Biocatalytic Oxidation Reactions: A Chemist's Perspective

Abstract

Oxidation chemistry using enzymes is approaching maturity and practical applicability in organic synthesis. Oxidoreductases (enzymes catalysing redox reactions) enable chemists to perform highly selective and efficient transformations ranging from simple alcohol oxidations to stereoselective halogenations of non-activated C-H bonds. For many of these reactions, no "classical" chemical counterpart is known. Hence oxidoreductases open up shorter synthesis routes based on a more direct access to the target products. The generally very mild reaction conditions may also reduce the environmental impact of biocatalytic reactions compared to classical counterparts. In this Review, we critically summarise the most important recent developments in the field of biocatalytic oxidation chemistry and identify the most pressing bottlenecks as well as promising solutions.

Keywords: Baeyer-Villiger oxidation; biocatalysis; halogenation; oxidation; oxyfunctionalisation.

Scheme 1

Biocatalytic oxidation by dehydrogenation. Dehydrogenases and oxidases mediate a hydride abstraction from alcohols and amines (X=O, NH); laccases and peroxidases mediate H atom abstraction reactions from phenolic starting materials.

Scheme 2

Biocatalytic oxidation by oxyfunctionalisation.

Scheme 3

The most common active species used by oxygenases: a) 4a‐peroxoflavins, b) oxyferryl heme species, and c) compound Q in non‐heme iron monooxygenases.5 The electrophilic oxygen atoms are shown in red.

Scheme 4

Generalised regeneration scheme for P450 monooxygenases. The reducing equivalents are derived from NAD(P)H and delivered sequentially via single electron mediators (e.g., ferredoxin or flavins) to the monooxygenase subunit.

Scheme 5

The two‐liquid‐phase system (2LPS) comprising an aqueous reaction medium with biocatalysts and a hydrophobic organic phase serving as the substrate reservoir and product sink.

Scheme 6

Selection of monooxygenase‐catalysed hydroxylation reactions: a) ω‐1 hydroxylation,16 b) ω hydroxylation,17 c) stereospecific 3‐ or 4‐hydroxylation of, for example, proline,18 d) allylic hydroxylation,19 e) steroid hydroxylation.20

Scheme 7

Selected examples of P450BM3 engineering to change the selectivity.19c, 20

Scheme 8

Controlling the regioselectivity of a macrolide hydroxylation with ester directing groups.28

Scheme 9

Examples of industrial P450 monooxygenase catalysed hydroxylation reactions.31

Scheme 10

Dioxygenase (DO)‐catalysed cis dihydroxylation of aromatic compounds and a representative selection of DO‐derived products. TDO: toluene dioxygenase;44 BDO: benzoate dioxygenase;45 NDO: naphthalene dioxygenase.46

Scheme 11

ortho‐Quinol products obtained by oxidative dearomatisation.51 TropB: FMO from Talaromyces stipitatus; AzaH: FMO from Aspegillus niger; SorbC: FMO from Penicillium chrysogenum.

Scheme 12

Chemoenzymatic halogenation of phenols by haloperoxidase‐catalysed hypohalite formation.

Scheme 13

Halogenation of “non‐natural” anthranilate substrates using the tryptophan halogenase from Pseudomonas fluorescens (PrnA) together with an FADH₂‐recycling system (Fre).58

Scheme 14

Enzymatic nitration of tryptophan with TxtE. The regioselectivity of the wild‐type enzyme was altered by protein engineering.62a

Scheme 15

Double oxidation of alkanes to ketones. The combination of NAD(P)H‐dependent monooxygenases with NAD(P)⁺‐dependent alcohol dehydrogenases enables overall redox‐neutral reactions.

Scheme 16

Cascade P450BM3‐catalysed, three‐step oxyfunctionalisation of cyclohexane with stereospecific ADH‐catalysed reduction of the resulting hydroxyketones to yield all possible cyclohexane‐1,2‐diol isomers.65

Scheme 17

Enzyme cascade for the transformation of cycloalkanes into secondary amines.69

Scheme 18

Enantiospecific nitrene transfer to alkyl‐substituted arenes catalysed by engineered P450 monooxygenases. The P411 enzyme has been generated from a P450 monooxygenase by ligand substitution. Note that the reduced (Fe^II) oxidation state is the catalytically active one. Below: Preliminary product scope; all chiral products are essentially enantiopure.70

Scheme 19

Selection of selective halogenation reactions mediated by halogenases. PCP: peptidyl carrier protein; ACP: acyl carrier protein; SyrB2: halogenase from Pseudomonas syringae;74 Barb2: halogenase from Lyngbya majuscule (involved in barbamide biosynthesis);75 CmaB: halogenase from Pseudomonas syringae (involved in coronatine biosynthesis);76 WelO5: halogenase from Hapalosiphon welwitschii (involved in welwitindolinone biosynthesis);77 CylC: non‐α‐KG‐dependent halogenase from Cylindrospermum licheniforme.78

Scheme 20

Most common biocatalytic epoxidation methods employing monooxygenases,80 peroxygenases,81 or lipases in the “perhydrolase” approach and some representative products.

Scheme 21

Engineered P450 BM3 (F87A/A328F) for highly (regio)selective epoxidation (as compared to the wild‐type (wt) enzyme).86

Scheme 22

Chemoenzymatic Prilezhaev reaction using hydrolase‐(re)generated peracids.

Scheme 23

Combinatorial “explosion” of products attainable from alkenes by the smart combination of different biocatalysts. PAL: phenylalanine ammonia lyase; PAD: phenylacetic acid decarboxylase; SMO: styrene monooxygenase; EH: epoxide hydrolase; ADH: alcohol dehydrogenase; AldDH: aldehyde dehydrogenase; ω‐TA: transaminase; SOI: styrene oxide isomerase.91 For reasons of clarity, cosubstrates and regeneration systems have been omitted.

Scheme 24

Enzymatic aziridination with P450BM3 mutants.93

Scheme 25

Haloperoxidase‐catalysed hypohalite formation followed by spontaneous, non‐enzymatic reactions with C=C and C≡C bonds.

Scheme 26

Aerobic cleavage of styrenes with AlkCE from Trametes hirsuta.99

Scheme 27

Use of an engineered P450 monooxygenase (aMOx) to catalyse the anti‐Markovnikov Wacker–Tsuji oxidation of alkenes.100

Scheme 28

Selected cascade reactions involving an oxidase‐catalysed alcohol oxidation step coupled to a further catalytic transformation of the aldehyde generated.

Scheme 29

Chemoenzymatic cascade for the production of methacrylic acid from 2‐methyl‐1,3‐propanediol.110

Scheme 30

(Chemo)enzymatic cascades to transform (benzyl) alcohols into carboxylic acids or amides, depending on the reaction conditions. GOase: evolved galactose oxidase; XDH: xanthine dehydrogenase.113

Scheme 31

Oxidative lactonisation of 1,n‐diols using ADHs exploiting spontaneous lactol formation.

Scheme 32

Bienzymatic synthesis of lactams from amino alcohols.106

Scheme 33

Generalised scheme for the oxidation of HMF to FDCA. For reasons of clarity, the actual catalysts (peroxygenase or oxidase) as well as the oxidants (H₂O₂ or O₂) have been omitted.

Scheme 34

ADH‐catalysed oxidation of 1,2‐diols to the corresponding (R)‐ or (S)‐α‐hydroxy ketones depending on the ADH used. BdhA: butane diol DH from B. subtilis;119 BCDD: cis‐diol DH from P. putida.120a

Scheme 35

Phenol radical polymerisation initiated by laccases or peroxidases.

Scheme 36

Embedding MAO‐N‐generated imines into more complex multistep chemoenzymatic syntheses.

Scheme 37

Conversion of amines into α‐aminonitriles using a combination of d‐amino acid oxidase (mutant, pkDAO) and cyanide.137

Scheme 38

Haloperoxidase‐initiated oxidative decarboxylation of amino acids leading to either the C1‐shortened nitrile or the aldehyde via the corresponding imine. VCPO: vanadium‐dependent haloperoxidase.138

Scheme 39

Dynamic kinetic resolution of profen aldehydes to the acids using ADHs.139

Scheme 40

Biocatalytic Baeyer–Villiger oxidations using flavin‐dependent Baeyer–Villiger monooxygenases (BVMO) or the perhydrolase approach. Below some selected preparative examples are shown.

Scheme 41

Examples of BVMO engineering to change the stereoselectivity.142a, 143

Scheme 42

Chemoenzymatic synthesis of Aerangis lactones with chemoenzymatic cascades in a flow process.147

Scheme 43

Redox‐neutral cascade transforming cycloalkanols into lactones by combining an ADH‐catalysed oxidation of the starting alcohol and BVMO‐catalysed lactonisation.

Scheme 44

Mixed‐culture approach to convert limonene (waste) into chiral carvolactone. The first hydroxylation step is catalysed by recombinant P. putida overexpressing a dioxygenase; this is followed by a sequence of alcohol oxidation (ADH), ene reduction (ER), and BV oxidation (BVMO) mediated by recombinant E. coli. For reasons of simplicity, the cofactors and cosubstrates have been omitted.153

Scheme 45

Convergent cascade transforming cyclohexanone and 1,6‐hexanediol into ϵ‐caprolactone.154

Scheme 46

Conversion of cyclohexanol into ω‐aminohexanoic acid (ester). The reaction sequence comprises two redox‐self‐sufficient cascades. First, the ADH/BVMO combination generates ϵ‐caprolactone; the second cascade comprises esterase‐catalysed methanolysis of the lactone followed by ADH‐catalysed oxidation of the terminal OH group and reductive amination catalysed by an ω‐TA. Alanine dehydrogenase (AlaDH) regenerates both the ADH and the ω‐TA (alanine/pyruvate system).155

Scheme 47

Artificial biotransformation pathway converting oleic acid into ω‐hydroxy acids or α,ω‐dicarboxylic acids. Hase: hydratase; ADH: alcohol dehydrogenase; BVMO: Baeyer–Villiger monooxygenase from P. putida or P. fluorescens).157a

Scheme 48

Chemoenzymatic Baeyer–Villiger oxidation using peracids in situ generated by hydrolases.

Scheme 49

Oxidative decarboxylation of carboxylic acids using the P450 peroxygenase OleT.

Scheme 50

Enantiospecific sulfoxidation of thioethers to generate enantiomerically pure active pharmaceutical ingredients (APIs).