Although the Diels–Alder reaction is a classic reaction, it is still at the forefront of the carbon–carbon bond formation toolbox in synthetic chemistry. In particular, the mechanism of the reaction has been well studied, and the scope has been expanded to a wide variety of diene/dienophile combinations. However, electronic matching needs to be considered; more specifically, that one reaction partners should be electron-rich and the other electron-deficient. To overcome this hurdle, "umpolung" strategy has been proven to be powerful approach. In this context, single electron transfer (SET) would be the most simple and straightforward process. For example, electron-deficient radical cation species can be produced from electron-rich reactant via SET oxidation. A typical example of the umpolung Diels-Alder reactions is the reaction between an electron-rich diene and a dienophile. These combinations are electronically mismatched, however, SET oxidation can produce a radical cation of one component, which is then able to be trapped by the other component.

In addition to the use of chemical redox agents, photocatalysis and electrocatalysis are currently recognized as effective approaches to induce SET processes. Umpolung Diels-Alder reactions have also been extensively studied for almost half a century, however, scope of the reactions has been limited. In many cases, the reactions were triggered by SET oxidation of diene components, generating the radical cation species, and which were then trapped by dienophiles. In 2011, Yoon elegantly demonstrated the opposite version, namely, the reaction triggered by SET oxidation of dienophile components using photocatalysis.

We have been developing carbon-carbon bond formations by electrocatalysis in lithium perchlorate (LiClO₄)/nitromethane (CH₃NO₂) electrolyte solution.¹ In most cases, the use of LiClO₄/CH₃NO₂ electrolyte solution is critical, which exhibits remarkable property as a Lewis acid to facilitate the reactions of carbon-centered radical cations with carbon nucleophiles. We also have shown that the electrocatalysis in LiClO₄/CH₃NO₂ electrolyte solution is effective to induce umpolung Diels-Alder reactions.² In this talk, recent advancement of the umpolung Diels-Alder reactions by electrocatalysis will be discussed in details.

(1) Imada, Y.; Yamaguchi, Y.; Shida, N.; Okada, Y.; Chiba, K. Chem. Commun. 2017, 53, 3960–3963.

(2) (a) Ozaki, A.; Yamaguchi, Y.; Okada, Y.; Chiba, K. ChemElectroChem 2017, 4, 1852–1855. (b) Okada, Y.; Yamaguchi, Y.; Ozaki, A.; Chiba, K. Chem. Sci. 2016, 7, 6387–6393.