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

Abstract

The formation of RP(X)(OH)OR′ (R = Prⁱ or Bu^t, R′ = Me or Prⁱ) from RP(X)(OH)NHBu^t and R′OH in CDCl₃ is insensitive to steric effects when X = S but not when X = O (>10³ times slower with R = Bu^t, R′ = Prⁱ than with R = Prⁱ, R′ = Me), pointing to a dissociative elimination–addition mechanism (metathiophosphonate intermediate) when X = S but an associative S_N2(P) mechanism when X = O.