Unveiling the Mechanism and Origins of Selectivity in Phosphine-Catalyzed Higher-Order Cycloadditions of Allenes with Tropone

A comprehensive computational study on the mechanism and selectivity of the phosphine-catalyzed higher-order cycloaddition reaction between allenes and tropone is presented. Density functional theory calculations reveal that the pathway leading to the [8 + 2] cycloadduct, which involves an initial nucleophilic addition to the readily formed β-phosphonium ylide followed by a cyclization reaction, is kinetically preferred over the possible alternative pathways (including [3 + 2] or [6 + 4] cyclizations). Exploration of the potential energy surface indicates that, unlike related additions to β-phosphonium ylides, which typically prefer α-additions, in this case, the γ-addition is favored. The origin of the (regio)selectivity of this transformation has been quantitatively elucidated using the activation strain model of reactivity in conjunction with reaction force analysis (RFA).