Vinylcyclopropane rearrangement is a reaction that can easily form multisubstituted cyclopentene from vinylcyclopropanes. Therefore, although this reaction was discovered in 1959, it is still studied extensively and intensively. However, since this reaction requires harsh reaction conditions, the use in synthetic organic chemistry has been limited. In order to realize the reaction under mild conditions, various studies have been carried out. Previsouly, we have developed carbon-carbon bond forming reactions by electrocatalysis and photocatalysis in lithium perchlorate (LiClO4)/nitromethane (MeNO2) solution. For example, we have demonstrated [2 + 2] cycloaddition reactions with electronically mismatched enol ethers and olefins, and [4 + 2] cycloaddition reactions with electronically mismatched dienophiles and dienes. Both cycloadditions were net redox neutral processes and were triggered by oxidative single electron transfer (SET) and complete by reductive SET. In this study, we demonstrate that TiO2 photocatalysis in LiClO4/MeNO2 can also promote vinylcyclopropane rearrangement. The image shows the vinylcyclopropane rearrangement using TiO2 photocatalyst. In analogy with our previous cycloadditions, the reaction is also net redox neutral and is expected to be triggered by oxidative SET, producing a styrene radical cation. The resulting radical cation causes tandem ring-opening/cyclization reaction to provide a cyclopentyl radical cation, which is reduced to give the desired product. The reaction realizes vinylcyclopropane rearrangement in transition metal-free manner by using TiO2 nanoparticles as a photocatalyst, and it has been confirmed that the reaction proceeds at room temperature. The reaction also proceed with the combination of variously substituted two aromatic rings such as electron rich – electron rich, electron rich – electron poor and electron poor – electron rich, and the product can be obtained in a good yield. In addition, substrate having functional groups such as monoester and diester were also synthesized and studied. From these results, multisubstituted cyclopentene can be obtained under mild conditions from vinylcyclopropane having a wide range of aromatic rings and functional groups. In this presentation, synthesis details, including scope and limitation and mechanistic aspect, will be discussed.

Radical cation vinylcyclopropane rearrangements by TiO2 photocatalysis

2019 Fall National Meeting

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