TEMPERATURE AND SOLVENT EFFECTS IN WITTIG REACTIONS

The rates of the Wittig reactions between 4-nitrobenzaldehyde and five stabilized phosphoranes, Ph 3 P=CHC(O)R, where R = OMe, OPh, NPh 2 , Me, and Ph, are studied in benzene, dimethyl formamid (DMF), and methanol, at 25, 35, and 45°C. The activation energies, E, when plotted as function of the corresponding activation entropies, ΔS*, are found in two different regions in the E - Δ S * diagram, approximately described by two parallel lines, one belonging to the reactions in benzene and DMF, the other to the reactions in methanol. The differences in the activation parameters in aprotic and protic solvents are most likely a consequence of strong H-bonding in protic solvents. Reaction mechanisms in aprotic and protic solvents are discussed.