Expedient synthesis of chiral α-amino acids through nickel-catalyzed reductive cross-coupling

Photoredox Nickel-Catalysed Stille Cross-Coupling Reactions

The Stille cross-coupling reaction is one of the most common strategies for the construction of C-C bonds. Despite notable strides in the advancement of the Stille reaction, persistent challenges persist in hindering its greener evolution. These challenges encompass multiple facets, such as the high cost of precious metals and ligands, the demand for various additives, and the slow reaction rate. In comparison to the dominant palladium-catalysed Stille reactions, cost-effective nickel-catalysed systems lag behind, and enantioconvergent Stille reactions of racemic stannanes remain undeveloped. Herein, we present a pioneering instance of nickel-catalysed enantioconvergent Stille cross-coupling reactions of racemic stannane reagents, resulting in the formation of C-C bonds in good to high yields with excellent stereoselectivity. This strategy provides a practical, scalable, and operationally straightforward method for the synthesis of C(sp3 )-C(sp3 ), C(sp3 )-C(sp2 ), and C(sp3 )-C(sp) bonds under exceptionally mild conditions (without additives and bases, ambient temperature). The innovative use of synergistic photoredox/nickel catalysis enables a novel single-electron transmetalation process of stannane reagents, providing a new research paradigm of Stille reactions.

Development of [18F]F-5-OMe-Tryptophans through Photoredox Radiofluorination: A New Method to Access Tryptophan-Based PET Agents

Although various radiolabeled tryptophan analogs have been developed to monitor tryptophan metabolism using positron emission tomography (PET) for various human diseases including melanoma and other cancers, their application can be limited due to the complicated synthesis process. In this study, we demonstrated that photoredox radiofluorination represents a simple method to access novel tryptophan-based PET agents. In brief, 4-F-5-OMe-tryptophans (l/d-T13) and 6-F-5-OMe-tryptophans (l/d-T18) were easily synthesized. The 18F-labeled analogs were produced by photoredox radiofluorination with radiochemical yields ranging from 2.6 ± 0.5% to 32.4 ± 4.1% (3 ≤ n ≤ 5, enantiomeric excess ≥ 99.0%) and over 98.0% radiochemical purity. Small animal imaging showed that l-[18F]T13 achieved 9.58 ± 0.26%ID/g tumor uptake and good contrast in B16F10 tumor-bearing mice (n = 3). Clearly, l-[18F]T13 exhibited prominent tumor uptake, warranting future evaluations of its potential usage in precise immunotherapy monitoring.

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.

Visible-Light Enabled C(sp3)-C(sp2) Cross-Electrophile Coupling via Synergistic Halogen-Atom Transfer (XAT) and Nickel Catalysis

We herein report the first visible-light-mediated cross-coupling of unactivated alkyl iodides with aryl bromides through synergistic halogen atom transfer (XAT) and nickel catalysis. This simple protocol operates under mild reaction conditions and tolerates a variety of functional groups affording C(sp³)-C(sp²) cross-coupling products in good to moderate yields.

Radical 1,4-Aryl Migration Enabled Remote Cross-Electrophile Coupling of α-Amino-β-Bromo Acid Esters with Aryl Bromides

We report an unprecedented, efficient nickel-catalysed radical relay for the remote cross-electrophile coupling of β-bromo-α-benzylamino acid esters with aryl bromides via 1,4-aryl migration/arylation cascades. β-Bromo-α-benzylamino acid esters are considered as unique molecular scaffolds allowing for aryl migration reactions, which are conceptually novel variants for the radical Truce-Smiles rearrangement. This reaction enables the formation of two new C(sp³ )-C(sp² ) bonds using a bench-stable Ni/bipyridine/Zn system featuring a broad substrate scope and excellent diastereoselectivity, which provides an effective platform for the remote aryl group migration and arylation of amino acid esters via redox-neutral C(sp³ )-C(sp² ) bond cleavage. Mechanistically, this cascade reaction is accomplished by combining two powerful catalytic cycles consisting of a cross-electrophile coupling and radical 1,4-aryl migration through the generation of C(sp³ )-centred radical intermediates from the homolysis of C(sp³ )-Br bonds and the switching of the transient alkyl radical into a robust α-aminoalkyl radical.

Synthesis of γ-Oxo-α-amino Acids via Radical Acylation with Carboxylic Acids

Herein we present a highly efficient, light-mediated, deoxygenative protocol to access γ-oxo-α-amino acid derivatives. This radical methodology employs photoredox catalysis, in combination with triphenylphosphine, to generate acyl radicals from readily available (hetero)aromatic and vinylic carboxylic acids. This approach allows for the straightforward synthesis of γ-oxo-α-amino acids bearing a wide range of functional groups (e.g., Cl, CN, furan, thiophene, Bpin) in synthetically useful yields (∼60% average yield). To further highlight the utility of the methodology, several deprotection and derivatization reactions were carried out.

Nickel-Catalyzed Enantioconvergent Reductive Hydroalkylation of Olefins with α-Heteroatom Phosphorus or Sulfur Alkyl Electrophiles

Substantial advances in enantioconvergent C(sp³)-C(sp³) bond formation reactions have been made in recent years through the use of transition-metal-catalyzed cross-coupling reactions of racemic secondary alkyl electrophiles with organometallic reagents. Herein, we report a general process for the asymmetric construction of alkyl-alkyl bonds adjacent to heteroatoms, namely, a nickel-catalyzed enantioconvergent reductive hydroalkylation of olefins with α-heteroatom phosphorus or sulfur alkyl electrophiles. Including the use of readily available olefins, this reaction has considerable advantages, such as mild reaction conditions, a broad substrate scope, and good functional group compatibility, making it a desirable alternative to traditional electrophile-nucleophile cross-coupling reactions.

A practical and scalable system for heteroaryl amino acid synthesis

Here, we describe a highly-effective catalytic method for the synthesis of heteroarene-containing unnatural amino acids.

A robust system for the preparation of β-heteroaryl α-amino acid derivatives has been developed using photoredox catalysis. This system operates via regiospecific activation of halogenated pyridines (or other heterocycles) and conjugate addition to dehydroalanine derivatives to deliver a wide range of unnatural amino acids. This process was conducted with good efficiency on large scale, the application of these conditions to amino ketone synthesis is shown, and a simple protocol is given for the preparation of enantioenriched amino acid synthesis, from a number of radical precursors.

Decarboxylative Cross-Electrophile Coupling of N-Hydroxyphthalimide Esters with Aryl Iodides

A new method for the decarboxylative coupling of alkyl N-hydroxyphthalimide esters (NHP esters) with aryl iodides is presented. In contrast to previous studies that form alkyl radicals from carboxylic acid derivatives, no photocatalyst, light, or arylmetal reagent is needed, only nickel and a reducing agent (Zn). Methyl, primary, and secondary alkyl groups can all be coupled in good yield (77% ave yield). One coupling with an acid chloride is also presented. Stoichiometric reactions of (dtbbpy)Ni(2-tolyl)I with an NHP ester show for the first time that arylnickel(II) complexes can directly react with NHP esters to form alkylated arenes.

Practical carbon-carbon bond formation from olefins through nickel-catalyzed reductive olefin hydrocarbonation

New carbon–carbon bond formation reactions expand our horizon of retrosynthetic analysis for the synthesis of complex organic molecules. Although many methods are now available for the formation of C(sp²)–C(sp³) and C(sp³)–C(sp³) bonds via transition metal-catalyzed cross-coupling of alkyl organometallic reagents, direct use of readily available olefins in a formal fashion of hydrocarbonation to make C(sp²)–C(sp³) and C(sp³)–C(sp³) bonds remains to be developed. Here we report the discovery of a general process for the intermolecular reductive coupling of unactivated olefins with alkyl or aryl electrophiles under the promotion of a simple nickel catalyst system. This new reaction presents a conceptually unique and practical strategy for the construction of C(sp²)–C(sp³) and C(sp³)–C(sp³) bonds without using any organometallic reagent. The reductive olefin hydrocarbonation also exhibits excellent compatibility with varieties of synthetically important functional groups and therefore, provides a straightforward approach for modification of complex organic molecules containing olefin groups.

Olefins are employed in many coupling procedures but direct hydrocarbonations of unactivated olefins remain to be developed. Here, the authors report the nickel-catalyzed reductive coupling of olefins with aryl and alky electrophiles under mild conditions and with a broad substrate scope.

Copper-catalyzed cross-coupling reactions of epoxides with gem-diborylmethane: access to γ-hydroxyl boronic esters

Herein, we describe a novel copper-catalyzed epoxide opening reaction with gem-diborylmethane.

A class of effective decarboxylative perfluoroalkylating reagents: [(phen)2Cu](O2CRF)

A class of effective reagents [(phen)₂Cu](O₂CR_F) for the decarboxylative perfluoroalkylation of aryl and heteroaryl halides have been invented.

Ligand-Controlled Regiodivergent Copper-Catalyzed Alkylboration of Alkenes

A novel copper-catalyzed regiodivergent alkylboration of alkenes with bis(pinacolato)diboron and alkyl halides has been developed. The regioselectivity of the alkylboration was controlled by subtle differences in the ligand structure. The reaction thus enables the practical, regiodivergent synthesis of two different alkyl boronic esters with complex structures from a single alkene.

Stereoselective Synthesis of Chiral β-Fluoro α-Amino Acids via Pd(II)-Catalyzed Fluorination of Unactivated Methylene C(sp(3))-H Bonds: Scope and Mechanistic Studies

The synthesis of fluorinated complex molecules via direct C(sp(3))-H fluorination is attractive yet remains challenging. Here we describe the Pd(II)-catalyzed fluorination of unactivated methylene C(sp(3))-H bonds by an inner-sphere mechanism. This method allows the site- and diastereoselective fluorination of β-methylene C(sp(3))-H bonds of α-amino acid derivatives. A range of substrates containing both aliphatic and benzylic C(sp(3))-H bonds were compatible with this protocol, leading to an array of β-fluorinated α-amino acids. Stoichiometric fluorination of an isolated palladacycle intermediate takes place rapidly under very mild reaction conditions (room temperature, 5-10 min). Data from preliminary mechanistic studies are consistent with direct C-F reductive elimination from a high-valent intermediate.

Practical synthesis of anti-β-hydroxy-α-amino acids by Pd(II) -catalyzed sequential C(sp(3) )-H functionalization

An improved and practical procedure for the stereoselective synthesis of anti-β-hydroxy-α-amino acids (anti-βhAAs), by palladium-catalyzed sequential C(sp(3) )-H functionalization directed by 8-aminoquinoline auxiliary, is described. followed by a previously established monoarylation and/or alkylation of the β-methyl C(sp(3) )-H of alanine derivative, β-acetoxylation of both alkylic and benzylic methylene C(sp(3) )-H bonds affords various anti-β-hydroxy-α-amino acid derivatives. As an example, the synthesis of β-mercapto-α-amino acids, which are highly important to the extension of native chemical ligation chemistry beyond cysteine, is described. The synthetic potential of this protocol is further demonstrated by the synthesis of diverse β-branched α-amino acids. The observed diastereoselectivities are strongly influenced by electronic effects of aromatic AAs and steric effects of the linear side-chain AAs, which could be explained by the competition of intramolecular C-OAc bond reductive elimination from Pd(IV) intermediates vs. intermolecular attack by an external nucleophile (AcO(-) ) in an SN 2-type process.

Direct access to functionalized benzotropones, azepanes, and piperidines by reductive cross-coupling of α-bromo enones with α-bromo enamides

High-yielding syntheses of functionalized azepenes and piperidines, bearing an α-benzotropone derivative, have been achieved through cobalt-catalysed reductive cross-coupling of α-bromo enamides with α-bromo enones.