Evidence for elimination–addition mechanisms in the reactions of N-t-butyl-P-alkylphosphonamidic chlorides with t-butylamine and isopropylamine

Abstract

The phosphonamidic chlorides RP(O)(Cl)NHBu^t(2; R = Me, Et, or Prⁱ) can be prepared directly from the corresponding phosphonic dichlorides RP(O)CI₂ and t-butylamine. t-Butylphosphonic dichloride reacts with t-butylamine only at high temperatures and then gives the diamide Bu^tP(O)(NHBu^t)₂ as the only detectable product. The phosphonamidic chloride (2; R = Bu^t) can, however, be prepared indirectly by oxidation (SO₂Cl₂) of Bu^tP(Cl)NHBu^t obtained from Bu^tPCl₂ and t-butylamine. The phosphonamidic chlorides react with t-butylamine and isopropylamine in dichloromethane at rates that are unusually insensitive to steric effects in both the substrate and the amine; the slowest reaction [(2; R = Bu^t)+ Bu^tNH₂] is only 30 times slower than the fastest [(2; R = Me)+ PrⁱNH₂]. This is attributed to the reactions proceeding by elimination-addition mechanisms rather than by nucleophilic attack at phosphorus. In competitive experiments with t-butylamine and isopropylamine a rather small preference (1.4–4.0) for formation of the product derived from the less hindered amine is consistent with the (partial) involvement of a reactive monomeric metaphosphonimidate intermediate.