Table 2 Oxidation of 4-nitrobenzyl alcohol (1 mmol) with different
amounts of NaNO2–Ac2O
An oxidation study of 4-nitrobenzyl alcohol with different
amounts of NaNO
2–Ac2O is shown in Table 2. If a catalytic
amount of NaNO2–Ac2O is used for oxidation of 4-nitrobenzyl
alcohol, the reaction does not go to completion. Three equiv-
alents of reagent are required for complete oxidation of
4-nitrobenzyl alcohol.
In conclusion, the present results demonstrate the novelty of
the NaNO2–Ac2O system which shows unique selectivity and
constitutes a useful alternative to the commonly accepted
procedures for the oxidation of primary, allylic and benzylic
alcohols. Moreover, this simple, inexpensive and rapid oxid-
ation technique affords products in good yields without
overoxidation to carboxylic acids.
Amount of
NaNO2/mmol
Yield of 4-nitro-
benzaldehyde (%)
Entry
1
2
3
4
5
6
0.5
1.0
1.5
2.0
2.5
3.0
40
55
70
80
87
92
The oxidation of alcohols into carbonyl compounds is an
important transformation in organic synthesis.1 Although a
large number of reagents are known in the literature2 for such
transformations there still appears a need either to improve the
existing oxidation methods3 or to introduce newer reagents4 to
permit better selectivity under milder conditions. More recently,
some novel reagents5 have been used for this purpose to get
relatively better results. Nitronium and nitrosonium fluoro-
borates are also reported as efficient reagents for similar
oxidations6–8 which are fast and exothermic.
We now report for the first time that NaNO2 in acetic
anhydride is a remarkably effective reagent for the rapid and
selective oxidation of a variety of primary, allylic and benzylic
alcohols into their corresponding carbonyl compounds under
mild conditions.
Experimental
A typical procedure for oxidation of 4-nitrobenzyl alcohol: a
mixture of NaNO2 (1035 mg, 15 mmol) and 4-nitrobenzyl alco-
hol (5 mmol) in acetic anhydride (2 mmol) was stirred. A spon-
taneous exothermic reaction took place which was controlled
by cooling the mixture in ice. After completion of the reaction
(TLC), the product was extracted with ether (2 × 10 ml).
Removal of the solvent under reduced pressure afforded crude
4-nitrobenzaldehyde which was further purified by column
chromatography (petroleum ether–ethyl acetate, 9:1).
Acknowledgements
NaNO2–Ac2O
RCH2OH
RCHO
VSS thanks CSIR, New Delhi for a junior research fellowship.
25 ЊC, <1 min
The rapid and selective formation of oxidation products
demonstrates the efficiency of this new method. In all cases, the
reactions are completed in <1 min. The results are summarized
in Table 1. Interestingly, no overoxidation to carboxylic acids
is observed (entries 1–15). Primary aliphatic alcohols (entries
2,13–15), a primary aliphatic diol (entry 14), allylic alcohols
(entries 12, 21–22) and benzylic alcohols (entries 1, 3–11) are
selectively oxidized to the corresponding carbonyl compounds.
No isomerization of double bonds is observed in the oxidation
of the α,β-unsaturated alcohols (entries 12, 21–22). Further-
more, cyano (entry 4), tert-amino (entry 6), THP ether (entry
17) and TBDMS (entry 18) groups remain unchanged during
the oxidation. In addition, it is worth commenting that second-
ary alcohols like cyclohexanol, menthol and benzoin are not
oxidized to the corresponding ketones under these reaction
conditions. It is also important to note that a primary alcoholic
group is oxidized selectively in the presence of a secondary
benzyl alcoholic group (entry 19) under these reaction condi-
tions. Therefore, it is a method of choice for the oxidation of
primary, allylic and primary benzylic alcohols in the presence
of secondary alcohols. Heterocyclic alcohols (entries 24, 25) are
also selectively oxidized without oxidation of heteroatoms like
N, S.
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Bandgar, Babasaheb P.
