Mild and selective oxidation of alcohols using Oxone as an oxidant catalyzed by palygorskite in water

Article abstract of DOI:10.1055/s-0030-1259005

Palygorskite, a natural, environmentally friendly and inexpensive fibrillar silicate clay mineral, was applied as a catalyst in the oxidation of alcohols in aqueous media. The oxidation transformation effectively proceeded in the presence of an environmentally benign oxidant Oxone. Benzyl alcohols, benzhydrols, cyclic and aliphatic alcohols could be oxidized to their corresponding acids or ketones in moderate to high yield and good selectivity at 50 °C or 70 °C in 21 hours.

Full text of DOI:10.1055/s-0030-1259005

2
818
LETTER
®
Mild and Selective Oxidation of Alcohols Using Oxone as An Oxidant
Catalyzed by Palygorskite in Water
O
S
xidation of
A
h
lcohols ang Wu, Hengchang Ma, Ziqiang Lei*
Key Laboratory of Polymer Materials of Gansu Province, Key Laboratory of Eco-Environment-Related Polymer Materials,
Ministry of Education, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, P. R. of China
Fax +86(931)7971687; E-mail: leizq@nwnu.edu.cn
Received 30 June 2010
The oxidation of alcohols to the corresponding carbonyl
Abstract: Palygorskite, a natural, environmentally friendly and in-
expensive fibrillar silicate clay mineral, was applied as a catalyst in
the oxidation of alcohols in aqueous media. The oxidation transfor-
compounds has profound importance in chemical, phar-
maceutical, and fragrances industries.1
5,16
The stable
tetraalkylnitroxyl radical TEMPO (2,2,6,6-tetramethyl-
mation effectively proceeded in the presence of an environmentally
®
benign oxidant Oxone . Benzyl alcohols, benzhydrols, cyclic and piperidine-1-oxyl) is a well-known oxidation catalyst,
aliphatic alcohols could be oxidized to their corresponding acids or whose numerous applications in organic synthesis, in-
ketones in moderate to high yield and good selectivity at 50 °C or
cluding oxidation of alcohols, sulfides, and organometal-
7
0 °C in 21 hours.
17
lic compounds have been recently reviewed. Of
Key words: alcohols, oxidation, ketenes, carboxylic acid, silicon
particular synthetic interest is the oxidation of alcohols to
carbonyl compounds. However, the separation of the
products from TEMPO can require lengthy workup proce-
Palygorskite is a clay mineral with a wide variety of in- dures, especially when reactions are run in large scale.
dustrial applications, which has found use as absorbent, Photo-oxidation reaction has attracted increasing atten-
pigment, catalyst, and rheological control agent.1 Tech- tion during the last decade of green chemistry. Itoh and
nological applications are based on its physicochemical co-workers investigated photo-oxidation of alcohols and a
properties, principally on composition, surface area, po- methyl group at the aromatic nucleus in the presence of
rosity, among others, and especially in its fibrous struc- organic or inorganic bromine sources in organic solvent.18
–6
7
ture. Bradley first described the structure of palygorskite, In recent years, hypervalent iodine reagents have been ex-
and referred it as attapulgite, a term now used in mining tensively explored in organic chemistry as mild, safe,
terminology.
highly efficient, commercially available, easy to handle,
and of low toxicity in preference to heavy metal reagents
Palygorskite – (Mg,Al) Si O (OH) (H O) ·4H O – is a
5
8
20
2
2
4
2
1
9
for oxidation. Whereas, most of these catalytic process-
es usually run in organic solvents, such as DMSO, chlori-
nated solvents and so on. Although homogeneous
catalysts have been extensively investigated for these
reactions, they suffer from the difficult separation from
reaction media and non-reusability during the reactions.20
2
:1 phyllosilicate in which the sheets of silica tetrahedral
are periodically inverted with respect to the tetrahedral
bases. As a result of this inversion, the octahedral sheets
are periodically interrupted and terminal cations must
complete their coordination sphere with water molecules.
As only four of five octahedral positions available are
filled, palygorskite appears to be intermediate between di- In recent years, clay minerals and other inorganic materi-
8
–14
octahedral and trioctahedral minerals (Figure 1).
als have been exploited as heterogeneous catalyst or sup-
ports for numerous applications. In our previous research
programs, we have also successfully prepared a series of
palygorskite-supported metal complexes by simple proce-
dures.2
1,22
Among the heterogeneous catalysts systems,
dispersed metal hydroxide species on an appropriate sup-
port turned out to be a powerful heterogeneous catalyst for
the alcohol oxidation. For representative work, the finding
by Mizuno and co-workers on the catalytic oxidation of
IV
alcohols by Al O -supported Ru OH species has stirred
2
3
up much interest and has generated the possibility of ap-
plications of MOH catalyst for several important reac-
tions.23 It is widely accepted that its powerful catalytic
ability is activated by a co-existing sites of Lewis acid and
Brønsted base collaborated effect. Despite the excellent
catalytic performance, the limitations for the adsorbed-
type catalyst system are obvious, such as expensive noble
Ru catalyst, catalyst leaching, use of harmful solvent and
more complicated synthesis procedure. Therefore, from
Figure 1 Schematic structure of palygorskite8–14
SYNLETT 2010, No. 18, pp 2818–2822
x
x
.x
x
.2
0
1
0
Advanced online publication: 14.10.2010
DOI: 10.1055/s-0030-1259005; Art ID: W10410ST
©
Georg Thieme Verlag Stuttgart · New York

LETTER
Oxidation of Alcohols
2819
the viewpoint of the environmental concern, it is neces- tant members in the future. Our preliminary study has dis-
sary to develop a transition-metal-free catalyst system that closed the efficient, environmentally friendly and
exhibits a wide range of substrate tolerance by using inexpensive catalyst system, which renders the oxidation
cheaper and less toxic reagents coupled with simpler reac- of alcohols with advantages such as green reaction media,
tion conditions and easier workup procedures.
In our ongoing research programs,21,22,24–27 we found that
mild reaction conditions, convenient product isolation and
good catalytic performance.
in water as medium, palygorskite is active in the oxidation In order to study the catalytic activities of palygorskite,
®
of
alcohols
in
the
presence
of
Oxone
oxidation of benzyl alcohol was selected as the model re-
(
2KHSO ·KHSO ·K SO ). This novel oxidation system action to optimize the influence factors such as tempera-
5
4
2
4
®
could convert a variety of alcohols to the corresponding ture, the amount of the catalyst and Oxone , as well as the
carbonyl compounds. As documented, the acid-leached reaction time. In the process of reaction, the mixture was
palygorskite produced a progressive dissolution of the oc- analyzed by gas chromatography. The results demonstrat-
®
tahedral sheet, creating microporosity between the tetra- ed that the optimum ratio of benzyl alcohol to Oxone was
hedral silicate sheets. These silicate sheets maintained the 1:1.8, and the appropriate amount of palygorskite catalyst
skeleton of the mineral and contained a small proportion was 16 mg (the content of Si is 17.36%, 2.78 mg). In a
of silanol groups, generated by the acid hydrolysis reac- controlled experiment, it was found that the reaction gave
1
4
tion. As we postulated that the naked silanol groups poor activity even after a long reaction time in the absence
could possibly be applied as the key intermediate for alco- of palygorskite.
hol oxidation, our attention was attracted by the metalloid
For the recycling study, the oxidation of alcohols was per-
elements, such as Al-, Si-containing substances, for the
formed with benzyl alcohol, maintaining the same reac-
reason that such structurally common units (AlOH, SiOH)
tion conditions as described above except for using the
are extensively present in such materials. For example,
recovered catalyst. We were pleased to find that the cata-
montmorillonite, loess, silica gel and so on. During the
lyst can be reused in at least six cycles with complete con-
screening experiments under specific conditions, mont-
version and selectivity without any need for further
morillonite, loess and silica gel all demonstrated average
treatment.
to excellent catalytic behavior for the oxidation of alco-
hols. Due to this pioneering work, we are convinced that
in the transition metal catalyst-dominated field of alcohol
oxidation, the metalloid elements Al and Si will be impor-
Table 1 Oxidation of a Series of Alcoholsa
b
Entry
1
Substrate
Product
Temp (°C) Conversion (%)b Selectivity (%) Yield (%)c Conversion (%)d
O
O
O
O
OH
OH
85e
30
OH
50
50
100
62
100
67
1
1
a
2
2
2
a
b
c
2
H
NR
b
OH
OH
10
OH
3
50
76
67
23
1
c
2
2
d
e
O
H
2
0
Synlett 2010, No. 18, 2818–2822 © Thieme Stuttgart · New York

2
820
S. Wu et al.
LETTER
Table 1 Oxidation of a Series of Alcoholsa (continued)
b
Entry
4
Substrate
Product
Temp (°C) Conversion (%)b Selectivity (%) Yield (%)c Conversion (%)d
O
OH
OH
92e
50
100
100
NR
O2N
O2N
1
d
2
f
O
OH
OH
88e
98f
96f
5
6
7
70
50
50
50
87
100
100
100
100
100
100
100
25
Cl
Cl
1
e
2
g
2h
2i
OH
O
NR
27
1
1
f
OH
O
g
OH
O
8
9
97f
96f
NR
Cl
Cl
1
h
2j
OH
O
70
70
100
75
100
90
NR
12
1
i
2k
2l
OH
OH
1
0
O
1
1
j
OH
OH
OH
OH
OH
OH
2
1
1
1
2
3
70
70
70
86
72
55
100
100
100
19
2
O
O
O
k
2
2
2
m
4
10
4
1
1
l
n
o
1
5
45f
NR
5
m
a
®
Reaction condition: alcohol (0.10 mmol), palygorskite (16 mg), Oxone /alcohol = 1.8: 1, H O (3 mL), 21 h. Percentage conversation and
2
reaction selectivity were determined by GC analysis.
b
The oxidation was carried out in the presence of catalyst.
Isolated yield.
The oxidation was carried out without catalyst. NR: no reaction.
Products were obtained after recrystallization.
c
d
e
f
Products were obtained after column chromatography.
To evaluate the scope of catalytic system, we investigated dized to aldehyde and carboxylic acid with conversions of
the oxidation of a number of alcohols (Table 1). Benzyl 62% and 76%, respectively. A similar transformation was
alcohol (1a) was readily transformed to benzoic acid (2a) also accomplished by using 4-nitrobenzyl alcohol (1d) to
with 100% conversion and selectivity. Alcohols substitut- afford the corresponding acid as a sole product (2f) with
ed with electron-donating groups, such as 2-methylbenzyl excellent conversion and selectivity. And the reaction also
alcohol (1b) and 3-methylbenzyl alcohol (1c) were oxi- provided good-to-excellent yields and purities (entries 6–
Synlett 2010, No. 18, 2818–2822 © Thieme Stuttgart · New York

LETTER
Oxidation of Alcohols
2821
8) in the oxidation of benzhydrol (1f) and substituted benz- The reaction went through a silyl ether intermediate II,
hydrols, such as 4-methylbenzhydrol (1g) and 4-chlo- which was formed between the alcohol and SiOH I. Sub-
robenzhydrol (1h). On the other hand, at 70 °C, 4- sequent b-hydrogen elimination produced the carbonyl
chlorobenzyl alcohol (1e) was oxidized to the correspond- compound and silicon hydride species III. In the presence
ing acid (2g) with 87% conversion (entry 5), and cyclo- of Mg and Al ions, exchange of oxygen atoms between
1
8
®
29
pentanol (1i) was converted to cyclopentanone (2k) with
excellent conversion and selectivity (entry 9). However,
H O and Oxone could be accelerated. As a result,
2
1
8
®
O-labled Oxone was used as an oxidant to convert the
for long-chain aliphatic alcohols (1k, 1l, 1m), the conver- reduced species III back to Si OH.
1
8
sions gradually reduced from 86% to 55% when the chain
length increased from six to nine, even at 70 °C (entries
1–13).
In order to determine a plausible mechanism, isotope O-
R1
18OH
1
Si
I
HO
R2
18
2
8
labeled benzyl alcohol was oxidized in the presence of
18O-labeled Oxone® (ref. 29)
®
palygorskite–Oxone –H O under N atmosphere. In the
2
2
H216
–
O
1
6
presence of H O, GC–MS detector indicated that the
2
6
1
abundance of O in product was >97%. That indicated the
oxidation process involving a selective cleavage of the C–
O bond of the alcohol with concomitant formation of a
new C=O bond. In the product, the oxygen atom was
H
18O
R1
R2
Si
Si
III
H
II
®
transferred from either Oxone or the solvent. Further
R1
study demonstrated that when the oxidation of isotope
18O
HO
Oxone®
R2 = H
ref. 30
1
8
18
18
O
O-labeled benzyl alcohol was carried out in H O, the
R2
2
R1
1
8
abundance of O in the product was found to increase to
9
2%. This interesting finding verified that in the process
Scheme 2 Proposed reaction mechanism
of alcohol oxidation, exchange of oxygen atoms between
alcohol and water had taken place (Scheme 1).
Based on the literature precedents,8–14,23,29–31 we proposed
a plausible mechanism for the alcohol oxidation
In summary, the heterogeneous clay catalyst shows great
advantage in the oxidation of alcohols in the presence of
32
water. A wide range of alcohols could be converted into
1
8
(
Scheme 2). Naked Si OH groups were formed via acid-
the corresponding carbonyl compounds in high yields. In
addition, this heterogeneous catalyst is stable, and can be
reused at least six times.
1
8
+
catalyzed cleavage with H3 O . The SiOH species then
prompted the oxidation of alcohols to the corresponding
carbonyl compounds (Figure 2).
CH218OH
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
palygorskite, Oxone®
H216O, N2
CH16
O
Acknowledgment
abundance of 16O: > 97%
This work was financially supported by the National Natural Sci-
ence Foundation of China (No. 20674063 and 20774074) and the
Specialized Research Fund for the Doctoral Program of Higher
Education (20050736001). We also thank the Key Laboratory of
Eco-Environment-Related Polymer Materials (Northwest Normal
University), Ministry of Education, for financial support.
CH218OH
CH18
O
CH16
O
palygorskite, Oxone®
H218O, N2
abundance of 18O and 16O: 92% and 8%, respectively
References and Notes
Scheme 1 The oxidation of isotope-labeled benzyl alcohol
(
(
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988, 631.
(
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Synlett 2010, No. 18, 2818–2822 © Thieme Stuttgart · New York

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822
S. Wu et al.
LETTER
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1
(28) Preparation of O-Enriched Benzyl Alcohol: Na (0.05 g)
1
8
18
(
(
was added to O-enriched water (0.75 mL, 98% H2 O,
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benzene (0.5 mL) was added into the flask. The mixture was
heated to 95 °C and refluxed for 48 h with continuous
stirring. The product was purified by column chromatog-
1
(
(
(
(
1
8
18
3
raphy and C H CH OH (0.2 g) was obtained. The O-
6 5 2
enriched benzyl alcohol was examined by GC–MS, and the
1
8
1
abundance of C H CH OH was determined to be 98%.
6 5 2
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(
(
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2
(
2
(
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2
3
under reduced pressure to give the desired crude product.
Analytically pure products were obtained after recrystal-
lization or column chromatography using petroleum ether
and EtOAc as eluent. Formation of products and
consumption of substrates were monitored by GC. The
identity of products was determined either by comparison
with authentic samples using gas chromatography or by GC–
MS or NMR analysis.
(
(
(
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4
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