Unconventional <i>β</i> ‐Hydride Elimination in Pd‐Catalyzed Migratory Cyclization: A Theoretical Study of Outer‐Sphere C(sp ² )–H Activation

The density functional theory (DFT) study reveals a novel mechanism for Pd-catalyzed C(sp²)-H activation through an unconventional outer-sphere deprotonation pathway. The canonical β-hydride elimination in alkenyl-Pd systems has been unequivocally demonstrated to be both kinetically and thermodynamically unfavorable. Our computational analysis reveals this energetic penalty stems from three key factors: (1) increased ring strain in the four-membered transition state, (2) stronger Pd-C bonding due to π-backdonation in alkenyl-Pd species, and (3) thermodynamic instability of the resulting Pd-H intermediate. This work elucidates a general mechanism for overcoming the longstanding challenge of β-C(sp²)-H activation in alkenyl-metal systems, providing new insights for future development of C-H functionalization reactions.