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

## TL;DR

This paper explains why gold-catalyzed reactions produce straight-chain products instead of branched ones, revealing a new reaction mechanism.

## Contribution

The study identifies a novel allylic deprotonation–coupling (ADC) mechanism as the reason for linear selectivity in gold-catalyzed reactions.

## Key findings

- The classical Heck pathway is energetically less favorable for gold-catalyzed reactions.
- The ADC mechanism is more energetically favorable and explains the observed regioselectivity.
- Gold supports the ADC mechanism, while palladium promotes the conventional Heck-type process.

## Abstract

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.