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)-CH3CH=CHCH2BF3]K (2a) by a catalyst prepared from Pd(OAc)(2) and D-t-BPF selectively provided gamma-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 (rho = -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 rho-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 fort-nation, 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-l-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.