Direct nitration and azidation of aliphatic carbons by an iron-dependent halogenase

Abstract

Iron-dependent halogenases employ cis-halo-Fe(IV)-oxo (haloferryl) complexes to functionalize unactivated aliphatic carbon centers, a capability elusive to synthetic chemists. Halogenation requires (1) coordination of a halide anion (Cl− or Br−) to the enzyme's Fe(II) cofactor; (2) coupled activation of O2 and decarboxylation of α-ketoglutarate to generate the haloferryl intermediate; (3) abstraction of hydrogen (H•) from the substrate by the ferryl oxo group; and (4) transfer of the cis halogen as Cl• or Br• to the substrate radical. This enzymatic solution to an unsolved chemical challenge is potentially generalizable to installation of other functional groups, provided that the corresponding anions can support the four requisite steps. We show here that the wild-type halogenase SyrB2 can indeed direct aliphatic nitration and azidation reactions by the same chemical logic. The discovery and enhancement by mutagenesis of these previously unknown reaction types suggests unrecognized or untapped versatility in ferryl-mediated enzymatic C–H-bond activation.

Catalytic Mechanisms of Fe(II)- and 2-Oxoglutarate-dependent Oxygenases

“…In particular, SyrB2 performs azidation and nitration of tethered L-2-aminobutyrate substrate, using azide and nitrite, respectively (68). On the basis of kinetic and spectroscopic results, a ferryl intermediate is proposed to be responsible for these transformations, similar to the halogenation and hydroxylation transformations (68).…”

Enzyme-Controlled Nitrogen-Atom Transfer Enables Regiodivergent C–H Amination

“…Combined with their ability to take on non-natural activities such as direct C–H amination, 24 enzymes represent a versatile platform for catalyst development to solve challenging selectivity problems in organic chemistry.…”

Opportunities for merging chemical and biological synthesis

“…The Arnold group applied related logic to engineer P450s that promote carbene insertions into N–H bonds and to develop a C-H amination reaction using sulfonyl azide substrates [41•,42]. Most recently, this general approach has been extended beyond P450s with the report of C–N bond-forming reactions promoted by wild-type and engineered variants of the non-heme iron(II)-dependent halogenase SyrB2 in the presence of both azide and nitrite anions [43•]. …”