27. Mechanism and Origin of Et2Al(OEt)-Induced Chemoselectivity of Nickel-Catalyzed Three-Component Coupling of One Diketene and Two Alkynes

Liu, Y.; Tang, Y.; Jiang, Y.-Y.; Zhang, X.; Li, P.; Bi, S.*  ACS Catal. 2017, 7, 1886–1896. Download Link

Abstract: Density functional theory (DFT) calculations have been performed to unravel the mechanism of Lewis-acid-induced Ni(cod)2-catalyzed selective coupling reactions of one diketene and two alkynes. Complex mixtures (unsymmetrical phenylacetic acid P1, symmetrical phenylacetic acid P2 and (3E)-4-ethyl-5-methylene-3- heptenoic acid P3) were obtained in the absence of Et2Al(OEt). P1 formation involves C(sp2)-O oxidative addition of diketene, twice alkyne insertion, intramolecular C=C insertion, acidolysis, and β-H elimination. For P2/P3 formation, the common key issue related to the C=C double bond cleavage of the substrate diketene was explored and found that it was accomplished via a four-membered-ring-closure/ four-membered-ring-opening process. And then, P2 was produced via the second alkyne insertion while P3 was accessed by a stoichiometric reaction with HCl. The Et2Al(OEt)-induced chemoselectivity was also probed. It is found that the Ni−O (from Al reagent) bonding facilitates the second alkyne insertion, and the Al−O (from carboxylate) bonding weakens the four-membered ring-closure step, which consequently leads to the formation of P1 exclusively. Additionally, HCl plays a promoting role as a cocatalyst in producing P1 and P2. The theoretical results not only well rationalize the experimental observations but provide insights into the mechanism of the Ni-catalyzed multicomponent coupling reactions.