Mechanistische Studien in der Photokatalyse

Abstract

Die sich rasant entwickelnden Felder der Photo‐ und Photoredoxkatalyse bieten neue Gelegenheiten, das Potential der synthetisch‐organischen Chemie zu erweitern. Fortschritte bei photokatalytischen Prozessen haben sich bisher vor allem aus empirischen Entdeckungen und der Suche nach neuen Reaktionen ergeben. Die allgemeine Wahrnehmung jedoch ist, dass die Photokatalyse in eine reifere Phase eintritt, in der die Kombination experimenteller und mechanistischer Studien eine Vorreiterrolle für weitere Innovationen spielt. Dieser Aufsatz beschreibt die mechanistischen Schlüsselstudien, die bei der Entwicklung photochemischer Prozesse in Betracht gezogen werden sollten, sowie die geeignetsten verfügbaren Techniken zur Gewinnung relevanter Informationen. Die Diskussion verweist auf ausgewählte Fallstudien, um zu zeigen, wie mechanistische Untersuchungen eine instrumentelle Rolle bei der Entdeckung und Entwicklung synthetisch nützlicher, photokatalytischer Transformationen spielen können.

P/N Heteroleptic Cu(I)-Photosensitizer-Catalyzed Deoxygenative Radical Alkylation of Aromatic Alkynes with Alkyl Aldehydes Using Dipropylamine as a Traceless Linker Agent

“…On the basis of the above-mentioned results and the previous literature, ,,,− a possible mechanism for the production of 5aa under the Cu­(I)-photoredox catalysis conditions is proposed in Scheme . Upon irradiation with blue light, the Cu­(I)-based PS5 was excited to its excited state Cu­(I)* ( E 1/2 [Cu I* /Cu 0 ] = 1.04 V versus SCE (saturated calomel electrode) in MeCN, see SI), which was quenched by dipropylamine 3c ( E p/2 [( n -Pr) 2 NH •+ /( n -Pr) 2 NH] = 0.89 V vs SCE in MeCN, see SI; literature value: E p [( n -Pr) 2 NH •+ /( n -Pr) 2 NH] = 0.90 V vs SCE) and/or Hantzsch ester 4 ( E [( HE •+ / HE ] = 0.79 V vs SCE; E­[( HE •+ )/ HE *] = −2.28 V vs SCE) through an SET process, , leading to the generation of dipropylamine radical cation species 3c′ and/or Hantzsch ester radical cation species 4′ , and the reactive Cu(0) complex. The above quenching processes were further confirmed by Stern–Volmer studies (see Figures S13 and S14 in SI).…”

Acridine Orange Hemi(Zinc Chloride) Salt as a Lewis Acid‐Photoredox Hybrid Catalyst for the Generation of α‐Carbonyl Radicals

“…To learn about the mode of action of the photocatalyst and to understand other mechanistic details, several spectrophotometric experiments were performed. [31] In the beginning, fluorescence emission spectra of the photocatalyst in the presence of different reactions components (1 a, 2 a, 4 a, 6 a and K 2 HPO 4 ) were recorded and analyzed in detail to estimate the light emission properties of the pure catalyst system and the distractions by external interferences (see SI for more details). The Stern-Volmer plot was depicted based on the photoluminescence quenching of the [a] Reaction conditions: 1 (0.3 mmol), 6 (0.3 mmol), AOBHZn…”

Selective 1,2‐Aminoisothiocyanation of 1,3‐Dienes Under Visible‐Light Photoredox Catalysis

“…A systematic survey of reaction conditions allowed us to identify the following optimum conditions: fac-Ir(ppy) 3 (1.0 mol %) as a photoredox catalyst in dichloromethane (DCM) under irradiation with blue LEDs at room temperature (Table 1, entry 1). [25] Under these conditions, the 1,2aminoisothiocyanation product 3 a was formed as single isomer in 75 % isolated yield. While 3 a was formed in similar yield with Ir(ppy) 2 (dtbbpy)(PF 6 ) (entry 2), other commonly used photocatalysts, such as Ru(bpy) 3 Cl 2 •6 H 2 O and eosin Y, gave inferior results (entries 2-4).…”