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

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 a-pinene, beta-pinene, camphene, (R)-(+)-limonene and (S)-(-)-limonene was evaluated with a dichloro-dioxoBipy molybdenum((VI)) complex (Bipy = 2,2' -bipyridine-4,4'-dicarboxylato) anchored on TiO2 nanotubes (labeled as Mo((VI))Cl(2)O(2)Bipy/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 a-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))Cl(2)O(2)Bipy/TiO2-NT catalyst to the desired epoxides with high selectivities due to the OAT with O-2 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))Cl(2)O(2)Bipy/TiO2-NT catalyst was shown to act stereoselectively during the OAT process. The high stability of the Mo((VI))Cl(2)O(2)Bipy -catalyst under the reaction conditions was also ascertained by recycling studies.

Automated summary

Selective oxidation of terpenes using a Mo((VI))Cl(2)O(2)Bipy/TiO2-NT catalyst under photo-assisted conditions was successful, leading to high selectivities in producing epoxides from monoterpenes. The chiral center in limonene was found to govern the formation of diastereoisomers during the reaction process, indicating stereoselectivity of the catalyst. Additionally, the Mo((VI))Cl(2)O(2)Bipy/TiO2-NT catalyst demonstrated high stability under the reaction conditions, as shown by recycling studies.