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

Abstract

In this study we use in situ infrared tracking to describe the kinetics of dual photo- and nickel-catalyzed carboxylate O-arylations. We examined both a state-of-the-art homogeneous (Ir(ppy)<sub>3</sub>) and heterogeneous (graphitic carbon nitride) photocatalyst, comparing their kinetics to each other, and to the existing mechanistic proposal. We argue against the current hypothesis, specifically that the photocatalyst is only involved to trigger a rate-limiting reductive elimination.<br>

Photocatalyst-free, visible-light-mediated nickel catalyzed carbon–heteroatom cross-couplings

“…Partial consumption of the starting material in the absence of a nickel salt (entry 15) might be a result of a photocatalytic activation of aryl iodides. 15 Product formation was not detected using bpy as ligand (entry 17), but significant amounts of 3 were obtained in presence of 9H-carbazole (entry 16) or a combination of 9H-carbazole and bpy (entry 16, 18). Although such reactivity was not observed in the other coupling protocols, this effect might result from formation of photoactive Ni-carbazole complexes.…”

“…A kinetic analysis of a related protocol using exogenous photocatalysts showed a rate dependence on the aryl halide, which was assigned to a direct photocatalytic activation. 15 Due to its nature (microwave power saturation and linewidth), the lightdependent EPR-signal is of organic origin without involvement of Ni. Therefore, we tentatively assign this signal to an elusive paramagnetic species that results from a light-induced reaction between a Ni•czbpy species and the aryl iodide, suggesting that the aryl iodide may play a role in the activation of the pre-catalyst.…”

“…All of these results are in agreement with previous reports on the dual nickel/photocatalytic cross-coupling of carboxylic acids with aryl halides, indicating that the photocatalyst-free strategy follows a similar mechanism. 11,15,26,34,35 Aromatic as well as aliphatic sulfonamides (3, 42-46) gave selective C-N cross-couplings with 4iodobenzotrifluoride (Table 2c), even though long reaction times were necessary in case of electronwithdrawing groups (43, 46). In contrast to the previous C-S and C-O coupling, reactivity is not affected by the substitution pattern of the aryl iodide (47-49, 54) or by the presence of either electronwithdrawing or electron-donating functional groups (3, 47-53, 56-58).…”

“…Commonly applied homogeneous noble-metal based photocatalysts that are mainly applied are expensive, not easily recyclable and unsustainable. Moreover, their high reactivity upon excitation can trigger unwanted side-reactions 15 and deactivation of the nickel catalyst 16 results in low selectivities and often poor reproducibility.…”

Weak arene/nickel interaction lights on monoarylation of alcohols

“…We next examined OA e ciency and possible OA pathway of our Et-DABDP/Ni catalyst [29][30][31] . Using electron-neutral bromide 4 as electrophile, in-situ generated "OA active species" consumed 4 in the presence of Ni(cod) 2 and lead to the formation of self-coupling product 5 via disproportionation of nickel(II) intermediate.…”