Light‐Promoted Nickel Catalysis: Etherification of Aryl Electrophiles with Alcohols Catalyzed by a Ni^II‐Aryl Complex

Abstract

A highly effective C−O coupling reaction of (hetero)aryl electrophiles with primary and secondary alcohols is reported. Catalyzed by a NiII‐aryl complex under long‐wave UV (390–395 nm) irradiation in the presence of a soluble amine base without any additional photosensitizer, the reaction enables the etherification of aryl bromides and aryl chlorides as well as sulfonates with a wide range of primary and secondary aliphatic alcohols, affording synthetically important ethers. Intramolecular C−O coupling is also possible. The reaction appears to proceed via a NiI–NiIII catalytic cycle.

Weak arene/nickel interaction lights on monoarylation of alcohols

“…2C). Furthermore, the combination of equal mole NiBr 2 and Ni(cod) 2 which is assumed to in-situ generate nickel(I) species via comproportionation 33 , didn't improve catalytic activity but gave lower yield compared to NiBr 2 alone as metal source. And we have not observed the electron paramagnetic resonance (EPR) signal of paramagnetic nickel(I) species 25 .…”

Deciphering the mechanism of the Ni-photocatalyzed C‒O cross-coupling reaction using a tridentate pyridinophane ligand

“…While inorganic bases such as K2CO3 were not effective, possibly due to the precipitation of halides required to stabilize the Ni I intermediate 14 than the loadings used in other Ni-photocatalyzed C-O cross-couplings. 10,11 Moreover, other alcohols such as 1-hexanol and benzyl alcohol could also be employed (Supplementary Table S7, 86-90% product yield). However, the combination of 1a and heterogenous Zn 0 or Mn 0 without light was ineffective in our conditions, likely due to the slow electron transfer kinetics or insufficient generation of the active species during catalysis.…”

“…In this regard, photoredox/nickel dual catalysis and light-promoted Ni catalysis have emerged as powerful strategies for challenging C-O bond formations. [8][9][10][11] Accordingly, an in-depth mechanistic understanding of Ni-mediated photocatalysis is essential for rational reaction design and optimization. From a mechanistic viewpoint, while the photocatalytic cycle is well-understood, the Ni-mediated redox cycle remains elusive as intermediates in various oxidation states (from Ni 0 to Ni IV ) have been proposed.…”

“…10,13,14 Our group has employed tetradentate azamacrocycle N,N'-dialkyl-2,11diaza [3.3](2,6)pyridinophanes ( R N4) ligands to isolate and characterize Ni III species capable of C-C [15][16][17][18] and C-heteroatom 15,19,20 bond formation reactions, and to probe the involvement of highvalent Ni species in these reactions. However, considering that the most ubiquitous ligands in Ni cross-coupling and photocatalytic reactions are the bidentate bipyridine ligands, [8][9][10][11]21 the R N4 ligands were thought to be less effective ligands owing to the crowded environment around the Ni center, which is also expected to disfavor the formation of Ni I species. By contrast, a bidentate ligand structure is not suitable for stabilizing high-valent Ni species, thus hampering the investigation of such Ni intermediates.…”

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

“…22,23 This observation was expanded to a synthetic protocol for C-O and C-N cross-couplings using UV-light irradiation. 24,25 More recently, pulse radiolysis together with spectroelectrochemistry indicated that Ni 0 /Ni II comproportonation generates a Ni I bipyridyl species that rapidly undergoes oxidative addition with iodobenzene. 21 Moreover, Nocera and coworkers showed that sub-stoichiometric amounts of zinc can be used instead of a photocatalyst and light for C-N and C-O cross-couplings.…”