Cycloaddition of styrene derivatives with quinone catalyzed by ferric ion; remarkable acceleration in an ionic liquid solvent system

Article abstract of DOI:10.1016/S0040-4039(02)00405-7

Ferric ion-catalyzed cycloaddition of styrene derivatives with quinone gave 2,3-dihydrobenzofuran derivatives in excellent yields with trans selectivity; remarkable acceleration was observed when the reaction was carried out in an ionic liquid solvent sys

Full text of DOI:10.1016/S0040-4039(02)00405-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 3041–3044
Cycloaddition of styrene derivatives with quinone catalyzed by
ferric ion; remarkable acceleration in an ionic liquid solvent
system
b
b
a,
Hiroyuki Ohara, Hiromi Kiyokane and Toshiyuki Itoh *
a
Department of Materials Science, Faculty of Engineering, Tottori University, Tottori 680-8552, Japan
Department of Chemistry, Faculty of Education, Okayama University, Okayama 700-8530, Japan
b
Received 22 January 2002; revised 20 February 2002; accepted 22 February 2002
Abstract—Ferric ion-catalyzed cycloaddition of styrene derivatives with quinone gave 2,3-dihydrobenzofuran derivatives in
excellent yields with trans selectivity; remarkable acceleration was observed when the reaction was carried out in an ionic liquid
solvent system. © 2002 Elsevier Science Ltd. All rights reserved.
Iron is recognized as an economical and pollution free
metal source, however, iron-catalyzed reaction has been
smoothly and the cycloaddition product was obtained
in an excellent yield (>90%), although the compound
was unprecedented 2,3-dihydrobenzofuran derivative
1
only limited. Recently, we demonstrated that alumina
2
4,5
supported iron(III) perchlorate (Fe(ClO ) ) catalyzed
7a and none of the [2+2]cycloaddition product 6 was
4
3
[
2+2]cycloaddition of (trans)-anethol (1) and gave the
obtained (Scheme 1). Further study showed that this
reaction was greatly accelerated when carried out in an
corresponding cyclobutane 2 in an excellent yield
(
2
6
Scheme 1). Because the reaction was believed to have
ionic liquid solvent system. Here, we report that
been caused by one-electron oxidation mediated by an
efficient synthesis of 2,3-dihydrobenzofuran derivatives
is accomplished using an iron(III) cation, which is
derived from iron(II) tetrafluoroborate (Fe(BF ) ) in
3
iron(III) cation, we further investigated the possibility
of the cross coupling reaction of (trans)-anethol (1)
with methyl vinyl ketone (3); the desired [2+2]cross
coupling product 5 was obtained, however, it was
unsuccessful in improving the chemical efficiency
4
2
situ by the reaction of styrene derivatives with 1,4-ben-
zoquinone in butylmethylimmidazolium hexafluoro-
7
phosphate ([bmim]PF6) as solvent with excellent
(
Scheme 1). On the other hand, the cross coupling
stereoselectivity.
reaction of 1 with 1,4-benzoquinone 4 proceeded
First, we evaluated the reaction of anethol 1a and
1
,4-benzoquinone 4 with 3 mol% of alumina-supported
2
Fe(ClO4)3 in acetonitrile (CH CN) at rt; the results are
3
summarized in Table 1. 2,3-Dihydrobenzofuran 7a was
8
obtained with high trans selectivity (trans:cis=13:1) in
9
3% yield (entry 1 in Table 1). It was reported that the
same compound 7a was obtained in 68% yield by a
Lewis acid-promoted reaction of (E)-1a with 4, but an
excess amount (1.2 equivalent) of Ti(IV) salt (a mixture
of TiCl :Ti(OiPr) =1.8:1) was necessary for the reac-
4
4
4,5
tion. In the present reaction, the same compound was
obtained through a truly catalytic reaction using
Fe(ClO ) . Interestingly, cycloadduct (trans)-7a was
4
3
obtained when (Z)-1a was subjected to the reaction,
though both the chemical and reaction rates dropped
significantly (entry 2). Styrene derivative 1b gave the
corresponding cycloadducts 7b with a moderate chemi-
cal yield (entry 3). Styrene derivatives 1c and 1d which
Scheme 1.
*
0
Corresponding author. E-mail: titoh@chem.tottori-u.ac.jp
040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00405-7

3
042
H. Ohara et al. / Tetrahedron Letters 43 (2002) 3041–3044
Table 1. Results of cycloaddition of styrene derivatives with 1,4-benzoquinone
Table 2. Iron salt-catalyzed cycloaddition in the ionic liquid solvent
Catalyst (3 mol%)
O
O
R
+
Ar
(2)
Ar
O
Ionic solvent rt
OH
R
1
4
7
1
7
Entry
1
Catalyst
Solvent
Time
Yield/ %a trans/cis
Ar
R
MeO
Me Fe(ClO4)3/Al2O3 [bmim]PF6 10 min 90
15:1
2
3
MeO
MeO
MeO
MeO
MeO
MeO
Me
Me
FeCl3
FeCl2
[bmim]PF6 10 min 91
11:1
12:1
---
[bmim]PF6 4 h
[bmim]PF6 24 h
94
4
5
6
7
8
Me FeSO ·7H O
4
2
0
Me Fe(BF4)2·6H2O [bmim]PF6 10 min
98
68
0
11:1
Me Fe(BF4)2·6H2O
Me Fe(BF4)2·6H2O
[bmim]BF4 3 h
[bmim]OTf 24 h
43:1
---
MeO
MeO
O
O
Me Fe(BF ) ·6H O [bmim]PF6 3 min
98
8:1
20:1
8:1
4
2
2
9
Me Fe(BF ) ·6H O [bmim]PF6 10 min
4
2
2
86
63
10
MeO
Et Fe(BF4)2·6H2O
[bmim]PF6 10 min
a
Isolated yield.

H. Ohara et al. / Tetrahedron Letters 43 (2002) 3041–3044
3043
6
have electron rich substituents gave the cycloadduct 7c
and 7d with excellent yields of 94 and 91%, respectively
chemical properties. We therefore next investigated the
1
0
reaction in the ionic liquid solvent system (Table 2).
A
(
entries 4 and 5).
remarkable acceleration was obtained when the reac-
tion was carried out in [bmim]PF and desired cycload-
6
The working hypothesis of the present reaction is illus-
duct 7a was obtained by only 10 min reaction with
excellent yield (entry 1), while it took several hours to
complete the same reaction if the reaction was carried
3
+
trated in Fig. 1. The Fe ion oxidized styrene 1a to
2
+
form radical cation species 1a* and Fe ion, which is
3
+
oxidized by quinone 4 to regenerate Fe ion with
radical anion 4*, and the catalytic cycle is completed as
shown in Fig. 1; cycloadduct 7a is thus obtained by the
result of rearrangement of [2+2]cycloadduct 6a* (Path
out in CH CN solvent. Investigation to switch the
3
counter anion part of the catalyst gave interesting
results; iron(III) chloride also catalyzed the reaction
very successfully (entry 2), and even iron(II) dichloride
worked very well as catalyst (entry 3). However,
9
A) or [2+3]cycloaddition of 1a* with 4* through 6a**
iron(II) sulfate (FeSO ) provided no cycloadduct com-
(
Path B).
4
pound (entry 4). We finally found that iron(II) tetra-
fluoroborate (Fe(BF ) ) was the best catalyst in this
It was anticipated that the cycloaddition reaction could
proceed more rapidly in a polar solvent system if
cycloadduct 7a was produced through the reaction
pathway, as illustrated in Fig. 1. Ionic liquids are a new
class of solvents which have attracted growing interest
over the past few years due to their unique physical and
4 2
reaction; the desired compound 7a was obtained in 98%
yield after just 10 min reaction (entry 5).
It was suspected that the reaction was promoted by the
acid. In fact, benzofuran 7a was obtained in ca. 80%
yield when the reaction was carried out in an acidified
[
bmim]PF by partial hydrolysis with water, while no
6
product 7a was obtained in pure [bmim]PF solvent
O
6
MeO
Me
without catalyst. It is well known that iron(II) cation is
oxidized easily by quinone to generate an iron(III)
cation; we therefore believe that the initial step of the
cycloaddition reaction might be initiated by a one-elec-
O
1a
4
Fe3+
Fe2+
Fe3+
11
tron oxidation of anethol 1, as hypothesized in Fig. 1.
We were then interested in the proper combination of
the counter anion of ionic liquid solvent with [bmim]
cation using Fe(BF ) as catalyst, and found that the
reaction rate was strongly dependent on the anionic
part of the immidazolium salt. The best results were
recorded when [bmim]PF6 was employed as solvent
O
MeO
Me
O
4
2
4
*
1
a*
(
entry 7); a significant drop in the reaction rate was
Path A
Path B
OH
obtained when the reaction was carried out in
12
[bmim]BF4 (entry 6), and no product was obtained in
[bmim]OTf (entry 7). It should be emphasized that
very rapid reaction was accomplished in the [bmim]PF
13
O
O
Ar
6
O
Ar
solvent system; desired cycloadducts 7c, 7d, and 7e were
obtained in good yield only by 3–10 min reactions
Me
Me
14
(
entries 8–10).
O
Ar
Me
O
Me
In conclusion, we demonstrated that Fe(BF ) -cata-
OH
4 2
O
Ar
lyzed cycloaddition of styrene derivatives 1 with 1,4-
6a**
benzoquinone
4
in [bmim]PF6 solvent afforded
H
6
a*
2
,3-dihydrobenzofuran derivatives 7 in excellent yields.
O
Ar
Me
This is not only the least recorded amount of catalyst
but also the record of extremely rapid reaction in this
type of reaction. Further determination of the scope
and limitations of this reaction will make it even more
beneficial.
OH
O
Ar
OH
H
Acknowledgements
Me
H
This research was supported by a Grant-in-Aid for
Scientific Research on Priority Areas (A) ‘Exploitation
of Multi-Element Cyclic Molecules’ from the Ministry
of Education, Culture, Sports, Science and Technology,
Japan. The authors are grateful to the SC-NMR Labo-
ratory of Okayama University for the NMR
measurements.
O
MeO
OH
Me
7a
Figure 1. Working hypothesis of the present reaction.

3
044
H. Ohara et al. / Tetrahedron Letters 43 (2002) 3041–3044
quinone and subsequent rearrangement by Engler et al.
4,5
References
1
0. The acidity of all ionic liquid solvents employed was
confirmed to be neutral prior to their use in each reac-
tion.
1
2
3
. For a review, see: Laszlo, P. Acc. Chem. Res. 1986, 19,
1
21.
. Ohara, H.; Itoh, T.; Nakamura, M.; Nakamura, E.
11. Although we hypothesize that iron(III) salt works as a
one electron transfer catalyst, as illustrated in Fig. 1,
Chem. Lett. 2001, 624.
3+
. It was proposed that [2+2]cycloaddition proceeds through
an electron transfer (ET) pathway: (a) a review see:
Ledwith, A. Acc. Chem. Res. 1972, 5, 133; (b) for the
most recent example, see: Reddy, G. D.; Wiest, O. J. Org.
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another mechanism may also be possible; Fe ion may
coordinate with the neutral quinone 4 and accelerating
electron transfer reaction to anethol.
12. [bmim]BF and [bmim]PF : Suarez, P. A. Z.; Dullius, J.
4
6
E. L.; Einloft, S.; de Souza, R. F.; Dupont, J. Polyhedron
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Kalyanasundaram, K.; Gr o¨ tzel, M. Inorg. Chem. 1996,
35, 1168.
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Am. Chem. Soc. 1988, 110, 7931.
zoquinone 4 (44.0 mg, 0.407 mmol) and (trans)-anethol
6
7
. For a recent review, see: Welton, T. Chem. Rev. 1999, 99,
1a (62.0 mg, 0.419 mmol) in [bmim]PF
(0.8 mL) was
6
2
071.
added an Fe(BF ·6H O (4.0 mg, 0.012 mmol) in one
4
)
2
2
. [bmim]PF : Huddleston, J. G.; Willauer, H. D.; Swat-
portion at rt. The mixture was stirred for 10 min and was
diluted with ether (4 mL), then extracted with ether (2
mL×5). The solvent was removed under reduced pressure
to give a crude mixture which was purified by TLC
(EtOAc/hexane=20%) to give the desired 2,3-dihy-
drobenzofrane 7a in 98% yield (102 mg, 0.398 mmol) as a
light yellow oil.
6
loski, R. P.; Visser, A. E.; Rogers, R. D. Chem. Commun.
1
998, 1765.
1
8. The ratio of trans–cis was determined by H NMR
analysis.
9
. It was proposed that 2,3-dihydrobenzofuran 7a was pro-
duced by [2+2]cycloaddition of anethol with 1,4-benzo-