Palladium-Catalyzed Stereoconvergent Formylation of (<i>E</i>/<i>Z</i>)-β-Bromo-β-fluorostyrenes: Straightforward Access to (<i>Z</i>)-α-Fluorocinnamic Aldehydes and (<i>Z</i>)-β-Fluorocinnamic Alcohols
We report here the stereoconvergent formylation of (E/Z)-beta-bromo-beta-fluorostyrene mixtures with carbon monoxide and sodium formate catalyzed by palladium. Optimization of reaction conditions leads to the corresponding pure (Z)-alpha-fluorocinnamaldehydes in good yields. The reaction was extended to styrenes bearing electro-attracting or electro-donating groups. The obtained alpha-fluoroaldehydes were smoothly reduced to the corresponding (Z)-beta-fluorocinnamic alcohol by NaBH4. The reaction could be performed on functionalized substrates as demonstrated by the access to the glucoside of beta-fluoroconiferyl alcohol, (Z)-beta-fluoroconiferin, a strong inhibitor of lignin polymerization.