Highly efficient and stereoselective Julia-Kocienski protocol for the synthesis of α-fluoro-α,β-unsaturated esters and Weinreb amides employing 3,5-bis(trifluoromethyl)phenyl (BTFP) sulfones

alpha-Fluoroacetates 3 and Weinreb amide 4, bearing a alpha-[3,5-bis(trifluoromethyl)phenyl]sulfonyl (BTFP-sulfonyl) group at the alpha-position, are employed in the highly stereoselective synthesis of alpha-fluoro-alpha,beta-unsaturated alkenoates and Weinreb amides, respectively. Aromatic and aliphatic aldehydes are condensed under extremely mild and simple reaction conditions using potassium carbonate in dimethylformamide. at room temperature under solid-liquid phase-transfer catalysis conditions in good yields and high Z-diastereoselectivities, specially in the case of the fluorinated Weinreb amides. A detailed computational mechanistic study suggests a final non-concerted elimination of sulfur dioxide and 3,5-bis(trifluoromethyl)phenoxide and explains the observed high stereoselectivities for the reaction on the basis of thermodynamic and kinetic considerations.