Optically active five-co-ordinated phosphoranes. Apicophilicity of the amino-group compared with hydrogen: a comparative kinetic study. Influence of the activation entropy

Abstract

Two diasteroisomeric pairs of optically active spirophosphoranes have been synthesized, which differ only in the nature of the monodentate phosphorus substituent: H for (1) and PhNH for (2). A kinetic study of the isomerization (epimerization) of each of these pairs has been carried out by polarimetry. The difference in apicophilicity of the exocyclic ligand fails to explain the small difference between the barriers observed for (1) and (2). An accurate determination of the activation parameters shows that in these cases the variation in the isomerization barrier is essentially due to entropy factors.