The first examples of a nickel-catalysed cross-coupling of simple aryl methyl ethers with boronic esters will further improve the utility of the Suzuki–Miyaura coupling
It is difficult to overstate the importance of metal catalysed cross-coupling strategies to the modern organic chemist. The Suzuki–Miyaura reaction — classically between an aryl halide and an aryl boronic acid — is extremely popular because of its wide substrate scope and the availability and low toxicity of the boronic acid reagent. It is only relatively recently that the scope of the reaction has been expanded with the development of methods that enable the use of aryl chlorides, as well as alkyl coupling partners. Despite these advances, the synthetic chemist is still limited by the availability of the desired aryl halide. A popular strategy thus far has involved the conversion of a phenol to a pseudohalide, such as a triflate, but this has its own drawbacks, including the expense and high reactivity of the sulphonating agent.
Now, Mamoru Tobisu and co-workers from Osaka University in Japan have demonstrated1 that simple methoxy aryl ethers will react with aryl boronic esters in the presence of a nickel catalyst. The mechanism of the reaction is likely to be the same as that accepted for the standard Suzuki coupling — oxidative addition of the ether to Ni(0), followed by transmetallation with a boronate and reductive elimination of the product. The oxidative addition of the methoxy aryl ether is probably rate-limiting and, therefore, the reaction is most effective with relatively electron-poor substrates, although following this initial discovery the scope of this coupling reaction will probably expand rapidly.
It seems unlikely that aryl methyl ethers will become the coupling partner of choice for the Suzuki reaction, but specific applications such as Tobisu and colleagues' example of non-symmetrical oligoarene synthesis could certainly prove its usefulness as an addition to the ever-expanding synthetic toolkit.
References
Tobisu, M., Shimasaki, T. & Chatani, N. Nickel-catalysed cross coupling of aryl methyl ethers with aryl boronic esters. Angew. Chem. Int. Ed. 10.1002/anie.200801447 (2008).
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Davey, S. Methoxy methodology. Nature Chem (2008). https://doi.org/10.1038/nchem.21
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DOI: https://doi.org/10.1038/nchem.21