| Abstract
| - 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 Lewisacid (borane) and the solvent polarity (dichloromethane) have been taken into account. Calculated activationparameters, regioselectivities (for 1,3-dipolar cycloaddition reactions), and endo/exo stereoselectivitiesshow good agreement with available experimental data. Gas-phase calculations show a varied behaviorof the s-cis/s-trans TS stability for noncatalyzed reactions (from the systematic s-cis preference for thecyclopentadiene reaction to the systematic s-trans predilection encountered in the diphenylnitronecycloaddition). BH3 coordination leads to a preferential stabilization of s-trans TSs in the reactions ofcyclopentadiene (exo approach) and diphenylnitrone but a larger stabilization of s-cis structures in theprocesses involving the parent nitrone or 1-pyrroline-1-oxide. Additionally, a rather systematic preferentialstabilization of s-trans structures is induced by solvent polarity in most reactions. As a consequence, ans-trans preference is predicted in solution for both thermal and catalyzed types of reactions in mostapproaches. Such a conclusion is consistent with some experimental results suggesting a preference fora particular conformation of the methacrolein−Lewis acid complexes.
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