate
ligand. Consequently, these ligands efficiently compete with
substrates for the place in the coordination sphere of the
catalyst.
The analysis of the reaction enantioselectivity enabled us to
discover a remarkable effect of the non-chiral cations on the
products composition (Table 1). Apparently, while the ee
values for the main product 3 were equal to 4–12 in the reac-
tion with [DDA][L-PRO], they increased to 15–23 when [BA]-
[L-PRO] was applied as the CIL. In both systems lower enantio-
selectivity was noted for product 2, namely 1–5 in the presence
of [DDA][L-PRO] and 4–10 in the reactions with [BA][L-PRO]. In
the case of [Bu4N][L-PRO] the ee values ranged from 1.5 to
7.5%.
It should be pointed out that the stereoselectivity obtained
with L-prolinate salts, especially with [BA][L-PRO], is remark-
ably higher than that observed by us earlier in the presence of
morpholinium salts bearing different chiral anions.22 These
promising results encouraged us to continue the investigations
and to check whether the studied systems are stable enough to
Results and discussion
Arylation of DHF
The arylation of DHF with iodobenzene was carried out using
Pd(OAc)2 as the catalyst precursor in DMF, under the
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Table 1 Heck arylation of DHF with Pd(OAc)2 and various CILs
[NBu4][L-LACT]
[BA][L-PRO]
[DDA][L-PRO]
[NBu4][L-PRO]
Yield of 2 and 3a (%) [ee%]
[CIL]/[Pd]
3
2
3
2
3
2
3
2
2
11.1
1.7 [2.1]
3.8 [3.3]
4.9 [3.6]
6.3 [4.0]
7.5 [4.0]
11.4 [15.3]
15 [22.6]
12.6 [20]
8.5 [21.3]
3.4 [18.8]
5.8 [4.0]
9.7 [6.5]
10.5 [4.8]
6.5 [8.3]
3.9 [10.0]
8.6 [4.5]
12.7 [6.5]
19.2 [7.2]
3.3 [12.0]
1.8 [11.5]
4.4 [0.7]
6.3 [2.1]
9.4 [2.6]
2.9 [3.4]
1.5 [4.7]
9.4 [7.5]
7.4 [3.0]
4.8 [4.7]
5.6 [7.1]
1.3 [1.5]
5 [5.7]
4
19.5 [0.4]
24.5 [0.4]
28.3 [0.4]
35.4 [0.4]
10.5 [3.2]
3.5 [7.2]
3 [6.7]
0.5
6
8
10
a Conditions: Pd(OAc)2 (1 mol%), 70 °C, 2 h, solvent DMF (6 mL), DHF (8.6 mmol), PhI (3.57 mmol), K2CO3 (4.34 mmol).
Pd–Ar bond (Fig. 5). This step, followed by β-hydrogen elimi-
nation, may be decisive in the formation of the particular
enantiomer. However, as it was already mentioned, the effect
of the non-chiral cation on the reaction course should be also
considered. For better confirmation of that statement, the
Heck reactions were performed under the same conditions
using L-proline and D-proline instead of quaternary
ammonium L-prolinates. From the data collected in Table 4 it
can be concluded that prolines have practically no influence
on the conversion of iodobenzene which is close to 20%, simi-
larly as for Pd(OAc)2 used without any additives. Also the ee
values were very low. Thus, the effect of the cations on the reac-
tion course was confirmed.
In a further step of our studies the catalytic system with
[Bu4N][L-PRO] salt was tested in the Heck cross-coupling of
DHF with differently substituted iodobenzenes (Table 5). In all
the cases good conversions (49–87%) were obtained after 6 h.
With an increase of the reaction time the ee values also
increased and the best result, 19.9%, was obtained for 4-iodo-
toluene. Merely slightly worse enantioselectivity was noted for
other substrates with the only exception of 4-iodoacetophe-
none, which formed the respective 2,3-dihydrofuran derivative
with the ee as low as 3.1%. Interestingly, the attempts to use
phenyl triflate as the substrate instead of iodobenzene failed
and only 3% of the coupling product was formed in the best
case. However, since iodobenzene is a cheaper and more com-
monly available substrate than aryl triflate, the applicability of
our catalytic system to aryl iodides shows its big advantage.
Fig. 3 Conversion in the Heck reaction over time at different [CIL] : [Pd] ratios
(CIL = [BA][L-PRO]).
be used for a longer time. In fact, the experiments performed
with [BA][L-PRO] gave very positive results illustrated by the
increase of the iodobenzene conversion up to 60% after 6 h in
the reactions carried out at the [CIL] : [Pd] ratio 4–10 (Fig. 3).
Slightly lower conversion, ca. 50%, was noted when a
higher excess of [BA][L-PROL] was used. Clearly, for gaining of
the optimal conversions the amount of CIL should not exceed
8, whereas the optimal reaction time is ca. 6 h. As far as the
enantioselectivity is concerned a decrease of the ee values in
time should be mentioned (Table 2). In almost all the reac-
tions carried out at the [CIL] : [Pd] in the range of 4–10, the ee
values of ca. 22% were obtained after 3–4 h for the product 3,
while after a longer time (6 h) they decreased to 12–17%. For
the minor product 2 up to 10% ee were obtained.
Similar experiments carried out for prolonged time with
[Bu4N][L-PRO] as the CIL enabled us to find the best result for
the studied Heck reaction, namely an excellent enantioselectiv-
ity at the reasonable yield of 3. At the 2-fold excess of [Bu4N]-
[L-PRO] to Pd(OAc)2 after 3 and 6 h reactions practically one
pure enantiomer of 3 was obtained with the yields of 57 and
83%, respectively. The application of higher amounts of [Bu4N]-
[L-PRO] resulted in the remarkable decrease of both, the yield
and enantioselectivity (Table 3).
Mechanistic considerations
According to the literature, the nature of the catalyst active in
the phosphane-free palladium system might be in part homo-
geneous and in part heterogeneous at the same time.24 In par-
ticular, the formation of Pd(0) nanoparticles under the
reaction conditions is very plausible.24,25 Such nanoparticles
can act as heterogeneous catalysts, however recently, it has
often been postulated in the literature that these nanoparticles
do not directly participate in the catalytic process but rather
act as a reservoir of soluble catalytically-active palladium
species. In such a case, a reaction mechanism might be based
on the homogeneous catalysis pathway.
The excellent enantioselectivity found in the studied reac-
tions might be explained by the influence of the chiral L-proli-
nate anion on the step involving insertion of alkene into the
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Fig. 4 TEM pictures and Pd(0) nanoparticles size distribution of : (A) Pd(OAc)2
+ K2CO3; (B) Pd(OAc)2 + K2CO3 + 2 [Bu4N][L-PRO]; (C) Pd(OAc)2 + K2CO3 + 2
[Bu4N][L-PRO] + PhI; (D) Pd(OAc)2 + K2CO3 + 2 [Bu4N][L-PRO] + PhI + DHF heated
30 min at 70 °C in DMF.
To get a closer insight into the mechanism of the catalytic
process in our system the Hg(0) test was performed using
500-fold excess of mercury to palladium. An analogous reaction
without Hg(0) addition was also performed for comparison. The
analysis of the products composition after 2 h showed a
decrease of the conversion from 62.3 in the reaction without
Hg(0) to 26.8% in the presence of Hg(0). Thus, the inhibiting
effect was clearly seen, however the reaction did not stop com-
pletely, as it would be expected for the heterogeneous process
catalyzed by Pd(0) nanoparticles or ligand-free Pd(0) species.
To verify the formation of palladium nanoparticles in the
system under studies four TEM analyses have been done for
the samples containing components of the Heck reaction in
DMF:
A. Pd(OAc)2 + K2CO3
B. Pd(OAc)2 + K2CO3 + 2 [Bu4N][L-PRO]
C. Pd(OAc)2 + K2CO3 + 2 [Bu4N][L-PRO] + PhI
D. Pd(OAc)2 + K2CO3 + 2 [Bu4N][L-PRO] + PhI + DHF
Sample A, used as the reference, contained palladium nano-
particles of the average diameter 4 nm. In the presence of
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Table 3 Heck arylation of DHF with Pd(OAc)2 and different amounts of [Bu4N][L-PRO] (CIL)a
[CIL] : [Pd] = 2
[CIL] : [Pd] = 4
Time (h)
Conversionb (%)
3 yield (%) [ee%]
2 yield (%) [ee%]
Conversionb (%)
3 yield (%) [ee%]
2 yield (%) [ee%]
2
3
6
14.4
56.7
83.0
9.4 [7.5]
39.5 [>99]
51.8 [>99]
5.0 [5.7]
12.7 [1.4]
26.4 [2.2]
17.9
39.3
41.0
7.4 [3.0]
27.7 [13.3]
25.6 [9.3]
10.5 [3.2]
11.2 [2.6]
14.9 [1.1]
a Conditions: Pd(OAc)2 (1 mol%), 70 °C, solvent DMF (6 mL), DHF (8.6 mmol), PhI (3.57 mmol), K2CO3 (4.34 mmol). b Conversion of
PhI.
Fig. 5 The proposed mechanism of the Heck reaction in the presence of [Bu4N][L-PRO].
[Bu4N][L-PRO] slightly smaller nanoparticles were formed with probably stabilized by CILs, represent the resting state of the
a narrower size distribution. In the presence of iodobenzene catalyst.
these nanoparticles undergo dissolution forming the soluble
The proposed mechanism of the Heck reaction, based on
active palladium catalyst. A partial dissolution of nanoparticles the literature26 and our observations, is presented in Fig. 5.
is in agreement with their redispersion observed in TEM. As is Similarly as previously observed,22 we propose the homo-
seen in Fig. 4 the nanoparticles are uniformly distributed, geneous halide-free pathway as a dominant one in the selective
whereas in sample D, containing all components of the Heck- formation of arylated dihydrofurans. The chiral carboxylate
reaction, some agglomeration was observed and the average anion, e.g. L-prolinate, competes efficiently with I− for a place
diameter of the nanoparticles increased to ca. 6 nm. It can be in the coordination sphere of palladium and is responsible for
concluded from these results that Pd(0) nanoparticles, most the asymmetric induction. The key palladium intermediate,
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Table 4 Heck arylation of DHF with Pd(OAc)2 and prolinesa
From mechanistic point of view, the remarkable influence
of the chiral anions on the enantioselectivity should be
pointed out. An important effect of the anions on the reactivity
of anionic Pd(0) complexes in the Heck reaction was also
demonstrated by Amatore and Jutand.25b,c According to our
results, L-prolinate is an anion of choice in the Heck arylation
of DHF with aryl iodides. However, the proper selection of the
non-chiral cation turned out to be also very important and par-
ticularly higher ee values were achieved with more bulky BA
than with smaller DDA L-prolinate. The effect of the cation
may be discussed in terms of its electrostatic interactions with
the negatively charged surface of Pd(0) nanoparticles. The for-
mation of nanoparticles, unequivocally confirmed by TEM,
enables us to consider them as a reservoir of catalytically active
soluble palladium species.
Conversionb
(%)
3 yield (%)
[ee%]
2 yield (%)
[ee%]
[L-Proline] : [Pd]
2
23.2
18.6
17.6 [1.3]
13.0 [1.8]
5.6 [2.2]
5.6 [4.9]
4
[D-Proline] : [Pd]
2
4
19.4
21.8
22.1
14.9 [4.2]
16.3 [6.5]
18.3
4.5 [1.2]
5.5 [1.3]
3.0
Pd(OAc)2
a Conditions: 1 mol% Pd(OAc)2, 70 °C, 2 h, solvent DMF (6 mL),
DHF (8.6 mmol), PhI (3.57 mmol), K2CO3 (4.34 mmol).
b Conversion of PhI.
containing aryl and carboxylate ligands ArPdL2(RCO2) (L =
solvent), is analogous to that identified by Amatore and Jutand
in the phosphane containing system for the Heck coupling.26c
The proposed reaction mechanism also illustrates the role
of Pd(0) nanoparticles as the catalyst resting state and the
source of soluble palladium species. The nanoparticles are
stabilized by CILs due to electrostatic interactions.27 Such
interactions may also influence the rate of nanoparticles dis-
solution and at the same time the rate of the catalytic process.
Consequently, the cationic part of the CIL plays an important
role in the process and a strong influence of the cation on the
reaction course was observed. In addition, the cations present
in CILs may also determine a nucleophilicity of the chiral
anion and its reactivity in the catalytic process. For example,
Hayashi stated that a stronger base may generate the highly
reactive acetate anion that induces the dissociation of (S)-2
from the palladium center and formation of almost enantio-
merically pure (R)-3.4a
Experimental
Materials
DHF, PhI, L-proline, D-proline, [Bu4N]OH and [Bu4N][L-LACT]
were obtained from Aldrich and used without further purifi-
cation. [BA][L-PRO] and [DDA][L-PRO] were obtained according
to the literature.23 [Bu4N][L-PRO] was obtained in the reaction
of [Bu4N]OH with L-proline.
Heck reaction
The Heck arylation of DHF with PhI was carried out under a
N2 atmosphere using a standard Schlenk technique. The
reagents were introduced into the Schlenk tube in the follow-
ing order: Pd(OAc)2 (8.0 mg, 0.0356 mmol, 1 mol%), solvent
DMF (6 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). The reac-
tion was carried out at 70 °C for 2 h. Afterwards, the reaction
mixture was quenched with H2O (3 mL) and the organic pro-
Conclusions
We have shown that the asymmetric Heck coupling of DHF ducts were separated by extraction with diethyl ether (3 times:
with aryl iodides can be efficiently performed under phos- 10 mL, 7 mL and 7 mL). The products were analyzed by
phane-free conditions using Pd(OAc)2 as a palladium source GC-FID (Hewlett Packard 8454A). Products 2, 3, 4 were ident-
and CILs as chiral agents. The excellent enantioselectivity ified by comparison of the MS spectra and the retention times
(>99% ee) obtained with the application of [Bu4N][L-PRO] with the literature data.
should be highlighted. The CILs tested in this work are easily
available, inexpensive and enable us to obtain very good GC-FID (Hewlett Packard 8454A) with a chiral β-cyclodextrin
results when used in small amounts. column.
The enantiomeric excess (ee) values were determined by
Table 5 Heck arylation of DHF with different iodobenzenes, using Pd(OAc)2 and [Bu4N][L-PRO]a
4-Iodotoluene
2-Iodoanisole
4-Iodoanisole
4-Iodoacetophenone
Conversionb (%) [ee%]
Time (h)
31
21
32
22
33
23
34
24
3
6
46 [18.3]
52 [19.9]
15 [7.5]
24 [11.0]
3.2 [1.5]
30 [18.0]
2.4 [7.2]
17[10.0]
27 [17.0]
46 [18.1]
13 [10.0]
21 [10.6]
22 [0.1]
38 [3.1]
8.6 [0.3]
14.5 [5.9]
a Conditions: Pd(OAc)2 (1 mol%), [CIL]:[Pd] = 2, 70 °C, solvent DMF (6 mL), DHF (8.6 mmol), ArX (3.57 mmol), K2CO3 (4.34 mmol).
b Conversion of ArI 31, 32, 33, 34 = 2-aryl-2,3-dihydrofuran 21, 22, 23, 24 = 2-aryl-2,5-dihydrofuran.
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TEM measurements
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Samples for TEM analyses were prepared at 70 °C using the
same amounts of reactants as in catalytic reactions. After
30 min, a droplet of the reaction mixture was placed on a
carbon coated microscope grid and dried for 40 min. TEM
measurements were carried out using an FEI Tecnai G2
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Acknowledgements
The authors are grateful to Mr Marek Hojniak (Faculty of
Chemistry, University of Wrocław) for GC and GC-MS analyses,
to Dr Wojciech Gil (Faculty of Chemistry, University of
Wrocław) for performing TEM measurements and to Dr
Andrzej Gniewek (Faculty of Chemistry, University of Wrocław)
for technical assistance.
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