TiO2 photocatalysis for C–C bond formation

Abstract

TiO₂-based nanocrystal semiconductor materials are attracting attention as novel promising catalysts for the application of solar energy in organic synthesis. Due to the strong oxidative ability of the photo-induced hole in the TiO₂ valence-band (h⁺_vb) (2.9 V vs. NHE), under UVA or sunlight irradiation, TiO₂ nanomaterials have enough ability to activate and cleave most C–H, C–C, and C–X bonds, including the most inert bonds such as the C–H bond of CH₄, the NN bond of N₂, and the CO bond of CO₂, to construct various and complex target organic compounds. On the other hand, the TiO₂ conductive-band electron (e⁻_cb) with a suitable redox potential (−0.3 V vs. NHE) can be readily eliminated by common oxidants, such as dioxygen, benzoquinone (BQ), and Ag⁺, and even reagents themselves; this readily allows the achievement of a very high catalytic turnover frequency. Recently, through finding the right substrates, fine-tuning the reaction conditions, as well as modifying the TiO₂ catalysts, a series of highly selective transformations in organic synthesis have been realized by TiO₂ photocatalysis. Some of the ideal examples provide novel alternative insights into the traditional single-electron-transfer (SET) mechanism and separate reactions between h⁺_vb oxidations and e⁻_cb reductions in TiO₂ photocatalysis. Especially, C–C bond formation mediated by TiO₂-based photocatalysts sometimes display a similar powerful ability as that mediated by the noble metal Pd-, Ru-, Ir-, and Rh-based catalysts. C–C bond formation reactions, despite being considered as the center of organic synthesis, have rarely been covered in previous reviews on TiO₂ photocatalytic organic transformations. In this minireview, we have mainly reviewed and discussed the state-of-the-art TiO₂-based photocatalysts for C–C bond formation application in organic synthesis; most of the C–C bond formation reactions are beyond the familiar SET oxidation transformations mediated by traditional TiO₂ photocatalysis. Herein, TiO₂ photocatalytic Michael addition, anti-Markovnikov addition, and [2 + 2 + 2] oxidative cyclizations were mainly considered. The unconventional roles of both photo-generated h⁺_vb and e⁻_cb and relevant reaction mechanisms have been highlighted for every kind of C–C bond formation reaction. Moreover, an enantioselective C–C bond formation reaction by TiO₂ modified by a chiral organocatalyst and an organic dye under visible-light irradiation is displayed. This review covers the current basic principles, statuses, challenges, and future directions of TiO₂-photocatalyzed C–C bond formation reactions.