Pathway for the Stereocontrolled <i>Z</i> and <i>E</i> Production of α,α-Difluorine-Substituted Phenyl Butenoates

An efficient preparation of pure ethyl Z- and E-α,α-difluoro-4-phenyl-3-butenoate 1a and 1b together with the corresponding acids 2a and 2b is described. The procedures involve stereocontrolled additions of •CF2CO2Et to phenylacetylene or β-bromostyrene. Compound 1a is easily obtained by addition of •CF2CO2Et to phenylacetylene via a mechanism where the stereochemistry is controlled by an electron-transfer process to produce predominantly the Z vinyl anion. The product 1b is obtained by •CF2CO2Et addition−elimination to Z- or E-β-bromostyrenes via a mechanism where the stereochemistry is controlled by steric factors in the conformational equilibration of the intermediates.