Associative and dissociative mechanisms for the reactions of N-t-butyl-P-phenylphosphonamidic chloride with isopropylamine and t-butylamine: competitive, kinetic, and stereochemical studies

The substitution reactions of N-t-butyl-P-phenylphosphonamidic chloride (4a) with isopropylamine and t-butylamine (or t-pentylamine in stereochemical experiments) in MeCN at O °C can apparently proceed by one associative and two dissociative mechanisms viz: SN2(P)[first-order in amine (nucleophile), high selectivity for PriNH2 in PriNH2–ButNH2 competitive experiments; complete stereospecificity]; simple elimination–addition with a free (symmetrically solvated) metaphosphonimidate intermediate [first-order in amine (base); low selectivity; practically complete racemisation]; preassociation elimination–addition [second-order in amine (nucleophile and base); low selectivity; extensive but not complete racemisation].