Amberlyst-15 in ionic liquid: an efficient and recyclable reagent for the benzylation and hydroalkylation of β-dicarbonyl compounds

Article abstract of DOI:10.1016/j.tetlet.2009.11.122

Benzylation and hydroalkylation of 1,3-dicarbonyl compounds using Amberlyst-15 immobilized in ionic liquid [Bmim][PF₆] as an efficient reusable reagent was studied. The reagent was compared with other solid acid reagents along with role of the ionic liquid. The effect of various reaction parameters like type of reagent, solvent, substrate molar ratio, reaction time, and temperature were studied. Present protocol is advantageous due to the ease in handling of reagent, simple work-up procedure, economical and environmentally benign process. The products were obtained in good to excellent yield and applicable to wide variety of substrates.

Full text of DOI:10.1016/j.tetlet.2009.11.122

Tetrahedron Letters 51 (2010) 724–729
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Amberlyst-15Ò in ionic liquid: an efficient and recyclable reagent for the
benzylation and hydroalkylation of b-dicarbonyl compounds
*
Ziyauddin S. Qureshi, Krishna M. Deshmukh, Pawan J. Tambade, Bhalchandra M. Bhanage
Department of Chemistry, Institute of Chemical Technology (Autonomous), N. Parekh Marg, Matunga, Mumbai 400 019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Benzylation and hydroalkylation of 1,3-dicarbonyl compounds using Amberlyst-15 immobilized in ionic
liquid [Bmim][PF6] as an efficient reusable reagent was studied. The reagent was compared with other
solid acid reagents along with role of the ionic liquid. The effect of various reaction parameters like type
of reagent, solvent, substrate molar ratio, reaction time, and temperature were studied. Present protocol
is advantageous due to the ease in handling of reagent, simple work-up procedure, economical and envi-
ronmentally benign process. The products were obtained in good to excellent yield and applicable to
wide variety of substrates.
Received 29 October 2009
Revised 20 November 2009
Accepted 27 November 2009
Available online 2 December 2009
Ó 2009 Elsevier Ltd. All rights reserved.
The alkylation of b-dicarbonyl compounds represents one of the
most important C–C bond formation methodology in organic syn-
thesis.1 Addition of b-dicarbonyl compounds to olefins results in a
highly atom efficient protocol.2 In general, these transformations
were performed by the reaction of alcohols3 or alkyl halides4 with
b-dicarbonyl compounds. In case of alcohols, hydroxy group is a
poor leaving group which necessitates its preactivation and which
can be achieved using high temperature or by suitable promoter.5
This leads to the formation of stoichiometric amount of salt-waste.
catalyzed transformations.8,10 This prompted us to initiate a
systematic exploration on benzylation and hydroalkylation of b-
dicarbonyl compounds with the help of ionic liquids (Scheme 1).
In continuation of our research on application of ionic liquids in
several organic transformations,11 we herein report the benzyla-
tion and hydroalkylation of b-dicarbonyl compounds by Amber-
lyst-15 in ionic liquid [Bmim][PF6].
The reagent Amberlyst-15 and solvent [Bmim][PF6] are air-
stable and applicable to the wide range of substrates.
The substitution with halides also requires
a
stoichiometric
Initially, the coupling reaction of acetylacetone 1 with 1-phen-
ylethanol 2 in the presence [Bmim][PF6] as a solvent was chosen as
a model reaction13 (Table 1). Various solid acid reagents such as
Amberlite-IR 120, Amberlyst-15, Indion-225H, Montmorillonite
K-10, Al2O3 were screened (Table 1, entries 1–5). Amberlyst-15
was found to be most effective reagent providing 66% yield of
the desired product (Table 1, entry 5) under screening conditions.
The probable reason for its higher activity can be explained on the
amount of base which limits its application for the larger scale. Re-
cently, many acid catalysts were employed to perform nucleophilic
substitution of benzyl alcohols with active methylene com-
pounds,6 whereas, the alkylation of b-dicarbonyl compounds with
alkenes was also explored with different types of acid catalysts.7
There has been an increasing interest in exploiting the potential
of ionic liquids as a reaction media to develop green methodolo-
gies; allowing reuse of the catalysts or reagents.8 Features that
make ionic liquids attractive include lack of vapor pressure and
the great versatility of their chemical and physical properties.
Using a judicious combination of cations and anions, it is possible
to tune the solvent properties as per the requirement of the reac-
tion and thus creating an almost indefinite set of ‘designer sol-
vents’. Besides the possibility of recycling the catalytic system,
potential interest in using ionic liquids result in the unique inter-
actions of these media with the active species and in the possibility
to modify the reaction activity and selectivity.9 Their successful use
as solvents has been demonstrated for a wide range of organic
reactions including acid-catalyzed reactions and transition metals
OH
2
O
O
O
O
Amberlyst-15
[Bmim] [PF6]
1
3
* Corresponding author. Tel.: +91 22 2414 5616; fax: +91 22 2414 5614.
E-mail address: bhalchandra_bhanage@yahoo.com (B.M. Bhanage).
Scheme 1. Benzylation and hydroalkylation of b-dicarbonyl compounds.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.11.122

Z. S. Qureshi et al. / Tetrahedron Letters 51 (2010) 724–729
725
Table 1
Effect of different catalyst on the addition of acetylacetone (1) to 1-phenylethanol (2)a
Entry
Reagent
H+ capacity
Surface area (m2/g)
Particle size (mesh)
Conversionb (%)
Yieldc (%)
1
2
3
4
5
Amberlite-IR 120
Indion-225H
Montmorillonite K-10
Al2O3
4.4 mequiv/g
1.8 mequiv/mL
3–4 mequiv/g
4.5 mequiv/g
4.2 mequiv/g
45
—
220
200
42
16–50
14–52
<200
150–300
20–50
96
97
96
44
97
61
36
27
—
Amberlyst-15
66
a
b
c
Reaction conditions: Catalyst (0.32 g), [Bmim][PF6] (2 mL), reaction temperature 80 °C.
The conversion was determined by GC with respect to 1-phenyl ethanol.
The yield was determined by GC, Reaction time 2 h.
basis of the physical properties like high H+ exchange capacity
(4.2 meq/g) and high surface area (42 m2/g).
3, entries 1–3). Benzoylacetone 1b also undergoes benzylation
2a–b, which gives a mixture of diastereomers (1.2:0.8) in good
yield (Table 3, entries 4 and 5). It was observed that bulky b-dike-
tone 1c also gives 81% yield of the expected product (Table 3, entry
6). Electron rich para-methoxy benzyl alcohol 2d undergoes
smooth conversion to the desired product with excellent yield (Ta-
ble 3, entry 7). The present protocol works well for the benzylation
2b of aliphatic, cyclic and aromatic b-ketoesters 1d–g (Table 3, en-
tries 8–11), as selectively alkylated products are obtained instead
of transesterification. Diethyl malonate does not undergo benzyla-
tion, which might be due to the inability of diethyl malonate for
enolization during the reaction (Table 3, entry 12). Acetyl acetone
1a also undergoes allylation with 2e to furnish the allylated prod-
uct in 81% (Table 3, entry 13). It is observed that aliphatic alcohol
2f is not favorable for the alkylation with acetyl acetone 1a (Table
3, entry 14).
The optimized reaction parameters were extended for a com-
parative study of 1,3-dicarbonyl compounds with styrene and its
derivatives (Table 4). In general we observed that yield of the de-
sired product was lower than the reaction with the subsequent
alcohols, which might be due to dimerization of styrene during
the reaction.
We explored the scope of the Amberlyst-15 reagent for hydro-
alkylation by varying styrene and its derivatives as well as 1,3-
dicarbonyl compounds in [Bmim][PF6] (Table 4). Styrene 3a was
effectively added to acetylacetone 1a to give the corresponding
product 6a in 79% (Table 4, entry 1), but 1,3-diphenylbut-1-ene 7
was also formed as side product. Various electron-donating and
electron-withdrawing groups such as –CH3, t-butyl, F, Cl, Br on sty-
rene 3b–e underwent hydroalkylation with smooth conversion to
give the desired products 6b–e in good yields (Table 4, entries 2–
6). Hindered 4-tert-butyl styrene 3f smoothly undergoes hydro-
alkylation with acetylacetone 1a giving 87% of the expected prod-
The influence of various parameters like solvent, substrates
molar ratio, reaction temperature, and time were examined
(Table 2). The influence of solvents on the coupling reaction of ace-
tylacetone with 1-phenylethanol using Amberlyst-15 as a reagent
of choice was studied (Table 2, entries 1–8). The reaction was more
favorable in ionic liquids such as [Bmim][PF6] and [Bmim][BF4] as
compared to conventional organic solvents, as the dimerization of
1-phenylethanol to 1,3-diphenylbut-1-ene is observe as major side
product. Since better results were obtained in [Bmim][PF6], it was
used for further studies. Substrate ratio of 1:4 (2:1) is found to be
best for this transformation (Table 2, entry 10) in order to get 4 as
single major product (84% yield) indicating the effect of solvent
and the Amberlyst-15 reagent. The self condensation of 1-phenyl-
ethanol to give symmetrical ether 5 is one of the side reactions
observed. Further, the reaction was carried out at different temper-
atures (80, 60 °C, and rt) (Table 2, entries 10–12). It was observed
that as the temperature decreased from 80 °C to rt the yield of the
desired product 4 also decreased. Increase in the reaction time did
not offer any significant advantages (Table 2, entry 13). Hence the
optimum reaction parameters were reagent: Amberlyst-15, sol-
vent: [Bmim][PF6], Substrate ratio of 2:1: 1:4, temperature: 80 °C,
and time: 2 h.
The optimized reaction conditions were used for the coupling
reaction of various substituted alcohols with substrates containing
active methylene group. This gives moderate to good yields of de-
sired products (Table 3, entries 1–14). The reaction of acetylace-
tone 1a with different types of benzyl alcohols 2a–c proceeded
smoothly providing 63–90% yield of the desired products (Table
Table 2
Effect of reaction parameters on the addition of acetylacetone (1) to 1-phenylethanol
(2)a
uct 6f (Table 4, entry 6). When
acetylacetone, it was observed that instead of expected product 6f,
-methyl styrene under acidic conditions, itself undergoes dimer-
a-methyl styrene was reacted with
Entry
Molar ratio 2:1
Solvent
Conversionb (%)
Yieldc (%)
a
1
2
3
4
5
6
7
8
9
10
11d
12e
13f
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:2
1:4
1:4
1:4
1:4
[Bmim][PF6]
Toluene
CH2Cl2
CH3CN
THF
CH3NO2
[Bmim][BF4]
Neat
[Bmim][PF6]
[Bmim][PF6]
[Bmim][PF6]
[Bmim][PF6]
[Bmim][PF6]
97
100
15
21
30
83
97
100
96
97
66
16
Trace
10
—
49
55
35
70
84
76
53
85
ization to form 4-methyl-2,4-diphenyl-2-pentene and 4-methyl-
2,4-diphenyl-1-pentene in 69% yield (1:1) (Table 4 entry 7). Previ-
ously Shirakawa and co-workers have successfully demonstrated a
palladium catalyzed dimerization of vinylarenes using Indium tri-
flate as a co-catalyst.12 similarly, 1b undergoes hydroalkylation
with styrene and gave a mixture of diastereomers 6h in 81% (Table
4, entry 8). The reaction of styrene and its derivatives with bulky b-
dicarbonyl compounds proceeded efficiently giving moderate to
good yields (Table 4, entries 9–11).
A recyclability study of present protocol was also studied. Reac-
tion of 1-phenyl ethanol and acetylacetone with Amberlyst-15–io-
nic liquid reagent was recycled up to three cycles (83%, 85%, 82%).
There was no significant decrease during the first cycle whereas
the yield declined up to 82% after the completion of the third cycle.
Thus the Amberlyst-15–ionic liquid system can be successfully
recycled and the reusability procedure was tested up to three
times and consistent results were obtained.
97
94
96
a
Reaction conditions: Amberlyst-15 (0.32 g), solvent (2 mL), reaction tempera-
ture 80 °C.
b
The conversion was determined by GC with respect to 1-phenylthanol.
The yield was determined by GC, Reaction time 2 h.
Reaction at 60 °C for 5 h.
Reaction at room temperature for 5 h.
Reaction at 80 °C for 4 h.
c
d
e
f

726
Z. S. Qureshi et al. / Tetrahedron Letters 51 (2010) 724–729
Table 3
Amberlyst-15 reagent for the addition of b-dicarbonyl compounds with benzylic alcoholsa
OH
O
O
O
O
O
+
+
1
2
4
5
Entry
1
Alcohol (2)
OH
Nucleophile (1)
Time (h)
Product
Yieldb,(Ref.) (%)
O
O
O
O
O
O
O
O
O
1a
3
636f
2a
4a
4b
OH
O
1a
2
3
2
3
5
5
846f
2b
OH
OH
O
O
1a
904
2c
4c
O
O
O
O
O
O
O
O
Ph
Ph
1b
4c
656f
2a
4d
4e
OH
OH
O
O
O
Ph
Ph
Ph
1b
5c
88c6f
2b
O
Ph
Ph
Ph
1c
6
7
6
816f
914
774
2b
4f
OH
O
O
O
O
1a
3.5
4g
O
O
OMe
MeO
2d
OH
OH
O
O
O
O
O
O
1d
8c
5
2b
2b
4h
O
O
O
O
1e
9c
5
854
4i

Z. S. Qureshi et al. / Tetrahedron Letters 51 (2010) 724–729
727
Table 3 (continued)
Entry
Alcohol (2)
OH
Nucleophile (1)
Time (h)
Product
Yieldb,(Ref.) (%)
O
O
O
O
Ph
O
Ph
O
10c
6
604
1f
2b
4j
OH
OH
O
O
O
O
O
O
11c
6
6
866e
O
4k
Ph
2b
2c
1g
O
O
O
O
O
O
O
12
—
1h
4l
O
O
O
O
O
OH
1a
13
14
5
5
816f
2e
2f
4m
O
O
O
OH
1a
—
4n
See Ref. 13 for experimental procedure.
a
Reaction conditions: alcohol 2 (1 mmol), 1,3-dicarbonyl compounds 1 (4 equiv), Amberlyst-15 (0.32 g), [Bmim][PF6] (2 mL) at 80 °C.
Isolated yield.
Mixture of diastereomers (1.2:0.8).
b
c
Table 4
Amberlyst-15 reagent for the addition of b-dicarbonyl compounds with alkenesa
O
O
O
O
+
+
1
3
6
7
Entry
1
Alkene (3)
Nucleophile (1)
Time (h)
Product
Yieldb,(Ref.) (%)
O
O
O
O
O
O
O
O
O
O
1a
1a
1a
3a
5
797d
6a
O
O
Cl
3b
3c
2
3
5
5
777d
Cl
6b
F
816g
F
6c
(continued on next page)

728
Z. S. Qureshi et al. / Tetrahedron Letters 51 (2010) 724–729
Table 4 (continued)
Entry
Alkene (3)
Nucleophile (1)
Time (h)
Product
Yieldb,(Ref.) (%)
O
O
O
O
O
O
O
O
O
O
1a
1a
1a
Br
3d
4
5
5
727d
Br
6d
O
O
3e
5
5
777d
6e
3f
6
87
6f
O
O
O
O
1a
3g
7
5
—
6g
O
O
O
O
Ph
Ph
1b
3a
8c
6
6
6
6
816g
657d
617d
556g
6h
O
O
O
O
O
O
O
Ph
Ph
Ph
Ph
Ph
Ph
Ph
1c
3a
9
6i
O
Ph
1c
Cl
3b
3c
10
11
Cl
6j
O
O
O
O
Ph
Ph
Ph
Ph
1c
F
F
6k
See Ref. 13 for experimental procedure.
a
Reaction conditions: styrene 3 (1 mmol), 1,3-dicarbonyl compounds 1 (3 equiv), Amberlyst-15 (0.32 g), [Bmim][PF6] (2 mL) at 80 °C.
Isolated yield.
Mixture of diastereomers (1.2:0.8).
b
c
In conclusion, we have demonstrated an efficient methodology
er solvent, simple of operation, mild reaction conditions, and reus-
ability of system.
for both benzylation and hydroalkylation with 1,3-dicarbonyl com-
pounds using Amberlyst-15 as reagent in [Bmim][PF6] ionic liquid.
The reaction was optimized with respect to various parameters
and enabled coupling reactions of various electron rich, electron
deficient and sterically hindered alcohols and alkenes with different
1,3-dicarbonyl compounds affording excellent yield of the desired
products. The reaction system employs use of ionic liquids as green-
Acknowledgments
The financial assistance from Indira Gandhi Centre for Atomic
Research (IGCAR) Kalpakkam, India is kindly acknowledged.

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729
12. Tsuchimoto, T.; Kamiyama, S.; Negoro, R.; Shirakawa, E.; Ka-wakami, Y. Chem.
Commun. 2003, 852–853.
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a well-stirred mixture of Amberlyst-15 (0.32 g) in
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[Bmim][PF6] (2 mL), nucleophile 1a–h (4 equiv) an alcohol 2a–e or styrene 3a–
g was added. The reaction mixture was stirred at 80 °C and the completion of
the reaction was monitored by GC/TLC. The products were confirmed by NMR
(Mercury Plus 300 MHz spectrometer), GC–MS(Shimadzu GC–MS QP 2010), IR
Perkin–Elmer FT-IR), and also using authentic samples. After completion, 5 mL
of di-iso propyl ether was added and vigorously shaken to extract all the
reactants and products in the ether phase. The extraction procedure was
repeated for about 3–4 times. The combined organic extracts were dried over
Na2SO4 and the solvent was evaporated in vacuo. The residue obtained was
purified using column chromatography (silica gel, mesh size 60–120) using pet
ether/ethyl acetate (90:10) as eluent to afford the pure products. After
extraction the reactor containing the recovered Amberlyst-15 in ionic liquid
was dried in vacuo for an hour and then charged with alcohol and nucleophile
again.
Physical and spectral data of selected compounds: 3-(1-Phenyl-ethyl)-pentane-
2,4-dione (4b): White solid, mp: 42–44 °C, IR (KBr)
m
max/cmÀ1 2926, 1727,
1678, 1452, 1360, 1148, 759, 701; 1H NMR (300 MHz; CDCl3; Me4Si) d = 1.20 (d,
3H, J = 6.9 Hz), 1.83 (s, 3 H), 2.27 (s, 3H), 3.54–3.64 (m, 1H), 4.02 (d, 1H,
J = 11.3 Hz), 7.17–7.32 (m, 5H); 13C NMR (75.43 MHz; CDCl3; Me4Si) 20.76,
29.64, 29.72, 40.34, 76.53, 126.89, 127.20, 128.73, 142.99, 203.29, 203.34 ppm;
(EI) m/z (%) 204 (M+, 1). 186 (10), 161 (100), 147 (33), 143 (14), 105 (33), 43
(26).
1-Phenyl-2-(1-phenyl-ethyl)-butane-1,3-dione (4e): Mixture of diastereomers,
colorless oil, IR (neat) m
max/cmÀ1 2926, 2854, 1727, 1678, 1454, 762, 699; 1H
NMR (300 MHz; CDCl3; Me4Si) d = 1.22 (d, 3H J = 6.6 Hz), 1.32 (d, 3H J = 7.3 Hz),
1.90 (s, 3H), 2.24 (s, 3H), 3.81–3.92 (m, 2H), 4.87 (d, 1H J = 10.9 Hz), 4.91 (d, 1H
J = 10.9 Hz), 7.01–7.07 (m, 1H), 7.08–7.25 (m, 6H), 7.28–7.34 (m, 6H), 7.37–
7.52 (m, 2H), 7.58–7.60 (m, 1H), 7.77–7.80 (m, 2H), 8.07–8.10 (m, 2H); 13C
NMR (75.43 MHz; CDCl3; Me4Si) 21.67, 22.77, 27.94, 29.78, 40.42, 41.01, 70.95,
71.59, 126.7, 127.08, 127.43, 127.57, 128.53, 128.66, 128.89, 128.95, 133.5,
133.9, 137.26, 143.22, 195.27, 203.3 ppm; (EI) m/z (%) 266 (M+, 1), 223 (100),
161 (28), 105 (83).
3-Oxo-2-(1-phenyl-ethyl)-butyric acid methyl ester (4h): Mixture of
diastereomers, colorless oil, IR (neat)
m
max/cmÀ1 2955, 1748, 1715, 1494,
1454, 1434, 1275, 1206, 1163, 765, 701; 1H NMR (300 MHz; CDCl3; Me4Si)
d = 1.24 (d, 3H, J = 6.9 Hz), 1.30 (d, 3H, J = 6.95 Hz), 1.88 (s, 3H), 2.25 (s, 3H),
3.35 (s, 1H), 3.52–3.57 (m, 1H), 3.52–3.57 (m, 1H), 3.69 (s, 1H), 3.76 (d, 1H,
J = 10.63 Hz), 3.81 (d, 1H, J = 10.63 Hz), 7.51–7.27 (m, 5H), 7.51–7.27 (m, 5H);
13C NMR (75.43 MHz; CDCl3; Me4Si) 19.72, 20.22, 29.24, 29.56, 39.36, 39.47,
51.67, 52.01, 66.25, 66.88, 126.48, 126.59, 126.99, 127.04, 128.13, 128.35,
142.68, 142.97, 168.21, 168.63, 201.74, 201.74 ppm; (EI) m/z (%) 220 (M+, 1),
202 (100), 177 (41), 77 (145), 131 (33), 105 (82), 43 (23).
3-[1-(4-tert-Butyl-phenyl)-ethyl]-pentane-2,4-dione (6f): White solid, mp: 45–
47 °C, IR (KBr) m
max/cmÀ1 2962, 1696, 1511, 1356, 1260, 1189, 972, 827, 702,
578; 1H NMR (300 MHz; CDCl3; Me4Si) d = 1.18 (d, 3H, J = 6.59 Hz), 1.28 (S, 9H),
1.83 (S, 3H), 2.26 (S, 3H), 3.54–3.6 (m, 1H), 4.01–4.05 (d, 1H, J = 11.36 Hz),
7.09–7.11 (d, 2H, J = 8.43 Hz), 7.28–7.31 (d, 2H, J = 8.43 Hz); 13C NMR
(75.43 MHz; CDCl3; Me4Si) 20.92, 29.75, 29.8, 31.33, 34.41, 39.99, 40.45,
76.67, 76.75, 77.1, 77.52, 125.66, 126.87, 127.28, 128.83, 139.84, 149.75,
203.67, 203.77 ppm; (EI) m/z (%) 260 (M+, 1), 242 (15), 217 (85), 161 (100), 143
(94), 131 (30), 127 (11), 99 (33), 77 (5), 43 (99); Ms–Ms (ESI+) m/z calcd for
(M+ Na+) 283.18; found: 283.20.
2-[1-(4-Chloro-phenyl)-ethyl]-1,3-diphenyl-propane-1,3-dione (6j): White solid,
mp: 110–111 °C, IR (KBr) m
max/cmÀ1 2926, 2853, 1697, 1665, 1596, 1492, 1448,
1260, 1193, 1094, 1014, 932, 797, 688; 1H NMR (300 MHz; CDCl3; Me4Si)
d = 1.30 (d, 3H, J = 6.96 Hz), 4.04 (dq, 1H, J = 10.26 Hz, 6.96 Hz), 5.58 (d, 1H
J = 10.26 Hz), 7.13–7.58 (m, 10H), 7.69–7.77 (m, 2H), 7.96–8.06 (m, 2H); 13C
NMR (75.43 MHz; CDCl3; Me4Si) 20.25, 40.57, 64.56, 128.48, 128.56, 128.8,
129.13, 132.23, 133.26, 133.69, 136.67, 136.96, 142.37, 194.36, 194.69 ppm;
(EI) m/z (%) 362 (M+, 1), 257 (100), 223 (3), 179 (3), 139 (8), 105 (75), 77 (49).
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Chem. Res. 2007, 370–372; (d) Qureshi, Z. S.; Deshmukh, K. M.; Bhor, M. D.;
Bhanage, B. M. Catal. Commun. 2009, 10, 833–837; (e) Deshmukh, K. M.;
Qureshi, Z. S.; Nandurkar, N. S.; Bhanage, B. M. Can. J. Chem. 2009, 87, 1–5.