Addition of alkynes to a gallium bis-amido complex: imitation of transition-metal-based catalytic systems

Acetylene, phenylacetylene, and alkylbutynoates add reversibly to (dpp‐bian)Ga–Ga(dpp‐bian) (dpp‐bian=1,2‐bis(2,6‐diisopropylphenyl)‐imino]acenaphthene) to give addition products dpp‐bian(R¹C?CR²)]Ga–Ga(R²C?CR¹)dpp‐bian]. The alkyne adds across the Ga? N? C section, which results in new carbon–carbon and carbon–gallium bonds. The adducts were characterized by electron absorption, IR, and ¹H NMR spectroscopy and their molecular structures have been determined by single‐crystal X‐ray analysis. According to the X‐ray data, a change in the coordination number of gallium from three in (dpp‐bian)Ga–Ga(dpp‐bian)] to four (in the adducts) results in elongation of the metal–metal bond by approximately 0.13 Å. The adducts undergo a facile alkynes elimination at elevated temperatures. The equilibrium between dpp‐bian(PhC?CH)]Ga–Ga(HC?CPh)dpp‐bian] and (dpp‐bian)Ga–Ga(dpp‐bian) + 2 PhC?CH] in toluene solution was studied by ¹H NMR spectroscopy. The equilibrium constants at various temperatures (298≤T≤323 K) were determined, from which the thermodynamic parameters for the phenylacetylene elimination were calculated (ΔG°=2.4 kJ mol^?1, ΔH°=46.0 kJ mol^?1, ΔS°=146.0 J K^?1mol^?1). The reactivity of (dpp‐bian)Ga–Ga(dpp‐bian) towards alkynes permits use as a catalyst for carbon–nitrogen and carbon–carbon bond‐forming reactions. The bisgallium complex was found to be a highly effective catalyst for the hydroamination of phenylacetylene with anilines. For instance, with (dpp‐bian)Ga–Ga(dpp‐bian)] (2 mol %) in benzene more than 99 % conversion of PhNH₂ and PhC?CH into PhN?C(Ph)CH₃ was achieved in 16 h at 90 °C. Under similar conditions, the reaction of 1‐aminoanthracene with PhC?CH catalyzed by (dpp‐bian)Ga–Ga(dpp‐bian) formed a carbon–carbon bond to afford 1‐amino‐2‐(1‐phenylvinyl)anthracene in 99 % yield.