Suzuki coupling

The Suzuki coupling is the single most popular way to join two aromatic rings in drug discovery. With a palladium catalyst as the matchmaker, it bonds a boron-bearing partner (a boronic acid or ester) to an aromatic halide, snapping two rings together at the exact carbons you choose. Ask any medicinal chemist for their go-to ring-joining reaction and this is usually the answer.

Mechanistically it is a cross-coupling: palladium inserts into the carbon–halide bond (oxidative addition), a base helps hand over the aryl group from boron to palladium (transmetalation), and the two carbons then bond as the catalyst is regenerated (reductive elimination). Its appeal is practical: the boron reagents are stable, often commercially abundant, and relatively non-toxic, the byproducts are benign, and the reaction tolerates a wide range of other functional groups. Akira Suzuki shared the 2010 Nobel Prize for it.

The same convenience that makes Suzuki coupling ubiquitous has a downside: because it is so easy, biaryl-rich molecules are overrepresented in screening collections, nudging chemists toward flat, similar structures. Practical caveats include removing residual palladium from the product, protodeboronation of unstable boronic acids, and sluggish reactivity with certain heteroaryl partners.