Iterative Flow Borylations Pump Out Complex Molecules

Chemists of late have developed a penchant for carrying out iterative cross-coupling reactions, which involve synthesizing a borylated product that in turn can be used in a subsequent reaction to prepare another borylated product. This cycle can be carried out multiple times, with the end result being a complex molecule with pharmaceutical applications in mind. A team led by Claudio Battilocchio and Steven V. Ley of the University of Cambridge has extended this strategy by using a flow reactor system to sequentially form up to four carbon-carbon bonds (Nat. Chem. 2016, DOI: 10.1038/nchem.2439). The researchers first pump a hydrazone precursor over manganese dioxide to create a diazo intermediate. Although diazo compounds are versatile reagents, chemists tend to avoid them because they are notoriously explosive. The flow system allows the team to manage small volumes of the compounds safely. The researchers next shunt the diazo compounds into flasks containing boronic acids, and in one step after another, complex molecules gradually take shape (one example shown). The diazo compounds permit the researchers to run metal-free cross-couplings, which avoid costly precious-metal catalysts and help prevent metal impurities from contaminating drug products.