Ytterbium(III) triflate: An efficient and simple catalyst for isomerization of 2′-hydroxychalcone and 2′-aminochalcones in ionic liquid

Article abstract of DOI:10.1002/jhet.760

Isomerization of 2′-hydroxychalcone and 2′-aminochalcone have been investigated using ytterbium(III) trifluromethanesulfonate {Yb(OTf) ₃} (30 mol %) as Lewis acid catalyst in [bmim][BF₄] ionic liquid. The effect of different metal triflates as Lewis acid, catalyst loading and reaction media was studied for this isomerization reaction. Advantages of the methodology include short reaction time, excellent yields, catalytic use of Lewis acid, and recovery and reuse of the catalyst. Copyright

Full text of DOI:10.1002/jhet.760

1356
Ytterbium(III) Triflate: An Efficient and Simple Catalyst for
Isomerization of 20-Hydroxychalcone and 20-Aminochalcones in
Ionic Liquid
Vol 48
V. Kameswara Rao, M. Sudershan Rao, and Anil Kumar*
Department of Chemistry, Birla Institute of Technology and Science, Pilani 333 031, India
*E-mail: anilkumar@bits-pilani.ac.in
Received July 16, 2010
DOI 10.1002/jhet.760
Published online 23 August 2011 in Wiley Online Library (wileyonlinelibrary.com).
Isomerization of 20-hydroxychalcone and 20-aminochalcone have been investigated using ytterbiu-
m(III) trifluromethanesulfonate {Yb(OTf)3} (30 mol %) as Lewis acid catalyst in [bmim][BF4] ionic liq-
uid. The effect of different metal triflates as Lewis acid, catalyst loading and reaction media was
studied for this isomerization reaction. Advantages of the methodology include short reaction time,
excellent yields, catalytic use of Lewis acid, and recovery and reuse of the catalyst.
J. Heterocyclic Chem., 48, 1356 (2011).
INTRODUCTION
of such organic transformations [26–29]. In view of
environment friendly methodologies, recovery and recy-
cling of the catalyst is highly desirable. In this regard,
neoteric solvents, such as ionic liquids, have played an
important role [30–33]. Lanthanide triflates immobilized
in ionic liquids have been used as efficient and recycla-
ble catalysts for various organic reactions [34]. In con-
tinuation of our efforts toward development of greener
and efficient reaction methodologies using Yb(OTf)3
[35–36], and ionic liquids [37–39], we decided to inves-
tigate the use of Yb(OTf)3 as a catalyst for isomeriza-
tion of 20-hydroxychalcones and 20-aminochalcones in
ionic liquid (Scheme 1).
Isomerization of 20-hydroxychalcones and 20-amino-
chalcones affords biologically important heterocyclic
compound flavonones and quinolinones, respectively.
Because of the presence of these moieties in various
natural products of diverse biological importance [1,2],
this process has received great attention. This isomeriza-
tion have been achieved using orthophosphoric acid [3],
sulfuric acid [4], zeolites [5], silica gel [6,7], silica-sup-
ported reagents [8–10], base catalysts [11–14], PEG-400
[15], microwave irradiation [16,17], and chiral Brønsted
acids and bases [18]. Isomerization of 20-hydroxychal-
cones into flavonone is never complete; the reaction is
very slow and generally proceeds with only moderate
yields. In addition to low yield, majority of these meth-
ods are associated with certain limitations such as long
reaction time or requirement of high energy source, use
of halogenated solvents and toxic agents, unwanted side
reaction, expensive catalyst, moisture sensitivity of the
catalyst, and tedious reaction work up. Thus, mild, sim-
ple, nonpolluting, and more efficient methods are still
desirable for the isomerization of 20-hydroxychalcones
and 20-aminochalcones.
RESULT AND DISCUSSION
In a first set of experiment, isomerization was carried
out with different Lewis acids as catalyst in ionic liquid
[bmim][BF4] using 20-hydroxychalcone (1a) as a model
substrate (Table 1). The ionic liquid [bmim][BF4] was
chosen as solvent for this reaction, as it has been found
that [bmim][BF4] itself acts as a catalyst for isomeriza-
tion of 20-hydroxychalcones and 20-aminochalcones
under microwave irradiation [17]. Among the catalysts
screened, Ce(OTf)3, Yb(OTf)3, In(OTf)3, Zn(OTf)2, and
pTSA were found to give good to excellent yield of 2a
(Table 1, entry 4, 8–10, and 14), and Yb(OTf)3 gave the
highest yield of 2a under these conditions and, thus,
was selected as choice of catalyst. The structure of 2a
Lanthanide triflates are unique Lewis acids, and they
have been extensively used to effectively promote sev-
eral carbon–carbon and carbon–heteroatom bond forma-
tion reactions due to their efficacy and their low envi-
ronmental impact [19–25]. Ytterbium triflate is probably
one of the most exploited lanthanide triflate for a variety
C
V
2011 HeteroCorporation

November 2011
Ytterbium(III) Triflate: An Efficient and Simple Catalyst for Isomerization of
20-Hydroxychalcone and 20-Aminochalcones in Ionic Liquid
1357
Scheme 1. Isomerization of 20-hydroxychalcones and 20-aminochalcones.
Table 2
Comparison yield of 2a in different solvents catalyzed by Yb(OTf)3
(30 mol %).
Entries
Solvents
Temperature (ꢁC)
Yield (%)a
1
2
3
4
5
6
7
8
9
Acetonitrile
H2O
THF
Reflux
100
Reflux
Reflux
Reflux
130
10
b
–
b
–
b
DCM
Toluene
DMSO
[bmim][BF4]
[bmim][PF6]
[bmim][Br]
–
b
–
79
86
61
45
was confirmed by spectroscopic data. In IR spectra, a
broad peak for hydroxyl group at 2995 cmꢀ1 disap-
peared along with a shift of carbonyl peak from 1660 to
1697 cmꢀ1, double doublets at d 5.43, 3.11, and 2.86
each for a single proton appeared along with other pro-
tons in 1H NMR spectra, and two peaks for m/z
258.9804 [M þ H]þ, 280.9562 [M þ Na]þ were
observed in HRMS spectra.
130
130
130
a Isolated yield.
b Trace of product formation was observed.
was detected (Table 2, entries 1–5), and reactions remained
incomplete even after heating at higher temperature for
long time. Moderate to good yield were observed in
DMSO and ionic liquid [bmim][Br] and [bmim][PF6]
(Table 2, entries 6–8). Significant solvent effect was
observed in our investigation with solvent screen, and,
thus, ionic liquid [bmim][BF4] was selected as the solvent
of choice for continuing this isomerization study.
Next, we optimized the catalysts quantity using differ-
ent amount of catalyst and found that increasing the cat-
alyst loading beyond 30 mol % did not lead to an
improved results (Table 1, entry 1–6). It is worthy to
mention that use of Yb(OTf)3 as Lewis acid in
[bmim][BF4] has dramatically increased the yield of 2a
when compared with that in only [bmim][BF4] (Table 1,
entry 1 and 4).
To examine the scope and generality of isomerization,
different 20-hydroxychalcones substituted with electron-
donating or electron-withdrawing groups were used as the
reaction substrates (Table 3). The substrates with elec-
tron-donating substituent in the aldehydic aromatic group
afforded the corresponding cyclic products (flavonones)
in excellent yields, whereas substrates with electron-with-
drawing substituent in the aldehydic aromatic group
resulted in poor yield of cyclized product. For instance,
1d required only 4 h to give 2d in 81% yield, whereas 1h
did not gave desired product (2h) even after heating at
160ꢁC for 12 h. This perhaps is due to reversal of polarity
of b-carbon in case of 1h by electron-withdrawing substit-
uent in the aromatic ring of aldehydic aryl group.
To demonstrate the advantage of the reaction in ionic
liquid [bmim][BF4], isomerization of 1a catalyzed by
Yb(OTf)3 was investigated in different organic solvents,
ionic liquids, and water. Interestingly, the best yield was
observed in ionic liquid [bmim][BF4] at 130ꢁC. When
shifting the solvent to acetonitrile, water, THF, DCM, or
toluene, only small amount or a trace of desired product
Table 1
Yields of 2a by different catalysts in [bmim][BF4].
Sr. No. Catalysts Catalyst (mol %) Time (h) Yield (%)a
Having ascertained the efficacy of Yb(OTf)3 catalyzed
isomerization of 20-hydroxychalcones in [bmim][BF4],
this methodology was extended to isomerization of 20-
aminochalcones (3a) to give cyclized product (4a) in
92% yield. The structure of 4a was characterized by
1
2
–
–
12
4
4
4
4
4
4
4
4
4
4
4
4
4
4
38b
61
73
86
79
63
53
58
76
55
52
66
54
64
57
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
Sc(OTf)3
Zn(OTf)2
Ce(OTf)3
Y(OTf)3
Ba(OTf)2
In(OTf)3
Er(OTf)3
PTSA
10
20
30
40
50
30
30
30
30
30
30
30
30
30
3
4
5
6
1
mass, H NMR, and IR spectroscopic data. Peaks at m/z
7
8
257.9977 and 279.9736 in HRMS for [M þ H]þ and [M
9
þ Na]þ ions, double doublets at d 4.73, 2.83, and 2.74
1
10
11
12
13
14
15
ppm in H NMR along with other protons, and a shift in
IR from 1643 and 3333 and 3456 cmꢀ1 to 1651 and
3302 cmꢀ1, respectively, confirmed the structure of 4a.
Encouraged with the excellent result for isomerization
of 3a, we performed reaction of different 20-aminochal-
cones (3b–g) under similar conditions, and the results
are summarized in Table 4. The influence of field effect
of substituent in the aromatic ring of aldehydic aryl
MgO
a Isolated yields.
b 1-Phenyl-3-(40-chlorophenyl)propenone (1a) gave 2a in 49% yield in
[bmim][BF4] under MW irradiation after 1.5 min.[17]
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

1358
V. K. Rao, M. S. Rao, and A. Kumar
Vol 48
Table 3
Isomerization of 20-hydroxychalcones.
philic character at this carbon, and it also allows rotation
through the O¼¼CACa bond to give trans-s-trans con-
formations required for cyclization from its original
trans-s-cis conformations [40] by disrupting intramolec-
ular hydrogen bonding.
Substrate
(1)
Product
(2)a
Time
(h)
Yield
(%)b
Sr. No.
Ar
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
4-ClPh
Ph
3-ClPh
2a
2b
2c
2d
2e
2f
4
4
4
86
79c
80
CONCLUSIONS
4-CH3OPh
4-CH3Ph
3-NO2Ph
2-Furyl
4-NO2Ph
4
81
4
6
78
In summary, we have developed a simple, economical,
and more efficient method for isomerization of 20-hydrox-
ychalcones and 20-aminochalcones using Yb(OTf)3 (30
mol %) as catalyst in ionic liquid. Apart from the efficient
and environmentally benign reaction conditions, excellent
yield, and short reaction time, possibility to recycle
Yb(OTf)3/[bmim][BF4] system offers significant advant-
age for this process. Synthetic applications of the reaction
and insight into its detailed mechanism are currently in
progress.
20d
74
1g
1h
2g
2h
4
12
e
–
a All the products gave satisfactory 1H NMR and mass data.
b Isolated yield.
c Yield of 2b was 86, 82, 79, 74, 76 and 73% in six successive runs,
respectively.
d Yield at 160ꢁC, almost quantitatively reactant was recovered at 80ꢁC.
e No product was observed even at 160ꢁC.
group was similar to that in case of 20-hydroxychalcones
(Table 4, entries 3–6). It was also observed that the
isomerization for 20-aminochalcone was faster when
compared with 20-hydroxychalcones. The higher reactiv-
ity of 20-aminochalcones relative to 20-hydroxychalcones
could be rationalized based on differential nucleophilic-
ity of amino group versus hydroxyl group and differen-
ces in the activation energy (Eact) for the cyclization,
which is 57 Kcal/mole for 20-aminochalcones when
compared with 117.886 Kcal/mole for 20-hydroxychal-
cones [17].
EXPERIMENTAL
Ytterbium triflate was purchased from Sigma–Aldrich. All
commercial reagents and solvents were used without further
purification unless otherwise specified. NMR spectra were
recorded on a Brucker Heaven Avance 11 400 spectrophotom-
eter using TMS as internal standard and CDCl3 as solvent, and
the chemical shifts were expressed in ppm. The IR spectra
were recorded using KBr pellets on Shimadzu Prestige-21
FTIR spectrophotometer, and mmax was expressed in cmꢀ1
.
R
Mass spectra were recorded on a QSTARV ELITE LX/MS/MS
mass spectrometer from applied biosystem. The purity of the
products was determined on silica-coated aluminum plates
(Merck). 20-Hydroxychalcone and 20-aminochalcones were pre-
pared using the appropriate aldehyde and the corresponding
ortho-substituted acetophenones [41]. The ionic liquid,
[bmim][BF4], was prepared from 1-methylimidazole by minor
modification of literature procedure [42].
Finally, recycling of the catalysts was studied using
1b as model substrate and found that recycled Yb(OTf)3
in [bmim][BF4] showed good catalytic activity yielding
2b in 86, 82, 79, 74, 76, and 73% yield in six succes-
sive runs, respectively.
The mechanism of the isomerization reaction seems
to be similar to the Lewis acid-catalyzed nucleophilic
addition to a,b-unsaturated ketones. A plausible mecha-
nism is shown in Scheme 2. The higher rate of reaction
is probably due to the higher acidity of Yb(OTf)3 com-
pared to other lanthanide triflates and ability of ionic
liquid to stabilize the formation of charged species by
coordination of Yb(OTf)3 with carbonyl oxygen. It is
expected that this coordination of Yb(OTf)3 activates
the nucleophilic attack at b-carbon by inducing electro-
General procedure for the isomerization. To a mixture of
20-hydroxychalcone/20-aminochalcone (0.712 mmol) and ionic
liquid [bmim][BF4] (2 mL) was added Yb(OTf)3 (81 mg, 30
mol %). The whole reaction mass was stirred at 130ꢁC. The
reaction progress was followed by TLC (hexanes/AcOEt 8:2).
After stirring the reaction mixture for appropriate time as men-
tioned in Tables 3 and 4, diethyl ether (10 mL) was added,
and the organic layer was separated. The solvent was dried
with anhydrous sodium sulfate and evaporated under reduced
pressure, and the residue was percolated through a band of
Scheme 2. A plasible mechanism for isomerization of 20-hydroxychalcones and 20-aminochalcones.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

November 2011
Ytterbium(III) Triflate: An Efficient and Simple Catalyst for Isomerization of
20-Hydroxychalcone and 20-Aminochalcones in Ionic Liquid
1359
Table 4
Isomerization of 20-aminochalcones.
8.21 (s, 1H), 8.16 (m, 1H), 7.97–7.70 (m, 2H), 7.06–7.00 (m,
2H), 6.45 (s, 1H), 6.39 (s, 1H), 5.53 (dd, J ¼ 3.63, 11.45 Hz,
1H), 3.25 (m, 1H), 2.97 (m, 1H). m/z (ESI-MS) Calcd. for
C15H11NO4 269.0688, found 270.0017 [M þ H]þ.
Substrate
(1)
Product Time Yield
(Min) (%)a
2-(Furan-20-yl)-2,3-dihydro-4H-chromen-4-one (2g). White
solid, mp 78–79ꢁC. IR (KBr) cmꢀ1: 1680 (C¼¼O), 1315, 1223
Sr. No.
Ar
(2)a
1
cmꢀ1. H NMR (300 MHz, CDCl3): d ¼ 7.91 (dd, J ¼ 7.60,
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
1f
4-ClPh
Ph
4a
4b
4c
4d
4e
4f
5
5
5
92
85
93
94
70
60c
89
2.40 Hz, 1H), 7.50–7.46 (m, 2H), 7.06–7.00 (m, 2H), 6.45 (s,
1H), 6.39 (s, 1H), 5.53 (dd, J ¼ 3.60, 11.60 Hz, 1H), 3.26 (m,
1H), 2.98 (m, 1H). HRMS: m/z Calcd. for C13H10O3 214.0629,
found 215.0155 [M þ H]þ.
4-CH3Ph
2,4-(CH3O)2Ph
3-NO2Ph
4-NO2Ph
2-Furyl
5
30
360
10
2-(40-Chlorophenyl)-2,3-dihydro-4(1H)-quinolinone
(4a). Pale yellow solid, mp 168–170ꢁC. IR (KBr) cmꢀ1: 3302
1g
4g
(NH), 1651 (C¼¼O) cmꢀ1
.
1H NMR (300 MHz, CDCl3): d ¼
a All the products gave satisfactory 1H NMR and mass data.
b
Isolated yield.
c Yield at 160ꢁC.
7.85 (d, J ¼ 7.60 Hz, 1H), 7.41–7.32 (m, 5H), 6.79 (dd, J ¼
7.20, 7.24 Hz, 1H), 6.72 (d, J ¼ 8.40 Hz, 1H), 4.73 (dd, J ¼
4.16, 13.20 Hz, 1H), 4.52 (brs, 1H), 2.83 (dd, J ¼ 13.20,
16.20 Hz, 1H), 2.74 (dd, J ¼ 4.16, 16.20 Hz, 1H). HRMS: m/z
Calcd. for C15H12ClNO 257.0607, found 257.9585 [M þ H]þ.
2-Phenyl-2,3-dihydro-4(1H)-quinolinone (4b). Pale yellow
solid, mp 150–151ꢁC. IR (KBr) cmꢀ1: 3325 (NH), 1648
silica gel (60–120) using hexane/AcOEt (9:1 v/v) as an eluent
to afford corresponding compounds in 20–94% yield.
2-(40-Chlorophenyl)-2,3-dihydro-4H-chromen-4-one
(2a). Colorless solid, mp 95–97ꢁC. IR (KBr) cmꢀ1: 1697
(C¼¼O), 1300, 1221. 1H NMR (300 MHz, CDCl3): d ¼ 7.92 (dd,
J ¼ 7.60, 1.60 Hz, 1H), 7.51 (dt, J ¼ 7.60, 1.60 Hz, 1H), 7.41–
7.39 (m, 4H), 7.08–7.04 (m, 2H), 5.46 (dd, J ¼ 2.80, 13.20 Hz,
1H), 3.04 (dd, J ¼ 13.20, 16.80 Hz, 1H), 2.88 (dd, J ¼ 2.80,
16.80 Hz, 1H). HRMS: m/z Calcd. for C15H11ClO2: 258.0448
[Mþ], found: 258.9804 [M þ H]þ, 280.9562 [M þ Na]þ.
2-Phenyl-2,3-dihydro-4H-chromen-4-one (2b). White solid,
1
cmꢀ1(C¼¼O) cmꢀ1. H NMR (300 MHz, CDCl3): d ¼ 7.87 (d,
J ¼ 7.66 Hz, 1H), 7.47–7.32 (m, 6H), 6.80 (dd, J ¼ 7.64, 7.23
Hz, 1H), 6.71 (d, J ¼ 8.41 Hz, 1H), 4.76 (dd, J ¼ 4.00, 13.62
Hz, 1H), 4.52 (brs, 1H), 2.88 (dd, J ¼ 13.62, 16.41 Hz, 1H),
2.77 (dd, J ¼ 3.98, 16.42 Hz, 1H). HRMS: m/z Calcd. for
C15H13NO 223.0997, found 224.0154 [M þ H]þ.
2-(p-Tolyl)-2,3-dihydro-4(1H)-quinolinone (4c). Pale yel-
low solid, mp 149–150ꢁC. IR (KBr) cmꢀ1: 3350 (NH), 1649
mp 75–77ꢁC. IR (KBr) cmꢀ1: 1692, 1310, 1218 cmꢀ1 1H
.
1
(C¼¼O) cmꢀ1. H NMR (300 MHz, CDCl3): d ¼ 7.85 (d, J ¼
NMR (300 MHz, CDCl3): d ¼ 7.91 (dd, J ¼ 8.00, 2.80 Hz,
1H), 7.47–7.36 (m, 6H), 7.05–7.04 (m, 2H), 5.48 (d, J ¼
13.60 Hz, 1H), 3.11–3.03 (m, 1H), 2.88 (dd, J ¼ 4.40, 16.80
Hz, 1H). HRMS: m/z Calcd. for C15H12O2 224.0837, found
225.0308 [M þ H]þ, 247.0061 [M þ Na]þ.
8.00 Hz, 1H), 7.34–7.19 (m, 5H), 6.79–6.75 (m, 1H), 6.70 (d,
J ¼ 8.00 Hz,1H), 4.69 (dd, J ¼ 3.40, 13.60 Hz, 1H), 4.57
(brs, 1H), 2.85 (dd, J ¼ 13.80, 16.20 Hz, 1H), 2.72 (dd, J ¼
2.40, 16.4 Hz, 1H), 2.36 (s, 3H). HRMS: m/z Calcd. For
C16H15NO 237.1153, found 238.0259 [M þ H]þ.
2-(30-Chlorophenyl)-2,3-dihydro-4H-chromen-4-one (2c). Pale
white solid, mp 97–99ꢁC. IR (KBr) cmꢀ1: 1694 (C¼¼O) 1302,
2-(20,40-Dimethoxyphenyl)-2,3-dihydro-4(1H)-quinolinone
(4d). Pale yellow solid, mp 117–118ꢁC. IR (KBr) cmꢀ1: 3326
1220 cmꢀ1
.
1H NMR (300 MHz, CDCl3): d ¼ 7.93 (d, J ¼
(NH), 1649 (C¼¼O) cmꢀ1
.
1H NMR (300 MHz, CDCl3): d ¼
6.80 Hz, 1H), 7.51 (m, 2H), 7.35 (m, 3H), 7.06 (d, J ¼ 7.60
Hz, 2H), 5.45 (dd, J ¼ 2.80, 13.20 Hz, 1H), 3.03 (dd, J ¼
13.60 Hz, J ¼ 16.80 Hz, 1H), 2.88 (dd, J ¼ 2.80, 16.80 Hz,
1H). HRMS: m/z Calcd. for C15H11ClO2 258.0448, found
258.9804, 280.9552 [M þ Na]þ.
7.85 (d, J ¼ 7.78 Hz, 1H), 7.11 (d, J ¼ 7.64, 1H), 6.79–6.75
(m, 1H), 6.73 (d, J ¼ 7.64, 1H), 6.70 (d, J ¼ 7.76 Hz, 1H),
6.63 (s, 1H), 4.69 (dd, J ¼ 3.46, 13.60 Hz, 1H), 4.54 (brs,
1H), 3.82 (s, 3H), 3.78 (s, 3H), 2.88 (dd, J ¼ 13.80, 16.44 Hz,
1H), 2.72 (dd, J ¼ 3.46, 16.44 Hz, 1H). HRMS: m/z Calcd.
2-(40-Methoxyphenyl)-2,3-dihydro-4H-chromen-4-one
(2d). White solid, mp 88–89ꢁC. IR (KBr) cmꢀ1: 1689 (C¼¼O),
for C17H17NO3 283.1208, found 284.0089 [M
þ
H]þ,
305.9772 [M þ Na]þ.
1
1311. 1222 cmꢀ1. H NMR (300 MHz, CDCl3): d ¼ 7.93 (d,
2-(30-Nitrophenyl)-2,3-dihydro-4(1H)-quinolinone (4e). Pale
J ¼ 6.4 Hz, 1H), 7.48 (d, J ¼ 6.8 Hz, 1H), 7.41 (d, J ¼ 6.40
Hz, 2H), 7.04–7.02 (m, 2H), 6.96 (d, J ¼ 6.40 Hz, 2H), 5.41
(d, J ¼ 11.20 Hz, 1H), 3.82 (s, 3H), 3.13–3.10 (m, 1H), 2.87–
2.83 (m, 1H). HRMS: m/z Calcd. for C16H14O3 254.0942,
found 255.0326 [M þ H]þ, 277.0081 [M þ Na]þ.
yellow solid, mp 159–160ꢁC. IR (KBr) cmꢀ1: 3351 (NH),
1
1653 (C¼¼O) cmꢀ1. H NMR (300 MHz, CDCl3): d ¼ 8.37 (s,
1H), 8.20 (td, J ¼ 8.20, 1.32, 1H), 7.97 (d, J ¼ 7.64 Hz, 1H),
7.70 (t, J ¼ 8.0 Hz, 1H), 7.60 (d, J ¼ 8.0 Hz,1 H), 7.36 (td, J
¼ 8.32, 1.56 Hz, 1H), 7.27 (s, 1H), 6.91 (d, J ¼ 8.32 Hz,1H),
6.68 (t, J ¼ 7.80 Hz,1H), 4.97 (dd, J ¼ 4.24, 11.52 Hz, 1H),
2.90 (dd, J ¼ 11.80, 16.08 Hz, 1H), 2.77 (dd, J ¼ 4.32, 16.08
Hz, 1H). HRMS: m/z Calcd. for C15H12N2O3 268.0847, found
269.0281 [M þ H]þ, 290.0031 [M þ Na]þ.
2-(p-Tolyl)-2,3-dihydro-4H-chromen-4-one (2e). Pale white
solid, mp 82–84ꢁC. IR (KBr) cmꢀ1: 1683, 1325, 1220 cmꢀ1
.
1H NMR (300 MHz, CDCl3): d ¼ 7.92 (d, J ¼ 6.40 Hz, 1H),
7.49 (m, 1H), 7.37 (d, J ¼ 5.60 Hz, 2H), 7.24 (d, J ¼ 5.60
Hz, 2H), 7.04 (m, 2H), 5.44 (d, J ¼ 9.2 Hz, 1H), 3.12–3.08
(m, 1H), 2.88–2.84 (m, 1H), 2.37 (s, 3H). HRMS: m/z Calcd.
for C16H14O2 238.0993, found 239.0426 [M þ H]þ, 261.0186
[M þ Na]þ.
2-(40-Nitrophenyl)-2,3-dihydro-4(1H)-quinolinone (4f). Orange
solid, mp 199–200ꢁC. IR (KBr) cmꢀ1: 3338 (NH), 1649
1
(C¼¼O) cmꢀ1. H NMR (300 MHz, CDCl3): d ¼ 8.26 (d, J ¼
8.64 Hz, 1H), 7.77 (d, J ¼ 8.64 Hz, 1H), 7.60 (d, J ¼ 7.88
Hz,1H), 7.35 (td, J ¼ 7.62, 1.24 Hz, 1H), 7.28 (s, 1H), 6.90
(d, J ¼ 8.32 Hz,1H), 6.67 (t, J ¼ 7.64 Hz,1H), 4.96 (dd, J ¼
2,3-Dihydro-2-(3-nitrophenyl)chromen-4-one (2f). Pale yel-
low solid, mp 101–103ꢁC. IR (KBr) cmꢀ1: 1681 (C¼¼O),
1547, 1327, 1223 cmꢀ1
.
1H NMR (300 MHz, CDCl3): d ¼
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

1360
V. K. Rao, M. S. Rao, and A. Kumar
Vol 48
[14] Sarvanamurugan, S.; Palanichamy, M.; Arabindoo, B.; Mur-
ugesan, V. Catal Commun 2005, 6, 399.
4.72, 10.88 Hz, 1H), 2.87–2.74 (m, 2H). HRMS: m/z Calcd.
for C15H12N2O3 268.0847, found 269.0282 [M
290.0029 [M þ Na]þ.
þ
H]þ,
[15] Kumar, D.; Patel, G.; Mishra, B. G.; Varma, R. S. Tetrahe-
dron Lett 2007, 49, 6974.
2-(Furan-20-yl)-2,3-dihydro-4(1H)-quinolinone (4g). Viscous
liquid. IR (KBr) cmꢀ1: 3331 (NH), 1658 (C¼¼O) cmꢀ1
.
1H
[16] Varma, R. S.; Saini, R. K. Synlett 1997, 857.
[17] Kumar, D.; Patel, G.; Kumar, A.; Roy, R. K. J Heterocyclic
Chem 2009, 46, 791.
NMR (300 MHz, CDCl3): d ¼ 7.85 (d, J ¼ 8.00 Hz, 1H),
7.38–7.26 (m, 2H), 6.79–6.69 (m, 2H), 6.32 (d, J ¼ 1.39 Hz,
1H), 6.25 (d, J ¼ 1.53 Hz, 1H), 4.84–4.78 (m, 2H), 3.07–2.92
(m, 2H). HRMS: m/z calcd. for C13H11NO2 213.0789, found
214.0136 [M þ H]þ, 235.0146 [M þ Na]þ.
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General procedure for recovery and reuse of catalyst. After
extracting the product using diethyl ether, the ionic liquid layer
containing Yb(OTf)3 was dried under vacuum. The flask con-
taining recovered Yb(OTf)3 in ionic liquid was again charged
with 2b (185 mg, 0.712 mmol), and the same procedure was
repeated as given in general procedure for isomerization.
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Acknowledgments. This work was supported by Department of
Science and Technology, New Delhi (SR-FTP-CS-34-2007) and
Council of Scientific and Industrial Research (CSIR), New Delhi
(01(2214)/08/EMR-II). The author MSR is thankful to Birla Insti-
tute of Technology and Science, Pilani, for institutional research
fellowship.
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet