Comparative Study of Diastereoisomer Interconversion in Chiral BINOL-ate and Diamine Platinum Complexes of Conformationally Flexible NUPHOS Diphosphines

Abstract

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 Δ H ⧧ and Δ S ⧧ 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 Δ H ⧧ value of 17 kcal mol - 1 has been determined for the conversion of δ-[(Ph 4 -NUPHOS)Pt{( S, S )-DPEN}]Cl 2 to λ-[(Ph 4 -NUPHOS)Pt{( S, S )-DPEN}]Cl 2, which could indicate that an alternative mechanism operates.