Efficient epoxidation of terminal and internal alkenes using t-butylhydroperoxide catalysed by polybenzimidazole-supported Mo(VI)center dot(PBI center dot Mo)

摘要

A polybenzimidazole-supported Mo complex (PBI.Mo) has been prepared by a method already reported. Extensive investigation of digestion procedures has shown a dry-ashing method using NaNO3/HNO3 (conc.) at 560 degrees C to be an optimal method for preparing samples for Mo analysis by atomic absorption spectrophotometric methodology. Mo loadings in the range 1.32-0.62 mmol Mo g(-1) polymer were demonstrated. PBI.Mo has been used as a heterogeneous catalyst in the epoxidation of cyclohexene, methylene-cyclohexane, 4-vinyl cyclohexene, styrene, 1,3-pentadiene and allyl chloride, bromide and alcohol using t-butylhydroperoxide as the oxidant. The catalyst is very effective for the first four substrates, somewhat less active than soluble MoO(2)acac(2), but providing final yields and purity of products generally better than using MoO(2)acac(2). The 1,3-pentadiene displays an overall conversion of similar to 35% with a distribution of the four possible monoepoxide isomers similar to that obtained with MoO(2)acac(2) as catalyst. The allylic substrates showed poor conversion probably as a result of secondary (oligomerisation) reactions involving the epoxide products. Running the epoxidations for extended periods in air allows in situ generation of alkyl hydroperoxides in the case of cyclohexene and 4-vinylcyclohexene and these are then effective internal oxidants for further Mo catalysed epoxidation of these alkenes. When run under anaerobic conditions the reactions are very clean with no evidence of any free radical processes contributing. In all cases Mo leaching is minimal. Good activity is seen in the recycling of PBI.Mo in the case of styrene and 1,3- [GRAPHICS] pentadiene, although with cyclohexene and 4-vinylcyclohexene steady deactivation is seen, probably as a result of catalyst fouling. Thermogravimetric analyses suggest that it might be possible to burn off the foulant without destroying the catalyst. [References: 18]