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

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 C-aryl-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 undoubtedly facilitate future discoveries in the development of novel Ni-catalysed transformations of aryl ethers.