Q6(b). Thermal cyclisation of pentadienyl anion and cation

Introduction. Ring closure of a pentadienyl ion to a cyclopentenyl ion is an electrocyclic reaction of a five-atom π system. The mode is fixed by the HOMO of the open ion: termini of the same phase on one face close disrotatorily, of opposite phase conrotatorily.

Figure: HOMO of the pentadienyl system; dots are nodes. Arrows: sense of rotation of the termini.

1. (i) Pentadienyl anion, C₅H₇⁻: 6 π electrons fill ψ₁–ψ₃; HOMO =ψ₃ (nodes at C-2 and C-4) with same-phase termini. Thermal closure is disrotatory (6π, 4n+2), giving the cyclopent-2-enyl (allyl) anion. Terminal substituents of an (E,E) anion end up cis on the new ring.
2. (ii) Pentadienyl cation, C₅H₇⁺: 4 π electrons; HOMO =ψ₂ (node at C-3) with opposite-phase termini. Thermal closure is conrotatory (4n), giving the cyclopentenyl (allyl) cation; terminal substituents of an (E,E) cation end up trans.
3. Photochemically both rules reverse (anion conrotatory, cation disrotatory).

Conclusion. Thermal cyclisation: pentadienyl anion – disrotatory (6π, HOMO ψ₃); pentadienyl cation – conrotatory (4π, HOMO ψ₂).

Application. The conrotatory closure of a pentadienyl cation is the key step of the Nazarov cyclisation (divinyl ketones to cyclopentenones), which fixes the trans relationship of substituents in the ring.