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.

Graphic Abstract

中文翻译：

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（R）-（+）-和（S）-（-）-柠檬烯的选择性光环氧化通过TiO 2-纳米管上固定的手性和非手性二氧杂钼（VI）配合物进行

摘要

使用UV-Vis光和O ₂作为氧化剂，通过共价锚定在TiO ₂纳米管上的二氧钼^（VI）络合物选择性选择性环氧化了柠檬烯的（R）和（S）异构体。研究配体：联吡啶，三联吡啶和席夫碱对柠檬烯通过光刺激氧原子转移（OAT）的立体选择性环氧化的影响是有趣的。观察到OAT活性按以下顺序增加：Mo ^（VI） O ₂（席夫碱）/ TiO ₂ -NT <Mo ^（VI） Cl ₂ O ₂（联吡啶）/ TiO ₂ -NT <Mo ^（VI） Cl ₂ O₂（三吡啶）/ TiO ₂ -NT。显示了对具有“非手性”配体如联吡啶和三联吡啶的配合物的顺式异构体的中等非对映选择性。与以席夫碱为“手性”配体的配合物相反，它假定非金属双氧杂配在过渡态与三取代烯烃发生侧向相互作用，与非对称双诱导有关，非对映异构体过量增加（52％）。

图形摘要