Page 503 - Advanced Organic Chemistry Part B - Reactions & Synthesis

a) Coefficient at C(2) is higher than at C(1) in the LUMO of a dienophile bearing and electron-withdrawing 477
substituent.
SECTION 6.1
1 1 Diels-Alder Reactions
2 2
EWG EWG

(b) Coefficient at C(4) is higher than at C(1) in HOMO of a diene bearing an electron-releasing substituent
at C(1).

3 2 3 2
4 ERG 4 ERG
1 1

(c) Coefficient at C(1) is higher than at C(4) in HOMO of a diene bearing an electron-releasing substituent
at C(1).
ERG
3 2 ERG
3 2
4 1 4 1

(d) The regioselectivity of the Diels-Alder reaction corresponds to matching the carbon atoms
having the largest coefficients of the frontier orbitals.

ERG ERG ERG
EWG EWG EWG
+ +
favored

“ortho”-like orientation:
ERG ERG ERG
+ +
EWG EWG EWG
favored
“para”-like orientation:
Fig. 6.2. HOMO-LUMO interactions rationalize regioselectivity of Diels-Alder reactions.

Stereospecificity also is exhibited for dienes having stronger electron-releasing groups,
such as trimethylsiloxy.

CO 2 C 2 H 5
C 2 H 5 O 2 C CO 2 C 2 H 5 TMSO
TMSO TMSO CO 2 C 2 H 5
CO 2 C 2 H 5
C 2 H 5 O 2 C CH 2
CO 2 C 2 H 5
CO 2 C 2 H 5 CH 2
77%
39%
Ref. 9
9
M. E. Jung and C. A. McCombs, Org. Synth., 58, 163 (1978); M. E. Jung and C. A. McCombs,
Tetrahedron Lett., 2935 (1976).