Q1(e). Photochemical [2+2] cycloaddition of methyl vinyl ketone and ethene

Introduction. Ethene and methyl vinyl ketone (but-3-en-2-one) combine into a cyclobutane: a [2+2] cycloaddition. A concerted [π2ₛ+π2ₛ] reaction is thermally forbidden (HOMO–LUMO phases do not match at both ends), so heating does not work. It is photochemically allowed: this is an enone–alkene photocycloaddition.

Conditions

• Light: UV of about 300–350 nm (mercury lamp, Pyrex filter) excites the enone (n→π*, π→π*); ethene does not absorb.
• Excess ethene (bubbled through or under pressure), in an inert solvent (CH₂Cl₂, hexane), often at low temperature (-70 to 0°C) to hold the ethene.
• Deoxygenated solution (O₂ quenches the triplet); a sensitiser (acetone, benzophenone) can be used to populate the triplet.

Steps

Scheme: Stepwise photocycloaddition through a triplet 1,4-biradical.

1. The enone absorbs: S₀→ S₁; intersystem crossing gives the triplet T₁ (ππ*), which lives long enough to meet ethene.
2. The triplet enone and ethene form an exciplex; one new C–C bond forms (at the α or β carbon) to give a triplet 1,4-biradical.
3. The biradical must flip one spin (ISC to the singlet) before the second C–C bond can form; it then closes to the cyclobutane, 1-cyclobutylethan-1-one, or partly fragments back to the starting materials.

Is it reversible?

For practical purposes no. The concerted retro-[2+2] is thermally forbidden for the same orbital reason, and a stepwise (biradical) cleavage of the cyclobutane needs a high temperature. The product is a saturated ketone: it has lost the conjugated enone chromophore and absorbs only weakly near 280 nm, so it is not re-excited back to the reactants under the reaction conditions. (The biradical stage itself is reversible, which lowers the quantum yield.) Acyclic enones such as methyl vinyl ketone react less efficiently than cyclic enones, because their excited state loses energy by twisting about the C=C.

Conclusion. The reaction needs UV light, excess ethene, an inert solvent and no O₂; it goes through the triplet enone, an exciplex and a 1,4-biradical to 1-cyclobutylethan-1-one. It is not reversible: thermal retro-[2+2] is forbidden and the product no longer absorbs at the exciting wavelength.

Application. Enone–alkene photocycloaddition (de Mayo type) builds the cyclobutane rings of natural products such as grandisol, the cotton boll weevil pheromone.