Equilibrium Acidities and Homolytic Bond Dissociation Enthalpies of the Acidic C−H Bonds in Phosphonates and Related Phosphorus Containing Compounds

Abstract

The homolytic bond dissociation enthalpies (BDEs) of the acidic C-H bonds in a series of phosphonates and related phosphorus-containing compounds have been determined by a combination (eq 3) of their equilibrium acidities (pK(HA)'s) and the oxidation potentials [E(ox)(A(-))'s] of their conjugate anions, which were measured in DMSO solution. The equilibrium acidity increases by 15.5, 14.9, and 10.9 pK units for the introduction of an alpha-P(O)(OEt)(2) group into toluene, acetonitrile, and ethyl acetate, respectively. Comparison with the published equilibrium acidity increases by 25.6, 24.4, and 21.0 pK units for the introduction of an alpha-(+)PPh(3) group into the same series of substrates indicates that the phosphoryl carbanions (Horner-Wadsworth-Emmons reagents) are about 10 pK units more reactive than the corresponding triphenylphosphonium ylides (Wittig reagents). However, both alpha-P(O)(OEt)(2) and alpha-(+)PPh(3) groups have negligible effects on the adjacent C-H BDEs, indicating that there is no resonance delocalization into the 3d vacant orbitals of phosphorus and that their acidifying effects are exclusively associated with the field/inductive (electrostatic) and polarizability effects. The acidifying effect of an alpha-CO(2)Et group is shown to be ca. 3.5 pK units stronger than that of an alpha-P(O)(OEt)(2) group in the same substrates, suggesting that the larger acidifying effect of the alpha-CO(2)Et group is predominantly associated with the resonance delocalization rather than with the field/inductive (electrostatic) and polarizability effects.