The isoimide–imide rearrangement

Abstract

When imidoyl chlorides (8) are solvolysed in the pH range 3–13 in aqueous dioxan in the presence of acetate or benzoate ions, the isoimides (10) are formed in situ. These isoimides undergo acid (pH <4.5) and base (pH >11.5) catalysed acyl transfer to the solvent, giving the amides (12). But at intermediate pH (6–11) rearrangement to the N-acyl form (11)(the Mumm rearrangement) alone occurs. The specific rate of this O → N acyl group migration is pH independent and shows a low solvent effect (m 0.175). Substituents attached to carbon or nitrogen have the same effect (ρ–0.84). When the migrating group R is varied a non-linear Hammett plot is observed, with ρ+0.60 for electron-withdrawing and +1.65 for electron-donating substituents. The changeover point is a function of the migrating terminus varying from R = Ph (when Y = H) to R =p-tolyl (when Y =p-NO2). Rate-determining O → N acyl transfer is suggested in all cases, but this is preceded by an equilibrium which favours the Z form (10a) when R is electron donating.