The Paternò-Büchi reaction - a comprehensive review.

The photochemical reaction between an excited state of carbonyl compounds and an alkene to give the oxetane deriving from a cycloaddition reaction (the Paternò-Büchi reaction) is presented. The mechanism of the reaction is discussed on the basis of kinetic, spectroscopic, and theoretical evidence, showing that the reaction can involve both singlet and triplet excited states of the carbonyl compounds, the high probable presence of an exciplex, and the possible presence of electron transfer processes, and when triplet excited carbonyl compounds are involved, the presence of a biradical intermediate is observed. The reactivity of electron-poor and electron-rich alkenes is discussed. Regio- and stereochemical behavior of the reaction are presented showing that, in some cases, very high regio- and stereoselectivities are obtained. The possible theories explaining this behavior are presented. In this field, the stereoselectivity observed, when chiral auxiliaries are used, when the reaction is performed in organized media, and when the presence of a hydroxyl directing effect can be supposed, is presented. The effect of light, temperature, solvent on the reaction is discussed. The possible application of this reaction in organic synthesis is discussed, presenting both previous synthetic schemes where the synthesis of an oxetane is a key step, the possible use of the oxetane in synthesis, and comparing the photochemical approach with the other approaches used to synthesize the oxetane ring.