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

Reactions of iridium(fluoroalkyl)hydride complexes Cp*Ir(PMe3)(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(PMe3)(CYFRF)X as variable mixtures of diastereomers. Solution conformations and rel...

Full description

Saved in:
Bibliographic Details
Published in:Journal of the American Chemical Society 2005-11, Vol.127 (44), p.15585-15594
Main Authors: Garratt, Shaun A, Hughes, Russell P, Kovacik, Ivan, Ward, Antony J, Willemsen, Stefan, Zhang, Donghui
Format: Article
Language:English
Subjects:
Chemistry
Exact sciences and technology
Kinetics and mechanisms
Organic chemistry
Reactivity and mechanisms
Citations: Items that this one cites
Items that cite this one
Online Access:Get full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary:Reactions of iridium(fluoroalkyl)hydride complexes Cp*Ir(PMe3)(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(PMe3)(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 (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/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.
ISSN:0002-7863
1520-5126
DOI:10.1021/ja0545012