Dowex polymer-mediated protection of carbonyl groups

Article abstract of DOI:10.1080/00397910500184651

Dowex (strongly acidic cationic exchange resin) polymer is employed as solid acid catalyst for the clean and less hazardous protection of carbonyl compounds as phenylhydrazones and 2,4-dintrophenylhydrazones in ethanol under reflux conditions. The reactions proceed very smoothly and the yields of the derivatives are excellent. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1080/00397910500184651

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Dowex Polymer–Mediated
Protection of Carbonyl Groups
Khodabakhsh Niknam a , Ali Reza Kiasat b & Sadegh
Karimi a
a Chemistry Department, Faculty of Sciences ,
Persian Gulf University , Bushehr, Iran
b Chemistry Department, College of Science , Shahid
Chamran University , Ahvaz, Iran
Published online: 18 Aug 2006.
To cite this article: Khodabakhsh Niknam , Ali Reza Kiasat & Sadegh Karimi (2005)
Dowex Polymer–Mediated Protection of Carbonyl Groups, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry,
35:17, 2231-2236
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Synthetic Communicationsw, 35: 2231–2236, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500184651
Dowex Polymer–Mediated Protection of
Carbonyl Groups
Khodabakhsh Niknam
Chemistry Department, Faculty of Sciences, Persian Gulf University,
Bushehr, Iran
Ali Reza Kiasat
Chemistry Department, College of Science, Shahid Chamran University,
Ahvaz, Iran
Sadegh Karimi
Chemistry Department, Faculty of Sciences, Persian Gulf University,
Bushehr, Iran
Abstract: Dowex (strongly acidic cationic exchange resin) polymer is employed as solid
acid catalyst for the clean and less hazardous protection of carbonyl compounds as phe-
nylhydrazones and 2,4-dintrophenylhydrazones in ethanol under reflux conditions. The
reactions proceed very smoothly and the yields of the derivatives are excellent.
Keywords: Dowex polymer, carbonyl compounds, 2,4-dinitrophenyl hydrazones,
phenyl hydrazones, protection
The main advantage of polymer-supported solid-phase organic reagents
over their monomeric counterparts is the easy separation of the excess reagent
and by-product from the desired reaction product.[1] The attachment of the
reagent to the insoluble macromolecular matrix can also solve the problems
of labile nature, toxicity, or odor often concerned with nonpolymeric
low-molecular-weight reagents.[2] In addition, in most cases the used
Received in the UK October 19, 2004
Address correspondence to Khodabakhsh Niknam, Chemistry Department, Faculty
of Sciences, Persian Gulf University, Bushehr 75168, Iran. E-mail: kh_niknam@
yahoo.com
2231

2232
K. Niknam, A. R. Kiasat, and S. Karimi
polymeric reagents can be regenerated. These advantages of insoluble polymeric
reagents have been reported in recent years for organic synthetic purposes.[1]
Protection of carbonyl groups is often a necessary requirement in
reactions involving substrates with multifunctional groups.[3,4] Derivatives
of carbonyl compounds such as phenylhydrazones and 2,4-dintrophenylhydra-
zones not only are used for the characterization and purification of carbonyl
compounds but also play an important role in the protection of carbonyl
compounds.[5] Phenylhydrazones are largely oils or possess low melting points
for many aliphatic aldehydes and ketones, but are generally crystalline for
aromatic carbonyl compounds and also for cycloaliphatic and heterocyclic
aldehydes and ketones. 2,4-Dinitrophenylhydrazones are generally crystalline
and are therefore eminently more suitable than phenylhydrazones for the
detection, characterization, and protection of carbonyl compounds.[6]
The formation of these derivatives involves Brønsted as well as Lewis
acid catalysis.[5,7] Because the conventional acids used in the preparation of
these derivatives are highly corrosive, they pose severe environmental hazards.
Although a protecting system consisting of phenylhydrazine or 2,4-dinitro-
phenylhydrazine and acidic zeolite in hexane/methanol was recently reported[3]
for the protection of carbonyl compounds, a literature survey reveals that no
attention has been focused on the protection of carbonyl compounds as
phenylhydrazones or 2,4-dintrophenylhydrazones using polymeric reagents.
Herein, we report a smooth and facile conversion of carbonyl compounds
to the corresponding phenylhydrazones and 2,4-dinitrophenylhydrazones in
the presence of a functionalized polymer (Dowex, strongly acidic) as catalyst
with phenylhydrazine and 2,4-dinitrophenylhydrazine in ethanol under reflux
conditions (Scheme 1).
The feasibility of the present protection of carbonyl compound was first
examined using benzaldehyde as a model substrate. Thus, benzaldehyde
was added to a mixture of 2 g of Dowex polymer and ethanol as solvent.
The mixture was refluxed for 45 min when phenylhydrazine was used as pro-
tecting agent and 60 min when 2,4-dinitrophenylhydrazine was used (Tables
1 and 2). We examined the catalytic ability of Dowex for protection of
carbonyl compounds as phenylhydrazones and 2,4-dintrophenylhydrazones.
This catalyst acted very efficiently, and it was observed that 2 g of the
polymer is sufficient to convert carbonyl compounds to their corresponding
arylhydrazones in excellent yields within 45–75 min. It is worth mentioning
Scheme 1.

Dowex Polymer–Mediated Protection of Carbonyl Groups
2233
Table 1. Dowex polymer–catalyzed conversion of carbonyl compounds to
corresponding phenylhydrazonea,b
Time
(min)
Yield
(%)
Entry
Substrate
Product
1
2
45
45
95
93
3
50
91
4
5
6
7
8
45
50
45
55
55
91
89
88
86
87
9
55
88
aYields refer to isolated products.
bProducts were characterized by comparison of their physical data, IR spectrum, and
NMR spectrum with known samples.
that the lower quantities of polymer (i.e., 0.5 g) also gave satisfactory results
with longer reaction times.
With the first successful result in hand, protection of other aldehydes and
ketones with phenyl or aryl hydrazine as protecting agents was carried out in
the presence of Dowex polymer as catalyst under similar reaction conditions.
Satisfactory results have been obtained in the reaction of variety of aromatic
and aliphatic aldehydes and ketones with phenylhydrazine and 2,4-dinitro-
phenylhydrazine in refluxing ethanol in the presence of Dowex polymer as
catalyst. However, the result of the hindered aliphatic ketones such as diiso-
propylketone was not satisfactory. Acid-sensitive substrates such as furfural
and cinnamaldehyde are also protected as 2,4-dinitrophenylhydrazone in

2234
K. Niknam, A. R. Kiasat, and S. Karimi
Table 2. Dowex polymer–catalyzed conversion of carbonyl compounds to
corresponding 2,4-dinitrophenylhydrazonesa,b,c
Time
(min)
Yield
(%)
Entry
Substrate
Product
1
2
60
60
96
95
3
65
91
4
5
63
65
92
90
6
7
8
65
60
65
91
91
90
CH3CHO
CH3CH¼NNHAr
9
10
11
65
70
70
89
89
90
12
75
89
13
14
CH3COCH3
70
70
87
88
aYields refer to isolated products.
bProducts were characterized by comparison of their physical data, IR spectrum, and
NMR spectrum with known samples.
cAryl is 2,4-dinitrophenyl.

Dowex Polymer–Mediated Protection of Carbonyl Groups
2235
very good yields (Table 2, entries 8, 9), without the formation of any side
products. It can be emphasized that the reaction is clean and the polymeric
acidic catalyst is regenerated and reusable without decrease in efficiency,
and so from the economical and environmental points of view, the use of
polymeric acidic catalyst is an improvement.
In summary, we have demonstrated an efficient, mild, and easy protection
methodology of the carbonyl group using phenylhydrazine and 2,4-
dinitrophenylhydrazine in the presence of Dowex polymeric acidic catalyst
in refluxing ethanol. The polymeric acidic catalyst has inherent advantages,
including operational simplicity, filterability, regenerability, and reuse.
EXPERIMENTAL
Dowex polymer (strongly acidic cationic exchange resin) (50-X8), aldehydes,
and ketones were purchased from Fluka, Aldrich, and Merck. All products are
known compounds and are identified by comparison of their physical and
1
spectral data with those of authentic samples. The H NMR spectra were
recorded on a Bruker Avance DPX instrument (250 MHz). IR spectra were
recorded on a Shimadzu 450 spectrophotometer. The purity determination of
the products and reaction monitoring were accomplished by TLC on silica-gel
polygram SILG/UV 254 plates.
GENERAL PROCEDURES
Dowex (strongly acidic cationic exchange resin) polymer (2 g) was added to a
solution of carbonyl compound (1 mmol) with reagent (phenylhydrazine or
2,4-dinitrophenylhydrazine) and (1.1 mmol) in ethanol (15 mL) as solvent.
The reaction mixture was refluxed with stirring for a specified time. The
progress of the reaction was monitored by TLC using n-hexane-ethyl acetate.
The polymer was filtered off and washed with warm ethanol. Then, the filtrate
was evaporated to give the product, which was recrystallized from a suitable
solvent in 86–96% isolated yields.
REGENERATION OF POLYMER
The spent polymer could be regenerated easily without any loss of efficiency
by putting it in HCl (6 M) for at least 6 h, and then filtering and air drying it.
ACKNOWLEDGMENTS
Authors acknowledge the partial support of this work by Persian Gulf Univer-
sity Research Council.

2236
K. Niknam, A. R. Kiasat, and S. Karimi
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