Mechanisms of the Nickel-Catalysed Hydrogenolysis and Cross-Coupling of Aryl Ethers

Abstract

The Ni-catalysed hydrogenolysis and cross-coupling of aryl ethers has emerged as a powerful synthetic tool to transform inert phenol-derived electrophiles into functionalised aromatic molecules. This has attracted significant interest due to its potential to convert the lignin fraction of biomass into chemical feedstocks, or to enable orthogonal reactivity and late-stage synthetic modification. Although the scope of nucleophiles employed, and hence the CC and Cheteroatom bonds that can be forged, has expanded significantly since Wenkert’s seminal work in 1979, mechanistic understanding on how these reactions operate is still uncertain since the comparatively inert CarylO bond of aryl ethers challenge the involvement of classical mechanisms involving direct oxidative addition to Ni(0). In this review, we document the different mechanisms that have been proposed in the Ni-catalysed hydrogenolysis and cross-coupling of aryl ethers. These include: i) direct oxidative addition; ii) Lewis acid assisted CO bond cleavage; iii) anionic nickelates, and; iv) Ni(I) intermediates. Experimental and theoretical investigations by numerous research groups have generated a pool of knowledge that will undoubtably facilitate future discoveries in the development of novel Ni-catalysed transformations of aryl ethers.

Shedding Light on the Hidden Roles of Lithium in the Nickel-Catalyzed Cross-Coupling of Aryl Ethers

“…Revised Catalytic Cycle(s). The set of energy profiles computed for the Ni-catalyzed cross-coupling under study suggests an interesting mechanistic scenario that differs from previously reported pathways 5,12,13 and that is summarized in Figures 7a and S15. It involves six key steps: (i) nickelate formation (1 → 4) through the complexation of two "ancillary" Ph groups to promote the release of one COD from Ni(COD) 2 and to enhance the electron donating ability of As mentioned above, the three nickelate complexes that feature all combinations of Ph and Naph groups are contributing to the overall selectivity and activity of the reaction and were thus included in kinetics simulations.…”

“…With this mechanistic knowledge in hand, we next sought to use these insights to try and overcome a current challenge in Ni-catalysis, the so-called "naphthalene problem". 5 This phenomenon generally restricts the electrophilic coupling partner to πextended systems, such as 2-methoxynaphthalene, meaning that C−C cross-couplings of unbiased aryl ethers such as anisole itself are rare. 8−10 Computationally, the oxidative addition of anisole to Ni 0 -ate complex 4 accordingly to a mechanism analogous to the one computed for 2-methoxynaphthalene in the presence of [PhLi(THF)] 2 in benzene was assessed.…”

“…Nicatalysis, on the other hand, has proven to be powerful for many types of C−O bond functionalization reactions, including the cross-coupling and hydrogenolysis of aryl ethers. 4,5 As early as 1979, Wenkert reported the first Nicatalyzed activation of inert C aryl −OMe bonds in a Kumada− Tamao−Corriu type cross-coupling reaction. 6 In more recent years, aryl ether derivatives have been used as electrophilic coupling partners in several types of cross-coupling reactions, including Suzuki, Negishi, and Murahashi.…”

“…11 Based on both experimental and computational mechanistic studies however, this simplified mechanism has been shown to be inappropriate for aryl ethers due to the high bond dissociation enthalpy of the C aryl −OMe bond. 5 Density functional theory (DFT) calculations by Wang and Uchiyama provided support for an alternative anionic pathway which instead involves anionic nickelate intermediates. 12−14 Nevertheless, the lack of experimental evidence, the use of a simplified chemical model, and the limited speciation of both nickelate complexes and polar organometallic aggregates temper the conclusions drawn.…”

Diphenylacetylene stabilised alkali-metal nickelates: synthesis, structure and catalytic applications

“…We have previously explored the rich co-complexation chemistry of Ni(COD) 2 and PhLi, which gives a series of lithium nickelates including II (Scheme 1), and demonstrated that these are key intermediates in the nickel-catalysed cross-coupling of aryl ethers. 19,24 When further assessing these co-complexation reactions, we found that using a larger excess of PhLi led to the formation of a polynuclear dinickel cluster containing a bridging C 6 H 4 dianion as a result of intramolecular C-H activation of a phenyl substituent (Scheme 1, IV). 25 The formation of IV suggests that the homoleptic tri-lithium nickelate "Li 3 NiPh 3 (solv) 3 " is too electron-rich to form, leading to the in situ formation of the π-accepting benzyne-type ligand.…”