Photocatalyzed Cycloaddition Reactions

Abstract

Visible light photocatalysis has emerged as a powerful tool in organic synthesis. The formation of radical species via electron transfer (ET) or energy transfer (EnT) mechanisms of photocatalysts readily enables various useful transformations. Multicomponent reaction (MCR) allows three or more reactants joined together in single step to form products with increased structural complexity. The advantages of MCR include step-economy and smaller environmental footprint. Visible-light photocatalyzed [4+2] cycloaddition has been explored in the context of MCRs by employing enamines, alkyl halides, and alkenes as coupling partners. Typically, a radical-polar crossover mechanism is involved in photocatalyzed MCRs owing to the quenching of radical intermediates to afford cationic intermediates. While these methods are useful, a photocatalyzed MCR in which bond formation occurs exclusively via radical species may allow an access to MCR products that are otherwise difficult to synthesize based on the radical-polar crossover mechanism. In this context, we developed a photocatalyzed cascade reaction in which bond formation exclusively occurs via radical species. On the other hand, alkyne-alkene [2+2] cycloaddition based on EnT-mediated photocatalysis has been developed to give diverse cyclobutene-containing products. Moreover, the efficient photometathesis was observed to afford 1,3-dienes via tandem triplet activation of the resulting cyclobutenes when intramolecular enyne cycloaddition was performed. Detailed mechanism studies will be discussed.