Full Papers
doi.org/10.1002/cctc.202000266
ChemCatChem
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Accumulation of Active Species in Silica Mesopore: Effect of
the Pore Size and Free Base Additives on Pd-catalyzed
Allylation using Allylic Alcohol
Ken Motokura,*[a, b] Sae Kawashima,[a] Masayuki Nambo,[a] Yuichi Manaka,[a, c] and
Wang-Jae Chun[d]
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A mesoporous silica-supported Pd complex was prepared using
various types of porous silica supports (pore size: 16–31 Å). The
effects of the pore size and base additive properties on the
catalytic allylation were investigated. The activity of the Pd-
catalyzed allylation of nucleophiles with specific base additives,
such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), is strongly
affected by the mesopore size. The best allylation performance
is obtained with a pore size of 23 Å. On the other hand, bases
with smaller/larger molecular size and/or high nucleophilicity,
such as K2CO3 and 1,4-diazabicyclo[2.2.2]octane (DABCO), do
not seem to show clear pore effect on the catalysis. The
performance of the catalytic allylation significantly decreases by
using such bases. Kinetic, spectroscopic, and control experi-
ments reveal that the effective accumulation of Pd species and
DBU in mesopores with appropriate pore sizes enhances their
frequency factors.
Introduction
has led to investigations of the specific effects of mesopores on
the catalytic activity and selectivity of immobilized and/or
accumulated active species inside mesopores. Size-selective
activation using mesoporous silica (MS)-supported metal cata-
lysts represents one of the most popular methods of mesopore-
induced selective catalysis.[4] Zapilko and co-workers reported a
size-selective MPV hydride transfer reaction of aldehyde to the
corresponding alcohol; the MPV reaction of benzaldehyde
preferentially occurs compared with pyrenecarboxaldehyde
with a MS-supported Al catalyst.[4a] Aleman and co-workers
developed a supported Pt complex catalyst for the size-selective
debromination reaction.[4b] The SBA-15-supported catalyst with
a pore size of 54 Å leads to a faster reaction rate of large
substrate molecules with tert-butyl groups than the other
support with 21 Å mesopores. Mesopores also induce regiose-
lective and enantioselective reactions.[5] Thomas and co-workers
reported a supported Pd complex for asymmetric CÀ N bond
formation.[5a] In the case of the MS-support, the selectivity for
the formation of the branched product increases with excellent
enantioselectivity. Raja and co-workers revealed that the
enantioselectivity in tight spaces is controlled by the pore
size.[5b] The catalytic activity and stability also strongly improve
by the introduction of active species into mesopores.[6]
Another advantage of the mesoporous support is its active
site accumulation in confined spaces.[7] Li and co-workers
presented the hydrolytic kinetic resolution of epoxide in a SBA-
16 cavity.[8] Two accumulated Co(salen) complexes showed
efficient cooperative catalysis. Jones and co-workers demon-
strated the amine-silanol cooperative catalysis for the aldol
reaction in mesopores.[9] The catalytic performance was signifi-
cantly affected by the pore size of the support. These findings
strongly indicate that the accumulation efficiency of two or
more active species with respect to catalysis is significantly
affected by the confined mesopore environment. Our group
also reported a silica-supported Pd-complex-tertiary amine
The design of a catalyst and catalytic reaction system for
catalysis in tight spaces is a unique methodology to achieve
efficient molecular transformation, that is, to increase the
activity, selectivity, and productivity.[1] The molecular sieving
effect is one of the most famous properties of zeolites, which
are microporous materials (pore size: <2 nm),[2] that are widely
applied in catalytic reactions, especially in the field of petro-
leum chemistry, and small molecule selective transformation.
On the other hand, the use of mesoporous materials for fine
chemical synthesis is strongly desired because their pore sizes
(pore size: 2 to 50 nm) are appropriate for functionalized
organic compounds.[3] The increase in the research activities in
the field of mesoporous supports for organic synthetic reactions
[a] Prof. K. Motokura, S. Kawashima, M. Nambo, Prof. Y. Manaka
Department of Chemical Science and Engineering
School of Materials and Chemical Technology
Tokyo Institute of Technology
Midori-ku, Yokohama, 226-8502 (Japan)
E-mail: motokura.k.ab@m.titech.ac.jp
[b] Prof. K. Motokura
PRESTO
Japan Science and Technology Agency (JST)
Saitama 332-0012 (Japan)
[c] Prof. Y. Manaka
Renewable Energy Research Center
National Institute of Advanced Industrial Science and Technology
2-2-9 Machiikedai
Koriyama, Fukushima 963-0298 (Japan)
[d] Prof. W.-J. Chun
Graduate School of Arts and Sciences
International Christian University
Mitaka, Tokyo, 181-8585 (Japan)
Supporting information for this article is available on the WWW under
This publication is part of a Special Collection on “Catalysis in Confined
Spaces”. Please check the ChemCatChem homepage for more articles in the
collection.
ChemCatChem 2020, 12, 1–10
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