Dynamic nuclear magnetic resonance investigation of phosphonamidothioic and phosphonamidous chlorides

Abstract

NN-Di-isopropyl-P-alkylphosphonamidothioic chlorides, RP(S)(Cl)NPrⁱ₂(R = Me, Bu^t, 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)–NR²₂ 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).