Transition Metal-Free Catalytic C−H Zincation and C−H Alumination

C−H metalation is the most efficient method to prepare aryl–zinc and –aluminum complexes that are highly useful nucleophiles. Virtually all C–H metalation routes to form Al or Zn organometallic reagents require stoichiometric, strong Brønsted bases with no base-catalyzed reactions reported, to our knowledge. Herein we present a catalytic C–H metalation process to form aryl-zinc and aryl-aluminum complexes that uses only simple amine bases (e.g., Et3N) in sub-stoichiometric quantity (10 mol%). Key to this approach is coupling an endergonic C–H metalation step using a [(-diketiminate)MNR3]+ (M = Zn or Al–Me) electrophile with a sufficiently exergonic dehydrocoupling step between the acidic ammonium salt by-product of C–H metalation ([(R3N)H]+) and a Zn–H or Al–Me containing complex. This step, forming H2/MeH, makes the overall cycle exergonic while also generating more of the key cationic metal electrophile. Mechanistic studies supported by DFT calculations revealed metal-specific dehydrocoupling pathways, with the divergent reactivity shown to be due to the different metal valency (which impacts the accessibility of amine-free cationic complexes) and steric environment. Notably, dehydrocoupling in the zinc system proceeds through a ligand-mediated pathway involving protonation of the -diketiminate C position. In this step the magnitude of the key barrier is dependent on the steric bulk of the spectator ligand, with bulkier ligands actually affording lower barriers. This catalytic approach to arene C−H metalation has the potential to be applicable to other main group metals and ligands, thus will facilitate the synthesis of these important organometallic compounds.