A tris(triazolate) ligand for a highly active and magnetically recoverable palladium catalyst of selective alcohol oxidation using air at atmospheric pressure

Article abstract of DOI:10.1002/chem.201500122

High efficiency and selectivity, easy magnetic recovery and recycling, and use of air as the oxidant at atmospheric pressure are major objectives for oxidation catalysis in terms of sustainable and green processes. A tris(triazolyl) ligand, so far only used in copper-catalyzed alkyne azide cycloadditions, was found to be extremely efficient in SiO₂/γ-Fe₂O₃-immobilized palladium complexes. It was characterized by inductively coupled plasma (ICP) analysis, transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectra (XPS) and found to fulfill the combined conditions for the selective oxidation of alcohols to aldehydes and ketones.

Full text of DOI:10.1002/chem.201500122

DOI: 10.1002/chem.201500122
Full Paper
&
Oxidation
A Tris(triazolate) Ligand for a Highly Active and Magnetically
Recoverable Palladium Catalyst of Selective Alcohol Oxidation
Using Air at Atmospheric Pressure
Dong Wang,[a] Christophe Deraedt,[a] Lionel Salmon,[b] Christine Labrugꢀre,[c]
Laetitia Etienne,[d] Jaime Ruiz,[a] and Didier Astruc*[a]
Abstract: High efficiency and selectivity, easy magnetic re-
covery and recycling, and use of air as the oxidant at atmos-
pheric pressure are major objectives for oxidation catalysis
in terms of sustainable and green processes. A tris(triazolyl)
ligand, so far only used in copper-catalyzed alkyne azide cy-
cloadditions, was found to be extremely efficient in SiO2/g-
Fe2O3-immobilized palladium complexes. It was character-
ized by inductively coupled plasma (ICP) analysis, transmis-
sion electron microscopy (TEM), scanning transmission elec-
tron microscopy (STEM), energy-dispersive X-ray spectrosco-
py (EDX), and X-ray photoelectron spectra (XPS) and found
to fulfill the combined conditions for the selective oxidation
of alcohols to aldehydes and ketones.
Introduction
dition to safety considerations, it is of particular importance to
develop an alternative approach to construct a cleaner and
more efficient catalytic system for the selective oxidation of al-
cohols. In this context, much attention has been recently paid
to the development of transition-metal-catalyzed aerobic alco-
hol oxidations that involve Ru, Pd, Au, Co, Cu, Pt, Rh, V, Os, Ce,
and Ni[4] by using “green” molecular oxygen (even air) as oxi-
dant.[2,5] Enhanced catalytic performances in both activity and
selectivity, high atom economy, and cheaper and less polluting
oxidants are involved in the latter processes.
As an essential development of organic synthesis, oxidation re-
actions play a pivotal role in the current chemical industry, as
well as in the establishment of green and sustainable chemical
processes.[1] In particular, the selective oxidation of primary or
secondary alcohols to the corresponding aldehydes or ketones,
which are ubiquitous precursors and intermediates in the syn-
thesis of agrochemicals, pharmaceuticals, and fine chemicals,
has been widely recognized as one of the most fundamental
transformations in organic chemistry.[2] Traditionally, stoichio-
metric oxidizing reagents including hypervalent iodine, per-
manganate, dichromate, peroxides, and other heavy-metal re-
agents have been employed for the oxidation of alcohols.[3]
These traditional procedures are quite useful in laboratory-
scale reactions. These stoichiometric oxidants are somewhat
expensive and/or toxic and hazardous, however, and release
a considerable amount of heavy-metal wastes and byproducts.
From the standpoint of green and sustainable chemistry, in ad-
Despite this progress, there are still persistent problems in
this field. In some cases, the high temperature and high
oxygen pressure are essential to obtain good catalytic efficien-
cy. The use of 1 atm of air is still limited in most of these aero-
bic oxidation systems. Also, numerous transition-metal cata-
lysts do not show good tolerance to alcohol substrates. Most
importantly, the economic concerns and metal contamination
of carbonyl compounds are extremely restrictive as they relate
to the application of homogeneous noble-metal catalysts in
aerobic alcohol oxidations. To overcome these problems, the
use of heterogeneous catalysts appears to be promising. Vari-
ous supports including polymers, Al2O3, mesoporous silica,
[a] Dr. D. Wang, C. Deraedt, Dr. J. Ruiz, Prof. D. Astruc
ISM, Universitꢀ de Bordeaux, 351 Cours de la Libꢀration
33405 Talence Cedex (France)
ionic liquids, resin, TiO2, CeO2, hydrotalcite, celite, kaolin, MgO,
E-mail: d.astruc@ism.u-bordeaux1.fr
[2,6]
activated carbon, carbon nanotubes, and ZrO2
have been
[b] Dr. L. Salmon
widely employed to immobilize homogeneous metal catalysts
or metal nanoparticle catalysts, thus forming recyclable catalyt-
ic species in the aerobic selective oxidation of alcohols. How-
ever, traditional heterogeneous catalysts usually suffer from
low catalytic activity relative to their homogeneous counter-
parts.
LCC, CNRS, 205 Route de Narbonne
31077 Toulouse Cedex (France)
[c] C. Labrugꢁre
PLACAMAT UMS CNRS 3626, Universitꢀ de Bordeaux
87 Avenue Albert Schweitzer, 33608 Pessac Cedex (France)
[d] L. Etienne
ICMCB, UPR CNRS No. 9048, Universitꢀ de Bordeaux
87 Avenue Albert Schweitzer, 33608 Pessac Cedex (France)
Research on magnetic nanoparticles (MNPs) has become
one of the hottest fields and has experienced extremely fast
growth in the past few years.[7] MNPs are considered to be the
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500122.
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most attractive and ideal support considering their specific
properties. MNPs are seemingly biocompatible and easily as-
sembled with low preparation cost. MNP-supported catalysts
generally exhibit excellent catalytic activity, selectivity, and sta-
bility owing to their nanoscale size, that is, large surface-to-
volume ratio, unique interaction between the catalytic species
and MNPs, and tunable characters such as size, shape, compo-
sition, electronic structure, and solubility. In addition, MNP cat-
alysts are efficiently and easily recoverable with an external
magnetic field for reuse, and they remain catalytically efficient
after many repeated reactions. Therefore MNP catalysts fully
embody the principles of green chemistry and sustainability.
chloroformate, followed by removal of the trimethylsilyl
groups. Then, CuAAC reactions of 1 with three equivalents of
benzyl azide were conducted in the presence of copper sulfate
and sodium ascorbate. It provided tris(1-benzyl-1H-1,2,3-
triazol-4-yl)methanol (2), which underwent further propargyla-
tion with NaH and propargyl bromide to give alkynyl-function-
alized tris(1-benzyl-1H-1,2,3-triazol-4-yl)methanol ligand
(Scheme 1).[11b]
3
The pre-synthesized azido-functionalized magnetic nanopar-
ticles (4) were obtained through the immobilization of 3-azido-
propyltriethoxysilane on the surface of robust SiO2/g-Fe2O3 by
means of heterogenization with the SiÀOH binding sites of
In
a related context, from
both environmental and eco-
nomic points of view, a few re-
ports on the preparation and
catalytic application of MNP cat-
alysts in aerobic oxidation of al-
cohols have emerged.[8a,d–f] How-
ever, these MNPs that involve
catalytic systems more or less
showed some imperfections,
such as narrow substrate scope,
poor selectivity towards alde-
hydes or ketones, and/or the re-
quirement of molecular oxygen
with high pressure. The use of
safe, clean, and available free air
as oxidant is still a major signifi-
Scheme 1. Synthesis of the alkynyl-functionalized ligand tris(1-benzyl-1H-1,2,3-triazol-4-yl)methanol (3).
cant challenge in alcohol oxidation reactions catalyzed by
MNPs. Therefore, it is highly desirable to develop an efficient
catalytic system that contains an MNP support for the aerobic
oxidation of various alcohols at atmospheric pressure of air.
Along with our continuous efforts to develop economical
and eco-friendly catalytic protocols for organic transformations
from the point of view of green and sustainable chemistry,[9]
herein we report the design and preparation of an MNP-immo-
bilized tris(triazolyl)–palladium(II) catalyst and its catalytic appli-
cation and recyclability in the selective oxidation of alcohols to
aldehydes and ketones by air at atmospheric pressure.
SiO2/g-Fe2O3.[9b,c,12] The CuAAC reaction was then successfully
carried out between 4 and an excess amount of 3 using CuI/
N,N-diisopropylethylamine (DIPEA) as catalyst in mixed DMF/
THF at room temperature (RT) to produce MNP-anchored tris-
(triazolyl) ligand 5 (Scheme 2). The reaction was monitored by
FTIR spectroscopy as indicated by the almost complete disap-
pearance of the IR signal of the azide group at 2102 cmÀ1 (Fig-
ures S1–S3 in the Supporting Information). The determination
of the nitrogen content (C, H, N elemental analysis) showed
that the tris(triazolyl) ligand loading was approximately
0.51 mmolgÀ1.
Results and Discussion
Preparation and characterization of MNP-immobilized Pd
catalysts
Since Fokin et al. pioneered the synthesis of tris(triazolyl) frag-
ments,[10] these compounds have been shown to be excellent
ligands with copper salts in CuI-catalyzed alkyne–azide cyclo-
addition (CuAAC).[10,11] To our surprise, however, tris(triazolyl) li-
gands have not been used in any other reaction. It was thus of
interest to functionalize, immobilize, and metalize a tris(triazol-
yl) ligand, and evaluate the catalytic property of its palladium
complexes in aerobic alcohol oxidation.
Complexation of 5 with Pd(OAc)2 (1.2 equiv) was conducted
overnight in toluene at 458C, and resulted in the assembly of
MNP-immobilized tris(triazolyl)ÀPd(OAc)2, Cat. 1. For compari-
son, the highly water-dispersible Cat. 1 was reduced by the ad-
dition of NaBH4 (10 equiv per Pd atom) in water to form MNP-
supported PdNPs,[13] Cat. 2 (Scheme 3). Both Pd catalysts were
characterized by transmission electron microscopy (TEM), in-
ductively coupled plasma (ICP) analysis, and X-ray photoelec-
tron spectroscopy (XPS).
Synthesis of a SiO2@g-Fe2O3-immobilized tris(triazolyl)
ligand
ICP analysis revealed that the Pd loadings of Cat. 1 and
Cat. 2 are 0.24 and 0.21 mmolgÀ1, respectively, which means
that the Pd species was partially released from the initial PdII
complex during the preparation of the PdNPs. The TEM images
In the primary steps, tris(alkynyl)carbinol intermediate 1 was
synthesized by the addition of trimethylsilylacetylide to ethyl
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Scheme 2. Synthesis of tris(triazolyl) fragment-modified SiO2@gÀFe2O3 nanoparticles.
indicated that the average diameter of the NPs of Cat. 1 was
approximately 30 nm and ranged from 15 to 40 nm, which is
similar to the sizes of both unloaded MNP SiO2/g-Fe2O3 and
Cat. 2 (Figure 1a–c). Moreover, the mean size of formed PdNPs
loaded onto the surface of MNP in Cat. 2 was around 4 nm
(Figure 1c).
by Cat. 2. For further characterization of the sample, scanning
transmission electron microscopy (STEM)-coupled quantified
energy-dispersive X-ray spectroscopy (EDX) mapping of Cat. 2
was also determined (Figure 3). On the compositional maps of
Si, Fe, Pd, and the combined composition images, the pres-
ence of the iron oxide nanoparticles is clearly distinguished in
the core of the MNP that is encapsulated by the silicon oxide
shell, whereas the palladium nanoparticle is localized at the
border of the MNP.
The elemental composition was measured by energy-disper-
sive X-ray spectroscopy (EDX) analysis and the results, shown
in Figure 2, indicate Si, O, Fe, and Pd signals that are produced
XPS analysis was performed on both Cat. 1 and Cat. 2 to in-
vestigate their composition and the oxidation states of the Pd
atoms. As shown in Figure 4c, the signals of Si, C, N, O, Fe, and
Pd were clearly observed in the XPS spectrum of Cat. 1, thus
proving the presence of the magnetic iron core coated with
a silica shell that is functionalized with C/N/Pd species. Two
binding energies (BEs) were observed for Pd 3d at 335.7 eV
(29.6%) and at 337.7 eV (70.4%), which indicate the presence
of both Pd0 and PdII, respectively (Figure 4a, Table S1 in the
Supporting Information). With the BE of the Pd(OAc)2 alone
being 338.4 eV, the BE of 337.7 eV belongs to PdII linked to the
triazole rings. It seems reasonable that some neutral Pd species
are released in situ from the initially formed tris(triazolyl)–palla-
dium(II) complexes owing to the treatment in toluene over-
night at 458C. The XPS spectrum of Cat. 2 showed the pre-
dominance of Pd0 (56%) over PdII (44%) (Figure 4b, Table S1 in
the Supporting Information). The relatively high amount of PdII
could be attributed either to the insufficient amount of reduc-
ing agent used for the reduction of all the PdII to Pd0, or to the
re-oxidation of the catalyst by air during storage and analysis.
Figure 1. a) TEM image of SiO2/gÀFe2O3 nanoparticles. b) TEM image of
Cat. 1. c) TEM image of Cat. 2. d) TEM image of Cat. 1 after four reaction
cycles in the aerobic oxidation of benzyl alcohol.
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Scheme 3. Preparation of MNP-immobilized Pd catalysts Cat. 1 and Cat. 2.
Investigation of the activity, selectivity, recyclability, and
substrate scope of MNP-immobilized Pd catalysts
In addition, the selectivity of 6 decreased to 82% with increas-
ing reaction time in the range of 3–5.5 h (Table 1, entries 1–3).
In these processes, the corresponding acid 7 and ester 8 were
also observed as side products. Under the same conditions,
the replacement of Cat. 1 by Cat. 2 caused clear decreases in
both conversion and selectivity towards 6 (Table 1, entry 4) to
76 and 85%, respectively. The oxidation of benzyl alcohol also
proceeded when Pd(OAc)2 was used as catalyst, but the con-
version was much lower than that of Cat. 1 (Table 1, entry 5).
Thus, the tris(triazolyl) group apparently played the role of an
activating ligand for the catalytic activity of the Pd atoms.
Moreover, the results of contrast tests showed that unloaded
g-Fe2O3@SiO2 nanoparticles were not active at all in this trans-
The Pd-catalyzed oxidation of alcohols using oxygen as the ox-
idant has been recognized as a potentially powerful transfor-
mation for organic synthesis.[6a,14] With the MNP-supported pal-
ladium catalysts in hand, the catalytic properties were investi-
gated for alcohol oxidation using benzyl alcohols as probe
substrate (Table 1). In these preliminary experiments, the reac-
tion was conducted in the presence of 40 mg of Cat. 1
(1.9 mol% of [Pd]) and K2CO3 in toluene at 85–908C using
1 atm of air as oxidant. It was found that the reaction was
completed within 4.5 h, with 93% selectivity and 84% isolated
yield towards desired aldehyde compound 6 (Table 1, entry 2).
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tained with excellent conver-
sions and selectivity, in the
range of 81–97 and 86–91%, re-
spectively. The results show that
no direct correlation can be
drawn between the electronic
nature of the alcohols and the
outcome of the reactions; in ad-
dition, primary alcohols exhibit-
ed higher reactivity than secon-
dary alcohols.
Mechanistic explanations for
Pd-catalytic aerobic oxidation of
alcohols have recently been
sought in several studies.[3e,15]
Briefly, three steps were includ-
ed in the catalytic cycle, which
are the coordination of the alco-
hol with Pd atom to form a Pd
alkoxide, b-hydride elimination
of the Pd alkoxide to produce
a carbonyl compound and a Pd
Figure 2. EDX spectrum of Cat. 2.
hydride, and oxidation of the Pd
hydride by molecular oxygen to
give the initial Pd catalyst for
formation. The mixture of g-Fe2O3@SiO2 and Pd(OAc)2 provided
moderate conversion but poor recyclability (Table 1, entry 7).
As a key issue for the evaluation of the heterogeneous cata-
lysts, the recyclability of MNP-anchored Pd catalysts was inves-
tigated (Table 2). After the completion of the first reaction
cycle, both Cat. 1 and Cat. 2 were readily removed from the
reaction medium by using an external magnet, and new reac-
tions were then performed with fresh reactants under the
same conditions. Remarkably, Cat. 1 was recycled for at least
five reaction runs without significant decrease in both activity
and selectivity. On the contrary, a clear decrease in the reaction
conversion was observed after two cycles in the case of Cat. 2.
ICP analysis indicated that the amount of Pd leaching after
four cycles from the initial Cat. 1 was negligible. The TEM
image revealed that the morphology and size of Cat. 1
changed over time, and a particle aggregation problem
emerged after four reaction cycles (Figure 1d).
the next cycle. In particular, in this transformation, Pd catalysts
are known to aggregate easily and form inactive Pd black. To
suppress the formation of Pd black, the use of both an excess
amount of ligand and sufficient oxygen partial pressure rather
than air are generally necessary. With Cat. 1, the tris(triazolyl)
moieties act as a strong ligand to stabilize PdII and Pd0 inter-
mediates, thus forming sufficiently stable Pd complexes, which
results in the inhibition of Pd atom aggregation. Chelated cage
structures that involve the formation of six-membered
rings[11a,16] are expected to lead to remarkable catalytic per-
formance in air at atmospheric pressure, as well as good recy-
clability.
Conclusion
Considering the increasing environmental and economic con-
cerns in large-scale preparation, the oxidation of alcohols
using dioxygen from air at atmospheric pressure over support-
ed transition-metal catalysts is particularly attractive.
The scope of alcohols was examined for oxidation to alde-
hydes and ketones using 1.9 mol% of the supported palladium
catalyst Cat. 1 under aerobic conditions. Representative results
are summarized in Scheme 4. The substituted primary alcohols
were oxidized smoothly under standard reaction conditions
and afforded the corresponding aldehydes 9a and 9b in excel-
lent conversions and selectivities. This protocol was extended
to primary alcohols that contained other electron-withdrawing
or electron-donating groups. When 2,3-dimethoxybenzyl alco-
hol and 3-nitrobenzyl alcohol were employed, the correspond-
ing aldehyde products 9b and 9c were obtained with lower
conversions and excellent selectivity. The catalyst was also ef-
fective for the oxidation of secondary benzylic alcohols with
electron-donating or electron-withdrawing groups under pro-
longed reaction times. A series of ketones 9e–9j were ob-
In this report, the catalytic oxidation of alcohols was ach-
ieved in air by using a novel MNP-immobilized tris(triazolyl)–
PdII catalyst that was designed and assembled with the aim of
using it as a magnetically recyclable and efficient catalyst. This
catalyst provided impressive and superior performances with
regard to activity, selectivity, and recyclability relative to its
PdNP counterpart. A series of primary and secondary benzyl al-
cohols were smoothly oxidized to afford the corresponding al-
dehydes and ketones in the presence of the magnetic tris(tria-
zolyl)–PdII catalyst (1.9 mol%) at 85–908C in toluene with 81–
97% conversions and 86–92% selectivity. In addition, the cata-
lyst was easily recoverable and reusable for at least five cycles
with only a slight decrease in catalytic performance.
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Figure 3. a) STEM dark-field image; elemental maps of Cat. 2 for b) Fe, c) Pd, d) Si, and e) mixtures of Fe and Pd, f) Fe, Pd, and Si. h) STEM dark-field image-ele-
mental maps of Pd and g) a mixture of Fe and Pd obtained by EDX. All scale bars=60 nm.
All commercially available reagents were used as received, unless
indicated otherwise. Flash column chromatography was performed
using silica gel (300–400 mesh). H NMR spectra were recorded by
Positive effects of this catalytic system in terms of cost as
well as associated safety and environmental benefits makes it
an ideal protocol for the selective oxidation of alcohols. This
work should significantly contribute to the development of
clean technology and establishing green and sustainable
chemical processes.
1
using a 300 MHz spectrometer, and 13C NMR spectra were recorded
at 75 MHz by using a 300 MHz spectrometer. Elemental analyses
were performed by the Center of Microanalyses of the CNRS at
Lyon Villeurbanne, France. The infrared spectra were recorded
using an ATI Mattson Genesis series FTIR spectrophotometer. The
inductively coupled plasma optical emission spectroscopy (ICP-
OES) analyses were carried out using a Varian ICP-OES 720ES appa-
ratus. Room temperature throughout the paper is 23–258C.
Experimental Section
General
Synthesis of tris(trimethylsilylethynyl)methanol[11a]
All reactions were performed under nitrogen by using standard
Schlenk techniques unless otherwise noted. Toluene was dried
over Na foil and freshly distilled under nitrogen immediately prior
to use. THF was dried over Na foil and freshly distilled from
sodium benzophenone under nitrogen immediately prior to use.
A solution of trimethylsilylacetylene (4.6 mL, 33.2 mmol) in anhy-
drous THF (40 mL) was added into a flame-dried round-bottomed
flask, and the mixture was cooled to À788C. Then, 2.5m nBuLi in
hexane (12.2 mL, 30.4 mmol) was added dropwise, and the solu-
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Table 2. Investigation of the recyclabilities of MNP–Pd catalysts in the
aerobic oxidation of benzyl alcohol.[a]
Cat. 1
Conversion[b] Selectivity
towards 6[b]
Cat. 2
Conversion[b] Selectivity
towards 6[b]
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
100
97
92
92
86
93
93
93
90
91
76
70
50
–
85
85
72
–
–
–
[a] The reaction was carried out with benzyl alcohol (0.5 mmol) in the
presence of freshly prepared or recycled Pd catalyst (Cat. 1 or Cat. 2) and
K2CO3 (1 mmol) in toluene (5 mL) at 85–908C in air at atmospheric pres-
sure. [b] Both conversion and selectivity towards 6 were measured by
1H NMR spectroscopic analysis.
tion was stirred for 2 h. Ethyl chloroformate (884 mL, 9.12 mmol)
was subsequently added, and the reaction was stirred overnight
(15 h) while warming to À308C. The reaction was quenched with
saturated NH4Cl solution, diluted with water, and extracted with
Et2O (3ꢂ60 mL). The combined organic phase was dried over
Na2SO4, and the solvent was removed under vacuum. Purification
by flash chromatography on a short pack of silica gel eluting with
hexanes and hexanes/ethyl acetate (90:10) afforded tris(trimethylsi-
lylethynyl)methanol in 67% yield (1.9 g). The product was recrystal-
lized from hexane at 08C. 1H NMR (300 MHZ, CDCl3): d=2.84 (s,
1H), 0.24 ppm (s, 27H).
Synthesis of benzyl azide
In a Schlenk tube, benzyl bromide (10 mmol) and sodium azide
(12 mmol) were added to DMSO (40 mL), and the mixture was
stirred at RT for 24 h. Then H2O (40 mL) was added to quench the
azidation reaction. The aqueous solution was extracted with dieth-
yl ether (3ꢂ20 mL), the combined organic phase was dried over
Na2SO4 and filtered, and the filtrate was removed under vacuum to
obtain the crude product that was further purified by silica-gel
chromatography (using pentane/ethyl acetate as eluent) to yield
benzyl azide.
Figure 4. a) XPS Pd 3d spectrum of Cat. 1. b) XPS Pd 3d spectrum of Cat. 2.
c) XPS survey of Cat. 1.
Synthesis of tris(1-benzyl-1H-1,2,3-triazol-4-yl)me-
thanol (2)[11a]
Table 1. Optimization of the oxidation of benzyl alcohol using Pd catalysts in air.[a]
In a round-bottomed flask, tris(trimethylsilylethynyl)me-
thanol (1.5 g) in methanol (15 mL) was stirred in the
presence of K2CO3 (8.0 g) at RT for 4 h. The solution was
filtered to remove an excess amount of K2CO3 and
Entry
Cat. [mol%]
t [h]
Conversion [%][b]
Selectivity [%][b]
added to a solution of benzyl azide (1.9 g) in methanol
(15 mL). CuSO4·5H2O (60 mg) and sodium ascorbate
(150 mg) were added, and the mixture was stirred for
72 h. Solvent was removed under reduced pressure. The
residue was dissolved in dichloromethane (CH2Cl2;
75 mL) and washed with a saturated Na2CO3 solution
(5ꢂ45 mL). At the end of the extraction, the aqueous
phase was not blue (the color of copper salts) any
longer. The organic phase was dried over Na2SO4, and
the solvent was removed under vacuum. A yield of 1.2 g
(52%) of the desired crude product 2 was obtained after
a very short silica-gel chromatography to remove the
excess amount of benzyl azide. The crude product was
6
7
8
1
2
3
4
5
6
7
Cat. 1 (1.9)
Cat. 1 (1.9)
Cat. 1 (1.9)
Cat. 2 (1.9)
Pd(OAc)2 (5)
g-Fe2O3@SiO2
3
90
100
100
76
55
>96
93
82
85
87
0
1
3
4
6
3
0
5
2
4.5
5.5
4.5
4.5
4.5
4.5
4
14
9
10
0
[c]
NR
49, 12[d]
g-Fe2O3@SiO2[c]/Pd(OAc)2 (5)
89
6
[a] The reaction was carried out with benzyl alcohol (0.5 mmol) in the presence of the
listed catalysts and K2CO3 (1 mmol) in toluene (5 mL) at 85–908C in air at atmospheric
pressure. [b] Both conversion and selectivity towards 6 were measured by 1H NMR
spectroscopic analysis. [c] g-Fe2O3@SiO2 (40 mg). [d] Conversion of the second recycle.
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ene (2ꢂ10 mL), CH2Cl2 (10 mL), a diluted aqueous solu-
tion of ammonia and a saturated Na2CO3 solution (until
there was no blue color in the solution), H2O (2ꢂ
20 mL), and acetone (2ꢂ10 mL) to remove any unanch-
ored species and then dried under vacuum.
Synthesis of the MNP-immobilized tris(triazolyl)
ligand by CuAAC reaction
The MNP-immobilized azide (600 mg) and tris(triazolyl)
compound 3 (300 mg) were mixed with CuI (8 mg) in
DMF/THF (1:1, 20 mL) under nitrogen. N,N-Diisopropyle-
thylamine (2 mL) was injected into the mixture, which
was then sonicated for approximately 30 min and
stirred at RT for 48 h. The reaction was monitored by
FTIR as indicated by the almost complete disappearance
of the IR signal of the azido group at 2102 cmÀ1. Then
the mixture was submitted to magnetic separation, and
the MNPs were washed sequentially with DMF (10 mL),
THF (10 mL), CH2Cl2 (10 mL), diluted aqueous ammonia
solution and saturated Na2CO3 solution (until there was
no blue color in the solution), H2O (2ꢂ10 mL), and ace-
tone (10 mL), and finally dried under vacuum.
Scheme 4. Investigation of the substrate scope in the presence of Cat. 1 in the oxidation
of alcohols.
Synthesis of MNP-immobilized tris(triazolyl)P-
d(OAc)2 complex (Cat. 1)
not further purified, and it was used directly for the next step.
1H NMR (300 MHZ, CDCl3): d=7.63 (s, 3H), 7.36–7.38 (m, 9H), 7.27–
7.30 (m, 6H), 5.49 (s, 6H), 4.71 ppm (s, 1H).
Under a nitrogen atmosphere, toluene (30 mL) was mixed with
MNP-immobilized tris(triazolyl) ligand (500 mg; the ligand loading,
0.51 mmolgÀ1
, was determined by elemental analysis) and
Pd(OAc)2 (1.2 equiv, 0.61 mmol, 16.2 mg). The mixture was then so-
nicated for approximately 30 min and stirred at 458C overnight.
After cooling to RT, the solution was colorless. Then the mixture
was submitted to magnetic separation, and the MNPs were
washed sequentially with DMF (10 mL), THF (10 mL), CH2Cl2
(10 mL), H2O (2ꢂ10 mL), and acetone (10 mL), and finally dried
under vacuum. The obtained catalyst was kept in N2 for further ap-
plications.
Synthesis of 3-[tris(1-benzyl-1H-1,2,3-triazol-4-yl)methoxy]-
propyne (3)[11b]
A solution of 2 (503 mg, 1.0 mmol) in DMF (2 mL) was added drop-
wise at 08C to a flame-dried flask that contained a suspension of
NaH (80 mg, 60% in oil, 2.0 mmol) in DMF (2 mL). After stirring for
2 h at RT, the suspension became a clean solution that was cooled
again to 08C. Then, a commercial solution of propargyl bromide in
toluene (0.220 mL, 80% solution in toluene, 2.0 mmol) was added
dropwise. The reaction mixture was allowed to warm to RT and
stirred for an additional 14 h. Water (10 mL) was added to the reac-
tion mixture, which was then extracted with CH2Cl2 (3ꢂ10 mL).
The combined organic phase was dried over Na2SO4 and concen-
trated under vacuum. Traces of DMF were removed by dissolving
the crude product in an ethyl acetate/hexane (4:1) mixture (40 mL)
and washing the solution with water (3ꢂ20 mL). The organic
phase was dried over Na2SO4 and concentrated under vacuum to
afford crude 3, which was further purified by flash column chroma-
tography using ethyl acetate as the eluent. The product was ob-
tained as a thick orange oil or pumiceous solid (0.46 g, 86%).
1H NMR (300 MHZ, CDCl3): d=7.88 (s, 1H), 7.29–7.41 (m, 15H), 5.53
(s, 6H), 4.15 (d, J=2.4 Hz, 2H), 2.08 ppm (t, J=2.4 Hz, 1H).
Synthesis of the MNP-immobilized Pd nanoparticles (Cat. 2)
Under a nitrogen atmosphere, in a Schlenk flask, the suspension of
MNP–Pd complex Cat. 1 (200 mg) in Milli-Q H2O (20 mL) was soni-
cated for approximately 20 min under nitrogen. An aqueous solu-
tion (10 mL) that contained 1.26 mmol of NaBH4 was then injected.
The mixture was stirred at RT for 2 h, and the color of the mixture
changed from brown to black, which indicated the reduction of
Pd2+ to Pd0 and PdNP formation. The mixture was submitted to
magnetic separation, the MNPs were washed with Milli-Q H2O (2ꢂ
10 mL) and acetone (10 mL) under nitrogen, and the catalyst was
dried at 458C for at least 4 h under vacuum and stored under ni-
trogen before use.
General procedures for the Cat. 1-catalyzed oxidation of
benzyl alcohol
Synthesis of MNP-immobilized azide
The Si(OMe)3-functionalized azido compound was freshly pre-syn-
thesized through a classic azidation method by using redistilled
DMF as solvent at 458C. Under an atmosphere of nitrogen, the ob-
tained Si(OMe)3-functionalized azido compound (350 mg) was
added to a suspension of MNPs SiO2/g-Fe2O3 (0.3 g) in anhydrous
toluene (30 mL). The mixture was then stirred at 1108C under a ni-
trogen atmosphere for 24 h. The dark brown solid material ob-
tained was magnetically separated, washed repeatedly with tolu-
A dried Schlenk tube equipped with a magnetic stirring bar was
charged with the MNP-immobilized tris(triazolyl) Pd(OAc)2 complex
(Cat. 1, 40 mg), benzyl alcohol (0.5 mmol), K2CO3 (1 mmol), and tol-
uene (10 mL). The mixture was sonicated for approximately 20 min
and stirred at 85–908C for 4.5 h in air. The catalyst was collected
by using a magnet and washed successively with toluene (2 mL),
CH2Cl2 (10 mL), and acetone (5 mL) with the protection of N2, then
dried at 458C under vacuum. The combined organic phase was
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Acknowledgements
Financial support from the China Scholarship Council (CSC) of
the People’s Republic of China (PhD grant to D.W.), the Minis-
tꢀre de la Recherche et de la Technologie (MRT, PhD grant to
C.D.), the University of Bordeaux and University of Toulouse 3,
the Centre National de la Recherche Scientifique (CNRS), and
L’Orꢃal are gratefully acknowledged.
Keywords: alcohols
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Received: January 12, 2015
Published online on && &&, 0000
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FULL PAPER
Coming up for air: A tris(triazolyl)
ligand, so far only used in Cu-catalyzed
alkyne azide cycloadditions, was found
to be extremely efficient in SiO2/g-
Fe2O3-immobilized palladium complexes
(see scheme). It was characterized by
several methods and found to fulfill the
combined conditions for the selective
oxidation of alcohols under air.
&
Oxidation
D. Wang, C. Deraedt, L. Salmon,
C. Labrugꢁre, L. Etienne, J. Ruiz,
D. Astruc*
&& – &&
A Tris(triazolate) Ligand for a Highly
Active and Magnetically Recoverable
Palladium Catalyst of Selective
Alcohol Oxidation Using Air at
Atmospheric Pressure
Chem. Eur. J. 2015, 21, 1 – 11
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These are not the final page numbers! ÞÞ