Hydrogallation of Alkynes with H−GaCl2: Formation of Organoelement Dichlorogallium Compounds Potentially Applicable as Chelating Lewis Acids

Treatment of trimethylsilylethynylbenzenes C6H6−x(CC−SiMe3)x (x = 1–3) with the hydridodichlorogallium compound H−GaCl2 afforded, almost quantitatively, the alkenylphenyl compounds C6H6−x[C(H)C(SiMe3)−GaCl2]x [x = 1 (6), 2 (7), and 3 (8)] by hydrogallation. Only compound 6 was readily soluble in n-hexane; it formed dimers via Ga−Cl bridges. The bisalkenyl compound 7 was only sparingly soluble; its molecular structure consisted of a singular dimeric formula unit with a cyclophane-type constitution and two bridging Ga2Cl2 heterocycles. The overall structure may be described by a molecular box formed by a large macrocycle comprising 22 Ga, C, and Cl atoms. Compound 8 proved to be insoluble in hydrocarbon solvents. Its molecular structure could not be detected. Extraction of the solid raw products of 7 and 8 with diethyl ether yielded small quantities of the ether adducts C6H6−x[C(H)C(SiMe3)−GaCl2(OEt2)]x (x = 2, 3) [7(OEt2)2 and 8(OEt2)3], both of which are monomeric because of the coordinative saturation of their gallium atoms. The tetraalkyne 1,2,4,5-tetrakis(trimethylsilylethynyl)benzene gave a different reaction course. Complete hydrogallation resulted in the release of 2 equiv of GaCl3, and neighboring alkenyl groups of the product 9 were connected by GaCl bridges to form seven-membered heterocycles and an overall tricyclic compound. Compound 9 was characterized as a diethyl ether adduct.