Insights into mechanism and selectivity in palladium-catalyzed oxidative carbocyclization-borylation of enallenes via olefin assistance

A systematic theoretical study has been carried out with the aid of density functional theory calculations on the mechanism for the Pd-catalyzed oxidative carbocyclization-borylation of enallenes. Our computational analysis reveals that the catalytic cycle comprises four principal stages, including CH bond activation, olefin insertion, transmetalation, and reductive elimination. The reaction initiates via C(sp³)H activation through a concerted metalation-deprotonation pathway, generating a vinylpalladium intermediate. Stereochemical control is governed by the regioselectivity of alkene insertion: proximal insertion leads to spirocyclic products, whereas distal insertion preferentially forms cyclohexene products, with the latter pathway exhibiting more favorable in both kinetics and thermodynamics. The transmetalation step follows two paths through four- or six-membered ring transition states, depending on which oxygen atom of the nucleophile attacks. Reductive elimination occurs and the catalyst is regenerated using benzoquinone as the oxidant to complete the catalytic cycle. Comparative analysis of alternative CC coupling pathways further validates the proposed mechanism.