Proton magnetic resonance non-equivalence of the enantiomers of alkylphenylphosphinic amides

The 1H n.m.r. spectra of optically active (but optically impure) samples of methylphenylphosphinic amide (3) and its N-phenyl (1) and N-p-nitrophenyl (2) analogues exhibit distinct signals for the P-methyl groups in the R- and S-enantiomers. The two enantiomers in a racemic sample of (1), (2), or (3) give rise to only one P-methyl resonance, but this separates into two distinct signals when a foreign optically active phosphinic amide is added. Similar enantiomer non-equivalence can be induced in racemic samples of phenyl-t-butylphosphinic amide (6) and (N-phenyl)phenyl-t-butylphosphinic amide (7), while the enantiotopic methyl groups in (N-phenyl)dimethylphosphinic amide (8) or dimethylphosphinic amide (9) become non-equivalent in the presence of optically active (1). The observed spectra can all be rationalised in terms of molecular association through hydrogen bonding.