NAD(P)+–NAD(P)H Models. 81. Temperature-Dependent Stereospecificity in the Interconversion between Central and Axial Chiralities

In the oxidation of (4R)-Me3MQPH or (4R)-Me3PNPH with a series of 1,4-benzoquinone and its derivatives, the R/S ratio with respect to the axial chirality in the product, i.e., 11R-/11S- isomer ratio in Me3MQP+ or 7R-/7S- isomer ratio in Me3PNP+, changes depending on the reaction temperature as well as the reactivity of quinone. It is confirmed that kinetic R-preference stems from the entropy-control, whereas kinetic S-preference comes out of the enthalpy-control. The relative importance of enthalpy and entropy is discussed in terms of the earliness and lateness of the transition state at the initial electron-transfer step.