Synthesis of 2H-chromenes in ionic liquid solvents

Article abstract of DOI:10.1055/s-2004-832823

Vinylboronic acids react with o-hydroxyaryl aldehydes in room temperature ionic liquids in the presence of secondary amines to generate 2H-chromenes.

Full text of DOI:10.1055/s-2004-832823

2194
LETTER
Synthesis of 2H-Chromenes in Ionic Liquid Solvents
Synthesis of
2
H
-C
e
hromenes orge W. Kabalka,* Bollu Venkataiah, Bhaskar C. Das
Departments of Chemistry and Radiology, The University of Tennessee, Knoxville, TN 37996-1600, USA
Fax +1(865)9742997; E-mail: kabalka@utk.edu
Received 31 January 2003
dibenzylamine. BmimPF6, an acidic ionic liquid, ap-
peared to degrade the starting reagents and produced low-
er yields (Scheme 1, Table 1). Because BmimBF4 is a
liquid at room temperature and is simple to prepare, it was
used in the remainder of the study. It is important to note
that the product is easily separated from the ionic liquid by
simple diethyl ether extraction or by distillation. A variety
of salicylaldehyde derivatives and vinyl boronic acids
were used to prepare the corresponding 2-substituted 2H-
chromenes. All the products were obtained in high yields
in BmimBF4 (Table 2).
Abstract: Vinylboronic acids react with o-hydroxyaryl aldehydes
in room temperature ionic liquids in the presence of secondary
amines to generate 2H-chromenes.
Key Words: ionic liquid, chromenes, Petasis reaction, boronic ac-
id, green chemistry
The past few years have witnessed a growing interest in
ionic liquids as solvent systems for organic syntheses.1
Reactions carried out in ionic liquids often exhibit differ-
ent thermodynamic and kinetic behaviors than those run
in conventional solvents. In addition, ionic liquids are air
and moisture stable and possess other properties that are
of importance in industrial applications.2 Their lack of va-
por pressure is most notable and their use in a wide variety
of environmentally friendly organic transformations
(green chemistry) has been widely reported.3
Table 1 Reaction of Salicylaldehyde and Styrenylboronic Acid in
the Presence of 20 mol% Dibenzylamine in Ionic Liquids
Entry
Ionic Liquid
BmimBF4
EmimBF4
BmimBr
Isolated yield (%)
1
2
3
4
92
91
89
7
Chromene and chroman ring systems are important oxy-
gen heterocycles4 occurring widely in nature. In view of
the recent interest in green chemistry in industrial applica-
tions employing room temperature ionic liquids, we wish
to report the synthesis of 2H-chromenes using boronic ac-
ids, amines, and o-hydroxyaryl aldehydes.5 The reaction
is complementary to one reported by Wang and Finn6 who
utilized 40 mol% of resin-bound amine at 90 °C for 24
hours in dioxane. Reaction yields are comparable to those
obtained by Wang and Finn, using the resin bound amine.
The new ionic liquid reaction was developed as part of our
investigation of boron reactions in non-traditional me-
dia.5d,7
BmimPF6
Although BmimBF4, EmimBF4 and BmimBr are easily
prepared,3b they are expensive, and recycling becomes
important. We found that the ionic liquid solvent could be
used repeatedly by dissolving them in acetone and filter-
ing off the solid reaction by-products.8 Generally, after
four cycles product yields were decreased by an average
of only 5%. NMR analyses indicated that the ionic liquids
are recovered essentially unchanged.
Many of the readily available imidazolium-based ionic
liquids can be utilized in the reaction. These include bu-
tylmethylimidazolium bromide (BmimBr), butylmeth-
ylimidazolium tetrafluoroborate (BmimBF4), and
ethylmethylimidazolium tetrafluoroborate (EmimBF4).
Using some of these solvents, 2-phenyl-2H-chromene can
be obtained in high yield in the presence of 20 mol% of
In summary, ionic liquids are excellent as solvents for pre-
paring 2H-chromenes. The reactions do not require long
reaction times and the isolation procedures are straightfor-
ward.
CHO
OH
B(OH)2
20 mol% dibenzylamine
+
BmimBF4, 80 °C, 3 h
O
Scheme 1
SYNLETT 2004, No. 12, pp 2194–2196
0
1
.1
0
.2
0
0
4
Advanced online publication: 21.09.2004
DOI: 10.1055/s-2004-832823; Art ID: S01203ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of 2H-Chromenes
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Table 2 Synthesis of 2H-Chromenes from Vinylboronic Acids and o-Hydroxyaryl Aldehydes in the Presence of 20 mol% Dibenzylamine in
BmimBF4
Vinylboronic acid
Aldehyde
Producta
Yield (%)b
92
CHO
B(OH)2
O
HO
91
90
O2N
O2N
B(OH)2
B(OH)2
CHO
OH
O
CHO
OH
O
88
Cl
Cl
B(OH)2
CHO
O
OH
94
87
CHO
B(OH)2
O
O
HO
CHO
B(OH)2
OH
CH3
O2N
CH3
O2N
91
CHO
OH
B(OH)2
O
6
(OH)2B
88c
87c
CHO
HO
O
O
(OH)2B
(OH)2B
6
O2N
NO2
O2N
CHO
OH
O
O
(OH)2B
6
a All products exhibited 1H NMR and 13C NMR characteristics in accord with assigned structures and literature values (for known compounds).
New products were analyzed by C, H, N analysis.
b Isolated yields, based on boronic acid.
c Two equivalents of aldehyde were utilized.
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G. W. Kabalka et al.
LETTER
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Synlett 2004, No. 12, 2194–2196 © Thieme Stuttgart · New York