Post-Transition-State Dynamic Effects in the Transmetalation of Pd(II)-F to Pd(II)-CF.

The observation of post-transition-state dynamic effects in the context of metal-based transformation is rare. To date, there has been no reported case of a dynamic effect for the widely employed class of palladium-mediated coupling reactions. We performed an experimental and computational study of the trifluoromethylation of PdF, which is a key step in the Pd/Pd-catalyzed trifluoromethylation of aryl halides or acid fluorides. Our experiments show that the / speciation of the formed PdCF is highly solvent- and transmetalation reagent-dependent. We employed GFN2-xTB- and B3LYP-D3-based molecular dynamics trajectory calculations (with and without explicit solvation) along with high-level QM calculations and found that depending on the medium, different transmetalation mechanisms appear to be operative. A statistically representative number of Born-Oppenheimer molecular dynamics (MD) simulations suggest that in benzene, a difluorocarbene is generated in the transmetalation with RSiCF, which subsequently recombines with the Pd via two distinct pathways, leading to either the - or PdCF. Conversely, GFN2-xTB simulations in MeCN suggest that in polar/coordinating solvents an ion-pair mechanism is dominant. A CF anion is initially liberated and then rebinds with the Pd cation to give a - or Pd. In both scenarios, a single transmetalation transition state gives rise to both - and -species directly, owing to bifurcation after the transition state. The potential subsequent - to isomerization of the PdCF was also studied and found to be strongly inhibited by free phosphine, which in turn was experimentally identified to be liberated through displacement by a polar/coordinating solvent from the -PdCF complex. The simulations also revealed how the variation of the Pd-coordination sphere results in divergent product selectivities.