Oxidation of enol acetate of flavanone with thallium(III) nitrate or phenyliodonium diacetate: A convenient new route to isoflavone and flavone

Article abstract of DOI:10.1007/s00706-014-1157-1

A new high-yield method has been described for the preparation of isoflavone by oxidation of enol acetate of flavanone with thallium(III) nitrate or phenyliodonium diacetate in trimethyl orthoformate in the presence of 70 % perchloric acid at room temperature. Flavone could also be prepared in high yield from the enol acetate by oxidation with the same reagents in glacial acetic acid at room temperature. Some key intermediates of these oxidations were investigated with quantum chemical (HF and DFT) methods. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s00706-014-1157-1

Monatsh Chem
DOI 10.1007/s00706-014-1157-1
ORIGINAL PAPER
Oxidation of enol acetate of flavanone with thallium(III) nitrate
or phenyliodonium diacetate: a convenient new route to isoflavone
and flavone
Istv a´ n N e´ meth
•
Attila Kiss-Szikszai
•
•
Katalin Gul a´ csi
•
Attila M a´ ndi
•
Istv a´ n Kom a´ romi
Tibor Kurt a´ n S a´ ndor Antus
•
Received: 16 October 2013 / Accepted: 10 January 2014
Ó Springer-Verlag Wien 2014
Abstract A new high-yield method has been described
for the preparation of isoflavone by oxidation of enol
acetate of flavanone with thallium(III) nitrate or phenyli-
odonium diacetate in trimethyl orthoformate in the
presence of 70 % perchloric acid at room temperature.
Flavone could also be prepared in high yield from the enol
acetate by oxidation with the same reagents in glacial
acetic acid at room temperature. Some key intermediates of
these oxidations were investigated with quantum chemical
via enol ether (± )-2 and thallium(III) intermediate (± )-3a
in good yield (75 %), but the relative configuration of the
2,3-dihidrobenzo[b]furan ring was not determined. It is
also noteworthy that flavone (5) could also be isolated in
low yield (15 %) as a by-product (Scheme 1). One year
later, Khanne [2] observed that the oxidation of (± )-1 by
lead tetraacetate (LTA) under the conditions used by
Kapoor et al. afforded (± )-4b via (± )-2 and (± )-3b in
85 % yield. In 1995, Prakash and Tanwar [3] reported
that (± )-4a or (± )-4b accompanied by cis-3-methoxyf-
lavanone [(± )-6] and flavone (5) could also be obtained
from (± )-1 by phenyliodonium diacetate (PIDA) in
TMOF in the presence of a catalytic amount of sulfuric
acid at room temperature in 50, 35, and 15 % yields,
respectively.
(
HF and DFT) methods.
Keywords Phenyl migration Á Dehydrogenation Á
Neighboring group participation Á Solvent effect
Introduction
In 2002, on the basis of NMR, ECD, and chiral HPLC
studies, Juh a´ sz et al. proved that the ring contraction of
(-)-(2S)-flavanone [(-)-1] under the conditions used by
Prakash and Tanwar [3] took place via 3c intermediate
stereoselectively, controlled by the configuration of the C-2
phenyl group to result in (?)-(2S,3S)-4a in moderate
(25 %) yield [4]. Finally, most recently, we have shown by
In 1993, Kapoor et al. [1] published that the oxidative rear-
rangement of racemic flavanone [(± )-1] by thallium(III)
nitrate (TTNÁ3H O) in trimethyl orthoformate (TMOF) in the
2
presence of a catalytic amount of 70 % perchloric acid at
room temperature resulted in racemic methyl 2,3-dihydro-2-
phenylbenzo[b]benzofuran-3-carboxylate [(± )-4a or (± )-4b]
HPLC monitoring of TTNÁ3H O-, PIDA-, and LTA-med-
2
iated ring contraction of (± )-1 in TMOF in the presence of
a catalytic amount of 70 % perchloric acid or concentrated
sulfuric acid at room temperature that only (± )-4a was
stereoselectively formed in good and moderate yields
I. N e´ meth Á A. Kiss-Szikszai Á K. Gul a´ csi Á A. M a´ ndi (&) Á
T. Kurt a´ n Á S. Antus (&)
Department of Organic Chemistry, University of Debrecen,
4032 Debrecen, Hungary
e-mail: mandi.attila@science.unideb.hu
(74–36 %), and by-products 5–9 also gave rise in low
yields or in traces (28–1 %) depending on the applied
conditions. Moreover, we supposed that the dimethyl ketal
function of intermediates 3a–3c might have an influence by
its neighboring group effect on the routes of this transfor-
mation [5].
S. Antus
e-mail: antus.sandor@science.unideb.hu
I. Kom a´ romi
Vascular Biology, Thrombosis and Hemostasis Research Group,
Hungarian Academy of Sciences, University of Debrecen,
H-4032 Debrecen, Hungary
In order to gain more information about the role of this
effect, we set our sights on the study of the oxidation of
123

I. N e´ meth et al.
enol acetate of (± )-1 [(± )-10] by TTNÁ3H O and PIDA
(5, 18 %) could be identified unequivocally by comparison
with their authentic samples. Surprisingly, rac-methyl 2,3-
dihydro-2-phenylbenzo[b]furan-3-carboxylate [(± )-4a or
2
using HPLC monitoring.
(
± )-4b], cis-3-methoxyflavanone (6), and phen-
ylbenzo[b]furan derivatives 8 and 9 did not form at all
entry 1 in Table 1). A very similar product profile could
be observed when PIDA was used as an oxidizing agent
entry 2). These results have very clearly indicated that the
Results and discussion
(
The HPLC monitoring of the transformation of enol acetate
of racemic flavanone (1) [(± )-10] prepared according to
the literature from (± )-1 [6] with 1.1 mol equivalent
(
substitution of the methyl group of (± )-2 with the acetyl
one resulting in (± )-10 prevented its ring contraction to
TTNÁ3H O in TMOF in the presence of a catalytic amount
2
of 70 % perchloric acid at room temperature clearly indi-
(
± )-4a or (± )-4b, and the formation of isoflavone (7)
cated that the conversion of (± )-10 reached 99 % in
3
became strongly preferred. This observation could be
explained by the neighboring group participation of acet-
oxy and methoxy groups at C-4 of (± )-11a, 11b formed
from (± )-10 as depicted in Scheme 2.
0 min to result in the mixture of products shown in Fig. 1.
All components of this mixture could be nearly base-line
separated, and besides enol acetate (± )-10 (\1 %), rac-
flavanone [(± )-1 (1 %)], isoflavone (7, 79 %), and flavone
Thus, in the first step, electrophilic addition of TTN or
PIDA on the enol acetate (± )-10 takes place at C-3 with the
thermodynamically preferred trans relative configuration.
This is followed by the addition of a methoxide anion
Scheme 1
formed from TMOF by TTNÁ3H O or PIDA in situ [7] at
2
C-4 resulting in the intermediates 11a and 11b, respec-
tively. Since the ring contraction of 11a or 11b yielding
(
± )-4a and/or (± )-4b did not take place at all, as men-
tioned above, the formation of the carbonium ion 12 by the
cleavage of the polarized C-3 and Tl(III) or I(III) bond of
intermediates 11a and 11b could be ruled out. Instead of
this carbonium ion 12, the O-methylepoxenium ion 13
formed by a neighboring group participation of the C-4
methoxy group of 11a or 11b. Subsequently, it would
rearrange smoothly to the thermodynamically more stable
1,3-dioxolanium ion 14. Since the stereochemical prereq-
uisites of a 2 ? 3 phenyl migration in 14 are given, its
phenyl group could shift to C-3 to yield the oxenium ion 15
whose transformation by a simple deprotonation would
lead to 4-acetoxy-3-phenyl-4-methyl-4H-chromene (16).
Finally, its reaction with water in the course of the workup
would give isoflavone (7) in 79 % yield as the main
product. An alternative transformation of the O-methyl-
epoxenium ion 13 involving its deprotonation at C-2,
Fig. 1 HPLC chromatogram of the crude product of the transformation of (± )-10 with TTN at room temperature
1
23

Oxidation of enol acetate of flavanone with thallium(III) nitrate or phenyliodonium diacetate
Table 1 Yields and retention times of the compounds detected in the transformation of (± )-10 by TTN and PIDA
Entry
Reag.
Time/h
Solv.
Yields/% of compounds (t
R
/min)
1
(7.26) 4a/4b (7.90/7.20)
5 (6.30)
6 (7.03)
7 (6.70)
8 (8.95)
9 (8.14)
10 (9.19)
1
2
3
4
TTN
PIDA
TTN
PIDA
2
A
A
B
B
1
–/–
–/–
–/–
–/–
18 [12]
13
–
–
–
–
79 [68]
81
–
–
–
–
–
–
–
–
\1
–
12
3.5
12
4
10
3
82
\1
2 [1]
\1
3
79 [65]
?
A, TMOF ? H ; B, glacial acetic acid; [%], isolated yields
˚
14b), possessing a value of more than 2.5 A for the longer
(
Scheme 2
C–O bond (for comparison, see Table 2). In both cases, the
C3–O bond is stronger than the C4–O.
It is important to note that solvents can stabilize the
slightly asymmetric form. The larger the e value of a sol-
vent is, the more it seems to make the slightly asymmetric
form more symmetrical, but it does not have the same
effect on the strongly asymmetric form (for comparison,
see Table 3). The energy difference of the two forms is not
too high, ca. 10–15 kJ/mol (14b being more stable). The
other aspect of the stabilization effect of the solvent on 14a
is that the energy difference is reciprocally proportional to
the e value of the solvent (see Table 3).
While 14 could be readily optimized, optimization of 13
surprisingly always resulted in a strongly asymmetric form
(
13b), where the OMe was strongly shifted from C4 toward
C3. In order to obtain the symmetrical form of 13, first a
model compound (18) was optimized at the B3LYP/
6–31 ? G(d) level of theory in Et O. Then the bond length
2
values from 18 were adopted for 13a; however, relaxed
optimizations resulted again in 13b. By fixing the bond
length values of C4–O(Me), C3–O(Me), and C3–C4 at the
values obtained in the model compound calculation and
optimizing the rest of the molecule, the relative energy of
13a could be estimated, being ca. 125–145 kJ/mol higher
followed by opening of the O-methylepoxxenium ion,
afforded the 4H-chromene derivative 17 as depicted in
Scheme 2. Its hydrolysis during the workup resulted in
flavone (5) as a by-product in 18 and 13 % yields
depending on the applied oxidizing agent TTN or
PIDA (Fig. 3), respectively (see entries 1 and 2 in Table 1).
In order to support the above-featured mechanism, we
performed quantum chemical calculations on its most
critical intermediates 13 and 14 to check their stability.
Both cations were optimized at various levels of theory.
Some of the calculations were performed not only in vacuo
but also by applying a PCM solvent model for a set of
solvents (in the range of e = 1.7–35.7) to gain information
on the impact of the solvent. Optimizations of 14 showed
that it can exist in two forms (Fig. 2), one of which is
slightly asymmetric (14a) where the longer C–O bond is
than that of 14b. The fully optimized 13b also has a much
higher relative energy (ca. 30–45 kJ/mol) than 14b (see
Table 3). Thus, the equilibrium is strongly shifted from 13
to 14, since 14 is strongly stabilized by the forming
dioxolane-type ring having time for the aryl shift to afford
isoflavone (7), while simple deprotonation of 13 can result
in 17 in low yield.
The HPLC monitoring of the transformation of (± )-10
by TTNÁ3H O in glacial acetic acid at room temperature
2
also gives some interesting information about the feature of
this reaction. The conversion of (± )-10 reached 90 % in
0.5 h, and flavone (5) was obtained as a main product in
82 % yield in addition to traces of isoflavone (7) (\1 %),
as shown in Fig. 4.
Since in this case methanol was not at all present in the
reaction mixture, in the course of the stereo-controlled
addition of the electrophilic reagent (TTN) at (± )-10 the
˚
ca. 1.5–1.7 A, while the other is strongly asymmetric
123

I. N e´ meth et al.
Fig. 2 Slightly (a) and strongly
b) asymmetrical forms of 14
obtained at the B3LYP/
–31 ? G(d) level of theory in
(
6
vacuo
Table 2 Selected bond lengths/
˚
A of 14a and 14b (where a is
Level
14a
14b
slightly and b strongly
asymmetric) calculated at
various levels of theory
rC3–O(Ac)
rC4–O(Ac)
rC3–O(Ac)
rC4–O(Ac)
HF/6–31G(d) in vacuo
1.452
1.519
1.509
1.498
1.493
1.490
1.396
1.396
1.397
1.398
1.399
1.428
1.427
1.427
1.428
1.428
1.427
1.429
1.429
1.429
1.430
1.430
1.428
2.656
2.705
2.730
2.737
2.739
2.496
2.532
2.577
2.594
2.603
2.500
2.539
2.574
2.619
2.638
2.652
2.546
HF/6–31G(d) PCM, e = 1.7
1.451
HF/6–31G(d) PCM, e = 4.2
1.448
HF/6–31G(d) PCM, e = 8.9
1.447
HF/6–31G(d) PCM, e = 35.7
B3LYP/6–31G(d) in vacuo
1.446
Does not exist
1.465
B3LYP/6–31G(d) PCM, e = 1.7
B3LYP/6–31G(d) PCM, e = 4.2
B3LYP/6–31G(d) PCM, e = 8.9
B3LYP/6–31G(d) PCM, e = 35.7
B3LYP/6–31G(d,p) in vacuo
B3LYP/6–31 ? G(d) in vacuo
B3LYP/6–31 ? G(d) PCM, e = 1.7
B3LYP/6–31 ? G(d) PCM, e = 4.2
B3LYP/6–31 ? G(d) PCM, e = 8.9
B3LYP/6–31 ? G(d) PCM, e = 35.7
B3LYP/6–31 ? G(d,p) in vacuo
1.674
1.622
1.608
1.595
1.467
1.467
1.467
Does not exist
1.466
1.702
1.646
1.609
1.595
1.584
1.691
1.468
1.468
1.468
1.468
1.467
Based on our quantum chemical calculations (Fig. 5),
20a has ca. 15–20 kJ/mol lower energy than 20b (see
Table 4). That is, 1,3-dioxolanium ion 20a due to a strong
charge transfer-type interaction between its C-2 phenyl of a
configuration and the 1,3-dioxolanium group attached at
C-3 and C-4 also with the a configuration is more stable
than 20b. Therefore, the equilibrium between 20a and 20b
had to be shifted toward 20a in which the prerequisites of a
reductive elimination were given, and the 4,4-diacetoxy-
4
H-flavene (21) was formed. In the last step, the hydrolysis
Fig. 3 Optimized model compound (18) of the three-membered ring
obtained at the B3LYP/6–31 ? G(d) level in Et
of 21 during the workup of the reaction mixture afforded
flavone (5) as a main product (82 %).
In the reaction mixture, the less stable carbonium ion
2
O
thallium(III) intermediate 19 was formed, as depicted in
Scheme 3. In the next step, its transformation by a neigh-
boring participation of its C-4 acetoxy group of a and b
configuration led to the mixture of 1,3-dioxolanium ions
2
0b must also be present in low concentration, in which the
C-2 phenyl group is in the anti position with the polarized
C-4 and 3-O bond of its 1,3-dioxolanium ring system.
Thus, 4,4-diacetoxyisoflavon (22) also formed by a 2 ? 3
phenyl migration followed by a deprotonation and
2
0a and 20b, respectively.
1
23

Oxidation of enol acetate of flavanone with thallium(III) nitrate or phenyliodonium diacetate
hydrolysis during the workup of the reaction mixture,
resulting in traces of isoflavone (7) as observed by us
experimentally (entry 3 in Table 1). It is noteworthy that
the conversion of (± )-10 induced by PIDA in glacial acetic
acid reached 85 %, and flavone (5) was obtained in 65 %
yield besides traces of isoflavone (7).
resulted in flavone (5) in 70, 88, and 96 % yields, respec-
tively. If [hydroxy(tosyloxy)iodo]benzene in acetonitrile at
100 °C [10] or TTA in the presence of three equivalents of
p-toluenesulfonic acid in propionitrile at room temperature
[11] was used as the oxidizing agent, isoflavone (7) was
obtained in 75 and 94 % yields, respectively.
A few efficient syntheses of flavone (5) and isoflavone
7) from racemic flavanone [(± )-1] have already been
(
reported in the literature. The oxidation of (± )-1 by TTN in
methanol at room temperature [8] and by thallium(III)
acetate (TTA) in methanol or in acetic acid at reflux [9]
Conclusion
The oxidation of easily available enol acetate of racemic
flavanone (± )-1 [(± )-10] by TTN or PIDA in TMOF in the
presence of a catalytic amount of sulfuric acid at room
temperature provides a new, useful, and efficient route to
isoflavone (7, in 79 and 81 %, respectively). If the oxida-
tion of (± )-10 by TTN or PIDA was performed in glacial
acetic acid instead of the TMOF-H SO system, flavone (5)
-1
Table 3 Relative energies/kJ mol of 13a–14b (where a is slightly
and b strongly asymmetric) calculated at various levels of theory
Level
13a
fixed)
13b 14a
14b
(
2
4
was obtained in high yield (82 and 79 %, respectively) in
addition to traces of 7 (*1 %). The study of the scope and
limitations of these processes ia in progress.
HF/6–31G(d) in vacuo
144.62 37.75 13.03
143.12 40.72 12.68
140.59 42.90 11.73
139.17 43.71 11.02
137.90 43.76 10.31
0
0
0
0
0
HF/6–31G(d) PCM, e = 1.7
HF/6–31G(d) PCM, e = 4.2
HF/6–31G(d) PCM, e = 8.9
HF/6–31G(d) PCM, e = 35.7
B3LYP/6–31G(d) in vacuo
Experimental
133.86 32.78 Does not 0
exist
Analytical RP-HPLC was performed on a Jasco HPLC
System. Data acquisition was performed using Borvin
chromatography software; UV detector: UV-2075 Plus,
B3LYP/6–31G(d) PCM, e = 1.7
B3LYP/6–31G(d) PCM, e = 4.2
B3LYP/6–31G(d) PCM, e = 8.9
B3LYP/6–31G(d) PCM, e = 35.7
B3LYP/6–31G(d,p) in vacuo
131.73 34.64 13.35
128.74 35.79 13.11
127.23 36.31 12.79
125.97 36.72 12.48
0
0
0
0
3
Pump PU-2089 Plus. The flow rate was 1 cm /min. An
3
injection volume of 20 mm and column temperature of
133.05 32.42 Does not 0
exist
2
5 °C were used. A C18 5 lm, 150 9 4.6-mm Gemini
chromatographic column (Phenomenex) was used for these
B3LYP/6–31 ? G(d) in vacuo
134.17 33.67 13.93
0
0
0
0
0
0
studies. A solution of KI (in CH CN) was used for dead
3
B3LYP/6–31 ? G(d) PCM, e = 1.7 132.21 35.37 14.40
B3LYP/6–31 ? G(d) PCM, e = 4.2 129.29 36.79 14.58
B3LYP/6–31 ? G(d) PCM, e = 8.9 127.74 37.42 14.49
B3LYP/6–31 ? G(d) PCM, e = 35.7 126.37 37.88 14.31
time (t ) determination. Retention times (t ) were mea-
D
R
sured in minutes. A mixture of CH CN (HPLC grade) and
3
H O (HPLC grade) was used as a mobile phase. The gra-
2
dient program was linear from 50 % of A to 100 % in
B3LYP/6–31 ? G(d,p) in vacuo
133.28 33.39 13.80
8
min and then held for 4 min. A detection wavelength of
Fig. 4 HPLC chromatogram of the crude product of the transformation of (± )-10 with TTN in glacial acetic acid at room temperature
123

I. N e´ meth et al.
Scheme 3
Isoflavone (7)
Procedure A To a stirred solution of (± )-10 (1 mmol) in
3
3
2
cm TMOF and 0.2 cm 70 % HClO at room temper-
4
ature, Tl(NO ) Á3H O (1.1 mmol) was added, and the
3
3
2
reaction mixture was stirred at r.t. for 2 h. It was diluted
with CH Cl and Tl(I)-salt filtered off. The organic
2
2
solution was washed with a saturated solution of sodium
hydrogen carbonate, then with water until neutral and
dried. The evaporation of the solution gave the crude
product as an oil [7 (79 %) and 5 (18 %) detected by
HPLC] whose purification was performed by preparative
TLC using n-hexane:ethyl acetate (9:1) as eluent affording
-
1
Table 4 Relative energies/kJ mol of 20a and 20b calculated at
various levels of theory
7
(151 mg, 68 %), m.p.: 132 °C (133–134 °C [12]), and 5
1
(25 mg, 12 %), m.p.: 96 °C (93–96 °C [6]). H [13, 14]
Level
20a
20b
13
and C [15, 16] NMR data of 5 and 7 were found identical
with those reported in the literature.
HF/6–31G(d) in vacuo
0
0
0
0
0
0
0
0
0
0
0
0
22.35
21.02
18.79
17.47
16.46
19.41
17.68
16.91
15.46
14.53
13.76
17.81
HF/6–31G(d) PCM, e = 1.7
HF/6–31G(d) PCM, e = 4.2
HF/6–31G(d) PCM, e = 8.9
HF/6–31G(d) PCM, e = 35.7
B3LYP/6–31G(d) in vacuo
Procedure B To a stirred solution of (± )-10 (1 mmol) in
3
3
2 cm TMOF and 0.5 cm conc. H SO at room temper-
2
4
3
ature, a solution of PIDA (1.1 mmol) in 1 cm TMOF was
dropped in 20 min, and the reaction mixture was stirred at
r.t. for 12 h. The workup was performed as in procedure A,
resulting in 7 (81 %) and 5 (13 %), detected by HPLC.
B3LYP/6–31 ? G(d) in vacuo
B3LYP/6–31 ? G(d) PCM, e = 1.7
B3LYP/6–31 ? G(d) PCM, e = 4.2
B3LYP/6–31 ? G(d) PCM, e = 8.9
B3LYP/6–31 ? G(d) PCM, e = 35.7
B3LYP/6–31 ? G(d,p) in vacuo
Flavone (5)
Procedure A To a stirred solution of (± )-10 (1 mmol) in
glacial acetic acid at room temperature, TTN
(
1.1 mmol) was added, and the reaction mixture was
stirred for 3.5 h. The reaction mixture was diluted with
water, and the products were extracted with CH Cl .
2
2
2
54 nm was chosen, and the measured lV s of the peak of
The organic phase was washed with a saturated solution
of sodium hydrogen carbonate, then with water until
neutral and dried. The evaporation of the solution gave
the crude product as an oil [5 (82 %) and 7 (0.5 %)
detected by HPLC].
the compounds was corrected on the basis of the ppm value
obtained from the HPLC chromatogram of the corre-
sponding analytically pure compound.
Analytical and preparative TLCs were done on Kiesel-
gel 60 F254 plates (Merck). Flavanone (1), flavone (5),
isoflavone (7), and the reagents (Tl(NO ) Á3H O and
Procedure B This was as described in procedure A,
but the reaction mixture was stirred for 12 h. The
similar workup gave a mixture of 5 (79 %) and 7 (2 %)
detected by HPLC. After its purification performed as in
procedure A, 5 (103 mg, 65 %) and 7 (1.6 mg, 1 %)
were obtained.
3
3
2
PhI(OAc) ) were purchased from Sigma-Aldrich. For
2
workup, the solutions were dried (MgSO ) and concen-
4
trated in vacuo. The enol acetate of (± )-1 [(± )-10] was
prepared from (± )-1 as described in the literature [6].
Fig. 5 Structures of 20a and
2
0b obtained at the HF/
–31G(d) level of theory in
6
vacuo
1
23

Oxidation of enol acetate of flavanone with thallium(III) nitrate or phenyliodonium diacetate
Computational methods
6. Patonay T, Jek }o J, Kiss-Szikszai A, L e´ vai A (2004) Monatsh
Chem 135:743
7
. Taylor EC, Robey RI, Liu KT, Favre B, Bozimo HT, Conley RA,
Chiang CS, McKillop A, Ford ME (1976) J Am Chem Soc
98:3037
B3LYP and HF level optimizations were performed with
the Gaussian 09 package [17] applying 6–31G(d),
6
–31G(d,p), 6–31 ? G(d), and 6–31 ? G(d,p) basis sets.
8. Varma RS, Varma H (1982) Synth Commun 12:927
. Singh OV, Kapoor RP (1990) Tetrahedron Lett 31:1459
0. Prakash O, Pahuja S, Goyal S, Sawhney SN, Moriarty RM (1990)
Synlett 7:337
11. Singh OV, Grag CP, Kapoor RP (1990) Tetrahedron Lett 32:2747
12. L e´ vai A, Balogh L (1975) Pharmazie 30:747
9
In the PCM solvent model calculations, ACN (e = 35.7),
CH Cl (e = 8.9), Et O (e = 4.2), and Xe (e = 1.7) were
applied as solvents. The Molekel software package [18]
1
2
2
2
was used to visualize the results.
1
1
3. Cui SL, Lin XF, Wang YG (2006) Org Lett 8:4517
4. Felpin FX (2005) J Org Chem 70:8575
Acknowledgments We are grateful to the National Research
´
Foundations (OTKA K-81701 and K105871 as well as TAMOP
15. Joseph-Nathan P, Mares J, Herna
Magn Res 16:447
1
1
´
ndez MC, Shoolery JN (1974) J
4
.2.2./B-10/1-2010-0024) for the financial support of our research.
6. Murthy MSR, Rao EV, Ward RS (1986) Magn Res Chem 24:225
We also thank the National Information Infrastructure Development
Institute (NIIFI 10038) for CPU time. A.M. thanks for the Zolt a´ n
Magyary postdoctoral fellowship program. The computational
7. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA,
Cheeseman JR, Scalmani G, Barone V, Mennucci B, Petersson
GA, Nakatsuji H, Caricato M, Li X, Hratchian HP, Izmaylov AF,
Bloino J, Zheng G, Sonnenberg JL, Hada M, Ehara M, Toyota K,
Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao
O, Nakai H, Vreven T, Montgomery JA, Peralta JE Jr, Ogliaro F,
Bearpark M, Heyd JJ, Brothers E, Kudin KN, Staroverov VN,
Kobayashi R, Normand J, Raghavachari K, Rendell A, Burant JC,
Iyengar SS, Tomasi J, Cossi M, Rega N, Millam JM, Klene M,
Knox JE, Cross JB, Bakken V, Adamo C, Jaramillo J, Gomperts
R, Stratmann RE, Yazyev O, Austin AJ, Cammi R, Pomelli C,
Ochterski JW, Martin RL, Morokuma K, Zakrzewski VG, Voth
GA, Salvador P, Dannenberg JJ, Dapprich S, Daniels AD, Farkas
O, Foresman JB, Ortiz JV, Cioslowski J, Fox DJ (2010) Gaussian
09, revision B.01. Gaussian Inc, Wallingford
´
research was realized in the frames of TAMOP 4.2.4. A/2-11-1-2012-
001 National Excellence Program—Elaborating and operating an
0
inland student and researcher personal support system convergence
program. The project was subsidized by the European Union and co-
financed by the European Social Fund.
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