Ce-Zr/SiO2: A versatile reusable heterogeneous catalyst for three-component synthesis and solvent free oxidation of benzyl alcohol

Article abstract of DOI:10.1039/c3ra47145d

Ce-Zr loaded on SiO₂ as catalyst provides an extremely efficient method to synthesize pyranoquinolines. This catalyst is found to be superb for the three-component synthesis of target compounds and for the solvent-free liquid-phase oxidation of benzyl alcohols. The catalyst is fully recoverable and reusable with no loss of catalytic activity even after multiple cycles.

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COMMUNICATION
Ce-Zr/SiO2: A versatile reusable heterogeneous catalyst for three-
component synthesis and solvent free oxidation of benzyl alcohol
Ramakanth Pagadala, Suresh Maddila, Surjyakanta Rana, Sreekantha B. Jonnalagadda,*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Functionalized heterocyclic building blocks are of great
50 importance in both medicinal and synthetic chemistry and
development of new efficient synthetic methodologies for these
The Ce-Zr loaded on SiO2 as catalyst provides an extremely
efficient method to synthesize pyranoquinolines. This catalyst
is found to be superb for the three-component synthesis of
target compounds and for the solvent-free liquid-phase
10 oxidation of benzyl alcohols. Catalyst is fully recoverable and
reusable with no loss of catalytic activity even after multiple
cycles.
scaffolds remains
a great challenge in modern organic
synthesis.25,26 The importance of quinoline and its annulated
derivatives is well recognized by synthetic and biological
55 chemists.27ꢀ30 Compounds possessing this motif have found broad
application in drug development material science31ꢀ35
bioorganometallic processes36,37 and biological activities covering
antimalarial, anticancer, antifungal, antibacterial, antiprotozoic
antibiotic and antiꢀHIV effects.38ꢀ40
The importance and popularity of green chemistry principles,
introduced significant advances in organic synthesis, such as
15 combinatorial
chemistry,
multicomponent
processes,
60 In continuation of our effort toward the development of new
protocols for the expeditious synthesis of biologically relevant
heterocyclic compounds,41ꢀ44 keeping these aspects in mind, we
organofluorine chemistry, organocatalysis, microwave synthesis
and sonochemistry to mention a few. Among these developments,
multicomponent reaction techniques played a leading role and the
field experienced tremendous progresses.1ꢀ7 It is pertinent to note
20 that multiꢀcomponent reactions have added advantages such as
high bond forming efficiency, convergence, operational
simplicity, and reduction in waste generation and hence conform
to the principles of green chemistry. Consequently, they are
regarded as viable synthetic routes toward both economic and
25 environmental benefits of chemical transformations.8,9
Multicomponent cascade reactions, which are ideal synthetic
methods, play an important role in the synthesis of heterocyclic
compounds.10ꢀ20 The oxidation of benzyl alcohol to
benzaldehyde, is also an important organic conversion because it
have designed
a
series of 2ꢀaminoꢀ4Hꢀpyranoquinoline
derivatives with using CeꢀZr/SiO2 as a heterogeneous solid
65 catalyst for the first time, with no earlier report of its kind. In
green chemistry point of view, the design of multicomponent
reactions using recyclable heterogeneous catalyst and ethanol as a
sole solvent is ideally the best choice owing to the easier workup
procedure and the inherent environmental and economic
70 advantages of such process.
To develop a clean three component protocol for the formation of
2ꢀaminoꢀ4Hꢀpyranoquinolines, we have installed a large number
of Brønsted and Lewis acid catalysts for the optimization of the
reaction condition. In a pilot experiment, benzaldehyde (1 mol),
75 malononitrile (1 mol), and 8ꢀhydroxyquinoline (1 mol) were
30 is
a valuable chemical in the perfumery, dyestuffs, and
agrochemical industries.21 Heterogeneous catalysts also have
received sizeable attention in organic synthesis because of
environmental, economic and industrial aspects. The use of solid
heterogeneous catalysts for the development of clean processes is
35 a theme of extreme importance in the chemical industry. These
catalysts make the synthetic process economically viable as they
can be easily recovered from the reaction mixture by simple
filtration and can be reused several times to achieve very high
turnover numbers compared to homogeneous catalysts.22
40 Recently, heterogeneous catalysts have received recognition as
the new generation of catalysts having their dual organic and
inorganic natures allows them to establish nearly all kinds of
interactions with reacting species including transition states, and
hence sometimes give rise to improved yields and rate
45 enhancements. In addition, silica supported reagents are
promising for both academic and industrial applications due to
their high mechanical and thermal stabilities, nonꢀcorrosive and
easy separation of the catalyst from the reaction mixture.23, 24
o
stirred in ethanol at 70 C temperature (Scheme 1). It is evident
that the three component coupling product, pyranoquinoline
derivative was not obtained in ethanol after 8 h at room
temperature as well as 70 oC temperature (Table 1, entries 1 & 2).
80 To accelerate the threeꢀcomponent reaction, we have applied a
wide spectrum of Brønsted and Lewis acid catalysts like NaOH,
pꢀtoluene sulphonic acid (pꢀTsOH) and [Bmim]BF4 were selected
(Table 1). This group of catalysts except [Bmim]BF4 has the
advantage low impact in the environment. We found that for the
85 threeꢀcomponent coupling protocol, Brønsted acids were not very
much efficient as compared to the Lewis acid catalysts desiring
the synthesis of the heterocyclic scaffold under the particular
experimental condition.
Developing environmentally benign and economical syntheses is
90 an area of research that is being vigorously pursued, and avoiding
the use of harmful organic solvents is a fundamental strategy to
achieving this. Hence, we have tried to synthesize the
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heterocyclic scaffold in less amount of ethanol applying suitable
catalyst combination and we have applied CeꢀZr/SiO2 catalyst
combination to enhance the rate of the reaction. The improvement
treated with malononitrile and 8ꢀhydroxyquenoline in order to
examine the scope of the reaction, and several representative
examples are summarized in Table 2. Notably, even sterically
of the yield of the desired product was noticed by using the 15 hindered substrates also participated effectively to afford the
5
catalyst combination (Table 1, entry 8ꢀ11). With the success,
focus was then shifted to optimization. The above model reaction
was carried out under different CeꢀZr/SiO2 quantities (20 mg, 30
mg, 40 mg and 50 mg Table 1). It was observed that 40 mg
desired 2ꢀaminoꢀ4Hꢀpyranoquinoline derivatives in good yield
(Table 2). The reaction mechanism may cautiously be visualized
to occur via a tandem sequence of the reactions depicted in the
reaction scheme in Mechanism 1. The resulting products were
1
loading of the CeꢀZr/SiO2 provided the best yield (Table 1, entry 20 characterized by FTIR, HNMR, 13CNMR and MS. Compounds
10 10).
were further identified by 15N NMR (GHSQC) to confirm the
presence of the ꢀNH2 group in their structure.
Under the optimal reaction conditions, a range of aldehydes were
Table 1. Optimization of reaction conditions of the threeꢀcomponent synthesis.
25
Entry
Product
No.
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
Catalyst
Amount
Solvent
Temp (°C)
Time (h)
Yield a(%)
1
2
3
4
5
6
7
8
9
ꢀ
ꢀ
ꢀ
ꢀ
EtOH
EtOH
H2O
EtOH
ꢀ
Room temp
8.0
8.0
6.0
10.0
8.0
5.0
5.0
5.0
4.0
4.0
4.0
b
b
c
b
b
50
30
55
70
94
95
70
NaOH
1.0 (eq.)
6 drops
6 drops
6 drops
30.0 mg
20.0 mg
30.0 mg
40.0 mg
50.0 mg
Room temp
Room temp
Room temp
[Bmim]BF4
[Bmim]BF4
[Bmim]BF4
PTSA
CeꢀZr/SiO2
CeꢀZr/SiO2
CeꢀZr/SiO2
CeꢀZr/SiO2
ꢀ
70
70
70
70
70
70
EtOH
EtOH
EtOH
EtOH
EtOH
10
11
a Isolated yields.
b Products was not found.
c Trace.
catalyst. The catalyst showed the pore diameter of 151 Ao along
30 Table 2. CeꢀZr/SiO2 catalyzed synthesis of the pyranoquinoline 45 with high pore volume of 1.055 cm3/g and surface area of 275
(4a-f) derivatives.
m2/g, which demonstrates its mesoporus character. The Ce (0.49
wt %) and Zr (0.48 wt %) obtained from Inductively Coupled
Plasmaꢀ Optical Emission Spectroscopy (ICPꢀOES) are in
correlation with the nominal weight loading. SEMꢀEDX (Fig 2 in
50 supplementary data) data also showed presence of Ce and Zr in
the catalysts with the appropriate nominal loading on the surface
of the support.47 In order to understand the pore structure of the
catalysts, the N2 adsorptionꢀdesorption measurements were
carried out. This could be attributed to the clogging of the narrow
55 pores of the support with the ceriumꢀzirconium making it
inaccessible to nitrogen molecules. The CeꢀZr loaded on SiO2
support catalyst exhibited high surface area due to the weak
interaction between the CeꢀZr and, CeꢀZr particles may be
agglomerated on the surface of the SiO2. The mesoporous size
60 has significant role on the adsorption properties of reactant
molecules and thereby it can play a major role in the activity. The
wide range distribution in the pore size may be correlated with
the activity exhibited by the catalyst.
Product
No.
R1
Time
(h)
2.0
Yield
a(%)
94.0
mp (oC)/(lit)
4a
H
224ꢀ225 (225ꢀ226)/45
218ꢀ220 (219ꢀ220)/45
231ꢀ232
4b
4c
4d
4e
4f
4ꢀOMe
4ꢀBr
2.0
3.0
2.5
3.0
3.0
92.0
95.0
92.0
91.0
90.0
4ꢀOH
2ꢀCl
242ꢀ243
285ꢀ286 (287ꢀ289)/46
264ꢀ265
2ꢀBr
a Isolated yields.
NH2
NC
O
OH
CHO
CN
N
N
Ce-Zr/SiO2
R
70 oC, EtOH
CN
R
2
3
1a-f
4a-f
35
The oxidation of benzyl alcohol is often used as an ideal reaction
65 for alcohol oxidation. The catalytic activities of CeꢀZr/SiO2 for
the oxidation of benzyl alcohol in the presence H2O2 are
presented in Table 3. The conversion of benzyl alcohol and the
selectivity to benzaldehyde increased to 87% and 97 %,
respectively, the oxidation reaction proceeding through the
70 formation of a peroxo complex by a reaction between the catalyst
and H2O2. This peroxo compound reacts with benzyl alcohol and
Scheme 1. Oneꢀpot threeꢀcomponent reaction of aromatic
aldehydes, malononitrile and 8ꢀhydroxyquenoline catalyzed by
CeꢀZr/SiO2 in ethanol media.
40 The corresponding N2 adsorptionꢀdesorption distribution
isotherms of the catalysts (Fig 1 in supplementary data). The
isotherms showed the hysteric loop (Type IV) which
demonstrates the presence of mesoporous character in the
2
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gives the product. Activity of the catalyst remained unchanged up
to 6th cycle in oxidation reaction. After that the activity
diminished due to possible cooke formation of the catalyst.
In multicomponent reaction, catalytic activity does not change up
of ceria and zirconia phases are in agreement with the ICDD files
no’s 01ꢀ078ꢀ0047, 01ꢀ73ꢀ6328, 38ꢀ1436 for CeO2, ZrO2 and Ceꢀ
ZrO2 respectively. There is an evidence of the mixed metal oxide
phase i.e. Ce50ꢀZr50O2, where dꢀspacing are in correlation with
5
to 5th cycle (Figure 1). The activity loss observed with the 50 ICDD file no: 28ꢀ0271. The catalyst showed the polycrystalline
regenerated catalyst later could be due to partial loss of acid sites
or surface area of the catalyst during reaction/regeneration. The
filtrate after reaction was analyzed for leached metal content by
ICPꢀOES. No trace of metal was detected, confirming no
nature, and the crystalline nature.
10 leaching.
55
60
65
H
N
O
CHO
NC
N
NC
CN
O
CN
CN
Catalyst
R
N
R
R
Cyclization
15
NH2
O
NH
NC
H
NC
O
N
Fig 2. Powder Xꢀray diffractogram of CeꢀZr/SiO2
N
R
R
Figure 3 shows the SEM images of the CeꢀZr supported on silica
catalyst. In the presence of CeꢀZr is distributed over silica
surface. The CeꢀZr particles are agglomerated on the surface of
20 Mechanism 1. Plausible reaction mechanism for the formation of
2ꢀaminoꢀ4Hꢀpyranoquinoline derivatives.
70 silica. A possible reason could be that silica has a large surface
area and therefore requires more CeꢀZr particles to cover its
surface.47, 48 The SEM images confirm the poly crystalline nature
of the catalyst, which is not affected by the different loadings of
ceriumꢀzirconium. SEMꢀEDX shows that the ceriumꢀzirconium is
75 evenly distributed on the surface of the supports, and SEMꢀEDX
results are in correlation with ICPꢀOES elemental analysis.
The TEM images of the catalysts showed an agglomeration of
particles on the surface of the supports (Figure 4).
Agglomeration of particles is the result of exposure of the
80 samples to a beam with the high energy resulting in the loss of
hydroxyl groups. The agglomeration of the particles is high on
the surface of the silica. On high magnification, the particles
appear as irregular a needleꢀlike shape which was also observed
in literature.48ꢀ50 The darker parts of the images show the
85 presence of CeꢀZr dispersed evenly on the surface which appear
as elongated rodꢀlike particles in the TEM images with particle
sizes around 40ꢀ80 nm.
Table 3. Conversions and Selectivity toward the Oxidation of
Benzyl alcohol.
25
Catalyst
Conversion
of benzyl
alcohol
Selectivity (%)
Benzaldehyde Benzoic
acid
CeꢀZr/SiO2
87
87
87
86.5
86.8
87
97
97.2
97
97
96
97
97
86
3
2.8
3
3
4
3
3
14
CeꢀZr/SiO2 (1st Cycle)
CeꢀZr/SiO2 (2nd Cycle)
CeꢀZr/SiO2 (3rd Cycle)
CeꢀZr/SiO2 (4th Cycle)
CeꢀZr/SiO2 (5th Cycle)
CeꢀZr/SiO2 (6th Cycle)
CeꢀZr/SiO2 (7th Cycle)
87
78
100
95
90
85
80
75
70
30
35
40
1
2
3
4
5
6
Catalyst Cycles
Fig 1. Recyclability of CeꢀZr/SiO2 catalyst
The XꢀRay diffraction patterns of prepared 1 wt % CeꢀZr
Fig 3. SEM image of CeꢀZr/SiO2
supported on SiO2 catalysts are shown in Figure 2. The phases of
45 support silica are correlating with literature.47, 48 The dꢀspacing’s
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further dried in an oven at 110ꢀ120 °C for 12 h. Then the catalysts are
calicined in the presence of air, at 550 °C for 3 h to obtain the 1% w/w
catalysts.
45 Typical procedure for the synthesis of pyranoquinolines catalyzed by
Ce-Zr/SiO2 (4a-f).
A mixture of freshly distilled benzaldehyde (1.0 mol) in ethanol (3 ml) at
room temperature, maloninitrile (1.0 mol), 8ꢀhydroxyquinoline (1 mol)
and CeꢀZr/SiO2 (40.0 mg) were added to a round bottom flask equipped
50 with a magnetic bar and condenser. Then, the reaction mixture was heated
at 70 °C for appropriate time as shown in Table 2. The reaction progress
was monitored by TLC (EtOAc/hexane = 5:5). After completion of the
reaction, the mixture was cooled to room temperature and extracted with
dichloromethane. After filtering the CeꢀZr/SiO2 solid, the solvent layer
55 was washed with water, dried with anhydrous sodium sulfate and the
solvent removed to obtain essentially pure product 4a (94% yield) as offꢀ
white solid. The recovered CeꢀZr/SiO2 solid was washed with
dichloromethane and evaporated solvent under reduced pressure and
reused in the next cycles.
Fig 4. TEM image of CeꢀZr/SiO2
Conclusions
60 Compound 4a
1
In summary, we have developed a very practical carbonꢀcarbon
and carbonꢀoxygen bond forming reaction in ethanol. The key is
the use of newly synthesized Lewis acid heterogeneous solid
catalyst, CeꢀZr/SiO2. A heterogeneous catalyst is of more utility,
Offꢀwhite solid: H NMR (400 MHz, DMSOꢀd6) δ = 4.94 (1H, s), 7.15
5
(2H, s, NH2), 7.19ꢀ7.33 (6H, m), 7.57ꢀ7.63 (2H, m), 8.30 (1H, dd, J = 8.2
Hz), 8.92 (1H, dd, J = 4.2 Hz); 13C NMR (100 MHz, CDCl3): δ 41.07,
55.99, 120.41, 121.93, 122.13, 123.56, 126.91, 126.99, 127.63, 128.72,
when it can be easily recovered and is reusable and the proposed 65 136.0, 137.43, 142.93, 145.57, 150.22, 160.27; IR (KBr, cmꢀ1): 3319
catalyst meets those criteria excellently. From green chemistry
(NH2), 2190 (CN); LCMS of [C19H13N3O + Na] (m/z): 322 (100%); Anal.
calcd: C 76.24, H 4.38, N 14.04%. Found: C 76.18, H 4.40, N 14.01%.
Compound 4b
10 point of view, to develop a work up procedure without using
large amount of any organic solvent is desirable. This simple,
environmentally benign and convenient methodology extends the
1
Offꢀwhite solid: H NMR (400 MHz, DMSOꢀd6) δ = 3.70 (3H, s), 4.89
scope towards a wide spectrum of novel compounds possessing 70 (1H, s), 6.86 (2H, d, J = 8.5 Hz), 7.10 (2H, s, NH2), 7.18 (3H, t, J = 8.2
an important structural subunit of a variety of biologically active
15 molecules. The solid support Lewis acid catalyst, CeꢀZr/SiO2
reported in this article will positively contribute to green
chemical processes by reducing the use of organic solvents and
reaction time.
Hz), 7.57ꢀ7.63 (2H, m), 8.31 (1H, d, J = 7.4 Hz), 8.92 (1H, d, J = 2.8 Hz);
13C NMR (100 MHz, CDCl3): δ 40.26, 55.0, 56.27, 114.06, 120.45,
122.07, 122.24, 123.46, 126.98, 127.59, 128.73, 135.97, 137.46, 137.76,
142.81, 150.16, 158.19, 160.10; IR (KBr, cmꢀ1): 3298 (NH2), 2187 (CN);
75 LCMS of [C20H15N3O2 + H+] (m/z): 330 (100%); Anal. calcd: C 72.94, H
4.59, N 12.76%. Found: C 72.89, H 4.52, N 12.80%.
20 Acknowledgments
The authors are grateful to the School of Chemistry & Physics
Compound 4c
1
Offꢀwhite solid: H NMR (400 MHz, DMSOꢀd6) δ = 4.99 (1H, s), 7.18ꢀ
and the College of Agriculture, Engineering
&
Science,
7.24 (5H, m, ArꢀH and NH2), 7.50ꢀ7.66 (4H, m), 8.32 (1H, dd, J = 8.3
University of KwaZuluꢀNatal, Westville campus, Durban, South 80 Hz), 8.93 (1H, dd, J = 4.0 Hz); 13C NMR (100 MHz, CDCl3): δ 40.43,
Africa for the facilities and financial support.
55.50, 120.14, 120.25, 121.31, 122.23, 123.69, 126.78, 127.76, 129.94,
131.24, 131.63, 136.02, 137.43, 142.99, 144.97, 150.27, 160.26; IR (KBr,
cmꢀ1): 3299 (NH2), 2193 (CN); LCMS of [C19H12BrN3O + Na] (m/z): 400
(100%); Anal. calcd: C 60.34, H 3.20, N 11.11 %. Found: C 60.39, H
85 3.27, N 11.08%.
25
Notes and references
School of Chemistry & Physics, University of KwaZulu-Natal, Westville
Campus, Chiltern Hills, Durban-4000, South Africa. Fax: +27 31 260
3091; Tel: +27 31 260 7325/3090; E-mail: jonnalagaddas@ukzn.ac.za
Compound 4d
Offꢀwhite solid: 1H NMR (400 MHz, DMSOꢀd6) δ = 4.82 (1H, s), 6.6
(2H, d, J = 8.3 Hz), 7.03ꢀ7.20 (5H, m, ArꢀH and NH2), 7.57ꢀ7.63 (2H, m),
8.31 (1H, d, J = 8.1 Hz), 8.92 (1H, d, J = 3.2 Hz), 9.35 (1H, s, OH); 13C
90 NMR (100 MHz, CDCl3): δ 40.32, 56.44, 115.37, 120.52, 122.02,
122.48, 123.40, 127.03, 127.54, 128.68, 129.94, 135.96, 136.10, 137.46,
142.77, 150.12, 156.30, 160.07, 161.13; IR (KBr, cmꢀ1): 3331 (NH2),
3210 (OH), 2199 (CN); LCMS of [C19H13N3O2 + H+] (m/z): 316 (100%);
Anal. calcd: C 72.37, H 4.16, N 13.33 %. Found: C 72.32, H 4.21, N
95 13.38%.
30
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
Catalyst preparation
35 The preparation of metal oxide supported catalysts was synthesized by
wet impregnation method.47, 51 The catalysts were prepared by dissolving
appropriate amount of Ce [Cerium nitrate, Aldrichꢀ99%], Zr [Zirconium
nitrate, Aldrichꢀ99%] in distilled water [50.0 mL] and adding it to 10 g of
Silica [SiO2, Aldrich] stirring for 4 hrs using a magnetic stirrer at room
40 temperature and again at room temperature for overnight. Catalysts are
Compound 4e
1
Offꢀwhite solid: H NMR (400 MHz, DMSOꢀd6) δ = 5.45 (1H, s), 7.07
(1H, d, J = 8.5 Hz); 7.21 (2H, s, NH2), 7.26ꢀ7.64 (6H, m), 8.31 (1H, dd, J
4
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= 8.2 Hz), 8.93 (1H, dd, J = 4.0 Hz); 13C NMR (100 MHz, CDCl3): δ
24 R. K. Sharma, S. Mittal and M. Koel, Crit. Rev. Anal. Chem.,
2003, 33, 183.
25 A. Nefzi, J. M. Ostresh, R. A. Houghten, Chem. Rev., 1997,
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26 S. J. Teagues, A. M. Davis, P. D. Leeson, T. Opera, Angew.
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27 C. W. Wright, J. AddaeꢀKyereme, A. G. Breen, J. E. Brown,
M. F. Cox, S. L. Croft, Y. Gokcek, H. Kendrick, R. M.
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28 N. P. Sahu, C. Pal, N. B. Mandal, S. Banerjee, M. Raha, A. P.
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38.48, 54.60, 120.04, 120.52, 122.25, 123.74, 126.12, 127.84, 127.95,
128.95, 129.84, 131.25, 131.99, 136.0, 137.33, 142.01, 143.27, 150.30,
160.47; IR (KBr, cmꢀ1): 3320 (NH2), 2195 (CN); LCMS of [C19H12ClN3O
+ H+] (m/z): 334 (100%); Anal. calcd: C 68.37, H 3.62, N 12.59 %.
Found: C 68.42, H 3.60, N 12.67%.
70
75
5
Compound 4f
1
Offꢀwhite solid: H NMR (400 MHz, DMSOꢀd6) δ = 5.46 (1H, s), 7.06
(1H, d, J = 8.5 Hz), 7.16ꢀ7.35 (5H, m, ArꢀH and NH2), 7.59ꢀ7.63 (3H, m),
10 8.31 (1H, dd, J = 8.3 Hz), 8.93 (1H, dd, J = 4.0 Hz); 13C NMR (100 MHz,
CDCl3): δ 40.58, 54.86, 119.95, 120.63, 122.27, 122.38, 123.78, 125.98,
127.86, 128.61, 129.22, 131.48, 133.0, 136.01, 137.37, 143.15, 143.78,
150.31, 160.37; IR (KBr, cmꢀ1): 3319 (NH2), 2195 (CN); LCMS of
[C19H12BrN3O + Na] (m/z): 400 (100%); Anal. calcd: C 60.34, H 3.20, N
15 11.11 %. Found: C 60.42, H 3.16, N 11.18%.
80
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Catalytic Activity towards Oxidation reaction:
The catalytic oxidation of benzyl alcohol was carried out in
a
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