Steric and Electronic Effects on the Stereochemistry of the Alkaline Hydrolysis of Acyclic Dialkoxyphosphonium Salts. Pseudorotation of Intermediates in Phosphorus Ester Reactions

The stereochemistry of the alkaline hydrolysis of three acyclic dialkoxyphosphonium salts, (R)- and (S)-menthoxymethoxymethylphenylphosphonium hexachloroantimonate (2) and ethoxymethoxymethylphenylphosphonium hexachloroantimonate (3), was investigated and compared to the previous results obtained for the alkaline hydrolysis of (R)- and (S)-ethoxymenthoxymethylphenylphosphonium hexachloroantimonate (1). The extent to which the intermediate phosphorane containing one apical alkoxy group and one equatorial alkoxy group can undergo pseudorotation prior to direct loss of the apical alkoxide increases by a factor of 10 and 70 as the apical alkoxy group is varied from methoxy to ethoxy or menthoxy, respectively, but decreases by a factor of 0.6 and 0.3, respectively, as the same variation is made in the equatorial alkoxy group. Thus, depending only on the nature of the alkoxy ligands, a reaction can proceed either with predominant (>90%) direct loss of an alkoxide or predominantly by a pathway involving pseudorotation. In contrast to previous explanations, the “bulky” menthoxy group exerts no observable steric driving force to occupy the less hindered equatorial position of a phosphorane. The extension of these results to the relative stability of intermediates in nucleophilic displacement reactions of phosphorus esters is discussed.