Gold Is Not Palladium: Linear Selectivity in Gold-Catalyzed Heck-Type Arylation via an Allylic Deprotonation-Coupling (ADC) Mechanism.

Gold-catalyzed arylation of alkenes has recently been reported by Patil and co-workers as a rare example of a Heck-type reaction exhibiting exclusive linear selectivity. However, no mechanistic rationale was provided for the complete suppression of the branched product. In this study, we address this gap through DFT calculations and show that if the reaction proceeds via the classical Heck-type pathway, formation of the branched product is energetically preferred, which is inconsistent with experimental observations. This discrepancy led us to identify a distinct mechanistic alternative: an allylic deprotonation-coupling (ADC) mechanism, in which alkene-gold(III) complexes undergo early allylic C-H deprotonation followed by C-C reductive elimination. The ADC mechanism is found to be energetically more favorable than the Heck pathway and accounts for the observed regioselectivity. This mechanism is also shown to be energetically far more favorable than an alternative involving nucleophilic attack by triflate, proposed for this catalytic transformation by Budzelaar et al. Comparative analysis further highlights the decisive role of the metal center in determining the reaction pathway: gold supports the ADC mechanism, while palladium promotes the conventional Heck-type process. The discovery of the ADC mechanism broadens the mechanistic understanding of transition-metal catalysis and provides a new perspective for future transformations.