Unexpected Hydrated Electron Source for Preparative Visible-Light Driven Photoredox Catalysis

Abstract

The hydrated electron is experiencing a renaissance as a superreductant in lab-scale reductions driven by light, both for the degradation of recalcitrant pollutants and for challenging chemical reactions. However, examples for its sustainable generation under mild conditions are scarce. By combining a water-soluble Ir catalyst with unique photochemical properties and an inexpensive diode laser as light source, we produce hydrated electrons through a two-photon mechanism previously thought to be unimportant for laboratory applications. Adding cheap sacrificial donors turns our new hydrated electron source into a catalytic cycle operating in pure water over a wide pH range. Not only is that catalytic system capable of detoxifying a chlorinated model compound with turnover numbers of up to 200, but it can also be employed for two novel hydrated electron reactions, namely, the decomposition of quaternary ammonium compounds and the conversion of trifluoromethyl to difluoromethyl groups.

Multi‐Photon Excitation in Photoredox Catalysis: Concepts, Applications, Methods

“…For instance, the dependence of the product yield on the excitation power ( P ) is a relatively straightforward experiment (Figure a). In the initial phase of a biphotonic reaction, the product yield is frequently expected to show a quadratic dependence on P . Thus, doubling the excitation power from P 1 to P 2 =2⋅ P 1 will not simply result in a doubled yield as for monophotonic reactions (green line in Figure a), but instead one expects a four‐fold increase of the yield for a biphotonic reaction (red line in Figure a).…”

“…Aiming to exploit the mechanism of Figure under continuous‐wave (non‐pulsed) irradiation conditions, we discovered that the [Ir(sppy) 3 ] complex (Table , entry 2) catalyzes the formation of hydrated electrons in the presence of triethanolamine (TEAO) or ascorbate . A diode laser (447 nm) sufficed for the 50‐mg‐scale photoreduction of 4‐(trifluoromethyl)benzoate to the corresponding difluoromethyl compound as well as for the decomposition of a benzylammonium cation.…”

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

“…Mit dem Ziel vor Augen, den Mechanismus von Abbildung unter cw‐Bestrahlungsbedingungen (nicht gepulst) anzuwenden, haben wir entdeckt, dass der Komplex [Ir(sppy) 3 ] (Tabelle , Eintrag 2) die Freisetzung hydratisierter Elektronen in Gegenwart von Triethanolamin (TEOA) oder Ascorbat katalysiert . Ein Diodenlaser (447 nm) genügt für die Photoreduktion von 4‐(Trifluormethyl)benzoat zur zugehörigen Difluormethylverbindung im 50‐mg‐Maßstab sowie für die Zersetzung eines Benzylammoniumkations.…”

“…Folglich ist die PC‐Photostabilität ein Schlüsselfaktor, dem gebührende Beachtung zu schenken ist. [Ir(sppy) 3 ] ist selbst bei intensiven Langzeitbestrahlungen bemerkenswert robust, was zum Teil die Erklärung für das Ablaufen der Reaktionen in Tabelle (Einträge 2 und 3) liefert. Dies trifft jedoch nicht für alle PCs zu, wie nachfolgend ausführlich diskutiert wird (Abschnitte 3, 4 und 8).…”

Laser‐Induced Wurtz‐Type Syntheses with a Metal‐Free Photoredox Catalytic Source of Hydrated Electrons

“…First, we access a" super-reductant" (the hydrated electron e À aq ;s tandard potential, À2.9 V) [6] through photon pooling but withoutt he necessity of storing the energy of the first photon in al ong-lived intermediate, as al ow-intensity source would mandate. [7,8] The advantage of the two-photon approachl ies in its longer operating wavelength, which is not absorbed so strongly and by so many substrates as is the UV-C ( % 250 nm) used by the iodide and sulfite processes of e À aq generation. [9][10][11] Second, we prepare e À aq in high local concentration such that its rapid interceptionb yachloro-organic to give ac hloride ion and a carbon-centred radicalf avoursd imerization of the latter over the usual decay pathwayso fh ydrogen abstraction from a donor or addition to ac oupling component.…”