3.(a) Would you expect the above conversion to require heat or light? Explain using molecular orbital diagram…

(a) Heat or light for the cycloocta-1,4-diene conversion

Light (hν) is required. The reactant is cycloocta-1,4-diene (C1=C2, C4=C5 with one CH₂ between them) and the product is a tricyclic C₈H₁₂ with a cyclopropane fused to a cyclobutane fused to a cyclopentane. Two new σ bonds form, C2–C4 and C1–C5, at the expense of two π bonds. This is an intramolecular [π2s + π2s] cycloaddition, the same type as norbornadiene to quadricyclane.

In the thermal reaction the HOMO (π) of one alkene must overlap the LUMO (π*) of the other. π has the same phase at both carbons, while π* changes sign across the C=C bond, so one terminus gives bonding and the other antibonding overlap. The thermal supra–supra route is symmetry-forbidden. On irradiation one electron goes to π*, so the SOMO of the excited alkene is π*. It overlaps the LUMO (π*) of the ground-state alkene in phase at both termini, and the reaction is allowed.

This agrees with the Woodward–Hoffmann rule: a supra–supra cycloaddition with 4n π electrons is photochemically allowed and thermally forbidden.

(b) Claisen rearrangement and decarboxylation

(i) The starting material is an allyl vinyl ether, HOOC–CH=CH–O–CH₂–CH=CH–Ph. On heating it undergoes a [3,3]-sigmatropic Claisen rearrangement through a chair-like six-membered transition state. It is concerted and thermally allowed. The all-carbon analogue would be a Cope rearrangement.

(ii) The Claisen product is 2-formyl-3-phenylpent-4-enoic acid, OHC–CH(COOH)–CH(Ph)–CH=CH₂ (C₁₂H₁₂O₃). It loses CO₂ (one carbon and two oxygens) to give the pentenal, C₁₁H₁₂O.

(iii) The aldehyde carbon is C-3 of the acrylic acid, the β-carbon bonded to the ether oxygen. It keeps its oxygen as C=O when the O–CH₂ bond breaks. C-1 (carboxyl) is lost as CO₂ and C-2 becomes the CH₂ next to CHO.

(iv) Step 1, Claisen rearrangement: the O–CH₂ bond breaks and a new C–C bond forms between acrylic C-2 and the phenyl-bearing allyl carbon. C-3 becomes CHO and a terminal CH=CH₂ appears. Step 2, decarboxylation: the intermediate is a β-formyl acid that behaves like a β-keto acid. Through a cyclic six-membered transition state the acid hydrogen moves to the aldehyde oxygen as CO₂ is lost, giving an enol. Step 3, tautomerisation: the enol becomes 3-phenylpent-4-enal, CH₂=CH–CH(Ph)–CH₂–CHO.

(c) Regioselectivity of Hofmann elimination

Hofmann elimination is the E2 decomposition of a quaternary ammonium hydroxide on heating, and it gives mainly the less substituted alkene. Take sec-butyltrimethylammonium hydroxide, CH₃(C-1)–CH(C-2)(⁺NMe₃)–CH₂(C-3)–CH₃(C-4): it gives chiefly but-1-ene and little but-2-ene. E2 needs the β-hydrogen anti-periplanar to ⁺NMe₃.

Towards C-3 (but-2-ene): in every staggered conformer that puts a C-3 hydrogen anti to N, the C-4 methyl must be gauche to the very bulky ⁺NMe₃, and in one conformer also gauche to the C-1 methyl. These conformers are high in energy and the base reaches the hydrogen with difficulty. Towards C-1 (but-1-ene): the C-1 methyl has three equivalent hydrogens and only H atoms are gauche to ⁺NMe₃, so the reactive conformer is unstrained.

⁺NMe₃ is a poor leaving group and strongly electron-withdrawing, so the transition state is carbanion-like (E1cB-like) and the more acidic primary β-hydrogen is removed faster. Steric crowding and C–H acidity together favour the Hofmann product, whereas halides follow Saytzeff's rule.

(d) Diels–Alder reactions with acrolein

Both reactions are thermal, concerted [4π s + 2π s] Diels–Alder cycloadditions through a cyclic six-membered transition state. The diene termini form two new σ bonds to the alkene carbons of acrolein and a new C=C appears between diene C-2 and C-3.

(i) 2-Methoxybuta-1,3-diene + acrolein: The product is 4-methoxycyclohex-3-ene-1-carbaldehyde, the "para" adduct. The +M effect of the 2-OMe group gives C-1 the largest HOMO coefficient, and the largest LUMO coefficient of acrolein is at its β-CH₂. Joining these two gives OMe and CHO in a 1,4-relationship. The product is a racemic enol ether.

(ii) 1-Methoxybuta-1,3-diene + acrolein: The product is cis-2-methoxycyclohex-3-ene-1-carbaldehyde, the "ortho" adduct. OMe at C-1 raises the HOMO coefficient at the far terminus C-4, which bonds to the β-carbon of acrolein. C-1 then bonds to the carbon bearing CHO, so OMe and CHO end up on adjacent carbons. The endo transition state is favoured by secondary orbital overlap, and suprafacial addition to the (E)-diene then gives OMe and CHO cis (racemic). The pattern is the ortho–para rule: a donor at C-1 gives 1,2-adducts and a donor at C-2 gives 1,4-adducts.