Inorganica Chimica Acta
Dendritic [RuCl2(BINAP)(DPEN)] catalysts with ‘Sandwich’ multi-layer
structure for asymmetric hydrogenation of simple aryl ketones
a,
Ji Liu a,b, Baode Ma a,b, Yu Feng a, Yanmei He a, , Qing-Hua Fan
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a Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Beijing 100190, PR China
b University of Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 7 June 2013
A new kind of chiral dendritic [RuCl2(BINAP)(DPEN)] catalysts with a ‘Sandwich’ multi-layer structure
have been synthesized via metal coordination reactions with dendritic chiral diphosphines and dendritic
chiral diamine ligands. The formation of the dendritic catalysts was confirmed by 31P NMR. The catalyst
performance was then investigated in the asymmetric hydrogenation of simple aryl ketones, and compa-
rable enantioselectivities were achieved with comparison to that of the corresponding small-molecular
catalyst. In addition, the recyclable property of these homogeneous catalysts was studied with the
third-generation catalyst, which was reused nine times by simple solvent precipitation without any loss
of enantioselectivity.
Metallodendrimers Special Issue
Keywords:
Metal catalysis
Asymmetric hydrogenation
Chiral dendrimer
Catalyst recycling
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
low catalyst loading. Especially for the higher generation catalyst,
it may result in low catalytic activity. In contrast to the core-func-
Dendrimers represent a new kind of macromolecules which
possess highly branched and well-defined molecular structures
with nano-scale sizes. These well-defined, discrete structures can
be precisely controlled at a molecular level [1,2]. The dendrimers
with these favorable properties have generated considerable inter-
ests for their application in the field of supported catalysis during
the past decades [3–9].
tionalized catalysts, the active sites of the periphery-functionalized
systems are located at the dendrimer surface, which are directly
available to substrate in addition to a high catalyst loading [26–
35]. Also, some reactions may benefit from the high local catalyst
concentration created by these catalysts. But sometimes, undesired
interactions between the neighboring peripheral catalytic sites
may cause ‘‘negative’’ dendrimer effects (a bimetallic deactivation
mechanism). Besides, catalyst recycling has not yet been achieved
for most of these periphery-functionalized dendritic metal cata-
lysts. Compared to the cases above, the catalysts with active sites
at the branching points of dendrimer (Fig. 1d) [36–38] will offer
an opportunity to combine the advantages of both. However, the
metal dendritic catalysts with this topology structure are rarely re-
ported due to the difficulties in synthesis, especially in asymmetric
catalysis. Therefore, developing new types of dendritic catalysts
that feature easy synthesis, high efficiency and reliable recyclabil-
ity is still remaining a challenging subject.
Recently, we reported a new kind of easily available chiral
BINAP-functionalized Janus dendrimers (Fig. 1e) for the Ru-cata-
lyzed asymmetric hydrogenation of 2-arylacrylic acids [39,40].
Based on this work and for our continuing interest in the appli-
cations of functionalized organometallic dendrimers as homoge-
neous catalysts, herein we report the synthesis of a novel series
of dendritic catalysts (Fig. 1f) of Janus type with the metal cata-
lytic centers located at the branching points of one dendron. The
dendritic catalysts are expected to provide the following advan-
tages: (1) By introducing the peripheral Fréchet dendrons, the
solubility and the molecular weight of the dendritic catalysts
could be fine-tuned to facilitate catalyst synthesis and recycling
Since van Koten and co-workers reported the first example of
dendritic catalyst in 1994 [10], a variety of dendritic catalysts have
been reported and successfully applied in various catalytic reac-
tions. Generally, the catalytically active sites are located at the
core, the focal point, the periphery or the branches of dendrimers
(Fig. 1a–d). In the case of core-functionalized dendritic catalysts
[11–24], on one hand, isolation effects created by the sterically
demanding dendritic wedges would be beneficial for some reac-
tions [25], in which a bimetallic deactivation mechanism is opera-
tive. On the other hand, the core-functionalized dendritic catalysts
might benefit from the local micro-environment along with
desolvation effects during the penetration of substrate into the
dendrimer shell, which is similar to the situation in enzyme catal-
ysis. In addition, the solubility of the core functionalized dendritic
catalysts can be well controlled by changing the surface groups,
which makes the recycle of catalyst much easier by solvent precip-
itation. Although good catalytic results and easy catalyst separa-
tion were achieved in most cases, placing a single catalytic site at
the core of a large dendrimer results in a catalytic system with
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0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.