Reactions of phosphonamidic acids and phosphonamidothioic acids with alcohols: mechanistic differences revealed by differing responses to steric effects.
The formation of RP(X)(OH)OR' (R = Pr(i) or Bu(t), R' = Me or Pr(i)) from RP(X)(OH)NHBu(t) and R'OH in CDCl3 is insensitive to steric effects when X = S but not when X = O (> 10(3) times slower with R = Bu(t), R' = Pr(i) than with R = Pr(i), R' = Me), pointing to a dissociative elimination-addition mechanism (metathiophosphonate intermediate) when X = S but an associative S(N)2(P) mechanism when X = O.