The Paternò–Büchi reaction, named after Emanuele Paternò and George Büchi, who established its basic utility and form,^[1][2] is a photochemical reaction, specifically a 2+2 photocycloaddition, which forms four-membered oxetane rings from an excited carbonyl and reacting with an alkene.^[3]

With substrates benzaldehyde and 2-methyl-2-butene the reaction product is a mixture of structural isomers:

Another substrate set is benzaldehyde and furan^[4] or heteroaromatic ketones and fluorinated alkenes.^[5]

The alternative strategy for the above reaction is called the Transposed Paternò−Büchi reaction.

• Aza Paternò−Büchi reaction - the aza-equivalent of the Paternò–Büchi reaction
• Enone–alkene cycloadditions - photochemical reaction of an enone with an alkene to give a cyclobutene ring unit