Vanadium-promoted epoxidation

The vanadium-promoted epoxidation method has proved to be valuable in synthetic routes leading to dl-Ci8 Cecropia juvenile hormone20 and lasalocid A.24 The mechanism of vanadium-mediated epoxidation has been elucidated25 but an evaluation of the use of other organometallic compounds as epoxidation reagents is required.26... 

Aiming at easier workup conditions, immobilization of several transition metal catalysts, which show activity for the epoxidation of allylic alcohols, on polymer support has been investigated. For example, Suzuki and coworkers incorporated an oxo-vanadium ion into cross-linked polystyrene resins functionalized with iminodiacetic acid or diethylenetri-amine derivatives (Scheme 57), which afforded a heterogeneous catalyst that can promote... 

Oxovanadium(V) and oxomolybdenum(VI) were incorporated into crosslinked polystyrene resins functionalized with iminodiacetic acid or diethylenetriamine derivatives 921 The polymer complexes were used as catalysts in the oxidation of olefins with f-butylhydroperoxide. Vanadium(V) complexes promote the epoxidation of allylic alcohols in a highly regioselective manner, e.g., 2,3-epoxide was obtained in 98 % selectivity from e-geraniol at 80 °C. The catalytic activity of the vanadium(V) complexes is generally higher than that of the molybdenium(VI) complexes in the oxidation of allylic alcohols, whereas an opposed trend holds for the epoxidation of cyclohexene. 

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. 

---