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)GaGa(dpp-bian) (dpp-bian=1,2-bis[(2,6-diisopropylphenyl)-imino]acenaphthene) to give addition products [dpp-bian(R1C?CR2)]GaGa[(R2C?CR1)dpp-bian]. The alkyne adds across the Ga?N?C section, which results in new carboncarbon and carbongallium 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)GaGa(dpp-bian)] to four (in the adducts) results in elongation of the metalmetal bond by approximately 0.13 angstrom. The adducts undergo a facile alkynes elimination at elevated temperatures. The equilibrium between [dpp-bian(PhC?CH)]GaGa[(HC?CPh)dpp-bian] and [(dpp-bian)GaGa(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 (Delta G degrees=2.4 kJ?mol-1, Delta H degrees=46.0 kJ?mol-1, Delta S degrees=146.0 J?K-1mol-1). The reactivity of (dpp-bian)GaGa(dpp-bian) towards alkynes permits use as a catalyst for carbonnitrogen and carboncarbon 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)GaGa(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 degrees C. Under similar conditions, the reaction of 1-aminoanthracene with PhC?CH catalyzed by (dpp-bian)GaGa(dpp-bian) formed a carboncarbon bond to afford 1-amino-2-(1-phenylvinyl)anthracene in 99?% yield.