Pinacol-type rearrangements in the phenylcyclohexane series: evidence for differences between the reaction mechanisms of epoxides and of the corresponding diols and halogenohydrins

Abstract

Boron trifluoride–ether complex in benzene rapidly converts (–)(S,S)-1,2-epoxy-1-phenylcyclohexane and the corresponding (–)-(S,S)-cis-1,2-diol and (+)-(R,S)-trans-1,2-diol into mixtures of 1-phenylcyclopentanecarbaldehyde and (–)-(S)-2-phenylcyclohexanone, while a slower secondary reaction transforms the aldehyde into the ketone. (–)-(S,S)-cis-2-Fluoro-2-phenylcyclohexanol, quantitatively formed from the epoxide with boron trifluoride–ether, is converted by the same reagent in benzene into the aldehyde and ketone. All the primary conversions into the ketone take place with a high optical yield, as expected for 1,2-hydride shifts. The large differences in the ketone–aldehyde ratios can be explained by assuming that, while the diols and fluorohydrin rearrange through open carbonium ions, the epoxide–aldehyde conversion takes place in a more concerted manner. A comparison with previous data on the same reaction applied to the corresponding diastereoisomeric 4-t-butyl derivatives strongly suggests that the diols and fluorohydrin react preferentially in their less stable conformations with an axial phenyl group.