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The application of the F{H} HOESY (heteronuclear Overhauser effect spectroscopy) technique to the determination of the solution conformations is presented. This technique is applied to the assignment of relative diastereomeric stereocenter configurations in complexes containing secondary fluoroalkyl ligands.

Solution conformations about the metal-carbon bond of the secondary fluoroalkyl ligands in iridium complexes [IrCp*(PMe3)(RF)X] [Cp* = C5Me5 RF = CF(CF3)2, X = I (1), CH3 (2); RF = CF(CF3)- (CF2CF3), X = I (4), CH3 (5)] have been determined using 19F{¹H} HOESY techniques. The molecules adopt the staggered conformation with the tertiary fluorine in the more hindered sector between the PMe3 and X ligands, with CF3 (and CF2CF3) substituents lying in the less hindered regions between PMe3 and Cp* or X and Cp*. In molecules containing the CF(CF3)2 ligand, these conformations are identical to those adopted in the solid state. For compound 4, containing the CF(CF3)(CF2CF3) ligand, two diastereomers are observed in solution. Solution conformations and relative stereocenter configuration assignments have been obtained using 19F{¹H} HOESY and correlated with the X-ray structure for the major diastereomer of 4, which has the (Slr, Sc) or (Rlr, Rc) configuration. Relative stereocenter configurations of analogue 5, for which no suitable crystals could be obtained, were assigned using 19F{¹H} HOESY and proved to be different from 4, with 5 preferring the (Slr, Rc) or (Rlr, Sc) configuration. [ABSTRACT FROM AUTHOR]

Reactions of iridium(fluoroalkyl)hydride complexes CpIr(PMe(3))(CF(2)R(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{(1)H}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(2)R(F))H and CpIr(PMe(3))(CF(2)R(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.

Solution conformations about the metal-carbon bond of the secondary fluoroalkyl ligands in iridium complexes [IrCp(PMe(3))(R(F))X] [Cp* = C(5)Me(5); R(F) = CF(CF(3))(2), X = I (1), CH(3) (2); R(F) = CF(CF(3))(CF(2)CF(3)), X = I (4), CH(3) (5)] have been determined using (19)F[(1)H] HOESY techniques. The molecules adopt the staggered conformation with the tertiary fluorine in the more hindered sector between the PMe(3) and X ligands, with CF(3) (and CF(2)CF(3)) substituents lying in the less hindered regions between PMe(3) and Cp or X and Cp. In molecules containing the CF(CF(3))(2) ligand, these conformations are identical to those adopted in the solid state. For compound 4, containing the CF(CF(3))(CF(2)CF(3)) ligand, two diastereomers are observed in solution. Solution conformations and relative stereocenter configuration assignments have been obtained using (19)F[(1)H] HOESY and correlated with the X-ray structure for the major diastereomer of 4, which has the (S(Ir), S(C)) or (R(Ir), R(C)) configuration. Relative stereocenter configurations of analogue 5, for which no suitable crystals could be obtained, were assigned using (19)F[(1)H] HOESY and proved to be different from 4, with 5 preferring the (S(Ir), R(C)) or (R(Ir), S(C)) configuration.