A simple and efficient hydrogenation of benzyl alcohols to methylene compounds using triethylsilane and a palladium catalyst

Article abstract of DOI:10.1016/j.tetlet.2009.08.043

Hydrogenolysis of benzyl alcohols using triethylsilane (Et₃SiH) and a catalytic amount of palladium(II) chloride (PdCl₂) is described. The reaction takes place under mild conditions affording high yields of the corresponding methylene compounds in short reaction times.

Full text of DOI:10.1016/j.tetlet.2009.08.043

Tetrahedron Letters 50 (2009) 5930–5932
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Tetrahedron Letters
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A simple and efficient hydrogenation of benzyl alcohols to methylene
compounds using triethylsilane and a palladium catalyst
b
a
Maryam Mirza-Aghayan a, , Rabah Boukherroub , Mahshid Rahimifard
*
a Chemistry and Chemical Engineering Research Center of Iran (CCERCI), PO BOX 14335-186, Tehran, Iran
b Institut de Recherche Interdisciplinaire (IRI, USR 3078), et Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN, UMR 8520), Cité Scientifique,
Avenue Poincaré–B.P 60069, 59652 Villeneuve d’Ascq, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 May 2009
Revised 22 July 2009
Accepted 14 August 2009
Available online 20 August 2009
Hydrogenolysis of benzyl alcohols using triethylsilane (Et3SiH) and a catalytic amount of palladium(II)
chloride (PdCl2) is described. The reaction takes place under mild conditions affording high yields of
the corresponding methylene compounds in short reaction times.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Hydrogenolysis
Benzyl alcohols
Palladium(II) chloride
Triethylsilane
Reduction
Methylene compounds
Heterogeneous and homogeneous catalytic hydrogenation reac-
tions have found widespread applications in the reduction of a
large variety of functional groups.1–3
kanes.11 The Raney Ni–sulfuric acid system was successfully ap-
plied for the reduction of several aryl bromides, benzyl alcohols,
benzyl ethers, and benzylamines under mild conditions. Although
details of the reaction mechanism for this reduction are not clear,
reductive cleavage of the substrate is believed to proceed by utiliz-
ing both the adsorbed hydrogen on the Raney Ni and the hydrogen
generated by the reaction between Raney Ni and sulfuric acid.12
Furthermore, organosilicon reagents in the presence of small
amounts of a catalyst are used for the reduction of functional
groups. The combination of chlorodimethylsilane and a catalytic
amount of an indium compound is effective for the deoxygenation
of aryl ketones and sec-benzylic alcohols to give the corresponding
hydrocarbons.13,14 Finally, ionic reduction of alcohols and ketones
with triethylsilane/trifluoroacetic acid (Et3SiH/CF3CO2H) was de-
scribed by Mayr.15
Benzyl alcohols are often used in organic synthesis, either as
starting materials or as protecting groups.4–7 Hydrogenolysis of
benzyl alcohols has been described in several reports. For example,
benzyl alcohols are readily converted into tetrazolyl and benziso-
thiazolyl ethers which can be catalytically hydrogenolyzed into
toluenes over palladium-on-charcoal in good yields using hydro-
gen donors.8 Hydrogen iodide, generated (and regenerated)
in situ by the action of iodine upon hypophosphorus acid, was
found to reduce a variety of aryl carbinols to the corresponding
diphenylmethanes under mild conditions.9 The reduction of pri-
mary, secondary, and tertiary benzylic alcohols to their corre-
sponding hydrocarbons was performed in a two-phase system
[apolar organic solvent and 50% KOH(aq)] at 50 °C, using 5% Pd/C
and H2 at atmospheric pressure. The reaction requires the presence
of aromatic halides. In the absence of the halide promoter, the
reaction takes place only if a halogen is present on the aromatic
ring of the alcohol.10 The use of trialkylborons with trifluorometh-
anesulfonic acid was found to be an effective reducing system for
the conversion of a variety of tertiary, secondary, and benzylic
alcohols into the corresponding alkanes, as well as alkylated al-
We have investigated the efficiency of the Et3SiH/PdCl2 system
for the hydrogenation of 1-alkenes under mild conditions,16,17 for
the transformation of alcohols into their corresponding silyl ethers
and for the cleavage of triethylsilyl ethers to the parent alcohols.18
Furthermore, the versatility of the Et3SiH/PdCl2 system was dem-
onstrated for the selective hydrogenation of the carbon–carbon
double bond of a,b-unsaturated ketones to afford the correspond-
ing saturated ketones under mild conditions.19 More recently, this
system was used for the reduction of carbonyl groups of aromatic
aldehydes and ketones into a methylene group.20 The results indi-
cated the formation of the corresponding alcohol as an intermedi-
* Corresponding author.
E-mail address: m.mirzaaghayan@ccerci.ac.ir (M. Mirza-Aghayan).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.08.043

M. Mirza-Aghayan et al. / Tetrahedron Letters 50 (2009) 5930–5932
5931
ate during the reduction of aromatic aldehydes. Based on the above
observation, the present work was aimed at exploring the effi-
ciency of molecular hydrogen, generated in situ by the reaction
of Et3SiH with EtOH in the presence of a catalytic amount of PdCl2,
for the reduction of benzyl alcohols to the corresponding methy-
lene compounds.
Reduction of various benzyl alcohols with Et3SiH in the pres-
ence of palladium dichloride in ethanol occurs at room tempera-
ture to yield the corresponding methylene products in high
yields (Scheme 1, Table 1).
The hydrogenation reaction requires the use of an inert atmo-
sphere and anhydrous solvent. In a typical experiment, PdCl2
(10 mol %) was added at room temperature to a stirred mixture
of alcohol (1 equiv) and Et3SiH (2 equiv) in dry ethanol (5 ml). An
exothermic reaction occurs during the first 5 min and then the
temperature decreases to room temperature. The resulting mixture
was stirred for the time indicated in Table 1 prior to GC–MS
analysis.
The reduction of the benzyl alcohols takes place quantitatively.
For example, the reaction of 1 equiv of benzyl alcohol (Table 1, en-
try 1), 4-methoxybenzyl alcohol (Table 1, entry 2) and 1-phenyl-
ethanol (Table 1, entry 7) with Et3SiH/PdCl2 (2 equiv/10%) in
ethanol (5 ml) for 10 min at room temperature led to the formation
of toluene, 4-methylanisole and ethylbenzene, respectively, in
100% yields. Likewise, the reaction of 4-hydroxy-3-methoxybenzyl
alcohol (Table 1, entry 4), 4-tert-butyl-benzyl alcohol (Table 1, en-
try 6) and 1-phenyl-1-propanol (Table 1, entry 8) gave 2-methoxy-
4-methylphenol, 4-tert-butyltoluene and propylbenzene in 96%,
90% and 95% yields, respectively. Increasing the reaction time to
30 min led to an increase in the yield to 95% for entry 6.
It should be noted that for certain compounds an excess of tri-
ethylsilane or longer reaction times were required to drive the
reaction to completion. For example, the reaction of 2-biphenyl-
4-ylpropan-2-ol (Table 1, entry 9) with Et3SiH (alcohol/Et3SiH: 1/
2 or 1/4) produced, respectively, 72% and 82% yields of the corre-
sponding product after 30 min. However, high conversion (96%)
was obtained when the reaction was conducted in the presence
of an excess of triethylsilane (alcohol/Et3SiH: 1/6). Similarly, the
reaction of the secondary alcohol benzhydrol (Table 1, entry 10)
with 2 equiv of Et3SiH in the presence of PdCl2 led to the formation
of diphenylmethane in 87% yield, after 30 min at room tempera-
ture. The yield was increased to 100% by increasing the reaction
time to 1 h under the same conditions. A yield of 100% was ob-
tained when the ratio of benzhydrol/Et3SiH was increased to 1/3,
after 30 min at room temperature. 1-Indanol was converted quan-
titatively into indane with 2 equiv of Et3SiH after 10 min at room
temperature. We also observed the reduction of conjugated alco-
hols such as cinnamyl alcohol to propylbenzene using 8 equiv of
Et3SiH in 94% yield after 10 min at room temperature. Finally, we
examined the hydrogenolysis of heterocylic alcohol derivatives. A
trace amount of 3-methylpyridine was obtained after 1 h at room
temperature in ethanol when the reduction of 3-pyridinylmetha-
nol was carried out using 2 equiv of Et3SiH. Using an excess of tri-
ethylsilane (8 equiv) and performing the reaction at reflux gave 3-
methylpyridine in high yield (95%) after 4 h. On the other hand,
thiophene-2-methanol was readily reduced to 2-methylthiophene
in 70% yield using 6 equiv of Et3SiH in the presence of PdCl2, after
4 h at room temperature.
OH
cat.PdCl2
Ar
70-100%
R
Ar
R
Et3SiH
EtOH
Next, we investigated the hydrogenolysis of aliphatic alcohols
such as 1-heptanol and 1-decanol using the Et3SiH/EtOH/PdCl2 sys-
tem. The reduction failed to proceed under these conditions, even
in the presence of a large excess of Et3SiH. Indeed, no reaction oc-
R= H, CH3, Et, Ph
Scheme 1.
Table 1
Reduction of benzyl alcohols to the corresponding methylene compounds using Et3SiH/PdCl2 in ethanol
Entry
Substrate
Product
Substrate/Et3SiH
Time (min)
Yielda (%)
1
2
3
4
5
6
7
8
9
Benzyl alcohol
Toluene
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/4
1/6
1/2
1/2
1/3
1/2
1/4
1/6
1/8
1/2
1/6
1/6
1/8
1/2
1/6
1/6
10
10
10
10
30
10
10
10
30
30
30
30
60
30
10
10
10
10
60
240
60
240
60
60
240
100
100
98
96
90
90
100
95
72
82
96
87
100
100
100
80
4-Methoxybenzyl alcohol
2-Methoxybenzyl alcohol
4-Hydroxy-3-methoxybenzyl alcohol
2-Hydroxybenzyl alcohol
4-tert-Butyl-benzyl alcohol
1-Phenylethanol
4-Methylanisole
2-Methylanisole
2-Methoxy-4-methylphenol21
o-Cresol
4-tert-Butyltoluene
Ethylbenzene
1-Phenyl-1-propanol
2-Biphenyl-4-ylpropan-2-ol
Propylbenzene
4-Isopropylbiphenyl
10
Benzhydrol
Diphenylmethane
11
12
1-Indanol
Cinnamyl alcohol
Indane
Propylbenzene
86
94
13
14
3-Pyridinylmethanol
3-Methylpyridine
2-Methylthiophene
Trace
34
85b
95b
24
Thiophene-2-methanol
54
70
a
Determined by GC–MS.
The reaction was conducted at reflux.
b

5932
M. Mirza-Aghayan et al. / Tetrahedron Letters 50 (2009) 5930–5932
Table 2
benzylic alcohols does not require the presence of a halogenating
agent. The only potential source of halogen is Et3SiCl, resulting
from reduction of PdCl2 by Et3SiH, as previously suggested.23 Thus
it is hard to draw a final conclusion regarding the exact reaction
mechanism at this stage.
Reduction of benzyl alcohol with Et3SiH (1/2) with different palladium catalysts in the
absence of a solvent at room temperature
Entry
Catalyst
Time
PhMe/BnOSiEt3/BnOHa (%)
1
2
3
4
5
PdCl2 (10%)
20 min
20 min
5 min
15 min
19 h
50/50/0
8/78/14
100/0/0
16/27/57
84/3/2c
Pd(OAc)2 (10%)
PdCl2 (1 equiv)
Pd(OAc)2 (1 equiv)
In conclusion, we have developed a simple and highly efficient
method for the hydrogenolysis of benzyl alcohols to the corre-
sponding methylene compounds using Et3SiH in ethanol in the
presence of a catalytic amount of PdCl2. The reaction is easy to car-
ry out affording high yields of the corresponding products.
General procedure for the reduction of alcohols: To a solution of
alcohol (1 mmol, 1 equiv) and triethylsilane (amount indicated in
Table 1) in ethanol (5 ml) was added a catalytic amount of palla-
dium(II) chloride (10 mol %) under an argon atmosphere. The
resulting mixture was stirred for the time indicated in Table 1 prior
to GC–MS analysis. The pure products in entries 1 and 7 were iso-
lated by distillation, and those for entries 2–6 and 8–10 were iso-
lated by column chromatography using hexane/ethyl acetate (9/1)
as eluent. The products24 were characterized by 1H NMR spectros-
copy and mass spectrometry.
b
Pd(OAc)2 (10%)
a
Determined by GC–MS.
The reaction was conducted in the presence of 1 equiv of Cl3C–CCl3 as halo-
b
genating agent.
c
11% of benzyl chloride was also obtained.
curred after 30 min at room temperature in ethanol in the presence
of 6 equiv of Et3SiH, or when using 10 equiv of Et3SiH in the pres-
ence of PdCl2 at reflux for 4 h, and only the starting material was
recovered. However, when the reaction was performed for 22 h
at room temperature in the presence of excess Et3SiH (alcohol/Et3-
SiH: 1/10), the corresponding triethylsilyl ethers were obtained in
62% and 56% yields along with 38% and 44% yields of the starting
alcohols, respectively.
Entry 9, Table 1, 1H NMR (CDCl3, 80 MHz): d = 1.42 (d, J = 5.3 Hz,
6H, CH3), 3.06 (m, 1H, CH), 7.34–7.77 (m, 9H, CH arom.). MS
(70 eV), m/z (%): 196 (62) (M+), 181 (100).
We also examined the reduction of benzyl alcohol by Et3SiH in
the presence of catalytic amounts of PdCl2 or Pd(OAc)2 in the
absence of solvent (ethanol). The reaction of benzyl alcohol with
Et3SiH (1/2) in the presence of 10% PdCl2 gave 50% of toluene
and 50% of the corresponding triethylsilyl ether after 20 min at
room temperature (Table 2, entry 1). Under the same conditions,
Pd(OAc)2 as the catalyst afforded only an 8% yield of toluene, 78%
of the corresponding triethylsilyl ether, and 14% of the starting
alcohol (Table 2, entry 2). On the other hand, the use of 1 equiv
of PdCl2 in the absence of ethanol yielded quantitatively, toluene,
after only 5 min (Table 2, entry 3). In contrast, only partial reduc-
tion of benzyl alcohol was observed when the reaction was per-
formed in the presence of 1 equiv of Pd(OAc)2 (Table 2, entry 4).
These results clearly indicate the influence of the chloride ions
(or Et3SiCl) on the reaction mechanism. Thus we investigated the
reduction of benzyl alcohol with Pd(OAc)2 as the catalyst in the
presence of a halogenating agent such as Cl3C–CCl3. The reaction
of benzyl alcohol with Et3SiH (1/2) in the presence of 10% Pd(OAc)2
as catalyst and 1 equiv of Cl3C–CCl3 yielded 84% of toluene, 3% of
benzyloxytriethylsilane, 11% of benzyl chloride, and 2% of the
starting alcohol under solvent-free conditions after 19 h at room
temperature (Table 2, entry 5). We also performed the reduction
of benzyl chloride using PdCl2 as the catalyst in the presence of
2 equiv of Et3SiH and the reduction to toluene was complete after
5 min at room temperature.
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The results suggest that chloride ions might play a role in the
reduction mechanism. This is corroborated by previous reported
work on the reduction of alcohols into their corresponding halides
and alkanes using the Et3SiH/PdCl2 system in the presence of a hal-
ogenating agent.22 Furthermore, addition of a halogenating agent
such as Cl3C–CCl3 to the reaction mixture led to an increase in
the yield for the reduction of benzyl alcohol by Et3SiH in the pres-
ence of Pd(OAc)2. However, in the present work, the reduction of
23. Boukherroub, R.; Manuel, G.; Chatgilialoglu, C. Organometallics 1996, 15, 1508.
24. Pouchert, C. J., The Aldrich Library of NMR Spectra, 2nd ed. Vol. 1 and 2, 1983.