A leading synthetic challenge for chemists is how to precisely target a single C–H bond for reaction among many similar-looking ones in an organic molecule. Carefully crafted transition-metal catalysts have led to some success in modifying molecules that contain a double bond or a functional group adjacent to the target C–H to guide the reaction. Some catalysts have even worked with alkanes that are only made up of C–H bonds. But complete control of C–H selectivity to generate single-enantiomer products has remained out of reach.
Emory University’s Kuangbiao Liao, Huw M. L. Davies, and coworkers have now shown that this full control is indeed possible by designing dirhodium catalysts that can distinguish between the different types of carbons in n-alkanes (Nature 2016, DOI: 10.1038/nature17651).
The Davies group has spent two decades developing dirhodium catalysts that interact with aryl diazoacetates to form a carbene intermediate that can insert into C–H bonds. The researchers have shown that modest C–H selectivity is possible for cyclic and branched alkanes. “We avoided n-alkanes because the mixtures of reaction products were a mess,” Davies says.
In the new work, the Emory researchers designed a precursor chiral dirhodium catalyst that allows them to streamline catalyst discovery. The new set of catalysts the team subsequently made encases the rhodium atoms within a three-dimensional scaffold that acts like a lock and key to allow only one particular type of C–H bond—primary, secondary, or tertiary—to approach the catalyst and undergo a reaction with high diastereo- and enantioselectivity.
For now, the new catalysts best target secondary C–H bonds at the C2 position of alkanes. Davies believes the approach can be expanded to target any C–H bond in an n-alkane. The Davies group and its collaborators are aiming to use the targeted approach in the total synthesis of a family of natural products and for converting polyethylene into functionalized polymers.
“This work provides an exceptional transformation of linear alkanes in a preferential manner with amazing site selectivity and an outstanding degree of enantioselectivity,” comments C–H activation specialist Pedro J. Pérez of the University of Huelva. This achievement suggests that the Emory team’s capabilities in rhodium-catalyzed C–H functionalization can go further, Pérez adds. “The Olympic motto ‘Citius, Altius, Fortius’—faster, higher, stronger—comes to mind.”