Chiral organoboranes built by bacteria

Although there are natural products that contain boron, the organisms that make these compounds do so by using small molecules that react spontaneously with boric acid in the environment. No enzymes are involved. What’s more, natural organisms that create carbon-boron bonds, or organoboranes, are unknown. But that didn’t stop Frances H. Arnold, S. B. Jennifer Kan, Xiongyi Huang, and coworkers at Caltech, who reasoned that with a little genetic tinkering they could create organoborane-making bacteria. The new approach could embellish current methods to make organoboranes, which are important synthetic building blocks for organic chemists. The researchers took Escherichia coli cells that were engineered with wild-type cytochrome c from the geothermal bacterium Rhodothermus marinus and incubated them with an N-heterocyclic carbene borane and a diazoester, finding they could create a chiral organoborane (reaction shown) in modest yield and with good enantioselectivity (Nature 2017, DOI: 10.1038/nature24996). After genetic adjustments to the amino acids around the enzyme’s heme unit, the Caltech team was able to boost both the yield, enantioselectivity, and turnover of the biocatalyst. The researchers applied the bacterial catalyst to different boron reagents and diazoesters, creating a series of organoboranes. They also used trifluoromethyl-substituted (diazomethyl)benzene as a substrate to make chiral α-trifluoromethylated organoboranes—a valuable chiral building block. “Microorganisms are powerful alternatives to chemical methods for producing pharmaceuticals, agrochemicals, materials and fuels,” the researchers write. “Borylation chemistry can now be added to the vast synthetic repertoire of biology.”