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Model could improve promising C-H activation

Theory reveals hidden energy costs and points to new chemistry

by Sam Lemonick
November 14, 2018 | A version of this story appeared in Volume 96, Issue 46

Reaction scheme showing c-h functionalization by intramolecular proton-couple electron transfer
Credit: C&EN
Theoretical modeling of C-H functionalization by proton-coupled electron transfer revealed a surprising energy penalty getting the oxygen and hydrogen lined up.

Carbon and hydrogen make stable bonds that are ubiquitous in organic molecules. That’s why chemists spend so much energy looking for ways to break them and replace hydrogens with more useful moieties. One group recently found a new way to make a carbon atom release a hydrogen and accept new substituents. But they didn’t understand why the intramolecular reshuffling worked as well as it did with relatively weak oxidants (Sci. Adv. 2018, DOI: 10.1126/sciadv.aat5776). Now their colleagues have made an accurate theoretical model that can explain the reaction’s dynamics (J. Am. Chem. Soc. 2018, DOI: 10.1021/jacs.8b10461). It points to ways to improve this method and hints at possible electrochemistry applications.

In July, James M. Mayer’s group at Yale University described a method for C-H functionalization under mild conditions by proton-coupled electron transfer (PCET) reaction.

Hammes-Schiffer’s theory explains that the reaction happens by simultaneous tunneling of the proton and electron between energy states. “All the transitions are downhill,” Hammes-Schiffer says, meaning they release energy. She thinks this is why the strength of the oxidant has relatively little effect on how quickly the reaction proceeds. Robert Knowles, a synthetic organic chemist at Princeton University, calls it a “beautiful theoretical study,” which he says “will serve as a road map for much future work in this area.” Hammes-Schiffer and Mayer think the unexpected ease of oxidizing the C-H bond could point researchers to possible new chemistry for for more efficient fuel cells that operate at lower voltages than current ones.

Mayer says he also discovered from the analysis that the hydrogen and oxygen atoms in his molecule are not as close together as he thought, meaning there’s an energy penalty incurred to twist the players into position. “If they can avoid that large conformational change,” the reaction may proceed faster, Hammes-Schiffer says. The two hope to collaborate to improve the molecule and the reaction with a combination of theoretical and experimental work. “I think there’s more to come,” says Mayer.


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