Light has the potential to serve as green and sustainable reagent to catalyze chemical reactions. Stemming from the pioneering work of Honda and Fujishima in 1972, titanium dioxide (TiO₂) has occupied a central place in the research field of semiconductor photocatalysis. The excited electron/hole pair generated upon irradiation in aqueous aerobic conditions produces reactive oxygen species (ROSs) at the surface, which are account for subsequent redox transformations. They rapidly convert organic compounds into CO₂ and H₂O via carbon-carbon bond cleavage, therefore, semiconductor photocatalysis has generally been recognized as redox options in inorganic chemistry rather than organic chemistry where carbon-carbon bond formation is a focal point for extensive research efforts.

While semiconductor photocatalysis is a heterogeneous process, a homogeneous alternative can be provided by the use of transition metal complexes, which is now well- known as “photoredox catalysis”. The early works by MacMillan, Yoon, and Stephonson have triggered the explosive growth in the general area of new reaction developments in this class. Arguably, heterogeneous semiconductor photocatalysis has taken a backseat to homogeneous photoredox catalysis in the field of organic chemistry. For TiO₂ photocatalysis to be effective in carbon-carbon bond formation, the reactions must be carried out in dry anaerobic conditions in order not to produce ROSs that can damage organic compounds.

We have been developing carbon-carbon bond formations by electrocatalysis in lithium perchlorate (LiClO₄)/nitromethane (CH₃NO₂) electrolyte solution.¹ Our reaction design involves the aromatic “redox tag” concept,² that is, an aromatic ring that provides both the oxidant and the reductant in the overall net redox neutral carbon-carbon bond formations. 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, even unactivated alkenes. Our reaction development is also characterized by the importance of an intramolecular electron transfer process, which is recently presented by Melchiorre as an elegant “electron relay” mechanism. We anticipated that the use of LiClO₄/CH₃NO₂ electrolyte solution would pave the way for carbon-carbon bond formations by TiO₂ photocatalysis. It should also be noted that overall net redox neutral processes would be benefited from the involvement of both reductive and oxidative electron transfers potentially enabled by excited electron and hole. In this talk, the aromatic redox tag-guided intermolecular formal [2 + 2] cycloadditions³ by TiO₂ photocatalysis will be discussed in details.

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

(2) (a) Okada, Y.; Chiba, K. Chem. Rev. 2018, accepted. (b) Okada, Y.; Yamaguchi, Y.; Ozaki, A.; Chiba, K. Chem. Sci. 2016, 7, 6387–6393. (c) Okada, Y.; Nishimoto, A.; Akaba, R.; Chiba, K. J. Org. Chem. 2011, 76, 3470–3476.

(3) Chiba, K.; Miura, T.; Kim, S.; Kitano, Y.; Tada, M. J. Am. Chem. Soc. 2001, 123, 11314–11315.