STEREOCHIMIE-LVII ETUDE STEREOCHIMIQUE ET CINETIQUE DE LA BROMATION D'ESTERS ET D'ETHERS ENOLIQUES STEROIDIQUES

Article abstract of DOI:10.1016/S0040-4020(01)97635-5

Absolute rates of bromination were measured for two series of derivatives of steroidal ketones 3, enol acetates 1 and enol methyl ether 2.Axial substituents exhibited a large effect on rates, which increased by 15,000 fold on going from (X=CH3; Y=CN) to (X=Y=H).From the bromide ion effect it was concluded that the first step (formation of an intermediate bromonium ion) was reversible and that the second step (formation of haloketones 4 or 5 or of haloacetals 8 or 9) was slow compared to the first step.The intermediate was concluded to be a highly unsymmetrical bromonium ion rather than a plain oxocarbenium ion.