Prussian blue as an efficient catalyst for rate accelerations in the transesterification of β-ketoesters

Article abstract of DOI:10.1080/15533174.2013.799488

Prussian blue triggered transesterification of ethylacetoacetate with various alcohols underwent efficiently. The reaction is mild, eco-friendly, and selective with good yields. The proposed reaction pathway depicts the formation of an intermediate by the interaction of β-ketoesters with catalytic site of the Prussian blue, followed by nucleophilic attack of the alcohol at the electrophilic center followed by successive elimination of the proton to give the product. Observed longer reaction times under conventional conditions reduced amazingly under sonication and microwave irradiation followed enhanced yield of products.

Full text of DOI:10.1080/15533174.2013.799488

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Prussian blue as an efficient catalyst for Rate
accelerations in the transesterification of β-Ketoesters
a
a
a
a
b
P. Srinivas , K. C. Rajanna , G. Krishnaiah , M. Satish Kumar & J. Narender Reddy
a
Department of Chemistry , Osmania University , Hyderabad , 500 007 , AP , India
b
Department of Chemistry , Govt. City College , Hyderabad , 500 001 , AP , India
Accepted author version posted online: 24 Sep 2013.Published online: 24 Sep 2013.
To cite this article: Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry (2013): Prussian blue as
an efficient catalyst for Rate accelerations in the transesterification of β-Ketoesters, Synthesis and Reactivity in Inorganic,
Metal-Organic, and Nano-Metal Chemistry, DOI: 10.1080/15533174.2013.799488
To link to this article: http://dx.doi.org/10.1080/15533174.2013.799488
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Prussian blue as an efficient catalyst for Rate accelerations in the transesterification of β-
Ketoesters
1
1
1
1
2
P. Srinivas , K. C. Rajanna *, G. Krishnaiah , M. Satish Kumar , J. Narender Reddy
1
Department of Chemistry, Osmania University, Hyderabad-500 007, A. P (India)
2
Department of Chemistry, Govt. City College, Hyderabad-500 001, A. P (India)
E-mail:kcrajannaou@yahoo.com
Abstract:
Prussian blue triggered transesterification of ethylacetoacetate with various alcohols underwent
efficiently. The reaction is mild, eco-friendly and selective with good yields. The proposed
reaction pathway depicts the formation of an intermediate by the interaction of β-ketoester with
catalytic site of the Prussian blue, followed by nucleophilic attack of the alcohol at the
electrophilic center followed by successive elimination of the proton to give the product.
Observed longer reaction times under conventional conditions reduced amazingly under
Sonication and Microwave irradiation followed enhanced yield of products.
Keywords: Prussian blue; efficient catalyst; transesterification; β-keto ester; sonication;
microwave irradiation
1
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Introduction
Transesterification is the process of exchanging the alkoxy group of an ester with the alkoxy
group of an alcohol. These reactions are often catalyzed by the addition of an acid or base
catalyst. The largest scale application of transesterification is in the synthesis of polyesters, in
[1]
which diesters undergo transesterification with diols to form macromolecules
.
This process has
[2]
been used to recycle polyesters into individual monomers. Recent findings of Marchetti et al
show that triglycerides and sulfuric acid were suitable to perform the transesterification and
direct esterification reaction to increase biodiesel production. Transesterification of ethyl
acetoacetate with higher alcohols  has been reported using molecular sieves, in which they
function as absorbents of alcohols. Over the years, quite a good amount of research has been and
[4-20]
is being pursued at present all over the world on the transesterification reactions
. At times
the attempts met with fruitful results, and sometimes the attempts demanded improvisations to
meet the demands of the chemical industry and bio-diesel processing. The contribution of
coordination complexes in the field of catalysis research has come a long way from the time of
[21]
Werner and Jorgensen
and is still marching ahead. The current trends in the area of catalysis
include the use of new types of catalysts that combine the practical advantages of both
homogeneous as well as heterogeneous catalysts, achieving high yields and selectivity, and safe
guard environmental safety. The β-ketoesters are versatile organic intermediates and have
practical importance in preparative organic synthesis and can be used in lubricants as efficient
[22]
[23]
friction modifiers
as potential antiviral drugs
and catalysts for various stereo selective
[24]
reactions . Prussian blue (PB) is a blue colored pigment and one of the most frequently used
[25]
inorganic dyes. It is used in the preparation
of paints, printing inks, and laundry dye etc. It
2
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exists as water soluble (KFe{Fe(CN)
6
}xH
2
O) and water insoluble (Fe
4
{Fe(CN)
6
}
3
yH
2
O)
[26]
forms . The micro-porous character of PB and its analogues find applications as adsorbent, as
[27]
[28]
molecular sieves for separation
and catalytic processes . It is used as an efficient tool for
[29]
the removal of heavy metal ions in wine production . Literature reports of PB indicated its
[30]
electrochemical applications for battery building , electronic switching and as electro chromic
[31]
devices . Moreover, Prussian blue complex (PB) is can be easily prepared from economically
cheap and laboratory desktop chemicals which are available even in undergraduate laboratories.
Recently our group explored the utility of Werner type complexes as efficient catalyst for
[32, 33]
selective nitration of aromatic compounds
. On the other hand we have also reported the
[34]
Chemo selective transesterification
of β-Keto Esters using Manganese (II) salts as efficient
Catalysts. Keeping in with these economically viable ongoing synthetic protocols, we want to
present certain results on transesterification using PB as catalyst under conventional (stirring at
[35]
[36]
room temp) and non-conventional methods such as Microwave irradiation
and Sonication
.
Results and discussion
O
O
O
O
1
R
Prussian Blue
1
R
+
+
OH
OH
O
O
(
i)Conventional/Reflux
ii)Sonication
(
(
iii)MW
In order to optimize the conditions, control experiments were performed with a reaction
mixture containing ethyl acetoacetate (5.0 mmol), phenol (5.0 mmol) and varied amounts of
3
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catalyst (P B) in the range of 0.2 to 3.0 mmol in toluene (20mL). Reaction mixture was stirred at
o
1
00-110 C in a round bottom flask, which was provided with a distillation condenser to remove
ethanol. Progress of the reaction was monitored by thin layer chromatography (TLC). Reactions
afforded very good yield of product only when 1.0 mmol catalyst was used. Reaction was too
sluggish below 1.0 mmol even after 24 hrs and under reflux conditions, and above 1.0 mmol
reaction times and yield of the product did not differ much as shown in table -1.
In view of these observations, we have performed the under ultrasonic and microwave
assisted (USA and MWA) conditions. A wide range of alcohols were used to explore PB
catalysed transesterification of β-Keto Esters. Experimental results are summarized in table-2.
Even though the reaction times are too long, very good yields of products are obtained.
[37]
According to Pearson’s HSAB theory , “Hard acids (HA) prefer to bind to hard bases (HB) and
soft acids (SA) prefer to bind to soft bases (SB)”. Fe (III) is a hard acid with sufficient number of
vacant orbitals can easily form adducts with hard bases such as ethyl acetoacetate and hydroxy
compounds. Ethyl acetoacetate being a harder base than hydroxy compound interaction between
hard acid Fe (III) present in PB and hard base ethyl acetoacetate through two carbonyl oxygen
atoms to form a cyclic intermediate is more likely. The cyclic intermediate thus formed then
probably reacts with hydroxy compound and affords product with the elimination of ethyl alcohol
and PB catalyst. Reaction sequence of the proposed mechanism is shown in Scheme-2. Further,
it is interesting to note that the catalyst could be recycled without any problem.
In order to have an insight into the mechanistic aspects, IR and UV-Visible spectroscopic studies
have been taken up. IR spectrum of ester (Fig.1 in the absence of catalyst) depicted a strong band
-1
-1
at 1735 cm in addition to a small hump around 1640 cm corresponding to carbonyl groups.
4
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Since the transesterification reaction is studied in presence of alcohol, IR spectrum of ester was
also scanned in presence of alcohol [isopropyl alcohol (IPA) as a specific case]. The IR spectrum
-1
of ester in presence of IPA (fig.2) furnished a broad band at 3705 cm (corresponding to O-H
-
1
stretching vibration), 1715and 1739 cm (attributed to carbonyl groups). However, it is
-1
-1
important to note that the band observed at 1640 cm (in fig.1) shifted to 1715 cm , which may
probably indicate the formation of enolate intermediate. This observation is in accordance with
[38]
literature reports
. In order to further strengthen the mechanism, UV-Visible spectroscopic
studies of PB were taken up in presence and absence of ester and IPA mixture (fig-3). The
spectrum of PB (observed around 700nm) underwent a blue shift in presence of ester and IPA
with a new band at 450 nm. This observation lends support for the formation of a cyclic
intermediate due to the interaction of enolate species with PB. The intermediate thus formed
1
further experiences the alkoxy (R -O) group attack at ester carbonyl before yielding final
product as shown in Scheme- 2.
Comparison of the results obtained under three procedures made it possible to draw some general
conclusions regarding the effect of sonication and MW irradiation on the transesterification. Data
presented in table -2 clearly indicate that even though the yields of end products are by and large
are similar with very good percentage, there is a remarkable change in the rates/ reaction times
from conventional method to sonication and/or microwave irradiation. Reaction times reduced
by almost ten times from conventional to Sonication conditions. This remarkable rate
acceleration could be attributed to the “Cavitation phenomenon”, which is generated during
ultra-sonication. A large amount of energy and pressure are released from the collapse of
[29-32]
cavitation bubbles during sonication of the reaction
, which may cause an increase in the
5
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fraction of activated molecules compared with normal reactions resulting in higher rates of the
reaction. On the other hand MW irradiation generates volumetric nature of power dissipation in
[33-35]
dielectric, which causes direct heating inside the sample
. This in situ mode of energy
conversion leads to a fast heating rate with minimized thermal gradients. This leads to the
reduction of the reaction times rapidly as observed in the present study from several hours (16-24
hrs) in conventional reaction to few minutes (45 min) in MWA reaction.
Conclusions
In conclusion, the authors demonstrated that Prussian blue, a typical Werner type complex, is an
efficient catalyst for transesterification of β-ketoesters. This catalyst can be prepared in any
undergraduate laboratory with inexpensive desktop chemicals. The reagent is operationally
simple and environmentally safe and thus has an advantage over many literature reports.
Ultrasonically assisted methodology reduced reaction times from 16-24 hrs (under reflux
conditions) to about 2.5 hrs; while microwave assisted reactions could complete the
transesterification within 8 to 10 minutes. These approaches are environmental friendly in terms
of Energy saving and time saving requirements.
Acknowledgment
The authors are indebted to UGC and CSIR, New Delhi (India) for financial support in the form
of fellowship (JRF) accorded respectively to P. Srinivas and G. Krishnaiah. The authors are
grateful to the constant encouragement and moral support rendered by Prof. P. K. Saiprakash
(
Former Dean, O. U Faculty of Science), Prof. T. Navaneeth Rao (Former Vice-Chancellor,
O.U), Principal, Nizam College and Head, Department of Chemistry, O.U).
Experimental
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All the chemicals used in the present study were reagent grade, which were purchased procured
from either from SD fine Chemicals (India), Ranbaxy (India) or Aldrich Chemicals. Prussian
blue complex was prepared from the reaction of ferric chloride (FeCl
3
) and potassium
ferrocyanide (K (Fe (CN) ) according to standard procedures.
4
6
General Procedure for Transesterification under conventional method: Five (5.0 mmol) of
alcohol (5.0 mmol) were added to five (5.0 mmol) of β - ketoester (5.0 mmol), and one (1.0
mmol) of Prussian blue catalyst in about twenty (20.0 mL) toluene in a round bottom (R.B) flask
0
was stirred at 100-110 C. The R. B. flask was provided with a distillation condenser to remove
ethanol. Progress of the reaction was monitored by thin layer chromatography (TLC). After
completion of the reaction, contents of the R.B. flask was filtered and the filtrate concentrated to
get crude product, which was purified by column chromatography on silica gel (ethyl acetate:
petroleum ether) to afford the ester as a viscous colorless liquid in good yields. Products are
characterized by spectroscopic data. Representative NMR spectroscopic data are presented in
table -3.
General Procedure for Ultrasonically Assisted Transesterification: Methodology for the
ultrasonically assisted transesterification is almost similar to the one discussed in the foregoing
section. Reaction mixture containing β - ketoesters (5.0 mmol), alcohol (5.0 mmol) and Prussian
blue catalyst (1.0 mmol) and toluene (20.0 mL) was placed in a Sonicator bath at room
temperature and progress of the reaction monitored by thin layer chromatography (TLC). After
completion, the reaction mixture was filtered and concentrated the filtrate to get crude product. It
was then purified by column chromatography on silica gel (ethyl acetate: petroleum ether) to get
the ester as a viscous colorless liquid in good yields.
7
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General Procedure for Microwave Assisted Transesterification: Microwave Assisted
Transesterification was performed in the MW oven with a MW irradiation source consisting of
magnetron tube operating at 2.45GHz.with power output that can be varied between 0 and 300
W. The flask containing mixture of β - ketoesters (5.0 mmol), alcohol (5.0 mmol) and Prussian
blue catalyst (1.0 mmol) in toluene (20.0 mL) was placed in MW oven. The solutions were
stirred by a magnetic stir bar to ensure high homogeneity. To prevent thermal gradients the
reactions were carried out in 5.0 mL tubes and the progress of the reaction was monitored by thin
layer.
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Table-1: Effect of [PB] on transesterification of Phenol
S.N.
[PB]
mmol)
0.10
0.25
0.50
0.75
1.00
2.00
5.00
Time Yield
(
(hr)
28
26
26
25
24
24
23
(%)
60
64
65
65
68
67
68
1
2
3
4
5
6
7
1
3
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Table -2: Prussian blue catalyzed Transesterification of β-Ketoesters under conventional
and Nonconventional conditions
Entry
Substrate
Product
Conventional
Sonication
R.T Yield
Microwave
R.T Yield
R.T
hr)
Yield
(%)
75
(
(hr)
(%)
74
(min)
08
(%)
84
1
2
.
.
HO
O
O
O
O
19
2.15
19
72
2.15
74
10
78
OH
O
O
3
4
.
.
O
O
20
24
75
68
2.15
2.00
75
70
10
10
80
75
OH
O
OH
O
O
O
Cl
5
.
22
68
2.15
70
10
75
Cl
O
O
O
HO
1
4
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6
.
OH
24
66
2.00
73
12
80
O
O
O
7
8
.
.
OH
24
22
68
66
2.15
2.15
74
72
11
10
75
80
O
O
O
O
O
OH
O
O
O
O
O
9
O
O
22
19
70
75
2.15
3.00
72
75
10
08
75
80
O
O
O
HO
O
O
1
1
0
1
O
O
OH
O
O
O
20
65
3.15
68
10
72
N
N
OH
O
O
O
1
1
2
3
OH
O
O
19
20
70
75
3.00
2.45
72
75
08
08
80
82
O
O
OH
O
O
1
5
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1
1
1
1
4
5
6
7
O
O
20
20
24
23
70
78
65
70
2.30
3.00
3.15
3.10
72
75
70
75
07
09
12
10
78
80
75
82
OH
O
OH
O
O
O
OH
O
O
O
o
o
HO
o
1
8
OH
23
72
3.15
72
10
11
80
O
O
O
1
2
9
0
OH
O
O
22
23
70
70
3.00
3.30
76
70
82
80
O
OH
O
O
10
O
1
6
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T able-3: NMR Spectroscopic Data for representative compounds
S. No
Compound name
NMR data
1
δ 4.18(t, 2H, -OCH
COCH
.32(m, 2H, c-CH
2
), 3.42(s, 2H, >CH
2
), 2.25(s, 3H, -
),
O
O
3
), 1.65(m, 2H, a-CH
2
), 1.38(m, 2H, b-CH
2
O
pentyl acetoacetate
1
2
), 0.92(t, 3H, d-CH
3
)
2
O
O
δ 4.26(t, 2H, -OCH
COCH ), 1.72(m, 2H, CH
H, CH
2
), 3.38(s, 2H, >CH
2
), 2.25(s, 3H, -
-), 0.92(t,
O
3
2
), 1.56(m, 2H, -CH
2
butyl 3-oxobutanoate
3
3
)
3
4
O
O
δ 4.32(t, 2H, -OCH
2
), 3.44(s, 2H, >CH
2
), 2.26(s, 3H, -
O
COCH ), 1.72(m, 2H, CH
3
2
), 0.96(t, 3H, CH )
3
Propyl acetoacetate
δ 7.42(m, 5H, Ar-H), 5.22(s, 2H, -OCH
2
), 3.44(s, 2H,
O
>
CH ), 2.22(s, 3H, -COCH
2
3
)
O
O
Benzyl acetoacetate
5
O
O
δ 3.92(d, 2H, -OCH
2
), 3.42(s, 2H, >CH ), 2.20(s, 3H, -
2
O
COCH ), 1.96(m, 1H, >CH-), 0.92(d, 6H, -CH )
3
3
Isobutyl acetoacetate
1
7
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6
7
8
9
O
O
δ 4.52(m, 1H, -O-CH), 3.46(s, 2H, >CH
2
), 2.25(s, 3H, -
O
COCH ), 1.24(d, 6H, -CH
3
3
)
isopropyl 3-oxobutanoate
δ 7.45(m, 4H, Ar-H), 3.46(s, 2H, >CH
CH ), 2.25(s, 3H, -COCH
2
2
2
), 2.35(s, 3H, Ar-
), 2.28(s, 3H, -
), 2.26(s, 3H, -
O
O
O
3
3
)
p-tolyl 3-oxobutanoate
δ 7.52(m, 5H, Ar-H), 3.48(s, 2H, >CH
COCH
O
O
O
3
)
phenyl 3-oxobutanoate
Cl
δ 7.55(m, 4H, Ar-H), 3.48(s, 2H, >CH
O
O
O
COCH3)
4
-chlorophenyl 3-oxobutanoate
1
0
δ 7.42-8.14(m, 7H, Ar-H), 3.40(s, 2H, >CH
H, -COCH
2
), 2.22(s,
O
O
O
3
3
)
Naphthalen-2-yl 3-oxobutanoate
1
8
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Fig.1: Infrared Spectrum of Ethylacetoacetate (EAA)
1
9
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Fig.2: Infrared Spectrum of Ethylacetoacetate (EAA) + Isopropyl alcohol (IPA)
2
0
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Fig.3: UV-Visible Spectrum of PB in presence and absence of (EAA+ IPA)
3
2
1
.0
.0
.0
.
s
b
A
0
3
.0
30.00
400.00
500.00
600.00
700.00
800.00
nm.
2
1
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O
O
O
O
Fe [{Fe(CN) } ]
4
6 3
Fe [{Fe(CN) } ]
4
6 3
O
O
Prussian Blue
1
R OH
H+
R1
O
O
O
O
H+
O
R1
O
O
OH
Fe [{Fe(CN) } ]
Fe [{Fe(CN) } ]
4 6 3
4
6 3
1
R = Alkyl, Aryl...etc
Scheme-2: Plausible mechanism of Prussian blue catalyzed Transesterification of β-
Ketoesters
2
2
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