Copper(II)triflate promoted highly chemoselective rearrangement of chalcone epoxides to β-keto aldehydes

Article abstract of DOI:10.2174/1570178611666141024005454

Highly chemoselective rearrangement of chalcone epoxides to β-keto aldehydes using catalytic amount of Cu(OTf)2 (1 mol%) is presented. Copper(II)triflate is a relatively cheap, inexpensive and commercially available catalyst. In this rearrangement selective migration of the acyl group takes place. The presence of an electron donating group on either of the phenyl rings favors the reaction. However, the presence of an electron withdrawing CN group leads to the corresponding β-keto aldehyde, along with an aryl ketone which is obtained through deformylation of the primary product.

Full text of DOI:10.2174/1570178611666141024005454

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Letters in Organic Chemistry, 2015, 12, 55-61
55
Copper(II)triflate Promoted Highly Chemoselective Rearrangement of
Chalcone Epoxides to ꢀ-keto Aldehydes
Balaso G. Jadhav, Ashish A. Vaidya and Shriniwas D. Samant*
Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai – 400 019, India
Received March 14, 2014: Revised June 03, 2014: Accepted October 23, 2014
Abstract: Highly chemoselective rearrangement of chalcone epoxides to ꢀ-keto aldehydes using catalytic amount of
Cu(OTf)2 (1 mol%) is presented. Copper(II)triflate is a relatively cheap, inexpensive and commercially available catalyst.
In this rearrangement selective migration of the acyl group takes place. The presence of an electron donating group on ei-
ther of the phenyl rings favors the reaction. However, the presence of an electron withdrawing CN group leads to the cor-
responding ꢀ-keto aldehyde, along with an aryl ketone which is obtained through deformylation of the primary product.
Keywords: ꢀ-keto aldehydes, Chalcone epoxides, Copper(II)triflate, Rearrangement of epoxides, Keto-enoltautomerism.
INTRODUCTION
ture sensitive, AuCl3 is costly, and Bi(OTf)3 is not commer-
cially available, and the method is not suitable for substrates
containing electron withdrawing group.
Rearrangement of epoxides to carbonyl compounds is an
important reaction in organic synthesis. Several catalysts like
BF3OEt2 [1], MABR [2], MgBr2 [3], lithium salts [4], indium
salts [5], BiOCl4 [6], Pd(OAc)2 [7], IrCl4.xH2O [8], meso-
porousaluminosilicate [9], Fe based catalyst [10], LPDE
[11], and Cu(BF4)2.nH2O [12] have been employed for this
rearrangement.
Metal triflates are efficient catalysts for many organic
transformations [19], but most of the metal triflates are
costly. Copper(II)triflate, a mild Lewis acid, is commercially
available and relatively cheap. It has been used successfully
in Diels-Alder reaction [20], Aldol condensation [21],
Friedel-Crafts reaction [22], Ullmann coupling [23], addition
of activated methylene compounds to alkenes [24], 1,3-
dioxalane formation from oxiranes [25], preparation of or-
thogonally protected glycosides [26], preparation of benzox-
azole from nitriles and o-aminophenols [27], and annulations
of phenols with 1,3-dienes [28].
Exhaustive study has been done on the rearrangement of
epoxides which are substituted with alkyl, aryl, and hydro-
gen. However, epoxides substituted with ꢁ-acyl group i.e.
ꢁ,ꢀ-epoxy ketones,have not been studied much. Chalcone
epoxides are ꢁ,ꢀ-epoxy ketones and undergo rearrangement
to 1,3-diketones [13], 1,2-diketones [14], and ꢀ-keto-
aldehydes [15-18], depending on the catalyst used; through
migration of the hydrogen, aryl ring, or the acyl group.
The product spread in the acid catalyzed rearrangement
of chalcone epoxides coupled with a challenge to develop an
effective and selective catalyst intrigued us and prompted us
to study the rearrangement of chalcone epoxides, with dif-
ferent substituents on the aryl rings, using more effective
catalyst and to study the selectivity in the migration. We
herein report the copper(II)triflatecatalyzed highlychemose-
lectivere arrangement of chalcone epoxides to ꢀ-keto-
aldehydes. The effect of substituents on either rings of the
chalcone epoxides has been investigated in detail using dif-
ferent chalcone epoxides.
The migration of the acyl group in this rearrangement is
fascinating. Further, the dicarbonyl products formed in such
a reaction are all important synthons; particularly for hetero-
cyclic synthesis.
House studied the rearrangement of chalcone epoxides
using classical Lewis acid catalyst BF3-OEt2, and isolated the
boron salts of the corresponding ꢀ-keto aldehydes, which
were converted into the corresponding pyrazoles for the
characterization purpose. The characterization of the prod-
ucts ꢀ-keto aldehydes was however not conclusive. Subse-
quently, while studying the rearrangement of epoxides in
general, a few catalysts, AuCl3 [16], Er(OTf)2 [17], Bi(OTf)2
[18] have been used. In this study a few chalcone epoxides
have also been tried and the reaction has not been studied
with respect to the substituents on the aryl rings. The cata-
lysts used have limitations; BF3-OEt2 is corrosive and mois-
RESULTS AND DISCUSSION
Chalcones 5 were prepared from the corresponding ace-
tophenones and benzaldehydes [29]. 5 were epoxidizedto
chalcone epoxides 1 using H2O2/NaOH [30].
The rearrangement of 1a (R1 = R2 = R3 = H) was carried
out, as a model reaction, using Cu(OTf)2 (5mol%) in DCM.
The product obtained was 6a only. The reaction was com-
pleted (by TLC) within 15 min at 25°C with excellent yield
(93%) of 6a.1a can rearrange with migration of any of the
four groups attached to the epoxide ring, i.e. COPh, Ph and
*Address correspondence to this author at the Department of Chemistry,
Institute of Chemical Technology, Matunga, Mumbai – 400 019, India; Tel:
+912233612606; Fax: +912233611020; E-mails: samantsd@yahoo.com,
sd.samant@ictmumbai.edu.in
1875-6255/15 $58.00+.00
© 2015 Bentham Science Publishers

56 Letters in Organic Chemistry, 2015, Vol. 12, No. 1
Jadhav et al.
O
O
1,3-diketones (2)
O
O
O
O
1a
1,2-diketones (3)
O
O
H
ꢀ-keto aldehyde (4a)
Scheme 1. Possible products in the rearrangement of chalcone epoxides.
R2
R2
R2
O
O
O
O
Cu(OTf)2 (1 mol%)
DCM (5-90 min)
O
H2O2/NaOH
MeOH
R3
H
90-95 %
R3
R1
R1
R1
5
1
4
R3
Scheme 2. Copper(II)triflate promoted rearrangement of chalcone epoxides to ꢀ-keto aldehydes.
O
OH
R2
O
O
R2
H
H
R1
R1
R3
R3
4
6
Scheme 3. Keto-enoltautomerism in 4.
1H-NMR showed two protons at ꢁ 15.92 (enol-OH) and
8.63(C=CH) which confirmed structure 6a. The quantity of
the catalyst was optimized (Table 1); the decrease in the
quantity of Cu(OTf)2 led to increase in the reaction time,
with out significantly affecting the yield of 6a. Only 1 mol%
of Cu(OTf)2 could effectively catalyze the reaction and gave
6a in 93% yield.
two types of H. Even though phenyl ring has a high migra-
tory aptitude, and the rearrangement is a rearrangement of
electron deficient carbon; the acyl group is reported to mi-
grate in the case of benzylideneacetophenone epoxide. The
mechanism proposed involves neighbouring group participa-
tion involving the carbonyl group. However, the effect of
substituent on the phenyl rings has not been reported by any
effective catalyst.
Different chalcone epoxides (1) containing electron do-
nating and electron withdrawing groups were rearranged to
the corresponding ꢀ-ketoaldehydes (4) under the optimized
conditions (Table 2).
4 can exist in keto (4) and enol form (6). Spectral data of
4a showed itto be solely in the enolform (6a). Very high enol
content in ꢀ-diketones with aryl rings is well-known. Thus, 4
is expected to be mainly in its enol form (6).

Copper(II)triflate Promoted Highly Chemoselective Rearrangement
Letters in Organic Chemistry, 2015, Vol. 12, No. 1 57
Table 1. Optimization of quantity of Cu(OTf)2in therearrangement of 1a to 4aa .
Entry
Cu(OTf)2 (mol%)
Time(min)
Yield of 4a(%)b
1
2
3
4
5
5
4
3
2
1
15
25
45
55
90
93
92
91
92
93
a Reaction conditions:1a (1 mmol), solvent= DCM (2 mL), temp = 25°C.
b Isolated yield
a
Table 2. Rearrangement of 1 to 6 in the presence of Cu(OTf)2 .
Entry
Epoxide (1)
Product (4)
Time (min)
Yield of 4(%)b
O
O
O
O
H
1
90
93
1a
4a
O
O
O
O
H
2
80
92
Cl
Cl
1b
4b
O
O
O
O
H
3
30
92
1c
4c
O
O
O
O
H
4
10
89
O
O
1d
4d
Cl
O
O
Cl
O
O
H
5
80
90
Cl
Cl
1e
4e

58 Letters in Organic Chemistry, 2015, Vol. 12, No. 1
Jadhav et al.
Table 2. contd…
Entry
Epoxide (1)
Product (4)
Time (min)
Yield of 4(%)b
O
O
O
O
H
6
60
92
Cl
Cl
4f
1f
O
O
O
O
H
7
5
89
O
O
1g
4g
O
O
O
O
H
8
50
93
1h
4h
O
O
O
O
H
O
9
30
85
O
1i
4i
a Reaction conditions:1(1 mmol), Cu(OTf)2 (1 mol%), DCM (2 mL) at 25°C, time = 5-180 min.
bIsolated yield.
reaction is reported in the case of indium chloride catalyzed
reaction of chalcone epoxide [5]. On increasing the quantity
of the catalyst (3 mol%) and with reflux 60% yield of 4j and
6b was obtained (Table 3, entry 3); while using 5 mol% of
the catalyst the reaction was fast and gave 90% yield of 4j
and 6b (Table 3, entry 4). Chalcone epoxide containing
naphthyl and furyl rings also underwent smooth rearrange-
ment with excellent yields of the products (Table 2, entries 8,
9).
In the rearrangement, irrespective of the substituent, only
the aroyl group attached to the epoxide ring migrated, and in
no case phenyl ring or one of the hydrogen atoms migrated.
Thus, the rearrangement was chemoselective. However, the
rearrangement was sensitive with respect to the substituent
on the phenyl ring. A substituent like –Cl on either ring does
not have any effect on the rearrangement (Table 2, entries
2,6). Electron donating methyl or methoxyl group has favor-
able effect and the reaction took place within a short time
(Table 2, entries 3, 4, 7). 2,4-Dichloro substituents did not
impede the reaction (Table 2, entry 5).
CONCLUSION
Interestingly, the reaction of 1j, containing strong elec-
tron withdrawing group –CN, showed a different behavior
(Table 3). The reaction of 1j at room temperature in DCE did
not afford the product even after a long time. On refluxing
for 12h, 30% yield of 4j and 6b was obtained (Table 3, entry
2). 6b possibly is formed by the deformylation of 4j. Such a
Copper(II)triflate is a chemoselective catalysts for the re-
arrangement of chalcone epoxides to ꢀ-keto aldehydes. The
rearrangement involves selective migration of the acyl
group. The products remain almost entirely in the enol form.
Irrespective of the substituents on the aryl rings the chalcone
epoxides undergo the rearrangement. In the case of the chal-

Copper(II)triflate Promoted Highly Chemoselective Rearrangement
Letters in Organic Chemistry, 2015, Vol. 12, No. 1 59
Table 3. Rearrangement of chalcone epoxide containing –CN group (1j) using copper(II) triflatea.
O
O
O
O
O
H
+
CN
1j
CN
4j
CN
6b
Quantity of Cu(OTf)2
mmol%)
Un reacted 4-cyano chalcone
epoxide (1j) (%)
Entry
Temp. (°C)
Time (h)
Yield 4j+6b(%)b
1
2
3
4
1
1
3
5
RT
12
12
12
2
90
60
30
00
--
--
Reflux
Reflux
Reflux
20 (4j)
40 (4j)
60 (4j)
10 (6b)
20 (6b)
30 (6b)
a Reaction conditions: 1j (1 mmol), Cu(OTf)2(1-5 mmol%), DCE(3 mL).
b Isolated yield and ratio determined by 1H-NMR.
cone epoxide with p-cyano substituent, the expected product
is accompanied by 4-cyanophenyl acetophenone (7b) ob-
tained through deformylation of the primary product. The
products ꢀ-keto aldehydes are important synthons for the
synthesis of heterocycles.
400MHz): ꢁ = 7.09-7.11 (m, 2H), 7.20-7.28 (m, 5H), 7.34-
7.37 (m, 3H), 8.63 (d, J=5.3Hz, 1H, =CH-O), 15.92 (d, J=
5.2 Hz, 1H, -OH).
3-(4-Chlorophenyl)-3-oxo-2-phenylpropanal (4b)[15a]
Solid; mp 95-96°C; FT-IR (KBr): 3514, 1688 cm-1; MS:
258.13 (M)+; 1H-NMR (CDCl3, 300MHz): ꢁ = 7.08-7.11 (m,
2H), 7.18-7.21(m, 2H), 7.29-7.32(m, 5H), 8.63 (d, J=5.4Hz,
1H, =CH-O), 15.85 (d, J= 5.4 Hz, 1H, -OH).
EXPERIMENTAL
All reactions were monitored by Thin Layer Chromatog-
raphy (TLC), Merck TLC plates (60F254). Visualization of
the developed chromatogram was performed by UV absor-
bance or iodine. Chromatography was performed on silica
gel (60-120 mesh). Petroleum ether refers to fraction boiling
in the range 60-80oC. Analytical grade dichloromethane was
used. NMR spectra were recorded on Bruker Advance/ Var-
ian 300 or 400 MHz instrument in CDCl3 [1H NMR ꢁ = 7.26
ppm (s).] using tetramethylsilane (TMS) as internal standard.
Coupling constants (J) were expressed in hertz (Hz) and spin
multiplicities were given as s (singlet), d (doublet), dd (dou-
ble doublet), t (triplet), m (multiplet) and br (broad). Infrared
spectra were obtained on SHIMADZU infrared spectrometer
as neat or KBr plate. Melting points recorded were uncor-
rected.
3-Oxo-2-phenyl-3-(p-tolyl)propanal (4c)
Solid; mp 90-92°C; FT-IR (KBr): 3507, 1684 cm-1; MS:
1
238.80 (M)+; H-NMR (CDCl3, 300MHz): ꢁ = 2.30 (s, 3H),
7.02-7.37(m, 9H), 8.60 (s, 1H, =CH-O), 16.02 (s, 1H, -OH).
3-(4-Methoxyphenyl)-3-oxo-2-phenylpropanal (4d) [11]
Solid; mp 104-106 °C; FT-IR (KBr): 3498, 1678 cm-1;
1
MS: 254.07 (M)+; H-NMR (CDCl3, 300MHz): ꢁ = 3.84 (s,
3H), 6.71(d, J=9.3Hz, 2H), 7.12-7.15 (m, 2H), 7.25-7.37 (m,
5H), 8.50 (d, J=3.6 Hz, 1H, =CH-O), 16.09 (d, J=4.8 Hz,
1H, -OH).
3-(2,4-Dichlorophenyl)-3-oxo-2-phenylpropanal (4e)
Solid; mp 50-52 °C; FT-IR (KBr): 3490, 1694 cm-1; EI-
General Procedure for Rearrangement of Chalcone
Epoxides to ꢀ-keto Aldehydes
1
MS: 293.1(M+H)+; H-NMR (CDCl3, 400MHz): ꢁ = 6.98-
7.00(m, 2H), 7.08(d, 1H, J= 8.4Hz), 7.13(d, 1H, J= 8.4Hz),
7.19-7.22(m, 3H), 7.31(d, 1H, J= 2.0Hz), 8.60 (d, J=4.8 Hz,
1H, =CH-O), 15.30 (d, 1H, -OH); 13C NMR (CDCl3,
75MHz): 117.9, 126.8, 127.1, 128.4, 128.6, 129.4, 129.8,
130.4, 130.6, 132.4, 133.8, 134.0, 136.2, 183.4, 185.0.
To a solution of chalcone epoxides (1.0 mmol) in DCM
(2 mL) was added Cu(OTf2) (1 mol%) at room temperature.
Reaction mixture was stirred for 5-90 min. After completion
of reaction, crude reaction mixture was passed through silica
gel column chromatography using 60-120 mesh size silica
gel and 5% ethyl acetate in hexane as an eluent to afford
pure product which were identified by IR, MS, 1H-NMR and
13C NMR.
2-(4-Chlorophenyl)-3-oxo-3-phenylpropanal (4f) [15a]
Solid; mp 118-120 °C (Lit. 132-133°C); FT-IR (KBr):
3505, 1690 cm-1; MS: 258.11 (M)+; 1H-NMR (CDCl3,
300MHz): ꢁ = 7.03 (d, J= 8.7 Hz, 2H), 7.24-7.28 (m, 4H),
7.35-7.39(m, 3H), 8.61 (d, J=4.2Hz, 1H, =CH-O), 15.93 (d,
J= 4.8 Hz, 1H, -OH).
3-Oxo-2,3-diphenylpropanal (4a) [31]
Solid; mp 110-112°C (Lit. 112-113°C); FT-IR (KBr):
3450, 1687 cm-1; MS: 224.12 (M)+; 1H-NMR (CDCl3,

60 Letters in Organic Chemistry, 2015, Vol. 12, No. 1
Jadhav et al.
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1
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1
273.30 (M-1); H-NMR (CDCl3, 300MHz): ꢀ = 7.03-7.08
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133.8, 135.0, 183.5, 187.3.
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The authors confirm that this article content has no con-
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ACKNOWLEDGEMENTS
The author BGJ is thankful to UGC-SAP for providing
fellowship.
SUPPLEMENTARY MATERIAL
Separate file has been provided which includes spectro-
1
scopic data, H-NMR, and 13C NMR spectra’s of the prod-
from
3-oxo-2,3-diaryl-propionaldehdyes.Tetrahedron,2011,67,
ucts, under the caption “Samant SD Supplementary mate-
rial”.
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Web site along with the published article.
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