Palladium-Catalyzed Divergent Cycloaddition of Arylalkynes with Norbornene Derivatives

A palladium-catalyzed (3 + 2) cycloaddition between arylalkynes and norbornene derivatives via C–H bond cleavage is reported. This reaction enables the selective synthesis of benzo-fused five-membered cyclic products from a variety of arylalkynes, including electronically biased and alkyl(aryl)alkynes. Notably, the use of a bulky bidentate ligand leads to the (2 + 2) cycloaddition, giving cyclobutene derivatives from the same starting materials. Mechanistic studies indicate that both cycloadditions proceed through hydropalladation of the arylalkynes by Pd(0) and AcOH to generate a vinyl–Pd intermediate. Density functional theory calculations suggest a ligand-to-ligand hydrogen transfer mechanism for this step. The subsequent turnover-determining step is proposed to vary with the ligand environment, involving either C–H bond cleavage or reductive elimination, thereby governing pathway selectivity.