Studies in carbonium ion chemistry

The meso-hydrobenzoin sulphites (94-96) have been prepared and in each case the stereo-isomers separated. Pyrolysis of these cyclic sulphites gave mixtures of aldehyde and ketone. For any pair of sulphites the product ratio has been shown to be dependent on the stereochemistry of the S-O bond, a greater yield of ketone being produced when the S-O is syn to a hydrogen on the heterocyclic ring. This is thought to be due to abstraction of the hydrogen by the syn S-O to form the enol of a ketone. The differences between the aldehyde to ketone ratios for a series of S-O isomers, where the aryl substituent changes, has been rationalized in terms of the mmigratory aptitude of the aryl group. The product ratios obtained when the unsubstituted (31), 1-methyl (33), 2-methyl (42) and 1,2-dimethyl (45) alkenes are oxidized by KMnO4, performic acid and iodine-silver acetate-water are discussed. The products from these oxidations are substituted cyclohexane-1,2-diols (46-57; 60-63). The cyclic sulphites of these diols have been prepared and the isomers separated. Pyrolysis of the cyclic sulphites produced aldehyde and (or) ketones. The products result from a 1,2-hydride, methyl or methylene shift. In only one case was a cis-1,2-hydride shift observed, implying a considerable degree of charge separation in the transition state. The major product aries from a 1,2-hydride shift where a favourable conformation is available