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(OtBu) 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.