Stereospecific synthesis of unsymmetrical (E)- and (Z)-1,2-difluorostilbenes

Abstract

The synthesis of 1,2-difluorostilbenes via a Susuki reaction using a benzene/ethanol/water solvent mixture required long reaction times and generated significant quantities of a homo-coupled boronic acid product, which was difficult to separate. Therefore, we attempted 1,4-dioxane as a solvent for these reactions, which showed remarkable increased reaction rates and minimized the amount of homo-coupled products to aid in purification. Both (E)-and (Z)-1-chloro-1,2difluorostyrenes stereospecifically gave the 1,2-difluorostilbenes in moderate to excellent yields. We investigated 4 protocols for the reaction. Protocols 2 and 3 were recommended for scale up, since the chlorostyrene starting material can be obtained in large quantities stereospecifically as described in previous reports. This work represents the first practical synthesis of unsymmetrical (E)-and (Z)-1,2difluorostilbenes from readily available starting materials.