DIELS-ALDER-Reaktionen. II. Über die Anwendung linearer Freier-Energie-Beziehungen auf DIELS-ALDER-Reaktionen

Investigation of the reaction mechanism of [4 + 2] cyclization of 2,3 dimethylbuta-1,3-diene to methylacrylate using the Michaelis-Menten equation

The cycloaddition reaction between 2,3‑dimethylbuta-1,3-diene and methylacrylate proceeds by the second order kinetics. The rate constants increase with the increase in the excess of one of the reactants. The change in the effective rate constants is described by the Michaelis–Menten equation indicating that the reaction proceeds through the initial equilibrium stage of formation of a molecular complex stabilized by van der Waals interaction which then transforms into the product. The limiting stage of the reaction is established and its mechanism is suggested.

Conformational Preferences of Methacrolein in Diels−Alder and 1,3-Dipolar Cycloaddition Reactions

A B3LYP/6-31G* study has been carried out for the reactions of methacrolein with cyclopentadiene, parent nitrone, 1-pyrroline-1-oxide, and (Z)-C,N-diphenylnitrone, in which the coordination of a Lewis acid (borane) and the solvent polarity (dichloromethane) have been taken into account. Calculated activation parameters, regioselectivities (for 1,3-dipolar cycloaddition reactions), and endo/exo stereoselectivities show good agreement with available experimental data. Gas-phase calculations show a varied behavior of the s-cis/s-trans TS stability for noncatalyzed reactions (from the systematic s-cis preference for the cyclopentadiene reaction to the systematic s-trans predilection encountered in the diphenylnitrone cycloaddition). BH3 coordination leads to a preferential stabilization of s-trans TSs in the reactions of cyclopentadiene (exo approach) and diphenylnitrone but a larger stabilization of s-cis structures in the processes involving the parent nitrone or 1-pyrroline-1-oxide. Additionally, a rather systematic preferential stabilization of s-trans structures is induced by solvent polarity in most reactions. As a consequence, an s-trans preference is predicted in solution for both thermal and catalyzed types of reactions in most approaches. Such a conclusion is consistent with some experimental results suggesting a preference for a particular conformation of the methacrolein-Lewis acid complexes.

Do secondary orbital interactions really exist?

A revision of the most typical examples used to illustrate the existence of secondary orbital interactions (SOI) has been achieved. However, our analysis indicates that no conclusive evidence can be obtained from these cases. All five examples proposed by Woodward and Hoffmann in The Conservation of Orbital Symmetry have been revisited. A combination of well-known mechanisms (such as solvent effects, steric interactions, hydrogen bonds, electrostatic forces, and others) can be invoked instead to justify the endo/exo selectivity of Diels-Alder reactions.