Q6(a). Norbornadiene with TCNE; ring opening of a cis-cyclobutene

Introduction. (i) Norbornadiene has two C=C bonds held parallel by the CH₂ bridge; with an electron-poor dienophile they act together as a “homo-diene” in a homo-Diels–Alder [π2ₛ+π2ₛ+π2ₛ] cycloaddition. (ii) is a thermal 4π electrocyclic ring opening, conrotatory (CON) as the arrow says.

(i) Homo-Diels–Alder

Scheme: Two new C–C bonds to TCNE and one new C–C bond across the ring (cyclopropane).

1. Six electrons take part: two from each C=C of norbornadiene and two from TCNE; the reaction is thermally allowed (like a Diels–Alder, 4n+2).
2. TCNE approaches from the endo face (under both C=C, away from the CH₂ bridge). Its two carbons bond to C-2 and C-6; at the same time C-3 and C-5 bond, forming a cyclopropane.
3. Product: a tetracyclic tetranitrile with the deltacyclane skeleton (rings of 3, 5, 5, 5 and 5 atoms; the four CN groups on the two carbons that came from TCNE), C₁₃H₈N₄.

(ii) Conrotatory opening of cis-3,4-disubstituted cyclobutene

Scheme: In a conrotatory opening the two cis groups end up one “out” and one “in”.

1. The thermal HOMO of the forming diene (ψ₂) has opposite-phase termini, so C-3 and C-4 rotate in the same direction.
2. With both R groups on one face, one R turns outward and the other inward: the product is the (E,Z)-diene (for R = Me: (2E,4Z)-hexa-2,4-diene).
3. Ring strain makes the open diene more stable, so the equilibrium lies on the diene side; (E,E) or (Z,Z) dienes would need a disrotatory (photochemical) path.

Conclusion. (i) Homo-Diels–Alder adduct: tetracyano-deltacyclane (cyclopropane-containing tetracyclic C₁₃H₈N₄). (ii) Conrotatory opening gives the (E,Z)-1,4-disubstituted buta-1,3-diene.

Application. Norbornadiene–quadricyclane and norbornadiene cycloadditions are studied for storing solar energy as strain energy; conrotatory cyclobutene opening is used to make (E,Z)-dienes stereospecifically.