Mechanisms and Synthetic Strategies in Visible Light‐Driven [2+2]‐Heterocycloadditions

Abstract

The synthesis of four membered heterocycles usually requires multi-step procedures and prefunctionalized reactants. A straightforward alternative is the photochemical [2+2]-heterocycloaddition between an alkene and a carbonyl derivative, conventionally based on the photoexcitation of this latter. However, this approach is limited by the absorption profile of the carbonyl, requiring in most of the cases the use of high-energy UV-light, that often results in undesired side reactions and/or the degradation of the reaction components. The development of new and milder visible light-driven [2+2]-heterocycloadditions is, therefore, highly desirable. In this Review, we highlight the most relevant achievements in the development of [2+2]-heterocycloadditions promoted by visible light, with a particular emphasis on the involved reaction mechanisms. The open challenges will also be discussed, suggesting new possible evolutions, and stimulating new methodological developments in the field. Mechanisms for Visible Light [2+2]-Hetero-CycloadditionsA prerequisite for visible light [2+2]-heterocycloadditions is the presence of a photoactive species able to absorb light

Roles of Nonadiabatic Processes, Reaction Mechanism, and Selectivity in Cu-Catalyzed [2 + 2] Photocycloaddition of Norbornene and Acetone to Oxetane

“…The [2 + 2] photocycloaddition reaction is a typical photochemical reaction for the formation of four-membered rings, which can occur either by direct excitation of substrates or through photocatalysts. − One of the most famous photocycloadditions is the Paternò-Büchi reaction, which involves carbonyl derivatives and alkenes to form oxetanes (Scheme a). − This reaction is activated by direct excitation of carbonyl derivatives and subsequent CC double bond engagement . Li et al advanced this photocycloaddition by direct excitation of noncovalent complexes comprising alkene and carbonyl substrates under visible-light irradiation, providing fresh insights into the extensively studied Paternò-Büchi reaction (Scheme b) .…”