Temperature and solvent effects in facial diastereoselectivity of nucleophilic addition: entropic and enthalpic contribution

Solvent and temperature effects on facial diastereoselectivity of nucleophilic addition have been long observed but often neglected. Temperature dependent measurements according to the modified Eyring equation allow the evaluation of stereoselectivity in terms of differential enthalpy and entropy of activation, and demonstrate the paramount importance of entropic contribution in directing the facial diastereoselectivity. Even the reaction solvent proved to be important in determining the isomer ratio. In many cases Eyring plots show a non-linear behavior consisting of two linear regions intersecting at a point called the inversion temperature (T-inv) which, for the same reaction, depends on the nature of the solvent and correlates with their melting points. We propose that T-inv is the temperature value for the interconversion between two different solvation clusters which should behave like two different molecules.