Highly efficient microwave-assisted substitution of β-dicarbonyl compounds with secondary alcohols using fluoroboric acid as the catalyst

Article abstract of DOI:10.1002/cjoc.201180359

The microwave-assisted substitution of β-dicarbonyl compounds with secondary alcohols has been achieved efficiently, using very cheap fluoroboric acid (HBF₄) as the catalyst. For various β-dicarbonyl compounds and a series of secondary alcohols

Full text of DOI:10.1002/cjoc.201180359

FULL PAPER
Highly Efficient Microwave-Assisted Substitution of
β-Dicarbonyl Compounds with Secondary Alcohols
Using Fluoroboric Acid as the Catalyst
Liu, Peinian*,a(刘培念)
Xia, Feia(夏斐)
Ren, Yujieb(任玉洁)
Chen, Junqina(陈君琴)
a Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and
Technology, Shanghai 200237, China
b
School of Chemistry and Environmental Engineering, Shanghai Institute of Technology, Shanghai 200235, China
The microwave-assisted substitution of β-dicarbonyl compounds with secondary alcohols has been achieved ef-
ficiently, using very cheap fluoroboric acid (HBF4) as the catalyst. For various β-dicarbonyl compounds and a series
of secondary alcohols, the direct substitution gives high yields after only 5 min of microwave irradiation.
Keywords substitution, alcohols, acid catalysis, dicarbonyl compounds, microwave chemistry
Introduction
tive catalyst for the substitution of β-diketones with
secondary alcohols, and illustrated unambiguously that
the mechanism of the Brønsted acid catalyzed substitu-
tion is an SN1 mechanism.15 Moreover, we have suc-
cessfully applied the silica gel supported TfOH as the
efficient catalyst to the heterogeneous substitution of
β-diketones with secondary alcohols.16
The carbon-carbon bond linkage is one of the most
fundamental strategies for the construction of molecular
framework in organic chemistry.1 The direct substitu-
tion of β-dicarbonyl compounds with alcohols has at-
tracted much attention in recent years because of the
feature of green chemistry. In this reaction, only H2O is
generated as a side product, and the preparation of the
active intermediates in the traditional protocol, such as
organometallic compounds and halides or a related spe-
cies, are not required.2 Since the pioneering work of
Baba et al. in 2006,3 the Lewis acidic metal catalysts,
Although remarkable progress has been achieved,
the substitution of β-dicarbonyl compounds with sec-
ondary alcohols catalyzed by various catalysts usually
consumes rather long reaction time (several hours to
dozens of hours) to achieve high yields in the traditional
thermal heating methods. In recent years, microwave
irradiation (MW) has emerged as a powerful technique
to promote a variety of chemical reactions.17 Compared
with the conventional thermal heating method, con-
trolled microwave heating approach often dramatically
reduces reaction time, increases product yields and en-
hances product purity by reducing unwanted side reac-
tions. The microwave-assisted substitution of
β-dicarbonyl compounds with secondary alcohols is still
very rare.18 Herein, we report the microwave-assisted
substitution of β-dicarbonyl compounds with secondary
alcohols in the presence of fluoroboric acid (HBF4) as
the catalyst.
such as InCl3,3 InBr3,4 FeCl3,5 Bi(OTf)3 and Ln(OTf)3
6
(Ln＝La, Yb, Sc, Hf),7 have been reported as effective
catalysts for the direct substitution of β-dicarbonyl
compounds with allylic and benzylic alcohols. Recently,
we have successfully applied a perchloric ruthenium
complex to the substitution of β-diketones with secon-
dary alcohols.8 Further mechanistic investigations of
this Lewis acid catalyzing substitution reveal that the
ruthenium complex reacts first with the β-diketone to
form a stable β-diketone chelate ruthenium complex and
concomitantly yields an equivalent of perchloric acid,
which has been proved to be the true catalyst for the
substitution reaction.
Recently, molecular iodine9 and various Brønsted
acids, such as dodecylbenzenesulfonic acid,10 p-toluene-
sulfonic acid,11 phosphotungstic acid,12 H-montmorillo-
nite13 and triflic acid (TfOH)14 have been disclosed to
be the effective catalysts for the substitution of
β-diketones with benzylic alcohols. As a very recent
progress, we have demonstrated that HClO4 is an effec-
Experimental
1-Phenyl-3-p-tolylpropane-1,3-dione (1c) was pre-
pared according to the literature.19 1-(4-Fluorophenyl)-
ethanol (2b), 1-(4-chlorophenyl)ethanol (2c), 1-(4-
methylphenyl)ethanol (2d), 1-(naphthalene-3-yl)etha-
nolhydrate (2e), benzhydrol (2f) and 1,3-diphenyl-2-
* E-mail: liupn@ecust.edu.cn; Tel.: 0086-021-64250552; Fax: 0086-021-64252758
Received December 1, 2010; revised March 21, 2011; accepted May 10, 2011.
Project supported by the National Natural Science Foundation of China (No. 20902020), the Shanghai Pujiang Talent Program (No. 09PJ1403500)
and the Fundamental Research Funds for the Central Universities.
Chin. J. Chem. 2011, 29, 2075— 2080
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Liu et al.
FULL PAPER
propenol (2g) were prepared by reductions of the corre-
sponding ketone precursors with NaBH4 in methanol.
The other reagents were obtained from Aldrich and used
without further purification. The microwave irradiation
reactions were performed with Microprocessor-Con-
troled Microwave Chemical Reactor (2450 MHz, 650 W)
manufactured by Nanjing Lingjiang Scientific Company.
1H NMR spectra were obtained from a Bruker DPX-400
spectrometer. Chemical shifts (δ) were reported using
TMS as the internal standard. 13C NMR spectra were
recorded with a Bruker DPX-400 spectrometer at
100.61 MHz; chemical shifts were internally referenced
to CDCl3 (δ＝77.0). HRMS was carried out with Waters
Micromass Q-Tof-2. Melting points were determined on
a Barnstead Electrothermal 9100 apparatus and were
uncorrected.
from the unisolable two compounds. White solid, m.p.
1
92—95 ℃ (petroleum ether); H NMR (CD3COCD3,
400 MHz) δ: 1.32 (t, J＝6.8 Hz, 3H), 2.29 (s, 1.5H),
2.38 (s, 1.5H), 3.93—4.01 (m, 1H), 6.12 (d, J＝10.0 Hz,
1H), 7.03—7.07 (m, 1H), 7.14—7.18 (m, 3H),
7.31—7.42 (m, 4H), 7.46—7.53 (m, 1.5H), 7.61—7.64
(m, 0.5H), 7.79 (d, J＝8.0 Hz, 1H), 7.88 (d, J＝8.0 Hz,
1H), 8.08 (d, J＝8.0 Hz, 1H), 8.16 (d, J＝8.0 Hz, 1H);
13C NMR (CDCl3, 100 MHz) δ: 20.3, 21.6, 21.7, 41.1,
64.8, 126.6, 127.7, 128.4, 128.5, 128.7, 128.8, 129.0,
129.2, 129.6, 133.0, 133.5, 134.4, 134.7, 137.0, 137.2,
143.9, 144.0, 144.6, 194.1, 194.5, 194.7, 195.1. HRMS-
＋
ESI calcd for C24H22NaO2 [M＋Na＋ ]: 365.1512,
found 365.1520.
2-Benzhydryl-1-phenyl-3-p-tolypropane-1,3-dione
(3m) White solid, m.p. 174—176 ℃ (petroleum
ether); 1H NMR (CDCl3, 400 MHz) δ: 2.34 (s, 3H), 5.32
(d, J ＝11.6 Hz, 1H), 6.32 (d, J ＝11.6 Hz, 1H),
7.03—7.07 (m, 2H), 7.12—7.16 (m, 6H), 7.24—7.26
(m, 4H), 7.30—7.34 (m, 2H), 7.44—7.48 (m, 1H), 7.75
(d, J＝7.2 Hz, 2H), 7.82 (d, J＝7.2 Hz, 2H); 13C NMR
(CDCl3, 100 MHz) δ: 21.6, 52.4, 62.3, 126.6, 128.3,
128.5, 128.6, 128.8, 129.3, 133.2, 134.5, 137.1, 141.8,
Typical procedure for the microwave-assisted sub-
stitution of β-dicarbonyl compounds with alcohols
catalyzed by HBF4
A mixture of β-dicarbonyl compound (1.5 mmol)
and alcohol (1.0 mmol) in nitromethane (2 mL) was
added HBF4 (7.8 µL, 40% aqueous solution, 0.05 mmol),
and the resulting mixture was stirred under the irradia-
tion of microwave (650 W, 70% with 30% interval) for
5 min. Then the solvent of the reaction mixture was re-
moved under reduced pressure and the residue was
passed through the flash column chromatography on
silica gel to afford the product.
＋
144.2, 193.5, 194.2. HRMS-ESI calcd for C29H24NaO2
[M＋Na＋]: 427.1669, found 427.1678.
Results and discussion
As the first stage to screen the effective Brønsted
acid catalysts and the reaction conditions under MW
(650 W, 70% with 30% interval), we chose 3-diphenyl-
propane1,3-dione (1a) and 1-phenylethanol (2a) as the
substrates for the model reaction. It can be seen from
Table 1 that 1a and 2a did not react under MW without
the catalyst (Table 1, Entry 1). When various Brønsted
acids such as CH3COOH, CF3COOH, 12-phosphomo-
lybdic acid (PMA), HNO3, HCl and CH3SO3H were
used as the catalysts, the yields of the reactions of 1a
and 2a varied remarkably in 13%—88% after 10 min of
MW (Table 1, Entries 2—7). Surprisingly, when
TfOH14 and HClO4,15 which are effective catalysts un-
der thermal heating conditions, were used as the cata-
lysts for the reaction of 1a and 2a under MW, only 22%
and 38% yields were obtained, respectively (Table 1,
Entries 8, 9). As HBF4 was applied as the catalyst for
the reaction of 1a and 2a, we were gratified to obtain
99% isolated yield after 10 min of MW (Table 1, Entry
10). Then the influence of solvent for the
HBF4-catalyzed reaction of 1a and 2a was investigated
under MW, and CH3NO2 stands out as the solvent of
choice. The reactions in other solvents tested only af-
forded negligible product except that the reaction in
CH2Cl2 gave moderate 52% yield (Table 1, Entries
11—16). If the catalyst loading of HBF4 was decreased
to 1 mol%, the yield of the reaction decreased to 53%
after 10 min of MW (Table 1, Entry 17). At last, the
molar ratio of 1a to 2a was reduced from 3∶1 to 1.5∶
3a (CAS No. 116140-58-0), 3b (CAS No.
960391-72-4), 3c (CAS No. 727401-27-6), 3d (CAS No.
945548-16-3), 3e (CAS No. 1129424-70-9), 3f (CAS
No. 60999-93-1), 3g (CAS No. 258881-76-4), 3h (CAS
No. 5870-49-5), 3i (CAS No. 727401-25-4), 3k (CAS
No. 33925-42-7), 3n (CAS No. 171927-27-8), 3o (CAS
No. 19289-28-2) and 3p (CAS No. 60999-96-4) are
known compounds and the spectral data of the new
compounds 3j, 3l, 3m are listed below.
2-(1-(Naphthalen-6-yl)ethyl)-1-phenylbutane-1,3-
dione (3j) Characterized as a 1∶0.5 diastereomeric
mixture and the spectral data reported here are arisen
from the unisolable two compounds. White solid, m.p.
1
132—135 ℃ (petroleum ether); H NMR (CDCl3, 400
MHz) δ: 1.29 (d, J＝7.2 Hz, 3H), 1.38 (d, J＝7.2 Hz,
1.5H), 1.92 (s, 3H), 2.28 (s, 1.4H), 4.01—4.10 (m,
1.5H), 4.94 (d, J＝11.2 Hz, 0.5H), 5.03 (d, J＝11.2 Hz,
1H), 7.29—7.40 (m, 2H), 7.42—7.54 (m, 6H), 7.60—
7.71 (m, 4H), 7.77—7.84 (m, 4H), 8.11—8.14 (m, 2H);
13C NMR (CDCl3, 100 MHz) δ: 20.4, 21.7, 27.5, 27.9,
40.4, 41.0, 70.8, 71.5, 125.5, 125.6, 125.8, 125.9, 126.2,
127.5, 127.7, 128.2, 128.5, 128.6, 128.7, 128.9, 132.3,
132.5, 133.4, 133.5, 133.6, 133.9, 136.9, 137.2, 140.7,
141.0, 195.1, 195＋.2, 203.1, 203.7; HRMS-ESI calcd for
C22H19O2 [M－H ]: 315.1391, found 315.1382.
1-Phenyl-2-(1-phenylethyl)-3-p-tolylpropane-1,3-
dione (3l) Characterized as a 1∶1 diastereomeric
mixture and the spectral data reported here are arisen
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Chin. J. Chem. 2011, 29, 2075— 2080

Highly Efficient Microwave-Assisted Substitution of β-Dicarbonyl Compounds
1 and the reaction time was reduced to 5 min, and simi-
lar 95% yield was still obtained under MW (Table 1,
Entries 18, 19). Note that the reaction of 1a with 2a did
not proceed in the absence of microwave heating even
after 1 h.
98% and 95% yields were obtained (Table 2, Entries 2,
3). But the reaction of 1a with 1-(4-nitrophenyl)ethanol
with a much stronger electron-withdrawing substituent
did not proceed, possibly because the strong elec-
tron-withdrawing substituent on the aromatic ring re-
duced the stability of the corresponding carbon cation,
the intermediate.15 The reaction of 1a with the benzylic
alcohol 2d bearing an electron-donating methyl group
gave only 70% yield (Table 2, Entry 4). When
1-(4-methoxyphenyl)ethanol was used as the substrate,
its reaction with 1a gave very low yield, probably be-
cause the methoxyl substitution on aromatic systems is
unstable under strong Brønsted acid conditions.20 The
reactions of 1a with 1-(2-naphthyl)ethanol (2e) and
benzhydrol 2f proceeded smoothly to generate the
products 3e and 3f in excellent yields (Table 2, Entries 5,
6). When the allylic alcohol 2g was applied to the reac-
tion with 1a, the allylation product 3g was generated in
70% yield (Table 2, Entry 7). Moreover, when the
β-diketone benzoylacetone 1b was employed in the re-
actions with secondary alcohols 2a, 2c, 2e and 2f, high
yields (84—94%) of the corresponding products were
obtained (Table 2, Entries 8—11). When the unsym-
metrical β-diketone 1c bearing an electron-donating
methyl group on one aromatic ring was used as the nu-
cleophile to react with 2a and 2f, the substitution prod-
ucts 3l, 3m were obtained in excellent yields (Table 2,
Entries 12, 13). Furthermore, the reaction of acetylace-
tone 1d with allylic alcohol 2g could also proceed
smoothly to provide the product 3n in 88% yield (Table
2, Entry 14). At last, with the hope to expand the sub-
strate scope, β-keto esters 1e and 1f were used as the
nucleophiles to react with benzhydrol 2f, and the prod-
ucts 3o and 3p were obtained successfully both in ex-
cellent 98% yields (Table 2, Entries 15 and 16). It is
noteworthy that under the present conditions, the reac-
tion of 1a with the primary alcohol such as benzylic
alcohol did not proceed and the reaction of 1a with the
propargylic alcohol such as 1,3-diphenylprop-2-yn-1-ol
only afforded rather low 52% yield.
Table 1 Microwave-assisted reactions of 1a and 2a using
Brønsted acids as catalysts under various conditionsa
OH
O
O
Bronsted acid catalysts
MW
+
Ph
Ph
1a
2a
O
O
Ph
Ph
3a
Entry
1
Catalyst
None
Solvent
n(1a)∶ n(2a) Yieldb/%
CH3NO2
3∶
3∶
3∶
3∶
3∶
3∶
3∶
3∶
3∶
3∶
3∶
3∶
3∶
3∶
3∶
3∶
3∶
1∶
1.5∶
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
＜
5
2
CH3COOH CH3NO2
CF3COOH CH3NO2
13
16
25
65
74
88
22
38
99
52
16
＜
3
4
PMA
HNO3
HCl
CH3NO2
CH3NO2
CH3NO2
5
6
7
CH3SO3H CH3NO2
8
TfOH
HClO4
HBF4
HBF4
HBF4
HBF4
HBF4
HBF4
HBF4
HBF4
HBF4
HBF4
CH3NO2
CH3NO2
CH3NO2
CH2Cl2
DCE
9
10
11
12
13
14
15
16
17c
18
19d
Toluene
Dioxane
Methanol
THF
5
5
5
5
＜
＜
＜
CH3NO2
CH3NO2
CH3NO2
53
92
95
1
Conclusions
a Reaction conditions: catalyst (0.05 mmol), 2a (1.0 mmol), sol-
In summary, we have successfully developed a
highly efficient protocol for the direct substitution of
β-dicarbonyl compounds with secondary alcohols using
very cheap HBF4 as the catalyst under microwave heat-
ing conditions. For various β-dicarbonyl compounds
and a series of secondary alcohols, the reactions afford
high yields in most cases after only 5 min. Moreover, it
is the first time that HBF4 was discovered to be the ef-
fective catalyst in the substitution of β-dicarbonyl com-
pounds with alcohols.
vent (2 mL), 10 min of MW at 650 W (70% with 30% interval),
unless noted. Isolated yields based on 2a. HBF4 (0.01 mmol)
was used. d MW time: 5 min.
b
c
Subsequently, the microwave-assisted direct substi-
tution of various β-dicarbonyl compounds with a series
of alcohols was examined in CH3NO2 using 5 mol%
HBF4 as the catalyst. The results after 5 min of MW
(650 W, 30% interval) are summarized in Table 2.
When β-dikeone 1a reacted with the benzylic alcohol 2b
and 2c bearing a fluoride or a chloride group, excellent
Chin. J. Chem. 2011, 29, 2075— 2080
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Liu et al.
FULL PAPER
a
Table 2 Microwave-assisted reactions of β-dicarbonyl compounds and various alcohols catalyzed by HBF4
O
O
OH
O
O
R1
5 mol% HBF4, CH3NO2
5 min, MW
R2
R3
R1
+
R2
R4
R3
R4
Entry
1
β-Dicarbonyl compound
Alcohol
Product
Yieldb/%
95
O
O
OH
O
O
Ph
Ph
Ph
Ph
Ph
1a
1a
2a
3a
O
O
OH
O
O
Ph
Ph
Ph
2
3
4
98
95
70
92
F
2b
F
3b
O
O
OH
O
O
O
O
O
O
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Cl
1a
1a
1a
2c
Cl
3c
O
O
OH
Ph
Me
2d
Me
3d
O
O
OH
Ph
O
Ph
5
6
2e
3e
O
OH
O
O
O
O
Ph
Ph
95
70
Ph
Ph
Ph
Ph
1a
1a
2f
3f
O
O
OH
Ph
Ph
7
8
Ph
Ph
Ph
Ph
2g
3g
O
O
OH
O
O
Ph
84
(1∶ 0.8)c
Ph
1b
1b
2a
3h
O
O
OH
O
O
Ph
90
(1∶ 0.8)c
9
Ph
Cl
2c
Cl
3i
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Chin. J. Chem. 2011, 29, 2075— 2080

Highly Efficient Microwave-Assisted Substitution of β-Dicarbonyl Compounds
Continued
Yieldb/%
Entry
10
β-Dicarbonyl compound
Alcohol
Product
O
O
OH
O
O
Ph
92
(1∶ 0.7)c
Ph
Ph
1b
2e
OH
2f
3j
O
O
O
O
Ph
11
12
94
1b
3k
O
O
OH
O
O
Ph
93
(1∶ 1)c
Ph
1c
2a
3l
O
O
OH
O
O
Ph
Ph
13
14
15
93
88
98
1c
2f
3m
O
O
OH
O
O
Ph
Ph
Ph
2g
OH
2f
1d
Ph
3n
O
O
O
O
OEt
OEt
1e
3o
O
O
OH
O
O
Ph
OEt
16
98
Ph
OEt
1f
2f
3p
a Reaction conditions: HBF4 (0.05 mmol), β-diketone (1.5 mmol), alcohol (1.0 mmol), MeNO2 (2 mL), 5 min of MW at 650 W (70% with
30% interval). b Isolated yield based on the alcohols used. c The ratio of the diastereomers was determined by 1H NMR.
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Chin. J. Chem. 2011, 29, 2075— 2080