2
28 J. CHEM. RESEARCH (S), 1999
J. Chem. Research (S),
A Rapid and Convenient Synthesis of Oximes in
1999, 228^229y
Dry Media under Microwave Irradiationy
Abdol Reza Hajipour,* Shadpour E. Mallakpour and
Gholamhasan Imanzadeh
College of Chemistry, Isfahan University of Technology, Isfahan, Iran
In a novel method, the reaction of hydroxylamine hydrochloride with a number of aldehydes and ketones under
microwave irradiation and solventless `d ry' condition gave oximes in excellent yield.
Oximes are highly crystalline and the oximation is a very
e¤cient method for characterisation and puri¢cation of car-
bonyl compounds. These compounds not only represent a
convenient series of derivatives of carbonyl compounds but
also may be used as intermediates for the preparation of
unsuccessful and the aldehye was recovered unchanged after
60 min grinding in a mortar (entry 2). When silica gel
was used as catalyst in aqueous solution (2 h, re£ux), the cor-
responding oxime was obtained in 63% yield (entry 4). The
reaction of benzaldehyde and hydroxylamine hydrochloride
in the presence of silica gel without irradiation was
unsuccessful and the yield of corresponding oxime after
30 min grinding in a mortar at 70 8C was only 20% (entry
3). Similarly when we used irradiation without silica gel
for 6 min the condensation proceeded in only 20% yield
(entry 5). Only in the case of dry media coupled with
microwave irradiation, taking advantage of the synergy
between both methodologies was the oxime produced in
excellent yield (entry 1).
1
2
amides by the Beckmann rearrangement,
nitrones,
3
3
hydroximinoyl chlorides, nitrile oxide and chiral a-sul¢nyl
4
oximes. The usual method for the preparation of oximes
involves
treatment of carbonyl
compounds
with
hydroxylamine hydrochlorides in a basic aqueous medium
with adjustment of pH. Because of improper levels of solvent
or reactant being used some oximes are liquid or oil-out and
the operating conditions of this method are di¤cult.
5
Since the early articles of Gedye and Giguere, application
of the microwave heating technique has been currently under
intensive investigation and has been recently reviewed. The
e¡ects usually observed are: (a) decreased reaction time
and (b) cleaner reaction with easier work-up. Especially
interesting was the coupling with dry media conditions which
The ability of silica gel in dry media coupled with
microwave irradiation was demonstrated using various
aldehydes and ketones with hydroxylamine hydrochloride
in the absence of any base, and the resulting data are
summarised in Table 2. Aliphatic, aromatic and a,
b-unsaturated aldehydes were converted to the corresponding
oximes in > 76% yield in < 4 min (entries a^j). For ketones
reactions were more di¤cult and lower yields were obtained
(entries k^q). The less reactive benzophenone was also found
to condense with hydroxamine hydrochloride in 50% yield in
6 min (entry k). The purity of the products were determined
6
allowed reaction on a preparative scale and open vessels
7
(avoiding the risk of high pressures and explosions).
We report here that silica gel, without any base, could be a
useful catalyst for the preparation of oximes in dry media
coupled with microwave irradiation. Hydroxylamine
hydrochlorides were reacted with several aliphatic and aro-
matic aldehydes and ketones (Scheme 1) a¡ording the desired
oximes.
1
by H NMR, IR spectra and melting point. It is very
interesting that in these reactions nitriles were not formed,
and in the IR spectra the CN group was not observed around
R1
R1
� 1
1
Silica gel
MW
2200 cm . In all H NMR spectra the OH group of oximes
appeared around d 8.0^9.8 as a broad singlet and in IR
spectra the OH and CNOH group were observed around
O
+
NH2OH•HCl
NOH
R2
R2
1
2
�
1
3
300^3500 and 1640^1690 cm respectively.
Scheme 1 Preparation of oximes 2
Another noteworthy feature of the method lies in the
exclusive reaction of aldehydes with hydroxylamine hydro-
chloride irrespective of the presence of ketones. When we
treated one equivalent of aldehydes in the presence of one
equivalent of ketones with two equivalents of hydroxylamine
hydrochloride only the aldehydes were selectively converted
In order to evaluate the synergy between dry media and
microwave irradiation in this reaction, several experiments
were tried. As shown in Table 1, the reaction of benzaldehyde
and hydroylamine hydrochloride without irradiation was
Table 1 Condensation of benzaldehyde with hydroxylamine hydrochloride
Entry
Catalyst
T or MW power
Time/min
Solvent
Oximea (%)
1
2
3
4
5
silica gel
silica gel
silica gel
silica gel
none
800 W
r.t.
70 8C
100 8C
800 W
1
60
30
120
6
None
None
None
Water
None
96
0
20
63
20
aEvaluated by TLC analysis.
to the corresponding oximes and ketones did not react at
all (Scheme 2). Therefore this methodology could be used
selectively for preparation of aldoximes of the compounds
that contain both aldehyde and ketone functional groups.
*
To receive any correspondence.
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).