Palladium chloride-catalyzed reductive cleavage of benzylic acetal, ketal and ether compounds with triethylsilane

Article abstract of DOI:10.1007/BF03249092

Reductive cleavage of benzylic acetal, ketal and ether compounds to the corresponding alkanes using triethylsilane and a catalytic amount of palladium(II) chloride is described. The reductive reaction took place under mild conditions, affording high yields of the corresponding alkane compounds in short reaction times.

Full text of DOI:10.1007/BF03249092

J. Iran. Chem. Soc., Vol. 8, No. 2, June 2011, pp. 570-573.
JOURNAL OF THE
Iranian
Chemical Society
Palladium Chloride-Catalyzed Reductive Cleavage of Benzylic Acetal, Ketal and Ether
Compounds with Triethylsilane
a,
b
a
a
M. Mirza-Aghayan *, R. Boukherroub , M. Rahimifard and R. Zadmard
Chemistry and Chemical Engineering Research Center of Iran (CCERCI), P. O. BOX 14335-186, Tehran, Iran
a
b
Institut de Recherche Interdisciplinaire (IRI, USR 3078), Parc de la Haute Borne, 50 Avenue de Halley- BP 70478,
9658 Villeneuve d'Ascq, France
5
(
Received 31 August 2010, Accepted 25 November 2010)
Reductive cleavage of benzylic acetal, ketal and ether compounds to the corresponding alkanes using triethylsilane and a
catalytic amount of palladium(II) chloride is described. The reductive reaction took place under mild conditions, affording high
yields of the corresponding alkane compounds in short reaction times.
Keywords: Reduction, Acetals, Ketals, Ethers, Palladium(II) chloride, Triethylsilane, Alkanes
INTRODUCTION
benzylic alcohols to yield the corresponding hydrocarbons
14-15]. Reduction of C-O bonds of primary alcohols or ethers
has taken place using triethylsilane (Et SiH) and catalytic
amounts of B(C . Gevorgyan et al. reported that certain
alcohols could be converted to alkanes by using Et SiH as
[
The development of new methods for the deprotection of
functional groups in one-step avoiding the intermediate step of
going back to the parent functionality has gained importance
in recent years. Aldehydes, ketones and diols are frequently
protected as acetals and ketals in organic synthesis, and a
number of techniques for their deprotection are known [1-7].
However, some of the reported methods for the deprotection
of acetals require high temperatures, long reaction times and
most of them involve acidic conditions. It is well known that
acidic treatment is often incompatible with the presence of
other acid-sensitive functional groups. Thus, milder conditions
and safer procedures have been developed for the deprotection
of acetals [8-13].
3
6
F
)
5 3
3
reductant, via initial formation and reduction of triethylsilyl
alkyl ethers [16-17]. Brookhart et al. used cationic iridium
pincer catalysts for the room temperature cleavage and
reduction of a broad range of alkyl ethers with triethylsilane
and investigated the mechanistic details governing this
catalytic transformation [18]. Similarly, the C-OH bond of
benzyl alcohols has been readily reduced to the corresponding
C-H bond with triethylsilane in the presence of Bi(OTf)3
catalyst [19]. The reduction of acetals to ethers and orthoesters
to acetals with hydrosilane using rhodium catalyst was
reported by Ohta et al. [20]. The deacetoxylation of
propargylic acetates with triethylsilane catalyzed by
indium(III) bromide to produce internal alkynes containing a
variety of functional groups was demonstrated by Sakai et al.
[21].
Organosilicon reagents in the presence of small amounts of
a catalyst are commonly used for the reduction of functional
groups. For example, the combination of chlorodimethylsilane
and a catalytic amount of indium trichloride (InCl ) is
effective for the deoxygenation of aryl ketones and sec-
3
We have previously investigated the efficiency of
*Corresponding author. E-mail: m.mirzaaghayan@ccerci.ac.ir
3
Et SiH/PdCl2 system for various chemical transformations

Mirza-Aghayan et al.
Table 1. PdCl -Catalyzed Reductive Cleavage of Benzylic
under mild conditions: hydrogenation of 1-alkenes [22,23],
selective hydrogenation of the carbon-carbon double bond of
α,β-unsaturated ketones to afford the corresponding saturated
ketones [24], and reduction of aromatic ketones, aldehydes
2
Acetal and Ketal Compounds to the Corresponding
Alkanes with Et SiH in Ethanol
3
Substrate/
Time
a
entry
1
Substrate
Ar
R
Et SiH
min Yield %
[
25] and alcohols [26,27]. We have also shown the versatility
3
O
O
H
of R SiH/PdCl system in the chemical transformation of
3
2
1a
CH
3
1/2
30
96
alcohols to silyl ethers and the subsequent deprotection of
triethylsilyl ethers to the parent alcohols under mild conditions
O
O
H
2
3
4
5
6
7
1b
1c
1d
1e
1f
CH3
1/2
30
60
70
82
1/2
[
28].
H C
H
3
C
3
O
O
CH
1/2
30
95
CH
3
3
EXPERIMENTAL
O
O
1
1
/10
/13
60b
60
75
81
b
All operations were carried out under an argon
atmosphere. Ethanol was distilled and stored under argon.
GC/MS analysis was performed on a FISON GC 8000 series
TRIO 1000 gas chromatograph equipped with a capillary
column CP Sil.5 CB, 60 M × 0.25 mm Id.
O
O
H
CH3
1/6
1/2
1/2
60b
30
78
96
90
N
N
O
O
H
CH
3
O
O
H
CH3
General Procedure for the Reduction
To a solution of an acetal, ketal or ether derivative (1
mmol, 1 equiv.) and triethylsilane (amount indicated in Tables
1g
30
HO
HO
aDetermined by GC/Ms analysis. bThe reaction was
conducted in refluxing ethanol.
1
and 2) in ethanol (5 ml) was added a catalytic amount of
palladium(II) chloride (10 mol%) under an argon atmosphere.
The resulting mixture was stirred for the time indicated in
Tables 1 and 2 prior to GC/MS analysis. The reaction mixture
was filtered and the filtrate was evaporated. The spectroscopic
data of the obtained products were compared with authentic
samples [29].
n
O
cat. PdCl2
O
Ar
R
Et SiH
3
Ar
R
EtOH
1a-g
2a-g
Scheme 1
RESULTS AND DISCUSSION
cat. PdCl2
ROR'
a-k
R''H
Et SiH
The present work was aimed at exploring the efficiency of
molecular hydrogen, generated in situ by the reaction of
3
3
4a-k
EtOH
Et SiH with EtOH in the presence of a catalytic amount of
Scheme 2
3
PdCl , for the reductive cleavage of benzylic acetal, ketal and
2
ether compounds to the corresponding alkanes. The reduction
of various benzylic acetal, ketal and ether compounds with
equiv.) in dry ethanol (5 ml). An exothermic reaction occurred
during the first five minutes and then the temperature
decreased to room temperature. The resulting mixture was
stirred for the time indicated in Tables 1 and 2 prior to GC/MS
analysis.
Et SiH in the presence of palladium dichloride in ethanol
3
occurred at room temperature to give the corresponding aryl
alkane products in high yields (Schemes 1 and 2). The
hydrogenation reaction requires the use of an inert atmosphere
and anhydrous solvent. In a typical experiment, PdCl (10
mol%) was added at room temperature to a stirred mixture of
The reductive cleavage of benzylic acetals and ketals took
place in high yields (entries 1-7, Table 1). For example, the
reaction of 1 equiv. of 2-phenyl-1,3-dioxolane (entry 1, Table
2
acetal, ketal or ether derivative (1 equiv.) and Et SiH (2
3
5
71

Palladium Chloride-Catalyzed Reductive Cleavage of Benzylic Acetal
Table 2. PdCl -Catalyzed Reductive Cleavage of Ether
2
1
, 1a) with Et SiH/PdCl (2 equiv./10 mol%) in ethanol (5 ml)
3
2
Compounds to the Corresponding Alkanes with
for 30 min at room temperature led to the formation of toluene
in 96% yield. Similarly, the reaction of 2-p-tolyl-1,3-dioxolane
Et SiH in Ethanol
3
(
(
9
entry 2, Table 1, 1b) or 2-methyl-2-phenyl-1,3-dioxolane
entry 3, Table 1, 1c) gave p-xylene or ethylbenzene in 82 and
5% yield after 60 and 30 min, respectively. It should be noted
Substrate/ Time
a
entry
1
ROR'
O
R"H
Yield %
Et SiH
3
min
3a
3b
1/3
1/4
30
99
that for certain compounds, an excess of triethylsilane or
longer reaction times were required to drive the reaction to
completion. For example, the reaction of 2,2-diphenyl-1,3-
dioxolane (entry 4, Table 1, 1d) with Et
(
OH
OH
OH
2
30
99
O
+
OEt
1
/15
60b
120
59
61
3
SiH
SiH: 1/10 or 1/13) produced diphenylmethane in
5 and 81% yield, respectively, after 60 min refluxing. The
3
4
3c
3d
b
1/15
O
substrate/Et
3
O
O
O
O
1/10
60
no reaction
7
reaction of 3-(1,3-dioxolan-2-yl)pyridine (entry 5, Table 1, 1e)
with 6 equiv. Et SiH produced 3-methylpyridine in 78% yield
after 60 min in ethanol refluxing. 2-Phenyl-1,3-dioxane (entry
CH
3
CH
3
5
6
3e
3f
1/4
1/2
60
10
b
99
98
COCH
O
3
CH2CH3
3
O
CH
3
CH3
CH
2
OH
CH
3
6
1
, Table 1, 1f) and 4-(1,3-dioxan-2-yl)phenol (entry 7, Table
, 1g) were successfully reduced to the corresponding toluene
O
O
CH
3
CH
3
7
8
9
3g
3h
1/2
1/2
1/2
10
10
30
100
96
HOH
HO
2
C
3
H C
and p-cresol in 82 and 95% yield after 30 min, respectively.
O
H
3
C
O
CH
OH
3
CH3
The results showed that benzylic acetal and ketal
OH
O
O
compounds are readily cleaved with Et SiH in the presence of
1
CH
3
3i
CH3
3
100
3
CHO
O
CH
O
0 mol% of palladium(II) chloride to yield aryl methane and
CH
3
CH
3
10
3j
1/2
30
86
mono and/or disilylated diols. Indeed, both mono and
disilylated diols were observed in the GC/MS spectra. The
formation of monosilylated diol is most likely due to the
partial hydrogenation of the disilylated diol. In fact, we have
previously demonstrated that triethylsilyl ethers can be easily
CHO
CH3
O
O
CH
3
CH3
11
3k
1/2
30
100
OHC
OHC
aDetermined by GC/Ms analysis. bThe reaction was
conducted in refluxing ethanol.
cleaved to the corresponding alcohols by Et SiH/EtOH system
3
in the presence of PdCl catalyst [28]. The formation of benzyl
2
alcohol as an intermediate during the reductive cleavage of
acetals is not excluded. Its subsequent reduction to the
corresponding methylene compounds takes place in very short
reaction time [27].
the reduction of similar ether. Indeed, (2-(benzyloxy)propan-
2-yl)benzene compound did not undergo the reduction using 4
equiv. Et SiH and B(C H ) or B(C F ) system as reductant
3
6
5 3
6 5 3
[16,17]. The reaction of 4-(benzyloxy)phenol (entry 2, Table
We have also examined the reduction of ether compounds
2, 3b) with 4 equiv. of Et SiH for 30 min produced toluene in
3
(
Table 2). The results indicate that benzylic ethers can be
99% yield. We also observed the presence of hydroquinone
readily converted to the corresponding alkanes in excellent
yields. For example, the reaction of 1 equiv. of dibenzylether
and its mono and/or disilylated form. However, for the
reduction of O-ethylbenzoine, an excess of triethylsilane and
longer reaction times in refluxing ethanol was required.
Indeed, the reaction of 1 equiv. of O-ethylbenzoine (entry 3,
Table 2, 3c) with Et SiH/PdCl (15 equiv./10 mol%) for 60
min in ethanol refluxing (5 ml) led to the formation of
dibenzyl in 59% yield. In this case, an increase of the reaction
time did not have any effect on the reaction yield.
(
entry 1, Table 2, 3a) with Et SiH/PdCl (3 equiv./10 mol%) in
3
2
ethanol (5 ml) for 30 min at room temperature led to the
formation of toluene in 99% yield. The formation of toluene is
most likely a stepwise process. We believe that the cleavage of
3
2
dibenzylether with Et SiH in the presence of palladium
3
catalyst yields aryl methane compound (toluene) and benzyl
alcohol. The latter is then reduced to toluene [27]. However, it
should be noted that Gevorgyan et al. reported no reaction for
This method of reduction appears to be limited to benzylic
ether compounds. In fact, when the reaction conditions
5
72

Mirza-Aghayan et al.
described above were applied to diphenylether, only the
starting material was recovered after 60 min reaction even
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3
in ethanol at room temperature.
In conclusion, we have developed a simple and efficient
method for the reduction of acetal, ketal and ether compounds
to the corresponding aryl alkanes using Et SiH/PdCl system
3
2
in ethanol. The proposed procedure has several advantages
including the ready availability of the catalyst and simple
workup. The adoption of this simple technique will be an
attractive addition to the range of procedures already known
for this general transformation.
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