The Development of Visible‐Light Photoredox Catalysis in Flow

Chem. Commun., 2013

Chemicals were either used as received or purified according to Purification of Common Laboratory Chemicals. All reactions were performed using common anhydrous, inert atmosphere techniques. Reactions were monitored by TLC and visualized by a dual short wave/long wave UV lamp and stained with an ethanolic solution of potassium permanganate or p-anisaldehyde. Column flash chromatography was performed using 230-400 mesh silica gel. NMR spectra were recorded on Varian Unity Plus 400 and Varian Mercury 500 spectrometers. Chemical shifts for 1 H NMR were reported as δ, parts per million, relative to the signal of CHCl 3 at 7.26 ppm. Chemical shifts for 13 C NMR were reported as δ, parts per million, relative to the CDCl 3 triplet at 77.0 ppm. Proton and carbon assignments were established using spectral data of similar compounds. The abbreviations s, br. s, d, dd, br. d, ddd, t, q, br. q, qi, m, and br. m stand for the resonance multiplicity singlet, broad singlet, doublet, doublet of doublets, broad doublet, doublet of doublet of doublets, triplet, quartet, broad quartet, quintet, multiplet and broad multiplet, respectively. IR spectra were recorded on an Avatar 360 FT-IR spectrometer. Mass spectra were recorded at the Mass Spectrometry Facility at the Department of Chemistry of the Boston University in Boston, MA on a Waters Q-Tof API-US with ESI high resolution mass spectrometer. Concentration refers to removal of solvent under reduced pressure (house vacuum at ca. 20 mmHg).

ACS Catalysis, 2019

A Hydrogen Atom Transfer (HAT) step from acetone allowed the smooth generation of acetonyl radical, that was then exploited as synthon in the mild formation of CC bonds under flow conditions. The process was promoted by aryl radicals photocatalytically generated via Single Electron Transfer (SET) reduction of arenediazonium salts. The mechanism has been investigated by a combined experimental and computational approach and further supported by deuterium labelling experiments.

Chemical Communications, 2010

Chemicals were either used as received or purified according to Purification of Common Laboratory Chemicals. Glassware was dried in an oven at 150°C or flame dried and cooled under a dry atmosphere prior to use. All reactions were performed using common dry, inert atmosphere techniques. Reactions were monitored by TLC and visualized by a dual short wave/long wave UV lamp and stained with an ethanolic solution of potassium permanganate or p-anisaldehyde. Column flash chromatography was performed using 230-400 mesh silica gel. NMR spectra were recorded on Varian Mercury 300, Varian Unity Plus 400, and Varian Mercury 400 spectrometers. Chemical shifts for 1 H NMR were reported as δ, parts per million, relative to the signal of CHCl 3 at 7.26 ppm. Chemical shifts for 13 C NMR were reported as δ, parts per million, relative to the center line signal of the CDCl 3 triplet at 77.0 ppm. Proton and carbon assignments were established using spectral data of similar compounds. The abbreviations s, br. s, d, dd, br. d, ddd, t, q, br. q, m, and br. m stand for the resonance multiplicity singlet, broad singlet, doublet, doublet of doublets, broad doublet, doublet of doublet of doublets, triplet, quartet, broad quartet, multiplet and broad multiplet, respectively. IR spectra were recorded on an Avatar 360 FT-IR spectrometer. Mass spectra were recorded at the Mass Spectrometry Facility at the Department of Chemistry of the Boston University in Boston, MA on a Waters Q-Tof API-US with ESI high resolution mass spectrometer. Concentration refers to removal of solvent under reduced pressure (house vacuum at ca. 20 mmHg). Ir(ppy) 2 (dtbbpy)PF 6 catalyzed reactions were carried out under visible light irradiation by a 14W household compact fluorescent lamp (CFL) clamped ~15 cm from the reaction vessel. Ru(bpy) 3 Cl 2 catalyzed reaction were carried out under irradiation by a 15 cm blue LED strip (available from , λ max = 435 nm) surrounding the reaction vessel. The Ru(bpy) 3 Cl 2 photoredox system is also active under irradiation by the CFL.

Catalysts

Photocatalytic organic synthesis/conversions and water treatment under visible light are a challenging task to use renewable energy in chemical transformations. In this review a brief overview on the mainly employed visible light photocatalysts and a discussion on the problems and advantages of Vis-light versus UV-light irradiation is reported. Visible light photocatalysts in the photocatalytic conversion of CO2, conversion of acetophenone to phenylethanol, hydrogenation of nitro compounds, oxidation of cyclohexane, synthesis of vanillin and phenol, as well as hydrogen production and water treatment are discussed. Some applications of these photocatalysts in photocatalytic membrane reactors (PMRs) for carrying out organic synthesis, conversion and/or degradation of organic pollutants are reported. The described cases show that PMRs represent a promising green technology that could shift on applications of industrial interest using visible light (from Sun) active photocatalysts.

Trends in Chemistry, 2019

Continuous-flow chemistry has recently attracted significant interest from chemists in both academia and industry working in different disciplines and from different backgrounds. Flow methods are now being used in reaction discovery/methodology, in scale-up and production, and for rapid screening and optimization. Photochemical processes are currently an important research field in the scientific community and the recent exploitation of flow methods for these methodologies has made clear the advantages of flow chemistry and its importance in modern chemistry and technology worldwide. This review highlights the most important features of continuous-flow technology applied to photochemical processes and provides a general perspective on this rapidly evolving research field.

Continuous flow reactor technology has a proven track record in enabling photochemical transformations. However, transfer of a photochemical batch process to a flow protocol often remains elusive, especially when solid reagents or catalysts are employed. In this work, application of an oscillatory plug flow photoreactor enabled a heterogeneous MacMillan-type C(sp²)-C(sp³) cross-electrophile coupling. Combination of an oscillatory flow regime with static mixing elements imparts exquisite control over mixing intensity and residence time distribution (RTD), pinpointing a mindset shift concerning slurry handling in continuous flow reactors. The C(sp²)-C(sp³) cross-electrophile coupling was successfully transferred from batch to flow, resulting in an intensified slurry process with significantly reduced reaction time and increased productivity (0.87 g/h).

Angewandte Chemie International Edition, 2015

Reported is the controllable selectivity syntheses of four distinct products from the same starting materials by visible-light photoredoxcatalysis.Byemploying adicyanopyrazine-derived chromophore (DPZ) as photoredoxcatalyst, an aerobic radical mechanism has been developed, and allows the reactions of N-tetrahydroisoquinolines (THIQs) with Nitaconimides to through four different pathways,i ncluding addition-cyclization, addition-elimination, addition-coupling, and addition-protonation, with satisfactory chemoselectivity. The current strategy provide straightforwarda ccess to four different but valuable N-heterocyclic adducts in moderate to excellent yields.

2019

While chemistry proceeding through radical intermediates has been known and studied for centuries, photoredox catalysis as a means of generating these species has only entered the forefront of synthetic organic chemistry within the past few decades. This approach has gained traction due to the use of visible light and generally mild conditions, making it compatible with a range of functional groups and late-stage functionalization. Chapters 1 and 2 of this work focus on photoredox catalysis in general. The first chapter covers the basic principles of photoredox catalysis, giving special attention to acridinium ion dyes as photoredox catalysts; in addition, select applications of acridinium ion photoredox catalysis to the synthesis of natural products are discussed. The second chapter details the importance of electrochemical potentials in planning and conducting photoredox transformations. Our work on compiling a database of experimental and computational potentials, and a collabora...

2012

Photoredox catalysis is now well-known in organic synthesis for the formation of free radicals under very soft irradiations conditions (e.g. sunlight, household fluorescence or LED bulbs, Xe lamp). This method has been introduced here to the polymer chemistry area to initiate ring opening polymerizations (ROP) or free radical polymerizations (FRP). The present paper will give an up-to date situation of the photocatalyst achievements in FRP and ROP.

ChemInform, 2016

This reaction can be also conducted by using a photo‐flow setup which leads to significantly decreased reaction times.