902
Chemistry Letters Vol.38, No.9 (2009)
Aquacatalytic Aerobic Oxidation of Benzylic Alcohols
with a Self-supported Bipyridyl–Palladium Complex
Takao Osako1 and Yasuhiro Uozumiꢀ1;2
1Institute for Molecular Science (IMS), 5-1 Higashiyama, Myodaiji, Okazaki 444-8787
2Advanced Science Institute, RIKEN, 2-1 Hirosawa, Wako 351-0198
(Received June 17, 2009; CL-090571; E-mail: uo@ims.ac.jp)
The aerobic oxidation of alcohols was promoted in water
under atmospheric molecular oxygen by a readily recyclable
self-supported bipyridyl–palladium polymeric complex, which
was prepared via construction of a metal–organic framework
(MOF) of a bipyridyl–palladium complex bearing carboxylic
groups and a copper(II) linker.
Figure 1. a) SEM and b) TEM images of Pd(TFA)2BPy–Cu.
Catalytic oxidation of alcohols to carbonyl compounds is
one of the most fundamental and important, yet undeveloped,
processes in organic chemistry.1 Recently, catalytic aerobic oxi-
dation of alcohols as well as organic reactions in water have at-
tracted much attention as safe and environmentally benign proc-
esses.2–5 Among the hitherto reported aerobic oxidation cata-
lysts, divalent palladium complexes of hydrophilic bipyridyl-
based ligands have been recognized as the most effective com-
plex catalysts for aquacatalytic aerobic oxidation.6 If the hetero-
geneous-switching of this class of palladium complexes could be
achieved, the resulting catalyst system would be safe, recyclable,
and environmentally benign, and would represent what may be
considered an ideal oxidation catalyst. We describe herein the
preparation of a novel self-supported bipyridyl–PdII complex
via the self-assembly of a palladium complex of a bipyridyl li-
gand bearing carboxylic groups and copper nitrate (Scheme 1),
and its catalytic application in the aerobic oxidation of benzylic
alcohols in water.
Self-supported catalysts have recently emerged as a new
type of immobilized catalyst7 and have been utilized for various
reactions such as hydrogenation, C–C coupling,8 and oxidation.9
One of the advantages of self-supported catalysts is their ease of
preparation. Thus, homogeneous catalysts are readily converted
into insoluble polymer composites via self-assembly without us-
ing any exogenous solid supports. However, to the best of our
knowledge, the aqueous aerobic oxidation of alcohols with
self-supported catalysts has not yet been achieved. The self-sup-
ported bipyridyl–PdII complex (Pd(TFA)2BPy–Cu) was pre-
pared by mixing Pd(TFA)2BPy containing two carboxylic
groups at the 5- and 50-positons of the bipyridyl ligand and cop-
per nitrate in the presence of Et3N in water at room temperature
to afford an insoluble polymeric composite in 71% yield
(Scheme 1).10 SEM and TEM analyses of Pd(TFA)2BPy–Cu re-
vealed that block-like nanoclusters (ca. 25–100 nm) were assem-
bled to form big cluster units (Figure 1). ICP analysis showed
Table 1. Screening of conditions for the aerobic oxidation of
1-phenylethanola
OH
O
catalyst (5 mol%)
Na2CO3 (1 equiv), O2 (1 atm)
solvent, 100 °C, 24 h
Entry
Catalyst
Solvent
Conv./%
Yield/%b
1
2c
3
4
5
6
7
8
9
Pd(TFA)2BPy–Cu
Pd(TFA)2BPy–Cu
Pd(TFA)2BPy–Cu
Pd(TFA)2BPy–Cu
Pd(TFA)2BPy–Cu
Pd(OAc)2BPy–Cu
PdCl2BPy–Cu
H2O
H2O
Toluene
DMF
DMSO
H2O
H2O
H2O
H2O
98
100
7
54
67
85
47
64
92
82
79d
0
29
53
57
21
34
70
Pd(TFA)2BPy
Pd(TFA)2BPy–Zn
aConditions: 1-phenylethanol (0.25 mmol), catalyst (5 mol %),
Na2CO3 (0.25 mmol), solvent (2 mL), 100 ꢁC, under 1 atm of O2.
c
bGC yields. 1-Phenylethanol (1 g, 8.2 mmol), catalyst (5 mol %),
Na2CO3 (8.2 mmol), and H2O (65 mL) were used. dIsolated yield.
that the ratio of Pd:Cu was 1:0.91 in the self-supported complex.
Combustion elemental analysis revealed that the self-supported
composite is comprised of Pd(OCOCF3)2[OC(O)–C5H3N–
11
. .
C5H3N–COO]Cu Et3N 0.5HNO3.
With the self-supported bipyridyl–PdII polymeric complex
in hand, we began to explore the optimal conditions for the aero-
bic oxidation of 1-phenylethanol in water (Table 1). Pd(TFA)2-
BPy–Cu catalyzed the aerobic oxidation of 1-phenylethanol in
the presence of Na2CO3 in H2O at 100 ꢁC for 24 h to afford ace-
tophenone in 82% yield (Entry 1).12 A gram scale oxidation of
1-phenylethanol also gave acetophenone in 79% isolated yield
(Entry 2). The catalyst was much less effective in organic sol-
vents (Entries 3–5).13 When the counter ion was replaced with
acetate and chloride,10 the catalytic activity decreased signifi-
cantly.14 It was expected that a cationic PdII–BPy complex
would be generated as the active species by dissociation of the
relatively stable trifluoroacetate anion from palladium in a polar
solvent. A control experiment with the parent palladium com-
plex Pd(TFA)2BPy gave acetophenone in low yield under other-
wise similar conditions (Entry 8).6 A similar self-supported
O
O
O
O
O
O
Cu
HO
N
O
N
O
OH
N
O
N
O
•
) 3H2O
2
Cu(NO3
n
O
O
O
O
Pd
Pd
Et3N
H2O, rt
F3
C
F3C
CF3
CF3
Pd(TFA)2BPy
Pd(TFA)2L−Cu
Scheme 1. Preparation of a self-supported bipyridyl complex.
Copyright Ó 2009 The Chemical Society of Japan