Thermal and microwave assisted [4+2] cycloadditions of 1,4-diaryl-1-aza-1,3-butadienes with allenicesters lead to cycloadducts, which after a 1,3-H shift afford variedly substituted unsymmetrical 2-alkyl-1,4-diaryl-3-ethoxycarbonyl-1,4-dihydropyridines in high yields. Reactions carried out under microwaveirradiation are cleaner and give higher yields with much shortened reaction times. Density functionaltheory (DFT) at the B3LYP/6-31G* level has been used to calculate geometric features of the reactants,barrier for s-trans to s-cis and reverse isomerization of azadienes (
5a-
d,
10a-
e), dihedral angles betweenN
1, C
2, C
3, and C
4 atoms of azadienes along with various indices such as chemical hardness (
), chemicalpotential (
), global electrophilicity (
), and the difference in global electrophilicity (
) between thereacting pairs and Fukui functions (
f + and
f -). The results revealed that s-trans is the predominantconformation of azadienes at ambient temperature and the barrier for conversion of the s-trans rotamerof 1-azadienes to s-cis may be the major factor influencing the chemoselectivity, i.e., [4+2] verses [2+2]cycloaddition. The regiochemistry of the observed cycloadditions is collated with the obtained localelectrophilicity indices (Fukui functions). Transition states for the formation of both [4+2] and [2+2]cycloadducts as located at the PM3 level indicate that the transition state for the formation of [4+2]cycloadducts has lower energy, again supporting the earlier conclusion that preferred formation of [4+2]cycloaaducts at higher temperature may be a consequence of barrier for s-trans to s-cis transformation of1-azadienes.
