Reply to “Photoredox Catalysis: The Need to Elucidate the Photochemical Mechanism”

Abstract

Spektroskopische Messungen und abgeschätzte thermodynamische Werte sind wichtige Hilfsmittel für mechanistische Untersuchungen von Photokatalysen. Messwerte, die nur unter idealisierten Bedingungen ermittelt werden und die Komplexität präparativer organischer Reaktionsgemische ausblenden, sind allerdings wenig geeignet, um mechanistische Hypothesen zu beweisen oder Syntheseausbeuten und ‐selektivitäten zu verbessern.

Birch‐Type Photoreduction of Arenes and Heteroarenes by Sensitized Electron Transfer

“…For triplet–triplet annihilation, either laser irradiation or very high intensity light is typically required . Our LED set up is not able to produce singlet arenes via a triplet–triplet annihilation process . From time‐dependent UV measurements (see Figure S8) we can conclude that an Ir II species is not stable for long in DMF under the reaction conditions.…”

The Tandem Photoredox Catalysis Mechanism of [Ir(ppy)2(dtb-bpy)]+ Enabling Access to Energy Demanding Organic Substrates

“…13,14 Energy is transferred from the metal catalyst excited state to an intermediary species, such as pyrene, which then undergoes electron transfer with a substrate, although the exact nature of this process is vigorously debated. 15,16 Reported systems where simple electron transfer between the catalyst and substrate proceeds efficiently despite being endergonic based on redox potentials are also poorly understood. 12 In substrates where hydrogen-bonding opportunities exist, this unexpected reactivity is often reconciled by concerted proton-coupled electron transfer (PCET).…”

Birch‐Type Photoreduction of Arenes and Heteroarenes by Sensitized Electron Transfer

“…Most arenes have comparatively long excited‐state life times, the immediate reduction of which generates the corresponding radical anion. This step should be exergonic (Δ G <0), and is followed by a fast hydrogen‐atom transfer (HAT) that should trap the radical anion and transform it into a stable anion. Subsequent protonation would result in the dearomatized product.…”