Reactions of carbonyl compounds in basic solutions. Part 9. Methoxide-catalysed cyclization of benzylidenephthalides and methyl o-phenylacetylbenzoates

Abstract

The detailed mechanism of the methoxide-catalysed rearrangement of substituted benzylidenephthalides and both normal and pseudo methyl o-phenylacetylbenzoates to form 2-phenylindane-1,3-diones has been studied. A rate–acidity function correlation for the reactions in methanolic dimethyl sulphoxide (DMSO) shows a linear increase in rate with increasing H_ to reach a maximum in rate before decreasing, except for the p-nitro substrate. This derivative shows a rate decrease throughout the H_ range. The ρ values in methanol and in 94 mol% methanolic DMSO are 1.7 and –1.6, respectively. The kinetic isotope effect has been observed with k_H/k_D 0.8–1.0. The equilibrium constants for ring–chain tautomerism of the methyl o-phenylacetylbenzoates have been determined and shown to be independent of substituent and solvent composition. The rate-determining step is the intramolecular attack of the anion of the normal ester on the ester carbonyl group. In methanol and methanolic DMSO of high methanol content this is preceded by the ionization equilibrium of the normal ester. In methanolic DMSO of low methanol content and for the p-nitro substrate, the initial state is the anion itself.