Mechanistic Insight into Palladium-Catalyzed Carbocyclization-Functionalization of Bisallene: A Computational Study

Density functional theory calculations were performed to reveal the mechanisms of Pd-catalyzed cascade carbocyclization-borylation and arylation reactions. The computational results indicate that the reactions start with allylic C-H cleavage through concerted metalation-deprotonation and an intramolecular exo-type allene insertion to form a six-membered carbocycle intermediate. The regioselectivity of insertion could be explained by frontier molecular orbital theory and natural population analysis calculation. In the absence of extra nucleophiles, eta(1)/eta(3)-isomerization followed by acetate-assisted deprotonation could yield polyene product. When nucleophile was added to the reaction system, transmetalation and subsequent reductive elimination could give the exo-substituted triene as major product. Meanwhile, eta(1)/eta(3)-isomerization, transmetalation, and reductive elimination could afford the endo-isomer as side product. The regioselectivity of further functionalization is controlled by the competition of transmetalation and eta(1)/eta(3)-isomerization. The computational results show that both exo- and endo-boronation product could be observed when bis(pinacolato)diboron is added as nucleophile. However, only exo-phenylation product is observed when phenylboronic acid is used as nucleophile because of the high free-energy barrier for reductive elimination from aryl eta(3)-allylic palladium.