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 I with alcohols (methanol, butanol, iso-propanol, tert-butanol) were measured in dichloromethane at 30 degrees C. The observed nitrogen isotope effect k(14)/k(15) 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 ((PrOH)-O-i), 1.0087 +/- 0.0007 ((BuOH)-O-t)]. The pre-equilibrium step, with proton transfer from oxygen to nitrogen was proved by the inverse hydrogen effect k(ROH)/k(ROD)[0.778 +/- 0.052 (MeOH), 0.863 +/- 0.063 (BuOH), 0.883 +/- 0.080 ((PrOH)-O-i), 0.746 +/- 0.084 ((BuOH)-O-t)]. 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. (c) 2004 Elsevier B.V. All rights reserved.