Carvacrol is a monoterpene which can be extracted from essential oils of plants such as oregano (Origanum vulgare). Although carvacrol has some interesting physiological properties (i.e. being anxiolytic-like), some of its derivatives, such as thymoquinone, present antitumor, hepatoprotective, and anticonvulsant activities, thus being a more valuable product than carvacrol itself. Due to the fact that thymoquinone can only be isolated from few species, its obtainment through carvacrol conversion employing low-cost reagents is of great interest. In this context, metalloporphyrins arise as promising catalyst candidates, as they can achieve high substrate conversion under mild temperature and pressure conditions. This work presents carvacrol oxidation by iodosylbenzene (PhIO) or iodosylbenzene diacetate [PhI(OAc)2] catalyzed by a classic manganese porphyrin, [MnIII(TPP)Cl] (MnP), while employing green solvents (ethyl acetate or dimethyl carbonate). The reactions were performed in microscale, under magnetic stirring, at 25 °C, for 90 minutes, using either dichloromethane (DCM), ethyl acetate (EtAc) or dimethyl carbonate (DMC) as solvent in a molar ratio 1:10:10 MnP:oxidant:carvacrol. The reaction mixtures were analyzed by gas chromatography and thymoquinone was identified through comparison with authentic samples. Carvacrol conversion neared 100% in systems with PhIO, and oscillated between 92% and 94% in systems with PhI(OAc)2. Regarding oxidant PhIO, the system in DCM has shown a yield of 7% of thymoquinone, whereas the systems in EtAc and DMC achieved yields of 1% and 0%, respectively. In systems with PhI(OAc)2, thymoquinone was obtained with 6% yield in DCM, 1% in EtAc and 0% in DMC. In general, despite the great substrate conversion, thymoquinone yields were low, suggesting that other products are being formed in the reaction – especially in systems with green solvents. Gunay et al. (2016) reported isolating two polymeric products out of thymol and carvacrol oxidation, which may present interesting chemical and/or biological properties that have yet to be studied. The products of our reactions are currently being analyzed by GC-MS whether to confirm or not the polymers (as well as other compounds with lower areas in the chromatogram) formation. This way, [MnIII(TPP)Cl] has shown great catalytic performance; despite the lack of selectivity towards thymoquinone, other formed products are potential precursors to important chemical substances.

Mn(III)-porphyrin as catalyst in carvacrol oxidation by iodosylarenes under an eco-friendly approach

2019 Spring National Meeting

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