Abstract

The employing of tetravalent chromium oxide (Magtrieve™) in oxidation of some simple alcohols and hydrocarbons under microwave irradiation were shown. Many advantages of such new oxidation protocol comparing to conventional heating and using another oxidants were pointed out.

Keywords

Magtrieve™; microwaves; oxidation

Introduction

Microwave processing of materials offers distinct advantages over conventional heating in some applications [1]. The energy of the microwave field can be dissipated directly into the desired media, without the convection and conduction associated with conventional heating [2]. The introduction of microwave energy into a chemical reaction which has at least one component which is capable of coupling strongly with microwaves can lead to much higher heating rates than those which are achieved conventionally [3]. Additionally the reactants do not interact equally with the commonly used microwave frequency for dielectric heating and consequently selective heating can be achieve [4]. Since early eighties there is a lot of reports on conducting organic transformation in the presence of microwave fields. The oxidation reaction are also a part of these investigation (using permanganate, dichromate, hydrogen peroxide, persulphates and others oxidant [5]). Magtrieve™ is DuPont's trademark for the oxidant based on tetravalent chromium dioxide (CrO₂) [6].

       Scheme 1

In our research on oxidation processes, we chose Magtrieve™ as an oxidant, because it has been proven to be a useful oxidant in some reactions including the oxidation of alcohols [7]. Magtrieve™ as an oxidant is very well suited reagent for microwave synthesis, because as an ionic and magnetically retrievable material, it carries a benefit of so efficient converting of electromagnetic energy into heat according to dielectric heating mechanism.

       Scheme 2

The investigation were focused on the oxidation of alcohols to corresponding carbonyl compounds (illustrated on Scheme 1 by example of 2-octanol) and side-chain hydrocarbons to equivalent ketons (illustrated on Scheme 2 by example of fluorene).

Experimental

All experiments were carried out in multimode and monomode microwave reactors (Plazmatronika Poland). The reaction procedure involves simple mixing of 1g of the substrate (alcohol or hydrocarbon), 5g of Magtrieve™ and 20mL of toluene. The heterogenic mixture were placed in the reactor and irradiated during specific period of time (see Table 1) under reflux. The power of applying microwaves were set in order to keep the reaction mixture boiling. After completing of the reaction the oxidant were separated by the magnet and the solvent were evaporated resulting the crude product which was purified by distillation or crystallization.

Result and Discusion

As a result we have obtained a number of desired carbonyl product showed in the table 1 and 2.

Table 1. Microwave oxidation of alcohols by Magtrieve™

Substrate	Product	Yield [%]	Time [min]
		99	25
		73	30
		96	5
		65	20
		85	30

The satisfactory yields of conducted reaction shows that Magtrieve™ is an useful microwave-working oxidant. Because its ionic structure and magnetic properties Magtrieve™ is strongly coupled with microwave irradiation.

Table 2. Microwave side-chain oxidation of hydrocarbons by Magtrieve™

Substrate	Product	Yield [%]	Time [min]
		54	90
		25	90
		65	70
		86	60
		96	45

The experiment of irradiating of pure oxidant showed dramatic increase in temperature of the material up to 370�C in 2 minutes (Scheme 3) what was mesured by infrared camera. The addition of nonpolar solvent (toluene) prevents the substrate to keep of burning (what was observed in additional experiment where only the liquid substrat i. e. alcohol and oxidant were irradiated) but the temperature of the reaction mixture after 2 minutes of irradiation reaches the boiling point of the solvent and the surface of the oxidant has about 140�C (Scheme 4).

Scheme 3.Pure Magtrieve™ after 2minutes of irradiation		Scheme 4.Reaction mixture after 2minutes of irradiation

In conclusion we can say that, however we have used a transition metal (chromium) oxidant, the describing procedure is environmental friendly because of short reaction time, easy set-up and separation of oxidant. Additionally the recycling of the oxidant (described in literature) ranks the described method of oxidation as a powerful and "green" tool in modern organic synthesis.