The Single‐Component Flavin Reductase/Flavin‐Dependent Halogenase AetF is a Versatile Catalyst for Selective Bromination and Iodination of Arenes and Olefins**

Abstract

Flavin-dependent halogenases (FDHs) natively catalyze selective halogenation of electron rich aromatic and enolate groups. Nearly all FDHs reported to date require a separate flavin reductase to supply them with FADH 2 , which complicates biocatalysis applications. In this study, we establish that the single component flavin reductase/flavin dependent halogenase AetF catalyzes halogenation of a diverse set of substrates using a commercially available glucose dehydrogenase to drive its halogenase activity. High site selectivity, activity on relatively unactivated substrates, and high enantioselectivity for atroposelective bromination and bromolactonization was demonstrated. Siteselective iodination and enantioselective cycloiodoetherification was also possible using AetF. The substrate and reaction scope of AetF suggest that it has the potential to greatly improve the utility of biocatalytic halogenation.Flavin-dependent halogenases (FDHs) natively catalyze site-selective halogenation of electron rich aromatic and enolate groups in a diverse range of halogenated natural products. [1][2][3] This unique capability has led to extensive efforts to understand FDH mechanism and the origins of their site selectivity. [4,5] Early studies established that FDH catalysis initially mirrors flavoprotein monooxygenase catalysis in that an enzyme-bound, reduced flavin adenine dinucleotide (FADH 2 ) cofactor reacts with O 2 to generate a hydroperoxy flavin intermediate. [6] In FDHs, this intermediate reacts with bound halide, typically bromide or chloride, to generate HOX, which migrates through the enzyme to a substrate binding pocket. [7][8][9] Most evidence now suggests that hydrogen bonding by a key active site lysine residue activates HOX for electrophilic halogenation, and precise substrate binding leads to site-selective halogenation by this species. [5,10,11] Nearly all FDHs reported to date require a separate flavin reductase to supply FADH 2 , [6,12] and this enzyme is typically driven by a glucose/glucose dehydrogenase cofactor regeneration system for biocatalysis applications (Figure 1A). [13] The need for a separate flavin reductase complicates biocatalysis since these enzymes are not widely available and are typically produced in-house, they add to the protein waste that must be removed during product isolation, and they can lead to undesired background reactions. [14] Previously, our group demonstrated that genetically fusing the flavin reductase RebF to the FDH RebH improved halogenation yields from whole-cell biocatalysis, suggesting that increased local concentration of FADH 2 can improve the efficiency of biocatalysis relative to the free enzymes (Figure 1A). [15] A recent family-wide sequence/

Asymmetric catalysis by flavin‐dependent halogenases

“…From our initial report of enantioselective desymmetrization in 2018 38 to recent efforts at atroposelective halogenation 42,54 and enantioselective halocyclization, 45,50,54 the studies outlined above show how FDHs can catalyze enantioselective transformations with diverse structural requirements. This capability is notable since, at least to date, no examples of enantioselective FDH catalysis have been characterized in natural products.…”

“…We recently reported that the single component flavin reductase/FDH AetF possesses remarkable substrate scope, including relatively electron deficient heterocycles and several substrates that undergo enantioselective transformations. 54 For example, AetF was able to brominate 26 to give 26a in good yield. While low enantioselectivity was observed, this result was notable since our initial desymmetrization study only investigated chlorination of 1.…”

“…We recently reported that the single component flavin reductase/FDH AetF possesses remarkable substrate scope, including relatively electron deficient heterocycles and several substrates that undergo enantioselective transformations 54 . For example, AetF was able to brominate 26 to give 26a in good yield.…”