Using this methodology, we prepared insoluble catalysts and
applied them to heterogeneous organic synthetic reactions.
These catalysts showed outstanding stability and reusability
in reaction media such as water and aqueous or anhydrous
organic solvents.
Scheme 2. Preparation of Solid-Phase Metal-Nanoparticles
Nano-Pd-V 3 via Self-Organization of PdCl2 and
Non-cross-linked Polymer 1
We planned to expand this methodology to the preparation
of solid-phase metal-nanoparticles via self-organization of
inorganic materials and non-cross-linked polymers. Anionic
inorganic materials self-assembled with non-cross-linked
cationic polymers to give insoluble supramolecular com-
plexes, which were converted by reduction into insoluble
self-organized metal-nanoparticles having the polymers as
matrixes (Scheme 1). These metal-nanoparticles should be
Scheme 1. Working Hypothesis for the Preparation of
Solid-Phase Metal-Nanoparticles via Self-Organization of
Inorganic Materials and Non-cross-linked Polymers
at 25 °C in EtOH to afford the black polymeric palladium
nanoparticles nano-Pd-V 3, which were insoluble in water,
ethanol, ethyl acetate, dichloromethane, toluene, and hexane.
To elucidate the structure of 3, several spectroscopic
measurements were carried out. The morphology of 3 was
evaluated by scanning electron microscopic (SEM) observa-
tion, which confirmed 3 to be a macroporous material (Figure
1, top and Figure S-1, Supporting Information). The images
revealed a spongelike structure with a disordered arrangement
of interconnected pores in the size range of approximately
1 µm. An energy dispersive spectrum (EDS) collected in
the SEM showed the presence of palladium and bromide with
a trace amount of chloride in 3. Transmission electron
microscopy (TEM) and scanning transmission electron
microscopy-energy dispersive spectroscopy (STEM/EDS)
analysis proved the existence of palladium nanoparticles in
3 (Figure S-2, Supporting Information). TEM analysis
showed that the palladium particles have a mean diameter
of 2 nm with a narrow size distribution (Figure 1, bottom,
and Figure S-2, Supporting Information).
stabilized by matrix polymers to prevent leaching, aggrega-
tion, and deactivation, so that they can become active and
recyclable solid-phase catalysts. In this Letter, we would like
to report the development of an environmentally benign
solid-phase polymeric catalyst of nanopalladium particles
(nano-Pd-V) and its application to the R-alkylation of
ketones with primary alcohols. It should be emphasized that
this novel catalyst was reused in the absence of organic
solvents under atmospheric conditions without any loss of
catalytic activity.
The novel nanopalladium catalyst nano-Pd-V 3 was
prepared as shown in Scheme 2. When PdCl2 and 20 mol
equiv of NaCl in water was added to an aqueous solution of
the main-chain viologen polymer 15 at 25 °C, the components
self-assembled to give the insoluble palladium complex 2
as an orange powder. Compound 2 was reduced by NaBH4
To demonstrate the potency of nano-Pd-V 3 as a reusable
heterogeneous catalyst for organic synthesis, R-alkylation of
ketones with primary alcohols was selected because this
reaction system is environmentally benign: it does not use
harmful alkyl halides as electrophiles, and the resulting
products are only the desired R-alkylated ketones and
water.6,7 In 2004, Kaneda et al. reported the first heteroge-
neous R-alkylation of ketones with alcohols at 180 °C in
toluene catalyzed by Ru-hydrotalcite.7a Recently, Cho re-
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Org. Lett., Vol. 8, No. 7, 2006