Comparative study of diastereoisomer interconversion in chiral BINOL-ate and diamine platinum complexes of conformationally flexible NUPHOS diphosphines

A thorough and detailed study of diastereointerconversion in the chiral platinum complexes [(NUPHOS)Pt{(S)-BINOL}] (3a-e) has been undertaken and compared with the results of a similar study with [(BIPHEP)Pt{(S)-BINOL}]. Rate data revealed that this process obeys first-order relaxation kinetics, and rate constants for conversion of the minor to the major diastereoisomer have been obtained. Eyring analysis of the data gave ΔH^‡ and ΔS^‡ values of 22-25 kcal mol^-1 and -1 to -16 eu, respectively. In combination with computational analysis, these studies indicate that atropinversion most likely occurs via an on-metal pathway involving a planar seven-membered transition state. Substitution of (S)-BINOL for (S,S)-DPEN results in a marked reduction in the barrier to atropinversion; a ΔH^‡ value of 17 kcal mol^-1 has been determined for the conversion of δ-[(Ph₄-NUPHOS)Pt{(S,S)-DPEN}]-Cl₂ to λ-[(Ph₄-NUPHOS)Pt{(S,S)-DPEN}]Cl₂, which could indicate that an alternative mechanism operates.