Kinetic isotope effects of nitrogen and hydrogen in reaction of N-tert-butyl-P-phenylphosphonamidothioic acid with alcohols

Nitrogen and hydrogen isotope effects for the reaction of N-tert-butyl-P-phenylphosphonamidothioic acid 1 with alcohols (methanol, butanol, iso-propanol, tert-butanol) were measured in dichloromethane at 30 °C. The observed nitrogen isotope effect k14/k15 is only slightly sensitive to a steric hindrance of the alcohol [1.0070 ± 0.0002 (MeOH), 1.0074 ± 0.0004 (BuOH), 1.0062 ± 0.0004 (PriOH), 1.0087 ± 0.0007 (ButOH)]. The pre-equilibrium step, with proton transfer from oxygen to nitrogen was proved by the inverse hydrogen effect kROH/kROD[0.778 ± 0.052 (MeOH), 0.863 ± 0.063 (BuOH), 0.883 ± 0.080 (PriOH), 0.746 ± 0.084 (ButOH)]. The experimental values are consistent with theoretical results of semiempirical calculations on PM3 level for an elimination–addition mechanism and metathiophosphonate PhPSO intermediacy. For the reaction with methanol the addition–elimination mechanism is also possible.