Regeneration of carbonyl compounds from oximes using BTBAD under microwave irradiation

Article abstract of DOI:10.1246/cl.2004.1038

Bis-tetrabutylammonium dichromate (BTBAD) has been found to be an efficient and new reagent for the conversion of oximes to the corresponding carbonyl compounds. The reaction was performed under microwave irradiation and gave excellent yields. It also facilitates the de-protection of acetals and ketals.

Full text of DOI:10.1246/cl.2004.1038

1
038
Chemistry Letters Vol.33, No.8 (2004)
Regeneration of Carbonyl Compounds from Oximes Using BTBAD
under Microwave Irradiation
Ã
R. Murugan and B. S. R. Reddy
Industrial Chemistry laboratory, Central Leather Research Institute, Adyar, Chennai - 600 020, India
(Received May 26, 2004; CL-040597)
Bis-tetrabutylammonium dichromate (BTBAD) has been
Table 1. Oxidative cleavage of oximes with BTBAD under
a
found to be an efficient and new reagent for the conversion of ox-
imes to the corresponding carbonyl compounds. The reaction
was performed under microwave irradiation and gave excellent
yields. It also facilitates the de-protection of acetals and ketals.
microwave irradiation
b
Entry
Oxime
Product
Time
Yield
/%
/
s
NOH
O
1
60
94
NOH
O
There is a growing interest for generating carbonyl com-
pounds from oximes since these can be prepared from non-car-
bonyl precursors as well as used for the protection and purifica-
2
3
MeO
Cl
MeO
7
7
5
5
93
90
NOH
O
1
tion of carbonyl compounds. Conversion of oximes back to
Cl
their corresponding carbonyl compounds is a very important
process in synthetic organic chemistry. Several oxidative de-ox-
imation methods have been developed and have found some def-
inite advantages over the conventional methods.2,3
NOH
O
4
120
89
NOH
O
NOH
O
BTBAD
5
6
60
92
87
Me
Me
NOH
NOH
O
1
R
2
R
1
R
2
R
MW, 1 2 min
120
N
H
N
H
O
Here, we report the conversion of oximes to the correspond-
ing carbonyl compounds using BTBAD as a reagent in dichloro-
methane. BTBAD has been already reported to be an oxidizing
agent in the conversion of alkyl halides to carbonyl compounds.
This reagent is readily prepared by stirring a heterogeneous mix-
ture of tetrabutylammonium hydrogen sulphate in dichlorome-
thane and potassium chromate in water at room temperature
for 2 h. BTBAD was obtained from the organic layer as orange
crystals (97% yield).
Microwave irradiation techniques are being extensively
used by the organic chemists to augment reaction rates. Micro-
wave assisted reactions are essentially easy to handle and de-
mand less tedious work-up procedures. Solvents with higher di-
electric constants are favoured in a microwave procedure as they
are proved to increase yields. Therefore, we have chosen di-
chloromethane which has a higher dielectric constant over many
other solvents.
OMe
MeO
7
8
N
N
120
89
92
H
H
NOH
O
60
N
N
NOH
O
4
9
1
1
20
20
87
89
85
NOH
O
1
1
0
1
NOH
O
6
120
120
6
NOH
O
O
1
2
87
NOH
The de-oximation was performed by mixing one equivalent
of the reagent with one equivalent of the oximes in a mortar and
ground with pestle until the formation of a homogeneous mix-
ture. The reaction mixture was dissolved in dichloromethane
and then transferred to an Erlenmeyer flask and irradiated in a
microwave oven, until complete disappearance of the starting
material.
We also rationalized that since it could cleave the oximes, it
could have a similar effect in deprotecting the acetals and ketals
for generation of carbonyl compounds. Deprotection of acetals
13
6
6
0
0
93
92
NOH
NO2
O
NO2
1
4
a
Products were confirmed by comparison with authentic sample
TLC, IR, NMR, and (solid) m.p).
Yield of isolated pure carbonyl compound.
(
b
Using this reagent under microwave irradiation, various oximes
have been successfully converted to their corresponding alde-
hydes and ketones in excellent yields of 85–95% within time pe-
riods of 1–2 min. The corresponding carbonyl compound ap-
peared as the sole product and no by-products were observed.
The same procedure was adopted for the acetals and ketals,
7
and ketals have been studied extensively by various methods.
This reaction is of considerable importance in many transforma-
tions.
BTBAD is an efficient and novel reagent for the oxidative
cleavage of C=N bond of oximes under microwave irradiation.
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.8 (2004)
1039
Table 2. Oxidative cleavage of ketals and acetals with BTBAD
under microwave irradiationa
reaction conditions and reduced reaction times with excellent
yields.
The reaction was carried out in the domestic microwave
oven, BPL microwave model BMO-700T, manufactured by
BPL-SANYO, India. Acetophenone oxime (1.48 mmol) and
BTBAD (1.48 mmol) was ground with pestle in a mortar to form
a homogeneous mixture, then transfered to an Erlenmeyer flask
in a microwave oven, until TLC showed complete disappearance
of starting materials. CH2Cl2 was added to the reaction mixture,
concentrated, charged on small silica gel column and eluted with
a mixture of EtOAc-petroleum ether (2:8) to afford the pure
product in 94% yield (Table 1). The same procedure was adopt-
ed for the ketals and the yields are given in Table 2.
b
Entry
Acetal/ketal Product
Time
Yield
/%
/s
O
O
o
1
2
3
120
120
120
85
89
92
O
O
O
MeO
MeO
O
O
O
Thanks are due to help rendered by R. Kamakshi and
M. Karthikeyan in the preparation of the manuscript.
N
N
N
NH
O
N
NH
References
4
5
Ph
Ph
120
120
90
87
O
CHO
1
2
3
T. W. Greene and P. G. M. Wuts, in ‘‘Protective Groups in
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O
o
OMe
OMe
1
903.
N
H
N
H
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a
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2
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b
4
5
and the reaction was found to complete in 2 min. The pure car-
1
bonyl compound was characterized by IR, H NMR, and melting
point analysis.
6
In conclusion, we report a new and efficient methodology
for the regeneration of aldehydes and ketones from oximes under
microwave irradiation. It is also found to facilitate deprotection
of acetals and ketals. The main advantage of this new method in-
cludes the availability of inexpensive BTBAD as a catalyst, mild
7
G. Sartori, R. Ballini, F. Bigi, G. Bosica, R. Maggi, and
P. Righi, Chem. Rev., 104, 199 (2004).
Published on the web (Advance View) July 19, 2004; DOI 10.1246/cl.2004.1038