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Chemie
Hydrogenation Catalysts
Hot Paper
Tailor-Made Ruthenium-Triphos Catalysts for the Selective
Homogeneous Hydrogenation of Lactams
Markus Meuresch, Stefan Westhues, Walter Leitner, and Jürgen Klankermayer*
Abstract: The development of a tailored tridentate ligand
enabled the synthesis of a molecular ruthenium-triphos
catalyst, eliminating dimerization as the major deactivation
pathway. The novel catalyst design showed strongly increased
performance and facilitated the hydrogenation of highly
challenging lactam substrates with unprecedented activity and
selectivity.
yield.[6] The reduction of lactams to cyclic amines is still
demanding, but the groups of Mashima and Saito could
already use a bis-bidentate (P,N)2-Ru system to accomplish
this transformation and obtained the cyclic products in low to
moderate yield.[7] Therefore, effective homogeneous catalysts
for the hydrogenolysis of lactams towards cyclic amines
remain largely elusive and the development of novel tran-
sition-metal compounds for this challenging catalytic trans-
formation needs to be established. Herein we describe
a rationally developed novel triphos-type ligand that enables
this transformation in hitherto unprecedented efficacy.
In our recent effort we could establish the highly versatile
and stable ruthenium complex [Ru(triphos)(tmm)] (1a,
tmm = trimethylenemethane) as active catalysts system.[2c,8]
Initial mechanistic evaluation of the active hydride species 1b
revealed two reaction pathways as important targets for
tailoring an improved catalyst (Scheme 1).[2c,d] The minor
C
atalytic hydrogenation using molecular catalysts based on
defined organometallic complexes has been advanced to be
an essential tool for the chemical synthesis in research
laboratories as well as on the industrial scale.[1] Very effective
catalysts could be tailored for the addition of hydrogen to
complex organic substrates, largely based on fundamental
mechanistic insight on a molecular level. Moreover, recent
research efforts have illustrated that ruthenium complexes of
the multidentate triphos (1,1,1-tri(diphenylphosphinome-
thyl)ethane) ligand demonstrate potential
for the development of highly active and
stable homogeneous species.[2] Especially for
the reduction of challenging functionalities,
the ruthenium-triphos systems could be es-
tablished as important molecular catalyst,[2b,c]
finding increasing application in numerous
research groups.[3] This important advance-
ment moved these molecular catalysts into the
spotlight for novel transformations and in
Scheme 1. Major reaction pathways of ruthenium-triphos-based catalysts in hydrogenation
reactions.
special cases closer to processing conditions of
heterogeneous catalyst systems.[4] Neverthe-
less, the hydrogenation of non-activated ali-
phatic amides remains an enormous challenge
for molecular catalysts and especially the
reductive cleavage of lactams requires novel dedicated
catalysts. The group of Bergens introduced the catalyst
[Ru(Ph2P(CH2)2NH2)2(m3-C3H5)]BF4 for the hydrogenation
of N-phenylpyrrolidin-2-one, enabling the formation of the
reaction pathway is strongly substrate dependent via the
formation of the dihydrido carbonyl complex [Ru(triphos)-
(CO)(H)2] (1c-CO), originating from the decarbonylation of
intermediate aldehydes or alcohols. However, 1c-CO can be
easily reactivated and recycled towards 1b.[2a] The major
pathway results in deactivation and is based on the irrever-
sible formation of a ruthenium dimer, resulting in the very
stable and catalytically inactive hydride bridged dimeric
complex (1c-Dimer).[2c,d]
À
respective amino alcohol with C N cleavage with high turn-
over number (TON).[5] Most recently the group of Milstein
presented N,N,P-pincer ruthenium complexes for the con-
version of glycine anhydride into ethanolamine in high
Catalyst deactivation via the formation of stable dimers,
trimers, or higher aggregates represents a wide-ranging
problem in homogeneous catalysis.[9] A general approach to
avoid the buildup of such structures is based on the design of
sterically demanding ligands using the respective repellent
forces for keeping the monomeric catalysts maintained in
solution.[10] However, the design of these enlarged ligands still
has to enable the coordination of the substrates, preserving
high catalytic activity.
[*] M. Meuresch, S. Westhues, Prof. Dr. W. Leitner,
Prof. Dr. J. Klankermayer
Institut für Technische und Makromolekulare Chemie
RWTH Aachen University
Worringerweg 2, 52074 Aachen (Germany)
E-mail: jklankermayer@itmc.rwth-aachen.de
Supporting information and ORCID(s) from the author(s) for this
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1392 –1395