Organolithium reagents are stalwarts of synthesis commonly used to forge carbon-carbon bonds. Now, researchers say they have developed an improved method for preparing the purified, highly reactive lithium metal needed to make organolithiums in the lab, an approach that could prove useful for making other lithium-based compounds (J. Am. Chem. Soc. 2022, DOI: 10.1021/jacs.2c07207).
The method came about after Andy A. Thomas of Texas A&M University began a project that required his group to make some organolithiums in-house. One common approach involves reacting an organohalide with lithium powder, sometimes sold as a dispersion in mineral oil. The high surface area of the ground-up metal particles makes them reactive enough to exchange the halogen atom for lithium. But Thomas’s team soon hit a snag.
“We couldn’t find any lithium powders to do these lithium-halogen exchange reactions,” Thomas recalls. He speculates that the dearth of commercial lithium powder might be due to high—and growing—demand for the metal from the battery sector.
Making lithium powder themselves would be onerous, especially on a small scale, Thomas says, so the researchers came up with a different approach. They started by dissolving lithium metal in liquid ammonia at –78 °C, which forms the soluble lithium-ammonia complex Li(NH3)4+ along with solvated electrons. As the metal dissolves, the solution turns deep blue and then fiery bronze as more electrons enter solution.
Thomas’s team then warmed the solution and used a vacuum pump to remove the ammonia. Gradually, pure lithium began to crystallize from the solution, forming spindly structures called dendrites that could be removed and stored under an inert atmosphere for later use. Thomas says that it’s a simpler procedure than making lithium powder, which involves mixing up molten lithium in hot oil, and it’s cheaper than buying the powder from a supplier. The whole process takes a couple of hours.
It also produces a product that is 20 times more reactive than the powder because the dendrites have roughly 100 times the surface area. “To my knowledge, it’s the most reactive form of lithium that’s available,” Thomas says. Another advantage is that this recrystallized lithium is free of the impurities that commercial lithium powders and dispersions can build up, such as lithium oxide.
The researchers used their lithium dendrites to make a wide variety of organolithium reagents from organohalides, in yields at least as good as those from lithium powder. They also prepared organolithium reagents labeled with the isotope lithium-6, useful for nuclear magnetic resonance studies. Lithium-6 is much pricier than normal lithium metal, so being able to prepare these compounds with high purity on a very small scale is a boon, Thomas says.
“It’s difficult to work at a very small scale with lithium powder, so I think from a practical perspective this is an innovation,” says Eva Hevia of the University of Bern, who develops and studies organolithium reagents for synthesis and was not involved in the new study. She adds that the method may not be confined to making organolithiums: “You’re generating a pure lithium source that is much more reactive,” Hevia says, “so this could really open the door to preparing other lithium compounds.”