9-Fluorenone acts as a metal-free and additive-free photocatalyst for the selective oxidation of primary and secondary alcohols under visible light. With this photocatalyst, a plethora of alcohols such as aliphatic, heteroaromatic, aromatic, and alicyclic compounds has been converted to the corresponding carbonyl compounds using air/oxygen as an oxidant. In addition to these, several steroids have been oxidized to the corresponding carbonyl compounds. Detailed mechanistic studies have also been achieved to determine the role of the oxidant and the photocatalyst for this oxidation.

A metal-free system has been discovered for the efficient α-oxygenation of tertiary amines to the corresponding amides using oxygen as an oxidant. This visible-light-mediated oxygenation reaction exhibited excellent substrates scope under mild reaction conditions and generated water as the only byproduct. The synthetic utility of this approach has been demonstrated by applying onto drug molecules. At the end, detailed mechanistic reactions clearly showed the role of oxygen and the photocatalyst.

Contents: 1. Materials and methods 2. General procedure for the oxidation of amines to imines 3. Setup for photocatalytic reactions 4. Optimization 5. Mechanistic experiments 6. Theoretical calculations 7. Characterization of products 8. References 9. NMR spectra Materials and methodsCommercial reagents were used without purification and reactions were run under CO2 atmosphere with exclusion of moisture from reagents using standard techniques for manipulating air-sensitive compounds. In case of dry DBN used for reactions, commercial DBN was dried over activated molecular sieves (3 Å) in a flame-dried Schlenk tube and degassed (several vacuum/argon cycles) prior to use. 1 H NMR spectra (300, 400 and 500 MHz) and 13 C NMR spectra (75.58, 100.62 and 125.71 MHz) were recorded using Bruker spectrometers AVANCE III 300, AVANCE III HD 400, AVANCE III 400, AVANCE III HD 500 and Varian spectrometers Mercury VX 300, VNMRS 300 and Inova 500 with CDCl3 and DMSO-d6 as solvent. NMR spectra were calibrated using the solvent residual signals (CDCl3: δ 1 H = 7.26, δ 13 C = 77.16; DMSO-d6: δ 1 H = 2.50, δ 13 C = 39.52; D2O: δ 1 H = 4.79). ESI mass spectra were recorded on Bruker Daltonic spectrometers maXis (ESI-QTOF-MS) and micrOTOF (ESI-TOF-MS). GC-MS mass spectra were recorded on Thermo Finnigan spectrometers TRACE (Varian GC Capillary Column; wcot fused silica coated CP-SIL 8CB for amines; 30 m x 0.25 mm x 0.25 µm) and DSQ (Varian FactorFour Capillary Column; VF-5ms 30 m x 0.25 mm x 0.25 µm). Gas chromatography was performed on an Agilent Technologies chromatograph 7890A GC System (Supelcowax 10 Fused Silica Capillary Column; 30 m x 0.32 mm x 0.25 µm). GC calibrations were carried out with authentic samples and ndodecane as an internal standard. Gas-phase GC measurements were conducted by a Shimadzu GC-2014 equipped with a TCD detector and a ShinCarbon ST 80/100 Silco column.Absorption-emission spectra were recorded on a Jasco FP-8500 Spectrofluorometer and UV/Vis spectra were recorded on a Jasco V-770 Spectrophotometer. General procedure for the dehydrogenation of amines to iminesA 10 mL two-necked flask containing a stirring bar was charged with 0.134 mmol substrate.After purging the flask three times with vacuum and two times with nitrogen the CO2 atmosphere was incorporated through a CO2-filled balloon. Afterwards dry DMSO (2.5 mL) and DBN (1.2 eq.; 0.16 mL of a 1 M solution in dry DMSO) were added. The resulting mixture was stirred for 48 h at irradiation of visible blue light (the progress can be monitored via GC-MS or TLC). Then, the resulting mixture underwent an aqueous workup (using distilled water; or brine in case of slurry phase separation) and was extracted three times with ethyl acetate.The combined organic layers were dried over anhydrous Na2SO4, filtered and concentrated in vacuo. Products were purified via silica gel chromatography with ethyl acetate and n-hexane and 1 V% triethylamine as solvents (typically 20:80 ethyl acetate:n-hexane).

The photochemistry of TiO2 has been studied intensively since it was discovered that TiO2 can act as a photocatalyst. Nevertheless, it has proven difficult to establish the detailed charge-transfer processes involved, partly because the excited states involved are difficult to study. Here we present evidence of the existence of hydroxyl-induced excited states in the conduction band region. Using two-photon photoemission, we show that stepwise photoexcitation from filled band gap states lying 0.8 eV below the Fermi level of rutile TiO2(110) excites hydroxyl-induced states 2.73 eV above the Fermi level that has an onset energy of ∼3.1 eV. The onset is shifted to lower energy by the coadsorption of molecular water, which suggests a means of tuning the energy of the excited state.

Dearomatisation of indole derivatives to the corresponding isatin derivatives has been achieved with the aid of visible light and oxygen. It should be noted that isatin derivatives are highly important for the synthesis of pharmaceuticals and bioactive compounds. Notably, this chemistry works excellently with N‐protected and protection‐free indoles. Additionally, this methodology can also be applied to dearomatise pyrrole derivatives to generate cyclic imides in a single step. Later this methodology was applied for the synthesis of four pharmaceuticals and a pesticide called dianthalexin B. Detailed mechanistic studies revealed the actual role of oxygen and photocatalyst.

Direct and selective oxygenation of C−H bonds to C−O bonds is regarded as an effective tool to generate high‐value products. However, these reactions are still subject to challenges such as harsh reaction conditions, use of expensive transition metal catalysts, and involvement of stoichiometric oxidants. To avoid these, molecular oxygen would be ideal as oxidant, as the byproduct is water or hydrogen peroxide. Additionally, achieving these reactions by using metal‐free catalysts would contribute to green and sustainable chemical synthesis. This Minireview summarizes recent reports on C−H oxygenation reactions with metal‐free catalysts and molecular oxygen under visible‐light conditions.

Carbon dioxide is a nontoxic and abundant chemical and has been widely used as a C1 building block for the synthesis of highly important chemicals. This greenhouse gas also has the ability to trigger changes in the chemical properties of certain chemicals without being incorporated into the product. CO2 as a promoter/mediator/catalyst can potentially replace toxic reagents that are used in the fine chemical and pharmaceutical sectors. Furthermore, CO2 is nonhazardous and enables easy isolation of the products and the respective catalysts, which in turn could make it suitable for industrial applications. Therefore, this Perspective is intended to address CO2-promoted reactions that can lead to the synthesis of pharmaceuticals and important molecules.

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