The hydroxylation of some simple cyclic alkenes i.e. cyclohexene and cyclooctene by hydrogen peroxide using microwaves as an energy source is described. The reaction results in the introduction of both hydroxy and alcoxy group depending on the alcohol used as a solvent.

 

The introduction of hydroxyl group into the organic molecules results in the obtaining of the very interesting group of compounds, which finds many application as both final products and intermediates [1] and was carried out in many different maners including apllication of hydrogen peroxide as the oxidant and tungstic acid as the catalyst [2]. The one of most interesting compounds on these field is cyclohexene, which could be transformed into w-caprolactame (via a cyclohexanol stage) or directly into adypic acid [3]. Unsaturated compounds could also be oxided into the corresponding oxiranes and the selectivity of the reaction depends on the reaction conditions [4].

From the other hand the application of microwaves in synthetic organic chemistry shown recently in many publication [5] leads to the conclusion, that this kind of the energy transport could have a big influence on both rate of the reaction and its selectivity. Such phenomenon is a consequence of the interaction of microwaves with the matter by dielectric and conducting mechanism [6]. As a continuation of our earlier research on microwave assisted oxidation reactions [7] we now report the microwave hydroxylation of some simple olefins using hydrogen peroxide as the oxidant and solid tungstic acid as a catalyst.

All the chemicals were purchased from Aldrich and used as received and the reactions were carried out in two phase system containing solid tungstic acid and the solution of hydrogen peroxide and substrate in different alcohols (Scheme 1). The processes were carried out in a multimode and monomode 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.



The comparable experiments were performed applying conventional conditions (thermostated water bath). All the experiments were carried out using tempereature and power programs listed in Table 1. In the hydroxylation of cyclohexene in methanol, which is representative for all the reactions the cyclohexene (25mmol) were dissolved in the methanol (20ml).


Then solid tungstic acid were added (1,3mmol). The mixture was then irradiated by microwave or heated conventionally and after its reaches 60°C, hydrogen peroxide (26mmol, 2,65ml of 30% water solution) were added. The mixture were then stirred and irradiated (MW)/heated up (conventionally) to the boiling under the reflux. After the reaction is finished the mixture were extracted by ether. The organic layer was evaporated resulting the crude mixture of 1,2-dihydroxycyclonehexan and 1-hydroxy-2-methoxy-cyclohexan. Products were separated by the vacuum destilation and characterized by ftir, HNMR and MS spectroscopy.