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Synthesis

Cooperative catalysts produce tough-to-make, nonnatural amino acids

2 catalysts working in tandem coax carbenes into the N–H bonds of aliphatic amines

by Bethany Halford
November 23, 2019 | APPEARED IN VOLUME 97, ISSUE 46

 

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Aliphatic amines are popular motifs in biologically relevant molecules, so chemists have myriad tools for assembling C–N bonds. Methods that make those bonds enantioselectively are particularly valuable because they can produce chiral drugs and unnatural amino acids, which are useful for studying biology. While chiral transition-metal catalysts are effective for making aromatic amines, they fall short when it comes to making aliphatic amines. These compounds tend to coordinate to the metal, poisoning the catalyst and scrambling its ability to work enantioselectively. Researchers led by Nankai University’s Shou-Fei Zhu and Qi-Lin Zhou have managed to get around these problems by creating a reaction that uses two catalysts in tandem (Science 2019, DOI: 10.1126/science.aaw9939). The reaction (example shown) features an achiral metal catalyst—a homoscorpionate copper complex—which forms a carbene from a diazo ester. The homoscorpionate ligand protects the copper from another reactant, an aliphatic amine, which attacks the carbene to make a new C–N bond. A second catalyst—a chiral amino thiourea—guides an enantioselective proton transfer, which produces a stereocenter in the final product. The reaction couples a wide variety of diazo esters with amines to make nonnatural α–amino acid derivatives. The same strategy could be used for other transition-metal-catalyzed asymmetric transformations that involve strongly coordinating substrates, the chemists say.

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