The influence of halogen substituents on the course of hydrogallation and hydroalumination reactions

Treatment of trimethylsilylethynylbenzene derivatives with HGaCl2 afforded products, [C6H6−x{C(H)C(SiMe3)GaCl2}x], in which by a very fast cis/trans-rearrangement the Ga and H atoms occupied opposite sides of the resulting CC double bonds. The stability of the cis-forms considerably increased upon application of 1,3-dibromo- and pentafluorophenylalkyne derivatives. Two pairs of cis/trans-isomers could be characterized by crystal structure determinations and allow the direct comparison of structural parameters. For the first time an equilibrium was detected between cis- and trans-forms in solution. Treatment of 1,4-di(tert-butylalkynyl)benzene with HAlR2 (R = CMe3, CH2CMe3) afforded cyclophane-type molecules by the release of AlR3. Only the neopentyl derivative could be isolated and characterized by crystal structure determination. In contrast, the dibromo compound, 1,4-Br2-2,5-(Me3CCC)2C6H2, yielded the simple addition product, C6H2Br2{C(AlR2)C(H)CMe3}2 (R = CMe3). Condensation was hindered in this case by intramolecular Al-Br interactions. Surprisingly, the simple addition product was also isolated from the reaction of 1,4-(Me3CCC)2C6H4 with the relatively small hydride HAlEt2. Solid-state NMR spectra of the product revealed strong intermolecular Al–C interactions involving the negatively charged terminal vinylic carbon atoms, to give one-dimensional coordination polymers.