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Mechanochemistry performs cross-coupling reactions

By Tetsuo Satoh |

Palladium-catalyzed cross-coupling reactions are one of the most powerful and versatile methods to synthesize a wide range of complex functionalized molecules. However, the development of solid-state cross-coupling reactions remains extremely limited. Now, Hajime Itoh, Koji Kubota and colleagues at Hokkaido University (Sapporo, Japan; https://labs.eng.hokudai.ac.jp/labo/organoelement) reported a rational strategy that provides a general entry to palladium-catalyzed Buchwald-Hartwig cross-coupling reactions in the solid state. The key finding of this study is that olefin additives can act as efficient molecular dispersants for the palladium-based catalyst in solid-state media to facilitate solid-state cross-coupling. Their strategy could inspire the development of industrially attractive, solvent-free palladium-catalyzed cross-coupling processes for other valuable synthetic targets.

Conventionally, palladium-catalyzed cross-coupling reactions of liquid and solid substrates are conducted in organic solvents. Researchers sought to re-design palladium-based catalyst systems for the solid state, which could potentially unlock versatile applications for solid-state synthesis.

They have developed a rational strategy for a potentially general and scalable solid-state palladium-catalyzed cross-coupling reaction using mechanochemistry. Whereas the palladium-catalyzed cross-coupling of neat liquids proceeds readily in ball mills, similar reactions using solid reactants remain challenging. However, they discovered that the addition of small amounts of olefins dramatically accelerates the C–N cross-coupling of such solid substrates. The examination of palladium nanoparticles, which were obtained from these reaction mixtures, by transmission electron microscopy (TEM) suggested that the olefin additives can act as efficient molecular dispersants for the palladium catalysts in solid-state media and thus facilitate this challenging solid-state cross-coupling. They expect that the strategy developed in this study could unlock broad areas of chemical space for palladium-catalyzed solid-state syntheses of valuable synthetic targets in various scientific fields.

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