the production of thymoquinone from thymol and carvacrol

Oxidations with H2O2 can involve homolytic pathways via free radical
intermediates and/or heterolytic oxygen transfer processes as seen in Figure 5.2
(Sheldon et al. 1998). In Figure 5.2, carvacrol is used as reactant which is the major
monoterpene component of many essential oil.
Heterolytic oxygen transfer process can be divided into two types based on the
active intermediate: a peroxometal or an oxometal complex. Peroxometal pathways
usually involve early transition elements with d 0 configuration, e.g. Mo(VI), W(VI),
V(V), Ti(IV). Late or first row transition elements, e.g. Cr(VI), V(V), Mn(V), Ru(VI),
Ru(VIII), Os(VIII), generally employ oxometal pathways. Some elements, e.g.
vanadium, can employ oxometal or peroxometal pathways depending on the reactant
(Arends et al. 2001).
In the presence of transition metal complexes oxidation reactions with hydrogen
peroxide are direct activation (unproductive oxidation) and productive oxidation
reactions as seen in Figure 5.3 and water is only by product. Catalysts (transition metal
complexes) are transferred oxygen from hydrogen peroxide to the reactant or substrate
by a homolytic cleavage of the metal oxygen bond (Salem et al. 2000).
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