Epoxidation vanadium-alkyl peroxide catalysts

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

Sharpless advanced this notion through using a metal-based epoxidation catalyst (Scheme 4.18) [44]. This approach took advantage of the capacity of VO(acac)j to coordinate simultaneously to alcohols and peroxides and promote regioselective epoxidations of allylic and homoal-lylic alcohols with iBuOOH as the oxidant, as shown by the conversion of 87 to 88. This approach also provides stereoselectivity, as shown by the transformation of 89 to 90 in which the hydroxyl group directs a yn-oxidation of the alkene. This procedure is selective because alkyl peroxides generally do not epoxidize alkenes but can be activated by coordination to the alcohol-coordinated vanadium Lewis acid. 

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