Reaction Pathway and Axial Ligand Effects of C(sp³)-H Primary Amination by Unprotected Iron-Porphyrin-Nitrenoid

Engineered hemoprotein-catalyzed unprotected nitrenoid C-(sp³)-H insertion has emerged as a highly promising method for the efficient synthesis of aliphatic primary amines. Despite its potential, the reaction mechanism, particularly the formation and the nature of nitrenoid species and the effects of axial ligands, still lacks understanding. Herein, we report a theoretical study on the unprotected nitrenoid C-(sp³)-H insertion using three iron-porphyrin complexes with distinct axial coordination environments. Our study reveals the formation of two reactive nitrenoid intermediates, [FeNH₂]⁺ and [FeNH], and elucidates their divergent mechanistic pathways and reactivity profiles. Notably, the [FeNH₂]⁺ species exists an unprecedented σ-channel/σ-channel reaction pattern, and exhibits an atypical radical rebound mechanism. Furthermore, we developed a multivariate linear regression (MLR) model for the hydrogen-atom transfer (HAT) step, identifying key factors governing reaction activity. Our study uncovers the critical role of axial ligands in modulating the stability and reactivity of iron-nitrenoid species, thereby steering the preferred reaction pathway in iron porphyrin-catalyzed C-(sp³)-H amination. In addition, QM/MM calculations on the P411_APA-catalyzed C-(sp³)-H amination confirmed that the [FeNH₂] ⁺ pathway is more favorable. Furthermore, the role of its axial ligand and the origin of stereoselectivity were elucidated. These mechanistic insights provide a valuable foundation for future catalyst design and protein engineering efforts.