Photochemical [2+2] and Paternò–Büchi template

Triplet cyclopent-2-enone adds to ethylene through a 1,4-diradical to give bicyclo[3.2.0]heptan-2-one; triplet benzophenone adds to 2-methylpropene through the more stable diradical to give 3,3-dimethyl-2,2-diphenyloxetane.

Mechanism step by step

Light makes cyclobutanes from enones and alkenes, and oxetanes from ketones and alkenes

1. Triplet enone + alkene – hν then intersystem crossing gives the triplet enone (drawn as a diradical); C3 bonds to the alkene
2. Ring closure – After spin inversion the 1,4-diradical closes: a cis-fused cyclobutane
3. Paternò–Büchi – Triplet benzophenone (O radical) adds to the CH₂ end, leaving the more stable tertiary radical
4. Oxetane formation – The 1,4-diradical closes to the four-membered oxetane ring

Key points

• Woodward–Hoffmann: a concerted suprafacial [π2s + π2s] cycloaddition is forbidden thermally but allowed photochemically (4 electrons, 4n).
• Enone [2+2] photocycloadditions go through the triplet excited state and a 1,4-diradical (either alkene carbon can bond first): stepwise, not concerted.
• Paternò–Büchi: the n,π* excited C=O adds to an alkene; the regiochemistry follows the more stable 1,4-diradical (benzophenone + 2-methylpropene → mainly the 3,3-dimethyl oxetane).
• Strained cyclobutanes and oxetanes are useful building blocks (De Mayo reaction, ring expansions, oxetanes in drug design).

Drawn with 80 atoms, 15 arrows, 6 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 34 Textbook
• Fleming, Molecular Orbitals and Organic Chemical Reactions, Wiley 2010 Textbook
• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook