Chemical shift non-equivalence of enantiomers in the proton magnetic resonance spectra of partly resolved phosphinothioic acids

Optically active samples of methylphenylphosphinothioic acid (2) and phenyl-t-butylphosphinothioic acid (3) give rise to distinct 1H n.m.r. spectra for the two enantiomers, with the separation between the two P-alkyl resonances depending on the enantiomer composition of the sample, the concentration and temperature of the n.m.r. solution, and the nature of the solvent. These optically active acids induce magnetic non-equivalence of enantiomers in racemic samples of other compounds, including O-methyl O-hydrogen phenylphosphonothioate (6), methyl-NP-diphenylphosphinic amide (8), and phenyl-t-butylphosphinic amide (9). The potential value of the optically active acids (2) and (3) as reagents for determining enantiomer ratios and configurations by n.m.r. spectroscopy is demonstrated using methylphenylphosphinic amide (7). The behaviour of all the systems examined can be understood in terms of molecular association arising from hydrogen bonding.