Dynamic nuclear magnetic resonance investigation of phosphonamidothioic and phosphonamidous chlorides

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Abstract
NN-Di-isopropyl-P-alkylphosphonamidothioic chlorides, RP(S)(Cl)NPri 2(R = Me, But, or Ph), show non-equivalent N-isopropyl groups in the proton n.m.r. spectra at low temperature. The barriers to isopropyl site-exchange have been determined by iterative bandshape analysis and lie in the range 9–12.6 kcal mol–1. These data are believed to refer to P–N bond rotation though alternative explanations are also considered. The corresponding NN-dimethyl compounds do not show any N-methyl non-equivalence at low temperature. Barriers to P–N bond rotation in some analogous phosphonamidous compounds, R1P(Cl)–NR2 2 have also been measured and are discussed in conjunction with literature data. The P–N rotational barriers generally increase with the steric bulk of the alkyl substituents, though the NN-dimethyl compounds show an unusual inverse steric effect of the P-alkyl substituent. The P-t-butyl compounds do not show any evidence of restricted rotation around the phosphorus-carbon bond at low temperature (–100 to –150 °C).

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