Multiphotonen‐Anregung in der Photoredoxkatalyse: Konzepte, Anwendungen und Methoden

Abstract

Die Energie sichtbarer Photonen und die zugänglichen Redoxpotentiale üblicher Photokatalysatoren schränken die Anzahl derjenigen Photoreaktionen ein, die durch sichtbares Licht angetrieben werden können. Da UV‐Anregung schädlich ist und häufig Nebenreaktionen auslöst, ist sichtbares Licht oder sogar NIR‐Strahlung vorzuziehen. Die Photochemie befasst sich momentan mit einer zweischneidigen Herausforderung, und zwar mit dem Wunsch, immer thermodynamisch anspruchsvollere Reaktionen mit immer kleineren Photonenenergien durchführen zu können. Das Akkumulieren der Energie zweier energiearmer Photonen kann dazu passende Lösungsansätze liefern. Obwohl die Multiphotonen‐Spektroskopie gut etabliert ist, haben Photoredox‐Synthesechemiker erst kürzlich begonnen, Multiphotonen‐Prozesse präparativ zu nutzen. Hier werfen wir einen kritischen Blick auf kürzlich entwickelte Reaktionen und mechanistische Konzepte, diskutieren einschlägige experimentelle Methoden und geben einen Ausblick auf mögliche zukünftige Entwicklungen dieses jungen Forschungsgebiets.

Minisci C−H Alkylation of Heteroarenes Enabled by Dual Photoredox/Bromide Catalysis in Micellar Solutions**

“…However, catalytic species of a much higher reducing power can be generated via the reductive quenching of the catalyst followed by the subsequent excitation with a second visiblelight-photon. 30,[32][33][34] Although the typically used reductive quenchers include tertiary amines, Hantzsch esters, alcohols, ascorbate anions etc., 35 it has recently been shown that the efficient quenching of excited Ir-complexes can also be achieved using simple halide anions, leading to Ir(II)-species and halide radicals. [36][37][38][39][40] We decided to test, if Branions, which are released upon the single-electron-reduction and fragmentation of alkyl bromides, can be recycled and used as mediators in the Minisci reaction -quench the excited Ir(III)-photocatalyst and thus promote the generation of alkyl radicals.…”

“…The latter species can undergo consecutive absorption of a second photon, resulting in the formation of a strongly reducing form of the iridiumcomplex 30,32 or a solvated electron. 34 SET to alkyl bromide A followed by fragmentation affords alkyl radical B and a bromide anion, which participates in subsequent catalytic cycles. An addition of alkyl radical B to pyridinium salt C provides the radical cation D, able to undergo hydrogen-atom-transfer (HAT) with electrophilic bromine radical.…”

Emerging Activation Modes and Techniques in Visible-Light-Photocatalyzed Organic Synthesis

“…In 2021, the Wenger group published a thorough investigation of the mechanism behind the hydrodehalogenation of selected aryl halides through sensitization-induced electron transfer (Scheme 31 ). 63 fac -Ir(ppy) 3 was applied as a photosensitizer, 2,7-di- tert -butylpyrene ( t BuPyr) as annihilator and N , N -dimethylaniline (DMA) as terminal reductant. They confirmed an sTTA-UC mechanism where the triplet 3 fac -Ir(ppy) 3 * catalyst performs a TTEnT towards t BuPyr resulting in the formation of 3 t BuPyr*.…”

“…ConPET and related e-PRC strategies allow the formation of excited radical anions/cations that possess exceptional redox potentials. 63 Although it unlocks the photocatalytic activation of conventionally unreactive substrates using a single catalyst, the excited radical species is often very short-lived. For this reason, bimolecular quenching usually requires high quencher concentrations or the presence of ground-state pre-aggregates.…”

Synthetic Molecular Photoelectrochemistry: New Frontiers in Synthetic Applications, Mechanistic Insights and Scalability

“…PRC reactions are limited by the redox window of photocatalysts, which is in most part defined by the energy of visible photons (ca. 1.8–3.1 eV) [3] . UV photons that access higher energy limits come with penalties of less energy‐ and cost‐efficient reactors, safety and the direct excitation of substrate molecules leading to deleterious pathways.…”