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Dimethylcalcium synthesis cracked

Chemists have struggled to make the organometallic compound for decades

by Celia Henry Arnaud
January 24, 2018 | A version of this story appeared in Volume 96, Issue 5

Structures of dimethylcalcium and two derivatives of it.

Dimethylcalcium may sound like a simple compound, but its synthesis has eluded chemists. Researchers reported a synthesis 60 years ago, but no one has reproduced it, casting doubt on whether dimethylcalcium was actually made that first time.

Now, Reiner Anwander and coworkers at Eberhard Karls University of Tübingen have succeeded in synthesizing dimethylcalcium via a metathesis reaction between methyllithium and calcium bis(trimethylsilyl)amide (J. Am. Chem. Soc. 2018, DOI: 10.1021/jacs.7b12984). A synthesis of methylcalcium compounds could lead to new catalysts or reagents for forming carbon-carbon bonds.

“Although methylcalcium compounds have been discussed for more than a century, they’ve always been ill-defined and probably impure,” says Michael S. Hill of the University of Bath. “The isolation and definitive characterization of these compounds is a real synthetic achievement, and the paper as a whole is a striking piece of work.”

The hardest part of making dimethylcalcium turned out to be finding appropriate starting materials. Dimethylmercury might have made a good transmethylating agent, but Anwander forbids the use of it in his lab because of its toxicity. His team turned to methyllithium instead.

But commercial sources of methyllithium are usually contaminated with lithium chloride, which precipitates during the reaction as inseparable lithium and calcium chlorides. Anwander’s team devised a purification method to remove the chloride. By adding potassium bis(trimethylsilyl)amide to a solution of methyllithium in ether, the chemists could trigger the precipitation of KCl. Lithium bis(trimethylsilyl)amide also forms, but it doesn’t interfere with the desired metathesis reaction.

Anwander thinks the methyllithium purification will be useful for the broader chemistry community. “Many people synthesize compounds from methyllithium that contains significant amounts of chloride,” he says. “If such compounds are then used as catalysts, the chloride can influence the chemistry,” he says.

After they made the dimethylcalcium, Anwander’s team used it in a variety of derivatization reactions. For example, they reacted the dimethylcalcium with calcium iodide to make a methylcalcium iodide-heavy Grignard reagent. By reacting dimethylcalcium with a bulky ligand, they were able to make complexes with a terminal calcium methyl. Anwander wants to test whether the terminal calcium methyl will make a good catalyst.



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