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Unveiling the Mechanism and Origins of Selectivity in Phosphine-Catalyzed Higher-Order Cycloadditions of Allenes with
Sebastián Gallardo-Fuentes1, Andrés F Flor-López2, Pablo Jaque2
1Instituto de Química, Facultad de Ciencias, Pontificia Universidad Católica de Valparaíso, Avenida Universidad 330, Curauma, Valparaíso 2373223, Chile.
Abstract:
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).
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