The kinetics and mechanisms of additions to olefinic substances. Part 13. Reactions of 3-substituted cholest-5-enes with sources of electrophilic bromine

Abstract

The kinetics of second-order bromination in acetic acid, and of third-order bromination in chlorobenzene, have been examined for a series of 3β-substituted cholest-5-enes; the results confirm that the rates of these reactions are subject to polar influences approximately of the magnitude expected for a substituent insulated by two saturated carbon atoms from the site of development of the carbocationic centre. The products of reactions of a number of these substrates with sources of electrophilic bromine and chlorine, in chlorobenzene, chloroform, and acetic acid as solvents, have been investigated by ¹H n.m.r. spectroscopy. In most cases the main products are the expected 5α-halogeno-6β-substituted adducts. 5β,6α-Adducts are formed in minor amounts also in the bromination of 3β-trifluoroacetoxycholest-5-ene, and in the reactions of a number of 3β-substituted cholest-5-enes with bromine chloride in deuteriochloroform and in chlorobenzene. Some of the mechanistic implications of these findings are discussed.