Optically active spirophosphoranes. Part VI. Kinetics of epimerization of the chiral skeleton unit

Abstract

The diastereoisomeric spirophosphoranes (1M) and (1P) possess two elements of chirality, an optically active ligand derived from (+)-pseudoephedrine and the (±)-phospha^vspironone system. The rates of epimerization of these compounds were obtained from a pure diastereoisomer [very likely (1M)] isolated by crystallization using the polarimetric kinetic method. Activation parameters at 278 K are: for (1M)→(1P);ΔG₁‡= 21·44 kcal mol^–1, ΔH₁‡= 19·8 kcal mol^–1, ΔS₁‡=–5·7 cal K^–1; for (1P)→(1M)ΔG_–1‡= 21·10 kcal mol^–1, ΔH_–1‡= 19·4 kcal mol^–1, ΔS_–1‡=–5·9 cal K^–1 mol^–1. These results, compared with those obtained previously for (2M)→(2P), show that the change in the rate constant of epimerization can be large (133:1) even though the atoms surrounding phosphorus are the same. Comments on the possible structure of the transition state are presented.