Water-Soluble Anthraquinone Photocatalysts Enable Methanol-Driven Enzymatic Halogenation and Hydroxylation Reactions

Abstract

Peroxyzymes simply use H₂O₂ as a cosubstrate to oxidize a broad range of inert C-H bonds. The lability of many peroxyzymes against H₂O₂ can be addressed by a controlled supply of H₂O₂, ideally in situ. Here, we report a simple, robust, and water-soluble anthraquinone sulfonate (SAS) as a promising organophotocatalyst to drive both haloperoxidase-catalyzed halogenation and peroxygenase-catalyzed oxyfunctionalization reactions. Simple alcohols, methanol in particular, can be used both as a cosolvent and an electron donor for H₂O₂ generation. Very promising turnover numbers for the biocatalysts of up to 318 000 have been achieved.

Scheme 1. Photoenzymatic Halogenation Combining Photocatalytic In Situ Generation of H₂O₂ Via O₂ Reduction in the Presence of Methanol to Drive CiVCPO-Initiated Halogenation of Thymol

Upper: overall reaction, lower: dissection into photochemical H₂O₂ generation (blue) and chemoenzymatic halogenation of thymol (red).

Figure 1

Halogenation of thymol by combining CiVCPO and visible light-driven in situ generation of H₂O₂ using SAS. (a) Conversion of thymol (1, ■) into 4-brominated thymol (1b, ⧫) in the presence of CiVCPO (100 nM) and SAS (0.5 mM), and control reactions in the dark (⧫) in the absence of CiVCPO (⧫) or SAS (⧫). (b) Influence of varied concentrations of SAS (⧫ = 0.25 mM, ● = 1 mM, and ▲ = 2 mM) and (c) CiVCPO on the reaction course. (d) Other cosolvent (as well as electron donor) investigated. Reaction conditions were as follows: [substrate] = 10 mM, [CiVCPO] = 25–100 nM, [SAS] = 0.5–2 mM, [NaBr] = 25 mM, pH 6.0 (NaPi buffer, 60 mM), 40% of cosolvent, and visible light illumination (λ > 400 nm). The concentration of methanol and formate was 100 mM. The yielded products were quantified by gas chromatography. Error bars represent the standard deviation of duplicate experiments.

Figure 2

Hydroxybromination and bomocyclization reactions by combining CiVCPO and visible light-driven in situ generation of H₂O₂ using SAS. Reaction conditions were as follows: [substrate] = 10 mM, [CiVCPO] = 50 nM, [SAS] = 0.5 mM, pH 6.0 (NaPi buffer, 60 mM), 40% of methanol, 32 h, and visible light illumination (λ > 400 nm). Experiments were performed in independent duplicates.

Figure 3

Hydroxylation of ethylbenzene into (R)–1-phenyl ethanol (⧫) and acetophenone (●) by combining rAaeUPO and visible light-driven in situ generation of H₂O₂ using SAS. Reaction conditions were as follows: [substrate] = 50 mM, [rAaeUPO] = 100 nM, [SAS] = 0.5 mM, 40% of methanol, and visible light illumination (λ > 400 nm). Experiments were performed as independent duplicates.

Figure 4

Self-sufficient, aerobic oxidation of cyclohexane to cyclohexanone. Reaction conditions were as follows: [cyclohexanol] = 2 mM, [cyclohexane] = 25 mM, [rAaeUPO] = 100 nM, [SAS] = 0.5 mM in NaPi buffer (pH 6.0, 60 mM). The values shown stem from one experiment (no duplicates).