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These drawbacks are fatal in modern catalysis because the de-
velopment of green reaction conditions is becoming more
urgent than ever in the face of stringent environmental stand-
ards and economic pressures. Moreover, the complex chemical
synthesis of dendrimers, especially for high-generation ones,
also largely hinders their industrialization and commercializa-
tion. In this regard, development of alternative stabilizers with
dendrimer-like characteristics but with a much easier prepara-
tion method, which can stabilize Pd NPs and catalyze Suzuki–
Miyaura reactions in water at room temperature, would be
highly desirable.
strates can afford the desired coupling products in excellent
yields. In particular, the Pd@triazole-DCCMs can tolerate many
functional groups under the same mild reaction conditions
and give the corresponding biphenyl products in high yields
as well. Further recycling experiments showed that the tria-
zole-containing DCCM-stabilized palladium catalyst system is
highly reusable and could be reused many times with negligi-
ble turnover frequency change. The simple catalyst prepara-
tion, the environmentally benign reaction conditions, and the
excellent catalytic performance and durability make this novel
triazole-containing DCCM-protected palladium nanoparticle
system an outstanding alternative for traditional dendrimer-
stabilized counterparts.
Our group has long been committed to the covalent capture
[
18]
of small-molecule self-assemblies. In a recent report, we em-
ployed for the first time the facile thiol–ene “click” reaction to
cross-link the neutral micelles formed by low molecular weight
[
19]
surfactants. The resulting core cross-linked micelles demon- Results and Discussion
strate dendrimer-like properties such as spherical shape, con-
Design, synthesis, and characterization of triazole-contain-
trollable size, hydrophobic interiors, and high chemical stabili-
ty, and have been proved to be an outstanding template for
gold nanoparticles. Importantly, the novel dendrimer-like core
cross-linked micelle (DCCM) stabilized noble gold clusters to
furnish excellent catalytic activity and give perfect reusability
in the aerobic oxidation of a-hydroxy ketones in water. In
open air, the oxidation could be repeated up to 48 times with
negligible turnover frequency (TOF) change. Following these
attractive properties, we herein report a new triazole-contain-
ing dendrimer-like core cross-linked micelle, synthesized
by ready available low molecular weight amphiphiles
ing dendrimer-like core cross-linked micelles (triazole-
DCCMs)
Astruc and co-workers did great work on dendrimer-stabilized
[21]
metal NPs. One of their main achievements was finding that
triazole groups are effective ligands to stabilize Pd nanopartic-
[17e]
les.
Inspired by this pioneering contribution, the neutral am-
phiphiles 1a–c with three alkynyl groups, which aim to con-
struct a triazole skeleton, were synthesized in this work
(Scheme 1 and see the Supporting Information for details).
With a hydrophobic alkyl head and a hydrophilic polyethy-
lene glycol (PEG) tail, the surfactants 1a–c could easily form
micelles in water. The critical micelle concentrations (CMCs)
were measured to be approximately 2.64, 1.48, and 0.55 mm,
respectively (Figure S1 in the Supporting Information). By
virtue of the alkynyl groups attached on the surfactant, the mi-
celles could be readily cross-linked by azide–alkyne cycloaddi-
tion in the presence of cross-linker (1,4-diazidobutane-2,3-diol,
2), copper chloride, and sodium ascorbate (Scheme 1, see the
Supporting Information for details). The successful capture was
first proved at the molecular level by the broadened proton
signals and disappearance of the alkenyl group signals in the
1
a–c through the efficient azide–alkyne cycloaddition
[
20]
(Scheme 1). The introduction of the triazole group aims to
stabilize palladium nanoparticles, which can be used as
a highly efficient heterogenized homogeneous catalyst for
Suzuki–Miyaura reactions. Arylboronic acids and aryl halides
with either electron-donating or -withdrawing substituting
groups on the aromatic ring all performed with excellent reac-
tivity under the green conditions of room temperature, water
as the sole solvent, and as low as 0.5% catalyst loading. Even
the sterically hindered ortho-substituted and hetero-aryl sub-
1
H NMR spectra (Figure S2 in the Supporting Information). The
cross-linking was also confirmed by dynamic light scattering
(
DLS) measurements, which showed a small size increase from
prior to cross-linking approximately 122.0, 8.0, and 58.0 nm to
after cross-linking approximately 142.0, 13.0, and 80.0 nm for
1
a–c, respectively (Figure 1). It should be noted that the size
of the DCCM from amphiphile 1a is abnormally large com-
pared with those formed by 1b and 1c, which occurred possi-
bly because the hydrophilic tail of 1a (PEG-350) is not long
enough to protect the relatively large hydrophobic head, con-
sequently leading to the formation of aggregates rather than
separate micellar self-assemblies. To get more intuitive infor-
mation, transmission electron microscopy (TEM) was further
conducted to investigate the morphology of the cross-linked
objects. As expected, all three samples revealed the formation
of spherical micelle-like particles with mean diameters of ap-
proximately 105.0, 10.0, and 65.0 nm (Figure 2), which is con-
sistent with the trend observed from the DLS results. Note that
Scheme 1. Preparation of triazole-containing dendrimer-like core cross-
linked micelle stabilized palladium nanoparticles (Pd@triazole-DCCMs).
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Chem. Asian J. 2016, 00, 0 – 0
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