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

Solvent and temperature effects on facial diastereoselectivity of nucleophi lic addition have been long observed but often neglected. Temperature depen dent measurements according to the modified Eyring equation allow the evalu ation of stereoselectivity in terms of differential enthalpy and entropy of activation, and demonstrate the paramount importance of entropic contribut ion in directing the facial diastereoselectivity. Even the reaction solvent proved to be important in determining the isomer ratio. In many cases Eyri ng plots show a non-linear behavior consisting of two linear regions inters ecting at a point called the inversion temperature (T-inv) which, for the s ame reaction, depends on the nature of the solvent and correlates with thei r melting points. We propose that T-inv is the temperature value for the in terconversion between two different solvation clusters which should behave like two different molecules.