Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

Potassium fluoride induced cis-trans isomerisation of the title compound I was investigated. The isomerisation takes place in a heterogeneous system and is enabled by solvation effect of dipolar aprotic solvents. Dimethyl 2,3,3,4-tetrafluoro-2-trifluoromethylpentanedioate (II) is formed as side product by proton transfer from the solvent to the intermediate 1,3-bis(methoxycarbonyl)perfluoro-1-butanide (III). The mechanism was deduced from the reaction conditions and results of the isomerisation. Composition of the equilibrium mixture of cis- and trans-isomer (Ia, Ib, respectively) studied in the interval 2-75 °C, shows an extreme. In the interval 2-30 °C the reaction Ia → Ib appears to be exothermic (ΔH = -21.2 kJ mol^-1), in the interval 35-47 °C the enthalpy equals practically zero and in the maximum of the curve at 42.5 °C the entropic factor ΔS = 2.82 J mol^-1 K^-1. In the interval 50-75 °C the reaction is endothermic (ΔH = 15.2 kJ mol^-1). In this context the effect of steric interactions between the substituents -COOCH₃ and -CF(CF₃)COOCH₃, as well as of the so called cis-effect in 1,2-dihalogenoethylenes, on the Ia ##e Ib equilibrium are discussed.