Asymmetric [3+2] Photocycloadditions of Cyclopropanes with Alkenes or Alkynes through Visible-Light Excitation of Catalyst-Bound Substrates

Visible light photocatalysis in the late-stage functionalization of pharmaceutically relevant compounds

The late stage functionalization (LSF) of complex biorelevant compounds is a powerful tool to speed up the identification of structure-activity relationships (SARs) and to optimize ADME profiles. To this end, visible-light photocatalysis offers unique opportunities to achieve smooth and clean functionalization of drugs by unlocking site-specific reactivities under generally mild reaction conditions. This review offers a critical assessment of current literature, pointing out the recent developments in the field while emphasizing the expected future progress and potential applications. Along with paragraphs discussing the visible-light photocatalytic synthetic protocols so far available for LSF of drugs and drug candidates, useful and readily accessible synoptic tables of such transformations, divided by functional groups, will be provided, thus enabling a useful, fast, and easy reference to them.

Recent developments in enantioselective photocatalysis

Enantioselective photocatalysis has rapidly grown into a powerful tool for synthetic chemists. This review describes the various strategies for creating enantioenriched products through merging enantioselective catalysis and photocatalysis, with a focus on the most recent developments and a particular interest in the proposed mechanisms for each. With the aim of understanding the scope of each strategy, to help guide and inspire further innovation in this field.

Preparation of Chiral Photosensitive Organocatalysts and Their Application for the Enantioselective Synthesis of 1,2-Diamines

Chiral phosphoric acid based organocatalysis and visible-light photocatalysis have both emerged as promising technologies for the sustainable production of fine chemicals. In this context, we have envisioned the design and the synthesis of a new class of chimeric catalytic entities that would feature both catalytic capabilities. Given their multitask nature, such catalysts would be particularly attractive for the development of new catalytic transformations, tandem processes in particular. Toward this goal, several BINOL-based chiral phosphoric acid backbones presenting one or two visible-light-sensitive thioxanthone moieties have been prepared and studied. The utility of these new photoactive chiral organocatalysts is then demonstrated in the enantioselective tandem three-component electrophilic amination of enecarbamates. Of note, the C₁-symmetric organo/photocatalyst has shown a better catalytic activity than those presenting a C₂ symmetry.

Catalytic Enantioselective Radical Transformations Enabled by Visible Light

Visible light has been recognized as an economical and environmentally benign source of energy that enables chemoselective molecular activation of chemical reactions and hence reveal a new horizon for the design and discovery of novel chemical transformations. On the other hand, asymmetric catalysis represents an economic method to satisfy the increasing need for enantioenriched compounds in the chemical and pharmaceutical industries. Therefore, combining visible light photocatalysis with asymmetric catalysis creates a wider range of opportunities for the development of mechanistically unique reaction schemes. However, there arise two main problems like undesirable photochemical background reactions and difficulties in controlling the stereochemistry with highly reactive photochemical intermediates which can pose a serious challenge to the development of asymmetric visible light photocatalysis. In recent years, several methods have been developed to overcome these challenges. This review summarizes the recent advances in visible light-induced enantioselective reactions. We divide our discussion into four categories: Asymmetric photoredox organocatalysis, asymmetric transition metal photoredox catalysis, asymmetric photoredox Lewis acid catalysis and asymmetric photoinduced energy transfer catalysis. Special emphasis has been given to different catalytic activation modes that enable the construction of challenging carbon-carbon and carbon-heteroatom bond in an enantioselective fashion. A brief analysis of substrate scope and limitation as well as reaction mechanism of these reactions has been included.

A Vinyl Cyclopropane Ring Expansion and Iridium-Catalyzed Hydrogen Borrowing Cascade

A vinyl cyclopropane rearrangement embedded in an iridium-catalyzed hydrogen borrowing reaction enabled the formation of substituted stereo-defined cyclopentanes from Ph* methyl ketone and cyclopropyl alcohols. Mechanistic studies provide evidence for the ring-expansion reaction being the result of a cascade based on oxidation of the cyclopropyl alcohols, followed by aldol condensation with the pentamethyl phenyl-substituted ketone to form an enone containing the vinyl cyclopropane. Subsequent single electron transfer (SET) to this system initiates a rearrangement, and the catalytic cycle is completed by reduction of the new enone. This process allows for the efficient formation of diversely substituted cyclopentanes as well as the construction of complex bicyclic carbon skeletons containing up to four contiguous stereocentres, all with high diastereoselectivity.

Enantioselective synthesis enabled by visible light photocatalysis

Enantioselective photoreaction has been a synthetic challenge for decades. With the continuous development of modern visible light photocatalysis and asymmetric catalysis, remarkable advances have been achieved through the synergistic action of these catalytic reactions, allowing the construction of various enantiomerically enriched molecules that were once inaccessible using photocatalytic reactions. This review presents some of the contemporary developments in enantioselective visible-light photocatalysis reactions, covering the period from 2008 to March 2020, with the contents classified by catalysis type.

Visible light-promoted ring-opening functionalization of three-membered carbo- and heterocycles

In this tutorial review, synthetic and mechanistic aspects of visible light-promoted ring-opening functionalization of three-membered carbo- and heterocycles are highlighted.

Visible-light-induced triple catalysis for a ring-opening cyanation of cyclopropyl ketones

A ring-opening cyanation of cyclopropyl ketones was developed via a synergistic triple catalysis strategy, providing a wide range of γ-cyanoketones in good reaction efficiencies under mild and eco-friendly conditions.

Enantioselective cyanation via radical-mediated C-C single bond cleavage for synthesis of chiral dinitriles

Ring-opening reaction via selective cleavage of C–C bond is known as a powerful strategy for construction of complex molecules. Complementary to the ionic process focusing on mostly small ring systems, radical-mediated C–C bond cleavage offers a solution for further diverse enantioselective functionalization benefited from its mild conditions, whereas such asymmetric transformations are still limited to three-membered rings so far. Herein, we describe radical-mediated ring-opening and enantioselective cyanation of four- and five-membered cycloketone oxime esters to access chiral 1,5- and 1,6-dinitriles. Employment of dual photoredox/copper catalysis is essential for the asymmetric ring-opening cyanation of cyclopentanone oxime esters. Both reactions proceed under mild conditions giving chiral dinitriles in high yields and enantioselectivity with low catalyst loading and broad substrate scope. The products dinitriles can be converted to valuable optically active diamides and diamines. Mechanistic studies indicate that the benzylic radical generated via C–C single bond cleavage is involved in the catalytic cycle.

Ring-opening reaction via selective cleavage of C–C bond is a powerful strategy to increase molecular complexity. Here the authors present the facile synthesis of chiral 1,5- and 1,6-dinitriles via the radical-mediated ring-opening and enantioselective cyanation of cycloketone oxime esters.

Photoredox-catalyzed oxo-amination of aryl cyclopropanes

Cyclopropanes represent a class of versatile building blocks in modern organic synthesis. While the release of ring strain offers a thermodynamic driving force, the control of selectivity for C–C bond cleavage and the subsequent regiochemistry of the functionalization remains difficult, especially for unactivated cyclopropanes. Here we report a photoredox-coupled ring-opening oxo-amination of electronically unbiased cyclopropanes, which enables the expedient construction of a host of structurally diverse β-amino ketone derivatives. Through one electron oxidation, the relatively inert aryl cyclopropanes are readily converted into reactive radical cation intermediates, which in turn participate in the ensuing ring-opening functionalizations. Based on mechanistic studies, the present oxo-amination is proposed to proceed through an S_N2-like nucleophilic attack/ring-opening manifold. This protocol features wide substrate scope, mild reaction conditions, and use of dioxygen as an oxidant both for catalyst regeneration and oxygen-incorporation. Moreover, a one-pot formal aminoacylation of olefins is described through a sequential cyclopropanation/oxo-amination.

The ring-opening and functionalization of electronically unbiased cyclopropanes is highly challenging to achieve in a regioselective fashion. Here, the authors report a mild photoredox-coupled oxoamination of electronically unactivated aryl cyclopropanes with simple azaarenes and molecular oxygen.

Asymmetric Photocatalysis by Intramolecular Hydrogen-Atom Transfer in Photoexcited Catalyst-Substrate Complex

3-(2-Formylphenyl)-1-pyrazol-1-yl-propenones undergo an asymmetric photorearrangement to benzo[d]cyclopropa[b]pyranones with up to >99 % ee, which is catalyzed by a bis-cyclometalated rhodium catalyst in the presence of visible light. Mechanistic experiments and DFT calculations support a mechanism in which a photoexcited catalyst/substrate complex triggers an intramolecular hydrogen-atom transfer followed by a highly stereocontrolled hetero-Diels-Alder reaction. In this reaction scheme, the rhodium catalyst fulfills multiple functions by 1) enabling visible-light π→π* excitation of the catalyst-bound enone substrate, 2) facilitating the hydrogen-atom transfer, and 3) providing the asymmetric induction for the hetero-Diels-Alder reaction.

Enantioselective Intermolecular Excited-State Photoreactions Using a Chiral Ir Triplet Sensitizer: Separating Association from Energy Transfer in Asymmetric Photocatalysis

Enantioselective catalysis of excited-state photoreactions remains a substantial challenge in synthetic chemistry, and intermolecular photoreactions have proven especially difficult to conduct in a stereocontrolled fashion. Herein, we report a highly enantioselective intermolecular [2 + 2] cycloaddition of 3-alkoxyquinolones catalyzed by a chiral hydrogen-bonding iridium photosensitizer. Enantioselectivities as high as 99% ee were measured in reactions with a range of maleimides and other electron-deficient alkene reaction partners. An array of kinetic, spectroscopic, and computational studies supports a mechanism in which the photocatalyst and quinolone form a hydrogen-bonded complex to control selectivity, yet upon photoexcitation of this complex, energy transfer sensitization of maleimide is preferred. The sensitized maleimide then reacts with the hydrogen-bonded quinolone-photocatalyst complex to afford a highly enantioenriched cycloadduct. This finding contradicts a long-standing tenet of enantioselective photochemistry that held that stereoselective photoreactions require strong preassociation to the sensitized substrate in order to overcome the short lifetimes of electronically excited organic molecules. This system therefore suggests that a broader range of alternate design strategies for asymmetric photocatalysis might be possible.

Enantioselective photoredox dehalogenative protonation

We report an enantioselective photoredox dehalogenative protonation as a new type of asymmetric protonation. As a paradigm, with a cooperative catalytic system consisting of a chiral H-bonding catalyst and a dicyanopyrazine-derived chromophore (DPZ) photosensitizer that is irradiated with visible light, a range of cyclic and acyclic ketones with labile chiral secondary C-F, C-Cl and C-Br bonds at the α-position were obtained in high yields with good to excellent enantioselectivities (up to >99% ee) by using a secondary amine as the terminal reductant. Given the ready accessibility of halides, the success of this work should provide inspiration for constructing diverse chiral α-tertiary carbonyls and their variants.

Enantioselective [2+2] Cycloadditions of Cinnamate Esters: Generalizing Lewis Acid Catalysis of Triplet Energy Transfer

We report the enantioselective [2+2] cycloaddition of simple cinnamate esters, the products of which are useful synthons for the controlled assembly of cyclobutane natural products. This method utilizes a cocatalytic system in which a chiral Lewis acid accelerates the transfer of triplet energy from an excited-state Ir(III) photocatalyst to the cinnamate ester. Computational evidence indicates that the principal role of the Lewis acid cocatalyst is to lower the absolute energies of the substrate frontier molecular orbitals, leading to greater electronic coupling between the sensitizer and substrate and increasing the rate of the energy transfer event. These results suggest Lewis acids can have multiple beneficial effects on triplet sensitization reactions, impacting both the thermodynamic driving force and kinetics of Dexter energy transfer.

Asymmetric Photocatalysis with Bis-cyclometalated Rhodium Complexes

Aspects of sustainability are playing an increasingly important role for the development of new synthetic methods. In this context, the combination of asymmetric catalysis, which is considered one of the most economic strategies to generate nonracemic chiral compounds, and visible light as an abundant source of energy to induce or activate chemical reactions has recently gained much attention. Furthermore, the combination of photochemistry with asymmetric catalysis provides new opportunity for the development of mechanistically unique reaction schemes. However, the development of such asymmetric photocatalysis is very challenging and two main problems can be pinpointed to undesirable photochemical background reactions and to difficulties in controlling the stereochemistry with photochemically generated highly reactive intermediates. In this Account, we present and discuss asymmetric photocatalysis using one of the currently most versatile photoactivatable asymmetric catalysts, namely, reactive bis-cyclometalated rhodium(III) complexes. The catalysts contain two inert cyclometalating 5-( tert-butyl)-2-phenyl benzoxazole or benzothiazole ligands together with two labile acetonitriles, and the overall chirality is due to a stereogenic metal center. The bis-cyclometalated rhodium complexes serve as excellent chiral Lewis acids for substrates such as 2-acyl imidazoles and N-acyl pyrazoles, which, upon replacement of the two labile acetonitrile ligands, coordinate to the rhodium center in a 2-point fashion. These rhodium-substrate intermediates display unique photophysical and photochemical properties and are often the photoactive intermediates in the developed asymmetric photocatalysis reaction schemes. This combination of visible light excitation to generate long-lived photoexcited states and intrinsic Lewis acid reactivity opens the door for a multitude of visible-light-induced asymmetric conversions. In a first mode of reactivity, bis-cyclometalated rhodium complexes function as chiral Lewis acids to control asymmetric radical reactions of rhodium enolates with electron-deficient radicals, rhodium-coordinated enones with electron-rich radicals, or rhodium-bound radicals generated by photoinduced single electron transfer. The rhodium-substrate complexes in their ground states are key intermediates of the asymmetric catalysis, while separate photoredox cycles initiate radical generations via single electron transfer with either the rhodium-substrate complexes or additional photoactive compounds serving as the photoredox catalyst (secondary asymmetric photocatalysis). In a second mode of reactivity, the rhodium-substrate complexes serve as photoexcited intermediates within the asymmetric catalysis cycle (primary asymmetric photocatalysis) and undergo stereocontrolled chemistry either upon single electron transfer or by direct bond forming reactions out of the excited state. These multiple modes of intertwining photochemistry with asymmetric catalysis have been applied to asymmetric α- and β-alkylations, α- and β-aminations, β-C-H functionalization of carbonyl compounds, [3 + 2] photocycloadditions between cyclopropanes and alkenes or alkynes, [2 + 2] photocycloadditions of enones with alkenes, dearomative [2 + 2] photocycloadditions, and [2 + 3] photocycloadditions of enones with vinyl azides. We anticipate that these reaction schemes of chiral bis-cyclometalated rhodium complexes as (photoactive) chiral Lewis acids will spur the development of new photocatalysts for visible-light-induced asymmetric catalysis.

Photoredox-catalysed formal [3+2] cycloaddition of N-aryl α-amino acids with isoquinoline N-oxides

This work realizes a new synthetic utility of N-aryl α-amino acids as a 1,2-synthon, a new strategy to achieve dearomatization of isoquinolines, and the synthesis of valuable diazabicyclo[3.2.1]octane-based compounds.

Photoredox asymmetric catalytic enantioconvergent substitution of 3-chlorooxindoles

An enantioconvergent substitution of 3-substituted 3-chlorooxindoles with N-aryl glycines under visible light irradiation is reported.

Enantioselective reduction of azaarene-based ketones via visible light-driven photoredox asymmetric catalysis

An enantioselective reduction of azaarene-based ketones through photoredox asymmetric catalysis is reported.

Advances in asymmetric visible-light photocatalysis, 2015–2019

Asymmetric visible-light photocatalysis has recently drawn considerable attention of the scientific community owing to its unique activation modes and significance for the enantioselective green synthesis.

Catalytic cascade reactions by radical relay

Catalytic radical relays are an attractive tool for the rapid construction of complex molecular architectures.

Mild dynamic kinetic resolution of amines by coupled visible-light photoredox and enzyme catalysis

Herein, we described photoenzymatic dynamic kinetic resolution (DKR) of amines under mild conditions. The racemization of amines via a photoredox-mediated hydrogen atom transfer (HAT) protocol in conjunction with an enzyme catalyst to achieve the DKR of amines allows a variety of primary amines to be converted into a single enantiomer in high yield and with excellent enantioselectivity. Notably, this protocol can also be extended to 1,4-diamine derivatives with high levels of diastereo- and enantioselectivity.

Synthesis of functionalized cyclopentenes through allenic ketone-based multicomponent reactions

A novel and efficient synthesis of diversely functionalized cyclopentene derivatives through the multicomponent reactions of 1,2-allenic ketones with 4-chloroacetoacetate and malononitrile/cyanoacetate under mild and metal-free conditions is presented. Mechanistically, the formation of title compounds involves a cascade process including nucleophilic substitution, Michael addition and intramolecular aldol type reaction. Interestingly, when 1-phenyl allenic ketones bearing electron-donating groups on the phenyl ring were reacted with 4-chloroacetoacetate and cyanoacetate, methylenecyclo-pentanes, the regioisomer of cyclopentenes, were formed with good selectivity and high efficiency.

Visible-light photocatalytic bicyclization of β-alkynyl propenones for accessing diastereoenriched syn-fluoren-9-ones

A novel visible-light photocatalytic bicyclization of β-alkynyl propenones with α-bromocarbonyls for highly diastereoselective synthesis of richly decorated syn-fluoren-9-ones is described. The reaction proceeds via a radical-triggered 5-exo-dig cyclization/1,6-H-abstraction/6-endo-trig cyclization cascade and offers a new and practical method for the assembly of 6/5/6 carbocyclic skeletons via C(sp3)-H alkenylation.

Photo-Organocatalytic Enantioselective Radical Cascade Reactions of Unactivated Olefins

Radical cascade processes are invaluable for their ability to rapidly construct complex chiral molecules from simple substrates. However, implementing catalytic asymmetric variants is difficult. Reported herein is a visible‐light‐mediated organocatalytic strategy that exploits the excited‐state reactivity of chiral iminium ions to trigger radical cascade reactions with high enantioselectivity. By combining two sequential radical‐based bond‐forming events, the method converts unactivated olefins and α,β‐unsaturated aldehydes into chiral adducts in a single step. The implementation of an asymmetric three‐component radical cascade further demonstrates the complexity‐generating power of this photochemical strategy.

Synthesis of 2,2-difluoro-homoallylic alcohols via ring-opening of gem-difluorocyclopropane and aerobic oxidation by photo-irradiation in the presence of an organic pigment

The aerobic oxidation took place after the visible light-mediated ring-opening reaction of gem-difluorocyclopropane in the presence of an organic dye and amine to furnish 2,2-difluoro-homoallylic alcohols in good yields.

Catalytic asymmetric synthesis of diphenylbutazone analogues

The asymmetric Michael addition of diphenylbutazone and its analogues to α,β-unsaturated 2-acyl imidazoles has been developed with a chiral-at-metal Rh(iii) complex as a catalyst.