Kinetics and mechanisms of the Bamberger rearrangement. Part 4. Rearrangement of sterically hindered phenylhydroxylamines to 4-aminophenols in aqueous sulphuric acid solution
The rates of the Bamberger rearrangement of sterically hindered phenylhydroxylamines have been determined in aqueous sulphuric acid solution and the substituent effects (in particular, the steric effects) are discussed. The rate constants for phenylhydroxylamines with 2-substituents (Me, Cl, l) satisfied the Taft equation: log krel=ρ*σ*–δES with ρ*– 1.93 and δ–1.16. The result shows that steric hindrance of the substituents, in addition to the electron-donating effect, has an accelerating effect on the rates of the Bamberger rearrangement. The rate constants for 3-substituted 2-methylphenylhydroxylamines were generally greater than those for 5-substituted 2-methylphenylhydroxylamines. The difference was attributed to the ‘buttressing effect’ of neighbouring 3-substituents. This is the first example of steric acceleration of the Bamberger rearrangement.