Tetra-n-alkylammonium bromates - New and efficient reagents for deoximation

Article abstract of DOI:10.1039/b409319d

Tetra-n-alkylammonium bromates were prepared from the corresponding bromides for the first time and were used for deoximation of oximes to the corresponding carbonyl compounds.

Full text of DOI:10.1039/b409319d

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L E T T E R
Tetra-n-alkylammonium bromates—new and efficient reagents
for deoximation
Utpal Nath, Satya Sandhy Das, Dibakar Deb and Pranab Jyoti Das*
Department of Chemistry, Gauhati University, Guwahati, 781014 India.
E-mail: unath123@rediffmail.com, pjd123@sify.com
Received (in Toulouse, France) 18th June 2004, Accepted 14th September 2004
First published as an Advance Article on the web 15th November 2004
1
9
20
21
Tetra-n-alkylammonium bromates were prepared from the cor-
responding bromides for the first time and were used for deoxi-
mation of oximes to the corresponding carbonyl compounds.
oxirane, Mn(O-Ac)2 and t-butylhydroperoxide. Despite
the great improvements reported, these methods suffer from
the disadvantages of using harsh reaction conditions, costly
reagents and elaborate recovery procedures. Herein, we wish to
report that tetra-n-alkylammonium and tetra-n-butylammo-
nium bromates, prepared for the first time, act as a new class
of cost-effective reagents for oxidative deoximation (Scheme 1,
Table 1).
Quaternary ammonium salts are versatile phase-transfer
catalysts and some of them have been used to assist oxidation
of a variety of organic compounds using inexpensive primary
It is noteworthy that, unlike other methods of deoximation,
the major drawbacks of overoxidation of aldehydes to car-
boxylic acid and the formation of by-products resulting from
oxidation of the solvent were not observed. Camphor oxime
could be converted to camphor, a,b-unsaturated oximes were
deoximated without oxidation of the double bond. Further-
more, acid- and base-sensitive groups remained unaffected and
finally the solvent toluene was not oxidized. The water solu-
bility of the unreacted reagent and by-products made product
recovery easy. In conclusion, bromates, which are not used
extensively in the oxidation of organic substrates primarily due
to their insolubility in organic solvents, can be used for this
purpose when present as a counterion in quaternary ammo-
nium salts.
2
1
2
3
4
5,6
2 2 4
oxidants such as O , NaOCI, H O , KMnO and others.
Research efforts are now directed towards modifying the usual
quaternary ammonium salts and using them as reagents rather
than as catalysts. In our search for low-cost, selective yet
efficient and easy-to-handle oxidizing agents, we observed that
the tetra-n-alkylammonium bromides can be converted to the
corresponding bromates by the simple procedure of passing
chlorine gas through an alkaline solution of the bromide.
Using this simple procedure two such bromates, namely tetra-
n-propylammonium bromate and tetra-n-butylammonium
bromate, were prepared. These bromates are low-melting
solids with excellent storage stability and responded to all
7
analytical test for bromates. These bromates were found to
be soluble both in water and in organic solvents. Bromates are
seldom used as oxidants in organic synthesis, primarily because
of their insolubility in organic solvents. The favourable solu-
bility characteristics of these tetra-n-alkylammonium bromates
not only offered the opportunity for their use as primary
oxidants for organic substrates but also made product recovery
easy. As a first application of these versatile oxidizing agents,
we used these reagents for the oxidative deoximation of several
oximes to the parent carbonyl compounds.
Experimental
All reagents and solvents were purified by methods reported in
7
the literature. The quaternary ammonium bromides were
procured from E. Merck, Inc. Melting points were recorded
in an apparatus from Scientific Devices, India, type MP-D, in
1
open capillaries and H NMR spectra were recorded in an EM
360, 60 MHz spectrometer. C,H,N analysis of the bromates
were obtained from the facility available at RRL, Jorhat.
Oximes are crystalline compounds and are used for the
isolation, purification and characterisation of the parent car-
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,9
10
bonyl compounds. Besides protecting a carbonyl group,
oximation gives the product oxime, which can be used as
Conversion of the tetra-n-alkylammonium bromide to bromate
1
intermediates for the preparation of nitriles, of amides via
1
The bromide (0.01 mol; 3.14 or 3.70 g for the propyl or butyl,
respectively) was dissolved in 50 ml of 5% aqueous NaOH and
chlorine gas was passed through the solution till a yellow
the Beckmann rearrangement or to activate the carbonyl
1
2
group. Considering that oximes can be prepared from non-
carbonyl precursors and from compounds having active
(
orange for the butyl case) oil separated, which solidified on
1
3,14
methylene groups by reported methods,
deoximation be-
standing. The yellow (orange) solid was filtered and dried. On
neutralising the filtrate with dilute HCl, a second crop of the
yellow (orange) bromate was obtained.
comes a very important procedure not only for the regenera-
tion of the parent carbonyl but also for conversion of
0
0
compounds of the type Z–CH –Z to Z–CO–Z where Z and
2
Z are electron-withdrawing groups, thus leading to new meth-
ods for the preparation of carbonyl compounds from hydro-
carbon sites.
The classical acid-catalysed hydrolysis of oximes as a meth-
od of deoximation proved to be unsatisfactory, especially when
1
5
acid-sensitive functional groups are present in the molecule.
Some methods have been reported for the oxidative deoxima-
1
6
tion and notable among them are the use of 2,6DCPCC,
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Scheme 1 Deoximation of oximes with tetra-n-alkylammonium bro-
mate.
1
7
TMSCI,
3-carboxypyridine chlorochromate,
dimethyl-
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 4 2 3 – 1 4 2 5
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Table 1 Deoximation of various oximes to the carbonyl compounds
a
Reflux time /h
a
b
% Yield
I
M.p. or b.p. /1C
6
Entry
1
Substrate
Product
I
II
3
II
Obs.
48
Lit.
3
85
82
48
2
3
2.5
2.5
3
3
89
79
85
82
46
72
47
73
4
5
6
2.5
1.5
2.5
3
84
92
78
80
78
76
108
46
106
44
2
2.5
74
71
7
8
2.5
1.5
3
2
92
78
81
74
116
117
c
c
249
247
9
2.5
2.5
92
90
56
58
1
1
1
0
1
2
2.5
1.5
2.5
1.5
1.5
2
80
86
76
76
81
80
42
52
42
53
c
c
158
156
1
424
N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 4 2 3 – 1 4 2 5

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Table 1 (continued )
a
Reflux time /h
a
b
% Yield
I
M.p. or b.p. /1C
6
Entry
Substrate
Product
I
II
II
Obs.
97
Lit.
13
1
1.5
89
87
95
c
c
1
4
1.5
1.5
2
2
88
83
78
180
179
1
5
71
179
179
a
b
I denotes tetra-n-propylammonium bromate and II is with tetra-n-butylammonium bromate. Products identified by their m.p./b.p., H NMR
1
c
and IR. Boiling point.
1
Tetra-n-propylammonium bromate. M.p. 114–116 1C;
H
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1
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assistance.
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N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 4 2 3 – 1 4 2 5
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