A comprehensive mechanistic investigation of sustainable carbene N-H insertion catalyzed by engineered His-ligated heme proteins

Engineered heme proteins possess excellent biocatalytic carbene N-H insertion abilities for sustainable synthesis, and most of them have His as the Fe axial ligand. However, information on the basic reaction mechanisms is limited, and ground states of heme carbenes involved in the prior computational mechanistic studies are under debate. A comprehensive quantum chemical reaction pathway study was performed for the heme model with a His analogue as the axial ligand and carbene from the widely used precursor ethyl diazoacetate with aniline as the substrate. The ground state of this heme carbene was calculated by the high-level complete active space self-consistent field (CASSCF) approach, which shows a closed-shell singlet that is consistent with many experimental works. Based on this, DFT calculations of ten main reaction pathways were compared. Results showed that the most favorable pathway involved the initial formation of the metal-bound ylide, followed by a concerted rearrangement/dissociation transition state to form the free enol, which then underwent a water-assisted proton transfer process to yield the final N-H insertion product. This computational prediction was validated via new experimental data using His-ligated myoglobin variants with different types of carbenes. Overall, this is the first comprehensive computational mechanistic study of heme carbene N-H insertions, particularly for neutral His ligated heme proteins and the first high-level CASSCF confirmation of the ground state of the used heme carbene. The experimental results are also the first in this field. Overall, these results build a solid basis for the proposed reaction mechanism to facilitate future biocatalytic carbene N-H insertion studies.