Carbon-fluorine bond activation coupled with carbon-hydrogen bond formation α to iridium: Kinetics, mechanism, and diastereoselectivity

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Reactions of iridium(fluoroalkyl)hydride complexes Cp*Ir(PMe 3)(CF2RF)Y (RF = F, CF3; Y = H, D) with LutHX (Lut = 2,6-dimethylpyridine; X = Cl, I) results in C-F activation coupled with hydride migration to give Cp*Ir(PMe 3)(CYFRF)X as variable mixtures of diastereomers. Solution conformations and relative diastereomer configurations of the products have been determined by 19F{1H}HOESY NMR to be (SC, SIr)(RC, RIr) for the kinetic diastereomer and (RC, SIr)(SC, RIr) for its thermodynamic counterpart. Isotope labeling experiments using LutDCl/Cp*Ir(PMe3)(CF2RF)H and Cp*Ir(PMe3)(CF2RF)D/LutHCl) showed that, unlike a previously studied system, H/D exchange is faster than protonation of the α-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. © 2005 American Chemical Society.

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Journal of the American Chemical Society

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