Unusual Diels-Alder Reactivity of Acyclic 2-Azadienes

Unusual Diels-Alder Reactivity of Acyclic 2-Azadienes 

A change of the substituent at the Cl position results in a radical modification of the reactivity of azadienes 1 towards dienophiles. Thus, azadiene 4 only participates in normal Diels-Alder processes, whereas azadienes 5 and 6 only take part in inverse cycloaddition reactions (Fig. 18.1). 

Considering the Frontier Molecular Orbitals (FMO) involved justifies the unexpected normal Diels-Alder reactivity of 2-azadiene 1 toward diethylfumarate. Compare the reactivities of butadiene and 2-azadiene 1 and discuss the dual reactivity of this compound with respect to both, electron-deficient and electron-rich dienophiles. Justify the observed reactivity of dienes 4, 5 and 6 in Diels-Alder reactions. 

Frontier orbital energies (obtained from AMI calculations)  

Ehomo = Ehomo Ehomo = Ehomo = Ehomo = Ehomo Ehomo = 

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