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Synlett 2025; 36(01): 8-14
DOI: 10.1055/s-0043-1775372
synpacts

Chiral Sulfones via Single-Electron Oxidation-Initiated Photoenzymatic Catalysis

Pandaram Sakthivel
,
Qinglong Shi
,
Juntao Ye∗

Financial support from the National Natural Science Foundation of China (Grant No. 22371180 and 22001163) and Shanghai Jiao Tong University are acknowledged.
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Abstract

We recently achieved an oxidation-initiated photoenzymatic enantioselective hydrosulfonylation of olefins through the utilization of a new Gluconobacter ene-reductase mutant (GluER-W100F-W342F). Our method simplifies the reaction system by eliminating the need for a cofactor regeneration mixture and, in contrast with previous photoenzymatic systems, does not depend on the formation of an electron donor–acceptor (EDA) complex between the substrates and enzyme cofactor. Moreover, the GluER variant exhibits good substrate compatibility and excellent enantioselectivity. Mechanistic investigations indicate that a tyrosine-mediated HAT process is involved and support the proposed oxidation-initiated mechanism. In this Synpacts article, we discuss the conceptual framework that led to the discovery of this reaction and reflect on the key aspects of its development.

1 Introduction

2 Conceptual Background

2.1 Intramolecular Photoenzymatic Reactions via Single-Electron Reduction

2.2 Intermolecular Photoenzymatic Reactions via Single-Electron Reduction

3 The Development of the Process

4 Conclusion


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Key words

photoenzymatic catalysis - hydrogen atom transfer - hydrosulfonylation - single-electron oxidation - directed evolution
1

Introduction

Hydrogen atom transfer (HAT) is an elementary step in radical transformations and many biological systems.[1] While significant advances have been made in photoinduced HAT over the last decade,[1`] [h] [i] several inherent features of HAT have hindered the development of stereoselective variants. This is mainly due to the high reactivity of free radicals, the smallest size of hydrogen atom, and favorable HAT reactions typically exhibit early transition states. To date, two strategies have been developed to achieve enantioselective HAT reactions: (1) chiral catalysts and achiral H-atom donors (Scheme [1b]),[2] and (2) chiral H-atom donors (Scheme [1c]).[3] While some progress has been made using each method, highly stereoselective HAT reactions remain rare. Moreover, it is highly desirable to avoid the use of hazardous tin reagents from the perspective of green chemistry.

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Juntao Ye studied chemistry as undergraduate student at Huazhong University of Science and Technology (HUST). He received his PhD in 2013 at Shanghai Institute of Organic Chemistry (SIOC) under the supervision of Prof. Shengming Ma. He then did postdoctoral research with Prof. Mark Lautens (University of Toronto), Prof. Tomislav Rovis (Columbia University), and Prof. Tristan Lambert (Cornell University). He joined Shanghai Jiao Tong University to start his independent career in July 2019 as an associate professor. His research interests include photocatalysis, photoenzymatic catalysis, and asymmetric catalysis.
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Scheme 1 Asymmetric hydrogen atom transfer (HAT) reactions

Photoenzymatic catalysis is an emerging field that utilizes enzymes in conjunction with light to enable non-natural reactivity of enzymes,[4] including stereoselective HAT reactions. This strategy has attracted a lot of interest in the field of organic synthesis as it can provide chiral molecules with high enantiopurity via a wide variety of non-natural reactions, which are otherwise challenging to achieve using small-molecule catalysts. In the following sections, we will discuss the conceptual background that stimulated our recent work on the synthesis of chiral sulfones via single-electron oxidation-initiated photoenzymatic hydrosulfonylation of olefins.


# 2

Conceptual Background

2.1

Intramolecular Photoenzymatic Reactions via Single-Electron Reduction

In 2016, the Hyster group reported a pioneering work in the field by combining visible-light with non-natural photoenzymes.[5a] Highly enantioselective HAT was accomplished in the context of radical hydrodehalogenation of lactones using nicotinamide-dependent ketoreductases (KRED). KRED exhibits remarkable selectivity in their native function for the reduction of ketones to alcohols via the transfer of a hydride. However, in the presence of an appropriate alkyl bromide substrate, the enzyme cofactor, NADPH, forms an electron donor–acceptor (EDA) complex with the substrate. Under visible-light irradiation, this EDA complex gets excited and undergoes single electron transfer to form radical intermediates, which eventually led to hydrodehalogenated products. Moreover, the author discovered that a short-chain dehydrogenase (LKADH) and another dehydrogenase with more extensive active sites (RasADH) exhibit enantiodivergent selectivity, providing both R- and S-configured products in high enantiopurity (Scheme [2], up).

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Scheme 2 Ketoreductase-catalyzed chiral radical reduction of α-bromolactones

The author proposed a mechanism for enantioselective radical dehalogenation (Scheme [2], bottom). Photoexcitation of the EDA complex I initiates a single-electron-transfer event, leading to the formation of radical ions pair II. Subsequent mesolytic cleavage of the C–Br bond results in the formation of a prochiral carbon radical, which then undergoes an enantioselective HAT with NADPH•+ within the active site of the enzyme to afford the enantioenriched product. Finally, NADP+ undergoes reduction to regenerate NADPH either by isopropyl alcohol or by glucose dehydrogenase and glucose.

Following this work, the Hyster group developed a wide range of photoenzymatic reactions by using flavin-dependent ‘ene’ reductases (ERs) as a catalyst and demonstrated the great potential of the field.[5]


# 2.2

Intermolecular Photoenzymatic Reactions via Single-Electron Reduction

In 2020, the Zhao group reported the use of flavin-dependent old yellow enzyme 1 (OYE1) to catalyze enantioselective intermolecular radical addition reactions between α-halogenated carbonyl compounds and olefins (Scheme [3]).[6a] This method yielded γ-chiral carbonyl compounds with high optical purity (up to 99.5:0.5 er) and good yields (up to 99%). Notably, the Hyster group independently reported a similar approach for the hydroalkylation of olefins using nicotinamide-dependent cyclohexanone reductase.[5d]

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Scheme 3 Photoenzymatic enantioselective hydroalkylation of olefins with α-halogenated carbonyl compounds

The proposed reaction mechanism also involves the formation of an EDA complex between an α-halogenated carbonyl compound and FMNhq (VI) in the enzyme’s active site (Scheme [3]). Upon photoexcitation, a single-electron-transfer event generates a substrate radical anion and FMNH• (VII). Subsequent halogen elimination forms an alkyl radical, which reacts with an olefin to generate a prochiral carbon radical (VIII). Final enantioselective HAT with FMNH•, which might be mediated by tyrosine (tyr196), then provides the final product (6).

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Scheme 4 Photoenzymatic intermolecular radical addition reactions

Following the work of Hyster[5] and Zhao,[6] there has been rapid progress in developing intermolecular photoenzymatic radical addition reactions (Scheme [4]). For instance, Xu and co-workers utilized flavin-dependent ene reductase OYE1 as a catalyst along with the radical precursor, α-diazo ketone/ester compounds 7, for the synthesis of S-configured γ-chiral ketone compounds 8.[7a] Shortly after, Xu’s group also employed phenylsulfonyl chloride 9 as a radical precursor to achieve asymmetric hydrosulfonylation reaction.[7b] In 2023, Zhao and co-workers utilized ene reductase XenB as a catalyst to accomplish an intermolecular hydroamination reaction.[6c] Recently, the Hyster[5i] and Zhao[6d] group independently achieved hydroalkylation of olefins 5 using azaarenes and 2- or 4-bromomethyl or chloromethyl pyridines 13.

In addition to the aforementioned work, others[8] have also made important contributions by using flavin-dependent ERs to achieve enantioselective HAT in the realm of hydrodehalogenation and hydrofunctionalization of olefins. It is noteworthy that all these processes are initiated by single-electron reduction of the substrate through the involvement of an EDA complex (Scheme [5a]). Consequently, a cofactor turnover system involving nicotinamide adenine dinucleotide phosphate (NADP+), glucose dehydrogenase (GDH), and glucose is required to regenerate the reduced state of flavin/nicotinamide cofactors, given that the oxidized form of these cofactors is generated after each catalytic cycle.

Hence, from a practical perspective, the development of photoenzymatic systems initiated by single- electron oxidation of the substrate would be highly desirable as it would obviate the addition of the ternary mixture for cofactor regeneration and simplify the reaction system (Scheme [5b]). More importantly, it would significantly expand the scope of photoenzymatic catalysis, allowing for the use of substrates incapable of forming EDA complexes with the cofactors.

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Scheme 5 Two strategies for radical generation under photoenzymatic catalysis

Notably, Beisson and collaborators have unveiled a novel photoenzyme named fatty acid photodecarboxylase sourced from Chlorella variabilis (CvFAP).[9a] Mechanistic investigations have elucidated that the pivotal step for decarboxylation involves the oxidation of the fatty acid carboxylate by the excited state of the flavin adenine dinucleotide (FAD) cofactor.[9`] [c] [d] [e] Expanding on these findings, Wu[10] and Hollmann[11] have successfully applied this enzyme class in various synthetic scenarios, such as kinetic resolution of α-functionalized carboxylic acids[10a] and decarboxylative deuteration,[10b] setting the stage for the advancement of asymmetric oxidative photoenzymatic reactions.


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# 3

The Development of the Process

In this context, we considered the possibility of developing an oxidation-initiated photoenzymatic hydrofunctionalization of olefins using readily available and oxidizable radical precursors as substrates. After extensive experimentation, we achieved this goal and developed an oxidation-initiated photoenzymatic enantioselective hydrosulfonylation of olefins utilizing an engineered gluconobacter ene reductase (GluER, Scheme [6a]).

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Scheme 6 (a) Oxidation-initiated photoenzymatic enantioselective hydrosulfonylation of olefins with sulfinates, (b) selected examples, (c) reaction mechanism, and (d) control experiments using enzymes with tyrosine (Y) 177 or Y343 being mutated to phenylalanine (F).

We started our research by using sodium benzenesulfinate 15a to test the feasibility of the oxidation-initiated hydrosulfonylation of olefins because of the low oxidation potentials of aryl and alkyl sulfinates (E p/2 ox = +0.4–0.5 V vs SCE in MeCN).[12] Based on the excited state reduction potentials of riboflavin-based photocatalysts such as riboflavin tetraacetate (* E 1/2 red = +1.67 V vs SCE in MeCN),[13] single-electron oxidation of 15a to the sulfonyl radical should be facile by the excited state of flavin cofactors. Through rational engineering and directed evolution,[14] GluER-W100F-W342F was found to be the optimal mutant for the hydrosulfonylation reaction. To the best of our knowledge, no previous photoenzymatic systems have reported the W100F and W342F mutations.[5] [6] [7] [8] With GluER-W100F-W342F acting as the catalyst, the scope of olefins and precursors to sulfonyl radicals is much wider than previous reductive system as the formation of an EDA complex with the substrate is not required.[7b] Both aryl and alkyl sulfinate salts are compatible in the present work. Furthermore, olefins with bulky substituents such as naphthyl and benzothienyl are also well tolerated (Scheme [6b]).

A possible mechanism was proposed for the hydrosulfonylation reaction based on our experimental observations and literature (Scheme [6c]).[12] [15] [16a] Reductive quenching of the excited flavoprotein by the easily oxidizable sulfinate generates a sulfonyl radical and the reduced form of the cofactor (FMNsq), which was observed by transient absorption spectroscopy. Subsequently, FMNsq is protonated to form the neutral species FMNH•. At the same time, the sulfonyl radical adds to the olefin to generate a prochiral carbon radical Int2, which then engages in enantioselective HAT with FMNH• to provide the final product and regenerate the ground state of the cofactor (path a). Alternatively, a tyrosine-mediated HAT process may also be operative, that is, the prochiral intermediate Int2 undergoes enantioselective HAT with a proximal tyrosine residue to give the final product, and the resulting tyrosine radical then reacts with FMNH• to regenerate FMN and tyrosine (path b). To shed some light on the HAT step, further mutations on nearby tyrosine residues Y177 and Y343 were carried out (Scheme [6d]). When the mutant GluER-Y177F was used as the catalyst for the model reaction, both the yield and enantioselectivity of 16a decreased but the sense of enantioinduction remained the same (79:21 er in favor of the S enantiomer). In contrast, when the mutant GluER-Y343F was employed, a very low yield (8%) and a reversed sense of enantioinduction (43:57 er favoring the R enantiomer) were observed. Collectively, these findings imply that the residue Y343 is crucial to the HAT step and that the tyrosine-mediated HAT mechanism (path b) is most likely operative in our system.

To give more insight on the mechanism of the HAT step, we conducted molecular dynamics (MD) simulations (Figure [1]). The OH proton of Y343 is located 4.0 Å from the prochiral carbon in WT GluER and the dihedral angle between the plane formed by the atoms C8, C9, and C10 is roughly 142° (Figure [1a]). However, in the optimal mutant GluER-W100F-W342F (Figure [1b]), with identical simulation parameters, the steric repulsion is decreased by smaller side-chains, leading to a reduced distance (3.0 Å) between the prochiral carbon and the OH proton of Y343, as well as a more suitable dihedral angle (105°) for the final HAT step.

Notably, many other research groups also showed their interests in oxidation-initiated photoenzymatic catalysis. For example, while our work was close to finish, Huang and co-workers reported the first example of oxidation-initiated photoenzymatic hydroarylation of olefins.[16a] Xu and co-workers recently developed a similar hydrosulfonylation reaction using sulfinates or sulfonyl hydrazines as the radical precursors and an old yellow enzyme 1 (OYE1) mutant as the catalyst,[16b] leading to R-configured sulfone products that are complementary to the S-configured products obtained in our study. Very recently, Yang and co-workers repurposed natural photoenzymes, fatty acid photodecarboxylases (FAPs), to enable new-to-nature decarboxylative radical cyclization reactions,[16c] which are also initiated by the oxidation of carboxylic acid substrates that are tethered with an α,β-unsaturated ester functionality. In addition, by adding an exogenous photocatalyst such as Eosin Y and Ru(bpy)3 2+, Huang and co-workers[16d] recently achieved an elegant enantioselective radical–radical coupling reaction while Hyster and co-workers developed a decarboxylative coupling of amino acids with vinylpyridines.[16e] Both studies also rely on single-electron oxidation of substrates or key intermediates by the excited-stated of flavoquinone cofactors.

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Figure 1 Molecular dynamics simulations (PDB 6008)

# 4

Conclusion

In summary, we have developed an oxidation-initiated photoenzymatic method for the hydrosulfonylation of styrene derivatives using aryl and alkyl sulfinates. Through directed evolution, a GluER variant with two new mutations, W100F and W342F, was identified as the optimal non-natural photoenzyme for this process. Unlike existing net-reductive photoenzymatic reactions, our oxidative system is simpler and does not rely on the formation of an EDA complex for radical generation. Mechanistic investigations confirmed the proposed oxidation-initiated mechanism and suggest the role of tyrosine 343 as a hydrogen atom shuttle in the final HAT step. With a wide array of readily available radical precursors and established protein engineering techniques, we believe that this oxidation-initiated photoenzymatic platform has the potential to enable a broader range of valuable and challenging transformations.


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Conflict of Interest

The authors declare no conflict of interest.


Corresponding Author

Juntao Ye
Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University
Shanghai 200240
P. R. of China   

Publication History

Received: 24 April 2024

Accepted: 31 May 2024

Article published online:
18 June 2024

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Juntao Ye studied chemistry as undergraduate student at Huazhong University of Science and Technology (HUST). He received his PhD in 2013 at Shanghai Institute of Organic Chemistry (SIOC) under the supervision of Prof. Shengming Ma. He then did postdoctoral research with Prof. Mark Lautens (University of Toronto), Prof. Tomislav Rovis (Columbia University), and Prof. Tristan Lambert (Cornell University). He joined Shanghai Jiao Tong University to start his independent career in July 2019 as an associate professor. His research interests include photocatalysis, photoenzymatic catalysis, and asymmetric catalysis.
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Scheme 1 Asymmetric hydrogen atom transfer (HAT) reactions
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Scheme 2 Ketoreductase-catalyzed chiral radical reduction of α-bromolactones
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Scheme 3 Photoenzymatic enantioselective hydroalkylation of olefins with α-halogenated carbonyl compounds
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Scheme 4 Photoenzymatic intermolecular radical addition reactions
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Scheme 5 Two strategies for radical generation under photoenzymatic catalysis
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Scheme 6 (a) Oxidation-initiated photoenzymatic enantioselective hydrosulfonylation of olefins with sulfinates, (b) selected examples, (c) reaction mechanism, and (d) control experiments using enzymes with tyrosine (Y) 177 or Y343 being mutated to phenylalanine (F).
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Figure 1 Molecular dynamics simulations (PDB 6008)