Popular coupling could turn into runaway reaction

The Suzuki-Miyaura cross-coupling reaction, which forges carbon-carbon bonds between organoboranes and electrophiles, has been an indispensable tool for molecule makers. But when vinylboron and aryl bromide reagents are used, this reaction has the potential to heat up uncontrollably. A team at Dow Agrosciences, led by Qiang Yang, performed a systematic evaluation of this reaction (example shown) in the presence of various bases, solvents, and catalysts to get a better handle on the scope of this exothermic behavior (Org. Process Res. Dev. 2018, DOI: 10.1021/acs.oprd.8b00001). The researchers found that palladium catalysts caused the reaction to heat up rapidly. This exothermic behavior was consistently higher and the reactions were faster when done in aqueous systems as compared with anhydrous conditions. The team also found that the choice of aryl electrophile, solvent, base, catalyst, and vinylboron species had an impact on the exothermic behavior of the reaction. Many of the reactions studied reached a temperature higher than the boiling point of the solvent or the onset of decomposition of the reaction mixture. Without an active cooling system, the chemists warn, these could become dangerous runaway reactions.