Efficient method for the reduction of carbonyl compounds by triethylsilane catalyzed by PdCl2

Article abstract of DOI:10.1016/j.jorganchem.2008.09.005

The versatility of the palladium(II) chloride and triethylsilane system has been tested in the reduction of aromatic carbonyl compounds. The reaction takes place under mild conditions. This facile and efficient method affords high yields for the reduction of aldehydes and ketones to the corresponding alkanes.

Full text of DOI:10.1016/j.jorganchem.2008.09.005

Journal of Organometallic Chemistry 693 (2008) 3567–3570
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Efficient method for the reduction of carbonyl compounds by triethylsilane
catalyzed by PdCl2
a,*
b
a
Maryam Mirza-Aghayan , Rabah Boukherroub , Mahshid Rahimifard
a
Chemistry and Chemical Engineering Research Center of Iran (CCERCI), P.O. Box 14335-186, Tehran, Iran
Institut de Recherche Interdisciplinaire (IRI, USR 3078), Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN, UMR 8520), Cité Scientifique,
Avenue Poincaré, BP 60069, 59652 Villeneuve d’Ascq, France
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 June 2008
Received in revised form 30 August 2008
Accepted 2 September 2008
Available online 10 September 2008
The versatility of the palladium(II) chloride and triethylsilane system has been tested in the reduction of
aromatic carbonyl compounds. The reaction takes place under mild conditions. This facile and efficient
method affords high yields for the reduction of aldehydes and ketones to the corresponding alkanes.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords:
Palladium(II) chloride
Triethylsilane
Reduction
Aldehydes
Ketones
1
. Introduction
powder and a catalytic amount of 4,4-di-tert-buthylbiphenyl
(
DTBB) in THF at room temperature, were successfully used to pro-
Hydrogenation of unsaturated organic functions such as olefins,
mote the reduction of a variety of ketones and aldehydes by trans-
fer hydrogenation using isopropanol as the hydrogen donor.
Modest to high yields of the corresponding alcohols were obtained.
However, the reduction of 4-methoxy benzaldehyde gave only 34%
of the corresponding alcohol under the same conditions after 24 h
[15]. On the other hand, good results have been obtained using
poly[N-(2-aminoethyl)acrylamido]trimethyl ammonium halide re-
carbonyls, and imines is currently becoming a standard procedure
in both academic laboratories and industrial applications [1,2]. The
development of a technique for the reduction of carbonyl com-
pounds is very important [3]. Conventional hydrogenation proce-
dure, although often offers reduction under mild conditions,
requires a special set of apparatus and is always associated with
the cautions of using hydrogen gas. Alternative methods to the
commonly used hydrogenation procedures such as heterogeneous
and homogeneous catalytic transfer hydrogenation have found
widespread applications in the reduction of a large variety of func-
tional groups [4–6]. Selective, mild and effective reducing agents in
transition metal catalyzed transfer hydrogenation have been of
considerable interest.
Transition metals such as Pd, Rh, Pt, Ni, Cu, Ir, Co and their com-
plexes are usually utilized as catalysts for reduction reactions [7].
For example, ammonium formate/palladium on activated charcoal
Pd/C) system has shown its versatile catalytic hydrogen transfer
and has been used for the reduction of various functionalities [8],
including heterocyclic ring in quinolines [9], aryl ketones to alco-
sin with NaBH
sterification of several aromatic ketones including methyl
phenylglyoxylate via hydride transfer using [Rh(C 10)Cl] as pre-
4
[16]. Gamez et al. have described that the transe-
8
H
2
cursor, N,N-1,2-diphenyl-1,2-ethanediamine as ligand and isopro-
panol as the reducing agent and solvent, is faster than the
reduction [17].
Trialkylsilanes are known to be poor reducing agents due to
their low capacity to donate hydrogen atoms or hydrides [18]. To
overcome these limitations, a variety of chemically modified si-
lanes with weaker Si–H bonds and composite reducing systems
based on the combination of a silane/transition-metal catalyst
have been developed [19]. The combination of silicon hydride with
palladium dichloride has been reported in few examples, i.e. for the
deprotection of aminoacids or peptides [20], for nucleophilic sub-
stitution at silicon atom [21], for the reduction of Schiff bases
[22], and for the preparation of halogenosilanes [23,24]. Further-
(
hols [10], benzyl glycosides [11], dibenzyl uracils [12],
urated nitroalkenes [13] and cyclic ,b-unsaturated ketones [14].
Nickel(0) nanoparticles, generated from nickel(II) chloride, lithium
a, b-unsat-
a
3 2
more, the system Et SiH/PdCl was successfully used for the reduc-
tion of alkyl halides to alkanes [25], conversion of alcohols to their
corresponding organic halides and alkanes [26], and for the effi-
*
Corresponding author. Tel.: +98 21 8036144 5; fax: +98 21 8037185.
E-mail address: m.mirzaaghayan@ccerci.ac.ir (M. Mirza-Aghayan).
0
022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.09.005

3
568
M. Mirza-Aghayan et al. / Journal of Organometallic Chemistry 693 (2008) 3567–3570
cient isomerization of 1-alkenes to 2- and 3-alkenes [27]. More-
over, the efficiency of the palladium catalyst/Et SiH system for
the hydrogenation of 1-alkenes under mild conditions [28,29], for
the transformation of alcohols to their corresponding silyl ethers
and for the cleavage of triethylsilyl ethers to the parent alcohols
3
(entry 10) using aldehyde/Et SiH (1/2 ratio) yielded 62% of 4-chlo-
rotoluene and 19% of toluene. There was a competition between
the carbonyl and C–Cl reduction. The reduction of alkyl halides
3
to the corresponding alkanes using Et
ously studied in detail [25]. When the ratio of aldehyde/Et
3
SiH/PdCl
2
system was previ-
SiH was
3
[
30] was demonstrated. More recently, we have shown the versa-
tility of the PdCl /Et SiH system for the selective hydrogenation
of the carbon–carbon double bond of ,b-unsaturated ketones to
increased from 1/2 to 1/6, the elimination of halide group increases
to yield 44% of 4-chlorotoluene and 56% of toluene.
2
3
a
The reaction of acetophenone and 2-methoxyacetophenone
the corresponding saturated ketones under mild conditions [31].
(entries 11 and 12) with Et
respectively 73.5 and 88% of the saturated compound after 5 h re-
flux of ethanol. When the ratio of the ketone/Et SiH was increased
to 1/4, ethylbenzene and 1-ethyl-2-methoxybenzene were ob-
tained in 99 and 99.6% yields after only 1 h reflux of ethanol,
respectively. In a similar way, the reaction of benzophenone (entry
3 3
SiH (ketone/Et SiH: 1/2) produced
This paper is a continuation of our previous work on the exploi-
tation of Et
ganic functional groups. Herein, we report our preliminary
results, which account for the versatility of PdCl /Et SiH couple
3
SiH/PdCl
2
system for chemical transformation of or-
3
2
3
in the chemical transformation of aromatic aldehydes and ketones
to saturated compounds under mild conditions. Direct reaction of
3 2
13) with Et SiH in the presence of PdCl led to the formation of
different aromatic aldehydes or ketones with Et
3
SiH in the pres-
diphenylmethane in 80% yield, after 30 min at room temperature.
A yield of 100% was obtained when the ratio of benzophenone/Et3-
SiH was increased to 1/6, after 30 min at room temperature.
We have next examined the reduction reaction of aliphatic ke-
tones such as diisopropylketone, 5-nonanone and 2-heptanone
ence of palladium dichloride in ethanol occurs at room tempera-
ture to yield the corresponding saturated compounds in high
yields. The reduction is most likely due to the reaction of the alde-
hydes or ketones with molecular hydrogen generated in situ by the
spontaneous reaction of ethanol and triethylsilane catalyzed by
palladium dichloride.
using Et
room temperature or reflux in ethanol even in the presence of a
large excess of Et SiH. Only the starting materials were recovered.
3 2
SiH/EtOH/PdCl . No reaction was observed after 6 h at
3
Finally, we have investigated the effect of the nature of the Pd
catalyst ligand on the reduction reaction yield and product distri-
2
. Results and discussion
bution. The reaction of 1 eq. of benzaldehyde with 2 eq. of Et
3
SiH
The aim of the present work is exploring the efficiency of the
system Et SiH/EtOH in the presence of catalytic amounts of PdCl
3 2
for the reduction of aromatic aldehydes and ketones under mild
conditions (Scheme 1). The reduction reaction requires the use of
an inert atmosphere and anhydrous solvent. In a typical experi-
in the presence of 10% Pd(OAc) as a catalyst led to the formation
2
of 86.4% of benzyl alcohol and 13.6% of toluene after 50 min at
room temperature in ethanol as solvent. The result is very different
with that obtained using PdCl as a catalyst (Table 1), and clearly
2
indicates the role of the chloride on the product distribution.
Based on our previous reports [28,29,31], it is assumed that
ment: PdCl
mixture of aldehyde or ketone (1 eq.) and Et
2
(10%) was added at room temperature to a stirred
SiH (2 eq.) in dry eth-
3
molecular hydrogen generated in situ by the reaction of Et
3
SiH
anol (2 ml). An exothermic reaction takes place in the first 5 min
and then the temperature decreases to room temperature. The
resulting mixture was further kept under stirring for indicated
time in Table 1 prior to GC/MS analysis. The obtained results are
summarized in Table 1.
and ethanol in the presence of PdCl
of the carbonyl group (Scheme 2).
2
is responsible of the reduction
2
The reaction of molecular hydrogen (H ), generated by the reac-
tion of Et SiH with EtOH catalyzed by Pd(0), with the carbonyl
3
function leads to the formation of the corresponding alcohol. The
transformation of alcohols to the corresponding halides and al-
kanes was previously studied in detail [25,26] and takes place
according to Scheme 3.
To emphasize the role of the chloride ligand in the reaction
mechanism, we have performed the reduction of benzyl alcohol
First, the reaction of aromatic aldehydes and ketones with Et3-
SiH/EtOH in the presence of PdCl catalyst was examined at room
2
temperature using the respective ratio 1/2/10 mol%. The results
indicate that the reduction of aldehydes (entry 1–8) is high at room
3
temperature in ethanol as solvent for an aldehyde/Et SiH = 1/2 ra-
tio. However, we have observed the presence of the corresponding
alcohol as intermediate in the reduction reaction. The reaction of
3 2
benzyl alcohol with Et SiH/PdCl system in presence of the solvent
gave toluene after 30 min. It should be noted that for substrates
with two unsaturated sites such as terephthaldehyde (entry 9),
eq. of triethylsilane were required to drive the reaction to com-
by Et
Pd(OAc)
Et SiH (1/2) in the presence of 10% PdCl
0 min at room temperature in ethanol as solvent. Under the same
conditions, the use of Pd(OAc) as a catalyst yielded only 18.5% of
toluene and 81.5% of the starting alcohol. This clearly shows the
influence of the chloride (or Et SiCl) on the reaction mechanism.
3
SiH/EtOH in the presence of catalytic amounts of PdCl
. As discussed above, the reaction of benzyl alcohol with
gave 100% of toluene after
2
or
2
3
2
3
4
2
pletion. On the other hand, the reduction of 4-chlorobenzaldehyde
3
However at this stage it is hard to draw a final conclusion regard-
ing the exact reaction mechanism without performing more
experiments.
In conclusion, we have developed a simple and highly efficient
method for the reduction of carbonyl (C@O) group of aromatic
O
cat. PdCl2
R
R
Et SiH
3
2 3
aldehydes and ketones into –CH – group by the use of Et SiH in
R'
R'
ethanol in the presence of PdCl
2
catalyst. The reaction is easy to
EtOH
6
0
2-100%
carry out and takes place with high conversion yields.
OH
R= H, CH ,Ph
3
R
3. Experimental
R'= H, 4-CH , 2 or 3 or 4-OCH ,
3
3
All manipulations were carried out under an argon atmosphere.
The aldehydes, ketones and triethylsilane were obtained from Al-
drich and used without further purification. Ethanol was distilled
and stored under argon. GC/MS analysis was performed on a FISON
2
or 3 or 4-OH, 4-CHO, 4-Cl
R'
.4-10%
Scheme 1.

M. Mirza-Aghayan et al. / Journal of Organometallic Chemistry 693 (2008) 3567–3570
3569
Table 1
Reduction of aromatic aldehydes and ketones using Et
3 2
SiH/PdCl in ethanol
Entry
Substrate
Product
Substrate/Et
3
SiH
Time (min)
Yielda,b
%
1
2
3
4
5
6
7
8
9
Benzaldehyde
Toluene
p-Xylene
2-Methylanisole
3-Methylanisole
4-Methylanisole
o-Cresol
m-Cresol
p-Cresol
p-Xylene
4-Chlorotoluene
Ethylbenzene
1-Ethyl-2-methoxybenzene
Diphenylmethane
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/4
1/2
1/4
1/4
1/4
30
30
30
30
30
30
30
30
30
30
30
60(reflux)
60(reflux)
95.5(4.5)
88.5(10.0)
100.0
86.0
100.0
4-Methylbenzaldehyde
2-Methoxybenzaldehyde
3-Methoxybenzaldehyde
4-Methoxybenzaldehyde
2-Hydroxybenzaldehyde
3-Hydroxybenzaldehyde
4-Hydroxybenzaldehyde
Terephthaldehyde
4-Chlorobenzaldehyde
Acetophenone
2-Methoxyacetophenone
Benzophenone
94.5(2.0)
92.0(6.5)
98.0(2.0)
100.0
10
11
12
13
62.0c
99(1.0)
99.6(0.4)
100.0
a
Determined by GC/Ms analysis.
The number between the parentheses show the percent of the corresponding alcohol.
The reduction of 4-chlorobenzaldehyde gave 62% of 4-chlorotoluene and 19% of toluene.
b
c
J = 8.6 Hz, 2H, CH arom.). MS (70 eV), m/z (%): 122 (100) (M+),
2
Et SiH + PdCl2
3
1
07 (45), 91 (30), 77 (62).
Entry 7, Table 1, H NMR (CDCl
2
Et SiCl + H
3 2
1
3
, 500 MHz): d = 2.36 (s, 3H, CH
3
),
5
.24 (s, 1H, OH), 6.69 (m, 2H, CH arom.), 6.80 (d, J = 7.73 Hz, 1H, CH
arom.), 7.18 (t, J = 7.73 Hz, 1H, CH arom.). MS (70 eV), m/z (%):108
Pd(0)
Et SiH
3
H2 + Et SiOEt
+
3
(90) (M ), 107 (100), 79 (45).
1
Entry 8, Table 1, H NMR (CDCl
3
, 80 MHz): d = 2.21 (s, 3H, CH
3
),
4
.43 (s, 1H, OH), 6.64 (d, J = 8.60 Hz, 2H, CH arom.), 6.97 (d,
+
J = 8.60 Hz, 2H, CH arom.). MS (70 eV), m/z (%): 108 (75) (M ),
1
07 (100), 77 (40).
Entry 12, Table 1, 1H NMR (CDCl
, 500 MHz): d = 1.37 (t,
), 3.96 (s, 3H,
), 6.98 (d, J = 7.50 Hz, 2H, CH arom.), 7.05 (t, J = 7.50 Hz, 1H,
CH arom.), 7.34 (m, 2H, CH arom.). MS (70 eV), m/z (%): 136 (56)
3
EtOH
Et SiPdH
3
J = 7.53 Hz, 3H, CH
3 2
), 2.81 (q, J = 7.53 Hz, 2H, CH
OCH
3
Scheme 2.
+
(
M ), 121 (100), 91 (50), 77 (12).
1
Entry 13, Table 1, H NMR (CDCl
3
, 80 MHz): d = 4.03 (s, 2H, CH
2
),
RCHO + H2
RCH -OH
2
+
7
.17 (s, 10H, CH arom.). MS (70 eV), m/z (%): 168 (87) (M ), 167
(
100), 153 (25), 91 (22).
RCH -OH + Et SiCl
RCH -Cl
2
2
3
RCH -Cl
+
Et SiH
R-CH3 + Et SiCl
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