COMMUNICATION
DOI: 10.1002/chem.201102276
A Convenient and General Ruthenium-Catalyzed Transfer Hydrogenation of
Nitro- and Azobenzenes
Rajenahally V. Jagadeesh, Gerrit Wienhçfer, Felix A. Westerhaus,
Annette-Enrica Surkus, Henrik Junge, Kathrin Junge, and Matthias Beller*[a]
Anilines are important intermediates and key precursors
in the synthesis of pharmaceuticals, dyes, pigments, agro-
chemicals, polymers, and herbicides.[1] Regarding their syn-
thesis, the selective hydrogenation of nitroarenes represents
a benign and versatile methodology. Less frequently, anilines
have also been synthesized by reduction from the corre-
sponding azo compounds. In addition to catalytic hydroge-
nations,[2,3] stoichiometric reducing agents such as decabor-
ane,[4] silanes,[5] sodium hydrosulphite,[6] hydrazine hydrate,[7]
and formates[8,9] have been described to reduce selectively
aromatic nitro[4–7,8] and azo[9] compounds in the presence of
various metal catalysts.
Despite notable achievements in the last decades, the
search for facile, chemoselective, cost-effective, and environ-
mentally benign procedures that avoid the use of expensive
and hazardous stoichiometric agents is of continuing interest
for catalytic reductions. Complementary to hydrogenations
are transfer hydrogenations (CTH), in which typically alco-
hols such as isopropanol or formic acid–amine mixtures are
used as hydrogen donors.[10,11] Advantageously, CTH reac-
tions do not require any elaborated experimental setup or
the use of high-pressure equipments compared with conven-
tional hydrogenation methods.[12]
yield. Notably, related reactions of azo compounds with
ruthenium catalysts are unknown.[15]
In the past decade, we investigated the catalytic activity
of homogeneous ruthenium catalysts for various organic
transformations.[16] Furthermore, we and others have applied
ruthenium catalysts for transfer-hydrogenation processes.[17]
Based on this experience and the limited knowledge on
ruthenium-catalyzed transfer hydrogenations of nitroarenes,
we became attracted to develop a more general and practi-
cal catalyst system. In this respect, the use of commercially
available ruthenium complexes in combination with nitrogen
ligands was especially attractive to us due to their easy prep-
aration, low price, and variability. Accordingly, we report
herein the application of a two component ruthenium cata-
lyst prepared in situ based on [{RuCl2ACTHNUTRGNE(UNG p-cymene)}2] and dif-
ferent nitrogen ligands. Notably, the in situ generated ruthe-
nium catalyst with ligand 1 allows for selective transfer hy-
drogenation of a variety of aromatic nitro and azo com-
pounds to corresponding anilines in good to excellent yields.
Preliminary catalytic transfer-hydrogenation experiments
have been carried out with nitrobenzene as the model sub-
strate. In exploratory experiments, we found that simple
[{RuCl2ACHTNUTRGNENG(U p-cymene)}2] in the presence of KOH as a base cat-
In the past, isopropanol has been used for the transfer hy-
drogenation of nitro and azo compounds in the presence of
heterogeneous catalyst systems.[13] However, some of these
procedures suffer from drawbacks such as poor selectivity,
low functional group tolerance, and require tedious catalyst
preparation and characterization techniques. In this respect,
the development of easily accessible and selective homoge-
neous catalyst systems for the synthesis of anilines is also
desirable. As a metal of choice ruthenium complexes prevail
in catalytic reductions of carbonyl and related functional
groups. Hence, it is surprising that only one ruthenium/phos-
phine catalyst[14] has been described for reduction of nitro-
benzene with isopropanol. In fact, only nitrobenzene was in-
vestigated as substrate under harsh conditions with poor
alyzed the reduction of nitrobenzene to produce aniline in
65% yield (Table 1, entry 1). However, in the presence of
Table 1. Ruthenium-catalyzed transfer hydrogenation of nitrobenzene in
the presence of different ligands.
Entry
Ligand
Yield[e]
[%]
1[a]
2[b]
3[b]
4[b]
5[b]
6[b]
7[c]
8[d]
–
1
2
3
4
5
–
1
65
98
91
90
85
72
3
[a] Dr. R. V. Jagadeesh, G. Wienhçfer, F. A. Westerhaus,
Dr. A.-E. Surkus, Dr. H. Junge, Dr. K. Junge, Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse e. V. an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
Fax : (+49)381-1281-5000
0
[a] Nitrobenzene (0.5 mmol), [{RuCl2ACHTNUGTRENUNG(p-cymene)}2] (0.0125 mmol), KOH
(0.075 mmol). [b] Same as [a] with ligand (0.025 mmol). [c] Same as [a]
without catalyst and ligand. [d] Same as [b] without base. [e] Yields were
determined by GC analysis.
Chem. Eur. J. 2011, 17, 14375 – 14379
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
14375