A general carbonyl alkylative amination for tertiary amine synthesis

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.

Versatile, Modular, and General Strategy for the Synthesis of α-Amino Carbonyls

Modulating the basicity of alkylamines is a crucial factor in drug design. Consequently, alkylamines with a proximal amide, ester, or ketone have become privileged features in many pharmaceutical candidates. The impact of α-amino carbonyls has made the development of new methods for their preparation a continuous challenge in synthesis. Here, we describe a practical strategy that provides a modular and programmable synthesis of a wide range of α-amino carbonyls. The generality of this process is made possible by an extremely mild method to generate carbamoyl radicals, proceeding via a Lewis acid-visible-light-mediated Norrish type-I fragmentation of a tailored carboxamide reagent and intercepted through addition to in situ generated unbiased imines. Aside from the reaction's broad scope in each component, its capacity to draw on plentiful and diversely populated amine and carbonyl feedstocks is showcased through a two-dimensional array synthesis that is used to construct a library of novel, assay-ready, α-amino amides.

Modular synthesis of α-branched secondary alkylamines via visible-light-mediated carbonyl alkylative amination

The development of methods for the assembly of secondary α-alkyl amines remains a central challenge to chemical synthesis because of their critical importance in modulating the physical properties of biologically active molecules. Despite decades of intensive research, chemists still rely on selective N-alkylation and carbonyl reductive amination to make most amine products. Here we report the further evolution of a carbonyl alkylative amination process that, for the first time, brings together primary amines, aldehydes and alkyl iodides in a visible-light-mediated multicomponent coupling reaction for the synthesis of a wide range of α-branched secondary alkylamines. In addition to exploring the tolerance and limitations in each reaction component, we also report preliminary applications to the telescoped synthesis of α-branched N-heterocycles and an N-alkylation protocol that is selective for primary over cyclic secondary amines. Our data support a mechanism involving addition of an alkyl radical to an uncharged alkyl imine which, to the best of our knowledge, has not previously been described. We believe that this method will enable practitioners of synthetic chemistry in academic and industrial settings to approach the synthesis of these important molecules in a manner that is streamlined compared to established approaches.

Titanium in photocatalytic organic transformations: current applications and future developments

Titanium, as an important transition metal, has garnered extensive attention in both industry and academia due to its excellent mechanical properties, corrosion resistance, and unique reactivity in organic synthesis. In the field of organic photocatalysis, titanium-based compounds such as titanium dioxide (TiO2), titanocenes (Cp2TiCl2, CpTiCl3), titanium tetrachloride (TiCl4), tetrakis(isopropoxy)titanium (Ti(OiPr)4), and chiral titanium complexes have demonstrated distinct reactivity and selectivity. This review focuses on the roles of these titanium compounds in photocatalytic organic reactions, and highlights the reaction pathways such as photo-induced single-electron transfer (SET) and ligand-to-metal charge transfer (LMCT). By systematically surveying the latest advancements in titanium-involved organic photocatalysis, this review aims to provide references for further research and technological innovation within this fast-developing field.

A General Three-Component Alkyl Petasis Boron-Mannich Reaction

Aryl amines are one of the most common moieties in biologically active molecules, and approximately 37% of drug candidates contain aromatic amines. Recent advancements in medicinal chemistry, coined "escaping from flatland", have led to a greater focus on accessing highly functionalized C (sp3)-rich amines to improve the physicochemical and pharmacokinetic properties of compounds. This article presents a modular and operationally straightforward three-component alkyl Petasis boron-Mannich (APBM) reaction that utilizes ubiquitous starting materials, including amines, aldehydes, and alkyl boronates. By adaptation of this transformation to high-throughput experimentation (HTE), it offers rapid access to an array of diverse C(sp3)-rich complex amines, amenable for rapid identification of drug candidates.

The first specific probe for pyrrolidine with multifunction by the interaction mechanism of atomic economic reaction

Pyrrolidine (PyD) has an important impact on the environment and human health. However, there is currently no method for trace detection of PyD. Here, we successfully designed diaminomethylene-4H-pyran (1) as the first specific fluorescent probe for PyD. Only by adding PyD to probe 1, there is blue fluorescence at 455 nm, and the color of the solution changes from colorless to yellow. The detection limit is 1.12 × 10-6 M, and the response time is less than 5 min. Meanwhile, probe 1 can also sense the gaseous PyD and detect PyD in actual water samples. Moreover, due to the low biological toxicity, probe 1 can detect the exogenous PyD in zebrafish. The preliminary mechanism shows that probe 1 and PyD undergo a combination-type chemical reaction to generate a new substance 1-PyD. Therefore, the 100% atom utilization reaction enables probe 1 to exhibit specific adsorption and removal of PyD.

Photoinduced Intermolecular Radical Hydroalkylation of Olefins via Ligated Boryl Radicals-Mediated Halogen Atom Transfer

Light-mediated Halogen-Atom Transfer (XAT) has become a significant methodology in contemporary synthesis. Unlike α-aminoalkyl and silyl radicals, ligated boryl radicals (LBRs) have not been extensively explored as halogen atom abstractors. In this study, we introduce NHC-ligated boranes as optimal radical chain carriers for the intermolecular reductive radical hydroalkylation and hydroarylation of electron-deficient olefins by using direct UV-A light irradiation. DFT analysis allowed us to rationalize the critical role of the NHC ligand in facilitating efficient chain propagation.

Acridine photocatalysis enables tricomponent direct decarboxylative amine construction

Amines are centrally important motifs in medicinal chemistry and biochemistry, and indispensable intermediates and linchpins in organic synthesis. Despite their cross-disciplinary prominence, synthetic access to amine continues to rely on two-electron approaches based on reductions and additions of organometallic reagents, limiting their accessible chemical space and necessitating stepwise preassembly of synthetic precursors. We report herein a homogeneous photocatalytic tricomponent decarboxylative radical-mediated amine construction that enables modular access to α-branched secondary amines directly from the broad and structurally diverse chemical space of carboxylic acids in a tricomponent reaction with aldehydes and aromatic amines. Our studies reveal the key role of acridine photocatalysis acting in concert with copper and Brønsted acid catalytic processes in facilitating the previously inaccessible homogeneous photocatalytic reaction and provide a streamlined segue to a wide range of amines and nonproteinogenic α-amino acids.

Silyl Radical as an Isocyanide Transfer Agent for Giese-Type Reactions Involving Aliphatic Amines

Herein we report silyl radicals serve as isocyanide transfer agents for Giese-type reaction from aliphatic amines and electron-deficient olefins. α-Primary, α-secondary, and sterically encumbered α-tertiary primary amines could be easily converted into isocyanides for coupling with electron-deficient olefins by employing latent silyl radicals under visible light irradiation. Notably, the abstraction of silane-mediated isocyanide not only enables voltage-independent activation of strong C-N bonds but also represents a mechanistic alternative Giese-type reaction in which single electron reduction and protonation processes are replaced by direct hydrogen atom transfer. This transformation occurs under photoinduced catalyst-free conditions and exhibits excellent functional group compatibility and mild reaction conditions.

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.

Cobalt-Catalyzed Enantioselective Reductive α-Chloro-Carbonyl Addition of Ketimine to Construct the β-Tertiary Amino Acid Analogues

β-Tertiary amino acid derivatives constitute one of the most frequently occurring units in natural products and bioactive molecules. However, the efficient asymmetric synthesis of this motif still remains a significant challenge. Herein, we disclose a cobalt-catalyzed enantioselective reductive addition reaction of ketimine using α-chloro carbonyl compound as a radical precursor, providing expedient access to a diverse array of enantioenriched β-quaternary amino acid analogues. This protocol exhibits outstanding enantioselectivity and broad substrate scope with excellent functional group tolerance. Preliminary mechanism studies rule out the possibility of Reformatsky-type addition and confirm the involvement of radical species in stereoselective addition process. The synthetic utility has been demonstrated through the rapid assembly of iterative amino acid units and oligopeptide, showcasing its versatile platform for late-stage modification of drug candidates.

Multicomponent Synthesis of α-Branched Amines via a Zinc-Mediated Carbonyl Alkylative Amination Reaction

Methods for the synthesis of α-branched alkylamines are important due to their ubiquity in biologically active molecules. Despite the development of many methods for amine preparation, C(sp3)-rich nitrogen-containing compounds continue to pose challenges for synthesis. While carbonyl reductive amination (CRA) between ketones and alkylamines is the cornerstone method for α-branched alkylamine synthesis, it is sometimes limited by the sterically demanding condensation step between dialkyl ketones and amines and the more restricted availability of ketones compared to aldehydes. We recently reported a "higher-order" variant of this transformation, carbonyl alkylative amination (CAA), which utilized a halogen atom transfer (XAT)-mediated radical mechanism, enabling the streamlined synthesis of complex α-branched alkylamines. Despite the efficacy of this visible-light-driven approach, it displayed scalability issues, and competitive reductive amination was a problem for certain substrate classes, limiting applicability. Here, we report a change in the reaction regime that expands the CAA platform through the realization of an extremely broad zinc-mediated CAA reaction. This new strategy enabled elimination of competitive CRA, simplified purification, and improved reaction scope. Furthermore, this new reaction harnessed carboxylic acid derivatives as alkyl donors and facilitated the synthesis of α-trialkyl tertiary amines, which cannot be accessed via CRA. This Zn-mediated CAA reaction can be carried out at a variety of scales, from a 10 μmol setup in microtiter plates enabling high-throughput experimentation, to the gram-scale synthesis of medicinally-relevant compounds. We believe that this transformation enables robust, efficient, and economical access to α-branched alkylamines and provides a viable alternative to the current benchmark methods.

Catalyst-free visible light-promoted defunctionalization of alkyl isocyanides with a hydrosilane through C-N bond cleavage

A radical initiator-free defunctionalization reaction of alkyl isocyanides with a hydrosilane has been established through C-N bond cleavage under catalyst-free visible light irradiation. Various alkyl isocyanides participated in the defunctionalization with tris(trimethylsilyl)silane under blue light irradiation at room temperature, delivering the reduced products in good yields with high chemoselectivity.

Modular α-tertiary amino ester synthesis through cobalt-catalysed asymmetric aza-Barbier reaction

Unnatural chiral α-tertiary amino acids containing two different carbon-based substituents at the α-carbon centre are widespread in biologically active molecules. This sterically rigid scaffold is becoming a growing research interest in drug discovery. However, a robust protocol for chiral α-tertiary amino acid synthesis remains scarce due to the challenge of stereoselectively constructing sterically encumbered tetrasubstituted stereogenic carbon centres. Herein we report a cobalt-catalysed enantioselective aza-Barbier reaction of ketimines with various unactivated alkyl halides, including alkyl iodides, alkyl bromides and alkyl chlorides, enabling the formation of chiral α-tertiary amino esters with a high level of enantioselectivity and excellent functional group tolerance. Primary, secondary and tertiary organoelectrophiles are all tolerated in this asymmetric reductive addition protocol, which provides a complementary method for the well-exploited enantioselective nucleophilic addition with moisture- and air-sensitive organometallic reagents. Moreover, the three-component transformation of α-ketoester, amine and alkyl halide represents a formal asymmetric deoxygenative alkylamination of the carbonyl group.

Design of Highly Efficient Electronic Energy Transfer in Functionalized Quantum Dots Driven Specifically by Ethylenediamine

The exploration of emerging functionalized quantum dots (QDs) through modulating the effective interaction between the sensing element and target analyte is of great significance for high-performance trace sensing. Here, the chromone-based ligand grafted QDs (QDs-Chromone) were initiated to realize the electronic energy transfer (EET) driven specifically by ethylenediamine (EDA) in the absence of spectral overlap. The fluorescent and colorimetric dual-mode responses (from red to blue and from colorless to yellow, respectively) resulting from the expanded conjugated ligands reinforced the analytical selectivity, endowing an ultrasensitive and specific response to submicromolar-liquid of EDA. In addition, a QDs-Chromone-based sensing chip was constructed to achieve the ultrasensitive recognition of EDA vapor with a naked-eye observed response at a concentration as low as 10 ppm, as well as a robust anti-interfering ability in complicated scenarios monitoring. We expect the proposed EET strategy in shaping functionalized QDs for high-performance sensing will shine light on both rational probe design methodology and deep sensing mechanism exploration.

Isonitriles as Alkyl Radical Precursors in Visible Light Mediated Hydro- and Deuterodeamination Reactions

Herein, we report the use of isonitriles as alkyl radical precursors in light-mediated hydro- and deuterodeamination reactions. The reaction is scalable, shows broad functional group compatibility and potential to be used in late-stage functionalization. Importantly, the method is general for Cα -primary, Cα -secondary and Cα -tertiary alkyl isonitriles. For most examples, high yields were obtained through direct visible-light irradiation of the isonitrile in the presence of a silyl radical precursor. Interestingly, in the presence of an organic photocatalyst (4CzIPN) a dramatic acceleration was observed. In-depth mechanistic studies using UV/Vis absorption, steady-state and time-resolved photoluminescence, and transient absorption spectroscopy suggest that the excited state of 4CzIPN can engage in a single-electron transfer with the isonitrile.

Oxidative Amination of Aldehydes with Amines into α-Amino Ketones

Oxidative amination for the installation of nitrogen functional molecules from nitrogen nucleophiles has always been a very challenging topic in organic synthesis. Here we report a novel conversion of different aldehydes with secondary amines for the synthesis of diversified α-amino ketones. This method can be achieved through oxidative rearrangement of an in situ-generated enamine intermediate promoted by commercially available sodium percarbonate. Furthermore, this one-pot process is also suitable for the functional modification of complex molecules.

Photocatalytic C-N bond construction toward high-value nitrogenous chemicals

The construction of carbon-nitrogen bonds is vital for producing versatile nitrogenous compounds for the chemical and pharmaceutical industries. Among developed synthetic approaches to nitrogenous chemicals, photocatalysis is particularly prominent and has become one of the emerging fields due to its unique advantages of eco-sustainable characteristics, efficient process integration, no need for high-pressure H2, and tunable synthesis methods for developing advanced photocatalytic materials. Here, the review focuses on potential photocatalytic protocols developed for the construction of robust carbon-nitrogen bonds in discrepant activation environments to produce high-value nitrogenous chemicals. The photocatalytic C-N bond construction strategies and involved reaction mechanisms are elucidated.

Photocatalytic Enantioselective Hydrosulfonylation of α,β-Unsaturated Carbonyls with Sulfonyl Chlorides

This research explores the enantioselective hydrosulfonylation of various α,β-unsaturated carbonyl compounds via the use of visible light and redox-active chiral Ni-catalysis, facilitating the synthesis of enantioenriched α-chiral sulfones with remarkable enantioselectivity (exceeding 99 % ee). A significant challenge entails enhancing the reactivity between chiral metal-coordinated carbonyl compounds and moderate electrophilic sulfonyl radicals, aiming to minimize the background reactions. The success of our approach stems from two distinctive attributes: 1) the Cl-atom abstraction employed for sulfonyl radical generation from sulfonyl chlorides, and 2) the single-electron reduction to produce a key enolate radical Ni-complex. The latter process appears to enhance the feasibility of the sulfonyl radical's addition to the electron-rich enolate radical. An in-depth investigation into the reaction mechanism, supported by both experimental observations and theoretical analysis, offers insight into the intricate reaction process. Moreover, the versatility of our methodology is highlighted through its successful application in the late-stage functionalization of complex bioactive molecules, demonstrating its practicality as a strategy for producing α-chiral sulfones.

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.

Selective and Functional-Group-Tolerant Photoalkylation of Imines by Energy-Transfer Photocatalysis

Basic amines show broad bioactivity and remain a promising source of new medicines. The direct photoalkylation of imines offers a promising strategy for complex amines. However, the lack of efficient imine photoreactivity hinders this reaction and remains a fundamental limitation in organic photochemistry. We report an efficient photoalkylation of imines that provides primary amines directly without protecting or leaving groups. The transformation effects C-H addition across N-H imines under energy-transfer photocatalysis by a ketone. Our method is distinguished from organometallic, metal-catalyzed, and photoredox approaches to imine alkylation by its lack of protecting groups and its broad scope, which includes unactivated alkanes, protic substrates, basic amines, heterocycles, and ketone imines. We highlight this scope through the condensation and alkylation of two pharmaceutical ketones, providing complex amines succinctly. Our mechanistic analysis supports a three-step process, involving hydrogen-atom transfer to an imine triplet excited state, intersystem crossing, and radical recombination, with photocatalytic enhancement through energy transfer. We further show that N-H imines are more photoreactive than N-substituted imines, a distinction partially explained by sterics and side reactions. To fully explain this distinction, we introduce the thermodynamic parameter excited-state hydrogen-atom affinity, which is highly effective at predicting the photoreactivity of imines.

Electrochemical halogen-atom transfer alkylation via α-aminoalkyl radical activation of alkyl iodides

Alkyl halides, widely recognized as important building blocks and reagents in organic synthesis, can serve as versatile alkyl radical precursors in radical-based transformations. However, generating alkyl radicals directly from unactivated alkyl halides under mild conditions remains a challenge due to their extremely low reduction potentials. To address this issue, α-aminoalkyl radicals were employed as efficient halogen-atom transfer (XAT) reagents in the photoredox activation of unactivated alkyl halides. Here, we report an effective electrooxidation strategy for generating alkyl radicals from unactivated alkyl iodides via an electrochemical halogen-atom transfer (e-XAT) process under mild conditions. The α-aminoalkyl radicals generated by anodic oxidation are demonstrated to be efficient XAT reagents in these transformations. This facile electricity-driven strategy obviates the need for sacrificial anodes and external chemical oxidants. The method successfully applies to a wide variety of alkyl iodides, including primary, secondary, and tertiary, as well as structurally diverse olefins, exhibiting excellent functional group tolerance. Moreover, we further demonstrate the utility of this strategy by rapidly functionalizing complex molecules and biomolecules.

Photocatalytic Multicomponent 1,n-Carboimination with Alkyl Iodides and O-Benzoyl Oxime through EnT and XAT Processes

In this study, we developed a strategy using commercially available alkyl iodides and O-benzoyl oxime to efficiently introduce alkyl and iminyl groups via energy transfer and halogen-atom transfer processes. We performed three-component 1,2-carboimination of olefins and four-component 1,4-carboimination across olefins and alkynes, resulting in the synthesis of over 60 nitrogen-containing molecules. Moreover, this transformation enables the synthesis of molecules with sensitive groups that were previously difficult to achieve.

Modular assembly of indole alkaloids enabled by multicomponent reaction

Indole alkaloids are one of the largest alkaloid classes, proving valuable structural moiety in pharmaceuticals. Although methods for the synthesis of indole alkaloids are constantly explored, the direct single-step synthesis of these chemical entities with broad structural diversity remains a formidable challenge. Herein, we report a modular assembly of tetrahydrocarboline type of indole alkaloids from simple building blocks in a single step while showing broad compatibility with medicinally relevant functionality. In this protocol, the 2-alkylated or 3-alkylated indoles, formaldehyde, and amine hydrochlorides could undergo a one-pot reaction to deliver γ-tetrahydrocarbolines or β-tetrahydrocarbolines directly. A wide scope of these readily available starting materials is applicable in this process, and numerous structural divergent tetrahydrocarbolines could be achieved rapidly. The control reaction and deuterium-labelling reaction are conducted to probe the mechanism. And mechanistically, this multicomponent reaction relies on a multiple alkylamination cascade wherein an unusual C(sp3)-C(sp3) connection was involved in this process. This method could render rapid access to pharmaceutically interesting compounds, greatly enlarge the indole alkaloid library and accelerate the lead compound optimization thus facilitating drug discovery.

C-H Functionalization of Pyridines via Oxazino Pyridine Intermediates: Switching to para-Selectivity under Acidic Conditions

para-Selective C-H functionalization of pyridines holds a significant value but remains underdeveloped. Site-switchable C-H functionalization of pyridines under easily tunable conditions expedites drug development. We recently reported a redox-neutral dearomatization-rearomatization strategy for meta-C-H functionalization of pyridines via oxazino pyridine intermediates. Here, we demonstrate that these oxazino pyridine intermediates undergo highly para-selective functionalization simply by switching to acidic conditions. A broad scope of para-alkylated and arylated pyridines is prepared through radical as well as ionic pathways. These mild and catalyst-free methods are applied to the late-stage para-functionalization of drugs using pyridines as the limiting reagents. Consecutive meta,para-difunctionalization of pyridines is also achieved with complete regiocontrol relying on the pH-dependent reactivity of oxazino pyridines.

Generation and reaction of alanyl radicals in open flasks

The generation and Giese-type reaction of alanyl radicals under metal-free reaction conditions is described. The procedure is operationally simple, occurring at ambient temperature in an open reaction vessel, and requiring short reaction times (≤5 min). The reaction occurs without epimerization and provides ready access to non-proteinogenic amino acids and peptides. Importantly, the process is tolerant of light absorbing groups including commonly used fluorescent tags.

C(sp3)-C(sp3) coupling of non-activated alkyl-iodides with electron-deficient alkenes via visible-light/silane-mediated alkyl-radical formation

Here, we present a remarkably mild and general initiation protocol for alkyl-radical generation from non-activated alkyl-iodides. An interaction between a silane and an alkyl iodide is excited by irradiation with visible light to trigger carbon-iodide bond homolysis and form the alkyl radical. We show how this method can be developed into an operationally simple and general Giese addition reaction that can tolerate a range of sensitive functionalities not normally explored in established approaches to this strategically important transformation. The new method requires no photocatalyst or other additives and uses only commerical tris(trimethylsilyl)silane and visible light to effectively combine a broad range of alkyl halides with activated alkenes to form C(sp3)-C(sp3) bonds embedded within complex frameworks.

Hydrogen Borrowing: towards Aliphatic Tertiary Amines from Lignin Model Compounds Using a Supported Copper Catalyst

Upcoming biorefineries, such as lignin-first provide renewable aromatics containing unique aliphatic alcohols. In this context, a Cu-ZrO₂ catalyzed hydrogen borrowing approach was established to yield tertiary amine from the lignin model monomer 3-(3,4-dimethoxyphenyl)-1-propanol and the actual lignin-derived monomers, (3-(4-hydroxyphenyl)-1-propanol and dihydroconiferyl alcohol), with dimethylamine. Various industrial metal catalysts were evaluated, resulting in nearly quantitative mass balances for most catalysts. Identified intermediates, side and reaction products were placed into a corresponding reaction network, supported by kinetic evolution experiments. Cu-ZrO₂ was selected as most suitable catalyst combining high alcohol conversion with respectable aliphatic tertiary amine selectivity. Low pressure H₂ was key for high catalyst activity and tertiary amine selectivity, mainly by hindering undesired reactant dimethylamine disproportionation and alcohol amidation. Besides dimethylamine model, diverse secondary amine reactants were tested with moderate to high tertiary amine yields. As most active catalytic site, highly dispersed Cu species in strong contact with ZrO₂ is suggested. ToF-SIMS, N₂ O chemisorption, TGA and XPS of spent Cu-ZrO₂ revealed that imperfect amine product desorption and declining surface Cu lowered the catalytic activity upon catalyst reuse, while thermal reduction readily restored the initial activity and selectivity demonstrating catalyst reuse.

Nickel-Catalyzed Reductive Alkylation of Heteroaryl Imines

The preparation of heterobenzylic amines by a Ni-catalyzed reductive cross-coupling between heteroaryl imines and C(sp3 ) electrophiles is reported. This umpolung-type alkylation proceeds under mild conditions, avoids the pre-generation of organometallic reagents, and exhibits good functional group tolerance. Mechanistic studies are consistent with the imine substrate acting as a redox-active ligand upon coordination to a low-valent Ni center. The resulting bis(2-imino)heterocycle⋅Ni complexes can engage in alkylation reactions with a variety of C(sp3 ) electrophiles, giving heterobenzylic amine products in good yields.

Switchable Radical Carbonylation by Philicity Regulation

Carbonylation reactions involving CO as readily available C1 synthons have become one of the most important tools for the construction of carbonyl compounds from feedstock chemicals. Despite numerous catalytic methods for carbonylation reactions proceeding via ionic or radical pathways, an inherent limitation to these methods is the need to control switchable single and double carbonylative formation of value-added products from the same and simple starting materials. Here, we describe a new strategy that exploits photoredox catalysis to regulate the philicity of amine coupling partners to drive switchable radical carbonylation reactions. In double carbonylation, amines were first transformed into nitrogen radical cations by single-electron transfer-oxidation and coupled with CO to form carbamoyl radicals, which further underwent radical cross-coupling with the incipient cyanoalkyl acyl radicals to afford the double carbonylation products. Upon the addition of stoichiometric 4-dimethylaminopyridine (DMAP), DMAP competitively traps the initially formed cyanoalkyl acyl radical to form the relatively stabilized cyanoalkyl acyl-DMAP salts that engaged in the subsequent substitution with the nucleophilic amines to produce the single carbonylation products. The reaction proceeded smoothly with excellent selectivity in the presence of various amine nucleophiles at room temperature, generating valuable amides and α-ketoamides in a versatile and controlled fashion. Combined experimental and computational studies provided mechanistic insights into the possible pathways.

Recent advances in visible light-induced C(sp3)–N bond formation

Synthetic chemists have long focused on selective C(sp ³)-N bond-forming approaches in response to the high value of this motif in natural products, pharmaceutical agents and functional materials. In recent years, visible light-induced protocols have become an important synthetic platform to promote this transformation under mild reaction conditions. These photo-driven methods rely on converting visible light into chemical energy to generate reactive but controllable radical species. This Review highlights recent advances in this area, mostly after 2014, with an emphasis placed on C(sp ³)-H bond activations, including amination of olefins and carbonyl compounds, and cross-coupling reactions.

Leveraging Electron‐Deficient Iminium Intermediates in a General Synthesis of Valuable Amines

The development of reactions converting alkenes and alkynes into valuable building blocks remains one of the main goals of synthetic chemistry. Herein, we present the leveraging of highly electron‐deficient iminium ions, rare and fleeting intermediates, into a general amine synthesis. This enables the preparation of amines bearing e.g. valuable α‐trifluoromethyl moieties under mild conditions. This broad concept is highlighted by the late‐stage amination of quinine into a biologically interesting new analogue.

Nutzung von elektronenarmen Iminiumintermediaten zur Synthese von wertvollen Aminen

Die Entwicklung von Reaktionen zur Umwandlung von Alkenen und Alkinen in wertvolle Bausteine ist nach wie vor eines der Hauptziele in der synthetischen Chemie. In dieser Arbeit berichten wir von der Nutzbarmachung von stark elektronenarmen, seltenen und kurzlebigen Iminiumionen zur Synthese von Aminen. Dies ermöglicht die milde Herstellung von Aminen mit z. B. trifluormethylierten Einheiten. Dieses umfassende Konzept wird durch die Aminierung von Chinin zu einem biologisch interessanten neuen Analogon verdeutlicht.

Merging Hydrogen Atom Transfer and Halogen Atom Transfer for Iodide-Catalyzed Radical Reductive Cyclization of 1,6-Enynes

Described herein is the development of a metal-free iodide-catalyzed radical reductive cyclization of 1,6-enynes. A strategy involving in situ iodination/radical cyclization/silyl radical-mediated halogen atom transfer/hydrogen atom transfer for the synthesis of functionalized pyrrolidines has been proposed. Using this halogen-atom abstraction protocol, 1,6-enynes with various vinyl halides including inert fluorides, chlorides, and reactive bromides could be transformed into substituted pyrroles via a multistep radical isomerization process.

α-Branched amines through radical coupling with 2-azaallyl anions, redox active esters and alkenes

α-Branched amines are fundamental building blocks in a variety of natural products and pharmaceuticals. Herein is reported a unique cascade reaction that enables the preparation of α-branched amines bearing aryl or alkyl groups at the β- or γ-positions. The cascade is initiated by reduction of redox active esters to alkyl radicals. The resulting alkyl radicals are trapped by styrene derivatives, leading to benzylic radicals. The persistent 2-azaallyl radicals and benzylic radicals are proposed to undergo a radical-radical coupling leading to functionalized amine products. Evidence is provided that the role of the nickel catalyst is to promote formation of the alkyl radical from the redox active ester and not promote the C-C bond formation. The synthetic method introduced herein tolerates a variety of imines and redox active esters, allowing for efficient construction of amine building blocks.

Multicomponent reactions and photo/electrochemistry join forces: atom economy meets energy efficiency

Visible-light photoredox catalysis has been regarded as an extremely powerful tool in organic chemistry, bringing the spotlight back to radical processes. The versatility of photocatalyzed reactions has already been demonstrated to be effective in providing alternative routes for cross-coupling as well as multicomponent reactions. The photocatalyst allows the generation of high-energy intermediates through light irradiation rather than using highly reactive reagents or harsh reaction conditions. In a similar vein, organic electrochemistry has experienced a fruitful renaissance as a tool for generating reactive intermediates without the need for any catalyst. Such milder approaches pose the basis toward higher selectivity and broader applicability. In photocatalyzed and electrochemical multicomponent reactions, the generation of the radical species acts as a starter of the cascade of events. This allows for diverse reactivity and the use of reagents is usually not covered by classical methods. Owing to the availability of cheaper and more standardized photo- and electrochemical reactors, as well as easily scalable flow-setups, it is not surprising that these two fields have become areas of increased research interest. Keeping these in view, this review is aimed at providing an overview of the synthetic approaches in the design of MCRs involving photoredox catalysis and/or electrochemical activation as a crucial step with particular focus on the choice of the difunctionalized reagent.

Multicomponent double Mannich alkylamination involving C(sp2)–H and benzylic C(sp3)–H bonds

Alkylamines are ubiquitous in pharmaceuticals, materials and agrochemicals. The Mannich reaction is a well-known three-component reaction for preparing alkylamines and has been widely used in academic research and industry. However, the nucleophilic components in this process rely on C(sp²)−H and activated C(sp³)−H bonds while the unactivated C(sp³)−H bonds involved Mannich alkylamination is a long-standing challenge. Here, we report an unprecedented multicomponent double Mannich alkylamination for both C(sp²)−H and unactivated benzylic C(sp³)−H bonds. In this process, various 3-alkylbenzofurans, formaldehyde and alkylamine hydrochlorides assemble efficiently to furnish benzofuran-fused piperidines. Mechanistic studies and density functional theory (DFT) calculations revealed a distinctive pathway that a multiple Mannich reaction and retro-Mannich reaction of benzofuran and dehydrogenation of benzylic C(sp³)−H bonds were key steps to constitute the alkylamination. This protocol furnishes a Mannich alkylamine synthesis from unusual C–H inputs to access benzofuran-fused piperidines with exceptional structural diversity, molecular complexity and drug-likeness. Therefore, this work opens a distinctive vision for the alkylamination of unactivated C(sp³)−H bonds, and provides a powerful tool in diversity-oriented synthesis (DOS) and drug discovery.

The Mannich reaction is a three-component reaction for preparing alkylamines, but the nucleophilic components rely on C(sp²)−H and activated C(sp³)−H bonds. Here, the authors report an unprecedented multicomponent double Mannich alkylamination for both C(sp²)−H and unactivated benzylic C(sp³)−H bonds.

Radical Carbonyl Umpolung Arylation via Dual Nickel Catalysis

The formation of carbon-carbon bonds lies at the heart of synthetic organic chemistry and is widely applied to construct complex drugs, polymers, and materials. Despite its importance, catalytic carbonyl arylation remains comparatively underdeveloped, due to limited scope and functional group tolerance. Herein we disclose an umpolung strategy to achieve radical carbonyl arylation via dual catalysis. This redox-neutral approach provides a complementary method to construct Grignard-type products from (hetero)aryl bromides and aliphatic aldehydes, without the need for pre-functionalization. A sequential activation, hydrogen-atom transfer, and halogen atom transfer process could directly convert aldehydes to the corresponding ketyl-type radicals, which further react with aryl-nickel intermediates in an overall polarity-reversal process. This radical strategy tolerates─among others─acidic functional groups, heteroaryl motifs, and sterically hindered substrates and has been applied in the late-stage modification of drugs and natural products.

Modular construction of functionalized anilines via switchable C–H and N-alkylations of traceless N-nitroso anilines with olefins

Switchable C–H or N-alkylations of N-nitroso anilines with olefins.

Redox-neutral access to 3,3′-disubstituted oxindoles via radical coupling reactions

An efficient radical protocol was developed for the assembly of valuable functional oxindoles in the absence of metals, photocatalysts, and bases.