Carbon-Fluorine Bond Activation Coupled with Carbon-Hydrogen Bond Formation a to Iridium: Kinetics,Mechanism,and Diastereoselectivity

摘要

Reactions of iridium(fluoroalkyl)hydride complexes CpIr(PMe_3)(CF_2R_F)Y (R_F=F,CF_3; Y=H,D) with LutHX (Lut=2,6-dimethylpyridine; X=Cl,I) results in C-F activation coupled with hydride migration to give CpIr(PMe_3)(CYFR_F)X as variable mixtures of diastereomers.Solution conformations and relative diastereomer configurations of the products have been determined by ~(19)F{~1H}HOESY NMR to be (S_c,S_(Ir))(R_c,R_(Ir)) for the kinetic diastereomer and (R_c,S_(Ir))(S_c,R_(Ir)) for its thermodynamic counterpart.Isotope labeling experiments using LutDCl/CpIr(PMe_3)(CF_2R_F)H and CpIr(PMe_3)(CF_2R_F)D/LutHCl) showed that,unlike a previously studied system,H/D exchange is faster than protonation of the alpha-CF bond,giving an identical mixture of product isotopologues from both reaction mixtures.The kinetic rate law shows a first-order dependence on the concentration of iridium substrate,but a half-order dependence on that of LutHCl; this is interpreted to mean that LutHCl dissociates to give HCl as the active protic source for C-F bond activation.Detailed kinetic studies are reported,which demonstrate that lack of complete diastereoselectivity is not a function of the C-F bond activation/H migration steps but that a cationic intermediate plays a double role in loss of diastereoselectivity; the intermediate can undergo epimerization at iridium before being trapped by halide and can also catalyze the epimerization of kinetic diastereomer product to thermodynamic product.A detailed mechanism is proposed and simulations performed to fit the kinetic data.