Evidence for Photocatalyst Involvement in Oxidative Additions of Nickel-Catalyzed Carboxylate O-Arylations

Abstract

Dual photocatalysis and nickel catalysis can effect cross-coupling under mild conditions, but little is known about the in situ kinetics of this class of reactions. We report a comprehensive kinetic examination of a model carboxylate O -arylation, comparing a state-of-the-art homogeneous photocatalyst (Ir(ppy) 3 ) with a competitive heterogeneous photocatalyst (graphitic carbon nitride). Experimental conditions were adjusted such that the nickel catalytic cycle is saturated with excited photocatalyst. This approach was designed to remove the role of the photocatalyst, by which only the intrinsic behaviors of the nickel catalytic cycles are observed. The two reactions did not display identical kinetics. Ir(ppy) 3 deactivates the nickel catalytic cycle and creates more dehalogenated side product. Kinetic data for the reaction using Ir(ppy) 3 supports a turnover-limiting reductive elimination. Graphitic carbon nitride gave higher selectivity, even at high photocatalyst-to-nickel ratios. The heterogeneous reaction also showed a rate dependence on aryl halide, indicating that oxidative addition plays a role in rate determination. The results argue against the current mechanistic hypothesis, which states that the photocatalyst is only involved to trigger reductive elimination.

LED‐NMR Monitoring of an Enantioselective Catalytic [2+2] Photocycloaddition

“…Many photocatalytic reactions conducted on preparative scale exhibit a linear relationship between photon flux and reaction rate because the solutions are optically dense, absorbing >99 % of the incident light . In such a photon‐limited regime, photocatalytic reaction rates would be controlled solely by the absorption of photons and should thus be zero‐order in the photocatalyst . This expectation is not consistent with the data in our reaction, as we observe a first‐order dependence on photocatalyst concentration under identical conditions.…”

“…This expectation is not consistent with the data in our reaction, as we observe a first‐order dependence on photocatalyst concentration under identical conditions. We conclude therefore that these experiments fall outside of the photon‐limited regime, and thus both photon flux and photocatalyst concentration have an influence on the reaction rate law. Indeed, even at the highest photocatalyst concentration, a simple Beer's law calculation shows that only 20 % of the photons will be absorbed over a 3 cm pathlength within the NMR tube (see Table S1 in the Supporting Information).…”

Photocatalytic (Hetero)Arylation of C(sp3)–H Bonds with Carbon Nitride

“…These experiments are indicative towards pathway B where the formation of an electronically excited Ni(II) complex is a prerequisite for bond formation. Pseudo-zero order and zero-order kinetic behaviour were also observed during time-course experiments37 (see theSupplementary Information). Nevertheless, we cannot rule out the possibility of a disproportionation reaction between photoexcited Ni(II)* and ground state Ni(II) to give Ni(I) and Ni(III) species followed by reductive elimination to afford the product.…”

“…2b). A wide variety of electron-poor nitrogen-containing heterocycles such as pyridine (33), quinolines (34,35), isoquinoline (36), pyrimidine (37), indole (38), benzothiazole worth mentioning that C−H arylation could also be performed in acetonitrile as solvent using 10 equivalents of amide C−H precursor and catalytic amounts of tetrabutylammonium chloride as an additive 32 . We were quite surprised to find that C(sp 3 (Fig.…”

Visible‐Light Promoted C–O Bond Formation with an Integrated Carbon Nitride–Nickel Heterogeneous Photocatalyst

“…Overall, the observed reactivity trend correlates with other transition metal catalyzed coupling reactions. [37] We further studied the coupling protocol between methanol and various aryl iodide and chlorobenzene substrates. Consistent with homogeneous transition metal catalysis reactions, a general inverse correlation between reaction rate and strength of CÀX bond (I < Br < Cl) was observed for a given functional group of aryl halides towards the coupling reaction.…”