Pd/C-catalyzed transfer reduction of aryl chlorides with sodium formate in water

Article abstract of DOI:10.1002/ejoc.200400258

Sodium formate catalytic transfer reduction, in the presence of palladium on carbon, accomplishes not only the rapid dehalogenation of aromatic chlorocarbons in water at room temperature, but also the reduction of the aromatic moieties to alicyclic rings at 100 °C. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejoc.200400258

SHORT COMMUNICATION
Pd/C-Catalyzed Transfer Reduction of Aryl Chlorides with
Sodium Formate in Water
Antonio Arcadi,*[a] Giorgio Cerichelli,[a] Marco Chiarini,[b] Raquel Vico,[a] and
Daniel Zorzan[a]
Dedicated to Professor Domenico Misiti on the occasion of his 70th birthday
Keywords: Reductive dechlorination / Palladium / Catalysis / Sodium formate / Aryl chlorides
Sodium formate catalytic transfer reduction, in the presence
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
of palladium on carbon, accomplishes not only the rapid de- Germany, 2004)
halogenation of aromatic chlorocarbons in water at room
temperature, but also the reduction of the aromatic moieties
to alicyclic rings at 100 °C.
Introduction
has some advantages over the use of molecular hydrogen
since it avoids the risks and constraints associated with hy-
drogen gas as well as the necessity of pressure vessels and
of other equipment. Therefore, the aid of a stable hydrogen
donor represents a useful alternative method to the catalytic
hydrodechlorination of aromatic chlorides by molecular hy-
drogen. These dechlorination systems involve the use of me-
tal hydrides,[17] metal alkoxides,[6,18] hydrosilanes,[19,20]
Grignard reagents,[21] alcohols,[22] and formic acid and its
salts[23,24] as the hydrogen source. Formic acid and its salts
are particularly versatile, and cheap reagents that have been
used as a source of hydrogen in catalytic transfer hydrogen-
ations and have frequently been applied in the reduction of
a large variety of functional groups.[25] Nevertheless, only
few reports are concerned with the catalytic transfer re-
duction of aromatic chlorides in water.[26]
The removal of halogens from aryl halides is an ex-
tremely important chemical transformation both from a
preparative[1,2] and an environmental point of view.[3,4] Aro-
matic chlorides are much less reactive than aromatic bro-
mides and iodides and hence, many efforts have been fueled
towards the goal of efficient and clean hydrodechlorination
methodologies.[5,6] Reductive cleavage is advantageous for
converting halogenated organic compounds, since oxidation
can generate highly toxic products such as phosgene, diox-
ins and chlorine, which may also poison most catalysts or,
at least, lead to their deactivation. General procedures for
the palladium on carbon (Pd/C) catalyzed hydrogenation of
aromatic chlorides that operate under mild conditions have
been applied to the hydrodechlorination of a variety of aro-
matic chlorides.[7,8] Multiphase systems composed of an al-
kaline aqueous phase and hydrocarbon solvent, such as iso-
octane in the presence of a phase-transfer (PT) agent, have
allowed facile and selective Pd- or Pt-catalyzed hydrodehalo-
genation to afford dehalogenated aromatics at 50 °C and at
a pressure of 1 atm of hydrogen.[9Ϫ16] However, H2 is a gas
that, for safety reasons, is not always convenient to use, and
when used losses must be avoided through a carefully as-
sembled gas delivery system. The use of a hydrogen donor
Results and Discussion
Herein, we describe a general alternative procedure for
the palladium on carbon (Pd/C) catalyzed hydrodechlori-
nation of aromatic chlorides in water, which operates under
mild conditions in the presence of sodium formate as the
hydrogen source. Among the noble metal catalysts tested
for the hydro treatment with H2 in the water phase of sev-
eral chloro organic compounds, Pd/C showed a higher ac-
tivity with respect to other noble metals.[27] In addition, Pd/
C was found to be a reusable catalyst in aqueous media.[28]
Initially, we investigated the Pd/C-catalyzed hydrodehalo-
genation reaction of 4-chlorobenzophenone (1a) in water
in the presence of different formic acid derivatives
(Scheme 1).
[a]
Dipartimento di Chimica, Ingegneria Chimica e Materiali,
`
Universita degli Studi dell’Aquila,
Via Vetoio, 67010 Coppito (AQ), Italy
Fax: (internat.) ϩ 39-0862-433753
E-mail: arcadi@univaq.it
[b]
`
Dipartimento di Scienze degli Alimenti, Universita degli Studi
di Teramo,
Via Spagna 1, 64023 Mosciano Stazione (TE), Italy
3404
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejoc.200400258
Eur. J. Org. Chem. 2004, 3404Ϫ3407

Pd/C-Catalyzed Transfer Reduction of Aryl Chlorides with Sodium Formate
SHORT COMMUNICATION
Table 1. Reduction of aryl chlorides in water with HCOONa and
catalytic 10% Pd/C
Scheme 1
The results obtained show that HCOONa is clearly su-
perior to the other two salts tested and to formic acid. The
remaining percentage is usually accounted for by un-
changed starting material. Evidence reported in the
literature[29Ϫ31] show that differences in the nature of formic
acid derivatives and in the reaction medium can signifi-
cantly affect the reaction course. Under the optimized con-
ditions determined above, dechlorination reactions of a var-
iety of aryl chlorides 1 to the corresponding aromatic hy-
drocarbon 2 were carried out (Scheme 2; Table 1, Entries 1,
3, 4, 6, 8, 10, 12, 14, and 18).
Scheme 2
Electron-withdrawing and -donating groups in the aryl
chlorides tested were well tolerated for the present catalytic
dechlorination to give the corresponding aromatic products.
The experimental procedure is very simple. After com-
pletion of the reaction, the catalyst was removed by fil-
tration, and the product was isolated by extraction into a
suitable organic solvent as ethyl acetate or diethyl ether. It
is worth noting that the recovered Pd/C shows sufficient
catalytic activity for the hydrodechlorination reaction. The
activity of the recovered Pd/C was monitored by the hydro-
dechlorination of 2-chloroaniline (1l). Although the cata-
lytic activity was gradually diminished, the yield was still
62% after it was reused for the third time. The additional
parallel path to produce biphenyls by reductive dimeri-
zation[32] was not observed. By contrast with the previously
reported results under our conditions, the reaction is not
sensitive to the amount of water present. The Pd/C-cata-
lyzed transfer hydrogenolysis of aryl halides by formate
salts in a triphase system required the absorption of all
three substrates, the aryl halide acceptor, the formate anion,
and water, to a single site on the catalyst surface in a
specific order.[33] No reaction was observed at all in the ab-
sence of water. It was also demonstrated that the amount
of the water present in the system should be optimized to
produce high reaction rates in solid/solid/liquid/liquid
triphase systems.[34] Interestingly, 1-chloroanthracene
(Table 1, Entry 16) leads to a mixture of anthracene (2c)
(16%), 1,2,3,4-tetrahydroanthracene (9%), and 9,10-dihy-
[a]
[b]
Isolated yield by flash chromatography.
Unless otherwise
stated, reactions were carried out in water with the molar ratio
1/HCOONa/10% Pd/C ϭ 1:5:0.05. [c] Unless otherwise stated, reac-
tions were carried out in water with the molar ratio 1/HCOONa/
[d]
10% Pd/C ϭ 1:15:0.05. GC yield.
Eur. J. Org. Chem. 2004, 3404Ϫ3407
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3405

A. Arcadi, G. Cerichelli, M. Chiarini, R. Vico, D. Zorzan
SHORT COMMUNICATION
droanthracene (11%). This result shows us that further re- Under the conditions in Table 1, the 4-fluorophenol is con-
duction steps could also occur. Dehalogenation and the verted into cyclohexanol (3a) in 75% yield (Scheme 4).
complete reduction of aromatic chlorocarbons to alicyclic
compounds is the key goal because benzene and its deriva-
tives, as well as polycyclic aromatic hydrocarbons (PAH),
are by themselves hazardous carcinogenic substances. The
development of the detoxification processes that also de-
stroy the aromatic moieties represent a target of current
interest.[35] Herein, we report that the palladium-catalyzed
reduction of aryl chlorides to the corresponding alicyclic
compounds can be accomplished in the presence of excess
Scheme 4
HCOONa in water at reflux (Scheme 3). To the best of our
knowledge, this is the first example of sodium formate cata-
Conclusion
lytic-transfer reduction of aromatic chlorocarbons to their
corresponding alicyclic derivatives. Hydrogenation of
chlorinated aromatic derivatives to alicyclic compounds
usually requires high hydrogen pressures.
In summary, we have shown that employment of Pd/C/
HCOONa in water at room temperature may operate as a
very mild and efficient system to accomplish the chemose-
lective hydrodechlorination of chloroaromatic compounds.
This general catalytic process may also serve as a facile way
to chemoselectively transform functionalized chloroarenes
into the corresponding alicyclic derivatives in water at 100
°C. In addition to the synthetic usefulness of the reaction,
demands for the treatment of industrial and natural con-
taminated waters is strongly motivating for the extension
of this method to practical use, toward the degradation of
structurally related chlorinated aromatic environmental pol-
lutants that form unfavored benzene derivatives through
conventional hydrodechlorination methods. The decontami-
nation of water polluted by chlororganic compounds re-
sulting from land storage of toxic wastes, industrial spills,
and use of agricultural products is still an environmental
emergency.
Scheme 3
The best conditions so far developed (1 equiv. of aryl
chloride, 15 equiv. of HCOONa in the presence of 0.05
equiv. of 10% Pd/C in H2O at 100 °C) were usually em-
ployed when we extended the procedure to a variety of aryl
chlorides as shown in Table 1. It is remarkable that under
these conditions the transfer reduction of chlorophenols
1cϪe and 4-chlororesorcinol (1h) leads chemoselectively to
the formation of cyclohexanol. By contrast with the results
previously reported[36] in the hydrogenation experiments of
chloroarenes in the presence of the sol-gel entrapped Pd/
[Rh(cod)Cl]2 or supported palladium[36] catalysts, the for-
mation of cyclohexane and cyclohexanone, which can be
regarded as the keto tautomer of the cyclohexenol, is not
observed with the present methodology. Surprisingly, 4-
chloroacetophenone (1g) can be selectively reduced to 1-
phenylethanol (4a). Under classical conditions using meth-
anol or ethanol as solvents and 10% Pd/C as the catalyst,
the reactions proceed without any selectivity, and aromatic
Experimental Section
General Remarks: All starting materials were commercially avail-
able and were used as purchased without further purification, un-
less otherwise stated. Solutions were prepared in redistilled deion-
ized water. The products, after conventional workup, were purified
by flash chromatography on silica gel, eluting with n-hexane/ethyl
acetate mixtures. The products are all known and were identified
by comparison of physical and spectroscopic data with those given
in the literature.
hydrocarbons are the sole final products.[37] The polycyclic Typical Experimental Procedure. Pd/C-Catalyzed Procedure for
Hydrodechlorination of Aryl Chlorides with HCOONa at Room
Temperature in Water: The aryl chlorides (1 mmol) and 10% Pd/C
(0.05 mmol) were added to an aqueous solution of HCOONa
(5 mmol, 4 mL). Reaction aliquots after filtration through a plug
of cotton wad were analyzed by GC-MS. After completion of the
reaction, the catalyst was removed by filtration, and the product
was isolated by extraction into a suitable organic solvent such as
ethyl acetate or diethyl ether and purified by flash chromatography
of the crude mixture (silica gel; n-hexane/ethyl acetate mixtures).
1-choroanthracene does not undergo exhaustive hydrogen-
ation, but rather proceeded in a manner similar to that of
previous results obtained in the reduction of anthracene;[38]
1,2,3,4,5,6,7,8-octahydroanthracene (6a) was isolated in
good yield. The exposure of 2-chloroaniline (1l) to the
standard conditions in Table 1 (Entry 19) leads to dicyclo-
hexylamine (7a). The formation of 7a can be viewed as
occurring through the reduction of N-cyclohexylaniline de-
rived by the palladium-catalyzed reductive amination of
cyclohexanone imine with aniline intermediates.[39] The for-
mation of the N-cyclohexylaniline was assessed by GC-MS.
Finally, although this study is concerned with the hydrode-
halogenation of chloroarenes, it is worth noting that fluoro-
Acknowledgments
`
We wish to thank, the Universita degli studi di L’Aquila, the Con-
sorzio Interuniversitario Nazionale La Chimica per l’Ambiente
arenes can easily undergo hydrodefluorination reaction. (INCA) ‘‘Green Chemistry Project’’, and the Ministero degli Affari
3406  2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjoc.org
Eur. J. Org. Chem. 2004, 3404Ϫ3407

Pd/C-Catalyzed Transfer Reduction of Aryl Chlorides with Sodium Formate
SHORT COMMUNICATION
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Received April 16, 2004
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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