A novel reduction reaction for the conversion of aldehydes, ketones and primary, secondary and tertiary alcohols into their corresponding alkanes

Article abstract of DOI:10.1016/j.tetlet.2006.06.007

A novel one-pot reaction has been developed for the reduction of aldehydes, ketones and primary, secondary and tertiary alcohols into their corresponding alkyl function. This is also the first reported method which can efficiently reduce primary, secondary, or tertiary alcohols, without affecting carbon-carbon double bonds, into their corresponding alkyl function in high yields. The reduction utilises either diethylsilane or n-butylsilane as the reducing agent in the presence of the Lewis acid catalyst tris(pentafluorophenyl)borane.

Full text of DOI:10.1016/j.tetlet.2006.06.007

Tetrahedron Letters 47 (2006) 5755–5758
A novel reduction reaction for the conversion of aldehydes,
ketones and primary, secondary and tertiary alcohols into
their corresponding alkanes
Rama D. Nimmagadda and Christopher McRae*
Department of Chemistry and Biomolecular Sciences, Division of Environmental and Life Sciences, Macquarie University,
Sydney, NSW 2109, Australia
Received 21 April 2006; revised 22 May 2006; accepted 1 June 2006
Abstract—A novel one-pot reaction has been developed for the reduction of aldehydes, ketones and primary, secondary and tertiary
alcohols into their corresponding alkyl function. This is also the first reported method which can efficiently reduce primary, second-
ary, or tertiary alcohols, without affecting carbon–carbon double bonds, into their corresponding alkyl function in high yields. The
reduction utilises either diethylsilane or n-butylsilane as the reducing agent in the presence of the Lewis acid catalyst
tris(pentafluorophenyl)borane.
Ó 2006 Elsevier Ltd. All rights reserved.
The reduction of aldehydes, ketones and alcohols to a
methyl function is a useful and widely reported synthetic
pathway. In the case of ketones and aldehydes, multi-
step reduction procedures using reagents such as hydra-
zine in basic polyethylene glycol,1–3 amalgamated zinc in
a hydroxylic solvent containing HCl,4 electrolytic reduc-
tion5 and diphenylsilane at high temperatures6 have
been shown to be quite effective. Single step reductions
of a carbonyl function to a methyl group have also been
reported using polymethylhydrosiloxane,7 triethyl-
silane8,9 and triphenylsilane10 in the presence of tris-
(pentafluorophenyl)borane (B(C6F5)3). Similarly, the
reduction of alcohols to alkanes has also been reported
via a range of procedures including acetic acid/HI,11
mixed hydrides from AlCl3 and LiAlH4,12 and photo-
chemical reduction.13 However, different reagents/meth-
ods are needed depending on whether the substrate is a
primary, secondary or tertiary alcohol. For instance, the
reduction of primary alcohols to hydrocarbons has been
reported using HSiEt3 in the presence of catalytic
amounts of B(C6F5)3, however, this reagent was unable
to reduce secondary and tertiary alcohols to alkanes.14,15
Primary and secondary acyclic alcohols can be reduced
to alkanes using sodium borohydride once they have
been activated by triphenylphosphonium anhydride.16
In addition, aliphatic primary alcohols can be converted
into hydrocarbons by condensation with samarium at
low temperatures at about 80–90 K.17 Secondary alco-
hols and tertiary alcohols were converted into their
corresponding hydrocarbons by hydrogenation in the
presence of Ni/Al2O3 catalyst at 190 °C.18 Catalytic
hydrogenation of alcohols to hydrocarbons in the pres-
ence of Pd/C was used after first converting the alcohols
into ureas using diamines.19 Chlorodiphenylsilane in the
presence of the catalyst, indium trichloride, has been
shown to directly reduce benzylic alcohols, secondary
alcohols and tertiary alcohols into their corresponding
alkanes.20 In most of these cases, however, carbon–car-
bon double bonds are also reduced along with the target
carbonyl/alcohol. In this work, we report the direct
reduction of primary, secondary and tertiary alcohols,
aldehydes and ketones to alkanes using either n-butyl-
silane (n-BS) or diethylsilane (DES) as reducing agents.
The proposed reaction mechanism for the reduction of
alcohols by n-BS is presented in Figure 1. Table 1 con-
tains a list of the alcoholic substrates studied, ordered
by class. The ability of n-BS and DES to reduce primary
alcohols is illustrated with the reduction of phenylmeth-
anol 1, and octadecanol 2, using 2 equiv of n-BS or
Keywords: Reduction; Aldehydes; Ketones; Alcohols; DES: diethyl-
silane; n-BS: n-butylsilane; B(C6F5)3: tris(pentafluorophenyl)borane.
*
Corresponding author. Tel.: +61 2 98508288; fax: +61 2 98508313;
e-mail: christopher.mcrae@mq.edu.au
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.06.007

5756
R. D. Nimmagadda, C. McRae / Tetrahedron Letters 47 (2006) 5755–5758
H
Si
H
H
H2 H2
Si Si
H
H
B(C6F5)3
CH2Cl2
I
+
R
R
R
H
+
O
OH
1 eq
1 eq
H
H
H
R
HO
H
H
Si
H2
Si
HO
H
OH
H
H
H
B(C6F5)3
R
H
Si
H
H
H
II
R
H
H
B(C6F5)3
Figure 1. Reduction mechanism for alcohols.
Table 1. Reduction of alcohols and phenols using either n-butylsilane (n-BS) or diethylsilane (DES)
Entry
Substrate
Product
GC yield (%)
n-BS
DES
41
CH2OH
CH3
1
2
97
1
CH2OH
CH3
(CH2)16
(CH2)16
91
83
H3C
H3C
2
3
4
OH
81
91
54
27
3
OH
(CH2)16
HOOC
4
5
OH
OH
5
97
34
6
94
72
73
67
41
43
17
64
48
52
6
(H2C)2
(H2C)2
(CH2)11
7
(CH2)11
(CH2)11
(CH2)11
C
C
H
OH
7
8
8a
9a
10a
OH
O
3 Si R1
R2
R
O
HOOC
OH
Si R3
R2
R1
9
O
HO
COOH
R1
Si
R2
Si R3
R2
R3
O
OH
10
R1
a R1 = R2 = H, R3 = butyl when using n-butylsilane and R1 = R2 = ethyl, R3 = H, when using diethylsilane as the reducing agent.
2 equiv of DES. Both substrates were converted to alk-
anes with either reagent. The reduction of secondary
and tertiary alcohols is illustrated with the reduction
of norbornen-2-ol 3, 12-hydroxystearic acid 4, 2-isoprop-
yl-5-methylcyclohexanol 5 and 1-phenylethanol 6. In
the case of norbornen-2-ol 3, it can be seen that the dou-

R. D. Nimmagadda, C. McRae / Tetrahedron Letters 47 (2006) 5755–5758
5757
ble bond was not affected during the reduction of the
secondary alcohol. 12-Hydroxystearic acid 4 could be
fully reduced to octadecane with 6 equiv of n-butyl-
silane. We have reported the conversion of carboxylic
acids to alkanes using n-BS and DES in a previous
paper.21 Full reduction of 12-hydroxystearic acid 4 to
octadecane could also be effected using 10 equiv of DES,
albeit in lower yield. The other major product from this
reduction was a silyl ether formed at the secondary alco-
hol. Higher yields of octadecane were obtainable by
using a large (12 equiv) excess of DES. The reduction
of tertiary alcohols is illustrated with the reduction of
13-propyl-hexacosane-13-ol 7. Although n-BS was able
to reduce this substrate in high yield, the yield of alkane
using DES was low. Again, the use of a large excess of
DES increased the yield to 68%. Both n-BS and DES
produce silyl ethers when reacted with phenols as
illustrated by the substrates 2,4,6-trimethylphenol 8,
4-hydroxybenzoic acid 9 and 2,4-dihydroxybenzoic acid
10.
en-2-one 15, hex-1-en-3-one 16 and 2-methyl-5-(prop-
1-en-2-yl)cyclohex-2-enone 17 were readily reduced to
methylenes without affecting the carbon–carbon double
bonds. The aromatic ketones, benzophenone 18, 3,4-
dihydronaphthalen-1(2H)-one 19 and anthracen-
9(10H)-one 20 were reduced to alkanes without
difficulty.
In summary, although there are numerous reducing
agents capable of converting alcohols, aldehydes and
ketones to alkanes, to date, there has not been a mild
reducing agent reported capable of converting all types
of alcohol, be they primary, secondary, or tertiary to
alkanes in a one-pot reaction and without affecting car-
bon–carbon double bonds. The method reported in this
work is capable of reducing all the above functional
groups to alkanes, even in the presence of carbon–car-
bon double bonds, in a single step and in very high
yields.
An oven dried 25 mL round bottom flask was purged
with nitrogen. Substrate (1 mmol) was weighed into
the flask which was then closed with a septum and
purged with argon. Catalyst (B(C6F5)3, 5–10 mol %)
was introduced via syringe as a freshly prepared solution
in 5 mL of anhydrous dichloromethane. Stirring was
continued for 10 min and then 1 equiv of n-butylsilane
(99% pure) or diethylsilane (99% pure) was added for
each alcohol group (2 equiv for each carbonyl group)
present in the substrate. A static atmosphere of argon
was established using an argon balloon. The reaction
was stirred for about 0.5–10 h depending upon the num-
ber of alcohol/carbonyl functions present. The reaction
progress was monitored by GC–MS. After complete
conversion of the starting material, the reaction mixture
was quenched with 0.1 mL of triethylamine. The crude
The proposed mechanism for the reduction of aldehydes
and ketones is presented in Figure 2. Table 2 contains a
list of the carbonyl substrates studied, ordered by class.
The reduction of aliphatic aldehydes is illustrated by the
reduction of octanal 11 which was converted to octane
in high yield. Similarly, the reduction of aromatic alde-
hydes both in the presence of electron donating groups,
as in 4-methoxybenzaldehyde 12 and electron withdraw-
ing groups, as in 4-nitrobenzaldehyde 13 were effected in
high yield. The rate of reaction and yield of product did
not appear to be affected by the electron donating/with-
drawing properties of the substituents. Ketones were
also successfully converted to alkanes. Further, the car-
bonyl carbons of 4-(2,6,6-trimethylcyclohex-2-enyl)but-
3-en-2-one 14, 4-(2,6,6-trimethylcyclohex-1-enyl)but-3-
H2 H2
Si Si
H
O
B(C6F5)3
CH2Cl2
H
H
H
Si
R
H
+
R
+
O
R1
R1
I
1 eq
2 eq
R = Alkyl, Aryl R1 = Alkyl, Aryl, H
O
R
C
R1
SiH2
SiH2
O
H
H
O
B(C6F5)3
Si
H
R
R
II
H
R1
R1
B(C6F5)3
H
H
H
H
B(C6F5)3
Si
SiH2
O
SiH2
R
R1
H
H
H
R1
H
H2 H2
Si Si
R
R
R1
O
H
B(C6F5)3
Figure 2. Reduction mechanism for aldehydes and ketones.

5758
R. D. Nimmagadda, C. McRae / Tetrahedron Letters 47 (2006) 5755–5758
Table 2. Reduction of aldehydes and ketones using either n-butylsilane (n-BS) or diethylsilane (DES)
Entry
Substrate
Product
GC yield (%)
n-BS
DES
81
O
1
2
79
11
O
O
91
87
89
71
O
12
NO2
O
3
4
NO2
O
13
93
64
14
15
O
5
6
7
85
84
73
78
67
86
O
O
16
O
17
8
9
87
87
41
41
O
18
19
O
10
93
56
20
11. Chichibabin, A. E. Ber. 1911, 44, 441–443.
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1962.
13. Saito, I.; Matsuura, T.; Ikehira, H. PCT Int. Appl., WO
86-JP506, 8702030, 1987.
mixture was purified by silica gel flash column chromato-
graphy with hexane as eluent or hexane–ethyl acetate
(50:1) to elute silyl ethers.
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