Selective endoperoxide formation by heterogeneous TiO 2 photocatalysis with dioxygen

A Mesoporous Lead-Doped Titanium Oxide Compound with High Performance and Recyclability in I2 Uptake and Photocatalysis

A three-dimensional (3D) mesoporous material with an atomically precise structure, Ti₁₆Pb₅O₁₆(C₆H₅CO₂)₂(OCH₃)₄₀ (Ti₁₆Pb₅), comprised of a novel high-nuclearity Pb-doped titanium oxide cluster (TOC), was synthesized. Ti₁₆Pb₅ exhibited a surface area of 45 m² g^-1 and a pore diameter of 3.5 nm. It exhibited an uptake capacity of I₂ of ≤2.2 g g^-1 in vapor, and the performance was maintained after seven uptake-release cycles. Ti₁₆Pb₅ also showed a high adsorption ratio and capacity (93% and 3.1 g g^-1) in hexane. The characterization data, including Fourier transform infrared, Raman, and powder X-ray diffraction, suggested the lattice structure of Ti₁₆Pb₅ was rigid and I₂ was accommodated in the pores of Ti₁₆Pb₅. To the best of our knowledge, this is the first example of using a TOC in I₂ adsorption. In addition, Ti₁₆Pb₅ showed excellent activity and recyclability in visible-light degradation of dye pollutants and photocurrent generation. Our structural analysis suggested the alkoxide ligands within the channels of Ti₁₆Pb₅ build up a confined polar environment and thereby facilitate I₂ accommodation, and meanwhile, the improved performances and stabilities of Ti₁₆Pb₅ are correlated with its cluster-based, 3D hierarchical structure.

A powerful azomethine ylide route mediated by TiO2 photocatalysis for the preparation of polysubstituted imidazolidines

Lewis- and Brønsted-acid catalyzed 1,3-dipolar cycloaddition between azomethine ylides and unsaturated compounds is an important strategy to construct five-membered N-heterocycles. However, such a catalytic route usually demands substrates with an electron-withdrawing group (EWG) to facilitate the reactivity. Herein, we report a TiO2 photocatalysis strategy that can conveniently prepare five-membered N-heterocyclic imidazolidines from a common imine (N-benzylidenebenzylamine) and alcohols along the route of 1,3-dipolaron azomethine ylide but without pre-installed EWG substituents on the substrates. Our EPR results uncovered the previously unknown mutual interdependence between an azomethine ylide and TiO2 photo-induced hvb+/ecb- pair. This transformation exhibited a broad scope with 21 successful examples and could be scaled up to the gram level.

TiO₂ Photocatalysis for Transfer Hydrogenation

Catalytic transfer hydrogenation reactions, based on hydrogen sources other than gaseous H₂, are important processes that are preferential in both laboratories and factories. However, harsh conditions, such as high temperature, are usually required for most transition-metal catalytic and organocatalytic systems. Moreover, non-volatile hydrogen donors such as dihydropyridinedicarboxylate and formic acid are often required in these processes which increase the difficulty in separating products and lowered the whole atom economy. Recently, TiO₂ photocatalysis provides mild and facile access for transfer hydrogenation of C=C, C=O, N=O and C-X bonds by using volatile alcohols and amines as hydrogen sources. Upon light excitation, TiO₂ photo-induced holes have the ability to oxidatively take two hydrogen atoms off alcohols and amines under room temperature. Simultaneously, photo-induced conduction band electrons would combine with these two hydrogen atoms and smoothly hydrogenate multiple bonds and/or C-X bonds. It is heartening that practices and principles in the transfer hydrogenations of substrates containing C=C, C=O, N=O and C-X bond based on TiO₂ photocatalysis have overcome a lot of the traditional thermocatalysis' limitations and flaws which usually originate from high temperature operations. In this review, we will introduce the recent paragon examples of TiO₂ photocatalytic transfer hydrogenations used in (1) C=C and C≡C (2) C=O and C=N (3) N=O substrates and in-depth discuss basic principle, status, challenges and future directions of transfer hydrogenation mediated by TiO₂ photocatalysis.

TiO2 Photocatalyzed C–H Bond Transformation for C–C Coupling Reactions

Fulfilling the direct inert C–H bond functionalization of raw materials that are earth-abundant and commercially available for the synthesis of diverse targeted organic compounds is very desirable and its implementation would mean a great reduction of the synthetic steps required for substrate prefunctionalization such as halogenation, borylation, and metalation. Successful C–H bond functionalization mainly resorts to homogeneous transition-metal catalysis, albeit sometimes suffering from poor catalyst reusability, nontrivial separation, and severe biotoxicity. TiO2 photocatalysis displays multifaceted advantages, such as strong oxidizing ability, high chemical stability and photostability, excellent reusability, and low biotoxicity. The chemical reactions started and delivered by TiO2 photocatalysts are well known to be widely used in photocatalytic water-splitting, organic pollutant degradation, and dye-sensitized solar cells. Recently, TiO2 photocatalysis has been demonstrated to possess the unanticipated ability to trigger the transformation of inert C–H bonds for C–C, C–N, C–O, and C–X bond formation under ultraviolet light, sunlight, and even visible-light irradiation at room temperature. A few important organic products, traditionally synthesized in harsh reaction conditions and with specially functionalized group substrates, are continuously reported to be realized by TiO2 photocatalysis with simple starting materials under very mild conditions. This prominent advantage—the capability of utilizing cheap and readily available compounds for highly selective synthesis without prefunctionalized reactants such as organic halides, boronates, silanes, etc.—is attributed to the overwhelmingly powerful photo-induced hole reactivity of TiO2 photocatalysis, which does not require an elevated reaction temperature as in conventional transition-metal catalysis. Such a reaction mechanism, under typically mild conditions, is apparently different from traditional transition-metal catalysis and beyond our insights into the driving forces that transform the C–H bond for C–C bond coupling reactions. This review gives a summary of the recent progress of TiO2 photocatalytic C–H bond activation for C–C coupling reactions and discusses some model examples, especially under visible-light irradiation.