Methyltrioxorhenium catalyzed aerobic oxidation of organonitrogen compounds

Article abstract of DOI:10.1016/S0040-4039(03)00639-7

A variety of tertiary, secondary and primary organonitrogen compounds have been efficiently and selectivity oxidized to their corresponding N-oxides, nitrones, and nitro compounds with molecular oxygen using methyltrioxorhenium as catalyst.

Full text of DOI:10.1016/S0040-4039(03)00639-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 3235–3237
Methyltrioxorhenium catalyzed aerobic oxidation of
organonitrogen compounds
Vishal B. Sharma, Suman L. Jain and Bir Sain*
Chemical and Biosciences Division, Indian Institute of Petroleum, Dehradun 248005, India
Received 13 January 2003; revised 25 February 2003; accepted 7 March 2003
Abstract—A variety of tertiary, secondary and primary organonitrogen compounds have been efficiently and selectivity oxidized
to their corresponding N-oxides, nitrones, and nitro compounds with molecular oxygen using methyltrioxorhenium as catalyst.
© 2003 Elsevier Science Ltd. All rights reserved.
The methyltrioxorhenium (MTO)/H2O2 oxidation sys-
tem first reported by Herrmann and co-workers1 in
1991 for epoxidation of olefins has emerged as an
efficient catalytic system for a variety of oxidation
reactions.2 Owing to its ease of synthesis, commercial
availability and stability in air, MTO has proved to be
a versatile catalyst for hydrogen peroxide oxidations in
both organic and aqueous solvents. Molecular oxygen
is an attractive oxidant from both environmental and
economic viewpoints,3 however, to the best of our
knowledge there is only one literature report on methyl-
trioxorhenium-catalyzed oxidation using molecular
oxygen as oxidant, which described the oxidation of
tertiary phosphines to phosphine oxides.4
gen as the sole oxidant where tertiary amines are
oxidized to N-oxides, and secondary and primary
amines to nitrones and nitro compounds, respectively
(Scheme 1).
The oxidations were carried out in refluxing acetonitrile
with molecular oxygen using 1 mol% of methyltri-
oxorhenium as catalyst.9 All the nitrogen compounds
studied were smoothly converted to their corresponding
oxidized products in very good yields. The results are
summarized in Table 1. As shown in the Table, two
aromatic primary amines were oxidized selectively to
the corresponding nitroarenes (entries 7 and 8), two
secondary amines were converted to their correspond-
ing nitrones (entries 9 and 10) and two tertiary aro-
matic amines and various pyridines were converted to
their N-oxides (entries 1–6) in near quantitative yields
without any evidence for the formation of other by-
products. Among the various nitrogen compounds
studied tertiary nitrogen compounds were found to be
the most reactive and required less time for their
oxidation.
Oxidation of organonitrogen compounds is important
both from industrial and synthetic viewpoints as N-
oxides obtained by oxidation of tertiary nitrogen com-
pounds find wide application as oxidants, and offer
functional group manipulation and structural modifica-
tion possibilities not accessible by other methods.5
Nitrones obtained by oxidation of secondary amines
are versatile synthetic intermediates for the synthesis of
heterocycles and natural products,6 and nitro, azo and
azoxy compounds obtained by oxidation of primary
amines are important synthetic intermediates.7 In con-
tinuation of our studies on oxidation with molecular
oxygen as the primary oxidant,8 we report herein for
the first time the methyltrioxorhenium catalyzed oxida-
tion of organonitrogen compounds with molecular oxy-
To evaluate the effect of solvents, the aerobic oxidation
of 4-picoline was carried out under similar conditions
using different solvents such as acetonitrile,
dichloroethane, ethanol and methanol. Acetonitrile and
dichloroethane were found to be good solvents for this
transformation, while the oxidation was found to be
Keywords: methyltrioxorhenium; molecular oxygen; oxidation;
organonitrogen compounds.
* Corresponding author. Tel.: +91-135-2660071; fax: +91-135-
2660202; e-mail: birsain@iip.res.in
Scheme 1.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00639-7

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V. B. Sharma et al. / Tetrahedron Letters 44 (2003) 3235–3237
Table 1. Oxidation of nitrogen compounds with the methyltrioxorhenium/O2 systema

V. B. Sharma et al. / Tetrahedron Letters 44 (2003) 3235–3237
3237
very slow in ethanol and methanol and the reaction was
not complete even after 24 h. The catalytic effect of
methyltrioxorhenium was also investigated using 4-
picoline as the representative example. In the absence of
catalyst, the reaction had not begun even after 12 h,
while increasing the catalyst concentration from 0.5 to
2 mol% had only a marginal effect in decreasing the
reaction time, indicating the effectiveness of catalyst
even at low concentration.
Cope, A. C.; Trumbull, E. R. Org. React. 1960, 11,
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In summary, the present method describes the second
use for an oxidation using the methyltrioxorhenium/O2
system. The versatility of the catalyst towards a variety
of nitrogen compounds, simple work-up, excellent
yields and selective synthesis of the products make this
procedure an attractive, environmentally friendlier syn-
thetic tool for the oxidation of various nitrogen com-
pounds by molecular oxygen.
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