P2O5/SiO2 as an efficient reagent for the preparation of Z-aldoximes under solvent-free conditions

Article abstract of DOI:10.1080/10426500590924148

A facile and efficient method for the preparation of Z-aldoximes is improved by means of P₂O₅/SiO₂ reagent in solvent-free media. Advantages of this method are the use of inexpensive and selective reagent, with high yields in simple operation, and short reaction time under solvent-free conditions. Copyright Taylor & Francis Inc.

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Phosphorus, Sulfur, and Silicon
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P O /SiO as an Efficient
2
5
2
Reagent for the Preparation of
Z -Aldoximes Under Solvent-
Free Conditions
a b
b
Hossein Eshghi & Zinat Gordi
a
Department of Chemistry, Sistan & Baluchestan
University , Zahedan, Iran
b
Department of Chemistry, Ferdowsi University ,
Mashhad, Iran
Published online: 01 Feb 2007.
To cite this article: Hossein Eshghi & Zinat Gordi (2005) P O /SiO as an Efficient
2
5
2
Reagent for the Preparation of Z -Aldoximes Under Solvent-Free Conditions,
Phosphorus, Sulfur, and Silicon and the Related Elements, 180:7, 1553-1557, DOI:
10.1080/10426500590924148
To link to this article: http://dx.doi.org/10.1080/10426500590924148
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Phosphorus, Sulfur, and Silicon, 180:1553–1557, 2005
Copyright © Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500590924148
P O /SiO as an Efficient Reagent for the Preparation
2
5
2
of Z-Aldoximes Under Solvent-Free Conditions
Hossein Eshghi
Department of Chemistry, Ferdowsi University, Mashhad, Iran and
Department of Chemistry, Sistan & Baluchestan University,
Zahedan, Iran
Zinat Gordi
Department of Chemistry, Ferdowsi University, Mashhad, Iran
A facile and efficient method for the preparation of Z-aldoximes is improved by
means of P O /SiO reagent in solvent-free media. Advantages of this method are
2
5
2
the use of inexpensive and selective reagent, with high yields in simple operation,
and short reaction time under solvent-free conditions.
Keywords Aldehydes; phosphorus pentoxide; protection; silica gel; z-aldoximes
INTRODUCTION
The oximation is a very efficient method for characterization, purifica-
tion, and protection of carbonyl 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 amides by
1
2
3
the Beckmann rearrangement, nitrones, hydroxyiminoyl chlorides,
3
4
nitrile oxide, and chiral α-sulfinyl oximes. The usual method for the
preparation of oximes involves treatment of carbonyl compounds with
hydroxylamine hydrochlorides in a basic aqueous medium with adjust-
ment of pH. Because of improper levels of solvent or reactant being
used some oximes are liquid or oil-out and the operating conditions
5
,6a
of this method are difficult. On the other hand, chemical methods
for the synthesis of oximes usually give a mixture of the two geomet-
rical isomers (Z and E), which have different physical properties and
6
biological activities and must be separated by chromatography or re-
crystalization techniques. However, the reagents that have been used
Received January 8, 2004; accepted February 10, 2004.
Address correspondence to Hossein Eshghi, Department of Chemistry, Ferdowsi Uni-
versity, Mashhad 91775-1436, Iran. E-mail: heshghi@hamoon.usb.ac.ir
1553

1
554
H. Eshghi and Z. Gordi
for oximation of ketones or Beckmann rearrangement also catalyze in-
7
terconversion of these geometrical isomers. The rate of equilibrition of
a mixture of Z and E isomers and the position of the equilibrium is tem-
perature dependent,7a therefore, temperature control of the experiment
is critical.
Very few methods are available for the synthesis of E and Z iso-
mer of oximes.7
d,8,9
In many cases, E isomers were obtained from
1
0
the Z forms by either hydrochloride method or purified by column
chromatography.5a As a result there is always considerable interest in
finding more selective methods of oximes synthesis or direct pathways.
Recently, the ability of silica gel in dry media was demonstrated using
various aldehydes and ketones with hydroxylamine hydrochloride in
the presence of a base1 or microwave irradiation, but these methods
suffer from lack of stereoselectivity. Our project have been continually
working on the basis of developing of new synthetic methods using
P2O5/SiO2 (Silicaphos) in a solid phase.12 Having the above facts in
mind, we wish to report a very simple and efficient method for selective
preparation of Z-oximes from aldehydes using the P2O5/SiO2 reagent
in solid phase.
1a
11b
RESULTS AND DISCUSSION
In order to prepare oximes, various types of aldehydes were ground with
hydroxylamine hydrochloride in the presence of P2O5/SiO2 reagent in
solvent-free media. In this approach, corresponding aldoximes were ob-
tained in quantitative yield. The general reaction is illustrated accord-
ing to the Scheme 1 and the results have been reported in Table I. All
reactions were performed in less than 10 min.
SCHEME 1
As shown in the Table I the reaction of hydroxylamine hydrochlo-
ride with different aromatic aldehydes, including those with electron-
withdrawing and donating substituents in the presence of this new
catalyst, gave Z-aldoximes in high yield and stereoselectivity. The pu-
1
rity of the products was determined by H-NMR, IR spectra, and melt-
1
ing point. In all H-NMR spectra the OH group of aldoximes appeared
around 8–10 as a broad singlet and in IR spectra the OH and C N
−
1
groups were observed around 3200–3500 and 1620–1660 cm , respec-
tively. The Z-stereochemistry of the products was determined from the

P2O5/SiO2 as an Efficient Reagent
1555
TABLE I Conversion of Aldehydes into Z-Aldoximes in the Presence
of P /SiO
2
O
5
2
◦
Bp/Torr or mp ( C)
Z-aldoximes
Yielda,b
(%)
1H chemical shift
of C(H) N group
Entry
R
Found
Reported [ref]
1208
1
2
3
4
5
6
7
8
9
Ph
95
94
90
78
75
88
80
85
75
87
65
8.15
8.47
8.1
8.2
8.1
8.5
8.1
8.1
8.2
8.1
7.6
118
89
130
101
145
70
1
1b
2-CH3O-C6H4
4-CH3O-C6H4
4-NO2-C6H4
4-Cl-C6H4
4-CH3-C6H4
4-Me2N-C6H4
4-HO-C6H4
2-HO-C6H4
4-HO-3-MeO-C6H3
2-furyl
88–90
8
132
100
146
8
8
8
72
1
1b
138
90
64
116
75
138–140
8
94
63
8
1
3
1
1
0
1
117
75–76
1
4
a
b
Isolated yield.
All the compounds give satisfactory spectral analysis (IR and 1H-NMR).
1
8
H-chemical shift of the C(H) N group which appeared around 8–8.5
as a singlet (Table I). It is very interesting that in these reactions nitriles
were not formed. Only in the case of these reactions coupled with mi-
crowave irradiation, the corresponding nitriles were obtained in high
1
2c
yields.
However, ketones such as benzophenone and acetophenone
did not afford the corresponding oximes under these conditions. In or-
der to show chemoselectivity of the presented reagent, a mixture of
one equivalent of aldehyde and one equivalent of ketone was treated
with two equivalents of hydroxylamine hydrochloride in the presence of
P2O5/SiO2 reagent at room temperature for 10 min. Only the aldehyde
was selectively converted to the corresponding oxime and ketones did
not react at all (Scheme 2). Therefore this methodology could be used
selectively for the preparation of aldoximes of compounds that contain
both aldehyde and ketone functional groups.
In conclusion, the reported procedure is an easy and novel method for
the preparation of aldoximes in solvent-free medium. Also this simple
and readily available reagent affords various aldoximes in a shorter
reaction time (10 min), with good to excellent yields (65–95%), and high
stereoselectivities (Z-isomers). Further studies of the new applications
of P2O5/SiO2 reagent in organic synthesis is in progress.
EXPERIMENTAL
All melting points recorded are uncorrected open-capillary mea-
surements. IR spectra were recorded on a Shimadzu-IR 470

1
556
H. Eshghi and Z. Gordi
SCHEME 2
1
spectrophotometer. H-NMR spectra were recorded on Bruker-80 MHz
and Bruker-500 MHz instruments using tetramethylsilane as an inter-
nal standard. Silica gel 60 (230–400 mesh) was obtained from Fluka
◦
and was dried in an oven at 120 C for 2 h. The P2O5/SiO2 reagent was
1
2
obtained according to the earlier reported procedure.
Preparation of Z-Aldoximes
General Procedure
In a typical reaction, a mixture of aldehyde (2 mmol), hydroxylamine
hydrochloride (4 mmol) and P2O5/SiO2 reagent (1 g) was ground thor-
oughly in a mortar for 10 minutes. Usually an immediate color change
was observed. The completion of the reaction was monitored by TLC
examination. After the completion of the reaction, 10 mL of 5% aque-
ous HCl was added to the mixture. The resulting solution was extracted
with CH2Cl2 (2 × 10 mL). The extracts were combined and dried over
CaCl2. Evaporation of the solvent under vacuum gave aldoximes with
1
high purity (based on TLC, H-NMR, IR, and melting point).
Z-2-Methoxybenzaldoxime
m.p.: 89 C (lit. [11b], 88–90 C); IR (KBr); 3200, 1625 cm−1. 1H-NMR
◦
◦
(
500 MHz, CDCl3): 8.82(bs, 1H), 8.47(s, 1H), 7.64(dd, 1H, J1 = 7.64 Hz,

P2O5/SiO2 as an Efficient Reagent
1557
J2 = 1.5 Hz), 7.35(dt, 1H, J1 = 7.70 Hz, J2 = 1.5 Hz), 6.96(t, 1H, J =
.48 Hz), 6.92(d, 1H, J = 8.35 Hz), 3.89(s, 3H).
7
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