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

Abstract

Selective epoxidation of the (R) and (S) isomers of limonene by dioxomolybdenum(VI) complexes anchored covalently on TiO2 nanotubes using UV–Vis light and O2 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: Mo(VI)O2(Schiff base)/TiO2-NT < Mo(VI)Cl2O2 (bipyridine)/TiO2-NT < Mo(VI)Cl2O2(terpyridine)/TiO2-NT. Moderate diastereoselectivity to the cis-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.