Selective Photo-epoxidation of (R)-(+)- and (S)-(-)-Limonene by Chiral and Non-Chiral Dioxo-Mo(VI) Complexes Anchored on TiO2-Nanotubes

Selective epoxidation of the (R) and (S) isomers of limonene by dioxomolybdenum((VI))complexes anchored covalently on TiO(2)nanotubes using UV-Vis light and O(2)as the oxidizing agent was evaluated. It is interesting to study the effect of the ligands: bipyridine, terpyridine, and Schiff base on the stereoselective epoxidation of limonene through photostimulated oxygen atom transfer (OAT). OAT activity observed to increase in the following order: (MoO2)-O-(VI)(Schiff base)/TiO2-NT < (MoCl2O2)-Cl-(VI)(bipyridine)/TiO2-NT < (MoCl2O2)-Cl-(VI)(terpyridine)/TiO2-NT. Moderate diastereoselectivity to thecis-isomer of complexes with non-chiral ligands like bipyridine and terpyridine was displayed. Contrary to the complex with the Schiff base as chiral ligand, it showed an increase in diastereoisomeric excess (52%), associated to an asymmetric double induction, assuming a complex metal-oxo sideway interaction with the trisubstituted olefins at the transition state. [GRAPHICS] .

Automated summary

The selective epoxidation of limonene using dioxomolybdenum((VI)) complexes anchored on TiO(2) nanotubes was studied, showing the influence of ligands on stereoselectivity. Complexes with non-chiral ligands exhibited moderate diastereoselectivity towards the cis-isomer of limonene. Contrary to this, the complex with Schiff base as chiral ligand showed an increase in diastereoisomeric excess, indicating an asymmetric double induction.