Evidence for Elimination-Addition Mechanisms in the Reactions of N-t-Butyl-P-alkylphosphonamidic Chlorides with t-Butylamine and Isopropylamine

Article abstract of DOI:10.1039/P19830002127

The phosphonamidic chlorides RP(O)(Cl)NHBu^t (2; R=Me, Et, or Prⁱ) can be prapared directly from the corresponding phosphonic dichlorides RP(O)Cl2 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.