Chiral Phosphines: The Effect of Asymmetric Carbon Centers α or β to Phosphorus on the31P NMR Spectra

Abstract

Chiral alkylphosphines containing asymmetric carbon centers either alpha or beta to the phosphorus are formed from the free radical addition of phosphine to prochiral olefins. The phosphorus NMR spectra of the primary phosphines have only one signal for the enantiomeric pair while the spectra of the secondary phosphines have three signals in a 1:2:1 ratio. This is the statistical ratio expected for the enantiomeric pair and two meso forms. Four enantiomeric pairs of tertiary phosphines can be formed from one prochiral olefin. One pair has alkyl groups of the same configuration while the remaining pairs have mixed configurations. The phosphorus NMR spectra show two signals in a 1:3 ratio. Dilution of the secondary phosphines with a polar solvent results in only one signal in the phosphorus NMR spectra. The chemical shifts and diluent effect can be explained in terms of the gamma shielding effect (1).