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

Abstract

Treatment of trimethylsilylethynylbenzene derivatives with HGaCl₂ afforded products, [C₆H_6−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.