Site-Selective C–H Alkylation of Piperazine Substrates via Organic Photoredox Catalysis

Programmable Piperazine Synthesis via Organic Photoredox Catalysis

Piperazine cores have long been identified as privileged scaffolds in the development of pharmaceutical compounds. Despite this, the facile synthesis of diverse C-substituted piperazines remains a challenge without prefunctionalized substrates/cores. Herein, we describe a programmable approach to highly diversifiable piperazine cores, which circumvents the typical need for radical precursors. The use of organic photoredox catalysis renders this method operationally simple, as direct substrate oxidation followed by 6-endo-trig radical cyclization with in situ generated imines may furnish the product. Additionally, the photoredox-catalyzed anti-Markovnikov hydroamination of readily accessible ene-carbamates provides a modular approach to functionalized diamine starting materials which are shown to generate more complex piperazine cores. A wide range of both carbonyl and amine condensation partners were shown to be compatible with this system in good to excellent yield.

Collaborative GSK-University of Strathclyde doctoral research and training programmes: Transforming approaches to industry-academia engagement

A global biopharma company, GSK, and the University of Strathclyde have developed an expansive and transformative research and training partnership originating in chemistry-aligned disciplines, with subsequent extensive expansion across further areas of the company. This has opened unique approaches for the delivery of collaborative research innovations while also enhancing the professional development and learning of GSK personnel, in addition to other embedded researchers and collaborating scientists, on a pathway towards more rapid and efficient discovery of new medicines.

Pioneering Metal-Free Late-Stage C-H Functionalization Using Acridinium Salt Photocatalysis

Using organic dyes as photocatalysts is an innovative approach to photocatalytic organic transformations. These dyes offer advantages such as widespread availability, adaptable absorption properties, and diverse chemical structures. Recent progress has led to the development of organic photocatalysts that can utilize visible light to modify chemically inert C-H bonds. These catalysts are sustainable, selective, and versatile, enabling mild reactions, late-stage functionalization, and various transformations in line with green chemistry principles. As catalysts in photoredox chemistry, they contribute to the development of efficient and environmentally friendly synthetic pathways. Acridinium-based organic photocatalysts have proved valuable in late-stage C-H functionalization, enabling transformative reactions under mild conditions. This review emphasizes their innovative features, such as organic frameworks, efficient light absorption properties, and their applications in modifying complex molecules. It provides an overview of recent advancements in the use of acridinium-based organic photocatalysts for late-stage C-H bond functionalization without the need for transition metals, showcasing their potential to expedite the development of new molecules and igniting excitement about the prospects of this research in the field.

Chemoselective Diazine Dearomatization: The Catalytic Enantioselective Dearomatization of Pyrazine

Despite much progress in the area of dearomatization, the enantioselective dearomatization of heterocycles is limited to those with a single heteroatom. Here we report a highly enantioselective copper-catalyzed dearomatization of pyrazine, a diazine, leading to chiral C-substituted piperazines. When exposed to a chloroformate and an alkyne in the presence of a catalyst derived from a copper salt and the chiral ligand StackPhos, pyrazine is readily dearomatized to provide a 2,3-disubstituted dihydropyrazine as single diastereomer in high enantiomeric excess. Mechanistic studies support a noninnocent involvement of chloride ion preventing a second iminium alkynylation, thus enabling subsequent functionalization at the second reactive site. The synthetically useful dihydropyrazine products, obtained in up to 95% yield and 99% ee, can be further manipulated to form optically active C-substituted piperazines and C1-symmetric 1,2-diamines.

Reactivity and Selectivity of Azaspirocycles in Radical C-H Functionalization

Investigations of saturated spirocycles toward selective C-H functionalization reactions are scarce, despite their potential applications. In this work, we uncovered fundamental reactivity and selectivity differences between saturated heterocycles and their spirocyclic analogues using a model radical C-H xanthylation coupled with computational analysis. Ultimately, this study sheds light on the fundamental, understudied radical reactivity of spirocycles, thereby allowing for a pronounced chemical tunability that will prove to be advantageous in the expansion of their chemical space and applications in medicinal chemistry.

Functionalized Polyamine Synthesis with Photoredox Catalysis

Polyamines, such as putrescine and spermidine, are pivotal in various biological processes across living organisms. Despite their significance, structurally modified polyamines offer a less-explored avenue for discovering bioactive compounds. The limitation is attributed to the synthetic difficulty of accessing functionalized polyamines. In this study, we accomplished photoredox-catalyzed functionalization of polyamines to diversify their structure. The rapid functionalization allows attaching fluorophores to the target polyamine, facilitating the development of molecular probes for advancing chemical biology studies.

Alicyclic-Amine-Derived Imine-BF3 Complexes: Easy-to-Make Building Blocks for the Synthesis of Valuable α-Functionalized Azacycles

A new strategy to access α-functionalized alicyclic amines via their corresponding imine-BF3 complexes is reported. Isolable imine-BF3 complexes, readily prepared via dehydrohalogenation of N-bromoamines in a base-promoted/18-crown-6 catalyzed process followed by addition of boron trifluoride etherate, undergo reactions with a wide range of organometallic nucleophiles to afford α-functionalized azacycles. Organozinc and organomagnesium nucleophiles add at ambient temperatures, obviating the need for cryogenic conditions. In situ preparation of imine-BF3 complexes provides access to α-functionalized morpholines and piperazines directly from their parent amines in a single operation. α-Functionalized morpholines can be elaborated further, for instance by installing a second substituent in the α'-position.

Oxidative Cross Dehydrogenative Coupling of N-Heterocycles with Aldehydes through C(sp3)-H Functionalization

Existing methodologies for metal-catalyzed cross-couplings typically rely on preinstallation of reactive functional groups on both reaction partners. In contrast, C-H functionalization approaches offer promise in simplification of the requisite substrates; however, challenges from low reactivity and similar reactivity of various C-H bonds introduce considerable complexity. Herein, the oxidative cross dehydrogenative coupling of α-amino C(sp3)-H bonds and aldehydes to produce ketone derivatives is described using an unusual reaction medium that incorporates the simultaneous use of di-tert-butyl peroxide as an oxidant and zinc metal as a reductant. The method proceeds with a broad substrate scope, representing an attractive approach for accessing α-amino ketones through the formal acylation of C-H bonds α to nitrogen in N-heterocycles. A combination of experimental investigation and computational modeling provides evidence for a mechanistic pathway involving cross-selective nickel-mediated cross-coupling of α-amino radicals and acyl radicals.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Chemoselective Decarboxylative Protonation Enabled by Cooperative Earth-Abundant Element Catalysis

Decarboxylative protonation is a general deletion tactic to replace polar carboxylic acid groups with hydrogen or its isotope. Current methods rely on the pre-activation of acids, non-sustainable hydrogen sources, and/or expensive/highly oxidizing photocatalysts, presenting challenges to their wide adoption. Here we show that a cooperative iron/thiol catalyst system can readily achieve this transformation, hydrodecarboxylating a wide range of activated and unactivated carboxylic acids and overcoming scope limitations in previous direct methods. The reaction is readily scaled in batch configuration and can be directly performed in deuterated solvent to afford high yields of d-incorporated products with excellent isotope incorporation efficiency; characteristics not attainable in previous photocatalyzed approaches. Preliminary mechanistic studies indicate a radical mechanism and kinetic results of unactivated acids (KIE=1) are consistent with a light-limited reaction.

Copper(I)-Catalyzed Three-Component Selenosulfonation of Maleimides with Sulfonyl Hydrazides and Diselenides via Radical Relay

By employing Cu(CH₃CN)₄PF₆ as the catalyst and tert-butyl hydroperoxide as the oxidant, we realized a three-component radical selenosulfonation of substituted maleimides, sulfonyl hydrazides, and diphenyl diselenides, providing a series of 3,4-selenosulfonylated succinimides in moderate to good yields. This reaction features broad substrate scopes, high functional-group tolerability, and feasibility of gram-scale synthesis, enabling one-step construction of C-SO₂ and C-Se bonds under mild reaction conditions. Preliminary mechanistic studies support the free-radical-induced pathway.

1,2-Radical Shifts in Photoinduced Synthetic Organic Transformations: A Guide to the Reactivity of Useful Radical Synthons

The exploration of 1,2-radical shift (RS) mechanisms in photoinduced organic reactions has provided efficient routes for the generation of important radical synthons in many chemical transformations. In this Review, the basic concepts involved in the traditional 1,2-spin-center shift (SCS) mechanisms in recently reported studies are discussed. In addition, other useful 1,2-RSs are addressed, such as those proceeding through 1,2-group migrations in carbohydrate chemistry, via 1,2-boron shifts, and by the generation of α-amino radicals. The discussion begins with a general overview of the basic aspects of 1,2-RS mechanisms, followed by a demonstration of their applicability in photoinduced transformations. The sections that follow are organized according to the mechanisms operating in combination with the 1,2-radical migration event. This contribution is not a comprehensive review but rather aims to provide an understanding of the topic, focused on the more recent advances in the field, and establishes a definition for the nomenclature that has been used to describe such mechanisms.

Synthetic Approaches to the New Drugs Approved During 2020

New drugs introduced to the market are privileged structures that have affinities for biological targets implicated in human diseases and conditions. These new chemical entities (NCEs), particularly small molecules and antibody-drug conjugates (ADCs), provide insight into molecular recognition and simultaneously function as leads for the design of future medicines. This Review is part of a continuing series presenting the most likely process-scale synthetic approaches to 44 new chemical entities approved for the first time anywhere in the world during 2020.

Hydroaminoalkylation for the Catalytic Addition of Amines to Alkenes or Alkynes: Diverse Mechanisms Enable Diverse Substrate Scope

Hydroaminoalkylation is a powerful, atom-economic catalytic reaction for the reaction of amines with alkenes and alkynes. This C-H functionalization reaction allows for the atom-economic alkylation of amines using simple alkenes or alkynes as the alkylating agents. This transformation has significant potential for transformative approaches in the pharmaceutical, agrochemical, and fine chemical industries in the preparation of selectively substituted amines and N-heterocycles and shows promise in materials science for the synthesis of functional and responsive aminated materials. Different early transition-metal, late transition-metal, and photoredox catalysts mediate hydroaminoalkylation by distinct mechanistic pathways. These mechanistic insights have resulted in the development of new catalysts and reaction conditions to realize hydroaminoalkylation with a broad range of substrates: activated and unactivated, terminal and internal, C-C double and triple bonds with aryl or alkyl primary, secondary, or tertiary amines, including N-heterocyclic amines. By deploying select catalysts with specific substrate combinations, control over regioselectivity, diastereoselectivity, and enantioselectivity has been realized. Key barriers to widespread adoption of this reaction include air and moisture sensitivity for early transition-metal catalysts as well as a heavy dependence on amine protecting or directing groups for late transition-metal or photocatalytic routes. Advances in improved catalyst robustness, substrate scope, and regio-/stereoselective reactions with early- and late transition-metal catalysts, as well as photoredox catalysis, are highlighted, and opportunities for further catalyst and reaction development are included. This perspective shows that hydroaminoalkylation has the potential to be a disruptive and transformative strategy for the synthesis of selectively substituted amines and N-heterocycles from simple amines and alkenes.

Photoinduced Chemo-, Site- and Stereoselective α-C(sp3 )-H Functionalization of Sulfides

The ubiquity of sulfur-containing molecules in biologically active natural products and pharmaceuticals has long attracted synthetic chemists to develop efficient strategies towards their synthesis. The strategy of direct α-C(sp³ )-H modification of sulfides provides a streamlining access to complex sulfur-containing molecules. Herein, we report a photoinduced chemo-, site- and stereoselective α-C(sp³ )-H functionalization of sulfides using isatins as the photoredox reagent and coupling partner catalyzed by a chiral gallium(III)-N,N'-dioxide complex. The reaction proceeds through a verified single-electron transfer (SET) mechanism with high efficiency, excellent functional group tolerance, as well as a broad substrate scope. Importantly, this cross-coupling protocol is highly selective for the direct late-stage functionalization of methionine-related peptides, regardless of the inherent structural similarity and complexity of diverse residues.

Synthesis of Polycyclic Isoindolines via α-C-H/N-H Annulation of Alicyclic Amines

Relatively unstable cyclic imines, generated in situ from their corresponding alicyclic amines via oxidation of their lithium amides with simple ketone oxidants, engage aryllithium compounds containing a leaving group on an ortho-methylene functionality to provide polycyclic isoindolines in a single operation. The scope of this transformation includes pyrrolidine, piperidine, azepane, azocane, and piperazines.

Photoredox-Catalyzed C-H Functionalization Reactions

The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class─either sp2 or sp3 C-H functionalization─lends perspective and tactical strategies for use of these methods in synthetic applications.

Switchable, Reagent-Controlled Diastereodivergent Photocatalytic Carbocyclisation of Imine-Derived α-Amino Radicals

A reagent-controlled stereodivergent carbocyclisation of aryl aldimine-derived, photocatalytically generated, α-amino radicals possessing adjacent conjugated alkenes, affording either bicyclic or tetracyclic products, is described. Under net reductive conditions using commercial Hantzsch ester, the α-amino radical species underwent a single stereoselective cyclisation to give trans-configured amino-indane structures in good yield, whereas using a substituted Hantzsch ester as a milder reductant afforded cis-fused tetracyclic tetrahydroquinoline frameworks, resulting from two consecutive radical cyclisations. Judicious choice of the reaction conditions allowed libraries of both single and dual cyclisation products to be synthesised with high selectivity, notable predictability, and good-to-excellent yields. Computational analysis employing DFT revealed the reaction pathway and mechanistic rationale behind this finely balanced yet readily controlled photocatalytic system.

Recent Advances in the Synthesis of Piperazines: Focus on C–H Functionalization

Piperazine ranks as the third most common nitrogen heterocycle in drug discovery, and it is the key component of several blockbuster drugs, such as Imatinib (also marketed as Gleevec) or Sildenafil, sold as Viagra. Despite its wide use in medicinal chemistry, the structural diversity of piperazines is limited, with about 80% of piperazine-containing drugs containing substituents only at the nitrogen positions. Recently, major advances have been made in the C–H functionalization of the carbon atoms of the piperazine ring. Herein, we present an overview of the recent synthetic methods to afford functionalized piperazines with a focus on C–H functionalization.

Photocatalysis in the Life Science Industry

In the pursuit of new pharmaceuticals and agrochemicals, chemists in the life science industry require access to mild and robust synthetic methodologies to systematically modify chemical structures, explore novel chemical space, and enable efficient synthesis. In this context, photocatalysis has emerged as a powerful technology for the synthesis of complex and often highly functionalized molecules. This Review aims to summarize the published contributions to the field from the life science industry, including research from industrial-academic partnerships. An overview of the synthetic methodologies developed and strategic applications in chemical synthesis, including peptide functionalization, isotope labeling, and both DNA-encoded and traditional library synthesis, is provided, along with a summary of the state-of-the-art in photoreactor technology and the effective upscaling of photocatalytic reactions.

C-H functionalisation tolerant to polar groups could transform fragment-based drug discovery (FBDD)

We have analysed 131 fragment-to-lead (F2L) examples targeting a wide variety of protein families published by academic and industrial laboratories between 2015-2019. Our assessment of X-ray structural data identifies the most common polar functional groups involved in fragment-protein binding are: N-H (hydrogen bond donors on aromatic and aliphatic N-H, amides and anilines; totalling 35%), aromatic nitrogen atoms (hydrogen bond acceptors; totalling 23%), and carbonyl oxygen group atoms (hydrogen bond acceptors on amides, ureas and ketones; totalling 22%). Furthermore, the elaboration of each fragment into its corresponding lead is analysed to identify the nominal synthetic growth vectors. In ∼80% of cases, growth originates from an aromatic or aliphatic carbon on the fragment and more than 50% of the total bonds formed are carbon-carbon bonds. This analysis reveals that growth from carbocentric vectors is key and therefore robust C-H functionalisation methods that tolerate the innate polar functionality on fragments could transform fragment-based drug discovery (FBDD). As a further resource to the community, we have provided the full data of our analysis as well as an online overlay page of the X-ray structures of the fragment hit and leads: https://astx.com/interactive/F2L-2021/.

α,α'-C-H Bond Difunctionalization of Unprotected Alicyclic Amines

A simple one-pot procedure enables the sequential, regioselective, and diastereoselective introduction of the same or two different substituents to the α- and α'-positions of unprotected azacycles. Aryl, alkyl, and alkenyl substituents are introduced via their corresponding organolithium compounds. The scope of this transformation includes pyrrolidines, piperidines, azepanes, and piperazines.

Direct, Catalytic α-Alkylation of N-Heterocycles by Hydroaminoalkylation: Substrate Effects for Regiodivergent Product Formation

Saturated N-heterocycles are prevalent in pharmaceutical and agrochemical industries, yet remain challenging to catalytically alkylate. Most strategies for C-H activation of these challenging substrates use protected amines or high loadings of precious metal catalysts. We report an early transition-metal system for the broad, robust, and direct alkylation of unprotected amine heterocycles with simple alkenes. Short reaction times are achieved using an in situ generated tantalum catalyst that avoids the use of bases, excess substrate, or additives. In most cases, this catalyst system is selective for the branched reaction product, including examples of products that are generated with excellent diastereoselectivity. Alkene electronic properties can be exploited for substrate-modified regioselectivity to access the alternative linear amine alkylation product with a group 5 catalyst. This method allows for the facile isolation of unprotected N-heterocyclic products, as useful substrates for further reactivity.

Recent advances in visible light-activated radical coupling reactions triggered by (i) ruthenium, (ii) iridium and (iii) organic photoredox agents

Photoredox chemistry with organic or transition metal agents has been reviewed in earlier years, but such is the pace of progress that we will overlap very little with earlier comprehensive reviews. This review first presents an overview of the area of research and then examines recent examples of C-C, C-N, C-O and C-S bond formations via radical intermediates with transition metal and organic radical promoters. Recent successes with Birch reductions are also included. The transition metal chemistry will be restricted to photocatalysts based on the most widely used metals, Ru and Ir, but includes coupling chemistries that take advantage of low-valent nickel, or occasionally copper, complexes to process the radicals that are formed. Our focus is on developments in the past 10 years (2011-2021). This period has also seen great advances in the chemistry of organic photoredox reagents and the review covers this area. The review is intended to present highlights and is not comprehensive.

α-C-H/N-H Annulation of Alicyclic Amines via Transient Imines: Preparation of Polycyclic Lactams

Polycyclic lactams are prepared in a single operation from o-toluamides and cyclic amines in a process that involves transient cyclic imines, species that are conveniently obtained in situ from the corresponding lithium amides and simple ketone oxidants. Imines thus generated, such as 1-pyrroline and 1-piperideine, engage lithiated o-toluamides in a facile annulation process. Undesired side reactions such as imine deprotonation and o-toluamide dimerization are suppressed through the judicious choice of reaction conditions.

Emergent synthetic methods for the modular advancement of sp3-rich fragments

Fragment-based drug discovery is an important and increasingly reliable technology for the delivery of clinical candidates. Notably, however, sp3-rich fragments are a largely untapped resource in molecular discovery, in part due to the lack of general and suitably robust chemical methods available to aid their development into higher affinity lead and drug compounds. This Perspective describes the challenges associated with developing sp3-rich fragments, and succinctly highlights recent advances in C(sp3)-H functionalisations of high potential value towards advancing fragment hits by 'growing' functionalised rings and chains from unconventional, carbon-centred vectors.

α-C-H Bond Functionalization of Unprotected Alicyclic Amines: Lewis-Acid-Promoted Addition of Enolates to Transient Imines

Enolizable cyclic imines, obtained in situ from their corresponding lithium amides by oxidation with simple ketone oxidants, are readily alkylated with a range of enolates to provide mono- and polycyclic β-aminoketones in a single operation, including the natural product (±)-myrtine. Nitrile anions also serve as competent nucleophiles in these transformations, which are promoted by BF3 etherate. β-Aminoesters derived from ester enolates can be converted to the corresponding β-lactams.

Diversification of Unprotected Alicyclic Amines by C-H Bond Functionalization: Decarboxylative Alkylation of Transient Imines

Despite extensive efforts by many practitioners in the field, methods for the direct α-C-H bond functionalization of unprotected alicyclic amines remain rare. A new advance in this area utilizes N-lithiated alicyclic amines. These readily accessible intermediates are converted to transient imines through the action of a simple ketone oxidant, followed by alkylation with a β-ketoacid under mild conditions to provide valuable β-amino ketones with unprecedented ease. Regioselective α'-alkylation is achieved for substrates with existing α-substituents. The method is further applicable to the convenient one-pot synthesis of polycyclic dihydroquinolones through the incorporation of a SN Ar step.

Recent developments in the difunctionalization of alkenes with C–N bond formation

Various alkene difunctionalization reactions involving nitridization, diamination, azidation, oxyamination, carboamination, aminohalogenation, and nitration are introduced in this review.

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.

Traceless Redox-Annulations of Alicyclic Amines

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with ortho-(nitromethyl)benzaldehyde. Benzoic acid acts as a promoter in these reactions, which involve concurrent amine α-C-H bond and N-H bond functionalization. Subsequent removal of the nitro group provides access to tetrahydroprotoberberines not accessible via typical redox-annulations. Also reported are decarboxylative annulations of ortho-(nitromethyl)benzaldehyde with proline and pipecolic acid.

Photocatalytic Hydrogen Evolution from Plastoquinol Analogues as a Potential Functional Model of Photosystem I

The recent development of a functional model of photosystem II (PSII) has paved a new way to connect the PSII model with a functional model of photosystem I (PSI). However, PSI functional models have yet to be reported. We report herein the first potential functional model of PSI, in which plastoquinol (PQH₂) analogues were oxidized to plastoquinone (PQ) analogues, accompanied by hydrogen (H₂) evolution. Photoirradiation of a deaerated acetonitrile (MeCN) solution containing hydroquinone derivatives (X-QH₂) as a hydrogen source, 9-mesityl-10-methylacridinium ion (Acr⁺-Mes) as a photoredox catalyst, and a cobalt(III) complex, Co^III(dmgH)₂pyCl (dmgH = dimethylglyoximate monoanion; py = pyridine) as a redox catalyst resulted in the evolution of H₂ and formation of the corresponding p-benzoquinone derivatives (X-Q) quantitatively. The maximum quantum yield for photocatalytic H₂ evolution from tetrachlorohydroquinone (Cl₄QH₂) with Acr⁺-Mes and Co^III(dmgH)₂pyCl and H₂O in deaerated MeCN was determined to be 10%. Photocatalytic H₂ evolution is started by electron transfer (ET) from Cl₄QH₂ to the triplet ET state of Acr⁺-Mes to produce Cl₄QH₂^•+ and Acr^•-Mes with a rate constant of 7.2 × 10⁷ M^-1 s^-1, followed by ET from Acr^•-Mes to Co^III(dmgH)₂pyCl to produce [Co^II(dmgH)₂pyCl]^-, accompanied by the regeneration of Acr⁺-Mes. On the other hand, Cl₄QH₂^•+ is deprotonated to produce Cl₄QH^•, which transfers either a hydrogen-atom transfer or a proton-coupled electron transfer to [Co^II(dmgH)₂pyCl]^- to produce a cobalt(III) hydride complex, [Co^III(H)(dmgH)₂pyCl]^-, which reacts with H⁺ to evolve H₂, accompanied by the regeneration of Co^III(dmgH)₂pyCl. The formation of [Co^II(dmgH)₂pyCl]^- was detected by electron paramagnetic resonance measurements.

Nucleophilic Aromatic Substitution of Unactivated Fluoroarenes Enabled by Organic Photoredox Catalysis

Nucleophilic aromatic substitution (SNAr) is a classical reaction with well-known reactivity toward electron-poor fluoroarenes. However, electron-neutral and electron-rich fluoro(hetero)arenes are considerably underrepresented. Herein, we present a method for the nucleophilic defluorination of unactivated fluoroarenes enabled by cation radical-accelerated nucleophilic aromatic substitution. The use of organic photoredox catalysis renders this method operationally simple under mild conditions and is amenable to various nucleophile classes, including azoles, amines, and carboxylic acids. Select fluorinated heterocycles can be functionalized using this method. In addition, the late-stage functionalization of pharmaceuticals is also presented. Computational studies demonstrate that the site selectivity of the reaction is dictated by arene electronics.

Regioselective, Photocatalytic α-Functionalization of Amines

Photocatalytic α-functionalization of amines provides a mild and atom-economical means to synthesize α-branched amines. Prior examples featured symmetrical or electronically biased substrates. Here we report a controllable α-functionalization of amines in which regioselectivity can be tuned with minor changes to the reaction conditions.

Recent progress toward the asymmetric synthesis of carbon-substituted piperazine pharmacophores and oxidative related heterocycles

The piperazine drugs are mostly N-substituted compared to only a few C-substituted drugs. To explore this unknown chemical space, asymmetric syntheses of C-substituted piperazines is the subject of this review.

The important requirement for approval of a new drug, in case it happens to be chiral, is that both enantiomers of the drug should be studied in detail, which has led synthetic organic and medicinal chemists to focus their attention on the development of new methods for asymmetric synthesis especially of relevant saturated N-heterocycles. On the other hand, the piperazine ring, besides defining a major class of saturated N-heterocycles, has been classified as a privileged structure in medicinal chemistry, since it is more than frequently found in biologically active compounds including several marketed blockbuster drugs such as Glivec (imatinib) and Viagra (sildenafil). Indeed, 13 of the 200 best-selling small molecule drugs in 2012 contained a piperazine ring. Nevertheless, analysis of the piperazine substitution pattern reveals a lack of structural diversity, with almost every single drug in this category (83%) containing a substituent at both the N1- and N4-positions compared to a few drugs having a substituent at any other position (C2, C3, C5, and C6). Significant chemical space that is closely related to that known to be biologically relevant, therefore, remains unexplored. In order to explore this chemical space, efficient and asymmetric syntheses of carbon-substituted piperazines and related heterocycles must be designed and developed. Initial, recent efforts toward the implementation of this particular target are in fact the subject of this review.