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

Abstract

AbstractThe 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).