GaCl₃-promoted addition reactions of carbon nucleophiles to alkyne

GaCl₃ promotes addition reactions of carbon nucleophiles to a C-C triple bond. Interaction of alkyne with GaCl₃ generates a highly reactive electrophile, which aromatic hydrocarbon attacks to give an alkenylated arene. Silylethyne reacts predominantly at the p-position of toluene, while disilylated 1,3-butadiyne exhibits o-selectivity. The behavior of a silylated 1,2-propadiene is intermediate between that of the silylethyne and the disilylated 1,3-butadiyne. In the presence of GaCl₃, electrophilic trimerization of silylethyne takes place to give a conjugated hexatriene. In this reaction, silylethyne attacks the GaCl₃-activated C-C triple bond. Carbometalation is another interesting addition reaction of an organogallium compound to alkyne. Alkynyldichlorogallium dimerizes in hydrocarbon solvents to give 1,1-dimetallo-1-buten-3-yne. In the presence of GaCl₃, silyl enol ether is ethenylated at the α-carbon atom with trimethylsilylethyne. Treatment of lithium phenoxide with silylethyne in the presence of GaCl₃ gives o-(β-silylethenyl) phenol. These reactions involve carbogallation of alkynylgallium, gallium enolate, or gallium phenoxide.