Deoxygenation of tertiary and secondary benzylic alcohols into alkanes with triethylsilane catalyzed by solid acid tin(IV) ion-exchanged montmorillonite

Article abstract of DOI:10.1016/j.tetlet.2014.05.042

We discovered an efficient protocol for the conversion of tertiary and secondary benzylic alcohols into the corresponding alkanes in good to quantitative yields by employing tin(IV) ion-exchanged montmorillonite (Sn-Mont) as a solid acid catalyst and Et3SiH as the hydride source. The reaction is likely to proceed via the S_N1-type reaction mechanism, that is, the formation of carbenium ions, followed by the addition of a hydride from the silane. The work-up of the reaction only requires simple filtration of the solid acid without any neutralization of the acid catalyst.

Full text of DOI:10.1016/j.tetlet.2014.05.042

Tetrahedron Letters xxx (2014) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Deoxygenation of tertiary and secondary benzylic alcohols
into alkanes with triethylsilane catalyzed by solid acid tin(IV)
ion-exchanged montmorillonite
Michael Andreas Tandiary a, Yoichi Masui b, Makoto Onaka b,
⇑
a Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
b Graduate School of Arts and Sciences, The University of Tokyo, Komaba 3-8-1, Meguro, Tokyo 153-8902, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
We discovered an efficient protocol for the conversion of tertiary and secondary benzylic alcohols into the
corresponding alkanes in good to quantitative yields by employing tin(IV) ion-exchanged montmorillon-
ite (Sn-Mont) as a solid acid catalyst and Et3SiH as the hydride source. The reaction is likely to proceed
via the SN1-type reaction mechanism, that is, the formation of carbenium ions, followed by the addition
of a hydride from the silane. The work-up of the reaction only requires simple filtration of the solid acid
without any neutralization of the acid catalyst.
Received 12 April 2014
Revised 26 April 2014
Accepted 9 May 2014
Available online xxxx
Keywords:
Ó 2014 Elsevier Ltd. All rights reserved.
Sn-Montmorillonite
Solid acid catalysis
Deoxygenation of benzylic alcohols
Triethylsilane
The direct conversions of alcohols to alkanes are often required
during the synthesis of organic molecules. Especially, tertiary and
secondary hydroxyl groups at the benzylic position are apt to be
replaced with a hydrogen atom by various reduction methods such
as the H2/Pd-C catalyst,1 Raney Ni/EtOH,2 alkali metals/NH3,3
NaBH4/CF3CO2H,4 NaBH3CN/ZnI2,5 LiAlH4,6 Et3SiH/CF3CO2H,7
sodium ions in the natural montmorillonite are substituted with
protons or multivalent metal ions, such as Al3+ and Fe3+, the clays
become strongly acidic. We have developed tin(IV) ion-exchanged
montmorillonite (Sn-Mont) and have revealed significant accelera-
tions of various acid-catalyzed organic reactions, such as the
cyanosilylation of aldehydes and ketones,12 the Strecker reaction,13
and the Mukaiyama aldol reaction.14
Recently, we have been interested in the Sn-Mont catalyzed
direct substitution of hydroxyls at the benzylic or allylic positions
with typical nucleophiles, such as cyanotrimethylsilane,15 allyltri-
methylsilane,16 and active methylene compounds,17 because the
hydroxyl of a typical poorly-leaving group was smoothly subject
to the direct nucleophilic substitution reactions. In these reactions,
once the carbenium ions are directly formed from the alcohols
upon contact with the acidic Sn-Mont, they subsequently react
with the nucleophiles to afford the corresponding products. We
envisioned that benzylic alcohols would be deoxygenated by
R3SiH in the presence of a catalytic amount of Sn-Mont to give
deoxygenated products, although we have previously reported that
aryl ketones, such as acetophenone and benzophenone, were
completely reduced to alkanes by Et3SiH in the presence of
Sn-Mont and Fe-Mont.18 In addition to the efficient catalysis, the
solid acid of Sn-Mont can be simply removed by filtration without
any neutralization process.
8
BF3ÁOEt2 or PdCl2,9 Me2SiHCl or Ph2SiHCl/InCl3,10 polymethylhy-
11
drosiloxane/FeCl3 etc.
Among such deoxygenation reagents, triethylsilane (Et3SiH) is
particularly suitable for small-scale experiments due to being a
liquid reductant with a relatively low boiling point. The deoxygen-
ation of benzylic hydroxyls with Et3SiH is normally carried out in
the presence of homogeneous acids, CF3CO2H7 and BF3ÁOEt2.8
Under such acidic conditions, the benzylic alcohols are trans-
formed into the corresponding carbenium ions through the proton-
ation or coordination of BF3 to the hydroxyls, followed by
subsequent hydride transfers from Et3SiH to yield the alkane prod-
ucts. For the homogeneous acid catalysts, basic work-up processes
are required to remove the acids from the reaction mixture.
Montmorillonite (Mont) is one of the abundant naturally-
occurring clays and composed of stacked, negatively charged,
two-dimensional aluminosilicate layers that contain exchangeable
cationic species, mostly sodium ions, between the layers. When the
We first performed an exploratory deoxygenation of a tertiary
alcohol, trityl alcohol (triphenylmethanol 1a), with Et3SiH in
CH2Cl2 at room temperature (rt) as shown in Table 1. Sn-Mont as
⇑
Corresponding author. Tel.: +81 354546595.
E-mail address: conaka@mail.ecc.u-tokyo.ac.jp (M. Onaka).
http://dx.doi.org/10.1016/j.tetlet.2014.05.042
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Tandiary, M. A.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.05.042

2
M. A. Tandiary et al. / Tetrahedron Letters xxx (2014) xxx–xxx
Table 1
Table 3
Deoxygenation of triphenylmethanol (1a) with silane in the presence of homoge-
Scope of substrates for deoxygenation with Et3SiH in the presence of Sn-Mont
neous and clay catalystsa
Entry
Alcohol
Conditiona
Yieldb
1
2
3
4
1a
1b
1c
1d
A
B
B
A
Quant
Quant
Quant
Quant
Catalyst
CH2Cl2
RT for 40 min
OH
+
Silane
2a
Solvent
1a
OH
Entry
Catalyst
Silane
Yield of 2b
5
6
B
B
Quant
Quant
1c
2d
3d
4d
5d
6d
7d
8d
9d
BF3ÁOEt2
Na-Mont
H-Mont
Sn-Mont
Sn-Mont
Sn-Mont
Sn-Mont
Sn-Mont
Sn-Mont
Et3SiH
Et3SiH
Et3SiH
Et3SiH
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
Et2O
Quant
0%
F
F
1e
OH
Quant
Quant
Quant
Quant
89%
Ph2MeSiH
Ph2MeSiH
Et3SiH
1f
OH
Et3SiH
Et3SiH
0%
8%
7
8
9
B
B
A
Quant
77%
Acetone
1g
a
b
c
The reaction was performed in CH2Cl2 using 1a (1 mmol) and silane (2 mmol).
1H NMR yield using mesitylene as internal standard.
Using 10 mol % of BF3ÁOEt2 as the catalyst.
OH
d
Using 30 mg of Sn-Mont as the catalyst.
1h
OH
well as H-Mont (proton-exchanged montmorillonite), which is also
an acidic montmorillonite and has been used for various acid-cat-
alyzed organic transformations,19 showed quantitative yields
(entries 3 and 4). Na-Mont, which is a pristine clay for the prepa-
ration of Sn-Mont20 and H-Mont,20 did not produce any products
(entry 2). It was found that the catalytic activity of Sn-Mont and
H-Mont was comparable to that of BF3ÁOEt2, a control homogenous
acid catalyst.
The effects of R3SiH and reaction solvents were then investi-
gated as shown in Table 1. Et3SiH, Ph2MeSiH, and PhMe2SiH
showed no differences in the deoxygenation efficiency. CH2Cl2
was determined to be the solvent of choice among CH2Cl2, CH3CN,
Et2O and acetone.
90%
MeO
1i
OH
O
O
79%
10
11
12
B
B
B
(75%)c
1j
OH
90%
MeO
1k
OH
Quant
We next surveyed the deoxygenation of a secondary alcohol,
benzhydrol 1b. In the case of 1b, diphenylmethane was obtained
in only 28% yield together with ether 3b in 15%, and 41% of 1b
remained (entry 1 of Table 2). The symmetrical ether 3b was
thought to be derived from the etherification of a benzhydryl car-
benium ion with 1b. Prolonging the reaction time to 3 h from
40 min slightly improved the yield of 2b to 33%, but the yield of
3b also increased to 33%, accompanied by complete disappearance
of benzhydrol 1b (entry 2 of Table 2). Benzhydrol 1c, bearing
electron-withdrawing chlorine atoms at both para-positions, was
1l
OH
13
14
A
A
0%
0%
1m
OH
MeO
1n
a
Condition A: Sn-Mont 30 mg,
1
1 mmol, Et3SiH 2 mmol, at rt for 40 min.
Condition B: Sn-Mont 30 mg, 1 1 mmol, Et3SiH 2 mmol, under reflux for 1 h.
1H NMR yields using mesitylene as internal standard.
b
c
H-Mont 30 mg instead of Sn-Mont was used.
Table 2
Deoxygenation of diphenylmethanol derivatives with Et3SiH in the presence of
Sn-Mont
converted into 2c and 3c in only 8% and 5% yields, respectively.
In sharp contrast, benzhydrol 1d bearing electron-donating meth-
oxy groups easily underwent the deoxygenation to afford 2d in a
quantitative yield. These results indicate that the efficiency of the
deoxygenation with Et3SiH is quite dependent on the stability of
the intermediary diarylmethyl cation, implying that the deoxygen-
ation reaction proceeds through the SN1-type mechanism.7
We observed that ether 3b was also reduced with Et3SiH under
reflux to yield 2b in a quantitative yield. This is the reason why the
deoxygenation of 1b with Et3SiH at rt afforded hydrocarbon 2b as
well as ether 3b, while the reaction under reflux only quantita-
tively produced 2b, as shown in entries 2 and 3 of Table 2.
We also investigated the scope of the deoxygenation of benzylic
alcohols with Et3SiH, as summarized in Table 3. In general, the Sn-
Mont-catalyzed deoxygenation with Et3SiH is applicable to a vari-
ety of benzylic alcohols: the deoxygenation of tertiary benzylic
alcohols, such as 1a, 1f and 1g, afforded the corresponding alkanes
R
OH
Sn-Mont
O
+
R
+
Et3SiH
2
CH2Cl2
RT
R
R
R
1
2
3
R
Entry
Alcohol
Reaction time
1b
2b
3b
1
1b (R = H)
1b (R = H)
1b (R = H)
1c (R = Cl)
40 min
3 h
1 h
40 min
40 min
41%
—
—
82%
—
28%
33%
Quant
8%
15%
33%
—
5%
—
2
3c
4
5
1d (R = OMe)
Quant
aThe reaction was performed in CH2Cl2 using 1b–1d (1 mmol), Et3SiH (2 mmol) and
Sn-Mont (30 mg).
b
1H NMR yields using mesitylene as internal standard.
The reaction was performed under reflux conditions.
c
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M. A. Tandiary et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
in quantitative yields. For the secondary benzylic alcohols, 1b, 1c,
1d, 1i, 1k and 1l were successfully reduced in excellent yields.
Especially, the benzhydrol bearing electron-withdrawing atoms
of chlorine 1c underwent the deoxygenation under reflux condi-
tions. However, 1h and 1j only afforded the corresponding alkanes
in the good yields of 77% and 79%, respectively. The more efficient
catalysis by Sn-Mont than H-Mont was confirmed for the deoxy-
genation of 1j.
Condition B: The same reaction mixture as the one described in
condition A was refluxed for 1 h. The mixture was then cooled to
room temperature, the Sn-Mont was filtered off, and the filtrate
was concentrated and analyzed by NMR.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.05.
042.
In contrast, the deoxygenation of primary benzylic alcohols 1m
and 1n was unsuccessful: 1m remained intact, while 1n produced
polymerized products.
Some advantages of using the solid acid instead of homogenous
conventional acids, such as CF3CO2H and BF3ÁOEt2, are as follows:
(1) Sn-Mont catalyst is removable by simple filtration without
neutralization with bases. (2) Sn-Mont can be reused for the next
reaction. We found no decrease in the yield for the deoxygenation
of 1d with Et3SiH in the second and third re-uses of Sn-Mont in
comparison to the yield for first use of Sn-Mont. (3) Dry reaction
conditions are not essential for the deoxygenation. In other words,
there is no need to activate Sn-Mont prior to its use and employ
dry solvents.
In summary, we have developed an efficient, easy-to-use
method for the deoxygenation of tertiary and secondary benzylic
alcohols to alkanes using a heterogenous acid, Sn-Mont as the cat-
alyst. In all the cases, the deoxygenation products were obtained in
high to excellent yields.
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Experimental methods
The deoxygenations of various alcohols were performed under
two different conditions.
Condition A: In a round-bottomed flask was placed 30 mg of
Sn-Mont, 1 mmol of 1a, 2 mmol of Et3SiH, and 5 mL of CH2Cl2.
The mixture was stirred at room temperature for 40 min. The
Sn-Mont was then filtered off, and the filtrate was concentrated
and analyzed by NMR using mesitylene as the internal standard.
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20. Detailed protocols for the preparation of Sn-Mont and H-Mont are shown in
the Supporting information.
Please cite this article in press as: Tandiary, M. A.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.05.042