Transfer hydrogenation of aldehydes with 2-propanol and Raney nickel

Article abstract of DOI:10.1080/00397910500281028

Raney nickel in refluxing 2-propanol containing a trace of HCl is an effective catalytic system for the reduction of aldehydes to primary alcohols. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1080/00397910500281028

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Transfer Hydrogenation of Aldehydes With
2‐Propanol and Raney® Nickel
Robert C. Mebane a & Amanda J. Mansfield a
a Department of Chemistry , University of Tennessee at Chattanooga ,
Chattanooga, Tennessee, USA
Published online: 20 Aug 2006.
To cite this article: Robert C. Mebane & Amanda J. Mansfield (2005) Transfer Hydrogenation of Aldehydes
With 2‐Propanol and Raney® Nickel, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 35:24, 3083-3086
To link to this article: http://dx.doi.org/10.1080/00397910500281028
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Synthetic Communicationsw, 35: 3083–3086, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500281028
Transfer Hydrogenation of Aldehydes With
2-Propanol and Raneyw Nickel
Robert C. Mebane and Amanda J. Mansfield
Department of Chemistry, University of Tennessee at Chattanooga,
Chattanooga, Tennessee, USA
Abstract: Raney nickel in refluxing 2-propanol containing a trace of HCl is an
effective catalytic system for the reduction of aldehydes to primary alcohols.
Keywords: Aldehyde reduction, Raney nickel, transfer hydrogenation
Reduction by heterogeneous catalytic transfer hydrogenation (CTH)
employing a hydrogen donor is an attractive alternative to using gaseous
hydrogen as the reductant.[1] Numerous combinations of catalysts and
hydrogen donors have been developed for CTH and are reviewed
elsewhere.[2,3] Recently we reported that Raney nickel with 2-propanol as
the hydrogen donor is an effective catalytic system for the hydrogenolysis
of aromatic alcohols,[4] the deiodination of iodolactones,[5] and the
reduction of nitriles to amines.[6] We have extended this catalytic system to
the reduction of aldehydes to primary alcohols, and we report our results
herein. In addition to being an effective method for reducing aldehydes, this
catalytic combination of Raney nickel and 2-propanol has the advantage of
not producing the metal sludge often encountered in metal hydride
reductions[7] and avoids the need for hydrogen containment and the use of
pressure vessels needed in traditional catalytic hydrogenation reactions.[8]
Received in the USA June 30, 2005
Address correspondence to Robert C. Mebane, Department of Chemistry, #2252,
University of Tennessee at Chattanooga, 615 McCallie Ave., Chattanooga,
Tennessee 37403-2598, USA. Fax: 423/425-5234; E-mail: robert-mebane@utc.edu
3083

3084
R. C. Mebane and A. J. Mansfield
Figure 1. Reduction of aldehydes by catalytic transfer hydrogenation.
The experimental procedure for the reduction of aldehydes by CTH
employing Raney nickel and 2-propanol is simple and straightforward and
affords primary alcohols in good yields (Fig. 1). In a typical run, the
aldehyde (1 g) is refluxed with the condenser open to the atmosphere in a mag-
netically stirred suspension of Raney nickel (2.5 g) in 2-propanol (10 mL) con-
taining one drop of concentrated aqueous HCl. The progress of the reaction is
conveniently monitored by GC/MS. A summary of our results is presented in
Table 1. Reaction products were confirmed by comparison of NMR and mass
spectra with authentic samples. No self-condensation products were detected
in any of our reactions, which were observed in the transfer hydrogenation of
aldehydes using 2-propanol and a nickel catalyst prepared from nickel
diisopropoxide.[9]
Straight-chain, branched, and cyclic aldehydes are readily reduced to
primary alcohols by this reductive method (entries 1–7). Steric bulk around
the aldehyde center does not appear to hinder the reductions (entries 5 and
6). The double bond in unsaturated aldehydes may also be reduced as
indicated by the formation of a minor amount of cyclohexane-1-methanol in
the CTH reduction of cyclohex-3-enecarbaldehyde (entry 8). Aromatic
aldehydes are not effectively reduced by this CTH method. For example,
Table 1. Reduction of aldehydes by catalytic transfer hydrogenation using Raney
nickel and 2-propanol as the hydrogen source
Yielda
(%)
Time
(min)
Entry
R
Product(s)
1
2
3
4
5
6
7
8
n-Pentyl
n-Hexyl
1-Hexanol
1-Heptanol
1-Octanol
1-Nonanol
66
71
78
93
63
90
77
73
60
15
n-Heptyl
n-Octyl
120
135
15
1-Methylbutyl
1-Ethylpentyl
Cyclohexyl
Cyclohex-3-enyl
2-Methylpentanol
2-Ethylhexanol
30
30
Cylcohexane-1-methanol
Cyclohex-3-enyl-1-methanol (83%)
Cyclohexane-1-methanol (17%)
780
1
aIsolated yield. Products were characterized by H NMR, 13C NMR, and MS and
were shown to be identical with authentic samples or literature spectra.

Transfer Hydrogenation of Aldehydes
3085
less than 30% reduction of benzaldehyde and furfural was realized after 4 h of
refluxing. We have previously shown[4] that the addition of benzene signifi-
cantly retards CTH reactions involving Raney nickel and 2-propanol. A
strong adsorption between the pi-system of the aromatic rings and the
nickel surface may be retarding the reactions.[10]
CTH of aldehydes does occur without the addition of hydrochloric
acid; however, the reaction is slower. For example, in the absence of acid
the reduction of heptanal requires 45 min to reach completion compared to
15 min with acid (entry 2). The addition of more than one drop of HCl to
the reactions does not result in further reductions in reaction times.
The acceleration by acid of catalytic hydrogenations using hydrogen gas
has been known.[11] This observed acceleration, as well as the acceleration we
observe in the CTH of aldehydes, could arise from a protonated carbonyl that
is strongly attracted to the metal surface (nickel in this account), followed by a
rapid transfer of hydride to the adsorbed activated carbonyl.[12] Further studies
are needed to fully elucidate the mechanism of the acceleration in the CTH of
aldehydes by acid.
The substrate to catalyst ratio (w/w) used to reduce aldehydes was
approximately 1 : 2 : 5. Although an excess of catalyst was used, as found in
our previous CTH studies,[4–6] the catalytic activity of the Raney nickel is
retained after repeated CTH reactions. Prior to each reuse the Raney nickel
was washed with 2-propanol (3 Â 10 mL).
In conclusion, we have shown that aldehydes are conveniently reduced to
alcohols by catalytic transfer hydrogenation using 2-propanol as a hydrogen
donor. The addition of small amounts of HCl accelerates the reductions,
and the catalyst can be used repeatedly. Thus, the CTH of aldehydes by this
method is an attractive alternative to the preparation of primary alcohols via
the reduction of aldehydes.
EXPERIMENTAL
Raneyw 2800 nickel was obtained from W. R. Grace Company, Chattanooga
Davison. The catalyst was washed prior to use with distilled water (6Â) and
1
2-propanol (3Â) and stored in 2-propanol. H and 13C NMR spectra were
recorded on a Varian Gemini 300 spectrometer using tetramethylsilane as
internal reference. Mass spectra were recorded on a ThermoFinnigan
PolarisQ GC-MS. The NMR and mass spectra of the alcohols prepared in
this work were identical to known spectra.
As an illustrative example of the hydrogenation procedure, octanal
(1.013 g, 7.91 mmol) was added to a mixture of Raney nickel (2.5 g),
2-propanol (10 mL), and one drop of concentrated aqueous HCl, which had
been refluxed for 5 min. While being magnetically stirred, this reaction
mixture was refluxed for an additional 120 min with the condenser open to
the atmosphere. After cooling to room temperature the organic layer was

3086
R. C. Mebane and A. J. Mansfield
decanted from the Raney nickel and the catalyst was washed with 2-propanol
(3 Â 15 mL). (Raney nickel is paramagnetic and sticks to the magnetic stir
bar, which facilitates the decantation.) The combined organic layers were
filtered through Celite, and the 2-propanol was removed by rotatory evapor-
ation followed by a high vacuum to give 0.80 g (78%) of a colorless liquid
1
whose H and 13C NMR spectra and the mass spectrum were identical to
authentic 1-octanol.
ACKNOWLEDGMENTS
The authors gratefully acknowledge financial support from the Research Cor-
poration, the University of Chattanooga Foundation, and the Grote Chemistry
Fund. In addition, we are indebted to the W. R. Grace Company, Chattanooga,
Tennessee, for the generous donation of Raney catalysts and to Benjamin
H. Gross for many helpful discussions.
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