FLUORO OLEFINS .2. ISOMERIZATION OF BETA-SUBSIITUTED PERFLUORO OLEFINS . KINETIC VS. EQUILIBRIUM CONTROL

The scope of the synthetic sequence for the preparation of β-substituted perfluoro olefins from triphenylphosphine, sodium chlorodifluoroacetate, and polyfluorinated ketones has been extended to the polyfluorinated ketone, 1-phenylpentafluoropropanone. The product mixture consisted of 2-phenylheptafluoro-l-butene and cis- and trans-2-phenylheptafluoro-2-butene. The latter arises by a fluoride ion catalyzed isomerization of 2-phenylheptafluoro-l-butene to the isomeric internal fluoro olefin. The product ratio of external olefin to internal olefin and the trans/cis ratio of the internal olefin were found to be highly dependent upon the choice of alkali metal chlorodifluoroacetate and reaction time. With sodium and potassium chlorodifluoroacetate, the isomerization yielded an initial trans/cis (k2/k1) kinetic ratio of 1.2 and 2.0, respectively, which is markedly different from the equilibrium ratio of ~6.7. The effect of added alkali metal fluoride salts greatly accelerated the decomposition of the alkali metal chlorodifluoroacetates in the Wittig reaction and enhanced the equilibration of the trans-cis isomers in the order Cs » Rb > K » Na. Two mechanisms are suggested to explain the initial kinetic ratio of trans/cis isomers. The first mechanism involves a linear transition state with or without the intermediacy of a “discrete” fluoro carbanion. The second mechanism involves a six-membered cyclic transition state consisting of fluoro olefin and alkali metal fluoride. © 1968, American Chemical Society. All rights reserved.