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.