DOI: 10.1002/chem.201406402
Communication
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Heterogeneous Catalysis
A Carbon Nanotube Confinement Strategy to Implement
Homogeneous Asymmetric Catalysis in the Solid Phase
Kazuki Hashimoto,[a] Naoya Kumagai,*[a] and Masakatsu Shibasaki*[a, b]
ports for heterogeneous Nd/Na catalysts for the anti-selective
Abstract: A readily recyclable asymmetric catalyst has
been developed based on the self-assembly of a homoge-
neous catalyst in a fibrous network of multiwalled carbon
nanotubes (MWNTs). Dimerization of an amide-based
chiral ligand with a suitable spacer allows for the efficient
formation of a heterogeneous catalyst by self-assembly on
addition of Er(OiPr)3. The self-assembly proceeds in the
MWNT fibrous network and small clusters of assembled
catalyst are confined in the MWNTs, producing an easily
handled solid-phase catalyst. The resulting MWNT-con-
fined catalyst exhibits a good catalytic performance in
a catalytic asymmetric Mannich-type reaction, which can
be conducted in a repeated batch system and in a contin-
uous-flow platform.
nitroaldol reaction.[5] An insoluble Nd/Na catalyst was formed
through self-assembly[6] of the catalyst component ligand 1,
NdO1/5(OiPr)13/5, and sodium hexamethyldisilazide (NaHMDS;
Scheme 1a).[5a,b] When the self-assembly was conducted in the
The development of heterogeneous catalysts is a continuous
area of research because of their value in industrial applica-
tions.[1,2] In contrast to the broad availability and intensive use
of achiral heterogeneous catalysts, their chiral counterparts,
which enable enantioselective catalysis, have been less ex-
plored because of the inherent difficulty of rendering stereo-
chemical differentiation by catalysts in the solid phase. A com-
monly exploited approach to producing heterogeneous chiral
catalysts involves utilization of the covalent linkage of the
chiral elements used in homogeneous catalysis, e.g., chiral or-
ganocatalysts or chiral ligands for metal-based catalysts, to
a given solid phase material. However, a decrease in catalytic
activity and/or stereoselectivity frequently arises, and is associ-
ated with the distinct chemical environment of the catalytically
active sites in the solid phase.
Scheme 1. a) Confinement of the heterogeneous Nd/Na catalyst; b) modifi-
cation of the chiral ligand for the production of new MWNT-confined cata-
lyst.
presence of MWNTs, the formation of catalyst clusters occurred
within the minute MWNT fibrous network. The assembled cata-
lyst was effectively confined within the MWNTs, thereby pro-
ducing a more stable and readily recoverable catalyst.[5c,d] The
confinement in MWNT bestowed an extended lifetime to the
less stable insoluble catalyst and the thus-obtained recyclable
solid-phase asymmetric catalyst was applied to a continuous-
flow reaction.[5e]
Therefore, robust and reliable chiral heterogeneous catalysts
are still underdeveloped, and are in high demand for prospec-
tive use in continuous-flow platforms in the chemical and
pharmaceutical industries.[3,4] Recently, we reported the use of
multiwalled carbon nanotubes (MWNTs) as suitable solid sup-
Although MWNT confinement is easily achieved, just by
mixing, and no covalent linkage is required,[7,8] this methodolo-
gy is only valid for heterogeneous complexes. We anticipated
that linking chiral ligands would lead to heterogeneity on com-
plexation with metals, and a similar MWNT-confined catalyst
would be produced. We reasoned that the chiral amide-based
ligand 2a would be well-suited for this study because: (1) Sev-
eral catalytic asymmetric reactions have been developed using
2a and rare earth (RE) metal catalysts,[9] and (2) 2a, containing
amide functionalities, is prone to form insoluble material on
addition of RE cations. Herein, we report on a strategic modifi-
cation of an amide-based chiral ligand to generate a heteroge-
[a] K. Hashimoto, Dr. N. Kumagai, Prof. Dr. M. Shibasaki
Institute of Microbial Chemistry (BIKAKEN) Tokyo
3-14-23 Kamiosaki, Shinagawa-ku, Tokyo 141-0021 (Japan)
Fax: (+81)3-3441-7589
[b] Prof. Dr. M. Shibasaki
JST, ACT-C, 3-14-23 Kamiosaki, Shinagawa-ku, Tokyo 141-0021 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406402.
Chem. Eur. J. 2015, 21, 1 – 6
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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