Reactions of phosphonamidic acids and phosphonamidothioic acids with alcohols: mechanistic differences revealed by differing responses to steric effects

The formation of RP(X)(OH)ORʹ(R = Pr^i or Bu^t, Rʹ= Me or Pr^i) from RP(X)(OH)NHBut and RʹOH in CDCl3 is insensitive to steric effects when X = S but not when X = O (>10^3 times slower with R = Bu^t, Rʹ= Pri than with R = Pr^i, Rʹ= Me), pointing to a dissociative elimination–addition mechanism (metathiophosphonate intermediate) when X = S but an associative SN2(P) mechanism when X = O.