Rare‐Earth Catalyzed C−H Bond Alumination of Terminal Alkynes

Organoaluminum reagents’ application in catalytic C−H bond functionalization is limited by competitive side reactions, such as carboalumination and hydroalumination. Herein, rare‐earth tetramethylaluminate complexes are shown to catalyze the exclusive C−H bond metalation of terminal alkynes with the commodity reagents trimethyl‐, triethyl‐, and triisobutylaluminum. Kinetic experiments probing alkyl‐group exchange between rare‐earth aluminates and trialkylaluminum, C−H bond metalation of alkynes, and catalytic conversions reveal distinct pathways of catalytic aluminations with triethylaluminum versus trimethylaluminum. Most significantly, kinetic data point to reversible formation of a unique [Ln](AlR 4 ) 2 ⋅AlR 3 adduct, followed by turnover‐limiting alkyne metalation. That is, C−H bond activation occurs from a more associated organometallic species, rather than the expected coordinatively unsaturated species. These mechanistic conclusions allude to a new general strategy for catalytic C−H bond alumination that make use of highly electrophilic metal catalysts.