Caro's acid supported on silica gel, Part VIII: An efficient and selective reagent for conversion of phenylhydrazones and semicarbazones to the corresponding carbonyl compounds

Article abstract of DOI:10.1080/10426500490466337

Deprotection of phenylhydrazones and semicarbazones to their parent aldehydes and ketones in high yields has been carried out using caro's acid supported on silica gel as a selective oxidant under mild conditions.

Full text of DOI:10.1080/10426500490466337

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Phosphorus, Sulfur, and Silicon and the Related
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CARO'S ACID SUPPORTED ON SILICA GEL, PART VIII: AN
EFFICIENT AND SELECTIVE REAGENT FOR CONVERSION
OF PHENYLHYDRAZONES AND SEMICARBAZONES TO THE
CORRESPONDING CARBONYL COMPOUNDS
Mahmood Tajbakhsh a , Moslem-Mansor Lakouraj a , Mohammad-Hosein Gholami a & Farhad
Ramzanian-Lehmali a
a University of Mazandaran , Babolsar, Iran
Published online: 16 Aug 2010.
To cite this article: Mahmood Tajbakhsh , Moslem-Mansor Lakouraj , Mohammad-Hosein Gholami & Farhad Ramzanian-
Lehmali (2004) CARO'S ACID SUPPORTED ON SILICA GEL, PART VIII: AN EFFICIENT AND SELECTIVE REAGENT FOR CONVERSION OF
PHENYLHYDRAZONES AND SEMICARBAZONES TO THE CORRESPONDING CARBONYL COMPOUNDS, Phosphorus, Sulfur, and Silicon
and the Related Elements, 179:9, 1731-1736, DOI: 10.1080/10426500490466337
To link to this article: http://dx.doi.org/10.1080/10426500490466337
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Phosphorus, Sulfur, and Silicon, 179:1731–1736, 2004
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Copyright Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490466337
CARO’S ACID SUPPORTED ON SILICA GEL,
PART VIII: AN EFFICIENT AND SELECTIVE
REAGENT FOR CONVERSION OF
PHENYLHYDRAZONES AND SEMICARBAZONES TO
THE CORRESPONDING CARBONYL COMPOUNDS
Mahmood Tajbakhsh, Moslem-Mansor Lakouraj,
Mohammad-Hosein Gholami, and Farhad Ramzanian-Lehmali
University of Mazandaran, Babolsar, Iran
(Received January 6, 2004; accepted March 1, 2004)
Deprotection of phenylhydrazones and semicarbazones to their parent
aldehydes and ketones in high yields has been carried out using caro’s
acid supported on silica gel as a selective oxidant under mild conditions.
Keywords: 2,4-Di-nitrophenylhydrazones; Caro’s acid; deprotection;
phenylhydrazones; semicarbazones
INTRODUCTION
Derivatives of carbonyl compounds such as phenylhyrazones and semi-
carbazones not only are used for the characterization and purification
of carbonyl compounds but also play an important role in the protec-
tion of carbonyl compounds, as they are highly crystalline and stable
compounds. Thus, the regeneration of carbonyl compounds from their
derivatives under mild condition is an important process in organic
synthetic chemistry.
The classical method for the cleavage of phenylhyrazones and semi-
carbazones to aldehydes and ketones includes acid hydrolysis, which is
not suitable for acid-sensitive compounds.1
We are thankful to the Mazandaran University Research Council for the partial sup-
port of this work.
Address correspondence to Mahmood Tajbakhsh, Department of Chemistry, School
of Sciences, University of Mazandaran, P.O. Box 453, Babolsar, Iran. E-mail: tajbaksh@
umz.ac.ir
1731

1732
M. Tajbakhsh et al.
Several oxidative deprotection methods have been developed that
have some advantages over the classical hydrolysis methods.2–13 Little
attention has been paid to the oxidative cleavage of phenylhyrazones
and semicarbazones, and only a few reports are available dealing with
the conversion of these derivatives to their corresponding carbonyl
compounds.14–18
RESULTS AND DISCUSSION
We have previously reported that Caro’s acid supported on silica gel is
an efficient reagent for oxidation of organic compounds.19,20
We now report a mild and convenient method for oxidative depro-
tection of semicarbazones and phenylhydrazones in high yields using
Caro’s acid/silica gel (Scheme 1).
SCHEME 1
Our experiments show that semicarbazones and phenylhydrazones
are converted to their corresponding aldehydes and ketones in reflux-
ing dichloromethane. Further oxidation of aldehydes to their carboxylic
acid is not observed. This reagent has wide applicability for regen-
eration of carbonyl compounds from aliphatic and aromatic semicar-
bazones and phenyl hydrazones (Table I).
It is noteworthy that the 2,4-dinitrophenylhydrazones are not de-
protected by this reagent in appropriate yield. These results suggest
that this procedure will show considerable selectivity between semi-
carbazones and phenylhydrazones with 2,4-dinitrophenylhydrazones.
This was established by an experiment in which a mixture of an
equal amount of the derivatives, semicarbazone of benzaldehyde
and 2,4-dinitrophenylhydrazone of p-chlorobenzaldehyde, was treated
with Caro’s acid on silica gel in CH2Cl2 at reflux temperature for
20 min. Working up the reaction mixture showed that only the semi-
carbazone was selectively oxidized to banzaldehyde, and the 2,4-
dinitrophenylhydrazone of p-chlorobenzaldehyde remained unchanged
(Scheme 2).
Similar competitive reaction between phenylhydrazone of ben-
zaldehyde and 2,4-dinitrophenyl-hydrazone of p-chlorobenzaldehyde

Deprotection of Hydrazones
1733
SCHEME 2
resulted only in the oxidation phenylhydrazone of benzaldehyde
(Scheme 3).
When we treated α, β-unsaturated semicarbazone, only the C N
bond was selectively oxidized to the corresponding carbonyl compound,
and the reagent was ineffective in oxidizing C C double bond (Table I,
Entry 8).
In order to show the efficiency of this method we have compared
some of the results with relevant ones reported in the literature14–18
(Table II). As indicated in Table II, when compared to other reagents
Caro’s acid on silica gel performs this transformation in higher yield,
shorter reaction times, and milder conditions.
In conclusion the methodology described here for the regeneration
of carbonyl compounds from their corresponding semicarbazones and
phenylhydrazones with Caro’s acid on silica gel is manipulatively sim-
ple, mild, highly selective, cheap, and it avoids disadvantages of other
methods.
SCHEME 3

1734
M. Tajbakhsh et al.
TABLE I Regeneration of Carbonyl Compounds from Semicarbazones and
Phenylhydrazones with Caro’s Acid/Silica Gel in Refluxing CH2Cl2
Time
(min)
Yielda,b
(%)
Entry
R1
R2
X
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
C6H5
H
H
H
H
H
H
H
H
CH3
C2H5
CH3
CH3
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHC6H5
NHC6H5
NHC6H5
NHC6H5
NHC6H5
NHC6H5
NHC6H5
NHC6H5
NHC6H5
20
35
5
8
5
50
80
39
5
7
4
10
45
25
15
120
60
50
75
105
85
90
150
125
100
100
100
100
100
90
p-MeO C6H4
o,p-(MeO)2 C6H3
p-ClC6H4
p-BrC6H4
p-NO2C6H4
m-NO2C6H4
C6H5CH CH
C6H5
90
100
100
100
100
100
90
100
100
95
90
90
90
95
C6H5
CH3
C2H5
(CH2)4
CH2CH2CH(C6H5)CH2CH2
Camphor
C6H5
H
H
H
H
H
H
H
CH3
C6H5
p-Me C6H4
o-OHC6H4
o-ClC6H4
m-ClC6H4
p-BrC6H4
p-NO2C6H4
o-OHC6H4
C6H5
95
90
90
90
aProducts were characterized by their physical constants, spectral characteristics (IR,
1H NMR) and comparison with authentic samples.
bYields are of pure isolated products.
EXPERIMENTAL
Products were isolated, and their physical data were compared with
those of known samples. Solvent was freshly distilled. Phenylhyra-
zones, 2,4-dinitrophenylhydrazones, and semicarbazones were pre-
pared according to the described procedures.21
General Procedure for Deprotection of Semicarbazones
and Phenylhydrazones
In a round-bottomed flask (50 ml) equipped with a condensor and a
magnetic stirrer, a mixture of the substrate (1 mmol) in CH2Cl2 (5 ml)
and Caro’s acid on silica gel (3 mmol) was placed. The reaction mixture
was stirred at reflux temperature for the specified time (Table I). The

Deprotection of Hydrazones
1735
TABLE II Comparison of Some of the Results Obtained by the
Desemicarbanozation with Caro’s Acid Supported on Silica Gel with Some of
Those Obtained by BTPPMS,a,17 Dowex-50,16 ACC/alumina,b,14 CuCl2.
2H2O,15 and QDCc,18
Time Yield
Reaction
condition
Entry
1
Reagent
Substrate
(min)
(%)
Caro’s acid/
Silica gel
BTPPMS
Dowex-50
Acetophenone semicarbazone
Benzaldehyde semicardazone
Acetophenone semicarbazone
Acetophenone semicarbazone
Benzaldehyde semicardazone
5
20
60
100
100
95
90
32
59
72
81
51
77
85
CH2Cl2, Reflux
2
3
MeCN, Reflux
H2O, Reflux
60
300
360
240
30
90
132
135
4
5
6
ACC/alumina Acetophenone semicarbazone
Benzaldehyde semicardazone
CuCl2.2H2O
CH2Cl2, Reflux
MeCN, Reflux
MeCN, Reflux
Acetophenone semicarbazone
Benzaldehyde semicardazone
Acetophenone semicarbazone
Benzaldehyde semicardazone
QDC
aBTPPMS, Benzyltriphenylphosphonium peroxymonosulfate.
bAmmonium chlorochromate.
cQuinolinium dichromate.
progress of the reaction was monitored by thin layer chromatography
(TLC) or gas chromatography (GC). After completion of the reaction
the solid material was filtered and washed with CH2Cl2, and dried by
Na2SO4. The combined filtrates were evaporated on a rotary evaporator.
The resulting crude material was purified on a silica gel plate (eluent:
hexane/ethyl acetate, 5:1) to afford the pure carbonyl compound, yield
90–100% (Table I).
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