Chemistry of small ring compounds. Part 16: Kinetics and mechanism of the isomerization of some 1,1-cyclopropanedicarboxylic acids in water and in aqueous sulfuric acid

The kinetics of the isomerization of 1,1-cyclopropanedicarboxylic acid 1a to αcarboxy-γ-butyrolactone 3a in D2O and in aqueous D2SO4 (up to 56%) has been studied at temperatures up to 160°C. The pseudo first-order reaction is progressively accelerated by increasing sulfuric acid concentrations and according to Bunnett's w, w* and ψ-values, the rate-limiting step is probably the nucleophilic attack of water on a cyclopropyl ring carbon atom of the protonated substrate. 2-Methyl-1,1-cyclopropanedicarboxylic acid 1b and 2,2-dimethyl- 1,1-cyclopropanedicarboxylic acid 1c isomerize faster than 1a by a factor of 31 and 80,000 respectively. The tremendous, rate-enhancement of 1c can be explained by a rate-limiting dissociation of a ring carbon-carbon bond of the protonated substrate (SN1-reaction).