Triflic acid adsorbed on silica gel as an efficient and recyclable catalyst for the addition of β-dicarbonyl compounds to alcohols and alkenes

Article abstract of DOI:10.1039/b926142g

The silica gel supported triflic acid was readily prepared via simple absorption of TfOH onto chromatographic silica gel. This solid acid was applied as an efficient catalyst for the heterogeneous addition of various β-dicarbonyl compounds to a series of alcohols and alkenes, which afforded moderate to excellent yields under solvent-free conditions or in nitromethane. Moreover, this silica gel supported catalyst surprisingly exhibited higher reaction yields in comparison with the homogeneous catalyst and can be readily recovered and reused up to 6 times with almost maintained reactivity and yields.

Full text of DOI:10.1039/b926142g

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www.rsc.org/greenchem | Green Chemistry
Triflic acid adsorbed on silica gel as an efficient and recyclable catalyst for
the addition of b-dicarbonyl compounds to alcohols and alkenes†
Pei Nian Liu,*a,b Fei Xia,a Qing Wei Wang,a Yu Jie Ren*a and Jun Qin Chena
Received 11th December 2009, Accepted 19th March 2010
First published as an Advance Article on the web 12th May 2010
DOI: 10.1039/b926142g
The silica gel supported triflic acid was readily prepared via simple absorption of TfOH onto
chromatographic silica gel. This solid acid was applied as an efficient catalyst for the
heterogeneous addition of various b-dicarbonyl compounds to a series of alcohols and alkenes,
which afforded moderate to excellent yields under solvent-free conditions or in nitromethane.
Moreover, this silica gel supported catalyst surprisingly exhibited higher reaction yields in
comparison with the homogeneous catalyst and can be readily recovered and reused up to 6 times
with almost maintained reactivity and yields.
Alkylation of b-dicarbonyl compounds is one of the most
common methodologies for C–C bond construction. Due to the
call of “green chemistry”, the direct reaction of alcohols and
active methylenes is an ideal transformation because only H2O
is generated as a side product. Besides, the preparation of alkyl
halides and the use of bases (as in the traditional protocol)
are not required.10 Although the advantages of enhancing atom
efficiency and avoiding waste are notable, successful examples of
such “green” transformations have seldom been explored until
recent years, due to the low reactivity of alcohol towards the
nucleophiles.11 It has been reported that addition of b-diketones
and b-keto esters to benzhydryl alcohols can be promoted
by a stoichiometric amount of the Lewis acid BF3·OEt2.12
Moreover, Pd,13 Co,14 Cu15 with additives, and various Lewis
acids such as InCl3,16 InBr3,17 FeCl3,18 Bi(OTf)3,19 Ln(OTf)3
(Ln = La, Yb, Sc, Hf),20 and a ruthenium complex21 have
been recently explored as effective catalysts for the addition
of b-dicarbonyl compounds to allylic and benzylic alcohols.
Additionally, molecular iodine22 and a series of Brønsted acids,
such as H-montmorillonite,23 dodecylbenzenesulfonic acid,24
p-toluenesulfonic acid,25 triflic acid26 and phosphotungstic acid27
have been disclosed as effective catalysts for the addition of
b-diketones to benzylic alcohols. Whereas much progress has
been made in the addition of b-dicarbonyl compounds to
benzylic alcohols, heterogeneous reactions catalyzed by reusable
supported catalysts remain unexplored. Herein, we would like
to present a triflic acid catalyst supported on silica gel (TfOH–
SiO2) by simple absorption, which demonstrates high reusability
and efficiency for the heterogeneous addition of b-dicarbonyl
compounds to alcohols and alkenes.
Introduction
Following the urgent demand of “green chemistry”, the search
for more environmentally friendly forms of catalysis has created
fast-growing interest, and one of the leading contenders for
environmentally acceptable alternatives is supported catalysis.
The catalyst immobilization provides easy separation of the
products from the catalysts without tedious experimental work-
up, and enables the efficient recovery of the catalysts, and
potentially allows the adaptation of the immobilized catalysts
to continuous flow-type processes.1 Triflic acid (TfOH), which is
a superacid, is the strongest protic acid with a large negative H0
value of -14.1,2 and has been used as perhaps the most versatile
Brønsted acid catalyst in a vast array of organic reactions.3
Because triflic acid is highly corrosive and is a fuming liquid,
difficulties remain in storage, transportation, handling and waste
disposal and severely restrict its application in industry. The
immobilization of triflic acid onto an inorganic material, such as
silica gel, affords solid acids which can be easily handled, as they
invariably are low toxicity, non-corrosive, free-flowing powders
with superior thermal and mechanical stability under catalytic
conditions.4 In the past several years, some Brønsted acids, such
as H2SO4,5 HBF4,6 HClO47 etc. have been successfully supported
onto silica gel and applied as solid acids in various catalytic
reactions. Although triflic acid supported on functionalized
silica has been reported,8 the study of triflic acid supported on
unmodified chromatographic silica gel is still very rare.9
aKey Lab for Advanced Materials and Institute of Fine Chemicals,
School of Chemistry and Molecular Engineering, East China University
of Science and Technology, 130 Meilong Road, Shanghai, 200237,
P. R. China. E-mail: liupn@ecust.edu.cn; Fax: +86-21-64252758;
Tel: +86-21-64250552
Results and discussion
bState Key Laboratory of Applied Organic Chemistry, Lanzhou
University, Lanzhou, 730000, P. R. China
† Electronic supplementary information (ESI) available: Analytical data
of the known products (3a–g, 3i–t) and crystallographic data of 3a in
CIF or other electronic format. CCDC reference number 651081. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/b926142g
As the first stage of our research, we attempted to immobilize
TfOH onto unmodified chromatographic silica gel and neutral
or alkaline alumina via simple adsorption. As shown in Table 1,
the resulting supported TfOH catalysts were used to catalyze
the reactions of acetylacetone (1a) and 1-phenylethanol (2a)
in air without solvent at 70 ◦C. The reaction results revealed
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Table 1 Reactions of 1a and 2a using acids as catalysts under various conditionsa
Entry
Catalyst
Loading of TfOH (mmol g-1)
Run
Temp. (◦C)
Time (h)
Yield (%)b
1c
2c
3c
4
5
6
7
8
9
10
11
12
13d
14
15
16
17
TfOH–neutral Al2O3
TfOH–alkaline Al2O3
TfOH–SiO2
TfOH–SiO2
TfOH–SiO2
TfOH–SiO2
TfOH–SiO2
TfOH–SiO2
TfOH–SiO2
TfOH
TfOH–SiO2
TfOH–SiO2
TfOH–SiO2
TfOH–SiO2
TfOH–SiO2
TfOH–SiO2
TfOH–SiO2
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
—
0.5
0.5
0.5
0.25
0.25
1.0
1.0
1
1
1
1
2
3
4
5
6
1
1
1
1
1
2
1
2
70
70
70
70
70
70
70
70
70
70
90
50
70
70
70
70
70
17
17
5
5
5
5
5
5
5
5
3
10
17
5
5
5
5
13
55
80
90
89
90
86
85
51
84
83
51
76
70
69
85
81
a Reaction conditions: supported TfOH (60 mg, containing 0.03 mmol TfOH), 1a (3.0 mmol), 2a (1.0 mmol), 70 ◦C, solvent-free conditions in air,
unless noted otherwise. b Isolated yield. c 1a (1.0 mmol) and 2a (2.0 mmol) were used and the yield was determined based on 1a. d TfOH–SiO2 (20 mg,
containing 0.01 mmol TfOH) was used.
that the unmodified chromatographic silica gel was a much
better support for TfOH in comparison with neutral or alkaline
alumina (Table 1, entries 1–3). When the pure oily product 3a
was placed in a sealed flask at room temperature for several
weeks, it was interesting to find that the compound sublimed to
form single crystals suitable for X-ray crystallography study.†
As the ratio of 1a to 2a was changed from 1 : 2 to 3 : 1, the
reaction catalyzed by TfOH–SiO2 gave a further increased yield
of 90% (Table 1, entry 4). Moreover, the silica gel supported
TfOH exhibited excellent reusability in the reaction of 1a and
2a and it could be readily recovered and reused for 6 runs
with almost maintained reactivities and yields (Table 1, entries
4-9). It is worth noting that when a different batch of TfOH–
SiO2 was used as the catalyst for the reaction of 1a and 2a,
similar reaction results were also obtained, which illustrated the
good catalytic reproducibility of TfOH–SiO2. For comparison,
a control experiment using liquid TfOH as the homogeneous
catalyst was carried out under the same conditions, and only
84% yield was obtained in the reaction of 2a and 1a. This
result suggested that the supported TfOH was a more effective
catalyst than the original homogeneous one (Table 1, entries
4, 10). If the reaction temperature was increased or decreased,
the reaction completion time was then shortened or prolonged,
but the yields were both affected negatively. (Table 1, entries
11, 12). This reaction was also effective when the catalyst loading
was decreased to 1%, but the reaction did not proceeds in the
second run (Table 1, entry 13). When the loading of TfOH in
TfOH–SiO2 was varied from 0.5 mmol g-1 to 0.25 mmol g-1
or 1.0 mmol g-1, worse reaction yields were observed and both
the catalysts were ineffective after two runs (Table 1, entries
14-17). Moreover, further investigations of leaching of TfOH
in the recycle reactions (Table 1, entries 4-9) were carried out
by elemental analysis of fluorine and sulfur in TfOH–SiO2. The
results showed that the amount of TfOH contained in TfOH–
SiO2 before reaction and after six runs was 7.1% and 4.2%,
respectively, which meant that approximately 40% of TfOH had
leached from the catalyst after six uses. The elemental analysis
results of the catalyst after each cycle also suggested that 5–8%
of TfOH leached out in one cycle of reaction.
Subsequently, the reactions of various b-dicarbonyl com-
pounds with a series of alcohols and alkenes were examined in
air at 70 ◦C, using 3 mol% TfOH–SiO2 as the catalyst. The results
of reactions carried out under solvent-free conditions are listed
in Table 2. The reactions of acetylacetone 1a with the benzylic
alcohols 2b and 2c bearing electron-donating groups on the
aromatic ring gave good to excellent yields (Table 2, entries 2, 3).
But the reaction of 1a with 1-(4-chlorophenyl)ethanol 2d showed
only 68% yield, which suggested that the electron-withdrawing
substitution on the aromatic ring affected the reaction negatively
(Table 2, entry 4). Acetylacetone 1a could also react with
1-(2-naphthyl)ethanol 2e and benzhydrol 2f smoothly to give the
products 3e and 3f in 93% and 96% yields, respectively (Table 2,
entries 5, 6). When the allylic alcohol 2g was applied to the
reaction with 1a, the reaction produced the product 3g in 82%
yield (Table 2, entry 7).
When the sterically hindered cyclic diketone 1b was used in
the reaction with 2f, the product 3h with a quaternary carbon
atom was obtained in excellent 96% yield, which illustrates that
the steric hindrance of the b-diketone had no obvious effect
on the reactivity and yield of this Brønsted acid catalyzed
heterogeneous reaction (Table 2, entry 8). With the hope to
expand the substrate scope, b-keto esters 1c and 1d were used as
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a
Table 2 Reactions of b-dicarbonyl compounds with alcohols or styrene catalyzed by TfOH–SiO2
Entry
1
Dicarbonyl compound
Alcohol
Product
Yield (%)b
90
2
3
4
84
94
68
5
6
93
96
7
8
82
96
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Table 2 (Contd.)
Entry
9
Dicarbonyl compound
Alcohol
Product
Yield (%)b
96
10
98
a Reaction conditions: TfOH–SiO2 (60 mg, containing 0.03 mmol TfOH), dicarbonyl compound (3.0 mmol), alcohol or alkene (1.0 mmol), solvent-free,
70 ◦C, 5 h. b Isolated yield based on alcohol or alkene used.
the nucleophiles to react with benzhydrol 2f, and the products
3i and 3j were obtained in 96% and 98% yields, respectively
(Table 2, entries 9 and 10).
(Table 3, entry 10).19,20 The TfOH–SiO2 catalyzed addition of
b-diesters to the alcohols was, unfortunately, not successful
under the present reaction conditions. As a further attempt to
expand the versatility of the solid acid TfOH–SiO2, we tried
the addition of 1f to the styrene 2j. We were delighted that the
reaction proceeded well, to afford the addition products 3n in
moderate yield (Table 3, entry 11). Moreover, the addition of
1f to 2-norbornene 2k catalyzed by TfOH–SiO2 also smoothly
produced the product 3s in 80% yield just after 5 h, while much
longer reaction time is needed in other homogeneous Brønsted
acid catalyzed reactions (Table 3, entry 12).21
When the solid b-diketones benzoylacetone 1e and 1,3-
diphenyl-1,3-propanedione 1f were used as the nucleophiles,
a small amount of CH3NO2 was needed as a solvent to
dissolve the solid reactants. The reaction of 1e and 2a afforded
the product 3k in 98% yield (Table 3, entry 1), while the
homogeneous reaction catalyzed by 5 mol% TfOH gave only
an 80% yield.26 The reaction of 1e with the alcohol 2d, which
bears an electron-withdrawing chloride on the aromatic ring,
generated the product 3l as two inseparable diastereoisomers in
94% yield (Table 3, entry 2). The reaction of 1e and benzhydrol
2f also showed excellent 98% yield (Table 3, entry 3).
Conclusions
When the b-diketone 1f with two phenyl groups was reacted
with the benzylic alcohols 2a, 99% yield was obtained after
5 h (Table 3, entry 4), whereas the reaction of 1f and 2a only
afforded 70% yield in the 5 mol% TfOH-catalyzed homogeneous
reaction.26 The reaction of b-diketone 1f with the benzylic
alcohols 2b and 2d both gave high yields, although one alcohol
bears electron-donating groups and the other bears electron-
withdrawing groups on the aromatic ring (Table 3, entries 5,
6). As for the reactions of 1f with 1-(2-naphthyl)ethanol 2e and
benzhydrol 2f, the corresponding products 3q and 3r were both
produced in excellent 98% or 97% yields (Table 3, entries 7, 8).
It is noteworthy that the reaction of 1f with exo-norborneol 2h
was accomplished after 5 h in a moderate yield (Table 3, entry
9), whereas a much longer time was needed in the previously
reported Brønsted acid-catalyzed reaction due to the rigid
structure of norborneol.21 When the benzyl alcohol 2i was used
as the reactant for this heterogeneous reaction, we were delighted
that the addition product 3t was isolated in 65% yield, which to
the best of our knowledge is the first example of the Brønsted
acid-catalyzed addition of a b-diketone to a primary alcohol
In summary, we have developed and optimized an easy-to-
handle, efficient and recyclable solid acid catalyst through direct
absorption of TfOH onto very cheap chromatographic silica gel.
This silica gel supported TfOH (TfOH–SiO2) has been applied
as the catalyst for the direct addition of various b-dicarbonyl
compounds to a series of alcohols or alkenes. The reactions can
be conveniently performed in air under solvent-free conditions
or in nitromethane, and usually give moderate to excellent yields
with H2O as the only by-product. In comparison with the
homogeneous reactions catalyzed by TfOH, the heterogeneous
reactions give better reaction yields with lower catalyst loading
in many cases. Moreover, the catalyst TfOH–SiO2 can be readily
recovered and reused in multiple consecutive catalytic runs
(up to 6 runs) with almost maintained reactivity and yields.
This TfOH–SiO2 catalyzed addition of b-dicarbonyl compounds
to alcohols or alkenes provides an efficient, environmentally
friendly protocol, which possesses the potential for application
in industry. Further investigations to expand the usage of this
readily available TfOH–SiO2 catalyst to more catalytic organic
reactions are underway in our lab.
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a
Table 3 Reactions of b-diketones with alcohols or alkenes catalyzed by TfOH–SiO2
Entry
1
Dicarbonyl compounds
Alcohol
Product
Yield (%)b
98 (1 : 0.8)c
2
3
94 (1 : 0.7)c
98
4
5
99
92
6
7
8
99
98
97
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Table 3 (Contd.)
Entry
9
Dicarbonyl compounds
Alcohol
Product
Yield (%)b
70
10
11d
12
65
65
80
a Reaction conditions: TfOH–SiO2 (60 mg, containing 0.03 mmol TfOH), dicarbonyl compound (3.0 mmol), alcohol or alkene (1.0 mmol), 2 mL
CH3NO2, 70 ◦C, 5 h unless noted. b Isolated yield based on alcohol or alkene used. c The ratio of the diastereoisomers was determined by 1H NMR.
d The reaction time was 10 h.
stirred magnetically for 60 min at r.t. The Et2O was removed
Experimental
under reduced pressure and the residue was dried at 110 ◦C for
1-(4-Methylphenyl)ethanol 2b, 1-(4-methoxyphenyl)ethanol 2c,
1-(4-chlorophenyl)ethanol 2d, 1-(2-naphthyl)ethanol 2e and
1,3-diphenyl-2-propenol 2g were prepared by reduction of the
corresponding ketone precursors with NaBH4 in methanol.
Chromatographic silica gel used as the support for TfOH was
obtained from Qingdao Haiyang Chemical Co. Ltd. The other
reagents were obtained from Aldrich and were used without
2 h to afford TfOH–SiO2 (0.5 mmol g-1) as a white powder.
Typical procedure for the catalytic reaction of b-dicarbonyl
compounds with alcohols (or alkenes)
A mixture of the b-dicarbonyl compound (3.0 mmol) and
alcohol (or alkene, 1.0 mmol) was combined and TfOH–SiO2
(60 mg, containing 0.03 mmol TfOH) was added. The mixture
was stirred at 70 ◦C and monitored by TLC. After the completion
of the reaction, CH2Cl2 (1 mL) was added and the mixture was
stirred for 1 min, then the reaction was centrifuged (2000 rpm)
for 1–2 min and the solution removed with a syringe. The
catalyst was then washed with CH2Cl2 (1 mL) twice and the
solution was removed; a new reaction could be conducted by
adding a new batch of b-dicarbonyl compound (3.0 mmol) and
alcohol (or alkene, 1.0 mmol) to the recovered catalyst. The
solution containing the product was passed through a flash
chromatography column on silica gel to afford the product.
1
further purification. H NMR spectra were obtained using a
Bruker DPX-400 spectrometer. Chemical shifts (d, ppm) were
reported using TMS as the internal standard. 13C{ H} NMR
1
spectra were recorded with a Bruker DPX-400 spectrometer
at 100.61 MHz; chemical shifts were internally referenced to
CHCl3 (d = 77.0 ppm). Mass spectrometry was carried out with
a Finnigan MAT 95S mass spectrometer and HRMS was carried
out using a Waters Micromass Q-Tof-2 instrument. Melting
points were determined on a Barnstead Electrothermal 9100
apparatus and are uncorrected.
Preparation of the silica gel supported triflic acid (TfOH–SiO2)
2-Acetyl-2-benzhydrylcyclopentanone (3h). White solid, Mp:
156–158 ◦C; 1H NMR (400 MHz, CDCl3) d 1.31-1.40 (m, 1H),
1.63-1.76 (m, 2H), 2.05-2.22 (m, 5H), 3.12-3.17 (m, 1H), 5.34
To a suspension of silica gel (10.0 g, 200–300 mesh) in Et2O
(35 mL), TfOH (0.765 g, 5 mmol) was added. The mixture was
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(s, 1H), 7.00-7.02 (m, 2H), 7.16-7.30 (m, 8H); 13C NMR
(100 MHz, CDCl3) d 19.6, 25.9, 27.4, 38.9, 55.1, 74.3, 126.9,
128.3, 128.8, 128.9, 129.8, 140.4, 202.6, 215.2; HRMS (+ESI)
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+
calcd for C20H20O2 (M+): 292.1458, found: 292.1460.
Acknowledgements
This work was supported financially by NSFC (Project
No. 20902020), the Shanghai Pujiang Talent Program (Project
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