High pressure, temperature, and solvent effect on the Diels–Alder cycloaddition reaction between thiobenzophenone and cyclopentadiene

The data on high hydrostatic pressure, temperature, and solvent influence on the Diels–Alder reaction rate of thiobenzophenone with cyclopentadiene have been obtained. Activation enthalpies, entropies, volumes, and reaction volumes in several solvents have been determined. The activation entropies and volumes of the Diels–Alder reaction of thiobenzophenone with cyclopentadiene are close to the corresponding activation parameters of Diels–Alder reactions involving dienophiles with C=C and N=N bonds. The rate of thiobenzophenone-cyclopentadiene reaction did not increase with increasing solvent polarity, which is also characteristic of other Diels–Alder reactions. The reaction of thiobenzophenone with cyclopentadiene is characterized by an “anomalous” ratio of the activation volume to the reaction volume ∆V^≠/∆V_r-n > 1. This can be explained by the less steric hindrances of transition-state molecules toward the solvent, compared with adduct molecules. The activities of a number of dienophiles with C=C, C=S, and N=N bonds have been compared, and the factors determining their reactivity in Diels–Alder reactions have been established.