Deoxygenation of aromatic ketones using transfer hydrogenolysis with Raney nickel in 2-propanol

Article abstract of DOI:10.1080/00397911.2010.515367

Aryl ketones are readily deoxygenated to their corresponding aryl alkanes upon treatment with Raney nickel catalyst in boiling 2-propanol. Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397911.2010.515367

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Deoxygenation of Aromatic Ketones
Using Transfer Hydrogenolysis with
Raney Nickel in 2-Propanol
Daniel R. Zuidema a , Sarah L. Williams a , Katherine J. Wert a ,
Karin J. Bosma a , Abigail L. Smith a & Robert C. Mebane b
a Department of Chemistry, Covenant College, Lookout Mountain,
Georgia, USA
b Department of Chemistry, University of Tennessee at Chattanooga,
Chattanooga, Tennessee, USA
Version of record first published: 29 Jun 2011.
To cite this article: Daniel R. Zuidema, Sarah L. Williams, Katherine J. Wert, Karin J. Bosma,
Abigail L. Smith & Robert C. Mebane (2011): Deoxygenation of Aromatic Ketones Using Transfer
Hydrogenolysis with Raney Nickel in 2-Propanol, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 41:19, 2927-2931
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Synthetic Communications1, 41: 2927–2931, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.515367
DEOXYGENATION OF AROMATIC KETONES USING
TRANSFER HYDROGENOLYSIS WITH RANEY NICKEL
IN 2-PROPANOL
Daniel R. Zuidema,1 Sarah L. Williams,1 Katherine J. Wert,1
Karin J. Bosma,1 Abigail L. Smith,1 and Robert C. Mebane2
1Department of Chemistry, Covenant College, Lookout Mountain,
Georgia, USA
2Department of Chemistry, University of Tennessee at Chattanooga,
Chattanooga, Tennessee, USA
GRAPHICAL ABSTRACT
Abstract Aryl ketones are readily deoxygenated to their corresponding aryl alkanes upon
treatment with Raney nickel catalyst in boiling 2-propanol.
Keywords Deoxygenation; hydrogenolysis; hydrogen transfer; ketone reduction; Raney
nickel
The process of reducing an aryl ketone to an aryl alkane is a widely used and
thoroughly studied synthetic technique.[1–3] Conventional reduction methods com-
monly used to reduce the aryl ketone functional group to an aryl alkane include
the Wolff–Kishner reduction and the Clemmensen reduction.[1] The Wolff–Kishner
reduction features basic conditions, whereas the Clemmensen reduction features
acidic conditions. This often precludes use of precursors that are base- or acid-
sensitive, respectively.
Catalytic transfer hydrogenolysis (CTH) has long been known as a method of
reducing a variety of functional groups.[4,5] This process features the catalytic trans-
fer of hydrogen from a hydrogen donor—various combinations of catalysts and
hydrogen donor agents have been studied for CTH and have been reviewed.[5,6]
Raney nickel has been shown to efficiently catalyze the transfer of hydrogen from
various hydrogen donors to effect a wide variety of reductions—some of these
include the conversion of aromatic nitro compounds to anilines,[7] nitriles to
Received June 1, 2010.
Address correspondence to Daniel R. Zuidema, Department of Chemistry, Covenant College,
14049 Scenic Highway, Lookout Mountain, GA 30750, USA. E-mail: zuidema@covenant.edu
2927

2928
D. R. ZUIDEMA ET AL.
Figure 1. Deoxygenation of aryl ketones by treatment with Raney nickel in refluxing 2-propanol.
amines,[8] and aryl carbonyl compounds to aryl methanes.[9] This is the first time that
2-propanol has been used as a CTH donor with Raney nickel to reduce aryl ketones
to aryl alkanes.
Mebane and coworkers have previously shown that ketones are readily reduced
to secondary alcohols using CTH.[10] Furthermore, Mebane and coworkers have also
shown that benzylic alcohols can be deoxygenated to their corresponding aryl
alkanes using CTH.[11] It remained for us to demonstrate that the conversion from
ketone to aryl alkane could be efficiently carried out as a one-pot procedure using
Raney nickel and refluxing 2-propanol. Our results are described herein.
In this report, we describe a simple, mild alternative to the harsh conditions
required in the Clemmensen and Wolff–Kishner reductions. Our procedure is carried
out under neutral conditions in refluxing 2-propanol with the reaction vessel open to
the atmosphere. The experimental procedure for this method of reduction is simple
and straightforward and affords aryl alkanes in good yields. The overall reaction is
described in Fig. 1, and a summary of our results is presented in Table 1. As seen in
Table 1, isolated yields for the less volatile aryl alkanes were very good (entries 1–5).
The more volatile aryl alkanes (entries 6 and 7) were not isolated because of exten-
sive product loss while removing the 2-propanol solvent by rotary evaporation.
Yields of all aryl alkanes as determined by GC were essentially quantitative. In a
typical run, the aryl ketone (0.5 g) was refluxed (container open to the atmosphere)
in a magnetically stirred suspension of Raney nickel (2.5 g) in 2-propanol (10 mL).
The progress of the reaction was conveniently monitored by thin-layer chromato-
graphy (TLC). Good yields were generally achieved within a few hours at the most.
Table 1. Deoxygenation of aryl ketones by treatment with Raney nickel in refluxing 2-propanol
Entry
Aryl ketone
Reduction product
Time (h)
Yield (%)
1
2
3
4
5
6
7
Benzophenone
Acetovanillone
Diphenylmethane
4-Ethyl-2-methoxyphenol
Hexylbenzene
1
1.5
1
98
82a
85
Hexanophenone
Decanophenone
o-Hydroxypropiophenone
Valerophenone
1-Phenyldecane
2-Propylphenol
Butylbenzene
0.3
2.5
1
99
94
b
—
b
—
Acetophenone
Ethylbenzene
1
aAnalysis of the product revealed that 4-ethylphenol was formed in 17% yield.
bNo isolated yield was determined because of the volatility of the product; gas chromatography indi-
cated complete conversion to deoxygenated product.

DEOXYGENATION OF ARYL KETONES
2929
Upon completion of the reaction, the aryl alkane could be isolated after suitable
workup as described in the experimental section. All of the aryl alkanes prepared
in this study are known. Therefore, product identities were confirmed by comparison
of NMR and mass spectral data with authentic samples or literature spectra. In some
instances, trace amounts of other reduction products were observed in addition to
the deoxygenated aromatic compound. In these cases, the aromatic ring moiety
underwent various degrees of reduction. As these products represented a very minor
component of the product mixture (<5%), this did not cause yields to suffer on the
reaction time scales we were using.
To demonstrate that the presence of Raney nickel was indeed required, a con-
trol experiment was run in which benzophenone (0.506 g, 2.78 mmol) was refluxed in
2-propanol (15 mL) for 3 h (no Raney nickel added). No conversion to diphenyl-
methane was observed. By comparison, in the presence of Raney nickel (2.5 g), after
1 h of heating, nearly quantitative conversion to diphenylmethane had been realized.
This established that the presence of Raney nickel is necessary for the reaction to
occur.
To verify that the mechanism of reduction was indeed CTH, an experiment was
set up to verify that acetone was being generated as a by-product of the redox reac-
tion. Hexanophenone (0.512 g, 2.64 mmol) was refluxed in a magnetically stirred sus-
pension of Raney nickel (2.5 g) in 2-propanol (15 mL) for 0.5 h with the container
open to the atmosphere. A Dean–Stark trap was used to collect distillate over the
course of the reaction. The distillate was tested for the presence of acetone by mixing
several drops of a 3% solution of 2,4-dinitrophenylhydrazine with a few drops of dis-
tillate. The formation of a yellow precipitate suggested a positive test for the presence
of acetone. Examination of the melting point of the precipitate revealed a melting
point identical to that of acetone 2,4-dinitrophenylhydrazone (126–128 ꢀC).[12]
A
source has reported that this test easily detects 1 part of acetone in 500 to 1000 parts
of 2-propanol.[13]
In our study, a 1:5 (w=w) substrate-to-catalyst ratio was used to deoxygenate
the aryl ketones. Because this is a relatively large excess of Raney nickel, we were
interested in determining whether the Raney nickel could be recycled. To verify that
the deoxygenation reaction was indeed catalytic with respect to the Raney nickel, an
experiment was set up to test whether the catalyst could be recycled without any loss
of activity. We found that a given batch of Raney nickel catalyst could be used
repeatedly (at least six times) without any loss of activity. Prior to reuse, the catalyst
was washed with 2-propanol (3 Â 10 mL). In a given run, benzophenone (0.50 g,
2.7 mmol) was refluxed in 2-propanol (10 mL) for 1 h. Yields for the reaction were
greater than 93% for each run.
Since other Raney catalysts have been observed to be effective at reducing
benzylic alcohols,[11] we were curious to see if these would also be effective at reduc-
ing aryl ketones. The two additional Raney catalysts explored were Raney cobalt
and Raney copper. For each catalyst, a reaction was set up in which benzophenone
(0.25 g, 1.4 mmol) was refluxed in a magnetically stirred suspension of the Raney
catalyst (1.25 g) in 2-propanol (10 mL) for 1 h with the container open to the atmos-
phere. No change was observed with either catalyst. The experiments were repeated
twice, once using acid catalyst (1 drop conc. HCl) and once using base catalyst (1 pel-
let solid NaOH), but again, in each case, unreacted starting material was recovered.

2930
D. R. ZUIDEMA ET AL.
In conclusion, aryl ketones are readily reduced to their corresponding aryl
alkanes when refluxed in a suspension of Raney nickel in 2-propanol. This affords
an attractive method of reducing ketones in that it offers cheap and readily available
reagents, operational simplicity, easy product isolation, and excellent, clean product
yields. Our laboratory continues to explore the CTH of aryl ketones using
2-propanol as a hydrogen donor source.
EXPERIMENTAL
Raney 2800 nickel was obtained from W. R. Grace Company, Chattanooga.
The catalyst was washed prior to use with distilled water (3x) and 2-propanol
(3x). Proton NMR spectra were recorded on a Varian 60-MHz spectrometer using
tetramethylsilane (TMS) as an internal reference. Mass spectra (MS) were recorded
on a ThermoElectron PolarisQ GC-MS. The proton NMR spectra and mass spectra
of the aryl alkanes prepared in this work were identical to authentic spectra.
As an illustrative example of the deoxygenation procedure, benzophenone
(0.499 g, 2.74 mmol) was refluxed in a magnetically stirred suspension of Raney
nickel (2.5 g) in 2-propanol (10 mL) for 1 h with the container open to the atmos-
phere. After cooling to room temperature, the organic layer was decanted from
the Raney nickel, and the catalyst was washed with 2-propanol (3 Â 10 mL). Raney
nickel is paramagnetic and clings to the magnetic stir bar, which facilitates decan-
tation. The combined organic layers were filtered through celite and concentrated
by rotary evaporation to yield a colorless oil (0.452 g, 2.69 mmol, 98.0%). Its spectral
data (NMR, MS) were identical to those of authentic diphenylmethane.
ACKNOWLEDGMENTS
The authors gratefully acknowledge financial support from the Covenant
College Kaleo Center. In addition, we are indebted to the W. R. Grace Company,
Chattanooga, for the generous donation of Raney nickel catalyst.
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DEOXYGENATION OF ARYL KETONES
2931
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