Potassium Carbonate to Unlock a GaCl3-Catalyzed C-H Propargylation of Arenes

A gallium-catalyzed C-H propargylation of electron-rich arenes using bromoallenes is described. The development of this reaction was first hampered by a side hydroarylation process catalyzed by adventitious protons, easily generated from the solvent (1,2-dichloroethane). This hidden Bronsted acid catalysis could be bypassed by using K2CO3 as an insoluble base. The unexpected compatibility of this base with the Lewis acid catalyst GaCl3 allowed a strictly gallium-catalyzed process that was thoroughly studied by kinetics, Ga-71 NMR experiments, and DFT computations. The GaCl3/K2CO3 system triggers the C-H propargylation of a wide range of arenes and heteroaromatics using a variety of bromoallenes.