Solvent and temperature effect in aldol condensation between the lithium enolate of tert-butyl acetate and 2-phenyl propanal: enthalpy and entropy contribution

An analysis of nucleophilic addition of the lithium enolate of tert-butyl a cetate and 2-phenyl propanal in two different solvents. such as THF and n-h exane, revealed the great importance of solvent effect in determining the s tereoselectivity. In particular, temperature-dependent measurements of dias tereomeric ratio allow the evaluation of diastereoselectivity in terms of d ifferential enthalpy and entropy of activation for this reaction. Either in THF or n-hexane we obtained a predominance of the anti isomer in all tempe rature ranges. but in THF the diastereoselection is controlled by the diffe rential activation enthalpy, whereas in n-hexane it is the sole differentia l activation entropy that accounts for the anti predominance. (C) 2001 Publ ished by Elsevier Science Ltd.