Discovery of an α-amino C-H arylation reaction using the strategy of accelerated serendipity

Bifunctional Imine Reductase Cascades for the Synthesis of Saturated N-Heterocycles

Saturated N-heterocycles constitute a vital scaffold for pharmaceutical chemistry but are challenging to access synthetically, particularly asymmetrically. Here, we demonstrate how imine reductases can achieve annulation through tandem inter- and intramolecular reductive amination processes. Imine reductases were used in combination with further enzymes to access unsubstituted, α-substituted, and α,α'-disubstituted N-heterocycles from simple starting materials in one pot and under benign conditions. This work shows the remarkable flexibility of these enzymes to have broad activity against numerous substrates derived from singlular starting materials.

Photoredox Product Selectivity Controlled by Persistent Radical Stability

The use of photoredox catalysis for the synthesis of small organic molecules relies on harnessing and converting the energy in visible light to drive reactions. Specifically, photon energy is used to generate radical ion species that can be harnessed through subsequent reaction steps to form a desired product. Cyanoarenes are widely used as arylating agents in photoredox catalysis because of their stability as persistent radical anions. However, there are marked, unexplained variations in product yields when using different cyanoarenes. In this study, the quantum yield and product yield of an α-aminoarylation photoredox reaction between five cyanoarene coupling partners and N-phenylpyrrolidine were characterized. Significant discrepancies in cyanoarene consumption and product yield suggested a chemically irreversible, unproductive pathway in the reaction. Analysis of the side products in the reaction demonstrated the formation of species consistent with radical anion fragmentation. Electrochemical and computational methods were used to study the fragmentation of the different cyanoarenes and revealed a correlation between product yield and cyanoarene radical anion stability. Kinetic modeling of the reaction demonstrates that cross-coupling selectivity between N-phenylpyrrolidine and the cyanoarene is controlled by the same phenomenon present in the persistent radical effect.

Radical Polarity

The polarity of a radical intermediate profoundly impacts its reactivity and selectivity. To quantify this influence and predict its effects, the electrophilicity/nucleophilicity of >500 radicals has been calculated. This database of open-shell species entails frequently encountered synthetic intermediates, including radicals centered at sp3, sp2, and sp hybridized carbon atoms or various heteroatoms (O, N, S, P, B, Si, X). Importantly, these computationally determined polarities have been experimentally validated for electronically diverse sets of >50 C-centered radicals, as well as N- and O- centered radicals. High correlations are measured between calculated polarity and quantified reactivity, as well as within parallel sets of competition experiments (across different radical types and reaction classes). These multipronged analyses show a strong relationship between the computed electrophilicity, ω, of a radical and its relative reactivity (krel vs Δω slopes up to 40; showing mere Δω of 0.1 eV affords up to 4-fold rate enhancement). We expect this experimentally validated database will enable reactivity and selectivity prediction (by harnessing polarity-matched rate enhancement) and assist with troubleshooting in synthetic reaction development.

Mechanism-Based Regiocontrol in SNAr: A Case Study of Ortho-Selective Etherification with Chloromagnesium Alkoxides

Although nucleophilic aromatic substitution (SNAr) is routinely employed as a practical alternative to transition-metal-catalyzed cross-coupling, the mechanistic basis for reactivity and regioselectivity remains underexplored and is an active area of research. This article reports a SNAr-based etherification of 2,4-difluoroarylcarboxamides as a model system and shows that the ortho/para regioselectivity spans >500:1 to 1:∼20 simply by varying the reaction conditions via high-throughput experimentation (HTE). An in-depth characterization of the ortho-selective lead conditions is presented, and these insights are used to build a reactivity model that self-consistently accounts for the regioselectivity and reaction scope. This article discusses synthetic implications of condition-dependent magnesium coordination and Schlenk equilibria and demonstrates that consideration of molecular-level stoichiometry and isomerism is an essential prerequisite for rationalizing reactivity and regioselectivity in SNAr.

Silyl Radicals as Single-Electron Reductants: α-Aminoalkyl Radical Formation via a Photocatalytic Oxidatively Initiated Radical Chain Process

The α-amino-radical constitutes a versatile reactive intermediate that has been used to great effect in the synthesis of complex amine-containing products. Here, we report the development of a multicomponent photocatalytic platform enabling access to all-alkyl α-amino-radicals, exploiting the oxidative formation of silyl-radicals from commercially available tris(trimethylsilyl)silane. A key design element of the new process involves the role of silyl-radicals in generating α-amino-radicals from iminium ions as part of an oxidatively initiated photocatalytic radical chain process. This distinct activation mode is showcased by engaging the ensuing radicals in cross-radical coupling with persistent arene radical anions, enabling the arylation of in situ-generated all-alkyl iminium ions to furnish alkyl-substituted benzylamines.

Synthesis of Diarylalkanes by Photoreductive 1,2-Diarylation of Alkenes with Aryl Halides and Cyanoaromatics

We report a visible light-induced photoreductive strategy for three-component diarylation of alkenes with aryl halides and cyanoaromatics. Upon photoredox catalysis and with tertiary alkyl amines as the electron transfer agent, aryl halides selectively undergo halogen atom transfer to generate the aryl radicals and two C(sp2)-C(sp3) bonds between the cabron atoms are created in a radical addition and radical-radical coupling fashion to rapidly assemble diverse functionalized polyarylalkanes with high regio- and chemoselectivity. This method can be applied to broad feedstocks, including terminal alkenes, internal alkenes, aryl iodides, aryl bromides, aryl chlorides, electron-deficient benzonitriles, and isonicotinonitriles.

CdS Quantum Dot Gels as a Direct Hydrogen Atom Transfer Photocatalyst for C-H Activation

Here, we report CdS quantum dot (QD) gels, a three-dimensional network of interconnected CdS QDs, as a new type of direct hydrogen atom transfer (d-HAT) photocatalyst for C-H activation. We discovered that the photoexcited CdS QD gel could generate various neutral radicals, including α-amido, heterocyclic, acyl, and benzylic radicals, from their corresponding stable molecular substrates, including amides, thio/ethers, aldehydes, and benzylic compounds. Its C-H activation ability imparts a broad substrate and reaction scope. The mechanistic study reveals that this reactivity is intrinsic to CdS materials, and the neutral radical generation did not proceed via the conventional sequential electron transfer and proton transfer pathway. Instead, the C-H bonds are activated by the photoexcited CdS QD gel via a d-HAT mechanism. This d-HAT mechanism is supported by the linear correlation between the logarithm of the C-H bond activation rate constant and the C-H bond dissociation energy (BDE) with a Brønsted slope α=0.5. Our findings expand the currently limited direct hydrogen atom transfer photocatalysis toolbox and provide new possibilities for photocatalytic C-H activation.

Carbonylation Reactions at Carbon-Centered Radicals with an Adjacent Heteroatom

Heteroatoms are essential to living organisms and present in almost all molecules with medicinal usage. The catalytic functionalization at the carbon-centered radical with an adjacent heteroatom provides an effective way to value added moiety while retaining the unique physicochemical and pharmacological properties of heteroatoms, which can promote the development of pharmaceutical and fine chemical production. Carbonylative transformation was discovered nearly a century ago which is an efficient method for the synthesis of carbonyl-containing molecules with potent applications in both industry and academia. Despite numerous advances in new reaction development, carbonylative transformation involving adjacent heteroatom carbon radical remain a subject that deserves to be discussed. In this minireview, we systematically summarized and discussed the recent advances in carbonylative transformations involving carbon-centered radicals with an adjacent heteroatom, including oxygen (O), nitrogen (N), phosphorus (P), silicon (Si), sulfur (S), boron (B), fluorine (F), and chlorine (Cl). The related reaction mechanism was also discussed.

Exploring the Potential of Al(III) Photosensitizers for Energy Transfer Reactions

Three homoleptic Al(III) complexes (Al1-Al3) with different degrees of methylation at the 2-pyridylpyrrolide ligand were systematically tested for their function as photosensitizers (PS) in two types of energy transfer reactions. First, in the generation of reactive singlet oxygen (1O2), and second, in the isomerization of (E)- to (Z)-stilbene. 1O2 was directly evidenced by its characteristic NIR emission at around 1276 nm and indirectly by the reaction with an organic substrate [e.g. 2,5-diphenylfuran (DPF)] using in situ UV/vis spectroscopy. In a previous study, the presence of additional methyl groups was found to be beneficial for the photocatalytic reduction of CO2 to CO, but here Al1 without any methyl groups exhibits superior performance. To rationalize this behavior, a combination of photophysical experiments (absorption, emission and excited state lifetimes) together with photostability measurements and scalar-relativistic time-dependent density functional theory calculations was applied. As a result, Al1 exhibited the highest emission quantum yield (64%), the longest emission lifetime (8.7 ns) and the best photostability under the reaction conditions required for the energy transfer reactions (e.g. in aerated chloroform). Moreover, Al1 provided the highest rate constant (0.043 min-1) for the photocatalytic oxygenation of DPF, outperforming even noble metal-based competitors such as [Ru(bpy)3]2+. Finally, its superior photostability enabled a long-term test (7 h), in which Al1 was successfully recycled seven times, underlining the high potential of this new class of earth-abundant PSs.

Stereocontrolled and time-honored access to piperidine- and pyrrolidine-fused 3-methylenetetrahydropyrans using lactam-tethered alkenols

Polycyclic oxygen-heterocycles bearing the 3-methylenetetrahydropyran (i.e., 3-MeTHP) motif are resident in bioactive molecules such as hodgsonox and iridoid. Meanwhile, the δ- and γ-lactam topologies as well as their reduced variants (i.e., piperidines and pyrrolidines) are at the core of several pharmaceuticals and fragrances. A stereocontrolled, time-honored, and cost-effective strategy that merges a 3-MeTHP motif with the aforementioned azaheterocyclic scaffolds could exponentially expand the 3D-structural space for the discovery of new small molecules with medicinal value. In these studies, readily affordable lactam-tethered alkenols have been interrogated in two complementary cascade approaches, leading to the regioselective and stereocontrolled synthesis of lactam-fused 3-MeTHPs. The first approach hinges on regioselective 6-endo-trig bromoetherification of the alkenols and concomitant elimination to arrive at the desired 3-MeTHPs. The methylene portion of the 3-MeTHP is unveiled at a late stage, which is noteworthy since all existing approaches to 3-MeTHPs rely on early-stage introduction of the methylene group. The second strategy involves transition metal-catalyzed alkoxylation of the tethered alkenol followed by base-induced double bond isomerization. The lactam-fused 3-MeTHPs are obtained in high site- and diastereo-selectivities. Post-modification of the bicycles has led to the construction of 3-MeTHP-fused saturated piperidines and pyrrolidines as well as 3-MeTHPs bearing four contiguous stereocenters.

Site-Selective Distal C(sp3)-H Bromination of Aliphatic Amines as a Gateway for Forging Nitrogen-Containing sp3 Architectures

Herein, we disclose a new strategy that rapidly and reliably incorporates bromine atoms at distal, secondary C(sp3)-H sites in aliphatic amines with an excellent and predictable site-selectivity pattern. The resulting halogenated building blocks serve as versatile linchpins to enable a series of carbon-carbon and carbon-heteroatom bond-formations at remote C(sp3) sites, thus offering a new modular and unified platform that expediates the access to advanced sp3 architectures possessing valuable nitrogen-containing saturated heterocycles of interest in medicinal chemistry settings.

Site-selective α-C(sp3)-H arylation of dialkylamines via hydrogen atom transfer catalysis-enabled radical aryl migration

Site-selective C(sp3)-H arylation is an appealing strategy to synthesize complex arene structures but remains a challenge facing synthetic chemists. Here we report the use of photoredox-mediated hydrogen atom transfer (HAT) catalysis to accomplish the site-selective α-C(sp3)-H arylation of dialkylamine-derived ureas through 1,4-radical aryl migration, by which a wide array of benzylamine motifs can be incorporated to the medicinally relevant systems in the late-stage installation steps. In contrast to previous efforts, this C-H arylation protocol exhibits specific site-selectivity, proforming predominantly on sterically more-hindered secondary and tertiary α-amino carbon centers, while the C-H functionalization of sterically less-hindered N-methyl group can be effectively circumvented in most cases. Moreover, a diverse range of multi-substituted piperidine derivatives can be obtained with excellent diastereoselectivity. Mechanistic and computational studies demonstrate that the rate-determining step for methylene C-H arylation is the initial H atom abstraction, whereas the radical ipso cyclization step bears the highest energy barrier for N-methyl functionalization. The relatively lower activation free energies for secondary and tertiary α-amino C-H arylation compared with the functionalization of methylic C-H bond lead to the exceptional site-selectivity.

Sequential closed-loop Bayesian optimization as a guide for organic molecular metallophotocatalyst formulation discovery

Conjugated organic photoredox catalysts (OPCs) can promote a wide range of chemical transformations. It is challenging to predict the catalytic activities of OPCs from first principles, either by expert knowledge or by using a priori calculations, as catalyst activity depends on a complex range of interrelated properties. Organic photocatalysts and other catalyst systems have often been discovered by a mixture of design and trial and error. Here we report a two-step data-driven approach to the targeted synthesis of OPCs and the subsequent reaction optimization for metallophotocatalysis, demonstrated for decarboxylative sp3-sp2 cross-coupling of amino acids with aryl halides. Our approach uses a Bayesian optimization strategy coupled with encoding of key physical properties using molecular descriptors to identify promising OPCs from a virtual library of 560 candidate molecules. This led to OPC formulations that are competitive with iridium catalysts by exploring just 2.4% of the available catalyst formulation space (107 of 4,500 possible reaction conditions).

Radical Deoxygenative Three-Component Reaction of Alcohols, Aryl Alkenes, and Cyanopyridines

We report herein a deoxygenative radical multicomponent reaction involving alcohols, aryl alkenes, and cyanopyridine under photoredox conditions. This method is photoredox-neutral, suitable for late-stage modification, and compatible with a wide array of alcohols as alkyl radical sources, including primary, secondary, and tertiary alcohols. This reaction comprises a radical relay mechanism encompassing the Giese addition of aryl alkenes by alkyl radicals, followed by the decyanative pyridination of benzyl radicals.

Calibration-free reaction yield quantification by HPLC with a machine-learning model of extinction coefficients

Reaction optimization and characterization depend on reliable measures of reaction yield, often measured by high-performance liquid chromatography (HPLC). Peak areas in HPLC chromatograms are correlated to analyte concentrations by way of calibration standards, typically pure samples of known concentration. Preparing the pure material required for calibration runs can be tedious for low-yielding reactions and technically challenging at small reaction scales. Herein, we present a method to quantify the yield of reactions by HPLC without needing to isolate the product(s) by combining a machine learning model for molar extinction coefficient estimation, and both UV-vis absorption and mass spectra. We demonstrate the method for a variety of reactions important in medicinal and process chemistry, including amide couplings, palladium catalyzed cross-couplings, nucleophilic aromatic substitutions, aminations, and heterocycle syntheses. The reactions were all performed using an automated synthesis and isolation platform. Calibration-free methods such as the presented approach are necessary for such automated platforms to be able to discover, characterize, and optimize reactions automatically.

Unlocking carbene reactivity by metallaphotoredox α-elimination

The ability to tame high-energy intermediates is important for synthetic chemistry, enabling the construction of complex molecules and propelling advances in the field of synthesis. Along these lines, carbenes and carbenoid intermediates are particularly attractive, but often unknown, high-energy intermediates1,2. Classical methods to access metal carbene intermediates exploit two-electron chemistry to form the carbon-metal bond. However, these methods are usually prohibitive because of reagent safety concerns, limiting their broad implementation in synthesis3-6. Mechanistically, an alternative approach to carbene intermediates that could circumvent these pitfalls would involve two single-electron steps: radical addition to metal to forge the initial carbon-metal bond followed by redox-promoted α-elimination to yield the desired metal carbene intermediate. Here we realize this strategy through a metallaphotoredox platform that exploits iron carbene reactivity using readily available chemical feedstocks as radical sources and α-elimination from six classes of previously underexploited leaving groups. These discoveries permit cyclopropanation and σ-bond insertion into N-H, S-H and P-H bonds from abundant and bench-stable carboxylic acids, amino acids and alcohols, thereby providing a general solution to the challenge of carbene-mediated chemical diversification.

Decyanative Heteroarylations of Glycine Derivatives

Amino acids and aromatic nitrogen heterocycles are widely used in pharmaceuticals. Herein, we present an effective visible-light-driven thiobenzoic acid (TBA)-catalyzed decyanative C(sp3)-H heteroarylation of glycine derivatives. This process occurs under mild and straightforward conditions, affording a range of valuable yet challenging-to-obtain α-heteroaryl amino acid derivatives. Moreover, this organocatalytic C(sp3)-C(sp2) bond formation reaction is applicable to the late-stage modification of various short peptides.

Electrochemical Synthesis of C(sp3)-Rich Amines by Aminative Carbofunctionalization of Carbonyl Compounds

Alkylamines form the backbone of countless nitrogen-containing small molecules possessing desirable biological properties. Despite advances in amine synthesis through transition metal catalysis and photoredox chemistry, multicomponent reactions that leverage inexpensive materials to transform abundant chemical feedstocks into three-dimensional α-substituted alkylamines bearing complex substitution patterns remain scarce. Here, we report the design of a catalyst-free electroreductive manifold that merges amines, carbonyl compounds and carbon-based radical acceptors under ambient conditions without rigorous exclusion of air and moisture. Key to this aminative carbofunctionalization process is the chemoselective generation of nucleophilic α-amino radical intermediates that readily couple with electrophilic partners, providing straightforward access to architecturally intricate alkylamines and drug-like scaffolds which are inaccessible by conventional means.

Metal Free Functionalization of Saturated Heterocycles with Vinylarenes and Pyridine Enabled by Photocatalytic Hydrogen Atom Transfer

Saturated heterocycles are important class of structural scaffolds in small-molecule drugs, natural products, and synthetic intermediates. Here, we disclosed a metal free, mild, and scalable functionalization of saturated heterocycles using vinylarenes as a linchpin approach. Key to success of this transformation is the employing of simple and cheap benzophenone as a hydrogen atom transfer (HAT) catalyst. This operationally robust process was used for the making of diverse functionalized saturated heterocycles. Furthermore, aldehydes, alkane, and alcohol have been functionalized under the optimized conditions. The potential pharmaceutical utility of the procedure has also been demonstrated by late-stage functionalization of bioactive natural compounds and pharmaceutical molecules. Initial mechanism studies and control experiments were performed to elucidate the mechanism of the reactions.

Artificial Photosynthases: Single-Chain Nanoparticles with Manifold Visible-Light Photocatalytic Activity for Challenging "in Water" Organic Reactions

Photocatalyzed reactions of organic substances in aqueous media are challenging transformations, often because of scarce solubility of substrates and catalyst deactivation. Herein, we report single-chain nanoparticles, SCNPs, capable of efficiently catalyzing four different "in water" organic reactions by employing visible light as the only external energy source. Specifically, we decorated a high-molecular-weight copolymer, poly(OEGMA300-r-AEMA), with iridium(III) cyclometalated complex pendants at varying content amounts. The isolated functionalized copolymers demonstrated self-assembly into noncovalent, amphiphilic SCNPs in water, which enabled efficient visible-light photocatalysis of two reactions unprecedentedly reported in water, namely, [2 + 2] photocycloaddition of vinyl arenes and α-arylation of N-arylamines. Additionally, aerobic oxidation of 9-substituted anthracenes and β-sulfonylation of α-methylstyrene were successfully carried out in aqueous media. Hence, by merging metal-mediated photocatalysis and SCNPs for the fabrication of artificial photoenzyme-like nano-objects─i.e., artificial photosynthases (APS)─our work broadens the possibilities for performing challenging "in water" organic transformations via visible-light photocatalysis.

Visible-Light-Promoted [4π + 2σ] Annulation of Dienes and Alkylamines via Dual Inert C(sp3)-H Bond Activation

Herein, visible-light-promoted [4π + 2σ] annulation of dienes and alkylamines was achieved via dual C(sp3)-H bond functionalization of alkylamines. The elusive enamine precursors are generated under mild conditions by photoredox catalysis, efficiently annulated by the diene, and simultaneously functionalized with two aliphatic C(sp3)-H bonds, resulting in the productive synthesis of new aromatic rings. The aromatic ring construction provides direct access to 2-hydroxybenzophenone derivatives in high yields (up to 90%). This [4π + 2σ] annulation reaction demonstrates mild reaction conditions, high reaction efficiency, and broad functional group tolerance, and this synthetic protocol has been made available for the late-stage transformation of natural products and commercial drugs.

Pt-S Bond-Enabled Temperature-Dependent Phosphorescence in S-Heteroaryl Tetradentate Pt(S^C^N^O) Complexes

This study presents the rare examples of S-heteroaryl tetradentate Pt(S^C^N^O) luminescent complexes (PtSZ and PtSZtBu) containing a Pt-S bond. The presence of the Pt-S bond allows the novel Pt(S^C^N^O) complexes to exhibit temperature-dependent phosphorescent emission behavior. The PtSZtBu exhibits dual-emission phenomena and biexponential transient decay spectra above 250 K, indicating the presence of two minimal excited states in the potential energy surface (PES) of the T1 state. Through complementary experimental and computational studies, we have identified changes in orbital composition between Pt(dxy)-S(px) and Pt(dyz)-S(pz) in excited states with increasing temperature. This results in two energy minima, enabling the excited states to decay selectively and radiatively at different temperatures. Consequently, this leads to remarkable steady-state and transient emission spectra changes. Our work not only provides valuable insights for the development of novel Pt-S bond-based tetradentate Pt(II) complexes but also enhances our understanding of the distinctive properties governed by the Pt-S bond.

Deciphering complexity in Pd-catalyzed cross-couplings

Understanding complex reaction systems is critical in chemistry. While synthetic methods for selective formation of products are sought after, oftentimes it is the full reaction signature, i.e., complete profile of products/side-products, that informs mechanistic rationale and accelerates discovery chemistry. Here, we report a methodology using high-throughput experimentation and multivariate data analysis to examine the full signature of one of the most complicated chemical reactions catalyzed by palladium known in the chemical literature. A model Pd-catalyzed reaction was selected involving functionalization of 2-bromo-N-phenylbenzamide and multiple bond activation pathways. Principal component analysis, correspondence analysis and heatmaps with hierarchical clustering reveal the factors contributing to the variance in product distributions and show associations between solvents and reaction products. Using robust data from experiments performed with eight solvents, for four different reaction times at five different temperatures, we correlate side-products to a major dominant N-phenyl phenanthridinone product, and many other side products.

Photoredox-Catalyzed Decarboxylative Cross-Coupling Reaction to Synthesis Unsymmetrical Diarylmethanes

The photoredox-catalyzed decarboxylative cross-coupling reaction of aryl acetic acids and aryl nitriles has been achieved under an argon atmosphere in high yields. This method provides a fast way to obtain prevalent aryl acetic acids from an abundant natural source. A tentative radical mechanism has been proposed.

Rapid reaction optimization by robust and economical quantitative benchtop 19F NMR spectroscopy

The instrumental analysis of reaction mixtures is usually the rate-determining step in the optimization of chemical processes. Traditionally, reactions are analyzed by gas chromatography, HPLC or quantitative NMR spectroscopy on high-field spectrometers. However, chromatographic methods require elaborate work-up and calibration protocols, and high-field NMR spectrometers are expensive to purchase and operate. This protocol describes an inexpensive and highly effective analysis method based on low-field benchtop NMR spectroscopy. Its key feature is the use of fluorine-labeled model substrates that, because of the wide chemical shift range and high sensitivity of 19F, enable separate, quantitative detection of product and by-product signals even on low-field, permanent magnet spectrometers. An external lock/shim device obviates the need for deuterated solvents, permitting the direct, noninvasive measurement of crude reaction mixtures with minimal workup. The low field-strength facilitates a homogeneous excitation over a wide chemical shift range, minimizing systematic integration errors. The addition of the optimal amount of the nonshifting relaxation agent tris(acetylacetonato) iron(III) minimizes relaxation delays at full resolution, reducing the analysis time to 32 s per sample. The correct choice of processing parameters is also crucial. A step-by-step guideline is provided, the influence of all parameters, including adjustments needed when using high-field spectrometers, is discussed and potential pitfalls are highlighted. The wide applicability of the analytical protocol for reaction optimization is illustrated by three examples: a Buchwald-Hartwig amination, a Suzuki coupling and a C-H arylation reaction.

Deconvolution and Analysis of the 1H NMR Spectra of Crude Reaction Mixtures

Nuclear magnetic resonance (NMR) spectroscopy is an important analytical technique in synthetic organic chemistry, but its integration into high-throughput experimentation workflows has been limited by the necessity of manually analyzing the NMR spectra of new chemical entities. Current efforts to automate the analysis of NMR spectra rely on comparisons to databases of reported spectra for known compounds and, therefore, are incompatible with the exploration of new chemical space. By reframing the NMR spectrum of a reaction mixture as a joint probability distribution, we have used Hamiltonian Monte Carlo Markov Chain and density functional theory to fit the predicted NMR spectra to those of crude reaction mixtures. This approach enables the deconvolution and analysis of the spectra of mixtures of compounds without relying on reported spectra. The utility of our approach to analyze crude reaction mixtures is demonstrated with the experimental spectra of reactions that generate a mixture of isomers, such as Wittig olefination and C-H functionalization reactions. The correct identification of compounds in a reaction mixture and their relative concentrations is achieved with a mean absolute error as low as 1%.

Direct S0 → Tn Transition under Visible Light Irradiation Enabling Synthesis of a Pseudoindoxyl Scaffold

Pseudoindoxyl is a partial skeleton found in various natural products. Its light-absorption properties make it useful for the design of functional molecules. However, versatile synthesis methods have not yet been reported. In this report, we present a versatile synthetic method for pseudoindoxyls using the direct S0 → Tn transition under visible light irradiation. We also discuss the application of pseudoindoxyls as photocatalysts.

C-H, N-H, and O-H Bond Activations to Prepare Phosphorescent Hydride-Iridium(III)-Phosphine Emitters with Photocatalytic Achievement in C-C Coupling Reactions

Complex IrH5(PiPr3)2 (1) activates two different σ-bonds of 3-phenoxy-1-phenylisoquinoline, 2-(1H-benzimidazol-2-yl)-6-phenylpyridine, 2-(1H-indol-2-yl)-6-phenylpyridine, 2-(2-hydroxyphenyl)-6-phenylpyridine, N-(2-hydroxyphenyl)-N'-phenylimidazolylidene, and 1,3-di(2-pyridyl)-4,6-dimethylbenzene to give IrH{κ3-C,N,C-[C6H4-isoqui-O-C6H4]}(PiPr3)2 (2), IrH{κ3-N,N,C-[NBzim-py-C6H4]}(PiPr3)2 (3), IrH{κ3-N,N,C-[Ind-py-C6H4]}(PiPr3)2 (4), IrH{κ3-C,N,O-[C6H4-py-C6H4O]}(PiPr3)2 (5), IrH{κ3-C,C,O-[C6H4-Im-C6H4O]}(PiPr3)2 (6), and IrH{κ3-N,C,C-[py-C6HMe2-C5H3N]}(PiPr3)2 (7), respectively. The activations are sequential, with the second generally being the slowest. Accordingly, dihydride intermediates IrH2{κ2-C,N-[C6H4-isoqui-O-C6H5]}(PiPr3)2 (2d), IrH2{κ2-N,N-[NBzim-py-C6H5]}(PiPr3)2 (3d), IrH2{κ2-N,N-[Ind-py-C6H5]}(PiPr3)2 (4d), and IrH2{κ2-N,C-[py-C6HMe2-py]}(PiPr3)2 (7d) were characterized spectroscopically. Complexes 3 and 5 are green phosphorescent emitters upon photoexcitation, exhibiting good absorption over a wide range of wavelengths, emission quantum yields about 0.70 in solution, long enough lifetimes (10-17 μs), and reversible electrochemical behavior. In agreement with these features, complex 3 promotes the photocatalytic α-amino C(sp3)-H arylation of N,N-dimethylaniline and N-phenylpiperidine with 1,4-dicyanobenzene and 4-cyanopyridine under blue LED light irradiation. The C-C coupling products are isolated in high yields with only 2 mol % of photocatalyst after 24 h.

Baldwin and Whitehead's Manzamine Alkaloids Biosynthesis Hypothesis Involves a Finely Tuned Reactivity of Acrolein: Automated Extraction of Reactivity Patterns from LC-MS2 Data

Inspired by the multicomponent reaction-type scenario involving fatty dialdehydes, a nitrogen source, and acrolein, as a key C3 unit, put forward by Baldwin and Whitehead to explain the formation of manzamine-type alkaloids, 96 multicomponent reactions were designed, and their analytical readouts were deconvoluted using a herein-provided chemoinformatic workflow. This strategy pinpointed relevant conditions tuning the reactivity of acrolein to fulfill Baldwin and Whitehead's manzamine alkaloids biosynthetic hypothesis. This strategy can become part of a general method for the high-content analysis of multicomponent reactions applied to a natural product biosynthetic scenario.

Computationally Guided Ligand Discovery from Compound Libraries and Discovery of a New Class of Ligands for Ni-Catalyzed Cross-Electrophile Coupling of Challenging Quinoline Halides

Although screening technology has heavily impacted the fields of metal catalysis and drug discovery, its application to the discovery of new catalyst classes has been limited. The diversity of on- and off-cycle pathways, combined with incomplete mechanistic understanding, means that screens of potential new ligands have thus far been guided by intuitive analysis of the metal binding potential. This has resulted in the discovery of new classes of ligands, but the low hit rates have limited the use of this strategy because large screens require considerable cost and effort. Here, we demonstrate a method to identify promising screening directions via simple and scalable computational and linear regression tools that leads to a substantial improvement in hit rate, enabling the use of smaller screens to find new ligands. The application of this approach to a particular example of Ni-catalyzed cross-electrophile coupling of aryl halides with alkyl halides revealed a previously overlooked trend: reactions with more electron-poor amidine ligands result in a higher yield. Focused screens utilizing this trend were more successful than serendipity-based screening and led to the discovery of two new types of ligands, pyridyl oxadiazoles and pyridyl oximes. These ligands are especially effective for couplings of bromo- and chloroquinolines and isoquinolines, where they are now the state of the art. The simplicity of these models with parameters derived from metal-free ligand structures should make this approach scalable and widely accessible.

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.

Unveiling Mechanistic Insights and Photocatalytic Advancements in Intramolecular Photo-(3 + 2)-Cycloaddition: A Comparative Assessment of Two Paradigmatic Single-Electron-Transfer Models

The synthesis of 1-aminonorbornane (1-aminoNB), a potential aniline bioisostere, through photochemistry or photoredox catalysis signifies a remarkable breakthrough with implications in organic chemistry, pharmaceutical chemistry, and sustainable chemistry. However, an understanding of the underlying mechanisms involved in these reactions remains limited and ambiguous. Herein, we employ high-precision CASPT2//CASSCF calculations to elucidate the intricate mechanisms regulating the intramolecular photo-(3 + 2)-cycloaddition reactions for the synthesis of 1-aminoNB in the presence or absence of the Ir-complex-based photocatalyst. Our investigations delve into radical cascades, stereoselectivity, particularly single-electron-transfer (SET) events, etc. Furthermore, we innovatively introduce and compare two SET models integrating Marcus electron-transfer theory and transition-state theory. These models combined with kinetic data contribute to recognizing the critical control factors in diverse photocatalysis, thereby guiding the design and manipulation of photoredox catalysis as well as the improvement and modification of photocatalysts.

Photoinduced Radical Desulfurative C(sp3)-C(sp2) Coupling via Electron Donor-Acceptor Complexes

Herein, we disclose a radical desulfurative C-C coupling protocol for the synthesis of 4-alkylpyridines. A variety of substituents on both benzyl thiols and 4-cyanopyridines are tolerated. The reaction is carried out under mild and photocatalyst- and transition-metal-free conditions. Preliminary mechanistic studies show that an electron donor-acceptor complex is formed between benzyl thiols and 4-cyanopyridines under alkaline conditions. Then, a variety of 1°, 2°, and 3° C(sp3)-centered radicals was formed by cleavage of the C-S bond, and the 4-alkylpyridines were achieved through a radical-radical coupling with the pyridyl radical anion.

Accelerated photochemical reactions at oil-water interface exploiting melting point depression

Water can accelerate a variety of organic reactions far beyond the rates observed in classical organic solvents. However, using pure water as a solvent introduces solubility constraints that have limited the applicability of efficient photochemistry in particular. We report here the formation of aggregates between pairs of arenes, heteroarenes, enamines, or esters with different electron affinities in an aqueous medium, leading to an oil-water phase boundary through substrate melting point depression. The active hydrogen atoms in the reactants engage in hydrogen bonds with water, thereby accelerating photochemical reactions. This methodology realizes appealingly simple conditions for aqueous coupling reactions of complex solid molecules, including complex drug molecules that are poorly soluble in water.

Development of a selective and scalable N1-indazole alkylation

N1-Alkyl indazoles are a ubiquitous and privileged motif within medicinal chemistry, yet methods to selectively furnish N1-alkyl indazoles with simple alkyl side chains remain sparse. Herein, negative data from high-throughput experimentation (HTE) enabled a confident pivot of resource from continued optimisation to the development of an alternative reaction. This workflow culminated in a methodology for the synthesis of N1-alkyl indazoles. The procedure is highly selective for N1-alkylation, practical, and broad in scope, with no N2-alkyl products detected at completion. Mechanistic understandings were consistent with attributing the high selectivity to thermodynamic control. Additional data-driven process development led to this reaction being safely demonstrated on a 100 g scale, with potential for further scale up. This study highlights pragmatic principles followed to develop a necessitated methodology, suitable for large scale manufacture.

Transition-Metal-Free, Reductive Csp2-Csp3 Bond Constructions via Electrochemically Induced Alkyl Radicals

Construction of the Csp2-Csp3 bond without the aid of transition metal catalysts has been achieved by coupling the electrogenerated alkyl radicals with electron deficient (hetero)arenes in an undivided cell. Simultaneous cathodic reduction of both unactivated alkyl halides and cyanobenzenes under high potential enables radical-radical cross-coupling to deliver alkylarenes in the absence of transition metals. Depending on the coupling partner, the electrogenerated alkyl radicals can also proceed the Minisci-type reaction with N-heteroarenes without redox agents.

In Operando Visualization of Elementary Turnovers in Photocatalytic Organic Synthesis

We report the in operando visualization of the photocatalytic turnovers on single eosin Y (EY) through a redox-induced photoblinking phenomenon. The photocatalytic cyclization of thiobenzamide (TB) catalyzed by EY was investigated. The analysis of the intensity-versus-time trajectories of single EYs revealed the kinetics and dynamics of the elementary photocatalytic turnovers and the heterogeneity of the activity of individual EYs. The quenching turnover time showed a fast population and a slow population, which could be attributed to the singlet and triplet states of photoexcited EY. The slow quenching turnovers were more dominant at higher TB concentrations. The activity heterogeneity of EYs was studied over a series of reactant concentrations. Excess quenching reagent was found to decrease the percentage of active EYs. The method can be broadly applied to studying the elementary processes of photocatalytic organic reactions in operando.

Enabling High Throughput Kinetic Experimentation by Using Flow as a Differential Kinetic Technique

Kinetic data is most commonly collected through the generation of time-series data under either batch or flow conditions. Existing methods to generate kinetic data in flow collect integral data (concentration over time) only. Here, we report a method for the rapid and direct collection of differential kinetic data (direct measurement of rate) in flow by performing a series of instantaneous rate measurements on sequential small-scale reactions. This technique decouples the time required to generate a full kinetic profile from the time required for a reaction to reach completion, enabling high throughput kinetic experimentation. In addition, comparison of kinetic profiles constructed at different residence times allows the robustness, or stability, of homogeneously catalysed reactions to be interrogated. This approach makes use of a segmented flow platform which was shown to quantitatively reproduce batch kinetic data. The proline mediated aldol reaction was chosen as a model reaction to perform a high throughput kinetic screen of 216 kinetic profiles in 90 hours, one every 25 minutes, which would have taken an estimated continuous 3500 hours in batch, an almost 40-fold increase in experimental throughput matched by a corresponding reduction in material consumption.

Electrochemical Vicinal C-H Difunctionalization of Saturated Azaheterocycles

A method to functionalize two vicinal C-H bonds of saturated azaheterocycles is described. The procedure involves subjecting the substrate to a mixture of hydrochloric acid, acetic acid, and acetic anhydride in an undivided electrochemical cell at a constant current, resulting in stereoselective conversion to the corresponding α-acetoxy-β-chloro derivative. The α-position can be readily substituted with a range of other groups, including alkyl, aryl, allyl, alkynyl, alkoxy, or azido functionalities. Furthermore, we demonstrate that the β-chloro position can be engaged in Suzuki cross-coupling. This protocol thus enables the rapid diversification of simple five-, six-, and seven-membered saturated azaheterocycles at two adjacent positions.

Accelerated chemical science with AI

In light of the pressing need for practical materials and molecular solutions to renewable energy and health problems, to name just two examples, one wonders how to accelerate research and development in the chemical sciences, so as to address the time it takes to bring materials from initial discovery to commercialization. Artificial intelligence (AI)-based techniques, in particular, are having a transformative and accelerating impact on many if not most, technological domains. To shed light on these questions, the authors and participants gathered in person for the ASLLA Symposium on the theme of 'Accelerated Chemical Science with AI' at Gangneung, Republic of Korea. We present the findings, ideas, comments, and often contentious opinions expressed during four panel discussions related to the respective general topics: 'Data', 'New applications', 'Machine learning algorithms', and 'Education'. All discussions were recorded, transcribed into text using Open AI's Whisper, and summarized using LG AI Research's EXAONE LLM, followed by revision by all authors. For the broader benefit of current researchers, educators in higher education, and academic bodies such as associations, publishers, librarians, and companies, we provide chemistry-specific recommendations and summarize the resulting conclusions.

Direct C(sp3)-H Arylation of Unprotected Benzyl Anilines and Alkylarenes by Organocatalysis under Visible Light

Reported herein is direct C(sp3)-H arylation of unprotected benzyl anilines and alkylarenes via consecutive photoinduced electron transfer by visible light irradiation. Reductive quenching cycles and radical-radical cross-coupling were involved, and electron paramagnetic resonance experiments provide evidence for the formation of radical intermediates formed in situ. The protocol highlights transition metal free, external oxidant free, broad substrate scope, and high efficiency (>60 examples, up to 96%).

Reactivity of radiolytically and photochemically generated tertiary amine radicals towards a CO2 reduction catalyst

Homogeneous solar fuels photocatalytic systems often require several additives in solution with the catalyst to operate, such as a photosensitizer (PS), Brønsted acid/base, and a sacrificial electron donor (SED). Tertiary amines, in particular triethylamine (TEA) and triethanolamine (TEOA), are ubiquitously deployed in photocatalysis applications as SEDs and are capable of reductively quenching the PS's excited state. Upon oxidation, TEA and TEOA form TEA•+ and TEOA•+ radical cations, respectively, which decay by proton transfer to generate redox non-innocent transient radicals, TEA• and TEOA•, respectively, with redox potentials that allow them to participate in an additional electron transfer step, thus resulting in net one-photon/two-electron donation. However, the properties of the TEA• and TEOA• radicals are not well understood, including their reducing powers and kinetics of electron transfer to catalysts. Herein, we have used both pulse radiolysis and laser flash photolysis to generate TEA• and TEOA• radicals in CH3CN, and combined with UV/Vis transient absorption and time-resolved mid-infrared spectroscopies, we have probed the kinetics of reduction of the well-established CO2 reduction photocatalyst, fac-ReCl(bpy)(CO)3 (bpy = 2,2'-bipyridine), by these radicals [kTEA• = (4.4 ± 0.3) × 109 M-1 s-1 and kTEOA• = (9.3 ± 0.6) × 107 M-1 s-1]. The ∼50× smaller rate constant for TEOA• indicates, that in contrast to a previous assumption, TEA• is a more potent reductant than TEOA• (by ∼0.2 V, as estimated using the Marcus cross relation). This knowledge will aid in the design of photocatalytic systems involving SEDs. We also show that TEA can be a useful radiolytic solvent radical scavenger for pulse radiolysis experiments in CH3CN, effectively converting unwanted oxidizing radicals into useful reducing equivalents in the form of TEA• radicals.

Organo-Photoredox Catalyzed C(sp3 )-H Bond Arylation of Aliphatic Amides

A C(sp3 )-H bond arylation of aliphatic amides has been achieved via organophotoredox catalysis. The reaction could be realized at room temperature with visible light source and metal-free catalyst. Quinuclidine is employed as an efficient HAT reagent and a range of aliphatic amides is employed as both substrate and solvent in the reaction. This photocatalyzed transformation provides a convenient protocol to afford a board range of N-benzyl amides.

β-C-H Allylation of Trialkylamines with Allenes Promoted by Synergistic Borane/Palladium Catalysis

Functionalization of the C(sp3 )-H bonds of trialkylamines is challenging, especially for reactions at positions other than the α position. Herein, we report a method for β-C(sp3 )-H allylation of trialkylamines. In these reactions, which involve synergistic borane/palladium catalysis, an enamine intermediate is first generated from the amine via α,β-dehydrogenation promoted by B(C6 F5 )3 and a base, and then the enamine undergoes palladium-catalyzed reaction with an allene to give the allylation product. Because the hydride and the proton resulting from the initial dehydrogenation are ultimately shuttled to the product by B(C6 F5 )3 and the palladium catalyst, respectively, these reactions show excellent atom economy. The establishment of this method paves the way for future studies of C-H functionalization of trialkylamines by means of synergistic borane/transition-metal catalysis.

Deoxygenative coupling of alcohols with aromatic nitriles enabled by direct visible light excitation

A general and practical protocol is presented for visible-light-driven deoxygenative coupling of alcohols with aromatic nitriles in the absence of external photocatalysts. Utilizing a hydroxyl activation strategy with carbon disulfide, this C(sp3)-C(sp2) constructing platform accommodates a broad scope of alcohols and aryl nitriles to deliver various alkyl-substituted arenes. Mechanism studies show that a single electron transfer event between a photoexcited aryl nitrile and a xanthate anion is key to the transformation.

Photoinduced α-Aminoalkylation of Sulfonylarenes with Alkylamines

α-Aminoalkylation of sulfonylarenes with alkylamines was found to be induced by photoirradiation. Here various types of alkylamines, such as trialkylamines, dialkylamines, N,N-dialkylanilines and N-alkylanilines as well as sulfonylarenes containing an azole, azine, heterole or benzene ring are available. The reaction proceeds through a homolytic aromatic substitution (HAS) process consisting of addition of an α-aminoalkyl radical to a sulfonylarene and elimination of the sulfonyl radical to give the α-arylalkylamine, where photoirradiation is considered to induce homolysis of sulfonylarenes leading to the generation of α-aminoalkyl radicals that make a radical chain operative.

Substrate-Mediator Duality of 1,4-Dicyanobenzene in Electrochemical C(sp2 )-C(sp3 ) Bond Formation with Alkyl Bromides

Electrochemical approaches to form C(sp2 )-C(sp3 ) bonds have focused on coupling C(sp3 ) electrophiles that form stabilized carbon-centered radicals upon reduction or oxidation. Whereas alkyl bromides are desirable C(sp3 ) coupling partners owing to their availability and cost-effectiveness, their tendency to undergo radical-radical homocoupling makes them challenging substrates for electroreductive cross-coupling. Herein, we disclose a metal-free regioselective cross-coupling of 1,4-dicyanobenzene, a useful precursor to aromatic nitriles, and alkyl bromides. Alkyl bromide reduction is mediated directly by 1,4-dicyanobenzene radical anions, leading to negligible homocoupling and high cross-selectivity to form 1,4-alkyl cyanobenzenes. The cross-coupling scheme is compatible with oxidatively sensitive and acidic functional groups such as amines and alcohols, which have proven difficult to incorporate in alternative electrochemical approaches using carboxylic acids as C(sp3 ) precursors.

Cobalt-catalyzed aminoalkylative carbonylation of alkenes toward direct synthesis of γ-amino acid derivatives and peptides

γ-Amino acids and peptides analogues are common constituents of building blocks for numerous biologically active molecules, pharmaceuticals, and natural products. In particular, γ-amino acids are providing with better metabolic stability than α-amino acids. Herein we report a multicomponent carbonylation technology that combines readily available amides, alkenes, and the feedstock gas carbon monoxide to build architecturally complex and functionally diverse γ-amino acid derivatives in a single step by the implementation of radical relay catalysis. This transformation can also be used as a late-stage functionalization strategy to deliver complex, advanced γ-amino acid products for pharmaceutical and other areas.

Photocatalyzed Aminomethylation of Alkyl Halides Enabled by Sterically Hindered N-Substituents

The catalytic C(sp3 )-C(sp3 ) coupling of alkyl halides and tertiary amines offers a promising tool for the rapid decoration of amine skeletons. However, this approach has not been well established, partially due to the challenges in precisely distinguishing and controlling the reactivity of amine-coupling partners and their product homologues. Herein, we developed a metal-free photocatalytic system for the aminomethylation of alkyl halides through radical-involved C(sp3 )-C(sp3 ) bond formation, allowing for the synthesis of sterically congested tertiary amines that are of interest in organic synthesis but not easily prepared by other methods. Mechanistic studies disclosed that sterically hindered N-substituents are key to activate the amine coupling partners by tuning their redox potentials to drive the reaction forward.

Quantum chemical calculations for reaction prediction in the development of synthetic methodologies

Quantum chemical calculations have been used in the development of synthetic methodologies to analyze the reaction mechanisms of the developed reactions. Their ability to estimate chemical reaction pathways, including transition state energies and connected equilibria, has led researchers to embrace their use in predicting unknown reactions. This perspective highlights strategies that leverage quantum chemical calculations for the prediction of reactions in the discovery of new methodologies. Selected examples demonstrate how computation has driven the development of unknown reactions, catalyst design, and the exploration of synthetic routes to complex molecules prior to often laborious, costly, and time-consuming experimental investigations.