Science of Synthesis

Katsuki, T., Science of Synthesis: Water in Organic Synthesis, (2012) 1, 63.

The above-described methods for epoxidation of allylic alcohols use alkyl hydroperoxides, which are monovalent, as the oxidants. Hydrogen peroxide, an atom-efficient oxidant, can be either a mono- or divalent ligand for complex formation.[‌43‌] Considering this fact, it was discovered that chiral μ-oxo[(oxo)niobium–salan]2 complexes, in which the niobium ion is pentavalent, serve as catalysts for the asymmetric epoxidation of allylic alcohols using urea–hydrogen peroxide adduct (UHP), although enantioselectivity is at best moderate to good (68–83% ee). This epoxidation is carried out under nonaqueous conditions because of the instability of the catalyst toward aqueous conditions.[‌44‌] In the ensuing study, it was found that the active species of this epoxidation is not dimeric but rather monomeric and that the epoxidation with a preprepared catalyst has an induction period, while the epoxidation with an in situ prepared catalyst does not. This suggests that dissociation of the preprepared dimeric complex to a catalytically active monomeric complex is slow under the reaction conditions. Indeed, in situ prepared catalysts from niobium(V) isopropoxide and the corresponding salan ligand ‌22‌ or ‌23‌ catalyze epoxidation using 30% aqueous hydrogen peroxide in a mixed medium of organic solvent/brine with higher enantioselectivity (74–95% ee) (‌Scheme 12‌).[‌45‌] The niobium–salan complexes slowly decompose in the presence of hydrogen peroxide. As such, better yields are obtained by adding hydrogen peroxide dropwise.

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References