Reactions of carbonyl compounds in basic solutions. Part 32.1 The Perkin rearrangement

The Perkin rearrangement of 3-halocoumarins to benzofuran-2-carboxylic acids, catalysed by base, proceeds in two separate stages. The first stage is a relatively rapid base-catalysed ring fission of the 3-halocoumarins to give (E)-2-halo-3-(2-hydroxyphenyl)acrylic acids. Rate coefficients have been measured for the base-catalysed ring fission of 6-substituted and 4-methyl-3-bromocoumarins and of 3-chlorocoumarin in 70% (v/v) dioxane–water at various temperatures. The second stage is a relatively slow cyclisation process. Rate coefficients have been measured for the cyclisation of the same series of substrates in 70% (v/v) dimethyl sulfoxide–water at various temperatures. The enthalpies and entropies of activation have been evaluated. The Hammett reaction constant for the ring fission at 30.0 °C is 2.34 and for the cyclisation at 60.0 °C is –3.54. The ring fission appears to occur by rate-determining addition of hydroxide anion to the carbonyl group, followed by a relatively rapid ring opening process; while the cyclisation probably proceeds by rate-determining fission of the carbon–halogen bond, following formation of a relatively unstable carbanion intermediate formed by intramolecular nucleophilic attack on the vinyl group by the phenoxide anion.