The Effect of Pressure on Retro Diels–Alder Reactions

The activation and reaction volumes (ΔV#/ΔV) were determined for the retro Diels–Alder reactions of the parent dihydrobarrelene 1a, its 2-cyano derivative 1b, the exo and endo Diels–Alder adducts of maleic anhydride to naphthalene exo-, endo-4, and the exo and endo Diels–Alder adducts of N-phenylmaleic imide to 6,6-dimethylfulvene exo-, endo-8 from the pressure dependence of the rate constants and the partial molar volumes (V) of reactants and products at various temperatures. The cleavage of exo-4, endo-4, and exo-8 are slightly accelerated by pressure showing negative volumes of activation (ΔV# < 0) whereas the others are slightly retarded (ΔV# > 0). From the analysis of the volume data including the van der Waals volumes (Vw) one can conclude that the packing coefficients of the pericyclic transition states are equal to or even larger than those of the corresponding Diels–Alder adducts. This finding may be explained with the restriction of the degrees of freedom in the transition states leading to a contraction of the expansion volume.