Catalytic reduction of the arene ring, and other functionalities, of organic substrates using formic acid and palladium on carbon

Article abstract of DOI:10.1016/S0040-4039(00)60800-6

The arene ring in a variety of compounds with functionalities which contain nitrogen is reduced in good yields using formic acid and Pd/C in methanol.

Full text of DOI:10.1016/S0040-4039(00)60800-6

Vol. 33. No. 49. pp. 7477-7480.1992
in Great Britain
+
Catalytic Reduction of the Arene Ring, and Other
of Organic Substrates
Using Acid and Palladium on Carbon
Howard
and Guiseppe
Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada
Centro CNR. Instituto di Chlmica del di Via
173.70126
Italy
The arene ring in a variety of compounds with functionalides which contain nitrogen
is reduced in good yields using formic acid and in methanol.
The reduction of organic substrates, using hydrogen and a transition metal catalyst, is a topic of ongoing
interest in organic chemistry.’ Formic acid and its sodium ammonium salts, amongst others, have been used
for the hydrogenation of
and acetylenes or in the removal of
The hydrogenation of the benzene rings in aromatic substrates is also of importance.
Methods to produce this
or homogeneous
oxlmes and
transformation include catalytic hydrogenation using
phase
or its salts are the reduction of
Recently
Examples using formic
compounds.
and coworkers studied the mechanism of
hydrogenation of nitroarenes.
reduction of
We have been interested in developing an alternative method for the
compounds the system formic We now describe the
reduction of the benzene ring and other functionalities in a numbers of nitrogen containing reactants.
Treatment of
as the major product (53% yield), with N-phenylcyclphexylamine (34%) and
(12%) as by-products (Table I). The best results were found using a 1 ratio of reactant
Use of a 7: 1 molar ratio of formic acid/substrate is critical to the success of the reaction.
with formic acid and a catalytic amount of
in methanol afforded
to
Little or no
reduction of the arene ring occurs when less formic acid is used (e.g. use of a
ratio of
7477

7478
TABLE I. Reduction of aromatic compounds by
NO
N3
NO2
NH2
NO2
0
CH3
NH2
(49)
(55)
(45)
NO2
NH2
0
a
Reaction conditions: Substrate (2.0 mmol); 10%
(0.05 mmol);
(0.5
14 mmol),
(10
70 hr,
Yields
determined by GC using an internal

gave aniline in 57% yield, the remainder being recovered nitro compound), while formanilide resulted (from
the reaction of aniline and formic acid) at higher ratios of formic acid to reactant (e.g. at
formanilide was isolated in 77% yield and
as well as aniline and phenyl
in 13% yield). The reduction is
applicable to
(Table I). Good selectivity for the benzene
No reaction
in the absence of either
ring reduction was obtained in the case of
formic acid or With one exception
and
afforded no reaction took place when an
atmosphere of air or carbon monoxide was used for the reaction. A nitrogen containing functionality is
needed in order to achieve reduction of the arene ring, although such a reduction does not occur in all cases.
No hydrogenation of the benzene ring is observed with compounds containing other functional groups, such
as
-OH, or with organophosphorus compounds (e.g.
We found that the reduction was rapidly terminated by the presence of an anion (e.g. Cl-) of a strong
acid. This observation is consistent with the results of Entwistle and
who investigated the
conversion of nitroarenes to aromatic
were run, for 15 minutes at
using 10% palladium-charcoal in formic acid. These reactions
using
acid as the solvent (i.e. a very high ratio of formic
acid/substrate).
It is likely that reduction of the arene ring in the present case is due to the use of a specific,
relatively low ratio of
acid/substrate
Also, the reaction was effected for an extended period of
time compared with the previous methodology.
The
acid likely acts as the hydrogen source in the reduction
with oxidative addition of
The sequence of events (i.e.
the formic acid to palladium resulting in the generation of a palladium hydride.
reduction of the arene and nitrogen functionalities) remains to be determined.
In conclusion the reduction of the arene ring of a number of organonitrogen compounds occurs under
exceptionally mild conditions using formic acid and palladium on carbon.
The following general procedure was used: a mixture of substrate (2 mmol),
and formic acid (0.5 14 mmol) in 10 of methanol was stirred at room temperature for 70 hr under a
nitrogen atmosphere. Work up was effected by filtration and the filtrate was concentrated by evaporation of
(50 mg, 0.05 mmol)
the methanol. The residue was extracted with ether and the solution concentrated to give the
was further purified by silica gel column chromatography.
which
Acknowledgment: We are grateful to British Petroleum and to the Natural Sciences and Engineering
Research of Canada for support of this research.

7480
References
1.
For
accounts, see Chapters by Siegel, S. (3.1) and by Takaya, H. and Noyoxi, R. (3.2) in
Comprehensive Organic Synthesis;
B.M., Ed.; Pergamon: New York, 1991; Vol. 8.
Volpin, M.E.;
V.O.; Kolomnikov, I.S. Tetrahedron
Gamer, T.; Selve, C.; Delpeuch, J.J.
A.G.M.; Spilling, C.D.
1609.
Rylander, P.N. Catalytic Hydrogenation over Platinum Metals, Academic Press: New York, 1967;
pp. 22, 181-186; Rao, V.S.; Perlin, A.S.
Chem.
Res.
175; Rao, V.S.; Perlin, A.S. Can. J.
6.
7.
8.
9.
H.; Januszkiewicz, K.R. Organometallics
Watanabe, Y.; Ota, T.; Tsuji, Y.
Kikugawa, Y.; Kashimura, M. Synthesis 1982,785.
R.
A.; Bemocchi, E.; Sacchi, S.;
Wiener, H.; Blum, J.; Y. J. Org. Chem.
I.D.; Jackson, A.E.; Johnstone, R.A.W.; Telford, R.P. J. Chem.
Ram, S.; Ehrenkaufer. R.E. Synthesis 1988.91;
Chim. Ital.
10.
F.
1991.27.
11.
12.
13.
I 1977,443.
G. Chem. Rev. 1974, 74, 567 and
cited therein; Johnstone, R.A.W.;
A.H.
Rev.
85, 129 and references cited
therein.
(Received in USA 27 July 1992; accepted 27 August 1992)