Photo-assisted O-atom transfer to monoterpenes with molecular oxygen and a dioxoMo(VI) complex immobilized on TiO2 nanotubes

Abstract

Selective oxidation of terpenes is a key enabling route to produce value-added epoxides from renewable raw materials. In the present work, the photo-assisted oxygen atom transfer (OAT) to renewable monoterpenes, such as α-pinene, β-pinene, camphene, (R)-(+)-limonene and (S)-(-)-limonene was evaluated with a dichloro-dioxo-Bipy molybdenum(VI) complex (Bipy = 2,2′-bipyridine-4,4′-dicarboxylato) anchored on TiO2 nanotubes (labeled as Mo(VI)Cl2O2Bipy/TiO2-NT) using molecular oxygen as a primary oxidant under benign ambient conditions. Supported MoO3 on TiO2 nanotubes were also used for comparison purposes. Photooxidation of α-pinene with that MoO3/TiO2-NT catalyst resulted in a low conversion and in products distribution similar to that of the parent titania nanotubes, with the ketone as the main product due to a radical process. In contrast, all monoterpenes were successfully oxidized by the Mo(VI)Cl2O2Bipy/TiO2-NT catalyst to the desired epoxides with high selectivities due to the OAT with O2 and UV–vis light. Additionally, the photo-stimulated OAT to (R)-(+)- and (S)-(-)-limonene with the immobilized dioxo-Mo(VI) complex resulted in a diastereomeric excess (d.e.) for the cis-limonene-1,2-epoxide isomers relative to the trans isomers (d.e. = 36 and 34 %, respectively), thereby suggesting that the chiral center in the (R)-(+)- and (S)-(-)-limonene governs the formation of the diastereoisomer. Although not chiral, the Mo(VI)Cl2O2Bipy/TiO2-NT catalyst was shown to act stereoselectively during the OAT process. The high stability of the Mo(VI)Cl2O2Bipy -catalyst under the reaction conditions was also ascertained by recycling studies.