The effect of pressure on retro Diels-Alder reactions

The activation and reaction volumes (Delta V-double dagger/Delta V) were de termined for the retro Diels-Alder reactions of the parent dihydrobarrelene 1a, its 2-cyano derivative 1b, the exo and endo Diels-Alder adducts of mal eic 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 t he 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 (Delta V-double dagger < 0) whereas the others are sl ightly retarded (Delta V-double dagger > 0). From the analysis of the volum e data including the van der Waals volumes (V-w) one can conclude that the packing coefficients of the pericyclic transition states are equal to or ev en larger than those of the corresponding Diels-Alder adducts. This finding may be explained with the restriction of the degrees of freedom in the tra nsition states leading to a contraction of the expansion volume.