Dual Nickel/Photoredox-Catalyzed Deaminative Cross-Coupling of Sterically Hindered Primary Amines

Mechanometallaphotoredox Catalysis: Utilizing Increased Throughput Mechanochemistry to Develop Solvent-Minimized Aryl Amination and C(sp2)-C(sp3) Cross-Coupling Reactions with Increased Tolerance to Aerobic Conditions

Photocatalysis as a tool used in organic synthesis has predominantly relied on the use of solvents, be it under homogeneous or heterogeneous conditions. In particular, metallaphotoredox catalysis reactions commonly use toxic organic solvents such as DMA and DMF. Herein, we demonstrate how mechanophotocatalysis, the synergistic union of mechanochemistry and photocatalysis, is compatible with this class of dual catalysis reactions involving both photocatalyst and nickel(II) cocatalysts. Using ball milling, these mechanistically complex reactions can be conducted in the absence of a bulk solvent and under air, affording high-yielding aryl aminations and C(sp2)-C(sp3) cross-couplings with alkyl carboxylic acids, alkyl trifluoroborate salts, and alkyl bromides. These advances are facilitated by the introduction of a novel reaction vessel design for conducting four mechanophotocatalysis reactions simultaneously. This work highlights the promise of solvent-minimized photocatalysis reactions, demonstrating that in these examples bulk solvent is redundant, thus significantly reducing this waste stream. Through time-resolved photoluminescence studies, we observed that the excited states of five different photocatalysts were quenched by oxygen more significantly in solution than in the solid state, providing evidence for the origin of the increased tolerance to aerobic conditions that these mechanophotocatalysis reactions experience.

Radical approaches for C(sp3)-N bond cleavage in deaminative transformations

Amines are versatile and widely available functional groups that serve as key building blocks in organic synthesis, particularly in pharmaceutical development. The deaminative functionalization of amines, where the C-N bond of amine is cleaved to function as a carbon source of the reaction, could be very useful for building the carbon framework of organic molecules. Conventional methods for C(sp3)-N bond cleavage often rely on two-electron ionic pathways or transition metal catalysis, which typically require activated amines or toxic reagents, limiting their broad applicability. In contrast, radical-based approaches provide a promising alternative, utilizing homolytic C-N bond cleavage to generate carbon-centered radicals that can participate in a range of transformations. These methods offer complementary substrate scopes and unique reactivity profiles, expanding the synthetic utility of amines. This review summarizes recent advancements in radical-mediated deaminative transformations, focusing on the mechanisms, substrate compatibility, and emerging applications in synthetic organic chemistry.

Heterocyclic Assembly: An Underutilized Disconnection with Potential to Maximize High Fsp3 Chemical Space Exploration

From a retrosynthetic standpoint, functionalization or synthesis of heterocyclic cores are fundamental disconnections that chemists make. This manuscript highlights heterocycle synthesis as the strategic bond disconnection by leveraging ubiquitous building blocks, carboxylic acids and amines, for preparation of heterocyclic cores in a library-friendly format. This heterocyclic formation strategy allows medicinal chemists to access much wider chemical space, especially for analogs with higher Fsp3 vs state-of-the-art heterocycle functionalization methods. The direct impact on medicinal chemistry programs is underscored by adapting and miniaturizing the synthesis of N2-indazoles and C2-benzimidazoles to μ-scale parallel medicinal chemistry (PMC) libraries, affording a similar success rate (80%) as venerable Suzuki and Buchwald-Hartwig libraries.

Benzothiazolium salts as versatile primary alcohol derivatives in Ni-catalyzed cross-electrophile arylation/vinylation

Herein, we report a Ni-catalyzed cross-electrophile coupling of aryl/vinyl halides with benzothiazolium salts derived from alcohols. Our findings demonstrate that primary alkyl benzothiazolium salts serve as effective C(sp3)-O substrates, facilitating coupling with aryl and vinyl halides. This method not only enables the formal functionalization of primary alcohols but also provides experimental support for previously established sequential alcohol halogenation and Ni-catalyzed reductive coupling platforms. Furthermore, this work introduces a new alcohol activation mode, presenting an alternative approach for engaging alcohols as coupling partners in organic synthesis.

Modular Synthesis of Dehydroprolines by an Energy-Transfer Enabled Cloke-Wilson Rearrangement

A one-pot photocatalytic method is reported for the generation of dehydroprolines, valuable precursors to 5-aryl prolines. Imines, obtained via simple condensation of aminocyclopropane carboxylates (ACPC) with a broad range of aldehydes, were employed in this transformation without purification. We demonstrate this energy-transfer (EnT) enabled Cloke-Wilson-type rearrangement affords both the rare 1,2-dehydroprolines or the more thermodynamically favored 1,5-isomers with up to >20 : 1 selectivity in both directions, using an identical catalytic system from the same starting material. Syntheses of intermediates of bioactive molecules in high yields and selectivity highlight this enabling transformation. Mechanistic studies support the proposed triplet-triplet EnT mode of activation, distinct from the previously developed singlet excited states accessed via UV excitation.

Low-energy photoredox catalysis

With the advent of photoredox catalysis, new synthetic paradigms have been established with many novel transformations being achieved. Nevertheless, modern photoredox chemistry has several drawbacks, namely, deficiencies in reaction efficiency and scalability. Furthermore, wavelengths of light in excess of the energy required for a chemical reaction are often used. In this Review, we document recent developments of low-energy light-absorbing catalysts and their cognate photochemical methods, advantageously mitigating off-cycle photochemical reactivity of excited-state species in the reaction mixture and improving batch scalability of photochemical reactions. Finally, developments in red-light photoredox catalysis are leading the next-generation applications to polymer science and biochemistry-chemical biology, enabling catalytic reactions within media composites - including mammalian tissue - that are historically recalcitrant with blue-light photoredox catalysis.

Csp3-Csp2 Coupling of Isonitriles and (Hetero)arenes through a Photoredox-Catalyzed Double Decyanation Process

Herein, we demonstrate the ability of isonitriles to be used as alkyl radical precursors in a photoredox-catalyzed transformation involving selective C-N cleavage and Csp3-Csp2 bond formation. This protocol allows for the preparation of functionalized heteroarenes from readily available isonitriles through a decyanation process. The reaction is general for primary, secondary, and tertiary substrates, including amino acid derivatives and druglike molecules.

Alkylthiolation of Aryl Halides under Electrochemical Conditions

An electrochemical protocol for the alkylthiolation of aryl halides was developed. By using dialkyl disulfides as the alkylthio sources, Mg plate as the sacrificial anode and graphite felt (GF) as the cathode, a series of aryl sulfides were obtained in moderate to good yields. The approach was also suitable for the synthesis of aryl selenides. This method has the features such as simple reaction conditions and good functional group compatibility, which makes it have a good application prospect in organic synthesis and drug synthesis.

Reductive deaminative cross-coupling of alkyl bistriflimides enabled by electrocatalysis

We present a versatile nickel-electrocatalytic deaminative cross-coupling platform for the efficient construction of C(sp3)-C(sp3) and C(sp3)-C(sp2) bonds from readily available alkyl bistriflimides. This methodology involves the assembly of two leaving groups on alkyl amines to form alkyl bistriflimides, followed by their effective coupling with a wide range of alkyl halides, alkyl pseudohalides, aryl halides, and alkenyl halides under electrochemical reductive conditions. Moreover, the successful application of electrochemical reductive relay cross-coupling and transition metal-free cross-electrophile coupling further demonstrates the versatility of alkyl bistriflimides as valuable building blocks in organic synthesis. Combined control experiments and density functional theory calculations provide insights into the reaction pathway and the crucial role of iodide in the catalytic process.

Photoinduced Nickel-Catalyzed Homolytic C(sp3)-N Bond Activation of Isonitriles for Selective Carbo- and Hydro-Cyanation of Alkynes

The exploration of strong chemical bonds as synthetic handles offers new disconnection strategies for the synthesis of functionalized molecules via transition metal catalysis. However, the slow oxidative addition rate of these covalent bonds to a transition metal center hampers their synthetic utility. Here, we report a C(sp3)-N bond activation strategy that bypasses thermodynamically challenging 2e- or 1e- oxidative addition via a distinct pathway in nickel catalysis. This strategy leverages a previously unknown activation pathway of photoinduced inner-sphere charge transfer of low-valent nickel(isonitriles), triggering a C(sp3)-N bond cleavage distal to the metal-ligand interaction to deliver nickel(cyanide) and versatile alkyl radicals. Utilizing this catalytic strategy, the selective intermolecular 1,2-carbocyanation reaction of alkynes with alkyl isonitriles as both alkylating and cyanating agents can be achieved, delivering a wide array of trisubstituted alkenyl nitriles with excellent atom-economy, regio-, and stereoselectivity under mild conditions. Furthermore, Markovnikov-selective hydrocyanation of aliphatic alkynes can be accomplished through the synergistic action of a photocatalyst utilizing isonitriles as the cyanation agents. Mechanistic investigations support the photogeneration of low-valent Ni(isonitrile) complexes that undergo photochemical homolysis of the C(sp3)-N bond to engage catalytic cyanation with alkynes.

Precision Propargylic Substitution Reaction: Pd-Catalyzed Suzuki-Miyaura Coupling of Nonactivated Propargylamines with Boronic Acids

Palladium-catalyzed Suzuki-Miyaura cross-coupling is an efficient approach for C-C bond construction. Here we report a deaminative Suzuki-Miyaura reaction to achieve chemo- and regioselectivity in the cross-coupling of nonactivated propargylamines with boronic acids, in which methyl propiolate is introduced to promote the cleavage of the C-N bond to form the C-C bond. This method features a wide range of substrates, good functional group tolerance, and ease of operation, providing an alternative approach to accessing valuable propargylated aromatic compounds.

Synthesis of Secondary Boronates via Deaminative Cross-Coupling of Alkyl Nitroso Carbamates and Boronic Acids

A strategy for transition metal-free cross-coupling of alkyl nitroso-carbamates and boronic acids is reported. The N-nitroso carbamates are easily prepared from the corresponding amine in two simple steps. This method allows for the synthesis of a wide variety of secondary boronates, benzylic boronates and formal Csp3-Csp2 cross-coupling products under operationally simple conditions. Functional group tolerance is also demonstrated and applied in the modification of lysine to make non-canonical amino acids.

Amine-Borane-Mediated, Nickel/Photoredox-Catalyzed Cross-Electrophile Coupling between Alkyl and Aryl Bromides

Nickel/photoredox catalysis has emerged as a powerful platform for exploring nontraditional and challenging cross-couplings. Herein, a metallaphotoredox catalytic protocol has been developed on the basis of a tertiary amine-ligated boryl radical-induced halogen atom transfer process under blue-light irradiation. A wide variety of aryl and heteroaryl bromides featuring different functional groups and pharmaceutical moieties were facilely coupled to rapidly install C(sp3)-enriched aromatic scaffolds. The compatibility of Lewis base-ligated borane with nickel catalysis was well exemplified to extend the chemical space for Ni-catalyzed cross-electrophile coupling.

Benzoquinone Ligand-Enabled Ruthenium-Catalyzed Deaminative Coupling of 2-Aminoaryl Aldehydes and Ketones with Branched Amines for Regioselective Synthesis of Quinoline Derivatives

The catalytic system generated in situ from the cationic Ru-H complex [(C6H6)(PCy3)(CO)RuH]+BF4- (1) with 2,3,4,5-tetrachloro-1,2-benzoquinone (L1) was found to be highly effective for promoting the deaminative coupling reaction of 2-aminoaryl aldehydes with branched amines to form 2-substituted quinoline products. The analogous deaminative coupling reaction of 2-aminoaryl ketones with branched amines led to the regioselective formation of 2,4-disubstituted quinoline products. A number of biologically active quinoline derivatives including graveolinine and a triplex DNA intercalator have been synthesized by using the catalytic method.

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.

Nickel-Electrocatalytic Decarboxylative Arylation to Access Quaternary Centers

There is a pressing need, particularly in the field of drug discovery, for general methods that will enable direct coupling of tertiary alkyl fragments to (hetero)aryl halides. Herein a uniquely powerful and simple set of conditions for achieving this transformation with unparalleled generality and chemoselectivity is disclosed. This new protocol is placed in context with other recently reported methods, applied to simplify the routes of known bioactive building blocks molecules, and scaled up in both batch and flow. The role of pyridine additive as well as the mechanism of this reaction are interrogated through Cyclic Voltammetry studies, titration experiments, control reactions with Ni(0) and Ni(II)-complexes, and ligand optimization data. Those studies indicate that the formation of a BINAPNi(0) is minimized and the formation of an active pyridine-stabilized Ni(I) species is sustained during the reaction. Our preliminary mechanistic studies ruled out the involvement of Ni(0) species in this electrochemical cross-coupling, which is mediated by Ni(I) species via a Ni(I)-Ni(II)-Ni(III)-Ni(I) catalytic cycle.

Site-Specific Deaminative Trifluoromethylation of Aliphatic Primary Amines

The introduction of trifluoromethyl groups into organic molecules is of paramount importance in modern synthetic chemistry and medicinal chemistry. While methods for constructing C(sp2 )-CF3 bonds have been well established, the advancement of practical and comprehensive approaches for forming C(sp3 )-CF3 bonds remains considerably restricted. In this work, we describe an efficient and site-specific deaminative trifluoromethylation reaction of aliphatic primary amines to afford the corresponding alkyl trifluoromethyl compounds. The reaction proceeds at room temperature with readily accessible N-anomeric amide (Levin's reagent) and bench-stable bpyCu(CF3 )3 (Grushin's reagent, bpy=2,2'-bipyridine) under blue light. The protocol features mild reaction conditions, good functional group tolerance, and moderate to good yields. Remarkably, the method can be applied to the direct, late-stage trifluoromethylation of natural products and bioactive molecules. Experimental mechanistic studies were conducted, and a radical mechanism is proposed, wherein the dual roles of Grushin's reagent have been elucidated.

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.

Photoredox-Catalyzed Generation of Tertiary Anions from Primary Amines via a Radical Polar Crossover

A method for the generation of tertiary carbanions via a deaminative radical-polar crossover is reported using redox active imines from α-tertiary primary amines. A variety of benzylic amines and amino esters can be used in this approach, with the latter engaging in a novel "aza-Reformatsky" reaction. Electronic trends correlate the stability of the resulting carbanion with reaction efficiency. The anions can be trapped with different electrophiles including aldehydes, ketones, imines, Michael acceptors, and H2 O/D2 O. Selective anion formation can be achieved in the presence of another equivalent or more acidic C-H bond in both an inter- and intramolecular fashion. Mechanistic studies suggest the intermediacy of a discrete carbanion intermediate.

Generation of Aromatic N-Heterocyclic Radicals for Functionalization of Unactivated Alkenes

Nitrogen-centered radicals (NCRs) have been widely recognized as versatile synthetic intermediates for the construction of nitrogen containing molecules of high value. As such, there has been a long-standing interest in the field of organic synthesis to develop novel nitrogen-based radicals and explore their inherent reactivity. In this study, we present the generation of aromatic N-heterocyclic radicals and their application in a novel and diverse functionalization of unactivated alkenes. Bench-stable aromatic N-heterocyclic pyridinium salts were employed as crucial NCR precursors, which enabled the efficient conversion of various unactivated alkenes into medicinally relevant alkylated N-heterocyclic amines. This approach offers an unexplored retrosynthetic disconnection for the synthesis of related molecules that commonly possess therapeutic value. Furthermore, this platform can be extended to the synthesis of densely functionalized heterocyclic amines by utilizing disulfides and diethyl bromomalonate as radical quenchers. Mechanistic investigations indicate an energy transfer (EnT) pathway involving the formation of a transient aromatic N-heterocyclic radical, radical addition to unactivated alkenes, and subsequent generation of the amination product through either hydrogen atom transfer (HAT) or radical addition processes.

Late-Stage Isotopic Exchange of Primary Amines

Stable isotopes such as 2H, 13C, and 15N have important applications in chemistry and drug discovery. Late-stage incorporation of uncommon isotopes via isotopic exchange allows for the direct conversion of complex molecules into their valuable isotopologues without requiring a de novo synthesis. While synthetic methods exist for the conversion of hydrogen and carbon atoms into their less abundant isotopes, a corresponding method for accessing 15N-primary amines from their naturally occurring 14N-analogues has not yet been disclosed. We report an approach to access 15N-labeled primary amines via late-stage isotopic exchange using a simple benzophenone imine as the 15N source. By activating α-1 and α-2° amines to Katritzky pyridinium salts and α-3° amines to redox-active imines, we can engage primary alkyl amines in a deaminative amination. The redox-active imines proceed via a radical-polar crossover mechanism, whereas the Katritzky salts are engaged in copper catalysis via an electron donor-acceptor complex. The method is general for a variety of amines, including multiple drug compounds, and results in complete and selective isotopic labeling.

Divergent Photosensitizer Controlled Reactions of 4-Hydroxycoumarins and Unactivated Olefins: Hydroarylation and Subsequent [2+2] Cycloaddition

Herein, we report a photoinduced approach for hydroarylation of unactivated olefins using 4-hydroxycoumarins as the arylating reagent. Key to the success of this reaction is the conversion of nucleophilic 4-hydroxycoumarins into electrophilic carbon radicals via photocatalytic arene oxidation, which not only circumvents the polarity-mismatch issue encountered under ionic conditions but also accommodates a broad substrate scope and inhibits side reactions that were previously observed. Moreover, divergent reactivity was achieved by changing the photocatalyst, enabling a subsequent [2+2] cycloaddition to deliver cyclobutane-fused pentacyclic products that are otherwise challenging to access in high yields and with high diastereoselectivity. Mechanistic studies have elucidated the mechanism of the reactions and the origin of the divergent reactivity.

Transition-Metal-Free Base-Promoted Deaminative Coupling of Gramines with Aminomaleimides

The direct utilization of amines for C-C bond formation without prefunctionalization remains a significant challenge. Herein, we report the base-promoted deaminative coupling of gramines with aminomalaimides under redox-neutral conditions. In this operationally simple reaction, a series of indolmethyl-substituted aminomaleimides that emitted fluorescence were synthesized in good-to-excellent yields. Biological evaluation revealed that some products exhibited antiproliferative activity against human cancer cell lines.

Orange Light-Driven C(sp2)-C(sp3) Cross-Coupling via Spin-Forbidden Ir(III) Metallaphotoredox Catalysis

We report the development and characterization of a library of Ir(III) photocatalysts capable of undergoing spin-forbidden excitation (SFE) under orange light irradiation (595 nm). These catalysts were successfully applied to the construction of synthetically valuable C(sp2)-C(sp3) bonds inaccessible with existing methods of low-energy light-driven dual nickel/photoredox catalysis, demonstrating the synthetic utility of this photocatalyst family. The photocatalysts are capable of accessing both oxidatively and reductively activated coupling partners, illustrated through deaminative arylation and potassium alkyl trifluoroborate cross-coupling reactions with aryl halides. We demonstrate diverse substrate scopes of both cross-coupling paradigms under mild conditions in the first example of low-energy light-driven C(sp2)-C(sp3) metallaphotoredox coupling.

Visible-Light/Nickel-Catalyzed Carboxylation of C(sp2) Bromides via Formate Activation

A new visible-light-driven method for the carboxylation of (hetero)aryl/vinyl bromides has been developed using catalytic 4CzIPN, nickel, phenyl triflimide, and sodium formate as a carboxylation agent. Interestingly, we found catalytic phenyl triflimide plays an essential role in promoting the reaction. While many C(sp²) carboxylation reactions require harsh reagents or gaseous carbon dioxide, we demonstrate the mild and facile construction of carboxylic acids from readily available starting materials.

Halide Noninnocence and Direct Photoreduction of Ni(II) Enables Coupling of Aryl Chlorides in Dual Catalytic, Carbon-Heteroatom Bond-Forming Reactions

Recent mechanistic studies of dual photoredox/Ni-catalyzed, light-driven cross-coupling reactions have found that the photocatalyst (PC) operates through either reductive quenching or energy transfer cycles. To date, reports invoking oxidative quenching cycles are comparatively rare and direct observation of such a quenching event has not been reported. However, when PCs with highly reducing excited states are used (e.g., Ir(ppy)3), photoreduction of Ni(II) to Ni(I) is thermodynamically feasible. Recently, a unified reaction system using Ir(ppy)3 was developed for forming C-O, C-N, and C-S bonds under the same conditions, a prospect that is challenging with PCs that can photooxidize these nucleophiles. Herein, in a detailed mechanistic study of this system, we observe oxidative quenching of the PC (Ir(ppy)3 or a phenoxazine) via nanosecond transient absorption spectroscopy. Speciation studies support that a mixture of Ni-bipyridine complexes forms under the reaction conditions, and the rate constant for photoreduction increases when more than one ligand is bound. Oxidative addition of an aryl iodide was observed indirectly via oxidation of the resulting iodide by Ir(IV)(ppy)3. Intriguingly, the persistence of the Ir(IV)/Ni(I) ion pair formed in the oxidative quenching step was found to be necessary to simulate the observed kinetics. Both bromide and iodide anions were found to reduce the oxidized form of the PC back to its neutral state. These mechanistic insights inspired the addition of a chloride salt additive, which was found to alter Ni speciation, leading to a 36-fold increase in the initial turnover frequency, enabling the coupling of aryl chlorides.

Harnessing Sulfur(VI) Fluoride Exchange Click Chemistry and Photocatalysis for Deaminative Benzylic Arylation

While among the most common functional handles present in organic molecules, amines are a widely underutilized linchpin for C-C bond formation. To facilitate C-N bond cleavage, large activating groups are typically used but result in the generation of stoichiometric amounts of organic waste. Herein, we report an atom-economic activation of benzylic primary amines relying on the Sulfur(VI) Fluoride Exchange (SuFEx) click chemistry and the aza-Ramberg-Bäcklund reaction. This two-step sequence allows the high-yielding generation of 1,2-dialkyldiazenes from primary amines via loss of SO2. Excitation of the diazenes with blue light and an Ir photocatalyst affords radical pairs upon expulsion of N2, which can be coaxed into the formation of C(sp3)-C(sp2) bonds upon diffusion and capture by a Ni catalyst. This arylative strategy relying on a traceless click approach was harnessed in a variety of examples and its mechanism was investigated.

Cross-Coupling of Amines via Photocatalytic Denitrogenation of In Situ Generated Diazenes

A method for C(sp3)-C(sp3) cross-coupling of amines is described. Primary amines are converted to 1,2-dialkyldiazenes by treatment with O-nosylhydroxylamines in the presence of atmospheric oxygen. Denitrogenation of the diazenes with an iridium photocatalyst then forges the C-C bond. The substrate scope accommodates a broad latitude of functionality, including heteroaromatics and unprotected alcohols and acids.

In(OTf)3-catalyzed reorganization/cycloaddition of two imine units and subsequent modular assembly of acridinium photocatalysts

Herein, we disclose a novel reorganization/cycloaddition between two imine units catalyzed by In(OTf)3 Lewis acid that differs from the well-known [4 + 2] cycloaddition version via the Povarov reaction. By means of this unprecedented imine chemistry, a collection of synthetically useful dihydroacridines has been synthesized. Notably, the obtained products give rise to a series of structurally novel and fine-tuneable acridinium photocatalysts, offering a heuristic paradigm for synthesis and efficiently facilitating several encouraging dihydrogen coupling reactions.

Direct Activation of the C(sp3)-NH2 Bond of Primary Aliphatic Alkylamines by a High-Valent CoIII,IV2(μ-O)2 Diamond Core Complex

Aliphatic alkylamines are abundant feedstock and versatile building blocks for many organic transformations. While remarkable progress has been made to construct C-N bonds on aliphatic and aromatic carbon centers, the activation and functionalization of C(sp³)-NH₂ bonds in primary alkylamines remain a challenging process. In the present work, we discovered an unprecedented method to directly activate the C(sp³)-NH₂ bond of primary alkylamines by a high-valent dinuclear Co^III,IV₂(μ-O)₂ diamond core complex. This reaction results in the installation of other functional groups such as halides and alkenes onto the α-carbon center concomitant with the 2-e^- oxidation of the nitrogen atom on the amino group to form NH₂OH. These results shed light on future development enabling versatile functionalization of primary alkylamines based on the dinuclear cobalt system. Moreover, our work suggests that a related high-valent copper-oxo intermediate is likely generated in the ammonia monooxygenase catalytic cycle to affect the oxidation of NH₃ to NH₂OH.

Direct Deaminative Functionalization

Selective functional group interconversions in complex molecular settings underpin many of the challenges facing modern organic synthesis. Currently, a privileged subset of functional groups dominates this landscape, while others, despite their abundance, are sorely underdeveloped. Amines epitomize this dichotomy; they are abundant but otherwise intransigent toward direct interconversion. Here, we report an approach that enables the direct conversion of amines to bromides, chlorides, iodides, phosphates, thioethers, and alcohols, the heart of which is a deaminative carbon-centered radical formation process using an anomeric amide reagent. Experimental and computational mechanistic studies demonstrate that successful deaminative functionalization relies not only on outcompeting the H-atom transfer to the incipient radical but also on the generation of polarity-matched, productive chain-carrying radicals that continue to react efficiently. The overall implications of this technology for interconverting amine libraries were evaluated via high-throughput parallel synthesis and applied in the development of one-pot diversification protocols.

Deaminative defluoroalkylation of α-trifluoromethylalkenes enabled by photoredox catalysis

Herein, we disclose a new photoredox-catalysed strategy to access gem-difluoroallylarenes from α-trifluoromethylalkenes with sterically hindered primary amines via C-N and C-F bond activation. This deaminative and defluorinative allylation is generally compatible with diverse functional groups and sterically hindered α-3° and 2° primary amines.

Deaminative Arylation and Alkenyaltion of Aliphatic Tertiary Amines with Aryl and Alkenylboronic Acids via Nitrogen Ylides

Transition-metal-catalyzed Suzuki-Miyaura coupling has significantly advanced C-C bond formation and has been well recognized in organic synthesis, pharmaceuticals, materials science and other fields. In this rapid development, cross coupling without transition metal catalyst is a big challenge in this field, and using widely existing tertiary amines as electrophiles to directly couple with boronic acids has great hurdles yet significant application prospects. Herein, we report an efficient and general deaminative arylation and alkenylation of tertiary amines (propargyl amines, allyl amines and 1H-indol-3-yl methane amines) with ary and alkenylboronic acids enabled by difluorocarbene under transition-metal-free conditions. Preliminary mechanism experiments suggest that in situ formed difluoromethyl quaternary amine salt, nitrogen ylide and tetracoordinate boron species are the key intermediates, the subsequent 1,2-metallate shift and protodeboronation complete the new coupling reaction.

Overcoming Photochemical Limitations in Metallaphotoredox Catalysis: Red-Light-Driven C-N Cross-Coupling

Aryl amination is an essential transformation for medicinal, process, and materials chemistry. In addition to classic Buchwald-Hartwig amination conditions, blue-light-driven metallaphotoredox catalysis has emerged as a valuable tool for C-N cross-coupling. However, blue light suffers from low penetration through reaction media, limiting its scalability for industrial purposes. In addition, blue light enhances unwanted side-product formation in metallaphotoredox catalysis, namely hydrodehalogenation. Low-energy light, such as deep red (DR) or near-infrared (NIR), offers a solution to this problem as it can provide enhanced penetration through reaction media as compared to higher-energy wavelengths. Herein, we show that low-energy light can also enhance the desired reactivity in metallaphotoredox catalysis by suppressing unwanted hydrodehalogenation. We hypothesize that the reduced side product is formed by direct photolysis of the aryl-nickel bond by the high-energy light, leading to the generation of aryl radicals. Using deep-red or near-infrared light and an osmium photocatalyst, we demonstrate an enhanced scope of (hetero)aryl bromides and amine-based nucleophiles with minimal formation of hydrodehalogenation byproducts.

Visible-Light-Induced C-F and C-N Bond Cleavage for the Synthesis of gem-Difluoroalkenes

Herein, we describe a novel and efficient photoredox catalytic radical addition/defluoroalkylation coupling reaction between primary amines and trifluoromethyl-substituted alkenes. A series of gem-difluoroalkenes were synthesized via C-N bond cleavage of α-3°, α-2°, and α-1° amines under visible light irradiation. This reaction is characterized by a broad substrate scope and good functional group tolerance.

Dual Nickel/Photoredox-Catalyzed Site-Selective Cross-Coupling of 1,2-Bis-Boronic Esters Enabled by 1,2-Boron Shifts

Site-selective transition-metal-catalyzed mono-deboronative cross-couplings of 1,2-bis-boronic esters are valuable methods for the synthesis of functionalized organoboron compounds. However, such cross-couplings are limited to reaction of the sterically less hindered primary boronic ester. Herein, we report a nickel/photoredox-catalyzed mono-deboronative arylation of 1,2-bis-boronic esters that is selective for coupling of the more sterically hindered secondary/tertiary position. This is achieved by taking advantage of a 1,2-boron shift of primary β-boryl radicals to the thermodynamically favored secondary/tertiary radicals, which are subsequently intercepted by the nickel catalyst to enable arylation. The mild conditions are amenable to a broad range of aryl halides to give β-aryl boronic ester products in good yields and with high regioselectivity. This method also allows stereodivergent coupling of cyclic cis-1,2-bis-boronic esters to give trans-substituted products.

Visible-Light-Promoted Desulfurative Alkylation of Alkyl Thianthrenium Salts with Activated Olefins

Reactions involving an alkyl radical generated from a primary alcohol by photochemistry are rare and challenging. Herein, we present a photocatalyst- and metal-free approach that enables the generation of an alkyl radical from the corresponding alcohol and the subsequent C(sp³)-C(sp³) bond formation with activated olefin, via an alkyl thianthrenium salt/Hantzsch ester electron donor-acceptor complex. This protocol for the conversion of a C-OH bond to a C-C bond is highly functionality tolerant and can successfully be used in late-stage functionalization of pharmaceuticals.

Photoredox Deaminative Alkylation in DNA-Encoded Library Synthesis

Herein, we report an on-DNA photoredox-mediated deaminative alkylation method for diversifying DNA-tagged acrylamide substrate with amine-derived radicals. The radicals can be conveniently generated from sterically hindered primary amines, and the deaminative alkylation can tolerate a broad array of radical precursors. Furthermore, the methodology is applicable to Boc-protected diamines, free amino acids, and aryl halides, which bear functional groups enabling additional rounds of diversification. The method is believed to offer a high potential for constructing DNA-encoded libraries, as was demonstrated by the production of a mock library in a 2 × 3 matrix format and confirmation of DNA stability by UPLC-MS and qPCR experiments.

Synergistic Brønsted/Lewis acid catalyzed aromatic alkylation with unactivated tertiary alcohols or di-tert-butylperoxide to synthesize quaternary carbon centers

Dual Brønsted/Lewis acid catalysis involving environmentally benign, readily accessible protic acid and iron promotes site-selective tert-butylation of electron-rich arenes using di-tert-butylperoxide. This transformation inspired the development of a synergistic Brønsted/Lewis acid catalyzed aromatic alkylation that fills a gap in the Friedel-Crafts reaction literature by employing unactivated tertiary alcohols as alkylating agents, leading to new quaternary carbon centers. Corroborated by DFT calculations, the Lewis acid serves a role in enhancing the acidity of the Brønsted acid. The use of non-allylic, non-benzylic, and non-propargylic tertiary alcohols represents an underexplored area in Friedel-Crafts reactivity.