Solid state cleavage of semicarbazones with cerium ammonium nitrate supported by wet alumina

Article abstract of DOI:10.1007/s007060270012

Cerium ammonium nitrate on wet alumina rapidly regenerates carbonyl compounds from their corresponding semicarbazone derivatives under solvent-free conditions; the process is expedited by microwave irradiation.

Full text of DOI:10.1007/s007060270012

Monatshefte fuÈr Chemie 133, 107±110 (2002)
Solid State Cleavage of Semicarbazones
with Cerium Ammonium Nitrate
Supported by Wet Alumina
1;2;Ã
1
Kioumars Aghapoor1, Majid M.Heravi
and Mitra Ghassemzadeh1
, Massoud A.Nooshabadi ,
1
Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran
2
Department of Chemistry, School of Sciences, Azzahra University, Vanak, Tehran, Iran
Summary. Cerium ammonium nitrate on wet alumina rapidly regenerates carbonyl compounds
from their corresponding semicarbazone derivatives under solvent-free conditions; the process is
expedited by microwave irradiation.
Keywords. Semicarbazones; Alumina; Cerium ammonium nitrate; Microwave irradiation; Solvent-
free system.
Introduction
Semicarbazones are useful protecting groups and are extensively used for isolation
and puri®cation of carbonyl compounds [1, 2]. Extensive studies on the deprotec-
tion of these derivatives have been carried out using various catalysts such as
copper(II) chloride [3a], cation exchange resin [3b], clayfen [3c], potassium bromate
[3d], zirconium sulfophenyl phosphonate, chlorotrimethylsilane/sodium nitrate or
nitrite [3e], and ammonium persulfate/clay [3f ]. These methods require high
temperatures [3a±b], long reaction times, or involve toxic metal ions [3e, g, h] as
catalysts. Consequently, there is a demand for the development of protocols using
readily available and safer reagents under environmentally friendly conditions.
Different salts of Ce(IV) have been employed as versatile reagents in organic
synthesis [4a±c]. In addition, Cerium(IV) as ceric ammonium nitrate has been used
for the regeneration of carbonyl compounds from oximes and semicarbazones in
nitric acid in a corrosive environment [5].
Reagents impregnated on mineral solid supports have gained popularity in
organic synthesis because of their selectivity and ease of manipulation [6, 7].
During the course of our investigations on organic manipulations in solvent-free
systems [8], we have observed a relatively useful microwave (MW) effect [8a±c].
The salient features of this MW approach are improved reaction rates and cleaner
Ã
Corresponding author. E-mail: mmheravi@azzahra.ac.ir

108
K. Aghapoor et al.
Scheme 1
reactions [9]. Microwave-assisted reactions under dry conditions [6, 7, 10] are
especially appealing as they provide an opportunity to work with open vessels, thus
avoiding the risk of high pressure, and with a possibility of upscaling the reaction
on the preparative scale. In continuation of our ongoing efforts [8a±d] we envisoned
the applicability of supported cerium ammonium nitrate to ful®l a variety of
oxidative needs under solvent-free reaction conditions using microwaves. Here, we
report a facile and rapid desemicarbazonation protocol using cerium ammonium
nitrate (CAN) doped wet alumina.
Results and Discussion
Among the various solid supports examined, such as montmorillonite K-10, silica
gel, and wet alumina, the latter proved to be superior. In the absence of wet alumina
the reaction was sluggish, and a mixture was obtained.
Wet alumina was prepared by shaking neutral aluminum oxide with distilled
water. The reagent was prepared by mixing cerium ammonium nitrate and a weight
equivalent of wet alumina using a pestle and mortar. The reaction was conducted
by mixing this reagent with neat semicarbazones.
In most cases, the reactions are completed upon simple mixing. However,
gentle warming by microwaves accelerates some others (Table 1). The reactions
are relatively clean; no tar formation was observed. Interestingly, no overoxidation
occured.
Table 1. Cleavage of semicarbazones with CAN supported by wet alumina in a solventless system
R1
R2
Reaction time
(s)
Yieldb
(%)
C6H5
H
10
90
86
82
85
88
82
75
85
88
85
68
C6H5
CH3
C6H5
H
20
C6H5
40a
60a
40a
40a
40a
300a
20a
20
p-Cl±C6C6H4
o-Cl±C6H4
m-Cl±C6H4
o-NO2±C6H4
p-OH±C6H4
Cyclohexyl
CH3
H
H
H
H
C2H5
H
±
±
CH CH CH
3
20
a
b
Reactions completed under microwave irradiation; isolated yield

Cleavage of Semicarbazones
109
The reaction seems to be quite general. The semicarbazones of not only aromatic
aldehydes and ketones, but also aliphatic aldehydes and cyclic ketones reacted
smoothly to give the corresponding aldehydes and ketones. However, the reaction
of crotonaldehyde semicarbazone gave a moderate yield of 60%, indicating
a possibility of carbon±carbon double bond cleavage as detected by GLC.
In conclusion, an environmentally benign and safe oxidant on a solid support is
introduced. Rapid reaction, high yield, and the use of inexpensive and non-corosive
alumina instead of corrosive nitric acid under solvent-free conditions are attractive
features of this protocol.
Experimental
All compounds are known and were identi®ed by comparison with authentic samples (physical and
spectroscopic data). Semicarbazones were prepared by reaction of aldehydes and ketones with
semicarbazide hydrochloride and identi®ed by their melting points and IR spectra.
Preparation of CAN/wet alumina
Aluminum oxide (1 g, Aldrich; Brockmann, ꢀ150 mesh) was shaken with 2 cm3 distilled H2O. To
this mixture, 1.09 g cerium ammonium nitrate (2 mmol) was added and crushed together in a mortar
to form an intimate mixture.
Regeneration of semicarbazones
An appropriate semicarbazone (2 mmol) was thoroughly mixed with the above catalyst (2.09 g,
equivalent to 2 mmol of CAN ) using a spatula. An exothermic reaction ensues with darkening of the
reagent and is completed almost immediately as con®rmed by TLC (hexane:AcOEt ˆ 8:2). The
product is extracted into CH2Cl2 (2 Â 25 cm3) and passed through a small bed of silica gel to afford
the pure carbonyl compound. In some cases, brief microwave irradiation (in an unmodi®ed household
microwave oven) completes the reaction (Table 1).
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Received April 25, 2001.Accepted (revised) July 9, 2001