ABSTRACT
A porphyrin-based photoexcited system has been revealed to be an efficient catalyst for d-limonene biotransformation under mild conditions and using molecular oxygen or/and H2O2 as oxidants. The influence of the oxidant, the wavelength of visible light, and the photoexcitation time on the catalytic system were studied for limonene oxidation with 5,10,15,20-tetraphenylporphyrin (H2TPP) as a catalyst. This porphyrin-catalyzed oxidation of limonene to three main products identified as carvone, an unknown product with a verbenone-like mass spectrum (1), and a (1S,4R)-p-mentha-2,8-diene 1-hydroperoxide (2). The highest conversion yield of these products was achieved at a very high molar ratio of H2TPP to limonene. The dependence of the biotransformation yield on the kind of solvent with different acceptor/donor electron properties was also investigated. Ethyl alcohol proved to be the best among the considered additives used for the reaction. Limonene photooxidation was not significantly dependent on wavelengths of visible light. It was concluded by UV-vis experiments that the reaction proceeds via a free-radical or/and molecular mechanism. Additional evidence for its radical nature was obtained from reactivity investigations. Maximal yield of carvone was obtained in the medium containing 90% of the substrate, within the period of 18 to 36 h of exposition to sunlight.
