Nanocrystalline ZnO for Knoevenagel condensation and reduction of the carbon, carbon double bond in conjugated alkenes

Article abstract of DOI:10.1002/hlca.200890072

A novel efficient and eco-friendly nanocrystalline ZnO catalyst was used for the Knoevenagel condensation and for the reduction of C=C double bonds at room temperature in the absence of solvents.

Full text of DOI:10.1002/hlca.200890072

Helvetica Chimica Acta – Vol. 91 (2008)
715
Nanocrystalline ZnO for Knoevenagel Condensation and Reduction of the
Carbon,Carbon Double Bond in Conjugated Alkenes
by Mona Hosseini-Sarvari*, Hashem Sharghi, and Samane Etemad
Department of Chemistry, Shiraz University, Shiraz 71545, I. R. Iran
(phone: þ 98-711-2284822; fax: þ 98-711-2280926; e-mail: hossaini@susc.ac.ir)
A novel efficient and eco-friendly nanocrystalline ZnO catalyst was used for the Knoevenagel
condensation and for the reduction of C¼C double bonds at room temperature in the absence of solvents.
Introduction. – Recently nanocrystalline inorganic oxides gained interest because
of their different topical characteristics [1 – 6]. Having variable properties depending
on the size of the particles, the inorganic oxides present enormous opportunities.
Nanocrystalline semiconducting II – IV metal oxides have been efficiently used as gas
sensors and photocatalysts, whereas the catalytic properties of these materials are
poorly explored [7 – 9]. Undoubtedly, one of the most important semiconductor
materials is zinc oxide (ZnO). ZnO is a versatile smart material that has key
applications as catalysts [10], sensors, piezoelectric transducers [11], transparent
conductors [12], and surface-acoustic-wave devices [13]. Nanometer-sized particles of
ZnO have very different physical and chemical properties compared with bulk
material. When used as catalysts, their catalytic activity is expected to be enhanced not
only because of their increased surface area, but also because of the changes of surface
properties such as surface defects.
Control over the synthesis of monosubstituted malononitriles (¼ propanedinitriles)
has been a challenging problem in organic chemistry. The most general and direct route
to monosubstituted malononitriles is the alkylation of malononitrile, but this method
generally produces various amounts of disubstituted malononitriles by over-alkylation
when malononitrile is the limiting agent. Indeed, symmetrically disubstituted
malononitriles can be efficiently prepared in this manner [14]. Keeping the alkylating
agent to a minimum causes the product to be contaminated with significant amounts of
unreacted malononitrile, which must be separated from the desired monosubstituted
malononitrile [15]. More selective methods for the synthesis of monosubstituted
malononitriles follow a two-step reaction sequence. The first step is a Knoevenagel
condensation between malononitrile and a ketone or aldehyde [16 – 20]. The
intermediate alkylidenedinitrile is reduced in a second step to afford the desired
monosubstituted malononitrile [21 – 24].
Knoevenagel condensations (KC) [16][17] which are one of the most important
CÀC bond forming reactions have been widely used in the synthesis of important
intermediates or products for perfumes [25], pharmaceuticals [26], calcium antagonists
[27], and polymers [28]. Thus, the development of new synthetic procedures for KC has
ꢀ 2008 Verlag Helvetica Chimica Acta AG, Zürich

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attracted the attention of organic chemists. Recently, various methodologies have been
developed for KC [16 – 20]. However, there still remains a need to develop a more
efficient method, particularly in view of the disadvantages associated with some of the
reported procedures such as the requirement of solvent, additional reagents, heating,
long reaction times, costly and moisture-sensitive catalysts, special apparatus, etc. While
designing a new catalyst, we thought that the use of a more effective electrophilic
activating agent should accelerate the overall reaction rate, and a metal oxide, in
particular nanocrystalline ZnO, which is composed of both Lewis acidic and Lewis
basic sites [29], might be an ideal candidate for a closer inspection (Scheme 1).
Scheme 1. Knoevenagel Condensation Catalyzed by Nano-Flake ZnO. For R1, R2, and Ar, see Table 2.
Results and Discussion. – Initially, various ZnO crystals (commercial ZnO (CM-
ZnO), nano-flake ZnO (NF-ZnO) (20 – 30 nm) [30], and nano-particle ZnO (NP-
ZnO) (35 Æ 5 nm)) [31] were screened in the KC between 4-chlorobenzaldehyde (1e;
Ar¼ 4-ClÀC6H4) and malononitrile (2; R1 ¼ R2 ¼ CN) (Scheme 1 and Table 1).
Among the ZnO samples screened in the KC, the NF-ZnO under solvent-free
conditions was found to be superior to the other ZnO in terms of yield of product 3e
(Ar¼ 4-ClÀC6H4, R1 ¼ R2 ¼ CN) and reaction time; this fast transformation did not
require any heating.
Table 1. Knoevenagel Condensation Catalyzed by Different Crystallites of ZnO (0.004 g) of 4-
Chlorobenzaldehyde (1e; Ar¼ 4-ClÀC6H4; 1 mmol) and Malononitrile (2; R1 ¼ R2 ¼ CN; 1 mmol) at 258
Catalyst
Solvent
Time
Yield [%]a)
NF-ZnO
NP-ZnO
CM-ZnO
NF-ZnO
NF-ZnO
none
none
none
CH2Cl2
MeCN
10 min
90 min
3 h
3 h
3 h
98
86
10b)
43
40
a) Yield of isolated 3e (Ar¼ 4-ClÀC6H4, R1 ¼ R2 ¼ CN). b) The yield was increased by raising the
temperature to 808 for 6 – 7 h.
To widen the scope, NF-ZnO was tested in the KC of various benzaldehydes 1a – 1e
with activated methylene compounds 2; the resulting products 3a – 3o were formed in
excellent yields (Table 2).
Spurred by the success of KC, the reduction of some activated conjugated alkenes 3,
which were synthesized by KC, was studied by using nanomaterials (Scheme 2). To the
best of our knowledge, most investigated reductions of C¼C bonds in conjugated
alkenes proceed much better in protic solvents such as alcohols, water, and/or aqueous
EtOH, and such reductions do not occur under solvent-free conditions [21 – 24].

Helvetica Chimica Acta – Vol. 91 (2008)
717
Table 2. Knoevenagel Condensation Catalyzed by NF-ZnO (0.004 g) at 258 under Solvent-Free
Conditions (see also Scheme 1)
Aldehyde
Ar
R1
R2
Product
Time [h]
Yield [%]a)
1a
1b
1c
1d
1e
1f
1g
1h
1i
Ph
CN
CN
CN
CN
CN
CN
CN
CN
CN
CN
CN
CN
CN
CN
CN
CN
3a
3b
3c
3d
3e
3f
3g
3h
3i
3.5
3
2.5
3
10 min
3
10 min
3
90
9
90
9
98
9
98
9
4-MeÀC6H4
4-MeOÀC6H4
2-MeOÀC6H4
4-ClÀC6H4
4-OHÀC6H4
4-NO2ÀC6H4
3-ClÀC6H4
2-ClÀC6H4
2-thienyl
0
5
0
0
0
CN
CN
CN
CN
30 min
2
90
9
1j
3j
1k
1l
1e
1e
1e
1e
pyridin-4-yl
furan-2-yl
4-ClÀC6H4
4-ClÀC6H4
4-ClÀC6H4
4-ClÀC6H4
CN
CN
CN
CN
CO2Et
CO2Et
CN
CN
CO2Et
MeO
Cl
3k
3l
3m
–
3n
3o
5 min
3
4
24
1
5
98
9
9
5
0
0
9
9
0
0
CO2Et
a) Yield of isolated product 3.
Among the ZnO samples screened in the reduction of 2-(4-chlorophenylmethylene)-
malononitrile (3e) with NaBH4 as reducing agent under solvent-free conditions, the
NF-ZnO was found to be superior to NP-ZnO and CM-ZnO in terms of yields and
reaction times; the yields of product 4e are 87, 68, and 60% by using the NF-ZnO, NP-
ZnO, and assorted crystals of CM-ZnO, respectively (Table 3).
Scheme 2. Reduction of C¼C Bonds Catalyzed by Nano-Flake ZnO. For R1, R2, and Ar, see Table 4.
Table 3. Reduction of 2-[(4-Chlorophenyl)methylene]malononitrile (3e; Ar¼ 4-ClÀC6H4, R1 ¼ R2 ¼
CN; 1 mmol) with NaBH4 (1 mmol) Catalyzed by Different Crystallites of ZnO (0.004 g) at 258 under
Solvent-Free Conditions
Catalyst
Time [min]
Yield [%]a)
NF-ZnO
NP-ZnO
CM-ZnO
2
30
90
87
68
60
a) Yield of isolated 4e (Ar¼ 4-ClÀC6H4, R1 ¼ R2 ¼ CN).
To widen the scope, a system composed of NF-ZnO under solvent-free conditions
was evaluated in the reduction of different activated alkene derivatives with NaBH4,
i.e. of 3e, 3h, 3i, 3f, 3g, 3m, 3n (Table 4). When the aryl substituent at the methylene

718
Helvetica Chimica Acta – Vol. 91 (2008)
moiety of the activated alkene 3 is substituted by a Cl-atom such as in 3e, 3h, 3i, 3m,
higher yields are observed than in the case of aryl substituents bearing other groups
(Entries 1 – 3 and 6). On the other hand, the NF-ZnO-promoted reductions of activated
alkenes 3 bearing 2- or 3-substituted aryl substituents with either electron-withdrawing
or electron-donating groups exhibit a decrease of yield when compared with the
corresponding 4-substituted analogs, which may be ascribed to the steric hindrance for
the unidirectional entry of reacting species (e.g., Entries 2 and 3). These catalytic
reductions are very impressive when compared with other reducing systems such as
InCl3/NaBH4 (i.e., CI2InH) and Al2O3/NaBH4, which require more than the
stoichiometric amount of reducing agent, offer low selectivity or conversions to the
desired products, and need protic solvents (Table 5).
Table 4. Reduction of Activated Alkene Derivatives 3 with NaBH4 Catalyzed by NF-ZnO at 258 under
Solvent-Free Conditions (see also Scheme 2)
Entry Knovenagel adduct Ar
R1
R2
Product Time [min] Yield [%]a)
1
2
3
4
5
6
7
3e
3h
3i
4-ClÀC6H4
3-ClÀC6H4
2-ClÀC6H4
4-HOÀC6H4
CN
CN
CN
CN
CN
CN
CN
CN
CN
4e
4h
4i
4f
4g
2
5
5
2
2
87
72
83
60
65
9
3f
3g
3m
3n
4-NO2ÀC6H4 CN
4-ClÀC6H4
4-ClÀC6H4
CN
CO2Et Cl
CO2Et 4m
4n
5
10
0
53
a) Yield of isolated product 4.
Table 5. Comparison of the Catalytic Efficiency of NF-ZnO and Various Catalysts for the Reduction of 3e
Catalyst
Time
Yield [%]a)
In, EtOH/H2O, NH4Cl [23a]
InCl3/NaBH4, MeCN, r.t., H2O [23b]
NF-ZnO, r.t.
10 h
3 h
2 min
82
83
87
a) Yield of isolated 4e.
Since the KC of malononitrile and 4-chlorobenzaldehyde (1e) in the presence of
NF-ZnO was accomplished without solvent, we hypothesized that reduction with
NaBH4 could be performed subsequently in the same pot also under solvent-free
conditions. Thus, the condensation of malononitrile and 4-chlorobenzaldehyde (1e)
was carried out at room temperature by absorption on 5 mol-% of NF-ZnO and
determined to be complete (TLC monitoring (CH2Cl2) of malononitrile) within ca.
10 minutes. Then NaBH4 was added, and all intermediate 3e was converted to product
4e within a couple of minutes (Scheme 3).
In an effort to determine the scope and limitation of this novel one-pot KC-
reduction methodology, various aldehydes 1 were treated with malononitrile under the
conditions developed for 1e, which resulted in excellent yields of the corresponding
products 4 (Table 6, Entries 1 – 11). We also found that the reaction of some other
activated methylene compounds with 1e were successful under these conditions, but the

Helvetica Chimica Acta – Vol. 91 (2008)
719
Scheme 3. One-Pot Synthesis of Monosubstituted Malononitriles. For R1, R2, and Ar, see Table 6.
rates for the condensation on NF-ZnO decreased with less reactive methylene compounds
(Entries 12 – 16), whereas the reduction with NaBH4 proceeded without incident.
Table 6. Overall Yield of the One-Pot Synthesis of 4 by the KC-Reduction Methodology (see also
Scheme 3)
Entry
Aldehyde
Ar
R1
R2
Product
Yield [%]a)
1
2
3
4
5
6
7
8
9
10
11
12
13
15
16
1a
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1e
1e
1e
1e
Ph
CN
CN
CN
CN
CN
CN
CN
CN
CN
CN
CN
CN
CN
CO2Et
CO2Et
CN
CN
CN
CN
CN
CN
CN
CN
CN
CN
CN
CO2Et
MeO
Cl
4a
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
–
3n
3o
95
98
95
98
95
98
95
95
95
98
95
90
0
4-MeOÀC6H4
2-MeOÀC6H4
4-ClÀC6H4
4-OHÀC6H4
4-NO2ÀC6H4
3-ClÀC6H4
2-ClÀC6H4
2-thienyl
pyridin-4-yl
furan-2-yl
4-ClÀC6H4
4-ClÀC6H4
4-ClÀC6H4
4-ClÀC6H4
90
90
CO2Et
a) Yield of isolated product 4.
NF-ZnO indeed displayed the highest activity compared to that of CM-ZnO, and
NF-ZnO has Lewis acid sites (Zn2þ) and Lewis basic sites (O2À) [29]. In the KC, an O2À
site of NF-ZnO takes up a proton of the malononitrile, and the resulting carbanion
forms a complex with the Zn2þ site (the Zn2þ sites are also interacting with the C¼O
groups of the aldehyde (Scheme 4). Hence, the KC proceeds via the dual activation by
NF-ZnO of both substrates, the nucleophile and the electrophile. The Lewis acid sites
of NF-ZnO also activate NaBH4 for the solvent-free reductions.
The above results indicate that the efficiency of the catalytic activity is dependent
on the particle size and particle shape of ZnO. The SEM (scanning electron
microscope) micrograph, FT-IR spectrum, and X-ray diffraction plot of NF-ZnO and
NP-ZnO are illustrated in Fig.1 and 2, respectively.
Conclusion. – We established that NF-ZnO is a highly active catalyst for both KC
and reductions of C¼C bonds in activated conjugated compounds. Thus, NF-ZnO with
its well-defined shape and size shows higher activity than other catalysts. The
advantages of this safe protocol include a simple reaction setup not requiring

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Helvetica Chimica Acta – Vol. 91 (2008)
Scheme 4. a) Wurtzite Structure of Crystalline ZnO. b) Wurtzite Structure of Crystalline ZnO in Front
View. c) Proposed Mechanism for Knoevenagel Condensation Catalyzed by NF-ZnO.
specialized equipment, very mild reaction conditions, high product yields, short
reaction times, and the limitation of solvents.
We gratefully acknowledge the support of this work by the Shiraz University Research Council.
Experimental Part
General. Commercial ZnO (CM-ZnO) was purchased from Merck. Samples of conventional ZnO, of
nano-flake ZnO (NF-ZnO (20 – 30 nm)), and of nano-particle ZnO (NP-ZnO (35 þ 5 nm)) were
synthesized as reported in [30][31], the latter two are characterized in Figs.1 and 2.
Nanoflake Zinc Oxide (NF-ZnO). In a typical experiment, zinc acetate dihydrate (10 mmol) and
CO(NH2)2 (0.2 mol) were dissolved in deionized H2O (200 ml) at r.t. (!transparent soln.), and the soln.
was refluxed for 12 h. The mixture was cooled in cold water to stop the reaction. The product was
centrifuged and washed with deionized H2O and abs. EtOH and dried at 808 for 8 h. The NF-ZnO (2 g)
was obtained by calcining the precursor in a furnace in air at 4008 for 2 h [30].
Nanoparticle Zinc Oxide (NP-ZnO). Zinc acetate dihydrate (10.98 g) was dissolved in EtOH
(300 ml) at 608. Oxalic acid dihydrate (12.6) which was previously dissolved in EtOH (200 ml) at 508 was
added slowly under stirring to the Zn(OAc)2 soln. at 608. A thick white gel formed, which was kept at 808
for drying. Then the solid was calcined at 5008 for 2 h to yield NP-ZnO (5 g) [31].
Knoevenagel Condensation of 1 and 2: General Procedure. A benzaldehyde (1; 1 mmol) and an
activated methylene compound 2 (1 mmol) were added to NF-ZnO (5 mol-%, 0.004 g) in a mortar. The

Helvetica Chimica Acta – Vol. 91 (2008)
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Fig. 1. a) SEM Micrograph of NF-ZnO. b) FT-IR Spectrum of NF-ZnO. c) X-Ray diffraction plot of NF-
ZnO.
mixture was ground with a pestle, and the progress of the reaction was monitored by TLC (AcOEt/hexane
2 :8). After the reaction was complete, AcOEt was added, and the mixture was centrifuged to separate
the catalyst. The org. solvent was evaporated, and the residue was purified by chromatography (silica gel).
The structures of the obtained Knoevenagel adducts 3 were confirmed by the 1H-NMR and IR data and
comparison with authentic samples obtained commercially or prepared by reported methods [18 – 20].

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Helvetica Chimica Acta – Vol. 91 (2008)
Fig. 2. a) SEM Micrograph of NP-ZnO. b) FT-IR Spectrum of NP-ZnO. c) X-Ray diffraction plot of NP-
ZnO.
Reduction of Knoevenagel Adducts 3: General Procedure. The Knoevenagel adduct 3 (1 mmol) and
NaBH4 (1 mmol) were added to NF-ZnO (5 mol-%, 0.004 g) in a mortar. The mixture was ground with a
pestle, and the progress of the reaction was monitored by TLC (silica gel, CH2Cl2; nearly identical Rf

Helvetica Chimica Acta – Vol. 91 (2008)
723
values of educt and product but only the educt was detected under UV light at 254 nm). CH2Cl2 (30 ml)
was added, and the mixture was centrifuged to separate the catalyst. Then the soln. was poured into a
separatory funnel with ice-water (20 ml). Excess hydride was destroyed, and the soln. was acidified with
3m HCl (8 ml). The org. phase was dried and concentrated, and the crude oily residue was purified by
column chromatography (hexane/AcOEt 9:1). The structures of the obtained colorless oily reduction
products 4 were confirmed by the 1H-NMR and IR data and comparison with authentic samples obtained
commercially or prepared by reported methods [21 – 24].
One-Pot Preparation of 4. A benzaldehyde 1 (1 mmol), an activated methylene compound 2
(1 mmol), and NF-ZnO (5 mol-%, 0.004 g) were mixed in a mortar. The mixture was ground with a
pestle, and the progress of the reaction was monitored by TLC (AcOEt/hexane 20 :80). After the
reaction was complete, NaBH4 (1 mmol, 0.04 g) was added, and the mixture was ground with a pestle, and
after 5 – 10 min the reaction was complete (TLC monitoring). Workup was carried out as described
above for the reduction of Knoevenagel adducts.
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Received October 5, 2007