Gibbs energies, Diels-Alder reactions

Figure 7.3 Standard Gibbs energies of transfer for reactants and activated complex for the Diels-Alder reaction of cyclopentadiene ( , ) with ethyl vinyl ketone (2, A) from 1-PrOH to 1-PrOH-water as a function of the mole fraction of water initial state (1 + 2, ) activated complex (o).

In the retro-Diels-Alder reaction of anthracenedione [42], the volume of activation is small. Acceleration in water cannot come from a change in the hydration shell of the molecule. Hydrophobic interactions are negligible and aqueous acceleration is caused by the hydrogen-bond donating ability of water, which stabilizes the polarized activated complex. The Gibbs energy of activation displays a fair linear correlation with the Ej parameter. Hexafluoroisopropanol with an Ej value of 65.3 is even more efficient as a solvent than water Ej= 63.1) which appears to be less polar [41]. 

The origin of the acceleration of the aqueous aza-Diels-Alder reaction comes from a large decrease in the Gibbs energy of the activated complex, since cycloreversion smoothly occurs when 2-azanorbornene hydrochlorides are kept in water at room temperature in the presence of N-methylmaleimide as a trapping agent for cyclopentadiene formed in the process (Scheme 11) [67]. 

A new approach to determination of partial rate constants for reactions of conformers has been developed on the basis of constituents of the Gibbs energy of activation for reactions of a series of conformationally heterogeneous substrates. The proposed model allows solving of formal kinetic tasks under conditions of thermodynamic control and in the absence of diastereoisomeric products. The p-values and partial rate constants have been determined for the chair and twist conformers of a series of 2-substituted l,3-dioxacyclohept-5-enes in the model Diels-Alder reaction with l,2,4>5-tetrazine-3,6-dicarboxylate in two solvents. In dioxane, the chair conformer reacts 3.4 times faster and, in acetone, 1.4 times faster than does the twist conformer <1996RJC477>. 

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