The stereoelectronic parameters of phosphites. The quantitative analysis of ligand effects (QALE)

Abstract

Through an analysis of pK_a values for HPZ₃⁺, ionization potentials (IP) for PZ₃, ν_CO for η-Cp(CO)(PZ₃)Fe(COMe)⁰, and E° (229 K) and ΔH° for the η-Cp(CO)(PZ₃)Fe(COMe)^+/0 couple, we determined the values of the three electronic parameters, χ_d, E_ar and π_p that are needed to describe the stereoelectronic properties of phosphites. The phenomenon we call the ‘aryl effect’, which is described by the parameter E_ar, is apparently not restricted to phosphines bearing pendant aryl groups but applies to the phosphites. π_p is a new parameter related to the π acidity of the phosphites. The phosphites studied are listed in terms of increasing π acidity: P(OMe)₃ ≈ P(OEt)₃ ≈ P(OBu)₃ ≈ P(O-i-Bu)₃ ≈ P(O-i-Pr)₃ < P(OCH₂CH₂Cl)₃ < P(O-p-XC₆H₄)₃ < P(OCH₂)₃CEt (X = MeO, Me, H, Cl, CN). Using these electronic parameters, along with Tolman’s cone angle, we have now incorporated the phosphites into the QALE (quantitative analysis of ligand effects) model and analyzed 32 sets of spectroscopic, thermodynamic, and kinetic data mostly taken from the literature. The analyses give a median r² value of 0.982.