Connecting Neutral and Cationic Pathways in Nickel-Catalyzed Insertion of Benzaldehyde into a C-H Bond of Acetonitrile

Nickel catalysts supported by diethylamine- or aza-crown ether-containing aminophosphinite (NCOP) pincer ligands catalyze the insertion of benzaldehyde into a C-H bond of acetonitrile. The catalytic activity of neutral (NCOP)Ni((OBu)-Bu-t) and cationic [(NCOP)Ni(NCCH3)](+) are starkly different. The neutral tert-butoxide precatalysts are active without any added base and give good yields of product after 24 h, while the cationic precatalysts require a base cocatalyst and still operate much more slowly (120 h in typical runs). A series of in situ spectroscopic studies identified several intermediates, including a nickel cyanoalkoxide complex that was observed in all of the reactions regardless of the choice of precatalyst. Reaction monitoring also revealed that the neutral tert-butoxide precatalysts decompose to form the cationic acetonitrile complex during catalysis; this deactivation involves alkoxide abstraction and can be hastened by the addition of lithium salts. While the deactivated cationic species is inactive under standard base-free conditions, catalysis can be reinitiated by the addition of catalytic amounts of base.