Michal Majek

On the mechanism of photocatalytic reactions with eosin Y

Summary A combined spectroscopic, synthetic, and apparative study has allowed a more detailed mechanistic rationalization of several recently reported eosin Y-catalyzed aromatic substitutions at arenediazonium salts. The operation of rapid acid–base equilibria, direct photolysis pathways, and radical chain reactions has been discussed on the basis of pH, solvent polarity, lamp type, absorption properties, and quantum yields. Determination of the latter proved to be an especially valuable tool for the distinction between radical chain and photocatalytic reactions.

Metal-Free Carbonylations by Photoredox Catalysis†

The synthesis of benzoates from aryl electrophiles and carbon monoxide is a prime example of a transition-metal-catalyzed carbonylation reaction which is widely applied in research and industrial processes. Such reactions proceed in the presence of Pd or Ni catalysts, suitable ligands, and stoichiometric bases. We have developed an alternative procedure that is free of any metal, ligand, and base. The method involves a redox reaction driven by visible light and catalyzed by eosin Y which affords alkyl benzoates from arene diazonium salts, carbon monoxide, and alcohols under mild conditions. Tertiary esters can also be prepared in high yields. DFT calculations and radical trapping experiments support a catalytic photoredox pathway without the requirement for sacrificial redox partners.

Ambient‐Light‐Mediated Copper‐Catalyzed CC and CN Bond Formation

Bringing to light: The rediscovery of visible light as an abundant energy source for organic reactions has most recently brought copper-catalyzed coupling reactions to the center of attention. This Highlight summarizes the most significant advancements in the field of C-C and C-N coupling reactions in which covalent copper–substrate complexes are photo-activated.

Application of Visible-to-UV Photon Upconversion to Photoredox Catalysis: The Activation of Aryl Bromides.

The activation of aryl-Br bonds was achieved by sequential combination of a triplet-triplet annihilation process of the organic dyes, butane-2,3-dione and 2,5-diphenyloxazole, with a single-electron-transfer activation of aryl bromides. The photophysical and chemical steps were studied by time-resolved transient fluorescence and absorption spectroscopy with a pulsed laser, quenching experiments, and DFT calculations.

Visible Light Driven Hydro-/Deuterodefunctionalization of Anilines

The defunctionalization of anilines is an important strategy in aromatic-substitution chemistry. Herein, we report on visible light mediated hydro- and deuterodediazonations in solutions of DMF. The mild reaction conditions (DMF, RT, no additives) tolerate various functional groups and allow the site-specific introduction of D atoms to the arene. Mechanistic investigations indicate the participation of photoredox and radical chain pathways and competing abstraction of methyl and formyl hydrogen atoms from DMF.

Aromatic Chlorosulfonylation by Photoredox Catalysis.

Visible-light photoredox catalysis enables the efficient synthesis of arenesulfonyl chlorides from anilines. The new protocol involves the convenient in situ preparation of arenediazonium salts (from anilines) and the reactive gases SO2 and HCl (from aqueous SOCl2 ). The photocatalytic chlorosulfonylation operates at mild conditions (room temperature, acetonitrile/water) with low catalyst loading. Various functional groups are tolerated (e.g., halides, azides, nitro groups, CF3 , SF5 , esters, heteroarenes). Theoretical and experimental studies support a photoredox-catalysis mechanism.

Metal-free radical thiolations mediated by very weak bases

Aromatic thioethers and analogous heavier chalcogenides were prepared by reaction of arene-diazonium salts with disulfides in the presence of the cheap and weak base NaOAc. The mild and practical reaction conditions (equimolar reagents, DMSO, r.t., 8 h) tolerate various functional groups (e.g. Br, Cl, NO2, CO2R, OH, SCF3, furans). Mechanistic studies indicate the operation of a radical aromatic substitution mechanism via aryl, acetyloxyl, thiyl, and dimsyl radicals.

Dichromatic Photocatalytic Substitutions of Aryl Halides with a Small Organic Dye

Photocatalytic bond activations are generally limited by the photon energy and the efficiency of energy and electron transfer processes. Direct two-photon processes provide sufficient energy but the ultra-short lifetimes of the excited states prohibit chemical reactions. The commercial dye 9,10-dicyanoanthracene enabled photocatalytic aromatic substitutions of non-activated aryl halides. This reaction operates under VIS-irradiation via sequential photonic, electronic, and photonic activation of the simple organic dye. The resultant highly reducing excited photocatalyst anion readily effected C-H, C-C, C-P, C-S, and C-B bond formations. Detailed synthetic, spectroscopic, and theoretical studies support a biphotonic catalytic mechanism.