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Chemistry Letters Vol.35, No.5 (2006)
Highly Selective Reduction of Cinnamaldehyde to Cinnamyl Alcohol
Using Nanometric Alkali Metal Hydrides
Ã1
1
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Yinheng Fan, Qiang Wu, Dan Jin, Yunling Zou, Shijian Liao, and Jie Xu
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School of Chemistry and Chemical Engineering, Institute of Chemistry for Functionalized Materials,
Liaoning Normal University, Dalian 116029, P. R. China
Dalian Institute of Chemical Physics, The Chinese Academy of Sciences, Dalian 116023, P. R. China
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(Received January 13, 2006; CL-060051; E-mail: fanyh@lnnu.edu.cn)
Under mild reaction conditions, cinnamaldehyde was re-
duced to cinnamyl alcohol with high selectivity and conversion
using nano-LiH or nano-NaH as a reducing agent. Selectivity of
1
C H CH=CHCH=O
C H CH=CHCH OH
6 5 2
6
5
CAL
CA
3
2
9
9
9.8% was obtained as reduced by nano-LiH with conversion of
9.4% in short reaction times.
C H CH CH CH=O
C H CH CH CH OH
6
5
2
2
6
5
2
2
2
4
HCAL
PP
Scheme 1.
The selective reduction of ꢀ,ꢁ-unsaturated aldehydes to the
corresponding alcohols is of great importance in the synthesis of
various fine chemicals as well as of academic interest. Selective
gas chromatograph with a PEG-20M capillary column of 30 m
and a FID detector.
The reduction of cinnamaldehyde (CAL) can give rise to
three possible products: cinnamyl alcohol (CA), hydrocinnamal-
dehyde (HCAL), and 3-phenylpropanol (PP). The possible reac-
tion pathways of reduction are shown in Scheme 1.
1
reduction reactions are problematic when the C=O and C=C
double bonds are conjugated further with the aromatic ring.
An example is the selective reduction of cinnamaldehyde.
Reduction of ꢀ,ꢁ-unsaturated aldehydes can be carried out by
a variety of methods, including catalytic hydrogenation and
the use of reducing agents, such as metal hydrides, dissolving
The conversion and selectivity were calculated on the base
of GC (gas chromatograph) analysis. As shown in Table 1, ex-
clusive formation of the target molecule (CA) was observed,
and the amount of side product (PP) is very small. No hydrocin-
namaldehyde was found in the GC analysis. These results show
that the reduction of an aldehyde group is much faster than that
of carbon–carbon double bond when MH is used as a reducing
agent. Cinnamaldehyde reacts with nano-MH to form metal al-
coholate, which generates cinnamyl alcohol after hydrolysis.
This addition reaction can be regarded as a nucleophilic addi-
tion. To the carbonyl carbon, a hydride ion from the nano-MH
acts as a nucleophile, which is the same as that in the reaction
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metals and aluminum isopropoxide. During the last few years,
various attempts have been done to develop the method for the
selective reduction in order to get either cinnamyl alcohol or hy-
drocinnamaldehyde.2 Platinum catalysts have been frequently
studied for the selective hydrogenation of cinnamaldehyde
to cinnamyl alcohol, and usually a fairly high pressure was
required.3 Liu et al. have reported the use of Pt catalyst for
the selective hydrogenation of cinnamaldehyde under 4 MPa
–14
–8
pressure. The catalyst efficiency TO (turnover) in an hour was
5
10
186 mol cinnamaldehyde/mol Pt. Classical reducing agents,
mechanism using LiAlH or NaBH as a reducing agent. From
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such as LiAlH4, NaBH4, and Na(i-PrO)BH, could reduce
Table 1, it can be seen that a selectivity of 99.8% is obtained as
reduced by nano-LiH, and 99.4% by nano-NaH, nano-KH exhib-
its lower selectivity, and the selectivity decreases further with
longer reaction time. When nano-LiH is used as a reducing
agent, the selectivity for cinnamyl alcohol almost keeps constant
(99.8%) with time, while a slight decrease in selectivity in the
case of nano-NaH as a reducing agent is observed. The selectiv-
ity decreases from 99.4% at 3 min to 99.1% at 30 min. When
nano-KH is used as a reducing agent, the selectivity decreases
from 98.8% at 3 min to 97.8% at 15 min. Nano-LiH gives the
highest conversion of cinnamaldehyde as compared with nano-
NaH and nano-KH, but the rate of reaction is the slowest among
the three metal hydrides. The conversion can be affected by
changing the molar ratio between the nano-LiH and cinnamalde-
hyde. The conversion increases by changing the ratio from
1.05:1 to 2:1 while the selectivity is unchanged. A maximum se-
lectivity (99.8%) and conversion (99.4%) can be obtained in
60 min at a ratio of LiH/CAL = 2:1.
9
–11,15
C=O bond to get cinnamyl alcohol.
ity was not higher than 99% with a cinnamaldehyde conversion
Usually, the selectiv-
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less than 92% based on either catalysts or reducing agents.
In
our previous study, PdCl2–0.5Co(OAc)2–PPh3 catalyst could se-
lectively hydrogenate cinnamaldehyde to hydrocinnamaldehyde
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under mild reaction conditions. This paper reports that cin-
namaldehyde can be selectively reduced to cinnamyl alcohol
at high selectivity and conversion by the simple nanometric
alkali metal hydrides for the first time.
Nano-MH is sensitive to air and moisture, therefore, all ma-
nipulations were carried out strictly under dry argon atmosphere
using Schlenk technique. Nanometric alkali metal hydrides were
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synthesized according to the method in our previous report.
Transmission electron micrographs (TEM) showed that the aver-
age primary particle sizes of LiH, NaH, and KH were 22, 23, and
2
1
9 nm and specific surface areas were 125, 90, and 50 m /g re-
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spectively. The reactions of cinnamaldehyde with nanometric
alkali metal hydrides were carried out in tetrahydrofuran
We have investigated the influence of temperature on the se-
lectivity and reduction rate of cinnamaldehyde to cinnamyl alco-
hol using nano-NaH as a reducer. The selectivity almost kept
(
THF) in a well-stirred glass flask. Typical reaction conditions
were: THF 10 mL, nano-MH 3 mmol, cinnamaldhyde: according
to the mole ratio to MH, refluxing THF, and normal pressure.
The hydrolyzed products were analyzed by Shimadzu GC-14A
ꢀ
constant in the temperature range from 15 to 60 C. An activa-
tion energy of 54 kJ/mol was obtained by the Arrhenius plot us-
Copyright Ó 2006 The Chemical Society of Japan