Solid state cleavage of semicarbazones with montmorillonite K-10 supported bis(trimethylsilyl)chromate under microwave irradiation

Article abstract of DOI:10.1007/PL00010275

Semicarbazones of aldehydes and ketones adsorbed on clay can be cleaved oxidatively to the corresponding carbonyl compounds under solvent-free conditions in high yields employing bis(trimethylsilyl)chromate and microwave irradiation.

Full text of DOI:10.1007/PL00010275

Monatshefte fuÈr Chemie 130, 933±936 (1999)
Solid State Cleavage of Semicarbazones
with Montmorillonite K-10 Supported
Bis(trimethylsilyl)chromate under
Microwave Irradiation
Majid M. Heravi1;Ã, Mahmood Tajbakhsh2, Hamid Bakooie2,
and Dariush Ajami1
1
Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran
2
Department of Chemistry, University of Mazandaran, Babolsar, Iran
Summary. Semicarbazones of aldehydes and ketones adsorbed on clay can be cleaved oxidatively to
the corresponding carbonyl compounds under solvent-free conditions in high yields employing
bis(trimethylsilyl)chromate and microwave irradiation.
Keywords. Semicarbazones; Carbonyl compounds; Microwave irradiation; Solvent-free conditions.
MikrowellenunterstuÈtzte Spaltung von Semicarbazonen mit an Montmorillonit K-10 adsor-
È
biertem Bis(trimethylsilyl)chromat im Festkorper
È
Zusammenfassung. An Ton adsorbierte Semicarbazone von Aldehyden und Ketonen konnen mit
Bis(trimethylsilyl)chromat unter Bestrahlung mit Mikrowellen in einer Festphasenreaktion in hohen
Ausbeuten oxidativ zu den ensprechenden Carbonylverbindungen gespalten werden.
Introduction
Semicarbazones are used not only to isolate and purify but also to protect carbonyl
compounds during syntheses [1, 2]. Several procedures for regenerating carbonyl
compounds from semicarbazones have been reported [1±19].
Microwave irradiation in organic synthesis is presently widely used. Its
application in the case of inorganic solid supported reactions has been recently
reviewed [20]. Solvent-free organic reactions or dry media techniques under
microwave irradiation are one of the main topics of research in our laboratory
[21, 22].
We have recently introduced montmorillonite K-10 supported bis(trimethyl-
silyl)chromate (BTSC) as an ef®cient reagent for oxidation of alcohols [23] and
oxidative deprotection of trimethylsilyl [24] and tetrahydropyranyl [25] ethers. In
this communication we demonstrate that montmorillonite K-10 supported BTSC
Ã
Corresponding author

934
M. M. Heravi et al.
can ef®ciently and rapidly regenerate aldehydes and ketones from their semi-
carbazones using microwaves under solvent-free conditions.
Results and Disscusion
When BTSC [23] was mixed with benzaldehyde semicarbazone and irradiated with
microwaves, after 5 min conversion to benzaldehyde was observed; however, a
considerable amount of starting material was still present. In recent years, organic
reactions with solid supported reagents [26] and the application of microwaves
[27], especially under solvent-free conditions [28], have attracted attention because
of enhanced selectivity, milder reaction conditions, and ease of manipulation.
Among the various supports examined, such as alumina, silica, and montmor-
illonite K-10, the latter was found to be the most ef®cient. Benzaldehyde
semicarbazone was mixed thoroughly with montmorillonite K-10 supported
bis(trimethylsilyl)chromate and irradiated with microwaves to afford the parent
aldehyde almost immediately and quantitatively. This reaction is rather general;
semicarbazones of aromatic aldehydes and ketones as well as of aliphatic and
unsaturated aldehydes reacted smoothly to give the corresponding aldehydes and
ketones (Table 1). Unlike to other oxidative hydrolytic methods, the major
drawback of overoxidation was not encountered. It should be mentioned that the
semicarbazone of crotonaldehyde gave only a moderate yield of aldehyde showing
that the reagent may cleave the carbon-carbon double bond.
In conclusion, montmorillonite K-10 supported BTSC mediated solvent-free
microwave thermolycic is a convenient, selective, and environmentally benign
desemicarbazonation protocol when compared to conventional solution phase or
Table 1. Cleavage of semicarbazones with montmorillonite K-10 supported BTSC under microwave
irradiation and solvent-free conditions
R1
R2
Reaction time
(sec)
Yiled
(%)
C6H5
H
60
60
96
82
80
90
92
80
82
82
85
80
62
C6H5
CH3
C6H5
H
C6H5
60
4-Cl-C6H4
2-Cl-C6H4
3-Cl-C6H4
2-NO2-C6H4
4-OH-C6H4
120
120
60
H
H
H
180
60
H
Cyclohexyl
120
120
120
CH3
C2H5
H
CH3CH=CH

Cleavage of Semicarbazones in the Solid State
935
heterogeneous reactions. In addition, by development of a continuous microwave
reactior for organic syntheses [29±30] this method may gain industrial application
where the absence of solvents and the low costs of the method may lead to
environmental and ®nancial advantages.
Experimental
All compounds empolyed are known and were identi®ed by comparison of their physical data with
those of authentic samples. Bis(trimethylsilyl)chromate supported on montmorillonite K-10 was
prepard according to Refs. [23±24].
General procedure
Montmorillonite K-10 supported BTSC (0.75 g, equivalent to 1.2 mmol of chromium(VI)) was mixed
thoroughly with 1 mmol of semicarbazone and irradiated by microwaves (900 W) for the indicated
time (Table 1). The progress of the reaction was monitored by TLC. After completion of the reaction,
the solid phase was taken up in CH2Cl2, ®ltered, and washed with an excess of CH2Cl2. The ®ltrate
was evaporated to dryness and puri®ed by column chromatography using hexane:ethylacetate ˆ 8:2
as eluent to afford the corresponding carbonyl compound (Table 1).
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Received August 21, 1998. Accepted October 7, 1998