Mechanistic investigation of sustainable heme-inspired biocatalytic synthesis of cyclopropanes for challenging substrates

Engineered heme proteins exhibit excellent sustainable catalytic carbene transfer reactivities toward olefins for value-added cyclopropanes. However, unactivated and electron-deficient olefins remain challenging in such reactions. To help design efficient heme-inspired biocatalysts for these difficult situations, a systematic quantum chemical mechanistic study was performed to investigate effects of olefin substituents, non-native amino acid axial ligands, and natural and non-natural macrocycles with the widely used ethyl diazoacetate. Results show that electron-deficient substrate ethyl acrylate has a much higher barrier than the electron-rich styrene. For styrene, the predicted barrier trend is consistent with experimentally used heme analogue cofactors, which can significantly reduce barriers. For ethyl acrylate, while the best non-native axial ligand only marginally improves the reactivity versus the native histidine model, a couple of computationally studied macrocycles can dramatically reduce barriers to the level comparable to styrene. These results will facilitate the development of better biocatalysts in this area. Engineered heme proteins exhibit excellent sustainable catalytic carbene transfer reactivities enabling the transformation of olefins to cyclopropanes, however, exploiting unactivated and electron-deficient olefins in such reactions remains challenging. Here, the authors perform a quantum chemical mechanistic study to investigate the effects of olefin substituents, non-native amino acid axial ligands, and natural and non-natural macrocycles with the widely used ethyl diazoacetate, revealing the potential to design an efficient heme-inspired biocatalyst.