Computational investigation of palladium-catalyzed allene-allene cross-coupling

The construction of [4]dendralenes poses a significant synthetic challenge. Palladium-catalyzed oxidative allene-allene cross-coupling offers high selectivity, but its mechanistic basis, competing pathways, and rate-determining step remain unclear. Herein, we investigate a palladium-catalyzed oxidative allene-allene cross-coupling mechanism using density functional theory (DFT) methods. Two competing pathways (Pathway 1 and Pathway 2) for R groups on the trisubstituted allene reactant, bearing either a -CH2-EWG (electron-withdrawing group) or -CH2-aryl substituent, were systematically evaluated. Computational results show that Pathway 2, involving selective allenic alpha-C-H bond cleavage in the beta-H elimination step, is kinetically favored (Delta Delta G double dagger = 7.3 kcal mol-1), strongly correlating with experimental observations. Carbopalladation (Delta G double dagger = 22.8 kcal mol-1) is identified as the rate-determining step (RDS) for both Pathway 1 and Pathway 2. Mechanistic analysis rationalizes the remarkable selectivities of this strategy, including (i) regioselective C-H activation, (ii) cross-selective carbocyclization-carbopalladation, and (iii) stereoselective cis/trans isomerism. The literature gap-specifically, the lack of mechanistic understanding of selectivity in palladium-catalyzed oxidative allene-allene cross-coupling, including unresolved questions about competing pathways and rate-determining steps-has been clearly explained. Furthermore, we reveal the pivotal role of the allylic directing group, which facilitates C-H activation through a synergistic Pd-pi interaction. Distortion-interaction (D/I) analysis revealed that higher distortion energy is responsible for this regioselectivity. This work provides atomic-level insights into the design of dendralene architectures and broadens the scope of stereocontrolled polyene synthesis.