Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

The vinylic nucleophilic substitution of methyl trifluoropropenoate (I) with potassium cyanide affords methyl 2,3-difluoro-3-cyanopropenoate (II) in the form of a mixture of cis- and trans-isomers in 95 : 5 ratio. Simultaneously the dimerization of propenoate I to dimethyl perfluoro-4-methyl-2-pentenedioate (III) also takes place to a lesser extent. On acid catalysed methanolysis of compound II dimethyl 2,3-difluoro-2-butenedioate (VI) is formed with the same content of configurational isomers. Diester VI with a relative cis : trans ratio of 35 : 65 was obtained on dehalogenation of dimethyl 2,3-dichloro-2,3-difluorobutanedioate (V), which was prepared by addition of chlorine to the cyano derivative II and subsequent acid catalyzed methanolysis. When reacting them with potassium fluoride in dimethyl sulfoxide compounds II and VIunderwent anionic cis-trans isomerization reaction. In the 0-80 °C interval the cis isomers prevail in the equilibrium mixture, and the conversion cis → trans is endothermic: in the 40-80 °C interval for IIa → IIb ΔH = 12.8 kJ mol^-1, for VIa → VIb ΔH = 38.6 kJ mol^-1. The influence of the cis-effect of the fluorine atoms on the thermodynamic stability of the configurational isomers of compounds II and VI is discussed.