Photosensitized, energy transfer-mediated organometallic catalysis through electronically excited nickel(II)

Abstract

Transition metal catalysis has traditionally relied on organometallic complexes that can cycle through a series of ground-state oxidation levels to achieve a series of discrete yet fundamental fragment-coupling steps. The viability of excited-state organometallic catalysis via direct photoexcitation has been demonstrated. Although the utility of triplet sensitization by energy transfer has long been known as a powerful activation mode in organic photochemistry, it is surprising to recognize that photosensitization mechanisms to access excited-state organometallic catalysts have lagged far behind. Here, we demonstrate excited-state organometallic catalysis via such an activation pathway: Energy transfer from an iridium sensitizer produces an excited-state nickel complex that couples aryl halides with carboxylic acids. Detailed mechanistic studies confirm the role of photosensitization via energy transfer.

Strongly Reducing, Visible‐Light Organic Photoredox Catalysts as Sustainable Alternatives to Precious Metals

“…Incorporating the photoredox cycle introduces redox or energy transfer [37] mechanisms with the nickel complexes to complete otherwise demanding catalytic cycles. Cross-coupling reactions have traditionally been catalyzed by palladium complexes at elevated temperatures to construct such critical bonds.…”

Sulfonamidation of Aryl and Heteroaryl Halides through Photosensitized Nickel Catalysis

“…In this context, our group has described several different transformations that employ photocatalyst excited states with nickel to enable otherwise disfavored reductive elimination steps involving C–O and C–N bonds (Figure 1). [10] Moreover, recent reports from the groups of Fu and Peters, Nocera, Molander, and Doyle describing direct photoexcitation of transition-metal catalysts provide elegant demonstrations of this paradigm. [11] Our group has also demonstrated that energy-transfer photosensitization can be used to access triplet excited states of Ni II complexes, specifically in the context of C–O bond formation between carboxylic acids and aryl halides.…”

“…[11] Our group has also demonstrated that energy-transfer photosensitization can be used to access triplet excited states of Ni II complexes, specifically in the context of C–O bond formation between carboxylic acids and aryl halides. [10b] A key benefit of this methodology is the separation of light-harvesting and cross-coupling roles between two different transition-metal complexes. Indeed, utilization of a discrete energy-transfer catalyst bypasses the question of whether an organometallic complex can efficiently undergo direct visible-light excitation, the capacity for which often varies on a case-by-case basis.…”

Photokatalyse mit sichtbarem Licht: Welche Bedeutung hat sie für die organische Synthese?

“…[141] Mit NiCl 2 und dem [Ir] 101 wurde eine Reihe von Arylbromiden 68 mit elektronenziehenden und elektronenschiebendenSubstituenten erfolgreich mit primären und sekundären Alkoholen 135 gekuppelt, was zeigt, dass die elektronische Natur der Substituenten am Arylhalogenid die Reaktivitätn icht beeinflusst (Schema 39 a). [142] Mit NiBr 2 ·Diglyme (5 Mol-%) und [Ir(ppy) 3 ] (2 Mol-%) als Photosensibilisator unter Bestrahlung mit sichtbarem Licht konnte eine Vielzahl aromatischer Verbindungen mit Alkyl-und Arylcarbonsäuren 137 gekuppelt werden, was die entsprechenden Ester 138 in guten Ausbeuten ergab.D ie Autoren schlagen vor,d ass Ir III als Sensibilisator dient, welcher sichtbares Licht absorbiert. [142] Mit NiBr 2 ·Diglyme (5 Mol-%) und [Ir(ppy) 3 ] (2 Mol-%) als Photosensibilisator unter Bestrahlung mit sichtbarem Licht konnte eine Vielzahl aromatischer Verbindungen mit Alkyl-und Arylcarbonsäuren 137 gekuppelt werden, was die entsprechenden Ester 138 in guten Ausbeuten ergab.D ie Autoren schlagen vor,d ass Ir III als Sensibilisator dient, welcher sichtbares Licht absorbiert.…”