| Popis: |
Like styrene, α-methylstyrene and cis- and trans-β-methylstyrene are oxidised by lead tetra-acetate by concurrent homolytic and heterolytic pathways. The relative significance of the two types of reaction is dependent on the conditions. For α-methylstyrene, the relative importance of the homolytic reaction is much smaller than for styrene, at least in part because the heterolytic reaction is so much faster (krel.= 19), whereas for the β-methylstyrenes both types of reaction are much slower than for styrene. The heterolytic reaction occurs by way of an adduct for which there are competitive reaction paths, namely, the migration of the phenyl group and the formation of an acetoxonium ion; for α-methylstyrene, the former dominates whereas, for the β-methylstyrenes, products to be expected from phenyl-migration could not be detected, and an explanation for this difference is suggested. The acetoxonium ion can be opened by reaction with water to give hydroxyacetates, and this has enabled the stereochemistry of the heterolytic reactions of the β-methylstyrenes with lead tetra-acetate to be studied; the results are compared with those for oxidation by the ‘wet’ Prevost procedure. Autoxidation is catalysed under the reaction conditions, and gives hydroxyacetates and the aromatic carbonyl compounds formed by cleavage of the olefinic bond. |
