γ-Selective Cross-Coupling Reactions of Potassium Allyltrifluoroborates with Haloarenes Catalyzed by a Pd(0)/D-t-BPF or Pd(0)/Josiphos ((R,S)-CyPF-t-Bu) Complex: Mechanistic Studies on Transmetalation and Enantioselection

Cross-coupling between bromoarenes and [( E )-CH 3 CH=CHCH 2 BF 3 ]K ( 2a ) by a catalyst prepared from Pd(OAc) 2 and D- t -BPF selectively provided γ-coupling products via S E 2′ substitution. Mechanistic study of transmetalation revealed a heretofore unknown process, namely, formation of a highly electrophilic [Pd(Ar)(D- t -BPF)] + before transmetalation with 2a . Thus, kinetic data in coupling of 4-substituted bromoarenes with 2a showed a linear positive correlation (ρ = −1.1) accelerated by donating substituents. This rate-determining role of transmetalation was further confirmed by kinetic data between oxidative adducts [Pd(Ar)(Br)(D- t -BPF)] and 2a that exhibited an analogous correlation with a negative ρ-value (−0.50). Theoretical study by density functional theory (DFT) calculation showed that transmetalation between [Ar−Pd] + and 2a via an S E 2′ (open) transition state is a slightly lower energy process than an S E 2′ (closed) process. Allylic substitutions with chiral catalysts are the current topics for enantioselective C−C bond formation, but the catalysts that are effective for allylic nucleophiles have remained unexplored. Among the ligands screened, ( R,S )-CyPF- t -Bu was found to achieve 77−90% ee for representative para- and meta-substituted bromoarenes and 2-bromo-1-alkenes in refluxing aqueous tetrahydrofuran (THF) or MeOH. To obtain mechanistic information on enantioselection, the mode of substrate coordination to a cationic phenylpalladium intermediate was calculated, that is, the reaction stage directly preceding the stereodetermining insertion step by DFT calculation. A stable adduct between [Pd(CyPF- t -Bu)(Ph)] + and 2a located at the C−C double bond from its re -face yielding the experimentally observed R -product is preferred thermodynamically rather than the corresponding si -coordination.