Correlation of 31P nuclear magnetic resonance chemical shifts in aryl phosphinates with Hammett substituent constants: Inductive versus resonance interactions and relevance to pπ–dπ bonding

Substituent-induced chemical shifts and coupling constants in the 31P, 13C, and 1H nuclear magnetic resonance spectra of meta- and para-substituted phenyl dimethylphosphinates (1), methylphenylphosphinates (2), and diphenylphosphinates (3) have been determined in CDCl3 solvent. For all three series, a correlation of δ 31P with Hammett–Taft σ0 (or σ) constants is preferred over σ− on the basis of the correlation coefficient and standard deviations of the slope and intercept values. Electron-withdrawing substituents induce downfield shifts in δ 31P, in contrast to the inverse trends observed for structurally related series of oxyphosphorus acids and their derivatives. It is proposed that electron-withdrawing substituents act to deplete the electron density on the aryl oxygen, thereby weakening a pπ–dπ bonding interaction between the aryl oxygen and phosphorus. The resultants loss of d-orbital density on phosphorus causes a downfield shift in δ 31P in each of the phosphinate series. Phenyl substituents attached directly to phosphorus in series 2 and 3 increase the phosphoryl pπ–dπ back-bonding interactions, either through inductive or resonance effects, which leads to shielding of the phosphorus atom, overriding the anticipated downfield shift through inductive electron withdrawal of the phenyl substituents in series 2 and 3, relative to the methyls in series 1. Trends in Hammett ρ values for the plots of δ 31P and δ 13C versus σ0 and differences in the shielding of 13C and 1H nuclei of the methyl attached to phosphorus in series 1 and 2 suggest that the phenyl groups may interact in π bonding with the phosphorus atom through a resonance interaction.