Scientists finally crack nature's most common chemical bond: Carbon-hydrogen bonds in hydrocarbon molecules have resisted functionalization until now:
Now, after nearly 25 years of work by chemists at the University of California, Berkeley, those hydrocarbon bonds -- two-thirds of all the chemical bonds in petroleum and plastics -- have fully yielded, opening the door to the synthesis of a large range of novel organic molecules, including drugs based on natural compounds.
"Carbon-hydrogen bonds are usually part of the framework, the inert part of a molecule," said John Hartwig, the Henry Rapoport Chair in Organic Chemistry at UC Berkeley. "It has been a challenge and a holy grail of synthesis to be able to do reactions at these positions because, until now, there has been no reagent or catalyst that will allow you to add anything at the strongest of these bonds."
Hartwig and other researchers had previously shown how to add new chemical groups at C-H bonds that are easier to break, but they could only add them to the strongest positions of simple hydrocarbon chains.
In the May 15 issue of the journal Science, Hartwig and his UC Berkeley colleagues described how to use a newly designed catalyst to add functional chemical groups to the hardest of the carbon-hydrogen bonds to crack: the bonds, typically at the head or tail of a molecule, where a carbon has three attached hydrogen atoms, what's called a methyl group (CH3).
"The primary C-H bonds, the ones on a methyl group at the end of a chain, are the least electron-rich and the strongest," he said. "They tend to be the least reactive of the C-H bonds."
UC Berkeley postdoctoral fellow Raphael Oeschger discovered a new version of a catalyst based on the metal iridium that opens up one of the three C-H bonds at a terminal methyl group and inserts a boron compound, which can be easily replaced with more complex chemical groups. The new catalyst was more than 50 times more efficient than previous catalysts and just as easy to work with.
"We now have the ability to do these types of reactions, which should enable people to rapidly make molecules that they would not have made before," Hartwig said. "I wouldn't say these are molecules that could not have been made before, but people wouldn't make them because it would take too long, too much time and research effort, to make them."
The payoff could be huge. Each year, nearly a billion pounds of hydrocarbons are used by industry to make solvents, refrigerants, fire retardants and other chemicals and are the typical starting point for synthesizing drugs.
Raphael Oeschger, Bo Su, Isaac Yu, Christian Ehinger, Erik Romero, Sam He, John Hartwig. Diverse functionalization of strong alkyl C–H bonds by undirected borylation. Science, 2020; 368 (6492): 736 DOI: 10.1126/science.aba6146
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