Mild and selective oxidation of alcohols to aldehydes and ketones using NaIO4/TEMPO/NaBr system under acidic conditions

Article abstract of DOI:10.1016/j.tet.2006.07.022

A TEMPO-catalyzed selective oxidation of alcohols to the corresponding aldehydes and ketones using NaIO₄ as the terminal oxidant is reported. The NaIO₄/TEMPO/NaBr system provides a mild and efficient method for the oxidation of alcohols that are sensitive to basic conditions. Furthermore, the recoverable ionic liquid immobilized TEMPO-catalyzed oxidation of benzyl alcohol in ionic liquid-H₂O medium is also developed.

Full text of DOI:10.1016/j.tet.2006.07.022

Tetrahedron 62 (2006) 8928–8932
Mild and selective oxidation of alcohols to aldehydes and ketones
using NaIO4/TEMPO/NaBr system under acidic conditions
*
Ming Lei, Rui-Jun Hu and Yan-Guang Wang
Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China
Received 5 April 2006; revised 6 July 2006; accepted 7 July 2006
Available online 28 July 2006
Abstract—A TEMPO-catalyzed selective oxidation of alcohols to the corresponding aldehydes and ketones using NaIO4 as the terminal
oxidant is reported. The NaIO4/TEMPO/NaBr system provides a mild and efficient method for the oxidation of alcohols that are sensitive
to basic conditions. Furthermore, the recoverable ionic liquid immobilized TEMPO-catalyzed oxidation of benzyl alcohol in ionic liquid–
H2O medium is also developed.
Ó 2006 Published by Elsevier Ltd.
1. Introduction
2. Results and discussion
Oxidation of alcohols to carbonyl compounds is among the
most important functional group transformations available
to the synthetic chemists.1 In recent years, nitroxyl radical
TEMPO (2,2,6,6-tetramethylpiperdine-1-oxyl) has been
extensively used as a catalyst for the catalytic oxidation of
alcohols to corresponding carbonyl compounds. Typically,
such oxidations are carried out in the presence of a catalytic
amount of TEMPO and a stoichiometric amount of a termi-
nal oxidant such as bleach (NaClO),2 sodium chlorite,3
sodium bromite,4 sodium or calcium hypochlorite,5
N-chlorosuccinimide (NCS),6 m-chloroperbenzoic acid
(m-CPBA),7 trichloroisocyanuric acid,8 tert-butyl hypochlo-
rite,9 [bis(acetoxy)iodo] benzene (BAIB),10 Oxone,11
iodine,12 oxygen or air.13 To the best of our knowledge,
sodium periodide (NaIO4), a stable and conventional inor-
ganic nonmetal oxidant, has not been used for these transfor-
mations. Herein, we describe a TEMPO-mediated oxidation
method using NaIO4 as the terminal oxidant and NaBr as
co-catalyst. Compared with the systems that performed
under basic conditions, especially the extensively used
NaClO/TEMPO/NaBr system that works at 0 ꢀC and pH
8.6–9.5,2a the present NaIO4/TEMPO/NaBr system could
work at room temperature without generation of over-
oxidized product. Also this buffer-free system provides an
alternative method for the oxidation of alcohols that are
sensitive to basic conditions.
2.1. NaIO4/TEMPO/NaBr system for the selective
oxidation of alcohols to aldehydes and ketones under
two-phase conditions (CH2Cl2–H2O)
Initially, we attempted the NaIO4/TEMPO/CH2Cl2–H2O
(1:1) system for the oxidation of benzyl alcohol, and the
results are summarized in Table 1. We found that benzyl
Table 1. Oxidation of benzyl alcohol under different conditions
CHO
CH2OH
Conditions
CH2CI2-H2O
Entry Conditions
Time Yieldd
(h) (%)
1
2
3
4
1.2 equiv NaIO4, 1 mol % TEMPOa
28
90
96
96
95
1.2 equiv NaIO4, 1 mol % TEMPO, 10 mol % NaBra 10
1.2 equiv NaIO4, 1 mol % TEMPO, 10 mol % NaCl 16
1.2 equiv NaIO4, 1 mol % TEMPO,
10
10 mol % NaBr, pH 2.0b
5
6
3 equiv NaIO4, 1 mol % TEMPO, 10 mol % NaBr
1.2 equiv NaIO4, 1 mol % TEMPO,
10 mol % (n-Bu)4NBr, 10 mol % NaBr
1.2 equiv NaIO4, 1 mol % TEMPO,
10 mol % NaBr, reflux
10
10
96
96
7
8
96
8
9
1 equiv NaIO4
1.2 equiv NaIO4, 1 mol % TEMPO,
48
24
10
5
10 mol % NaBr, pH 8.6c
a
b
c
d
Keywords: Oxidation; Alcohols; Aldehydes; Ketones; Sodium periodide;
TEMPO; Ionic liquids.
* Corresponding author. Tel./fax: +86 571 87951512; e-mail: orgwyg@zju.
edu.cn
Aqueous layer pH 4.0.
Aqueous layer pH was adjusted with 0.5 M H2SO4.
Aqueous layer pH was adjusted with NaHCO3.
Isolated yield.
0040–4020/$ - see front matter Ó 2006 Published by Elsevier Ltd.
doi:10.1016/j.tet.2006.07.022

M. Lei et al. / Tetrahedron 62 (2006) 8928–8932
Table 2. Oxidation of various primary and secondary alcohols
8929
alcohol was completely oxidized to benzaldehyde by
NaIO4 (1.2 equiv) and TEMPO (0.01 equiv) in CH2Cl2–
H2O (1:1) two-phase system at room temperature (Table 1,
entry 1). Furthermore, NaBr and NaCl (10 mol %) could
significantly promote the transformation (Table 1, entries 2
and 3) without generation of overoxidized product. In the
NaIO4/TEMPO/NaBr system, the aqueous layer was main-
tained at pH 4.0 during the reaction. When the aqueous layer
was adjusted at pH 2.0 with 0.5 M H2SO4 (Table 1, entry 4),
the reaction gave a similar yield (Table 1, entry 2). Increas-
ing the amount of NaIO4 (Table 1, entry 5) or adding
phase transfer catalyst (Table 1, entry 6) did not improve
the yield, but refluxing could accelerate the reaction some-
what (Table 1, entry 7). In all cases, TEMPO was necessary.
Otherwise, the reaction proceeded very slowly and gave
poor yield (Table 1, entry 8). It is noteworthy that when
the aqueous layer was buffered at pH 8.6 (Table 1, entry 9),
only 5% yield was obtained.
NaIO4 (1.2 equiv)
TEMPO (1 mol%)
OH
R2
O
NaBr (10 mol%)
CH2CI2-H2O, rt
R1
R1
R2
Entry
1
Alcoholsa
Products
Time (h) Yield (%)c
C6H5CH2OH
CH2OH
C6H5CHO
CHO
10
8
96
2
20
96
95
6b
NO2
NO2
CH2OH
CHO
3
4
15
15
95
96
OMe
OMe
CHO
CH2OH
As the next step, we used the NaIO4/TEMPO/NaBr system
to oxidize various alcohols under the optimized reaction
conditions (Table 2). It was found that primary benzylic
alcohols could be oxidized to the corresponding aldehydes
in excellent yields. The electron-withdrawing group sub-
stituted benzylic alcohols (Table 2, entry 2) reacted faster
than the electron-donating group substituted benzylic alco-
hols (Table 2, entries 3–5). The chloro-substituted benzylic
alcohols (Table 2, entries 6 and 7) exhibited a similar reac-
tivity with benzylic alcohol (Table 2, entry 1). Refluxing
could promote the reactions (Table 2, entries 20 and 70). Pri-
mary aliphatic alcohols (Table 2, entries 8 and 9) as well as
secondary aliphatic and benzylic alcohols (Table 2, entries
10 and 11) could also be oxidized to the corresponding alde-
hydes and ketones in excellent yields after a prolonged reac-
tion time (20–28 h).
OCH3
OCH3
OCH3
OCH3
CH2OH
CHO
O
O
O
O
5
6
15
12
95
95
CH2OH
Cl
CHO
Cl
CH2OH
CHO
7
70
12
95
96
10b
Cl
Cl
8
9
C6H5CH2CH2OH
C6H5CH2CHO
20
28
95
We also examined the unsaturated alcohol, cinnamic alcohol
(1) (Scheme 1). It was found that besides cinnamaldehyde
(2, 18%) and unreacted cinnamic alcohol (1, 45%), an
epoxide 3-phenyl oxiranemethanol (3) was also obtained
in 21% yield (determined by GC–MS). Taking into account
the NaIO4-mediated selective halogenation of alkenes and
aromatics using alkali metal halides,14 we attributed the
formation of the epoxide to the electrophilic addition of
hypohalogenous acid to the double bond of cinnamic alco-
hol, which was followed by an intramolecular nucleophilic
substitution in one-pot procedure.
96d
OH
6
O
6
10
28
18
95d
95
OH
OH
O
O
11
a
Reaction conditions: alcohol (50 mmol), TEMPO (0.5 mmol), NaIO4
(60 mmol), NaBr (5 mmol), DCM (100 ml), water (120 ml), rt.
Under reflux conditions.
Isolated yield.
Determined by GC.
b
c
d
CHO
18%
2
NaIO4 (1.2 equiv)
+
TEMPO (1 mol%)
O
NaBr (10 mol%)
OH
at pH 8.6–9.5 and requires 0 ꢀC reaction temperature, our
NaIO4/TEMPO/NaBr system works efficiently under
slightly acidic conditions (pHw4.0). Therefore, our proce-
dure provides an alternative methodology for the oxidation
of alcohols that are sensitive to basic conditions. Using the
NaIO4/TEMPO/NaBr system, natural product podophyllo-
toxin (4), which is unstable under basic conditions, was
oxidized to ketone 5 in 60% isolated yield (not optimized)
with a 30% recovery of substrate 4 (Scheme 2). Under the
reaction conditions, both the lactone ring opened and C-2
isomerized products were not observed.
OH
CH2CI2-H2O
24 h, rt
21%
3
1
+
OH
45%
1
Scheme 1. Oxidation of cinnamic alcohol.
Compared with the extensively used and more active
NaOCl/TEMPO/KBr system,2a,15 which must be buffered

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M. Lei et al. / Tetrahedron 62 (2006) 8928–8932
OH
O
IL-TEMPO-catalyzed NaIO4 oxidation of benzyl alcohol in
[bmim]PF6–H2O medium (Table 3). Compared with the
CH2Cl2–H2O medium version, the IL-TEMPO-catalyzed
oxidation in water–IL two-phase conditions showed similar
activity and selectivity. The crude product could easily be
separated from the IL medium by simple extraction with
diethyl ether. The IL solution containing the catalyst 6 could
be reused up to six times without significant decrease of
catalytic activity and selectivity.
O
O
O
O
NaIO4 (1.2 eq)
TEMPO (1 mol%)
NaBr (10 mol%)
O
O
O
O
4
+
CH2CI2-H2O
24 h, rt
30%
OCH3
OCH3
CH3O
CH3O
OCH3
5
OCH3
60%
4
Scheme 2. Oxidation of podophyllotoxin (4) to ketone 5.
3. Conclusion
2.2. Recoverable ionic liquid immobilized TEMPO-
catalyzed oxidation of benzyl alcohol using NaIO4 in
ionic liquid–H2O two-phase condition
In summary, we have developed a novel and efficient
TEMPO-catalyzed oxidation of alcohols to the correspond-
ing aldehydes and ketones using NaIO4 as the terminal
oxidant. The reaction could be performed at room tempera-
ture under two-phase conditions. The procedure provides an
alternative method for the oxidation of alcohols that are sen-
sitive to basic conditions. Furthermore, the NaIO4/IL immo-
bilized TEMPO/RTIL system was also developed and could
be recycled up to six times without remarkable deactivation.
Although low catalyst concentrations are required for an
efficient transformation, product separation and TEMPO
recovery remain key issues in TEMPO-catalyzed oxidation.
Consequently, several solid-supported TEMPO moieties
have been developed, which include silica-supported
TEMPO,16 sol–gel entrapped TEMPO17 and polyamine
immobilized piperidinyl oxyl (PIPO).18 These catalysts are
heterogeneous in nature and are readily recovered by simple
filtration from the reaction medium. Recently, it was reported
that soluble polymer poly(ethylene glycol)-supported
TEMPO catalysts (PEG-TEMPOs) exhibited high activity
for the chemoselective oxidation of alcohols.19 This class
of catalysts could be readily recovered via precipitation
with diethyl ether.
4. Experimental
4.1. General
All chemicals were of reagent grade and used as purchased.
1H NMR spectra were recorded in deuterated solvent on
Bruker Avance-500 spectrometer operating at 500 MHz
or on Bruker Avance-300 spectrometer operating at
300 MHz. 13C NMR spectra were recorded in deuterated
solvent on Bruker Avance-500 spectrometer operating at
125 MHz. Infrared spectra were recorded on a Nicolet
Nexus 470 FT-IR spectrometer and measured as thin film
or in KBr. Melting points were measured on WRS-1B digital
melting point apparatus. Capillary gas chromatography was
performed on a Hewlett–Packard HP 6890 gas chromato-
graph/mass spectra instrument (GC–MS). Mass spectra
(EI, 70 eV) were recorded on an HP5989B mass spectro-
meter. ESI-MS spectra were recorded on a Bruker Daltonics
Esquire 3000 plus instrument. HRMS data were obtained on
a Bruker FT-ICR-MS Apex III apparatus.
In the last decade, room temperature ionic liquids (RTILs)
have become promising candidates to replace traditional
solvents used in organic chemistry for their interesting
properties such as high thermal stability, nonvolatility,
high loading capacity, easy recyclability and tunable pola-
rity. A number of chemical reactions can be performed in
ionic liquids.20 Recently, the so-called ‘task-specific ionic
liquids’ immobilized catalysts have been demonstrated to
be an effective strategy for facilitating product separation
and catalyst recovery.21 More recently, a TEMPO-derived
task-specific ionic liquid (IL-TEMPO) (6) for oxidation of
alcohols by the Anelli protocol has been reported.22
In order to develop a recoverable TEMPO-catalyzed and
environmentally friendly oxidation process, we investigated
4.2. Typical experimental procedure for oxidation of
alcohols using NaIO4/NaBr/TEMPO system
Table 3. Oxidation of benzyl alcohol to benzaldehyde using NaIO4 in the
recovered IL immobilized TEMPO-IL solutiona
4-Nitrobenzyl alcohol (7.65 g, 50 mmol) and TEMPO
(78 mg, 0.5 mmol) were dissolved in DCM (100 ml). Then
the aqueous NaIO4 (12.84 g, 60 mmol) and NaBr (0.51 g,
5 mmol) solution (120 ml) were added. The reaction mixture
was vigorously stirred at room temperature and monitored
by TLC. For 10 h, after completion, the organic layer was
washed by NaS2O3 aqueous solution and then dried over
anhydrous sodium sulfate and concentrated in vacuum.
The residue was purified by silica gel chromatography elut-
ing with EtOAc–hexane (1:3) to give 4-nitrobenzyl aldehyde
(7.25 g, 96% yield).
IL-TEMPO (6, 1mol%)
[bmim] PF6 -H2O
CHO
CH2OH
NaIO4 (1.2eq.)
NaBr (10mol%)
O
.
N
N
IL-TEMPO =
O
N O
PF6
6
Run
1
2
3
4
5
6
4.2.1. Benzaldehyde. Colourless liquid; IR (neat): n¼
1
1700 cmꢁ1; H NMR (300 MHz, CDCl3, ppm): d¼7.48–
Time (h)
Isolated yield (%)
15
98
15
95
16
95
16
94
16
93
16
93
7.52 (m, 2H), 7.60–7.65 (m, 1H), 7.85–7.90 (m, 2H), 9.95
In 20 mmol scale.
(s, 1H); MS (EI): m/z 106 (M+).
a

M. Lei et al. / Tetrahedron 62 (2006) 8928–8932
8931
4.2.2. 4-Nitrobenzaldehyde. Light yellow solid; mp 104–
106 ꢀC; IR (KBr): n¼1710 cmꢁ1
8.6 Hz, 2H), 9.90 (s, 1H); MS (EI): m/z 151 (M+).
127.5, 137.8, 138.6, 139.9, 149.2, 154.7, 155.2, 175.8,
193.5; ESI-MS: m/z 435 ([M+Na]+).
;
1H NMR (300 MHz,
CDCl3, ppm): d¼8.10 (d, J¼8.6 Hz, 2H), 8.40 (d, J¼
4.4. Synthesis of IL immobilized TEMPO (6)
4.2.3. 4-Methoxybenzaldehyde. Colourless liquid; IR
Compound 6 was synthesized according to the literature pro-
cedures.22 Red solid; mp 53–54 ꢀC; IR (KBr): n¼3055,
2977, 2933, 1736, 1567, 1266, 1173 cmꢁ1; HRMS (ESI):
calcd for C15H25N3O3 (MꢁPF6): 295.1890, found:
295.1898.
1
(neat): n¼1688 cmꢁ1; H NMR (300 MHz, CDCl3, ppm):
d¼3.85 (s, 3H), 7.00 (d, J¼8.2 Hz, 2H), 7.85 (d, J¼
8.2 Hz, 2H), 9.80 (s, 1H); MS (EI): m/z 136 (M+).
4.2.4. 3,4-Dimethoxybenzaldehyde. Colourless liquid; IR
1
(neat): n¼1690 cmꢁ1; H NMR (300 MHz, CDCl3, ppm):
4.5. Experimental procedure for oxidation of benzyl
alcohol to benzaldehyde using NaIO4 in the recoverable
IL immobilized TEMPO-IL solution
d¼3.80 (s, 3H), 3.85 (s, 3H), 7.20–7.30 (m, 3H), 9.85
(s, 1H); MS (EI): m/z 166 (M+).
4.2.5. 4-Chlorobenzaldehyde. White solid; mp 45–46 ꢀC;
To a solution of benzyl alcohol (2.16 g, 20 mmol) and IL-
TEMPO (90 mg, 0.2 mmol) in [bmim]PF6 (20 ml) was
added the solution of NaIO4 (5.14 g, 24 mmol) and NaBr
(0.21 g, 2 mmol) in water (20 ml). The mixture was vigor-
ously stirred at room temperature. After the oxidation
(TLC monitoring) was completed, the product was separated
from the IL medium by extraction with ether (3ꢂ30 ml). The
organic layer was dried over anhydrous sodium sulfate and
concentrated under vacuum. The product was purified by
flash silica gel chromatography eluting with EtOAc–hexane
(1:5). The recovered IL containing the IL-TEMPO could be
reused for consecutive recycling experiments.
IR (KBr): n¼1703 cmꢁ1
;
1H NMR (300 MHz, CDCl3,
ppm): d¼7.55 (d, J¼7.8 Hz, 2H), 7.85 (d, J¼7.8 Hz, 2H),
9.95 (s, 1H); MS (EI): m/z 141 (M+).
4.2.6. 2-Chlorobenzaldehyde. Colourless liquid; IR (neat):
1
n¼1705 cmꢁ1 (C]O); H NMR (300 MHz, CDCl3, ppm):
d¼7.33–7.48 (m, 3H), 7.75 (d, J¼8.0 Hz, 1H), 10.25
(s, 1H); MS (EI): m/z 141 (M+).
4.2.7. 3,4-Methylenedioxybenzaldehyde. White solid; mp
1
83–84 ꢀC; IR (KBr): n¼1682 cmꢁ1; H NMR (300 MHz,
CDCl3): d 6.00 (s, 2H), 6.95–7.35 (m, 3H), 9.82 (s, 1H);
MS (EI): m/z 150 (M+).
Acknowledgements
4.2.8. Phenylacetaldehyde. Colourless liquid; IR (neat):
n¼1710 cmꢁ1 (C]O); 1H NMR (300 MHz, CDCl3,
ppm): d¼3.25 (d, J¼2 Hz, 2H), 7.05–7.10 (m, 2H), 7.25–
7.30 (m, 3H), 9.75 (t, J¼2 Hz, 1H); MS (EI): m/z 120
(M+).
Authors would like to thank the National Natural Science
Foundation of China (no. 20272051) as well as the Teaching
and Research Award Programme for Outstanding Young
Teachers in Higher Education Institutions of MOE, PR
China.
4.2.9. Acetophenone. Colourless liquid; IR (neat):
n¼1680 cmꢁ1; H NMR (300 MHz, CDCl3, ppm): d 2.58
(d, J¼8.5 Hz, 2H); MS (EI): m/z 120 (M+).
1
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