Chapter 39 — Quiz

Twenty-five questions on pericyclic reactions and Woodward-Hoffmann rules. ∗ marks questions answered in the answer key.

Multiple choice

1.∗ A pericyclic reaction is: (a) concerted, going through a cyclic transition state with no intermediates (b) stepwise, involving cation or radical intermediates (c) only photochemical (d) only thermal

2.∗ The Diels-Alder cycloaddition [4+2] is thermally: (a) allowed (4n+2 = 6 electrons; aromatic-like TS) (b) forbidden (c) only works with UV light (d) only with Pd catalyst

3.∗ The [2+2] cycloaddition (two alkenes) is thermally: (a) allowed (b) forbidden (4n = 4 electrons; antiaromatic-like TS) (c) only with high pressure (d) always works at room T

4.∗ The [2+2] cycloaddition is photochemically: (a) allowed (one electron promoted; orbital symmetry favorable) (b) forbidden (c) only thermal (d) requires Pd catalyst

5.∗ Electrocyclic closure of 1,3-butadiene (4 π electrons) is thermally: (a) conrotatory (4n electrons rule) (b) disrotatory (c) doesn't work (d) only with UV

6.∗ Electrocyclic closure of 1,3,5-hexatriene (6 π electrons) is thermally: (a) conrotatory (b) disrotatory (4n+2 electrons rule) (c) doesn't work (d) only photo

7.∗ A [3,3] sigmatropic rearrangement (e.g., Cope, Claisen): (a) is the family of 6-electron sigmatropic shifts; thermal allowed via suprafacial-suprafacial (b) is a hydride shift (c) involves 4 electrons (d) requires light

8.∗ Diels-Alder is: (a) [4+2] cycloaddition (b) [3,3] sigmatropic (c) electrocyclic (d) [1,5] hydrogen shift

9.∗ The Woodward-Hoffmann rules and Fukui's FMO theory were recognized by Nobel: (a) 1981 (Fukui and Hoffmann; Woodward had passed) (b) 1965 (Woodward, Hoffmann, Fukui) (c) 1979 (Woodward only) (d) 2005

10.∗ Pericyclic reactions usually have: (a) concerted mechanism (no intermediates) (b) radical intermediate (c) carbocation intermediate (d) carbanion intermediate

11.∗ A [3,3] Cope rearrangement of (E,E)-1,5-hexadiene goes through: (a) chair-like TS (preferred) (b) only boat-like TS (c) random (d) doesn't have a TS

12.∗ The Claisen rearrangement converts an allyl vinyl ether to: (a) γ,δ-unsaturated carbonyl (b) α,β-unsaturated alcohol (c) cyclobutane (d) ester

13.∗ Why is the Diels-Alder so widely used in synthesis? (a) builds a 6-membered ring with multiple stereocenters in one step (b) thermally allowed (c) stereospecific (d) all of the above

14.∗ A [1,5]-H shift in a 1,3-pentadiene is thermally: (a) allowed (6 electrons total; suprafacial) (b) forbidden (c) only with UV (d) requires Pd catalyst

15.∗ A [1,3]-H shift is thermally: (a) allowed (b) forbidden suprafacially (4 electrons; would have to be antarafacial, which is geometrically difficult) (c) only with light (d) only with Pd

16.∗ The vitamin D photosynthesis involves a: (a) photochemical electrocyclic ring-opening of 7-dehydrocholesterol's B-ring (conrotatory) followed by [1,7]-H sigmatropic shift to give vitamin D₃ (b) only enzyme catalysis (c) only oxidation (d) only fatty acid metabolism

17.∗ Why does the vitamin D photosynthesis require UVB light specifically (280-315 nm)? (a) the energy gap of the conjugated steroid π system corresponds to UVB photons (b) any UV works (c) only sunlight at high noon (d) only red light

18.∗ A 1,3-dipolar cycloaddition (e.g., azide + alkyne; the "click" reaction) is thermally: (a) allowed (6 total electrons: 4 from dipole + 2 from dipolarophile) (b) forbidden (c) only photochemical (d) requires Pd

19.∗ Why do Diels-Alder reactions often go through a chair-like TS? (a) the 6-membered TS minimizes torsional and steric strain (like a chair cyclohexane) (b) only with bulky dienes (c) it doesn't; boat is preferred (d) random TS

20.∗ Pericyclic reactions are stereospecific because: (a) the cyclic TS imposes specific geometric constraints; product stereochemistry is determined by reactant stereochemistry (b) of orbital symmetry (c) of substrate steric effects (d) all of the above

Short answer

21. Sketch the cyclic TS for a Diels-Alder ([4+2]). Identify which orbitals interact (HOMO of diene + LUMO of dienophile). Why is this 6-electron, aromatic-like TS thermally allowed?

22. Predict whether each is thermally allowed, photochemically allowed, or both: (a) [4+2] (Diels-Alder) (b) [2+2] (c) Electrocyclic closure of 4π (conrotatory or disrotatory?) (d) [3,3] Claisen

23. Sketch the chair-like TS for a [3,3] Cope rearrangement of 1,5-hexadiene.

24. Explain the vitamin D photosynthesis: 7-dehydrocholesterol + UVB → previtamin D₃ → vitamin D₃. What is the photochemical step? What is the thermal step?

25. Why is the [2+2] cycloaddition thermally forbidden but photochemically allowed?

Answer key

  1. a — Pericyclic = concerted, cyclic TS.
  2. a — [4+2] = 6 electrons = thermally allowed.
  3. b — [2+2] = 4 electrons = thermally forbidden.
  4. a — [2+2] photochemically allowed (HOMO-LUMO* interaction works).
  5. a — 4n electrons → conrotatory thermal.
  6. b — 4n+2 electrons → disrotatory thermal.
  7. a — [3,3] sigmatropic = Cope/Claisen family.
  8. a — Diels-Alder = [4+2].
  9. a — 1981 Nobel.
  10. a — Pericyclic = concerted.
  11. a — Cope chair-like TS preferred.
  12. a — Claisen → γ,δ-unsaturated carbonyl.
  13. d — All listed.
  14. a — [1,5]-H shift thermally allowed (6 electrons).
  15. b — [1,3]-H shift thermally forbidden (suprafacial).
  16. a — Vitamin D photosynthesis description.
  17. a — UVB matches steroid π* energy.
  18. a — Click 1,3-dipolar; 6 electrons total; thermal allowed.
  19. a — Chair TS minimizes strain.
  20. d — All factors contribute.

21. Diels-Alder TS: 6-membered cyclic arrangement with the diene's two ends and the dienophile's two ends forming the new bonds. The HOMO of the diene (a 4-π MO with 2 nodes) overlaps with the LUMO of the dienophile (a 2-π MO with 1 node). The total π system in the TS has 6 electrons distributed in 3 occupied orbitals — analogous to benzene's 6 π electrons in 3 occupied orbitals. This aromatic transition state is thermally allowed (4n+2 = 6 electrons).

22. (a) [4+2] Diels-Alder: thermally allowed (4n+2 = 6 electrons). (b) [2+2]: thermally forbidden; photochemically allowed (4n = 4 electrons). (c) Electrocyclic closure of 4π: thermal conrotatory (4n rule); photo disrotatory. (d) [3,3] Claisen: thermally allowed (4n+2 = 6 electrons; suprafacial-suprafacial).

23. Cope chair TS: The 6-carbon chain of 1,5-hexadiene wraps into a chair-like 6-atom TS where: - C1 and C6 are the migrating ends. - C2, C3, C4, C5 form the central core. - The C1-C2 σ bond breaks; a new C1-C6 σ bond forms. - C3=C4 becomes a single bond; a new C5=C6 double bond forms. The chair conformation places substituents in equatorial positions; this is the lower-energy TS.

24. Vitamin D photosynthesis: - Photochemical step: 7-dehydrocholesterol absorbs UVB light (~290 nm). This excites a π → π transition in the steroid's B-ring (which has a conjugated triene system). The excited state undergoes a conrotatory photochemical electrocyclic ring-opening (4π photochemical = conrotatory, by Woodward-Hoffmann rules). The B-ring of the steroid opens; the new molecule is previtamin D₃ (a hexatriene with the steroid skeleton intact except for the open B-ring). - Thermal step: previtamin D₃ undergoes a [1,7]-sigmatropic hydrogen shift (thermally allowed; suprafacial; 8 electrons total = 4n, but actually [1,7] over a 7-atom span is 8 electrons, antarafacial — wait, let me recheck). Actually [1,7]-H shift is allowed antarafacially-suprafacially. Thermal allowed with 8 electrons: this is correct; antarafacial. The [1,7]-shift gives vitamin D₃* (cholecalciferol), the active form. Why UVB? The photon energy ~3-4 eV (~280-315 nm) matches the HOMO-LUMO gap of the conjugated triene in 7-dehydrocholesterol's B-ring.

25. [2+2] thermal: 4 electrons total. By Woodward-Hoffmann (suprafacial-suprafacial assumption), 4n electrons → forbidden thermally. The orbital-symmetry mismatch: HOMO of one alkene + HOMO of the other doesn't have the right phase to form two new σ bonds at both ends.

[2+2] photochemical: One electron is promoted from HOMO to LUMO upon UV absorption. The HOMO + LUMO of the two alkenes now have the right phase to form the two new bonds. The reaction goes through an excited-state intermediate. The 4-electron problem becomes a 4-electron-but-with-promotion → allowed.

Mechanistically: in the excited state, one alkene has 1 electron in its π and 1 in its π. The π of the excited alkene + π of the ground-state alkene → bond. This is the orbital symmetry argument for photochemical [2+2].