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). BH
3 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, an
s-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.
