Lewis Acidity Scale of Diaryliodonium Ions toward Oxygen, Nitrogen, and Halogen Lewis Bases

摘要

Equilibrium constants for the associations of 17 diaryliodonium salts Ar_2I~+X~- with 11 different Lewis bases (halide ions, carboxylates, p-nitrophenolate, amines, and tris(p-anisyl)-phosphine) have been investigated by titrations followed by photometric or conductometric methods as well as by isothermal titration calorimetry (ITC) in acetonitrile at 20 ℃. The resulting set of equilibrium constants Kt covers 6 orders of magnitude and can be expressed by the linear free-energy relationship lg K_I = s_I LA_I + LB_I, which characterizes iodonium ions by the Lewis acidity parameter LA_I, as well as the iodonium-specific affinities of Lewis bases by the Lewis basicity parameter LB_I and the susceptibility s_I. Least squares minimization with the definition LA_I = 0 for Ph_2I~+ and s_I = 1.00 for the benzoate ion provides Lewis acidities LA_I for 17 iodonium ions and Lewis basicities LB_I and s_I for 10 Lewis bases. The lack of a general correlation between the Lewis basicities LB_I, (with respect to Ar_2I~+) and LB (with respect to Ar_2CH~+) indicates that different factors control the thermodynamics of Lewis adduct formation for iodonium ions and carbenium ions. Analysis of temperature-dependent equilibrium measurements as well as ITC experiments reveal a large entropic contribution to the observed Gibbs reaction energies for the Lewis adduct formations from iodonium ions and Lewis bases originating from solvation effects. The kinetics of the benzoate transfer from the bis(4-dimethylamino)-substituted benzhydryl benzoate Ar_2CH—OBz to the phenyl(perfluorophenyl)iodonium ion was found to follow a first-order rate law. The first-order rate constant k_(obs) was not affected by the concentration of Ph(C_6F_5)I~+ indicating that the benzoate release from Ar_2CH—OBz proceeds via an unassisted SNl-type mechanism followed by interception of the released benzoate ions by Ph(C_6F_5)I~+ ions.