Page 863 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

X: C X: 847
3.0 Z
1.5 C 2.5 C Z :X
1.0 1.0 Z 2.3 SECTION 10.2
0.0 0.5 0.7
–0.5 –0.3 The Diels-Alder Reaction
C X:
C X: –8.2 Z C Z :X
–9.1 –9.1 –9.0 –8.5 –8.5 –8.7
–9.1
–10.5 Z –9.5 –9.3
–10.9
Unsubstituted system Substituted Dienophiles 1-Substituted Dienes 2-Substituted Dienes
Fig. 10.5. Coefficients and relative energies of dienophile and diene frontier MOs. Orbital energies
are given in eV. The sizes of the circles give a relative indication of the orbital coefficient. Z stands
for a conjugated EWG, e.g., C=O, C≡N. NO 2 ; C is a conjugated substituent without strong electronic
effect, e.g., phenyl, vinyl; X is a conjugated ERG, e.g., OCH 3 ,NH 2 . From J. Am. Chem. Soc., 95,
4092 (1973).

From these ideas, we see that for substituted dienes and dienophiles there is
charge transfer in the process of formation of the TS. The more electron-rich reactant
acts as an electron donor (nucleophilic) and the more electron-poor reactant accepts
electron density (electrophilic). It also seems from the data in Tables 10.1 and 10.2
that reactions are faster, the greater the extent of charge transfer. The reactivity of
cyclopentadiene increases with the electron-acceptor capacity of the dienophile. Note
also that the very strongly electrophilic dienophile, tetracyanoethene, is more sensitive
to substituent effects in the diene than the more moderately electrophilic dienophile,
maleic anhydride. These relationships can be understood in terms of FMO theory by
noting that the electrophile LUMO and nucleophile HOMO are closer in energy the
stronger the substituent effect, as illustrated schematically in Figure 10.6.
The FMO considerations are most reliable when one component is clearly more
electrophilic and the other more nucleophilic. When a diene with a 2-EWG substituent

1.5
1.0

HOMO – LUMO gap narrows
as the substituent effect increases

– 9.1
–10.5

unsubstituted unsubstituted
dienophile EWG ERG diene
increasing increasing
electrophilicity nucleophilicity

Fig. 10.6. Schematic diagram illustrating substituent effect on reactivity
in terms of FMO theory. HOMO-LUMO gap narrows, transition state is
stabilized, and reactivity is increased in normal electron-demand Diels-
Alder reaction as the nucleophilicity of diene and the electrophilicity of
dienophile increase.