Pd/C-Catalyzed Transfer-Hydrogenation of Benzaldehydes to Benzyl Alcohols Using Potassium Formate as the Selective Hydrogen Donor

Article abstract of DOI:10.1081/SCC-120027711

Aromatic and aliphatic aldehydes are selectively reduced to primary alcohols under mild conditions with potassium formate in the presence of Pd/C catalyst. The high selectivity obtained is attributed due to the presence of the basic potassium bicarbonate generated in situ from the formate salt via the hydrogen transfer process. Molecular hydrogen shows similar activity in the presence of potassium carbonate salts. Ketones did not react under these conditions; therefore, this method is used to separate aldehydes selectively from a mixture of aldehydes and ketones.

Full text of DOI:10.1081/SCC-120027711

This article was downloaded by: [Duke University Libraries]
On: 30 July 2012, At: 00:34
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer
House, 37-41 Mortimer Street, London W1T 3JH, UK
Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic
Chemistry
Publication details, including instructions for authors and subscription information:
http://www.tandfonline.com/loi/lsyc20
Pd/C‐Catalyzed Transfer‐Hydrogenation of
Benzaldehydes to Benzyl Alcohols Using Potassium
Formate as the Selective Hydrogen Donor
a
a
b
a
Mubeen Baidossi , Ashutosh V. Joshi , Sudip Mukhopadhyay & Yoel Sasson
a
Casali Institute of Applied Chemistry, Hebrew University, Givat Ram Campus,
Jerusalem, 91904, Israel
b
Department of Chemical Engineering, University of California, Berkeley, California, USA
Version of record first published: 16 Aug 2006
To cite this article: Mubeen Baidossi, Ashutosh V. Joshi, Sudip Mukhopadhyay & Yoel Sasson (2004): Pd/C‐Catalyzed
Transfer‐Hydrogenation of Benzaldehydes to Benzyl Alcohols Using Potassium Formate as the Selective Hydrogen Donor,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 34:4,
643-650
To link to this article: http://dx.doi.org/10.1081/SCC-120027711
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Any substantial or systematic
reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to
anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the contents
will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses
should be independently verified with primary sources. The publisher shall not be liable for any loss, actions,
claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or
indirectly in connection with or arising out of the use of this material.

w
SYNTHETIC COMMUNICATIONS
Vol. 34, No. 4, pp. 643–650, 2004
Pd/C-Catalyzed Transfer-Hydrogenation of
Benzaldehydes to Benzyl Alcohols Using
Potassium Formate as the Selective
Hydrogen Donor
1
Mubeen Baidossi, Ashutosh V. Joshi, Sudip Mukhopadhyay,
1
2
1
,
*
and Yoel Sasson
1
Casali Institute of Applied Chemistry, Hebrew University,
Jerusalem, Israel
2
Department of Chemical Engineering, University of California,
Berkeley, California, USA
ABSTRACT
Aromatic and aliphatic aldehydes are selectively reduced to primary
alcohols under mild conditions with potassium formate in the presence of
Pd/C catalyst. The high selectivity obtained is attributed due to the
presence of the basic potassium bicarbonate generated in situ from the
formate salt via the hydrogen transfer process. Molecular hydrogen shows
similar activity in the presence of potassium carbonate salts. Ketones did
*
Correspondence: Yoel Sasson, Casali Institute of Applied Chemistry, Hebrew
University, Givat Ram Campus, Jerusalem, 91904, Israel; Fax: (þ972) 2 6586761;
E-mail: ysasson@huji.ac.il.
6
43
DOI: 10.1081/SCC-120027711
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

ORDER
REPRINTS
6
44
Baidossi et al.
not react under these conditions; therefore, this method is used to separate
aldehydes selectively from a mixture of aldehydes and ketones.
Key Words: Pd/C; Transfer-hydrogenation; Potassium formate; Hydro-
gen; Aldehyde; Base; Alcohol.
INTRODUCTION
Heterogeneous palladium catalyzed reduction of aromatic aldehydes to
alcohols are often unselective due to the consecutive hydrogenolysis reaction
yielding toluenes as the major product (Sch. 1).
Therefore, Raney Nickel, copper-chromium or Adam’s platinum oxide
(
promoted with ferric chloride) catalysts have been used for alcohol reduction
[1]
to aldehydes with low to moderate selectivity. In a German patent, it has
been claimed that addition of an aqueous sodium carbonate or sodium
hydroxide solution to the hydrogenation mixture of benzaldehyde in the
presence of supported nickel catalyst increased the benzyl alcohol yield from
[
2]
4
8.7% to 91.5%.
A similar base effect has also been observed in the ruthenium and
rhodium black catalyzed hydrogenation of acetophenone to ethylcyclohexane
and cyclohexylmethylcarbinol, where addition of a base such as lithium
hydroxide or lithium acetate substantially lowered the hydrogenolysis activity
[
3]
[4]
of both the catalysts. Ohkuma and co-workers have pointed out that the
relative activity of the homogeneous RuCl (PPh ) catalyst in olefin
2
3 3
hydrogenation can be altered by a huge factor upon the addition of a catalytic
amount of ethylene diamine and potassium hydroxide as compared to
[
5]
aldehyde hydrogenation.
Nonetheless, heterogeneous Pd-catalyst is preferred over other transition
[6]
metal catalysts such as Pt, Rh, and Ru for the reduction reaction at ambient
temperature and pressure due to its: (a) high activity (b) higher turn over
frequency, and (c) easy reusability when used in a protic solvents such as
methanol or acetic acid. As mentioned above the only disadvantage with Pd as
a catalyst is the poor selectivity to alcohols. This has been overcome partially
[7]
by employing a very high loading of expensive Pd-metal catalyst or in the
Scheme 1. Hydrogenolysis of aldehydes to toluenes.

ORDER
REPRINTS
Pd/C-Catalyzed Transfer-Hydrogenation
645
Scheme 2. Selective reduction of aldehydes to alcohols.
[8]
presence of an expensive inhibitor such as N, N-diethylnicotine amide or
ethylenediamine. These modifications have exhibited high selectivity mainly
[9]
for the hydrogenation of ketones and aldehydes to alcohols.
There,
therefore, is incentive to find a simple process scheme using Pd/C catalyst for
the selective hydrogenation of aldehydes to alcohols as this reaction is
extensively used in day-to-day pharmaceutical and fine chemical industries.
Thus, here we will report a simple method for reducing aldehydes
selectively to alcohols by using a reductant such as potassium formate salts
which are acting as the reductant as well as the source of an in situ generated
base additive, such as KHCO and K CO (Sch. 2).
3
2
3
RESULTS AND DISCUSSIONS
In a typical reaction (see also experimental section) benzaldehyde was
a
reduced by potassium formate in ethanol using 5% Pd/C as a catalyst at 208C
for 2 hours. After 2 h of reaction time, GC analysis showed that the only
product obtained was benzyl alcohol. The same procedure was successfully
extended to the reduction of several aromatic aldehydes which all yielded the
corresponding benzyl alcohols with high yield.
Results are presented in Table 1. Phenylacetaldehyde behaves similarly
to benzaldehyde producing mainly 2-phenylethanol but 1-hexanal reacts
moderately (10% conversion after 2 hours), though in high selectivity to
1-hexanol. Both o- and p-hydroxybenzaldehydes are very susceptible to over
reduction.
[
10]
With both substrates the major products, under the above
reaction conditions, were the corresponding cresols.
However, no conversion was observed when aromatic ketones such as
acetophenone and benzophenone were exposed to the above conditions.
a
Pd/C catalyst from different sources was used in this study: 5% Pd/charcoal T37
(
Johnson Matthey batch no. F153); 5% Pd/graphite type 450 powder (Johnson Matthey
Batch No. 97180A); 5% Pd/C (Engelhard, Lot No. 05201). No difference in activity is
observed while using various batches of these catalysts.

ORDER
REPRINTS
6
46
Baidossi et al.
a
Table 1. Reduction of various carbonyl compounds with potassium formate salts.
%
Conv. to
% Conv. to methylene
%
Overall
conv.
Number
Substrate
alcohol
compound
1
2
3
4
5
6
7
8
9
Phenyl acetaldehyde
Benzaldehyde
Anisaldehyde
90
100
100
92
98
99
100
99
100
95
95
99
30
3
2
1
0
2,4-Dimethoxy benzaldehyde
3,5-Dimethoxy benzaldehyde
3,4-Dimethoxy- benzaldehyde
2-Toluylaldehyde
1-Naphthaldehyde
2-Hydroxy benzaldehyde
4-Hydroxy benzaldehyde
Hexanal
1
100
100
100
50
0
5
5
1
100
100
10
70
97
0
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
100
0
Acetophenone
Benzophenone
0
0
0
10
0
0
Cyclohexanone
Benzaldehyde
90
99
99
24
10
1
b
c
d
100
100
100
Benzaldehyde
Benzaldehyde
1
76
a
Reaction conditions: substrate, 10 mmol; potassium formate, 30 mmol; 5% Pd/C,
0
.5 g (0.235 mmol of Pd); water, 1 g; ethanol, 15 mL; temparature, 208C; time, 2 h.
b
2
H is used instead of formate salts for a reaction time of 4 h in the presence of 10 mol%
additives such as potassium carbonate, potassium bicarbonate or sodium carbonate.
c
Reaction time, 6 h.
d
H is used for a reaction time of 4 h in the absence of any additives.
2
Conversely, cyclohexanone was reduced slowly to cyclohexanol (10%
conversion after 2 h). The reaction was also studied in view of a possible
reactive separation protocol of aldehyde from a mixture of aldehyde and
ketone. Thus, when an equimolar mixture of benzaldehyde and acetophenone
was reacted under the best conditions as mentioned in the experimental
section, only a selective reduction of the aldehyde was achieved.
Ethanol was the most effective solvent for the reaction. Similar results
were obtained with dimethylformamide. Methanol, however, resulted in a
poor performance with formation of only traces of product. This could be due
to the less solubility of potassium formate in methanol. The inactivity of
methanol was also realized in our previous study on the transfer hydrogenation
[11]
of nitroarenes by formate salts.

ORDER
REPRINTS
Pd/C-Catalyzed Transfer-Hydrogenation
647
The presence of water is necessary for the hydrogen transfer process to
take place; however, excess of water is detrimental to the reaction. Thus the
reaction was optimized with 4.5 equivalents of water in ethanol. A similar
optimum water/substrate ratio was beforehand measured in the transfer
dehydrohalogenation of aryl halides to arenes using formate salts as hydrogen
[
12]
donor.
Formate salts are known hydrogen carriers and donors
decompose to molecular hydrogen and bicarbonate salt in presence of water
[13,14]
and generally
and a Pd/C catalyst. Thus, the reversible Pd/C-catalyzed decomposition of
can be
[15,16]
potassium formate to hydrogen and potassium bicarbonate
represented as (Sch. 3):
Scheme 3. Pd/C-catalyzed decomposition of potassium formate.
Similarly the Pd/C catalyzed hydrogen transfer reaction of potassium
formate with an acceptor (A) such as nitroarenes, haloarenes, olefins and
[
17,18]
aldehydes can be shown as written in Sch. 4. The products are the
hydrogenated form of the starting materials and bicarbonate salts.
Scheme 4. Pd/C-catalyzed hydrogen transfer.
It is realized that the in situ generation of bicarbonate salts during Pd/C-
catalyzed transfer-hydrogenation reaction of aldehydes to alcohols via
hydrogen transfer from formate salts is beneficial for enhancing the selectivity
to alcohols. This indeed is the case as we characterize the salt obtained after
the reaction as KHCO (Sch. 5).
3
Noteworthy, the reduction of aldehydes was less effective when
potassium formate was replaced with sodium formate. Interestingly, ammon-
ium formate yielded toluenes along with amino derivatives as by-products
Scheme 5. Selective hydrogen-transfer reaction of aldehydes to alcohols.

ORDER
REPRINTS
6
48
Baidossi et al.
upon reaction with ketones or aldehydes in a similar system. These results are
[
19]
consistent with the literature.
Interesting results were obtained when the reduction of benzaldehyde was
carried out using hydrogen gas at ambient pressure with Pd/C in the presence
10 mol% of potassium carbonate or potassium bicarbonate (Table 1, entry 15).
The selectivity to alcohol was .99% after a reaction period of 4 h. The
selectivity remained unchanged while running the reaction for 6 h (Table 1,
entry 16). Identical activity and selectivity were obtained upon the addition of
10 mol% of sodium carbonate as an additive. On the contrary, in the absence
of carbonate or bicarbonate additives a considerable reduction of benzaldehyde
to toluene was observed in 4 h. (Table 1, entry 17). All in all the Pd-catalyst was
successfully reused up to 10 times without losing activity (see experimental
section).
CONCLUSIONS
We have, therefore, demonstrated a simple and straightforward reaction
scheme for the selective reduction of aldehydes to the corresponding alcohols
using potassium formate as the hydrogen donor. Heterogeneous Pd/C is used
as a catalyst. The inhibition of the consecutive hydrogenolysis of the alcohol
to toluene is attributed to the presence of the basic potassium bicarbonate,
which is generated in situ by the reaction of potassium formate and water in
the presence of a catalytic amount of Pd/C. Ketones are inert under these
reaction conditions. Thus, it is applied with high selectivity for the reactive
separation of aldehyde from a mixture of aldehyde and ketone.
EXPERIMENTAL
Transfer-Hydrogenation of Aldehydes with Formate Salts
In a typical procedure, 50 mmol benzaldehyde (5.3 g), 75 mL ethanol,
mL water, 75 mmol potassium formate (6.3 g), and 0.94 mmol of 5% Pd/C
4
catalyst (2.0 g) were mixed in a 250 mL glass reactor at 208C. The reaction
temperature was maintained for another 2 hours at this temperature. After the
reaction period, the reaction mixture was worked up by adding 60 mL water
and 20 mL dichloromethane followed by mixing and filtration through a G3
filter for the separation of the catalyst. The aqueous layer was separated
and extracted three times with 20 mL portions of dichloromethane. The
combined batches of dichloromethane were dried over 5 g of magnesium

ORDER
REPRINTS
Pd/C-Catalyzed Transfer-Hydrogenation
649
sulfate and evaporated to yield 4.85 g (45 mmol, 90% yield) of benzyl alcohol
of 99% purity (GC analysis).
Direct Hydrogenation of Aldehydes with Molecular H2
A 5.3 g (50 mmol) of benzaldehyde, 75 mL ethanol, 4 mL water, 10 mol%
of potassium carbonate or potassium bicarbonate, and 2.0 g of 5% Pd/C
catalyst (0.94 mmol) were charged in a 300 mL high-pressure autoclave at
208C. The reactor was then charged with 0.1 Mpa (gauge) hydrogen gas and
stirred for 4 h. The reaction progress was monitored by GC analysis of samples
taken from the reaction mixture at fixed intervals. A complete conversion of
the starting aldehyde to alcohol was achieved after 4 h and the selectivity to
alcohol was .99%.
Catalyst Reusability
The catalyst was filtered off the reaction mixture and washed thoroughly
with 3 Â 15 mL of hot water and then stirred with methanol at 458C for 1 h in
presence of 3 atm of H . The catalyst thus obtained after pretreatment even
2
showed better activity than the initial batch and was used upto 10 batches
without loosing any activity.
REFERENCES
1
. Nishimura, S. Hydrogenation of aldehydes and ketones. In Handbook of
Heterogeneous Catalytic Hydrogenation for Organic Synthesis; Wiley-
Interscience: New York, 2001; 170–225.
2
. Braband, J.; Richter, G. DD 277,270. Chem. Abstr. 1990, 113, 211575f.
. Nishimura, S. Hydrogenation of aromatic compounds. In Handbook of
Heterogeneous Catalytic Hydrogenation for Organic Synthesis; Wiley-
Interscience: New York, 2001; 450.
3
4. Ohkuma, T.; Ooka, H.; Ikariya, T.; Noyori, R. Preferential hydrogenation
of aldehydes and ketones. J. Am. Chem. Soc. 1995, 117 (41),
10417–10418.
5
. Bar, R.; Bar, L.K.; Sasson, Y.; Blum, J. Phase transfer-catalyzed reduc-
tion of aromatic aldehydes by aqueous sodium formate in the presence of
dichlorotris(triphenylphosphine)ruthenium(II): a kinetic-study. J. Mol.
Catal. 1985, 33 (2), 161–177.
6. Rylander, P.N. Hydrogenation of aldehydes. In Catalytic Hydrogenation
over Platinum Metals; Academic Press: New York, 1967; 238–257.

ORDER
REPRINTS
6
50
Baidossi et al.
7
8
. Freifelder, M. Catalytic Hydrogenation in Organic Synthesis. Procedures
and Commentary; Wiley Interscience: New York, 1978; 81.
. Kindler, K.; Helling, H.-G.; Sussner, E. The mechanism of chemical
reactions. XVI. The assistance of metalloids in catalytic hydrogenation
with palladium and platinum. Liebigs Ann. Chem. 1957, 605, 200.
. Hattori, K.; Sajiki, H.; Hirota, K. Chemoselective control of hydro-
genation among aromatic carbonyl and benzyl alcohol derivatives using
Pd/C(en) catalyst. Tetrahedron 2001, 57 (23), 4817–4824.
9
1
1
1
0. Nightingale, D.; Radford, H.D. The hydrogenation of aromatic ketones
with hydrogen and copper-chromium oxide catalyst. J. Org. Chem. 1949,
14 (6), 1089–1093.
1. Wiener, H.; Blum, J.; Sasson, Y. Studies on the mechanism of transfer
hydrogenation of nitroarenes by formate salts catalyzed by Pd/C. J. Org.
Chem. 1991, 56 (14), 4481–4486.
2. Wiener, H.; Blum, J.; Sasson, Y. Transfer hydrogenolysis of aryl halides
and other hydrogen acceptors by formate salts in the presence of Pd/C
catalyst. J. Org. Chem. 1991, 56 (21), 6145–6148.
13. Wiener, H.; Zaidman, B.; Sasson, Y. Storage of energy by solutions of
alkali formate salts. Solar Energy 1989, 43 (5), 291–296.
1
4. Wiener, H.; Zaidman, B.; Sasson, Y. Application of aqueous formate salts
as hydrogen donors in reduction processes: a techno-economic com-
parison with pressurized and electrically generated hydrogen gas. Int.
J. Hydrogen Energy 1989, 14 (6), 365–370.
1
1
1
1
1
5. Wiener, H.; Sasson, Y.; Blum, J. Palladium-catalyzed decomposition of
aqueous alkali metal formate solutions. J. Mol. Catal. 1986, 35 (3),
277–284.
6. Wiener, H.; Blum, J.; Feilchenfeld, H.; Sasson, Y.; Zalmanov, N. The
heterogeneous catalytic-hydrogenation of bicarbonate to formate in
aqueous-solutions. J. Catal. 1988, 110 (1), 184–190.
7. Arkad, O.; Wiener, H.; Garti, N.; Sasson, Y. Catalytic transfer hydro-
genation of soybean oil methyl-ester using inorganic formic-acid salts as
donors. J. Am. Oil Chem. Soc. 1987, 64 (11), 1529–1532.
8. Zoran, A.; Sasson, Y.; Blum, J. Catalytic transfer hydrogenation of
unsaturated-compounds by solid sodium formate in the presence of
palladium on carbon. J. Mol. Catal. 1984, 26 (3), 321–326.
9. Ram, S.; Ehrenkaufer, R.E. Ammonium formate in organic synthesis: a
versatile agent in catalytic hydrogen transfer reductions. Synthesis 1988
(
2), 91–95.
Received in the USA August 14, 2003

Request Permission or Order Reprints Instantly!
Interested in copying and sharing this article? In most cases, U.S. Copyright
Law requires that you get permission from the article’s rightsholder before
using copyrighted content.
All information and materials found in this article, including but not limited
to text, trademarks, patents, logos, graphics and images (the "Materials"), are
the copyrighted works and other forms of intellectual property of Marcel
Dekker, Inc., or its licensors. All rights not expressly granted are reserved.
Get permission to lawfully reproduce and distribute the Materials or order
reprints quickly and painlessly. Simply click on the "Request Permission/
Order Reprints" link below and follow the instructions. Visit the
U.S. Copyright Office for information on Fair Use limitations of U.S.
copyright law. Please refer to The Association of American Publishers’
(
AAP) website for guidelines on Fair Use in the Classroom.
The Materials are for your personal use only and cannot be reformatted,
reposted, resold or distributed by electronic means or otherwise without
permission from Marcel Dekker, Inc. Marcel Dekker, Inc. grants you the
limited right to display the Materials only on your personal computer or
personal wireless device, and to copy and download single copies of such
Materials provided that any copyright, trademark or other notice appearing
on such Materials is also retained by, displayed, copied or downloaded as
part of the Materials and is not removed or obscured, and provided you do
not edit, modify, alter or enhance the Materials. Please refer to our Website
User Agreement for more details.
Request Permission/Order Reprints
Reprints of this article can also be ordered at
http://www.dekker.com/servlet/product/DOI/101081SCC120027711