Reactions of carbonyl compounds in basic solutions. Part 23.1 The mechanism of the base-catalysed ring fission of 2,2-dihydroxyindane-1,3-diones

Abstract

The base-catalysed ring fission of a series of substituted 2,2-dihydroxyindane-1,3-diones and phenalene-1,3-dione has been studied in 30% (v/v) dioxane–water. The reaction proceeds in two distinct steps. The first is a relatively fast base-catalysed ring fission to give substituted o-carboxyphenylglyoxals, and the second is a rearrangement of the latter resulting in formation of substituted o-carboxymandelic acids. Rate coefficients for the ring fission of a limited series of substrates have been determined at 25.0 °C. The reaction is first order in the mono-anion of the substrate alone. The Hammett reaction constants, ρ, have been obtained from both the kinetic and product studies. The rate- and product-determining step appears to be an intramolecular nucleophilic attack. The rate coefficients for the rearrangement have been determined for all substrates at 25.0, 35.0 and 45.0 °C. The activation parameters have been calculated. The kinetic solvent isotope, solvent and salt effects have been studied. The effects of the 5-substituted and 5,6-disubstituents on the rates have been correlated using a modified Hammett equation giving a reaction constant, ρ, equal to 3.4 at 25 °C.