Level 2 – Case 18
120
N
CO 2 Me
Ph
CO 2 Et
EtO 2 C
N
CO 2 Me
Ph
N
N
CO 2 Me
Ph
EtO 2 C
EtO 2 C
NH
H
N
CO 2 Me
Ph
normal Diels-Alder
l
inverse Diels-Alder
Scheme 18.1
A change of the substituent at the C1 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
l
5 and 6 only take part in
inverse cycloaddition reactions (Fig. 18.1).
N
CO 2 Me
NMe 2
NO 2
N
CO 2 Me
N
CO 2 Me
CO 2 Et
1
2
3
4
5
6
1
2
3
4
4
1
2
3
4
Figure 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.
E Ex xp pe er ri im me en nt ta al l D Da at ta a
Frontier orbital energies (obtained from AM1 calculations)
1
1 The AM1 calculations described above were done in all cases considering the s-cis conformation of the diene.