DFT Investigation on Palladium-Catalyzed [2 + 2 + 1] Spiroannulation between Aryl Halides and Alkynes: Mechanism, Base Additive Role, and Solvent and Ligand Effects.

Abstract

Transition metal-catalyzed spiroannulations are practical strategies for constructing spirocyclic skeletons of pharmaceutical and biological significance, yet the microscopic mechanism still lacks in-depth explorations. Here, the palladium-catalyzed [2 + 2 + 1] spiroannulation between aryl halides and alkynes was studied by employing the density functional theory (DFT) method. Based on comprehensive explorations on a couple of possible reaction pathways, it is found that the reaction probably experiences C-I oxidative addition, alkyne migration insertion, Cs₂CO₃-assisted aryl C-H activation, C-Br bond oxidative addition, C-C coupling, arene dearomatization and reductive elimination in sequence and leads to the formation of the spiro[4,5]decane pentacyclic product (P) ultimately. Among these, the C-Br bond oxidative addition step acts as the rate-determining step (RDS) of the whole reaction, featuring a practical free energy barrier of 32.4 kcal·mol^-1 at 130 °C. Computationally predicted kinetics such as half-life transferred from the RDS step's barrier on the optimal reaction pathway (1.2 × 10¹ h) coincides well with corresponding experimental results (91% yield of the spiro[4,5]decane pentacyclic product P after reacting 10 h at 130 °C). In addition, theoretical predictions regarding the solvent/ligand effects and base additive role in the reaction, rationalized by distortion-interaction/natural population/noncovalent interaction analyses, are also in good agreement with experimental data and trend. This good agreement between experiment and theory makes sense for new designations and further experimental improvements of such Pd-catalyzed transformations.