Boron Trihalide Mediated Alkyne-Aldehyde Coupling Reactions: A Mechanistic Investigation

A boron trihalide mediated alkyne-aldehyde coupling reaction leading to stereodefined 1,3,5-triaryl-1,5-dihalo-1,4-pentadienes is described. The study led to the discovery of a direct substitution of hydroxyl
groups by stereodefined alkenyl moieties using alkenylboron dihalides. During the investigation, it was
also discovered that, at low temperatures, the reaction of BCl3 with alkynes produces monovinylboron
dichloride rather than the reported divinylboron chloride. A modified reaction mechanism for the boron
trichloride mediated alkyne-aldehyde coupling reaction is now proposed. The reaction temperature and
mode of addition have been found to have dramatic affects on the stereochemistry of the diene products.