Profiling and Application of Photoredox C(sp3)-C(sp2) Cross-Coupling in Medicinal Chemistry

Hydrophobic Surfactant-DNA Complex (Surf-DNA) Enables DNA-Encoded-Library-Compatible Decarboxylative Arylation under Anhydrous Conditions

DNA-encoded libraries (DELs) are a key technology for identifying small-molecule hits in both the pharmaceutical industry and academia, but their chemical diversity is largely limited to water-compatible reactions to aid in the solubility and integrity of encoding DNA tags. To broaden the DEL chemical space, we present a workflow utilizing DNA-cationic surfactant complexation that enables dissolution and reactions on-DNA in anhydrous organic solvents. We demonstrate its utility by developing DEL-compatible photoredox decarboxylative C(sp2)-C(sp3) coupling under water-free conditions. The workflow is optimized for the 96-well format necessary for large-scale DEL productions, and it enables screening and optimization of DEL-compatible reactions in organic solvents.

Light-Driven Radiochemistry with Fluorine-18, Carbon-11 and Zirconium-89

This review discusses recent advances in light-driven radiochemistry for three key isotopes: fluorine-18, carbon-11, and zirconium-89, and their applications in positron emission tomography (PET). In the case of fluorine-18, the predominant approach involves the use of cyclotron-produced [18F]fluoride or reagents derived thereof. Light serves to activate either the substrate or the fluorine-18 labeled reagent. Advancements in carbon-11 photo-mediated radiochemistry have been leveraged for the radiolabeling of small molecules, achieving various transformations, including 11C-methylation, 11C-carboxylation, 11C-carbonylation, and 11C-cyanation. Contrastingly, zirconium-89 photo-mediated radiochemistry differs from fluorine-18 and carbon-11 approaches. In these cases, light facilitates a postlabeling click reaction, which has proven valuable for the labeling of large biomolecules such as monoclonal antibodies (mAbs). New technological developments, such as the incorporation of photoreactors in commercial radiosynthesizers, illustrate the commitment the field is making in embracing photochemistry. Taken together, these advances in photo-mediated radiochemistry enable radiochemists to apply new retrosynthetic strategies in accessing novel PET radiotracers.

Utilization of High-Throughput Experimentation (HTE) and ChemBeads Toward the Development of an Aryl Bromide and Benzyl Bromide Photoredox Cross-Electrophile Coupling

The discussion herein describes a metallaphotoredox reaction that allows for efficient exploration of benzyl structure-activity relationships in medicinal chemistry. The use of HTE (high-throughput experimentation) and ChemBeads allows for rapid reaction optimization. The formation of di(hetero)arylmethanes via cross-electrophile coupling between aryl bromides and benzyl bromides provides access to diverse chemical space. The breadth of the substrate scope will be discussed, along with the utilization of batch photochemistry for the preparation of this di(hetero)arylmethane motif on a larger scale.

At the Speed of Light: The Systematic Implementation of Photoredox Cross-Coupling Reactions for Medicinal Chemistry Research

The adoption of new and emerging techniques in organic synthesis is essential to promote innovation in drug discovery. In this Perspective, we detail the strategy we used for the systematic deployment of photoredox-mediated, metal-catalyzed cross-coupling reactions in AbbVie's medicinal chemistry organization, focusing on topics such as assessment, evaluation, implementation, and accessibility. The comprehensive evaluation of photoredox reaction setups and published methods will be discussed, along with internal efforts to build expertise and photoredox high-throughput experimentation capabilities. We also highlight AbbVie's academic-industry collaborations in this field that have been leveraged to develop new synthetic strategies, along with discussing the internal adoption of photoredox cross-coupling reactions. The work described herein has culminated in robust photocatalysis and cross-coupling capabilities which are viewed as key platforms for medicinal chemistry research at AbbVie.

MicroCycle: An Integrated and Automated Platform to Accelerate Drug Discovery

We herein describe the development and application of a modular technology platform which incorporates recent advances in plate-based microscale chemistry, automated purification, in situ quantification, and robotic liquid handling to enable rapid access to high-quality chemical matter already formatted for assays. In using microscale chemistry and thus consuming minimal chemical matter, the platform is not only efficient but also follows green chemistry principles. By reorienting existing high-throughput assay technology, the platform can generate a full package of relevant data on each set of compounds in every learning cycle. The multiparameter exploration of chemical and property space is hereby driven by active learning models. The enhanced compound optimization process is generating knowledge for drug discovery projects in a time frame never before possible.

Structural Modification of Noscapine via Photoredox/Nickel Dual Catalysis for the Discovery of S-Phase Arresting Agents

Herein, we disclose a powerful strategy for the functionalization of the antitumor natural alkaloid noscapine by utilizing photoredox/nickel dual-catalytic coupling technology. A small collection of 37 new noscapinoids with diverse (hetero)alkyl and (hetero)cycloalkyl groups and enhanced sp3 character was thus synthesized. Further in vitro antiproliferative activity screening and SAR study enabled the identification of 6o as a novel, potent, and less-toxic anticancer agent. Furthermore, 6o exerts superior cellular activity via an unexpected S-phase arrest mechanism and could significantly induce cell apoptosis in a dose-dependent manner, thereby further highlighting its potential in drug discovery as a promising lead compound.

Nickel-Catalyzed Electroreductive Coupling of Alkylpyridinium Salts and Aryl Halides

An electrochemical, nickel-catalyzed reductive coupling of alkylpyridinium salts and aryl halides is reported. High-throughput experimentation (HTE) was employed for rapid reaction optimization and evaluation of a broad scope of pharmaceutically relevant structurally diverse aryl halides, including complex drug-like substrates. In addition, the transformation is compatible with both primary and secondary alkylpyridinium salts with distinct conditions. Mechanistic insights were critical to enhance the efficiency of coupling using secondary alkylpyridinium salts. Systematic comparisons of the electrochemical and non-electrochemical methods revealed the complementary scope and efficiency of the two approaches.

Enabling Deoxygenative C(sp2)-C(sp3) Cross-Coupling for Parallel Medicinal Chemistry

Herein we report the development of an automated deoxygenative C(sp²)-C(sp³) coupling of aryl bromide with alcohols to enable parallel medicinal chemistry. Alcohols are among the most diverse and abundant building blocks, but their usage as alkyl precursors has been limited. Although metallaphotoredox deoxygenative coupling is becoming a promising strategy to form C(sp²)-C(sp³) bond, the reaction setup limits its widespread application in library synthesis. To achieve high throughput and consistency, an automated workflow involving solid-dosing and liquid-handling robots has been developed. We have successfully demonstrated this high-throughput protocol is robust and consistent across three automation platforms. Furthermore, guided by cheminformatic analysis, we examined alcohols with comprehensive chemical space coverage and established a meaningful scope for medicinal chemistry applications. By accessing the rich diversity of alcohols, this automated protocol has the potential to substantially increase the impact of C(sp²)-C(sp³) cross-coupling in drug discovery.

Formal Cross-Coupling of Amines and Carboxylic Acids to Form sp3-sp2 Carbon-Carbon Bonds

Amines and carboxylic acids are abundant synthetic building blocks that are classically united to form an amide bond. To access new pockets of chemical space, we are interested in the development of amine-acid coupling reactions that complement the amide coupling. In particular, the formation of carbon-carbon bonds by formal deamination and decarboxylation would be an impactful addition to the synthesis toolbox. Here, we report a formal cross-coupling of alkyl amines and aryl carboxylic acids to form C(sp³)-C(sp²) bonds following preactivation of the amine-acid building blocks as a pyridinium salt and N-acyl-glutarimide, respectively. Under nickel-catalyzed reductive cross-coupling conditions, a diversity of simple and complex substrates are united in good to excellent yield, and numerous pharmaceuticals are successfully diversified. High-throughput experimentation was leveraged in the development of the reaction and the discovery of performance-enhancing additives such as phthalimide, RuCl₃, and GaCl₃. Mechanistic investigations suggest phthalimide may play a role in stabilizing productive Ni complexes rather than being involved in oxidative addition of the N-acyl-imide and that RuCl₃ supports the decarbonylation event, thereby improving reaction selectivity.

ChemBeads-Enabled Photoredox High-Throughput Experimentation Platform to Improve C(sp2)-C(sp3) Decarboxylative Couplings

Enthusiasm surrounding nickel/photoredox C(sp2)-C(sp3) cross-couplings is very high; however, these methods are sometimes challenged by complex drug-like substrates in discovery chemistry. In our hands this has been especially true of the decarboxylative coupling, which has lagged behind other photoredox couplings in internal adoption and success. Herein, the development of a photoredox high-throughput experimentation platform to optimize challenging C(sp2)-C(sp3) decarboxylative couplings is described. Chemical-coated glass beads (ChemBeads) and a novel parallel bead dispenser are used to expedite the high-throughput experimentation process and identify improved coupling conditions. In this report, photoredox high-throughput experimentation is utilized to dramatically improve low-yielding decarboxylative C(sp2)-C(sp3) couplings, and libraries, using conditions not previously identified in the literature.

Picolinamide Ligands: Nickel-Catalyzed Reductive Cross-Coupling of Aryl Bromides with Bromocyclopropane and Beyond

The arylcyclopropane motif as the combination of aryl and cyclopropyl ring systems can be found in an increasing amount of approved and investigational drugs. Herein, we have developed a mild, efficient nickel-catalyzed reductive cross-coupling protocol, featuring a simple Ni(II) precatalyst and a novel picolinamide NN₂ pincer ligand. A variety of (hetero)aryl bromides could successfully couple with cyclopropyl bromide to furnish the valued arylcyclopropanes in good to excellent yields. This method is applicable to other alkyl bromides as well. Notably, the reaction is tolerant of a broad range of functionalities including free amines. Furthermore, the synthesis of several significant intermediates of bioactive molecules was achieved in grams, proving the practicability of this method.

Enantioselective Synthesis of N-Benzylic Heterocycles by Ni/Photoredox Dual Catalysis

An asymmetric cross-coupling of α-N-heterocyclic trifluoroborates with aryl bromides using Ni/photoredox dual catalysis has been developed. This C(sp2)-C(sp3) cross-coupling provides access to pharmaceutically relevant chiral N-benzylic heterocycles in good to excellent enantioselectivity when bioxazolines (BiOX) are used as the chiral ligand. High-throughput experimentation significantly streamlined reaction development by identifying BiOX ligands for further investigation and by allowing for rapid optimization of conditions for new trifluoroborate salts.

ART─An Amino Radical Transfer Strategy for C(sp2)-C(sp3) Coupling Reactions, Enabled by Dual Photo/Nickel Catalysis

Introducing the novel concept of amino radical transfer (ART) enables the use of easily accessible and commercially available alkyl boronic esters as cross-coupling partners for aryl halides in dual photoredox/nickel catalysis mediated by visible light. Activation of otherwise photochemically innocent boronic esters by radicals generated from primary or secondary alkylamines gives rise to an outstanding functional group tolerance in a mild, fast, and air-stable reaction. As shown in more than 50 examples including unprotected alcohols, amines, and carboxylic acids, this reaction allows quick access to relevant scaffolds for organic synthesis and medicinal chemistry. In comparison with existing methods for C(sp²)-C(sp³) couplings an extraordinary generality could be realized via the ART concept, employing a single set of optimized reaction conditions. Due to its selectivity, the transformation can also be used for late-stage functionalization, as demonstrated with three exemplary syntheses of drug molecules. Furthermore, the successful one-to-one scalability of this reaction up to gram scale without the necessity of any further precautions or flow systems is demonstrated.

Elucidating the Mechanism of Excited-State Bond Homolysis in Nickel-Bipyridine Photoredox Catalysts

Ni 2,2'-bipyridine (bpy) complexes are commonly employed photoredox catalysts of bond-forming reactions in organic chemistry. However, the mechanisms by which they operate are still under investigation. One potential mode of catalysis is via entry into Ni(I)/Ni(III) cycles, which can be made possible by light-induced, excited-state Ni(II)-C bond homolysis. Here, we report experimental and computational analyses of a library of Ni(II)-bpy aryl halide complexes, Ni(^Rbpy)(^R'Ph)Cl (R = MeO, t-Bu, H, MeOOC; R' = CH₃, H, OMe, F, CF₃), to illuminate the mechanism of excited-state bond homolysis. At given excitation wavelengths, photochemical homolysis rate constants span 2 orders of magnitude across these structures and correlate linearly with Hammett parameters of both bpy and aryl ligands, reflecting structural control over key metal-to-ligand charge-transfer (MLCT) and ligand-to-metal charge-transfer (LMCT) excited-state potential energy surfaces (PESs). Temperature- and wavelength-dependent investigations reveal moderate excited-state barriers (ΔH^‡ ∼ 4 kcal mol^-1) and a minimum energy excitation threshold (∼55 kcal mol^-1, 525 nm), respectively. Correlations to electronic structure calculations further support a mechanism in which repulsive triplet excited-state PESs featuring a critical aryl-to-Ni LMCT lead to bond rupture. Structural control over excited-state PESs provides a rational approach to utilize photonic energy and leverage excited-state bond homolysis processes in synthetic chemistry.

A compact, practical photoreactor for multi-reaction arrays

We report here the BMS-PR460: a photoreactor designed for multi-reaction arrays at various scales and temperatures with irradiation at 460 nm. Transformations of interest to pharmaceutical research have been performed to demonstrate reactor utility.

Au–Pd@ZnO alloy nanoparticles: a promising heterogeneous photocatalyst toward decarboxylative trifluoromethylation under visible-light irradiation

Au–Pd@ZnO alloy nanoparticles have promising potential as heterogeneous photocatalysts for decarboxylative trifluoromethylation reactions under visible light irradiation.

Development of a high intensity parallel photoreactor for high throughput screening

Enhanced reactivity of decarboxylative cross-coupling using a new parallel photoreactor.

Tunable and Practical Homogeneous Organic Reductants for Cross-Electrophile Coupling

The syntheses of four new tunable homogeneous organic reductants based on a tetraaminoethylene scaffold are reported. The new reductants have enhanced air stability compared to current homogeneous reductants for metal-mediated reductive transformations, such as cross-electrophile coupling (XEC), and are solids at room temperature. In particular, the weakest reductant is indefinitely stable in air and has a reduction potential of -0.85 V versus ferrocene, which is significantly milder than conventional reductants used in XEC. All of the new reductants can facilitate C(sp²)-C(sp³) Ni-catalyzed XEC reactions and are compatible with complex substrates that are relevant to medicinal chemistry. The reductants span a range of nearly 0.5 V in reduction potential, which allows for control over the rate of electron transfer events in XEC. Specifically, we report a new strategy for controlled alkyl radical generation in Ni-catalyzed C(sp²)-C(sp³) XEC. The key to our approach is to tune the rate of alkyl radical generation from Katritzky salts, which liberate alkyl radicals upon single electron reduction, by varying the redox potentials of the reductant and Katritzky salt utilized in catalysis. Using our method, we perform XEC reactions between benzylic Katritzky salts and aryl halides. The method tolerates a variety of functional groups, some of which are particularly challenging for most XEC transformations. Overall, we expect that our new reductants will both replace conventional homogeneous reductants in current reductive transformations due to their stability and relatively facile synthesis and lead to the development of novel synthetic methods due to their tunability.

Applications of Halogen-Atom Transfer (XAT) for the Generation of Carbon Radicals in Synthetic Photochemistry and Photocatalysis

The halogen-atom transfer (XAT) is one of the most important and applied processes for the generation of carbon radicals in synthetic chemistry. In this review, we summarize and highlight the most important aspects associated with XAT and the impact it has had on photochemistry and photocatalysis. The organization of the material starts with the analysis of the most important mechanistic aspects and then follows a subdivision based on the nature of the reagents used in the halogen abstraction. This review aims to provide a general overview of the fundamental concepts and main agents involved in XAT processes with the objective of offering a tool to understand and facilitate the development of new synthetic radical strategies.

Enabling Metallophotoredox Catalysis in Parallel Solution-Phase Synthesis Using Disintegrating Reagent Tablets

Compressed tablets containing a mixture of a photocatalyst, a nickel catalyst, an inorganic base, and an inert excipient are employed as a fast, safe, and user-friendly chemical delivery system for two different metallophotoredox-catalyzed reactions. This delivery method simplifies the preparation of compound libraries using photoredox chemistry in a parallel setting. The reagent tablets were successfully applied to late-stage functionalization of drug-like intermediates. These tablets can be prepared with various reagents and catalysts in different sizes and be stored on the bench thanks to blister packaging.

The Chosen Few: Parallel Library Reaction Methodologies for Drug Discovery

Parallel library synthesis is an important tool for drug discovery because it enables the synthesis of closely related analogues in parallel via robust and general synthetic transformations. In this perspective, we analyzed the synthetic methodologies used in >5000 parallel libraries representing 15 prevalent synthetic transformations. The library data set contains complex substrates and diverse arrays of building blocks used over the last 14 years at AbbVie. The library synthetic methodologies that have demonstrated robustness and generality with proven success are described along with their substrate scopes. The evolution of the synthetic methodologies for library synthesis over the past decade is discussed. We also highlight that the combination of parallel library synthesis with high-throughput experimentation will continue to facilitate the discovery of library-amenable synthetic methodologies in drug discovery.

Saturated Boronic Acids, Boronates, and Trifluoroborates: An Update on Their Synthetic and Medicinal Chemistry

This review discusses recent advances in the chemistry of saturated boronic acids, boronates, and trifluoroborates. Applications of the title compounds in the design of boron-containing drugs are surveyed, with special emphasis on α-amino boronic derivatives. A general overview of saturated boronic compounds as modern tools to construct C(sp³ )-C and C(sp³ )-heteroatom bonds is given, including recent developments in the Suzuki-Miyaura and Chan-Lam cross-couplings, single-electron-transfer processes including metallo- and organocatalytic photoredox reactions, and transformations of boron "ate" complexes. Finally, an attempt to summarize the current state of the art in the synthesis of saturated boronic acids, boronates, and trifluoroborates is made, with a brief mention of the "classical" methods (transmetallation of organolithium/magnesium reagents with boron species, anti-Markovnikov hydroboration of alkenes, and the modification of alkenyl boron compounds) and a special focus on recent methodologies (boronation of alkyl (pseudo)halides, derivatives of carboxylic acids, alcohols, and primary amines, boronative C-H activation, novel approaches to alkene hydroboration, and 1,2-metallate-type rearrangements).

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.

Metallaphotoredox-enabled deoxygenative arylation of alcohols

Metal-catalysed cross-couplings are a mainstay of organic synthesis and are widely used for the formation of C-C bonds, particularly in the production of unsaturated scaffolds¹. However, alkyl cross-couplings using native sp³-hybridized functional groups such as alcohols remain relatively underdeveloped². In particular, a robust and general method for the direct deoxygenative coupling of alcohols would have major implications for the field of organic synthesis. A general method for the direct deoxygenative cross-coupling of free alcohols must overcome several challenges, most notably the in situ cleavage of strong C-O bonds³, but would allow access to the vast collection of commercially available, structurally diverse alcohols as coupling partners⁴. We report herein a metallaphotoredox-based cross-coupling platform in which free alcohols are activated in situ by N-heterocyclic carbene salts for carbon-carbon bond formation with aryl halide coupling partners. This method is mild, robust, selective and most importantly, capable of accommodating a wide range of primary, secondary and tertiary alcohols as well as pharmaceutically relevant aryl and heteroaryl bromides and chlorides. The power of the transformation has been demonstrated in a number of complex settings, including the late-stage functionalization of Taxol and a modular synthesis of Januvia, an antidiabetic medication. This technology represents a general strategy for the merger of in situ alcohol activation with transition metal catalysis.

The Application of Pulse Radiolysis to the Study of Ni(I) Intermediates in Ni-Catalyzed Cross-Coupling Reactions

Here we report the use of pulse radiolysis and spectroelectrochemistry to generate low-valent nickel intermediates relevant to synthetically important Ni-catalyzed cross-coupling reactions and interrogate their reactivities toward comproportionation and oxidative addition processes. Pulse radiolysis provided a direct means to generate singly reduced [(dtbbpy)NiBr], enabling the identification of a rapid Ni(0)/Ni(II) comproportionation process taking place under synthetically relevant electrolysis conditions. This approach also permitted the direct measurement of Ni(I) oxidative addition rates with electronically differentiated aryl iodide electrophiles (k_OA = 1.3 × 10⁴-2.4 × 10⁵ M^-1 s^-1), an elementary organometallic step often proposed in nickel-catalyzed cross-coupling reactions. Together, these results hold implications for a number of Ni-catalyzed cross-coupling processes.

Focused Libraries for Epigenetic Drug Discovery: The Importance of Isosteres

Epigenetic drug discovery provides a wealth of opportunities for the discovery of new therapeutics but has been hampered by low hit rates, frequent identification of false-positives, and poor synthetic tractability. A key reason for this is that few screening collections consider the unique requirements of epigenetic targets despite significant medicinal chemistry interest. Here we analyze the suitability of some commercially available screening collections in the context of epigenetic drug discovery, with a particular focus on lysine post-translational modifications, and show that even privileged motifs found in U.S. Food and Drug Administration (FDA)-approved drugs are not present in these collections. We propose that the incorporation of epigenetic bioisosteres should become central in the design of new focused screening collections and highlight some opportunities for the development of synthetic methods which may improve the tractability of hit molecules.

Chemistry Informer Libraries: Conception, Early Experience, and Role in the Future of Cheminformatics

The synthetic chemistry literature traditionally reports the scope of new methods using simple, nonstandardized test molecules that have uncertain relevance in applied synthesis. In addition, published examples heavily favor positive reaction outcomes, and failure is rarely documented. In this environment, synthetic practitioners have inadequate information to know whether any given method is suitable for the task at hand. Moreover, the incomplete nature of published data makes it poorly suited for the creation of predictive reactivity models via machine learning approaches. In 2016, we reported the concept of chemistry informer libraries as standardized sets of medium- to high-complexity substrates with relevance to pharmaceutical synthesis as demonstrated using a multidimensional principle component analysis (PCA) comparison to the physicochemical properties of marketed drugs. We showed how informer libraries could be used to evaluate leading synthetic methods with the complete capture of success and failure and how this knowledge could lead to improved reaction conditions with a broader scope with respect to relevant applications. In this Account, we describe the progress made and lessons learned in subsequent studies using informer libraries to profile eight additional reaction classes. Examining broad trends across multiple types of bond disconnections against a standardized chemistry "measuring stick" has enabled comparisons of the relative potential of different methods for applications in complex synthesis and has identified opportunities for further development. Furthermore, the powerful combination of informer libraries and 1536-well-plate nanoscale reaction screening has allowed the parallel evaluation of scores of synthetic methods in the same experiment and as such illuminated an important role for informers as part of a larger data generation workflow for predictive reactivity modeling. Using informer libraries as problem-dense, strong filters has allowed broad sets of reaction conditions to be narrowed down to those that display the highest tolerance to complex substrates. These best conditions can then be used to survey broad swaths of substrate space using nanoscale chemistry approaches. Our experiences and those of our collaborators from several academic laboratories applying informer libraries in these contexts have helped us identify several areas for potential improvements to the approach that would increase their ease of use, utility in generating interpretable results, and resulting uptake by the broader community. As we continue to evolve the informer library concept, we believe it will play an ever-increasing role in the future of the democratization of high-throughput experimentation and data science-driven synthetic method development.

Reagent and Catalyst Capsules: A Chemical Delivery System for Reaction Screening and Parallel Synthesis

Commercially available hydroxypropyl methylcellulose capsules are employed as a fast, safe, and user-friendly chemical delivery system containing all reagents (catalyst, ligand, and base) for three important transition-metal-catalyzed reactions: Buchwald-Hartwig, Suzuki-Miyaura, and metallophotoredox C-N cross-coupling reactions. This encapsulation methodology simplifies the screening of reaction conditions and the preparation of compound libraries using parallel synthesis in organic solvents or aqueous media. These reagents-containing HPMC capsules are easy to prepare, come in different sizes, and can be stored on the bench under noninert conditions.

Increasing saturation: development of broadly applicable photocatalytic Csp2–Csp3 cross-couplings of alkyl trifluoroborates and (hetero)aryl bromides for array synthesis

HTE was used to systematically investigate the reaction of alkyl trifluoroborates and aryl bromides under photocatalytic conditions. General conditions were identified for coupling of activated primary, benzylic and secondary alkyl trifluoroborates.

Cross-Electrophile Coupling of Unactivated Alkyl Chlorides

Alkyl chlorides are bench-stable chemical feedstocks that remain among the most underutilized electrophile classes in transition metal catalysis. Overcoming intrinsic limitations of C(sp³)-Cl bond activation, we report the development of a novel organosilane reagent that can participate in chlorine atom abstraction under mild photocatalytic conditions. In particular, we describe the application of this mechanism to a dual nickel/photoredox catalytic protocol that enables the first cross-electrophile coupling of unactivated alkyl chlorides and aryl chlorides. Employing these low-toxicity, abundant, and commercially available organochloride building blocks, this methodology allows access to a broad array of highly functionalized C(sp²)-C(sp³) coupled adducts, including numerous drug analogues.

Regioselective Cross-Electrophile Coupling of Epoxides and (Hetero)aryl Iodides via Ni/Ti/Photoredox Catalysis

A cross-electrophile coupling reaction of epoxides and (hetero)aryl iodides that operates via the merger of three catalytic cycles involving a Ni-, Ti-, and organic photoredox catalyst has been developed. Three distinct classes of epoxides, styrene oxides, cyclic epoxides, and terminal aliphatic epoxides, all undergo coupling in moderate to good yield and high regioselectivity with the use of three different nitrogen-based ligands for Ni under otherwise identical reaction conditions. The mild reaction conditions accommodate a broad scope of abundant and complex coupling partners. Mechanistic studies suggest that when styrene oxides are employed radical intermediates are involved via Ti-radical ring-opening of the epoxide. Conversely, for terminal aliphatic epoxides, involvement of an iodohydrin intermediate enables the formation of the unexpected linear product.

Expanding the Medicinal Chemist Toolbox: Comparing Seven C(sp2)-C(sp3) Cross-Coupling Methods by Library Synthesis

Despite recent advances in the field of C(sp²)-C(sp³) cross-couplings and the accompanying increase in publications, it can be hard to determine which method is appropriate for a given reaction when using the highly functionalized intermediates prevalent in medicinal chemistry. Thus a study was done comparing the ability of seven methods to directly install a diverse set of alkyl groups on "drug-like" aryl structures via parallel library synthesis. Each method showed substrates that it excelled at coupling compared with the other methods. When analyzing the reactions run across all of the methods, a reaction success rate of 50% was achieved. Whereas this is promising, there are still gaps in the scope of direct C(sp²)-C(sp³) coupling methods, like tertiary group installation. The results reported herein should be used to inform future syntheses, assess reaction scope, and encourage medicinal chemists to expand their synthetic toolbox.

The Symbiotic Relationship Between Drug Discovery and Organic Chemistry

All pharmaceutical products contain organic molecules; the source may be a natural product or a fully synthetic molecule, or a combination of both. Thus, it follows that organic chemistry underpins both existing and upcoming pharmaceutical products. The reverse relationship has also affected organic synthesis, changing its landscape towards increasingly complex targets. This Review article sets out to give a concise appraisal of this symbiotic relationship between organic chemistry and drug discovery, along with a discussion of the design concepts and highlighting key milestones along the journey. In particular, criteria for a high-quality compound library design enabling efficient virtual navigation of chemical space, as well as rise and fall of concepts for its synthetic exploration (such as combinatorial chemistry; diversity-, biology-, lead-, or fragment-oriented syntheses; and DNA-encoded libraries) are critically surveyed.

Photoredox Radical/Polar Crossover Enables Construction of Saturated Nitrogen Heterocycles

Photoredox-mediated radical/polar crossover (RPC) processes offer new avenues for the synthesis of cyclic molecules. This process has been realized for the construction of medium-sized saturated nitrogen heterocycles. Photocatalytically generated alkyl radicals possessing pendant leaving groups engage imines in C-C bond formation, and subsequent reduction of the intermediate nitrogen-centered radical triggers anionic ring closure. With the aid of visible light irradiation, substituted pyrrolidines, piperidines, and azepanes can be prepared under mild, redox-neutral conditions.

Alkyl Carbon-Carbon Bond Formation by Nickel/Photoredox Cross-Coupling

The union of photoredox and nickel catalysis has resulted in a renaissance in radical chemistry as well as in the use of nickel-catalyzed transformations, specifically for carbon-carbon bond formation. Collectively, these advances address the longstanding challenge of late-stage cross-coupling of functionalized alkyl fragments. Empowered by the notion that photocatalytically generated alkyl radicals readily undergo capture by Ni complexes, wholly new feedstocks for cross-coupling have been realized. Herein, we highlight recent developments in several types of alkyl cross-couplings that are accessible exclusively through this approach.