Enantioselective Synthesis of Chiral Amines via Biocatalytic Carbene N—H Insertion

Optically active amines represent highly valuable building blocks for the synthesis of advanced pharmaceutical intermediates, drug molecules, and biologically active natural products. Hemoproteins have recently emerged as promising biocatalysts for the formation of C—N bonds via carbene transfer, but asymmetric N—H carbene insertion reactions using these or other enzymes have so far been elusive. Here, we report the successful development of a biocatalytic strategy for the asymmetric N—H carbene insertion of aromatic amines with 2-diazopropanoate esters using engineered variants of myoglobin. High activity and stereoinduction in this reaction could be achieved by tuning the chiral environment around the heme cofactor in the metalloprotein in combination with catalyst-matching and tailoring of the diazo reagent. Using this approach, an efficient biocatalytic protocol for the synthesis of a broad range of substituted aryl amines with up to 82% ee was obtained. In addition, a stereocomplementary catalyst useful to access the mirrorimage form of the N—H insertion products was identified. This work paves the way to asymmetric amine synthesis via biocatalytic carbene transfer, and the present strategy based on the synergistic combination of protein and diazo reagent engineering is expected to prove useful in the context of these as well as other challenging asymmetric carbene transfer reactions.