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, an...
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