Photocatalytic hydrogenation of acetophenone on a titanium dioxide cellulose film

A previously developed sustainable immobilization concept for photocatalysts based on cellulose as a renewable support material was applied for the photocatalytic hydrogenation of acetophenone (ACP) to 1-phenyl ethanol (PE). Four different TiO₂ modifications (P25, P90, PC105, and PC500) were screened for the reaction showing good performance for PC25 and PC500. PC500 was selected for a detailed kinetic study to find the optimal operating conditions, and to obtain a better understanding of the photocatalytic pathway in relation to conventional and transfer hydrogenation. The kinetic data were analyzed using the pseudo-first-order reaction rate law. A complete conversion was obtained for ACP concentrations below 1 mM using a 360 nm filter and argon as the purge gas within 2–3 hours. High oxygen concentrations slow down or prevent the reaction, and wavelengths below 300 nm lead to side-products. By investigating the temperature dependency, an activation energy of 22 kJ mol⁻¹ was determined which is lower than the activation energies for conventional and transfer hydrogenation, because the light activation of the photocatalyst turns the endothermic to an exothermic reaction. PC500 was immobilized onto the cellulose film showing a 37% lower activity that remains almost constant after multiple use.

The photocatalytic hydrogenation of acetophenone to 1-phenylenthanol was investigated with cellulose-immobilized titanium oxide (TiO₂) particles.

Site-selective D2O-mediated deuteration of diaryl alcohols via quantum dots photocatalysis

Owing to the high synthetic value of deuteration in the pharmaceutical industry, we describe herein the conversion of a range of aromatic ketones to deuterium-labeled products in good to excellent yields. Efficient and site-selective deuteration of benzyl alcohols by D₂O with visible light irradiation of quantum dots (QDs), together with gram-scale synthesis and photocatalyst recycling experiments indicated the potential of the developed method in practical organic synthesis.

Asymmetric hydrogenation of trifluoromethyl ketones: application in the synthesis of Odanacatib and LX-1031

The asymmetric hydrogenation of trifluoromethyl ketones via iridium catalysis to access chiral secondary 2,2,2-trifluoroethanols is presented. The key intermediates of Odanacatib and LX1301 were prepared with high yields and enantioselectivities.

Inactivation and change of tetracycline-resistant Escherichia coli in secondary effluent by visible light-driven photocatalytic process using Ag/AgBr/g-C3N4

Control of antibiotic-resistant bacteria (ARB) and their related genes in secondary effluents has become a serious issue because of increased awareness of their health risks. A considerable number of techniques have been developed in recent years, particularly in relation to advanced oxidation. However, limited information is known about cellular behavior and resistance characteristic change during photocatalytic treatment. In this study, the inactivation of tetracycline (TC)-resistant Escherichia coli (TC-E. coli), removal of TC-resistant genes (TC-RGs), and antibiotic susceptibility were evaluated by employing photocatalytic treatment using Ag/AgBr/g-C₃N₄ with visible light irradiation. The effects of light intensity, photocatalyst dosage, and reaction ambient temperature on photocatalysis were modelled and investigated. The rate of TC-E. coli removal was also optimized. Results demonstrated that the optimal conditions for TC-E. coli removal included light intensity of 96.0 mW/cm², photocatalyst dosage of 211.0 mg/L, and reaction ambient temperature of 23.7 °C. Under such conditions, the ARB removal rate was 6.1 log after 90 min and the related TC-RG removal rates were 49%, 86%, 69%, and 86% for tetA, tetM, tetQ, and intl1, respectively. The minimum inhibitory concentration test after photocatalysis shows that the antibiotic resistance of TC-E. coli was enhanced, which may be mainly due to the changes in the membrane potential and resulted in difficulty in destroying the bacteria through antibiotic contact. Hence, photocatalytic treatment could be an ideal method for ARB and antibiotic-resistant gene (ARG) control in wastewater, but the health risks of the remaining ARB and ARG should be investigated further.

Stereoselective Organic Reactions in Heterogeneous Semiconductor Photocatalysis

The most significant feature of heterogeneous semiconductor photocatalysis is that both oxidation and reduction occur in a one-pot process. Thus, photocatalysis leads to unique redox organic reactions that cannot be achieved by conventional techniques using oxidants or reductants. Semiconductor photocatalysis is expected to be a new method for fine chemical syntheses of highly valuable molecules such as chiral medicines. However, the use of semiconductor photocatalysts in stereoselective reactions has been limited so far. This mini-review highlights recent progress in stereoselective organic reactions using semiconductor photocatalysts, briefly summarizing the enantio- and diastereoselective reactions based on the currently available literature.

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.

Metal-free virucidal effects induced by g-C3N4 under visible light irradiation: Statistical analysis and parameter optimization

Waterborne viruses with a low infectious dose and a high pathogenic potential pose a serious risk for humans all over the world, calling for a cost-effective and environmentally-friendly inactivation method. Optimizing operational parameters during the disinfection process is a facile and efficient way to achieve the satisfactory viral inactivation efficiency. Here, the antiviral effects of a metal-free visible-light-driven graphitic carbon nitride (g-C₃N₄) photocatalyst were optimized by varying operating parameters with response surface methodology (RSM). Twenty sets of viral inactivation experiments were performed by changing three operating parameters, namely light intensity, photocatalyst loading and reaction temperature, at five levels. According to the experimental data, a semi-empirical model was developed with a high accuracy (determination coefficient R² = 0.9908) and then applied to predict the final inactivation efficiency of MS2 (a model virus) after 180 min exposure to the photocatalyst and visible light illumination. The corresponding optimal values were found to be 199.80 mW/cm², 135.40 mg/L and 24.05 °C for light intensity, photocatalyst loading and reaction temperature, respectively. Under the optimized conditions, 8 log PFU/mL of viruses could be completely inactivated by g-C₃N₄ without regrowth within 240 min visible light irradiation. Our study provides not only an extended application of RSM in photocatalytic viral inactivation but also a green and effective method for water disinfection.

Chiral α-hydroxy acid-coadsorbed TiO2 photocatalysts for asymmetric induction in hydrogenation of aromatic ketones

In enantioselective photohydrogenation of aromatic ketones on TiO₂, the enantioselectivity is strongly affected by not only chiral reagents but also the crystalline phase, surface structure, and morphology of TiO₂.

Preparative semiconductor photoredox catalysis: An emerging theme in organic synthesis

Heterogeneous semiconductor photoredox catalysis (SCPC), particularly with TiO2, is evolving to provide radically new synthetic applications. In this review we describe how photoactivated SCPCs can either (i) interact with a precursor that donates an electron to the semiconductor thus generating a radical cation; or (ii) interact with an acceptor precursor that picks up an electron with production of a radical anion. The radical cations of appropriate donors convert to neutral radicals usually by loss of a proton. The most efficient donors for synthetic purposes contain adjacent functional groups such that the neutral radicals are resonance stabilized. Thus, ET from allylic alkenes and enol ethers generated allyl type radicals that reacted with 1,2-diazine or imine co-reactants to yield functionalized hydrazones or benzylanilines. SCPC with tertiary amines enabled electron-deficient alkenes to be alkylated and furoquinolinones to be accessed. Primary amines on their own led to self-reactions involving C-N coupling and, with terminal diamines, cyclic amines were produced. Carboxylic acids were particularly fruitful affording C-centered radicals that alkylated alkenes and took part in tandem addition cyclizations producing chromenopyrroles; decarboxylative homo-dimerizations were also observed. Acceptors initially yielding radical anions included nitroaromatics and aromatic iodides. The latter led to hydrodehalogenations and cyclizations with suitable precursors. Reductive SCPC also enabled electron-deficient alkenes and aromatic aldehydes to be hydrogenated without the need for hydrogen gas.

Photocatalysis with TiO2 Applied to Organic Synthesis

Titanium dioxide is a versatile heterogeneous catalyst. Absorption of light by a TiO2 particle leads to the formation of an electron–hole pair. Electron transfer from or to the particle induces redox reactions. Although mainly applied in the context of environmental chemistry, these processes are also used to selectively transform organic compounds. Oxidations and reductions have been carried out. Applications to the synthesis of heterocycles have been reported. Many C–C bond formation reactions have been performed. Owing to adsorption of the substrates or by different surface modifications, visible light can be used to excite the catalytic system, which generates mild reaction conditions.

Probing the role of surface energetics of electrons and their accumulation in photoreduction processes on TiO₂

We address the role of the energetics of photogenerated electrons in the reduction of 4-nitrobenzaldehyde on TiO2. This model molecule bears two functional groups featuring different reducibilities. Electrochemistry shows that reduction to 4-aminobenzyl alcohol occurs in entirely distinct potential ranges. Partial reduction of the -NO2 group, affording 4-aminobenzaldehyde, takes place through surface states at potentials positive of the flatband potential (E(fb)). Dark currents caused by reduction of the aldehyde group are observed only at potentials more negative than E(fb), and the process requires an electron accumulation regime. Photocatalysis with TiO2 suspensions agrees with the electrochemical data. In particular, reduction of the nitro group is a relatively fast process (k=0.059 s(-1)), whereas that of the aldehyde group is slower (k=0.001 s(-1)) and requires electron photoaccumulation. Control of the photogenerated charge is a prospective means for achieving chemoselective reductions.