The proton magnetic resonance spectra of chiral phosphinate esters. Chemical shift non-equivalence of enantiomers induced by optically active phosphinothioic acids
The 1H n.m.r. spectrum of racemic methyl methylphenylphosphinate [MePhP(O)OMe] recorded in CCl4 solution in the presence of 1 mol. equiv. of (+)-(R)-phenyl-t-butylphosphinothioic acid or (–)-(S)-methylphenylphosphinothioic acid contains well separated P-methoxy signals (Δδ 7.0 or 5.4 Hz at 100 MHz) for the two enantiomers. For non-racemic samples of the phosphinate ester the enantiomer ratio can be easily and accurately measured. Other phosphinates [e.g. RPhP(O)OR′; R = Me, Et, Pri, But, Ph; R′= Me, Ph] and thiophosphinates [RPhP(O)SR′ but not RPhP(S)OR′] exhibit similar induced chemical shift non-equivalence of enantiomers. The non-equivalence is attributed to the formation of diastereoisomeric complexes in which the optically active acid is hydrogen bonded to the PO group of the enantiomeric esters.