posted on 2013-03-15, 00:00authored byLuis R. Domingo, Patricia Pérez, Daniela
E. Ortega
The reactions of bicyclic enone (BCE, 1) with cyclopentadiene
(Cp, 2) and the five-membered heterocyclic compounds
(FHCs) furan 3 and N-methyl pyrrole 4 for the construction of polycyclic heterocyclic compounds
have been studied at the B3LYP/6-31G* level. No reaction takes place
in the absence of Lewis acid (LA) catalysts as a consequence of the
high activation energy associated with these reactions. Electrophilic
activation of BCE 1 by formation of a complex with the
BF3 LA, 1-BF3, and
solvent effects favor the reactions. However, a different reactivity
is manifested by Cp 2 and FHCs 3 and 4. Thus, while the reaction of 1-BF3 with Cp 2 yields the expected exo [4 + 2] cycloadduct, the reactions of these FHCs yield
Michael adducts. In any case, the reactions are characterized by the
nucleophilic/electrophilic interaction between the most nucleophilic
centers of these dienes and the most electrophilic center of complex 1-BF3. The greater ability of FHCs 3 and 4 to stabilize positive charges opposed
to Cp 2 favors a stepwise mechanism with formation of
a zwitterionic intermediate. Although in most stepwise Diels–Alder
reactions, the subsequent ring closure has unappreciable barriers,
in these FHCs the abstraction of a proton with regeneration of the
aromatic ring becomes competitive. Thermodynamic calculations suggest
that the exergonic character of the formation of the Michael adducts
could be the driving force for the reactions involving FHCs.