Uniting Amide Synthesis and Activation by PIII/PV-Catalyzed Serial Condensation: Three-Component Assembly of 2-Amidopyridines

Mechanistic Approach Toward the C4-Selective Amination of Pyridines via Nucleophilic Substitution of Hydrogen

The development of site-selective functionalization of N-heteroarenes is highly desirable in streamlined synthesis. In this context, direct amination of pyridines stands as an important synthetic methodology, with particular emphasis on accessing 4-aminopyridines, a versatile pharmacophore in medicinal chemistry. Herein, we report a reaction manifold for the C4-selective amination of pyridines by employing nucleophilic substitution of hydrogen (SNH). Through 4-pyridyl pyridinium salt intermediates, 4-aminopyridine products are obtained in reaction with aqueous ammonia without intermediate isolation. The notable regioselectivity was achieved by the electronic tuning of the external pyridine reagents along with the maximization of polarizability in the proton elimination stage. Further mechanistic investigations provided a guiding principle for the selective C-H pyridination of additional N-heteroarenes, presenting a strategic avenue for installation of diverse functional groups.

Synthesis, antimicrobial activity and application of polymers of praseodymium complexes based on pyridine nitrogen oxide

The traditional pyridine nitrogen oxide-based antimicrobial agents are often associated with health risks due to heavy metal enrichment. To mitigate this concern, we synthesized two novel complexes, Pr2(mpo)6(H2O)2 and Pr(hpo)(mpo)2(H2O)2, and integrated rare-earth salts, Hhpo (2-hydroxypyridine-N-oxide) and Nampo (2-mercapto-pyridine-N-oxide sodium salt). These complexes were characterized through infrared analysis, elemental analysis, thermogravimetric analysis, and X-ray crystallographic analysis. Our comparative analyses demonstrate that the synthesized rare-earth complexes exhibit stronger antimicrobial activity against Staphylococcus aureus (S. aureus ATCC6538) and Escherichia coli (E. coli ATCC25922) compared to the ligands and rare-earth salts alone. Quantitative results revealed the lowest inhibitory concentrations of the two complexes against S. aureus ATCC6538 and E. coli ATCC25922 at 3.125 μg mL-1, 6.25 μg mL-1, 3.125 μg mL-1 and 6.25 μg mL-1, respectively. Preliminary investigations indicated that the antibacterial mechanism of these complexes involved promoting intracellular substance exudation to achieve antibacterial effects. Incorporation of these complexes into polymeric antimicrobial films resulted in a potent antimicrobial effect, achieving a 100% inhibition rate against S. aureus ATCC6538 and E. coli ATCC25922 at a low addition level of 0.6 wt%. Our results suggest that nitrogen oxide-based praseodymium complexes have potential for various antimicrobial applications.

Nickel-Catalyzed Highly Selective Radical C-C Coupling from Carboxylic Acids with Photoredox Catalysis

Controlling the cross-coupling reaction between two different radicals is a long-standing challenge due to the process occurring statistically, which would lead to three products, including two homocoupling products and one cross-coupling product. Generally, the cross-coupling selectivity is achieved by the persistent radical effect (PRE) that requires the presence of a persistent radical and a transient radical, thus resulting in limited radical precursors. In this paper, a highly selective cross-coupling of alkyl radicals with acyl radicals to construct C(sp2)-C(sp3) bonds, or with alkyl radicals to construct C(sp3)-C(sp3) bonds have been achieved with the readily available carboxylic acids and their derivatives (NHPI ester) as coupling partners. The success originates from the use of tridentate ligand (2,2' : 6',2''-terpyridine) to enable radical cross-coupling process to Ni-mediated organometallic mechanism. This protocol offers a facile and flexible access to structurally diverse ketones (up to 90 % yield), and also a new solution for the challenging double decarboxylative C(sp3)-C(sp3) coupling. The broad utility and functional group tolerance are further illustrated by the late-stage functionalization of natural-occurring carboxylic acids and drugs.

Chiral Bisphosphine-Catalyzed Asymmetric Staudinger/aza-Wittig Reaction: An Enantioselective Desymmetrizing Approach to Crinine-Type Amaryllidaceae Alkaloids

An unprecedented chiral bisphosphine-catalyzed asymmetric Staudinger/aza-Wittig reaction of 2,2-disubstituted cyclohexane-1,3-diones is reported, enabling the facile access of a broad range of cis-3a-arylhydroindoles in high yields with excellent enantioselectivities. The key to the success of this work relies on the first application of chiral bisphosphine DuanPhos to the asymmetric Staudinger/aza-Wittig reaction. An effective reductive system has been established to address the challenging PV═O/PIII redox cycle associated with the chiral bisphosphine catalyst. In addition, comprehensive experimental and computational investigations were carried out to elucidate the mechanism of the asymmetric reaction. Leveraging the newly developed chemistry, the enantioselective total syntheses of several crinine-type Amaryllidaceae alkaloids, including (+)-powelline, (+)-buphanamine, (+)-vittatine, and (+)-crinane, have been accomplished with remarkable conciseness and efficiency.

Combined Computational and Experimental Study Reveals Complex Mechanistic Landscape of Brønsted Acid-Catalyzed Silane-Dependent P═O Reduction

The reaction mechanism of Brønsted acid-catalyzed silane-dependent P═O reduction has been elucidated through combined computational and experimental methods. Due to its remarkable chemo- and stereoselective nature, the Brønsted acid/silane reduction system has been widely employed in organophosphine-catalyzed transformations involving P(V)/P(III) redox cycle. However, the full mechanistic profile of this type of P═O reduction has yet to be clearly established to date. Supported by both DFT and experimental studies, our research reveals that the reaction likely proceeds through mechanisms other than the widely accepted "dual activation mode by silyl ester" or "acid-mediated direct P═O activation" mechanism. We propose that although the reduction mechanisms may vary with the substitution patterns of silane species, Brønsted acid generally activates the silane rather than the P═O group in transition structures. The proposed activation mode differs significantly from that associated with traditional Brønsted acid-catalyzed C═O reduction. The uniqueness of P═O reduction originates from the dominant Si/O═P orbital interactions in transition structures rather than the P/H-Si interactions. The comprehensive mechanistic landscape provided by us will serve as a guidance for the rational design and development of more efficient P═O reduction systems as well as novel organophosphine-catalyzed reactions involving P(V)/P(III) redox cycle.

PIII/PV-Catalyzed Beckmann Reaction and Sequential [2,3]-Sigmatropic Rearrangement to Construct 2-Amidopyridines

An organophosphorus catalytic method for the synthesis of substituted 2-amidopyridines is reported. The method employs a small-ring organophosphorus-based catalyst and a hydrosilane reductant to drive the conversion of ketoximes and pyridine-N-oxides into 2-amidopyridines through sequential Beckmann rearrangement followed by [2,3]-sigmatropic rearrangement. The readily available ketoximes could be activated to nitrilium ions in PIII/PV redox catalysis and could efficiently participate in the domino reaction of pyridine-N-oxides, thus providing various substituted 2-amidopyridines in moderate to excellent yields. This presented strategy features excellent functional group tolerance and a broad substrate scope.

(3+2) Annulation of 4-Acetoxy Allenoate with Aldimine Enabled by AgF-Assisted P(III)/P(V) Catalysis

A phosphine-catalyzed (3+2) annulation of 4-acetoxy allenoate and aldimine with the assistance of AgF is described. The success of this reaction hinges on the metathesis between the enolate-phosphonium zwitterion and AgF, leading to a key intermediate comprising of silver enolate and a fluorophosphorane P(V)-moiety. The former is able to undergo a Mannich reaction with aldimine, whereas the latter initiates a cascade sequence of AcO-elimination/aza-addition, thus furnishing the P(III)/P(V) catalysis. By taking advantage of the silver enolate, a preliminary attempt at an asymmetric variant was conducted with the combination of an achiral phosphine catalyst and a chiral bis(oxazolinyl)pyridine ligand (PyBox), giving moderate enantioselectivity.

Tf2O-Promoted Regioselective Heteronucleophilic Ring-Opening Approaches of Tetrahydrofuran

The ring-opening functionalization strategy in tetrahydrofuran (THF) represents an ideal approach to access different valuable structures. Herein, we report different operationally simple, efficient, unique, and practical regioselective heteronucleophilic ring-opening strategies for the THF system. Tf2O, which is a strong electrophilic activator, was found to generate a THF triflate intermediate that triggers the nucleophilicity of nitriles (Nu1) and led to regioselective ring opening in the presence of different nucleophiles (Nu2). Furthermore, the synthesis of different heteronucleophilic ring-opening dimerization products was attributed to the nucleophilicity of Nu2. We also demonstrated that use of borane-tetrahydrofuran (BTHF) can achieve challenging hydride addition in a similar manner.

Atroposelective PIII /PV =O Redox Catalysis for the Isoquinoline-Forming Staudinger-aza-Wittig Reaction

Herein, we describe the feasibility of atroposelective PIII /PV =O redox organocatalysis by the Staudinger-aza-Wittig reaction. The formation of isoquinoline heterocycles thereby enables the synthesis of a broad range of valuable atropisomers under mild conditions with enantioselectivities of up to 98 : 2 e.r. Readily prepared azido cinnamate substrates convert in high yield with stereocontrol by a chiral phosphine catalyst, which is regenerated using a silane reductant under Brønsted acid co-catalysis. The reaction provides access to diversified aryl isoquinolines, as well as benzoisoquinoline and naphthyridine atropisomers. The products are expeditiously transformed into N-oxides, naphthol and triaryl phosphine variants of prevalent catalysts and ligands. With dinitrogen release and aromatization as ideal driving forces, it is anticipated that atroposelective redox organocatalysis provides access to a multitude of aromatic heterocycles with precise control over their configuration.

Organophosphorus-Catalyzed "Dual-Substrate Deoxygenation" Strategy for C-S Bond Formation from Sulfonyl Chlorides and Alcohols/Acids

A green method to construct C-S bonds using sulfonyl chlorides and alcohols/acids via a P^III/P^V═O catalytic system is reported. The organophosphorus-catalyzed umpolung reaction promotes us to propose the "dual-substrate deoxygenation" strategy. Herein, we adopt the "dual-substrate deoxygenation" strategy, which achieves the deoxygenation of sulfonyl chlorides and alcohols/acids to synthesize thioethers/thioesters driven by P^III/P^V═O redox cycling. The catalytic method represents an operationally simple approach using stable phosphine oxide as a precatalyst and shows broad functional group tolerance. The potential application of this protocol is demonstrated by the late-stage diversification of drug analogues.

Challenges and Breakthroughs in Selective Amide Activation

In contrast to ketones and carboxylic esters, amides are classically seen as comparatively unreactive members of the carbonyl family, owing to their unique structural and electronic features. However, recent decades have seen the emergence of research programmes focused on the selective activation of amides under mild conditions. In the past four years, this area has continued to rapidly develop, with new advances coming in at a fast pace. Several novel activation strategies have been demonstrated as effective tools for selective amide activation, enabling transformations that are at once synthetically useful and mechanistically intriguing. This Minireview comprises recent advances in the field, highlighting new trends and breakthroughs in what could be called a new age of amide activation.

Ligand‐Enabled Disproportionation of 1,2‐Diphenylhydrazine at a P V ‐Center**

We present herein the synthesis of a nearly square‐pyramidal chlorophosphorane supported by the tetradentate bis(amidophenolate) ligand, N,N′‐bis(3,5‐di‐tert‐butyl‐2‐phenoxy)‐1,2‐phenylenediamide. After chloride abstraction the resulting phosphonium cation efficiently promotes the disproportionation of 1,2‐diphenylhydrazine to aniline and azobenzene. Mechanistic studies, spectroscopic analyses and theoretical calculations suggest that this unprecedented reactivity mode for P^V‐centres is induced by the high electrophilicity at the cationic P^V‐center, which originates from the geometry constraints imposed by the rigid pincer ligand, combined with the ability of the o‐amidophenolate moieties to act as electron reservoir. This study illustrates the promising role of cooperativity between redox‐active ligands and phosphorus for the design of organocatalysts able to promote redox processes.

Relative rates of alkylation for B-substituted triarylphosphines: an ortho-Boron group enhances reactivity on phosphorus

Advancements in main-group catalysis are contingent on our ability to quantify effects that enhance reactivity in these systems. Herein we report the rates of alkylation for several substituted phosphines. We report that by incorporating a single pinacol boronic ester group in the ortho-position on triphenylphosphine, the rate of substitution with benzyl bromide is approximately 4.7 times faster than the parent compound as measured by initial rates. The corresponding meta- and para-isomers are only 1.3 and 1.5 times as fast, respectively. Using X-ray crystallographic data and quantum chemical calculations, we propose this rate acceleration occurs from an O to P electrostatic interaction that stabilizes the transition state.

Mechanistic Details of Asymmetric Bromocyclization with BINAP Monoxide: Identification of Chiral Proton-Bridged Bisphosphine Oxide Complex and Its Application to Parallel Kinetic Resolution

The mechanism of our previously reported catalytic asymmetric bromocyclization reactions using 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP) monoxide was examined in detail by the means of control experiments, NMR studies, X-ray structure analysis, and CryoSpray electrospray ionization mass spectrometry (ESI-MS) analysis. The chiral BINAP monoxide was transformed to a key catalyst precursor, proton-bridged bisphosphine oxide complex (POHOP·Br), in the presence of N-bromosuccinimide (NBS) and contaminating water. The thus-formed POHOP further reacts with NBS to afford BINAP dioxide and molecular bromine (Br₂) simultaneously in equimolar amounts. While the resulting Br₂ is activated by NBS to form a more reactive brominating reagent (Br₂─NBS), BINAP dioxide serves as a bifunctional catalyst, acting as both a Lewis base that reacts with Br₂─NBS to form a chiral brominating agent (P═O⁺─Br) and also as a Brønsted base for the activation of the substrate. By taking advantage of this novel concerted Lewis/Brønsted base catalysis by BINAP dioxide, we achieved the first regio- and chemodivergent parallel kinetic resolutions (PKRs) of racemic unsymmetrical bisallylic amides via bromocyclization.

C2-Selective, Functional-Group-Divergent Amination of Pyrimidines by Enthalpy-Controlled Nucleophilic Functionalization

Synthesis of heteroaryl amines has been an important topic in organic chemistry because of their importance in small-molecule discovery. In particular, 2-aminopyrimidines represent a highly privileged structural motif that is prevalent in bioactive molecules, but a general strategy to introduce the pyrimidine C2-N bonds via direct functionalization is elusive. Here we describe a synthetic platform for site-selective C-H functionalization that affords pyrimidinyl iminium salt intermediates, which then can be transformed into various amine products in situ. Mechanism-based reagent design allowed for the C2-selective amination of pyrimidines, opening the new scope of site-selective heteroaryl C-H functionalization. Our method is compatible with a broad range of pyrimidines with sensitive functional groups and can access complex aminopyrimidines with high selectivity.

Photomediated reductive coupling of nitroarenes with aldehydes for amide synthesis

In view of the widespread significance of amide functional groups in organic synthesis and pharmaceutical studies, an efficient and practical synthetic protocol that avoids the use of stoichiometric activating reagents or metallic reductants is highly desirable.

Generation of Oxyphosphonium Ions by Photoredox/Cobaloxime Catalysis for Scalable Amide and Peptide Synthesis in Batch and Continuous-Flow

Phosphine-mediated deoxygenative nucleophilic substitutions, such as the Mitsunobu reaction, are of great importance in organic synthesis. However, the conventional protocols require stoichiometric oxidants to trigger the formation of the oxyphosphonium intermediates for the subsequent nucleophilic additions. Through dual catalysis of photoredox and cobaloxime, we realized a radical strategy for the catalytic formation of acyloxyphosphonium ions that enables direct amidation. The deoxygenative protocol exhibits a broad scope and has been used in the late-stage amidation of drug molecules. In addition to batch reactions, a continuous-flow reactor was developed, enabling rapid peptide synthesis on gram scale. The successful assembly of a tetrapeptide on the solid support further demonstrated the versatility of this photocatalytic system. Moreover, experimental and computational studies are consistent with the hypothesis of acyloxyphosphonium ions being formed as the key intermediates.