Boron Trichloride Mediated Alkyne-Aldehyde Coupling Reactions

Abstract

The boron trichloride mediated reaction of aryl aldehydes and alkynes can be used to selectively generate either (Z,Z)- or (Z,E)-1,5-dichloro-1,4-dienes as major products, depending on the reaction conditions employed. To investigate the mechanism of this important carbon-carbon bond forming reaction, the boron trichloride mediated reaction of phenylacetylene with p-bromobenzaldehyde was chosen as the model system. Interestingly, when the coupling reactions are carried out below ―60 °C, (E,E)-1,5-dichloro-1,4-dienes are also formed. The ratios of (Z,Z)-, (Z,E)-, and (E,E)-1,5-dichloro-1,4-dienes were correlated to the reaction conditions using gas chromatography-mass spectrometry. The results of the investigation can be used to explain why the ratio of (Z,E)-, (Z,Z)-, and (E,E)-1,5-dichloro-1,4-pentadiene products change very dramatically when changes are made in either the sequence of addition of the reagents or the reaction temperature.