The microwave assisted oxidation of nitrogen containing heterocyclic compounds was presented. The hydrogen peroxide was used as an oxidant employing sodium tungstate as a catalyst and the reaction was carried out in the multimode microwave reactor, at the boiling of the reaction mixture. As a result the appropriate N-oxide with a very good yield and purity were obtained.

The recent development in so called "green chemistry" shows that alternative methods of carrying out chemical transformation could minimize the environmental harmfulness of classical reactions. One of the most popular and interesting approach on this field is employing the microwave energy for conducting many chemical transformations [1]. Using this method the reaction mixture is irradiated by microwaves with the frequency of 2,45GHz (usually). The interaction of the matter with such kind of electromagnetic waves results in: higher speed of heating (because of volumetric heating) [2], different distribution of temperature [3] and in the case of the mixture containing compounds which differ with polarity in selective heating of more polar molecules [4]. Examples of microwave assisted synthesis already described in the literature show much shorter reaction time and very often the higher selectivity of desired products [1].

From the other hand the application of hydrogen peroxide as an oxidant could also improved the environmental aspect of organic syntheses becouse of its high oxidation potential and non-toxic reduction product (i.e. water) [5]. The combination of accelerating of chemical reactions and using such "environment friendly" reagents (oxidants) seems to enlarge the field of "green organic chemistry".

As a continuation of our earlier research on microwave assisted oxidation reactions [6] we now report the microwave oxidation of some azaheterocyclic compounds using hydrogen peroxide as the oxidant (shown on Scheme 1 for oxidation of quinaldine) which is a very important transformation on the field of obtaining substituted azaheterocyclic compounds.

All the chemicals were purchased from Aldrich and used as received. The reactions were carried out in a multimode microwave reactor with a continuous power regulation (PLAZMATRONIKA, Poland), which is equipped with magnetic stirrer and two inlets on the top and one side of the reactor. The inlets allowed applying an upright condenser and introducing a fibre-optical sensor (ReFlex, Nortech) which was used to control temperature during microwave experiments. IR spectra were recorded on FT-IR BIORAD FTS-165 spectrophotometer as liquids on NaCl disks. H-NMR spectra were collected on Tesla 487C (80MHz) spectrometer using TMS as an internal standard. GC/MS spectra were determined on GC/MS 5890 SERIES II HEWLETT-PACKARD gas chromatograph equipped with Ultra 2 (25m x 0.25mm x 0.25 mm) column with HEWLETT-PACKARD 5971 Series Mass Selective Detector.

General method for the oxidation
All the reactions were carried out according the oxidation procedure given for quinaldine, which was representative of the general procedure employed for microwave conditions. Quinaldine (20mmol, 2,86g) was placed in a 50 mL round-bottom reaction flask. Then hydrogen peroxide (30mmol, 1,02g) as a 30% water solution was added following the sodium tungstate (0,3mmol 0,1g) which was used as a catalyst. Next the magnetically stirred suspension was irradiated (250W of microwave power) in an open vessel at reflux using multimode microwave reactor (Plazmatronika, Poland) during 30min. After completion of the process products was isolated by means of high performance liquid chromatography what afford 2,38g (75%) of quinaldine N-oxide. In order to proove the structure of the product the following characterisation were done: FTIR, HNMR and MS.

As a result of carried out experiments the appropriate N-oxides of the azaheterocyclic compounds were obtained with the yield listed in a Table 1.



The obtained results lead us to the conclusion that the microwave process of oxidation of azahetrocyclics using hydrogen peroxide as an oxidant could be very useful for obtaining of apriopriate N-oxides even the yields of the reactions do not exceed 80%. The simple set-up of described transformation, employing the environmental source of energy and toxicless oxidant seems to give the prompt for improving the methodology in order to rise the yields of the reaction.