Role of Engineered Iron-haem Enzyme in Reactivity and Stereoselectivity of Intermolecular Benzylic C–H Bond Amination

A recent success in which the engineered iron-haem enzymes P411CHA′ aminate the intermolecular benzylic C–H bond with both high efficiency and stereoselectivity solves a long-standing challenge in synthetic chemistry (Arnold and co-workers Nat. Chem. 2017, 9, 629−634). The mechanism, reactivity, and stereoselectivity of this reaction were studied by quantum mechanical (QM)/molecular mechanical (MM) calculations in this work. To understand better the origin of such an excellent catalytic performance of biocatalyst P411CHA′, iron-cofactor FePIX alone for the intermolecular C–H bond amination was also theoretically investigated as a comparison. The catalytic cycle includes two processes: N2 dissociation and nitrene transfer. The calculation results show that P411CHA′ enzyme can catalyze intermolecular C–H amination with high reactivity and stereoselectivity, whereas the FePIX-catalyzed reaction has much higher barriers for both N2 dissociation and nitrene transfer compared to P411CHA′. The reason for this dramatic difference in catalytic reactivity between P411CHA′ and FePIX is that the former but not the latter allows the formation of precursors B-5PR1 and B-3PR2, which are structurally close to transition states B-3TS1 and B-3TS2 and accelerate N2 dissociation and nitrene transfer, respectively. The mutated residues (A82L A78V F263L) assist the formations of B-5PR1 and B-3PR2 via reducing effectively the size of the haem distal pocket. High stereoselectivity of P411CHA′ stems from the steric effect in H-abstraction. A theoretical analysis on how para substituent R affects reactivity was also carried out. A strong π-type electron-donating group on the substrate enhances significantly the reactivity of P411CHA′-catalyzed intermolecular C–H amination. These results provide valuable information for designing and constructing environmentally friendly biocatalytic C–H amination systems with high reactivity and stereoselectivity.