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, η 1 /η 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, η 1 /η 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 η 1 /η 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 η 3 ‐allylic palladium.