Studies of some reactions between electrophilic halogens and various conjugated systems in which intermediates or products having rearranged double-bond systems are concerned

Abstract

PART I A STUDY OF MECHANISTIC VARIATIONS IN THE BROMINATION AND CHLORINATION OF SOME ALKYL PHENOLS, PHENYL ETHERS AND PNENYL ESTERS. The brominations of some 2,6-dialkylphenyl acetates in nitro-methane and in acetic acid have been shown to give (a) products of substitution at both the C-3 and C-4 nuclear sites (b) products of substitution with rearrangement leading to the formation of various brominated alkylphenols and (c) uniquely in acetic acid, a product of alkyl side-chain substitution. The bromination of the corresponding benzoate derivatives realised only the products of 3- substitution. Various products of (a) substitution and (b) substitution with rearrangement were determined in the bromination of 3,4-dimethylphenyl acetate, phenyl acetate, and 2,3,5,6-tetramethylphenyl acetate. The bromination of 2,4,6-dimethylphenyl methyl ether gave only the expected product of nuclear substitution. Evidence for the likely involvement of addition-elimination sequences in the formation of products of substitution with rearrangement, has been adduced. The possible role of various other processes leading to the formation of deacylated products is discussed. The chlorination of 2,6-dimethylphenyl acetate, 2,6-dimethylphenyl benzoate and of 2,3,5,6-tetramethylphenyl acetate in both acetic acid and nitromethane gave only the expected product of nuclear substitution. The reaction of chlorine with 3,4-dimethylphenol, its acetate and methyl ether in acetic acid led to the formation of certain enone and dienone adduct derivatives which could be shown to rearrange under the conditions of the reaction to give (a) in the case of the phenol-products of substitution and (b) for the acetate and methyl ether derivatives, products of substitution with rearrangement. The structures of the intermediates were determined from their spectral properties. Various mechanisms likely to be involved in processes of chloro-deacylation and of chloro-demethylation are discussed; a consideration of certain structural features of the substrates studied led to the conclusion that chloro-deacylation and chloro-demethylation were essentially processes initiated by ortho- attack of the chlorine molecule. PART II REACTION PATHS IN THE BROMINATION OF CHOLEST-4-EN-3-ONE, CHOLEST-5-EN-3-ONE AND 3-ACETOXYCHOLESTA-3,5-DIENE The bromination of cholest-4-en-3-one can occur by a number of reaction paths. Conditions have been established under which reaction can be established as occurring in part by electrophilic attack on (a) 3-hydroxycholesta-4,6-diene (b) 3-hydroxycholesta-2,4-diene or its anion and (c) cholest-4-en-3-one; among the products can be included 2α-, 4-, 6β-, and 6α-bromocholest-4-en-3-one; 2α,6β-, 2α,6α-, and 4,6β-dibromocholest-4-en-3-one; and 2α,4,6β-tribromocholest-4-en-3-one. Unstable adducts involving the 4,5-double bond are implicated in a number of cases. The bromination of cholest-5-en-3-one is equally complicated, and can involve addition to the 5,6-double bond. Knowledge of the range of conditions and reagents through which 3-acetoxycholesta-3,5-diene is brominated to give 6β-bromocholest-4-en-3-one has been extended. Intermediate adducts involving the 5,6-double bond have been shown to be components of the reaction mixture from the acetoxydiene and bromine in acetic acid. Acid catalysed isomerisation and rearrangements of the kinetically controlled products, and dibromination, can be controlled by selective choice of the conditions of bromination and work-up. Complications associated with the reaction paths have been partly elucidated. The rates and mechanism of reversible isomerisation of 6β- to 6α-bromocholest-4-en-3-one, and of 2α,β- to 2α,6α-dibromocholest-4-en-3-one, have been studied and a proposed mechanism is outlined.