Selective oxidation of alcohols using benzyldimethyltelluronium dichromate

Article abstract of DOI:10.1080/00397910500408712

Selective oxidation of benzylic, allylic, and saturated secondary alcohols to the corresponding aldehydes or ketones by benzyldimethyltelluronium dichromate is described. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910500408712

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Synthetic Communications , 36: 631–635, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500408712
Selective Oxidation of Alcohols Using
Benzyldimethyltelluronium Dichromate
Yang-Heon Song
Department of Chemistry, Mokwon University, Daejeon, South Korea
Abstract: Selective oxidation of benzylic, allylic, and saturated secondary alcohols to
the corresponding aldehydes or ketones by benzyldimethyltelluronium dichromate is
described.
Keywords: Alcohols, benzyldimethyltelluronium dichromate, selective oxidation
The oxidation of alcohols to the corresponding carbonyl compounds is one of
the most important of all chemical transformations.
[1,2]
The representative
chromium-based reagents play a vital role in organic chemistry as oxidants
for alcohols. For instance, pyridinium chlorochromate (PCC, Corey’s
[
3]
[4]
[5]
reagent),
[
pyridinium dichromate (PDC),
reagent, pyridinium fluorochromate (PFC), and bipyridinium chloro-
Jones’ reagent,
Collins’
6]
[7]
[
8]
chromate (BPCC) have been developed for this purpose. Some of these
reagents are mild and effective oxidizing agents for alcohols, but other
reagents are associated with several drawbacks such as overoxidation, poor
selectivity, and use of acidic media.
Herein we report an efficient method for the selective oxidation of
benzylic, allylic, and saturated secondary alcohols using benzyldimethyl-
telluronium dichromate. To the best of our knowledge, no attempts to use
benzyldimethyltelluronium dichromate as an oxidant for oxidation of alcohols
were successful.
Received in Japan July 7, 2005
Address correspondence to Yang-Heon Song, Department of Chemistry, Mokwon
University, Daejeon 302-729, South Korea. E-mail: yhsong@mokwon.ac.kr
6
31

6
32
Y.-H. Song
Scheme 1.
Benzyldimethyltelluronium dichromate was readily and safely prepared
by adding an aqueous solution of potassium dichromate to an aqueous
solution of benzyldimethyltelluronium bromide at room temperature. The
resulting orange-yellow solid is slightly soluble in acetonitrile or dimethyl-
formamide, air stable, and effective after long storage times.
The reactions and results are shown in Scheme 1 and Table 1, respect-
ively. At first, we investigated the reactivity of the oxidizing agent prepared
with benzylic alcohols. The oxidation of benzyl alcohol (1a) with 2 equiv.
of benzyldimethyltelluronium dichromate in boiling acetonitrile gave benzal-
dehyde (2a) in 1 h in 95% yield. At room temperature, oxidation of benzylic
alcohols with the oxidizing agent in acetonitrile or dimethylformamide was
very sluggish. For example, in acetonitrile at room temperature, benzhydrol
was oxidized to benzophenone in 45% yield for 24 h. During investigation,
it was found that boiling acetonitrile is a suitable solvent for oxidation
reaction because of its lower boiling point, higher polarity, and increased solu-
bility of the oxidizing agent. The other solvent such as dichloromethane or
Table 1. Oxidation of alcohols (1) with benzyldimethyl-
telluronium dichromate
(2),
Yield
Time
(h)
a
(
1)
R
1
R
2
(%)
a
b
c
d
e
f
C H
6
H
H
1
2
95
93
5
6 5
m-Cl–C H
b
C
C
6
H
H
5
5
C
6
H
5
2
93
6
CH
H
3
1
90
92
80
82
p-CH O–C H
6 5
1
3
C
6
H
5
CH 55 CH
C 55 CH(CH
CCH 55 CH
H
10
10
g
(CH
3
)
2
2
)
2
–
H
3
h
i
(CH
CH
CH (CH ) CH
2
)
5
10
10
20
20
68
75
5
3
(CH
2
)
3
CH
2
2
CH
H
3
j
3
2 9
k
Cyclohexyl
H
15
a
b
GC yields based on alcohols (1).
Isolated yield.

Selective Oxidation of Alcohols
633
benzene, where the oxidizing agent was suspended, could be used but the
yields and the rates were very low. This shows that the oxidation ability of
the oxidizing agent apparently depends on its solubility in solvent.
Other benzylic alcohols were also transformed into the corresponding
aldehydes or ketones in good to high yields. Further oxidation of aldehydes
to their carboxylic acids was not observed. The oxidation of allylic alcohols
(1f, 1g) afforded the corresponding aldehydes with reaction times longer
than that of benzylic alcohols. However, one limitation of this reaction was
to give product mixtures, which appear to have been caused by (E)/(Z)
isomerization during oxidation.
Interestingly, the oxidation of primary and secondary aliphatic alcohols
under the same conditions revealed selectivity. Cyclohexanol (1h) and
2-heptanol (1i) as secondary alcohols were oxidized to cyclohexanone and
2-heptanone, respectively, in 68% and 75% yields in 10 h, whereas
1-dodecanol (1j) and cyclohexylmethanol (1k) as primary alcohols gave the
corresponding aldehydes, respectively, only 5% and 15% in 20 h. This
showed a possibility of selective oxidation potential, different from other
oxidizing agents. The chemoselective oxidation of a specific alcohol in the
presence of other oxidizable hydroxyl groups is particularly useful in organic
synthesis. Therefore, the chemoselectivity of this oxidizing agent to the
oxidation of diol compounds was further examined. As shown in Scheme 2,
1
hydroxyl group under the same reaction conditions was oxidized to
-phenyl-1,3-propandiol (3) having a benzylic and a saturated primary
[
9]
3
-hydroxy-1-phenyl-1-propanone (4) in 4 h in 75% yield without affecting
a saturated primary hydroxyl group. Similarly, compound (5) was also trans-
[9]
formed into the corresponding hydroxyl ketone (6) in 5 h in 67% yield.
It is noteworthy that good selectivity was observed in the selective
oxidation of benzylic and allylic alcohols that have saturated aliphatic
alcohols.
Scheme 2.

6
34
Y.-H. Song
In conclusion, benzyldimethyltelluronium dichromate as an oxidizing
agent was effective for the selective oxidation of benzylic, allylic, and
saturated secondary alcohols.
EXPERIMENTAL
Melting points were determined on a Thomas Hoover apparatus and are uncor-
1
rected. H NMR spectra recorded in CDCl on a Varian VXR-300 instrument
3
at 300 MHz with TMS as internal standard. IR spectrum was measured in KBr
on an FT Alpha Centauri FT-IR spectrophotometer.
Preparation of Benzyldimethyltelluronium Dichromate
[
10]
Dimethyltelluronium
(3.14 g, 20 mmol) was added slowly to a solution of
benzyl bromide (3.42 g, 20 mmol) in methanol (20 mL) and the mixture was
stirred at room temperature for 2 h. On standing, benzyldimethyltelluronium
bromide (white solid) was formed, filtered off, and dried. Yield: 6.10 g
(
92%); mp 149–1508C. A solution of potassium dichromate (1.47 g,
mmol) in water (50 mL) was added slowly to a solution of benzyldimethyl-
5
telluronium bromide (3.27 g, 10 mmol) in water (180 mL) at room tempera-
ture. After stirring the resulting solution for 1 h, the precipitate was
collected by filtration and dried in vacuo for 3 h to give the orange-yellow
solid. Yield: 3.14 g (88%); mp 142–1438C. IR (KBr): 935, 880, 770,
2
1
[11]
730 cm (dichromate ion).
C H O Cr Te : C, 30.29; H, 3.67.
Anal. found: C, 30.10; H, 3.88. Calcd. for
1
8
26
7
2
2
Oxidation of Alcohols (1) to Carbonyl Compounds (2):
General Procedure
Benzyldimethyltelluronium dichromate (2 mmol) was added to a stirred
solution of an alcohol (1 mmol) in acetonitrile (25 mL). The reaction mixture
was stirred for 1–20 h under reflux. After completion of the reaction, the
mixture was filtered and extracted with ether. The extract was dried and evap-
orated to dryness, and a portion was analyzed by GC with the authentic
samples.
Selective Oxidation of 1,3-Diols (3), (5) to Carbonyl
Compounds (4), (6)
Benzyldimethyltelluronium dichromate (2 mmol) was added to a stirred
solution of (3) or (5) (1 mmol) in acetonitrile (25 mL). The reaction mixture

Selective Oxidation of Alcohols
635
was stirred for 4–5 h under reflux. After completion of the reaction, workup as
described previously followed by flash column chromatography on silica gel
(ethyl acetate/hexane, 1:1) gave the product (4) in 75% yield or the product
(6) in 67% yield, respectively. Spectral data of the products were identical
with those reported.
[
9]
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