Efficient solvent-free oxidation of benzylic and aromatic allylic alcohols and biaryl acyloins by manganese dioxide and barium manganate

Article abstract of DOI:10.1039/a806954i

Active manganese dioxide and commercially available barium manganate are used for the efficient oxidation of benzylic and aromatic allylic alcohols and biaryl acyloins under solvent-free conditions.

Full text of DOI:10.1039/a806954i

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236 J. CHEM. RESEARCH (S), 1999
J. Chem. Research (S),
Efficient Solvent-free Oxidation of Benzylic and
Aromatic Allylic Alcohols and Biaryl Acyloins by
Manganese Dioxide and Barium Manganatey
1999, 236^237y
Habib Firouzabadi,* Babak Karimi and Mohammad Abbassi
Department of Chemistry, College of Sciences, Shiraz University, Shiraz, Iran
Active manganese dioxide and commercially available barium manganate are used for the efficient oxidation of
benzylic and aromatic allylic alcohols and biaryl acyloins under solvent-free conditions.
Manganese dioxide as a heterogeneous oxidizing agent has
been used extensively for the oxidation of a variety of func-
tional groups, especially for the transformation of benzylic
and allylic alcohols to their corresponding carbonyl
manganese dioxide could be used also for the oxidation
of the compounds under study in the absence of solvents,
but it requires much longer reaction times.
In all reactions under our studies, the amount of the
oxidant did not exceed a ratio of 2 molar equivalents with
respect to the substrates. The oxidized products were all
known compounds and identi¢ed by spectroscopic compari-
son with authentic samples. The methods were also applicable
for large-scale laboratory operations.
1
compounds. .
The solvent plays a crucial role upon the chemical
reactivity of manganese dioxide in oxidation reactions.1
Therefore, conducting reactions in the absence of solvent
could be considered as a useful practical achievement for
the oxidation with MnO2.
In conclusion, the presented methods showed some advan-
4b
Stable and commercially available barium manganate
BaMnO4) is a strong competitor to active manganese
tages with respect to similar reactions in solution; (a) The
molar ratios of the oxidants to substrates are much less than
those reported and did not exceed 2, and (b) work-up is
straight forward.
(
dioxide and has been used for the oxidation of a variety
2
of functional groups. In view of economical, practical,
and environmental demands, solvent-free reactions have been
3
receiving interest in recent years in organic synthesis.
Table 1 Oxidation of benzylic alcohols by MnO2 and BaMnO4
Recent publications on chemical manganese dioxide for
the oxidation of alcohols in hexane and dichloromethane
4
prompted us to report our results with manganese dioxide
and barium manganate for the selective oxidation of aromatic
allylic and benzylic alcohols and biaryl acyloins under
solvent-free conditions with excellent yields. Reactions were
carried out easily by mixing alcohols with the oxidants
and heating the resulting mixtures in an oil bath at 90 8C
under magnetic agitatation for appropriate times. Work-up
of the reaction mixtures was not time consuming and the
products were isolated readily in high purity by applying
the reaction mixtures on a silica gel pad and washing them
with a small amount of the appropriate solvent.
OH
O
R
MnO2 or BaMnO4
Solvent-free
R
X
X
1
2
MnO2 a
BaMnO4 a
Product
X
R
Time/h Yield (%) Time/h Yield (%)
2a
H
H
H
H
H
H
5
5
5
5
5
92
95
95
94
95
96
96
96
95
100
4
4
4
4
5
9
4
4
4
2
93
95
95
95
94
95
96
96
97
100
2
2
2
2
2
b
c
d
e
f
4-MeO
4-Me
3-Me
3-CI
3,4-OCH2O H 10
H
4-Br
H
H
BaMnO4 and MnO2 in the absence of solvent have been
applied successfully for the oxidation of benzylic alcohols
a ^ j (Table 1) to give the corresponding aldehydes or ketones
1
2
a^j in excellent yields. High yield (> 95%) formation of
piperonal from If shows that the hydrolytically labile
methylenedioxy linkage survives the solvent-free oxidation
process.
Aromatic allyl alcohols 3a,b (Table 2) were also oxidized
by the reagents to a¡ord the corresponding carbonyl com-
pounds in excellent yields in the presence of 2 molar equiv-
alents of the reagents.
Oxidation of oct-1-en-3-ol as a non-aromatic allylic alcohol
was also studied under similar reaction conditions with
MnO2 and BaMnO4. Work-up of the reaction mixture after
2g
2h
Me
Me
Et
5
5
5
2
2
2
i
j
Ph
aMolar ratio of the oxidants towards substrates = 1:1.5.
Table 2 Oxidation of aromatic allyl alcohols by MnO2 and
BaMnO4
OH
O
9
6 h a¡orded the corresponding ketone in only 20^25%
MnO2 or BaMnO4
Solvent-free
yields. Saturated alcohols remain intact by these methods.
Oxidation of biaryl acyloins 5a^c to their corresponding
a-diketones 6a^c (Table 3) proceeded readily using MnO2
and BaMnO4 oxidants under solvent-free conditions in quan-
titative yields. It is also noteworthy that commercial
R
R
3
4
a R = H, b R = Ph
4
a
4b
Oxidanta
Time (h)
Yield (%)
Time (h)
Yield (%)
*
To receive any correspondence.
MnO2
BaMnO4
a
15
15
97
98
72
72
95
95
y This is a Short Paper as de¢ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1999, Issue 1]; there is
therefore no corresponding material in J. Chem. Research (M).
2 equivalents of oxidant used.

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J. CHEM. RESEARCH (S), 1999 237
Table 3 Oxidation of biaryl acyloins by MnO2 and BaMnO4
agitated in an oil bath at 90 8C for 4 h. The progress of the reaction
was monitored by TLC. The reaction mixture was applied on a silica
gel pad (3 g) and washed with Et2O (20 ml) to a¡ord pure benzil 6a
O
O
quantitatively (mp 94 8C, lit.5 94^95 C). The same reaction with
BaMnO4 proceeded to completion after 2 h using 1.5 mmol of
the oxidant.
ꢀ
MnO2 or BaMnO4
Solvent-free
R
R
R
R
OH
O
5
6
We appreciate partial ¢nancial support of National
Research Council of Iran for this work by Grant no. 464.
a R = Ph, b R = 4-MeOPh, c R = Fur
6a
6b
6c
Received 7th September 1998; Accepted 17th December 1998
Paper E/8/06954I
Oxidant Time/h Yield (%) Time/h Yield (%) Time/h Yield (%)
MnO2
BaMnO4
a
4a
2
100
100
8
10
a
a
98
98
24
15
a
a
98
98
b
2
equivalents of oxidant used. b 1.5 equivalents of oxidant used.
References
1
2
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Experimental
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prepared according to the reported procedures.
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chromatography or preparative TLC. Purity of the products was
accomplished by GLC on a Shimadzu model GC-8A instrument
or by TLC on silica-gel polygram SIL G/UV254 plates, IR spectra
were recorded on a Perkin-Elmer 781 spectrophotometer and were
compared with those reported for authentic samples.
1
989, 30, 2559; (e) M. Hudlicky, Oxidations in Organic
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1;2
All yields refer
3
4
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Oxidation of Benzoin to Benzil by MnO2 as a Typical Procedure for
the Oxidation of Biaryl Acyloins.öA mixture of benzoin (0.212 g,
5
Aldrich Catalogue Handbook of Fine Chemicals, Sigma^Aldrich,
Gillingham, 1992^93.
1
mmol) and MnO2 (0.174, 2 mmol) was prepared and magnetically