COMMUNICATION
DOI: 10.1002/chem.201203059
Discrete Iron Complexes for the Selective Catalytic Reduction of Aromatic,
Aliphatic, and a,b-Unsaturated Aldehydes under Water–Gas Shift Conditions
Anis Tlili, Johannes Schranck, Helfried Neumann, and Matthias Beller*[a]
The water–gas shift reaction (WGSR) of carbon monox-
ide and water to give carbon dioxide and hydrogen [Eq. (1)]
is a very important industrial process.[1] For instance, it plays
a major role in steam reforming of alkanes to produce hy-
drogen. Therefore, it is the basis of important bulk hydroge-
nation processes, most notably the catalytic reduction of mo-
lecular nitrogen to form ammonia in the Haber-Bosch proc-
Scheme 1. Iron-catalyzed reduction of carbonyl compounds with various
reductants.
ess.[2]
cat:
!
CO þ H O
CO þ H
ð1Þ
2
2
2
tively reduce a variety of aldehydes by using cheap carbon
monoxide and water as the source of hydrogen in the pres-
ence of discrete cyclopentadienyliron–tricarbonyl complexes.
Previously, this type of complex had been synthesized by
Knçlker and co-workers. Later, Casey and co-workers ele-
gantly demonstrated that such iron complexes can be used
for the reduction of ketones under a low pressure of molec-
ular hydrogen.[10,11,12]
In general, WGSRs are performed in the presence of
stable heterogeneous catalysts at high temperatures and
pressures. As an example, Fe3O4 or copper metal are em-
ployed as catalysts at temperatures up to 3508C.[3] However,
since the early 1970s, the development of homogenous cata-
lysts based on Ru, Rh, Ir, and Pt has also been investigated
in order to perform the WGSR under milder reaction condi-
tions, typically at temperatures around 1008C.[1] In addition
to noble-metal catalysts, less expensive discrete molecular
iron complexes have also been investigated for use in this
reaction. Hence, as early as 1978, King and co-workers dem-
onstrated that simple iron pentacarbonyl catalyzes the
WGSR,[4] although temperatures as high as 1808C were re-
quired.
Initial experiments were carried out with benzaldehyde as
a model substrate, 10 bar of carbon monoxide in the pres-
ence of various bases and solvents, and the stable cyclopen-
tadienyliron–tricarbonyl complex 2a. It is noteworthy that
2a is the precursor of the “Knçlker iron complex” (2b),
which has mainly been applied to ketone hydrogenations,
but is also significantly more sensitive to oxygen than 2a
and therefore difficult to handle. In the presence of potassi-
um carbonate in pure water, benzyl alcohol was obtained in
24% yield (Table 1, entry 1). Addition of THF as a cosol-
vent improved the reactivity significantly and the desired
product was observed in 77% yield (Table 1, entry 3).
Benzyl alcohol was formed in an even better yield (95%)
when DMF was employed as the cosolvent (Table 1,
entry 4). Interestingly, in the case of DMF, a byproduct re-
sulting from the reductive amination of the decomposed
DMF and benzaldehyde was also observed (5% yield). The
most efficient and selective system is formed when the reac-
tion is carried out in DMSO and an excellent yield of 99%
was achieved (Table 1, entry 5).
From a conceptual point of view, the WGSR allows the
use of carbon monoxide and water as a source of hydrogen.
Therefore, catalytic hydrogenations might be performed
with a mixture of CO and water as the reductant. Surpris-
ingly, this approach has not been widely studied.[5] Based on
our general interest in developing selective iron-catalyzed
reductions,[6] we started to investigate the reduction of unsa-
turated compounds with the WGSR as a source of hydrogen
in the presence of discrete iron complexes.[7] During the last
decade, several elegant iron-catalyzed hydrogenations, trans-
fer hydrogenations and hydrosilylations, especially of car-
bonyl compounds (Scheme 1), have been developed.[7,8]
However, to the best of our knowledge, no iron-catalyzed
reductions under WGSR conditions have been reported to
date.[9] Herein, we demonstrate that it is possible to selec-
To investigate the role of the base in our system, various
inorganic bases (Cs2CO3, KOH, and LiOtBu) and one or-
ganic base (NEt3) were tested in the model system (Table 1,
entries 6–9). From these experiments, it can be concluded
that K2CO3 is the base of choice for this system. Control ex-
periments showed that the iron catalyst and base are neces-
sary for the reduction (Table 1, entries 10 and 11). The cata-
lyst showed good activity (94% yield) even when lower cat-
[a] Dr. A. Tlili, J. Schranck, Dr. H. Neumann, Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse an der Universitꢁt Rostock
Albert-Einstein-Straße 29a, 18059 Rostock (Germany)
Fax : (+49)381-1281-5000
Chem. Eur. J. 2012, 18, 15935 – 15939
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
15935