Catalysis with a Skip: Dynamically Coupled Addition, Proton Transfer, and Elimination during Au- and Pd-Catalyzed Diol Cyclizations

Au and Pd complexes have emerged as highly effective π-bond cyclization catalysts to construct heterocycles. These cyclization reactions are generally proposed to proceed through multistep addition–elimination mechanisms involving Au– or Pd–alkyl intermediates. For Au- and Pd-catalyzed allylic diol cyclizations, while the density functional theory (DFT) potential energy surface landscapes show a stepwise sequence of alkoxylation π-addition, proton transfer, and water elimination, quasiclassical direct dynamics simulations reveal dynamical mechanisms that depend on the metal center. For Au, trajectories reveal that after π-addition the Au–alkyl intermediate is always skipped because addition is dynamically coupled with proton transfer and water elimination. In contrast, for Pd catalysis, due to differences in the potential energy landscape shape, only about half of trajectories show Pd–alkyl intermediate skipping. The other half of the trajectories show the traditional two-step mechanism with the intervening Pd–alkyl intermediate. Overall, this work reveals that interpretation of a DFT potential energy landscape can be insufficient to understand catalytic intermediates and mechanisms and that atomic momenta through dynamics simulations are needed to determine if an intermediate is genuinely part of a catalytic cycle.