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.
A mild protocol has been developed using polymeric carbon nitrides (PCN) as metal-free heterogeneous photocatalyst to convert olefins into the corresponding carbonyls which even can be applied in the gram scale synthesis using direct solar energy.
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).
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.
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