Enzymes are ideal for use in asymmetric catalysis by the chemical industry, because their chemical compositions can be tailored to a specific substrate and selectivity pattern while providing efficiencies and selectivities that surpass those of classical synthetic methods. However, enzymes are limited to reactions that are found in nature and, as such, facilitate fewer types of transformation than do other forms of catalysis. Thus, a longstanding challenge in the field of biologically mediated catalysis has been to develop enzymes with new catalytic functions. Here we describe a method for achieving catalytic promiscuity that uses the photoexcited state of nicotinamide co-factors (molecules that assist enzyme-mediated catalysis). Under irradiation with visible light, the nicotinamide-dependent enzyme known as ketoreductase can be transformed from a carbonyl reductase into an initiator of radical species and a chiral source of hydrogen atoms. We demonstrate this new reactivity through a highly enantioselective radical dehalogenation of lactones-a challenging transformation for small-molecule catalysts. Mechanistic experiments support the theory that a radical species acts as an intermediate in this reaction, with NADH and NADPH (the reduced forms of nicotinamide adenine nucleotide and nicotinamide adenine dinucleotide phosphate, respectively) serving as both a photoreductant and the source of hydrogen atoms. To our knowledge, this method represents the first example of photo-induced enzyme promiscuity, and highlights the potential for accessing new reactivity from existing enzymes simply by using the excited states of common biological co-factors. This represents a departure from existing light-driven biocatalytic techniques, which are typically explored in the context of co-factor regeneration.

Strategies that provide enzymes with the ability to catalyse non-natural reactions are of considerable synthetic value. Photoredox catalysis has proved adept at expanding the synthetic repertoire of existing catalytic platforms, yet, in the realm of biocatalysis it has primarily been used for cofactor regeneration. Here we show that photoredox catalysts can be used to enable new catalytic function in nicotinamide-dependent enzymes. Under visible-light irradiation, xanthene-based photocatalysts enable a double-bond reductase to catalyse an enantioselective deacetoxylation. Mechanistic experiments support the intermediacy of an α-acyl radical, formed after the elimination of acetate. Isotopic labelling experiments support nicotinamide as the source of the hydrogen atom. Preliminary calculations and mechanistic experiments suggest that binding to the protein attenuates the reduction potential of the starting material, an important feature for localizing radical formation to the enzyme active site. The generality of this approach is highlighted with the radical dehalogenation of α-bromoamides catalysed by ketoreductases with Eosin Y as a photocatalyst.

The development of non-natural reaction mechanisms is an attractive strategy for expanding the synthetic capabilities of substrate promiscuous enzymes. Here, we report an “ene”-reductase catalyzed asymmetric hydroalkylation of olefins using α-bromoketones as radical precursors. Radical initiation occurs via ground-state electron transfer from the flavin cofactor located within the enzyme active site, an underrepresented mechanism in flavin biocatalysis. Four rounds of site saturation mutagenesis were used to access a variant of the “ene”-reductase nicotinamide-dependent cyclohexanone reductase (NCR) from Zymomonas mobiles capable of catalyzing a cyclization to furnish β-chiral cyclopentanones with high levels of enantioselectivity. Additionally, wild-type NCR can catalyze intermolecular couplings with precise stereochemical control over the radical termination step. This report highlights the utility for ground-state electron transfers to enable non-natural biocatalytic C–C bond forming reactions.

Biocatalysis has revolutionized chemical synthesis, providing sustainable methods for preparing various organic molecules. In enzyme-mediated organic synthesis, most reactions involve molecules operating from their ground states. Over the past 25 years, there has been an increased interest in enzymatic processes that utilize electronically excited states accessed through photoexcitation. These photobiocatalytic processes involve a diverse array of reaction mechanisms that are complementary to one another. This comprehensive review will describe the state-of-the-art strategies in photobiocatalysis for organic synthesis until December 2022. Apart from reviewing the relevant literature, a central goal of this review is to delineate the mechanistic differences between the general strategies employed in the field. We will organize this review based on the relationship between the photochemical step and the enzymatic transformations. The review will include mechanistic studies, substrate scopes, and protein optimization strategies. By clearly defining mechanisticallydistinct strategies in photobiocatalytic chemistry, we hope to illuminate future synthetic opportunities in the area.

Allylations are practical transformations that forge C–C bonds while introducing an alkene for further chemical manipulations. Here, we report a photoenzymatic allylation of α-chloroamides with allyl silanes using flavin-dependent “ene”-reductases (EREDs). An engineered ERED can catalyze annulative allylic alkylation to prepare 5, 6, and 7-membered lactams with high levels of enantioselectivity. Ultrafast transient absorption spectroscopy indicates that radical termination occurs via β-scission of the silyl group to afford a silyl radical, a distinct mechanism by comparison to traditional radical allylations involving allyl silanes. Moreover, this represents an alternative strategy for radical termination using EREDs. This mechanism was applied to intermolecular couplings involving allyl sulfones and silyl enol ethers. Overall, this method highlights the opportunity for EREDs to catalyze radical termination strategies beyond hydrogen atom transfer.