Copper-catalyzed aerobic alcohol oxidation under air in neat water by using a water-soluble ligand

Article abstract of DOI:10.1039/c3ra43366h

A copper-catalyzed primary benzylic and allylic alcohols oxidation system has been developed under air in neat water by use of a water-soluble ligand (pytl-β-CD). The ligand was designed and synthesized via a click conjunction between functionalized β-cyclodextrin (β-CD) and 2-ethinylpyridine.

Full text of DOI:10.1039/c3ra43366h

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Copper-catalyzed aerobic alcohol oxidation under air in
neat water by using a water-soluble ligand3
Cite this: RSC Advances, 2013, 3, 19255
a
a
a
a
a
b
Guofu Zhang,* Xingwang Han, Yuxin Luan, Yong Wang, Xin Wen, Li Xu,*
Received 2nd July 2013,
a
a
Accepted 12th August 2013
Chengrong Ding and Jianrong Gao
DOI: 10.1039/c3ra43366h
www.rsc.org/advances
A copper-catalyzed primary benzylic and allylic alcohols oxidation
system has been developed under air in neat water by use of a
water-soluble ligand (pytl-b-CD). The ligand was designed and
ligand, which provided aldehydes from primary benzylic alcohols
in CH CN/H O. Another Cu/ligand/TEMPO catalytic system was
rendered by Knochel and co-workers, which contained a
fluoroalkyl-substituted bpy ligand with CuBr?Me S as catalyst in
a fluorous biphasic condition (C 17Br/PhCl), and expressed good
3
2
12
synthesized via
a
click conjunction between functionalized
2
b-cyclodextrin (b-CD) and 2-ethinylpyridine.
8
F
catalytic activity. However, these ligand-assisted oxidation meth-
ods were unavoidably performed in organic solvents or mixed
solutions partially due to the hydrophobicity of these ligands.
Water, a readily available and non-toxic solvent, has attracted
much attention due to its features as improving reactivity and
Copper is an inexpensive, abundant metal and its widespread
applications in organic synthesis have been impressively demon-
strated in recent publications. Of particular interest is the
1
development of copper-catalyzed oxidation systems with econom-
ically and environmentally benign oxidants such as oxygen, whose
13
selectivity, simplifying workup procedures and so on.
2
reduction product is water. In fact, copper has been discovered in
Consequently there has been a growing demand for the design
of efficient ligands that could promote copper-catalyzed reactions
in water. However, relevant systems are rare, for example only
Timo Repo reported copper(II) diimine-based and copper(II) 2-
N-arylpyrrolecarbaldimine-based catalysts for the aerobic oxidation
of benzylic alcohols mediated by the 2,2,6,6-tetrame-thylpiperi-
3
various metalloproteins, that mediate the transfer of oxygen-atom
to organic substrates, which indicated its enticing potential for
4
oxygen activation. Since the report by Semmelhack and co-
5
workers in 1984, copper-catalyzed aerobic alcohol oxidations have
gained enormous attention and many efforts have been made to
6
14
develop efficient oxidation systems. Compared with noble metals
dine-1-oxyl (TEMPO) radical in alkaline water solution.
7
8
9
Therefore, the design of novel ligands that can be used in
aqueous alcohol oxidation, to facilitate the synthesis of aldehydes
and ketones environmentally, remains an important pursuit from
the viewpoint of sustainable chemistry.
such as Pd, Au, Ru, etc., copper catalysts not only show similar
excellent activity for the oxidation of a broad range of alcohols, but
can also overcome the deactivation problems caused by the
coordination between metal catalysts and heteroatoms in tested
7a
substrates. However, since copper has a lower oxidizing ability
Cyclodextrins (CDs) are cyclic oligomers of D(+)-glucopyranosyl
units linked by a-1,4-glycosidic bonds. Due to their hollow
truncated conical structures with a hydrophobic cavity and two
hydrophilic external faces, cyclodextrins (CDs) and their derivatives
have been widely applied in fields such as supramolecular,
bioinorganic, pharmaceutical, material chemistry, separation
2+
(
Cu/Cu Eox = +0.34 V), ligands are usually needed to promote the
efficient transformation of copper-catalyzed alcohol oxidation.
Traditionally, these ligands are confined to 1,10-phenanthroline
1
0
(
phen), 2,29-bipyridine (bpy) and their derivatives. For instance,
6a
Mark o´ and co-workers employed phen as a ligand in the CuCl-
catalyzed system, and the alcohols were oxidized into desired
carbonyl compounds in toluene. Later on, Sheldon and co-
15
sciences etc. In recent years, modified CDs have been introduced
into organometallic chemistry and showed perfect activities as
ligands in catalytic reactions. For example, b-cyclodextrin dimers
with a linked bipyridyl, 1,10-phenanthroline-2,9-dimethyl and 2,6-
dimethylpyridine groups were all reported as efficient ligands to
form Cu(II)/Zn(II) complexes, which could accelerate the hydrolysis
1
1
workers developed a CuBr/TEMPO/O
2
system with bpy as the
a
College of Chemical Engineering and Materials Science, Zhejiang University of
Technology, Hangzhou 310014, People’s Republic of China.
16
E-mail: gfzhang@zjut.edu.cn; Fax: (+86)-571-88320147; Tel: (+86)-571-88320147
rate of several esters.
b
Department of Chemistry, Zhejiang University, Hangzhou 310014, People’s Republic
In this context, we wish to demonstrate an efficient copper-
catalyzed aerobic alcohol oxidation system in neat water assisted
by a water-soluble ligand pytl-b-CD (Scheme 1). The novel ligand
containing a ‘‘2,29-bipyridine analogue’’ fragment (pytl-b-CD) was
of China. E-mail: xuli2003@263.net
3
Electronic supplementary information (ESI) available: Detailed experimental
procedures, the optimization of copper-catalyzed primary alcohol oxidation and
NMR data for products. See DOI: 10.1039/c3ra43366h
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a
Table 2 Aerobic alcohol oxidation of primary alcohols to aldehydes
T
Yield
Selec.
(%)
b
c
Entry
R
C
4-CH
4-CH
1-naphthyl
3,4-CH
3,4-CH
(h) (%)
1
2
3
4
5
6
7
8
9
6
H
5
10 94
10 95
10 93
12 95
12 95
10 93
12 91
16 92
12 93
10 93
14 93
14 95
12 93
16 93
12 95
12 80
12 81
10 58
24 13
24 NR
.99
.99
.99
.99
.99
.99
.99
.99
.99
.99
.99
.99
.99
.99
.99
.99
.99
63
Scheme 1 Synthesis of the water-soluble ligand pytl-b-CD. Reaction conditions:
3
C
OC
6
H
4
(
a) TsCl, NaOH, H
2
O, 0–5 uC, 4 h, 24.3%; (b) NaN
3
, DMF, 75 uC, 4 h, 93.6%; (c)
3
6
H
4
2
-ethynylpyridine, CuSO
4
/NaAsc, DMSO/H O, r.t., 1 d, 92.1%.
2
3
C
6
H
6
3
H
3
OC
OC
3
2-CH
2-ClC
3-ClC H4
3
6
H
4
17
designed and synthesized via a click reaction between 2-ethy-
nylpyridine and monoazido b-CD (II). Notably, the designed ligand
showed excellent water-solubility as expected.
6
H
4
6
1
1
1
0
1
2
4-ClC
2-BrC
3-NO
4-FC
6 4
H
6
H
3
Initially, p-tolylmethanol was selected as the model substrate to
optimize the oxidation conditions, and the results are presented in
Table 1. An 88% yield of the product was obtained when the
2
C
6
H
4
13
6
H
4
1
1
1
4
5
6
6
2,4-ClC H
3
3-pyridyl
2-thienyl
2-furyl
t
oxidation was carried out with CuCl/pytl-b-CD/TEMPO/ BuOK in
water under reflux (Table 1, entry 1). Notably, neither carboxylic
acid nor ester as byproduct was detected. Since the copper source
has always shown a significant influence on organic transforma-
tions, different copper sources were screened. From Table 1, it was
found that the oxidations proceeded smoothly as well by using
17
1
1
2
21
2
8
9
0
6 5
C H CHLCH
1-phenethyl alcohol
n-octyl alcohol
.99
—
—
d
e
benzyl alcohol + 1-phenethyl alcohol 24 A–B 92 : 0
4-CH 15 93
2
3
C
6
H
4
.99
a
other copper salts, such as CuCl
2
, CuBr, CuBr
2
, CuSO
4
and CuI, yet
2 2
Reaction condition: substrate (1.0 mmol), Cu(OAc) ?H O (5 mol%),
pytl-b-CD (5 mol%), TEMPO (5 mol%), Na
2
CO
3
(1.0 mmol), H
2
O (4.0
with similar or slightly lower conversions (87–96%, entries 2–6). To
our delight, a nearly quantitative conversion was achieved when
the reaction was catalyzed by Cu(OAc) ?H O (entry 7). So
b
c
d
mL), in air under reflux. Determined by GC-MS. Isolated yield.
A
e
represents benzaldehyde, B indicates acetophenone. The reaction
2
2
was carried out on a 20.0 mmol-scale.
considering its easier accessibility and lower cost, Cu(OAc) ?H O
2
2
was chosen as the ideal catalyst. Next, we turned our attention to
investigate the effect of base. Na CO led to further improvement
of the green transformation and furnished full consumption of
2
3
the starting material under similar conditions (entry 11). Other
bases such as K CO , NaHCO , K PO and NaOH were inferior to
Na CO (entries 8–10, 12). Finally, control experiments showed
2
3
3
3
4
2
3
a
that the starting material gave rare product in the absence of
Cu(OAc) ?H O or TEMPO, and a drastic decrease in yield was
Table 1 The optimization of copper-catalyzed alcohol oxidation
2
2
observed when pytl-b-CD was omitted (entries 13–15).
Having obtained the optimized reaction conditions: 1.0 mmol
alcohol, 5 mol% TEMPO, 5 mol% ligand, 5 mol% Cu(OAc)
2 2
?H O,
b
c
Entry
Cat.
Base
Conv.(%)
Yield(%)
1.0 mmol Na CO , 4.0 mL water, reflux under air, we turned our
2
3
attention to explore the substrate scope. The results are
summarized in Table 2. To our delight, most tested substrates
could be selectively transformed into the corresponding aldehydes
with high yields and over oxidation products such as acids and
esters were not observed during the reaction. Substrates contain-
ing electron-donating substituents like methyl and methoxy
groups on the phenyl ring turned into the corresponding
benzaldehyde in excellent yields with high selectivity (Table 2,
entries 1–7). Similar results were also observed for electron-
deficient benzylic alcohols yet with a prolonged reaction time
t
1
2
3
4
5
6
7
8
9
CuCl
CuCl
CuBr
CuBr
CuSO
CuI
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
—
Cu(OAc)
Cu(OAc)
BuOK
BuOK
BuOK
BuOK
BuOK
BuOK
BuOK
93
92
93
92
87
96
98
94
85
86
100
87
5
88
86
87
88
81
90
93
91
80
82
95
82
—
—
—
t
2
t
t
2
t
4
t
t
2
2
2
2
?H
?H
?H
?H
2
2
2
2
O
O
O
O
K
2
CO
3
NaHCO
K
Na
NaOH
Na
Na
Na
3
10
11
12
13
14
15
3
PO
4
2
?H
2
O
2
CO
3
2
?H
2
O
d
e
f
2
CO
2
CO
2
CO
3
3
3
(entries 8–14). It is noteworthy to mention that the oxidation
2
?H
?H
2
2
O
O
8
23
2
protocol also allowed the smooth transformation of heterocyclic
alcohols under reflux with good to excellent yields (entries 15–17).
However, the aerobic oxidative transformation of secondary
alcohols like 1-phenylethanol oxidation was inhibited (entry 19),
which might be ascribed to the steric effect of the methyl group in
a
Reaction conditions: p-tolylmethanol (1.0 mmol), copper salt (5
mol%), TEMPO (5 mol%), base (1.0 mmol), pytl-b-CD (5 mol%), H O
2
4.0 mL), reflux for 10 h. Determined by GC. Isolated yield.
Copper salt was omitted. The reaction was carried out in the
b
c
(
d
e
f
absence of TEMPO. No pytl-b-CD was employed.
1
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and water as the solvent, made the system attractive for
environmentally sustainable processes.
Acknowledgements
We acknowledge financial support from the National Natural
Science Foundation of China (No. 20702051), the Natural
Science Foundation of Zhejiang Province (LY13B020017) and
the Key Innovation Team of Science and Technology in
Zhejiang Province (No. 2010R50018).
Notes and references
1
2
3
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nol.
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activity, which is a prerequisite for its practical application
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efficiently and selectively in refluxed water using air as the
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1
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