Light has the potential to catalyze various chemical reactions. A wide variety of organic chromophores have been developed and used as photocatalysts to promote unique reactions through single electron transfer (SET). Furthermore, currently transition metal complexes have extensively used as photocatalysts to realize a number of new reaction developments in the field of synthetic organic chemistry. Since the chromophores and complexes are small molecules and soluble in typical organic solvents, they can both undergo homogeneous SET. Since the pioneering work by Fujishima and Honda, heterogeneously dispersed semiconductors have also been studied as photocatalysts, generally in aqueous media. Although semiconductor photocatalysis can achieve various useful redox transformations, its application in the field of synthetic organic chemistry is somewhat less widespread. This is probably due to reactive oxygen species (ROS) produced at the surfaces in aqueous aerobic conditions, causing the decomposition of organic compounds via carbon-carbon bond cleavage. However, it was recently demonstrated that effective carbon-carbon bond formations were also possible if the reaction of the carbon-centered radical cation with a carbon nucleophile was sufficiently more favorable than the undesired oxygenation by ROS. We have been developing novel carbon-carbon bond forming cycloadditions, including [2 + 2] and Diels-Alder reactions, by electrocatalysis in lithium perchlorate/nitromethane (LiClO4/CH3NO2) solution, which has proven to be beneficial in reactions between carbon-centered radical cations and carbon nucleophiles. Recently, we demonstrated that TiO2 photocatalysis in combination with this solution was also a powerful option to promote [2 + 2] cycloadditions. The reactions are net redox neutral and are expected to be triggered by hole oxidation and completed by excited electron reduction. In this study, we have applied a same photocatalytic system to electronically mismatched Diels-Alder reactions and have demonstrated that a wide variety of beta-methyl styrenes can be used as electron-rich dienophiles. Less-electron-rich styrenes that were not previously usable in this reaction are also found to be productive substrates. Scope and limitation, including mechanistic details, will be discussed in this presentation.

Electron-catalyzed Diels-Alder reactions by TiO2 photocatalysis

2019 Fall National Meeting

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