Metal-Free Synthesis of 6-Phosphorylated Phenanthridines: Synthetic and Mechanistic Insights

Thermal and Photoinduced Radical Cascade Annulation using Aryl Isonitriles: An Approach to Quinoline-Derived Benzophosphole Oxides

Herein, we present a radical cascade addition cyclization sequence to access quinoline-based benzophosphole oxides from ortho-alkynylated aromatic phosphine oxides using various aryl isonitriles as radical acceptors and inexpensive tert-butyl-hydroperoxide (TBHP) as a terminal oxidant in the presence of a catalytic amount of silver acetate. Alternatively, the same cascade can be realized through a sustainable photochemical approach utilizing 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as an organic photocatalyst at room temperature. The introduced modular approach shows broad functional group tolerance and offers straightforward access to complex P,N-containing polyheterocyclic arenes. These novel π-extended benzophosphole oxides exhibit interesting photophysical and electrochemical properties such as absorption in the visible region, emission and reversible reduction at low potentials, which makes them promising for potential materials science applications. The photophysical properties can further be tuned by the addition of external Lewis and Brønsted acids.

Silver-catalyzed P-centered anion nucleophilic addition to isocyanide: access to 2-phosphinoyl indoles/indol-3-ols

A silver-catalyzed chemoselective cascade nucleophilic addition of a P-centered anion to isocyanides and cyclization reaction was developed for the efficient and practical synthesis of a wide range of 2-phosphinoyl indole and indol-3-ol derivatives. Unlike the well-documented synthesis of phosphorus-functionalized heterocycles via a P-centered radical, an anionic reactivity profile of phosphine oxides is most likely involved in this domino transformation.

Visible-Light-Mediated Annulation/Thiolation of 2-Isocyanobiaryls with Disulfides to Organoylthiophenanthridines Derivatives

A visible-light-induced annulation/thiolation of 2-isocyanobiaryls with dialkyl(aryl)disulfides has been established, delivering a sustainable and atom-economic route to 6-organoylthiophenanthridines with wild functional group tolerance and good to excellent yields under oxidant-, base-, and transition-metal-free conditions.

Transition Metal-Free Regioselective Phosphonation of Pyridines: Scope and Mechanism

Regioselective phosphonation of pyridines is an interesting transformation in synthesis and medicinal chemistry. We report herein a metal-free approach enabling access to various 4-phosphonated pyridines. The method consists of simply activating the pyridine ring with a Lewis acid (BF₃·OEt₂) to facilitate the nucleophilic addition of a phosphine oxide anion. The formed sigma complex is subsequently oxidized with an organic oxidant (chloranil) to yield the desired adducts in good to excellent yields. We furthermore showed that access to C2-phosphoinated pyridines can be achieved in certain cases with strong Lewis basic phosphorus nucleophiles or with strong Lewis acidic pyridines. Both experimental and computational mechanistic investigations were undertaken and allowed us to understand the factors controlling the reactivity and selectivity of this reaction.

A Visible-Light-Induced α-Aminoalkyl-Radical-Mediated Halogen-Atom Transfer Process: Modular Synthesis of Phenanthridinone Alkaloids

A halogen-atom transfer (XAT) strategy utilizing α-aminoalkyl radicals allows the generation of aryl radicals at room temperature, which is applied for intramolecular cyclization reactions en route to biologically relevant alkaloids. Starting from simple halogen-substituted benzamides under visible light irradiation in the presence of an organophotocatalyst (4CzIPN) and nBu₃N allows the modular construction of the phenanthridinone core, which gives facile access to drug analogs and alkaloids, e.g., from the Amaryllidaceae family. The reaction pathway most likely involves a quantum mechanical tunneling enabled transfer event to achieve aromatization-halogen-atom transfer.

Unveiling the Three-Component Phosphonylation on Alkynylaldehydes: Toolbox toward Fluorescent Molecules

A regioselective tandem approach for annulated napthyridines/isoquinolines embedded with the phosphine oxide group under mild reaction conditions has been achieved in good to excellent yields. The designed strategy involves the triflate-induced formation of new C sp³-P and C sp²-N bond formation in one pot. This protocol was also well tolerated for the construction of densely functionalized organo-phosphorylated chromenes in good yields. Further, phosphino-derived sulfamethazine and sulfamethoxazole drugs were also successfully synthesized in good yields. The mechanistic studies revealed that the ionic pathway and the formation of regioselective 6-endo dig cyclized products were confirmed through X-ray crystallographic studies. Interestingly, photophysical studies of selectivity selected compounds revealed their stimulating fluorescence properties.

Visible Light-Promoted Radical-Mediated Ring-Opening/Cyclization of Vinyl Benzotriazoles: An Alternative Approach to Phenanthridines

A visible light-promoted radical-mediated ring-opening/cyclization of vinyl benzotriazoles has been developed. The method provides an efficient and practical approach to synthesize functionalized phenanthridines from vinyl benzotriazoles with alkyl bromides under mild conditions. Significantly, the readily available and bench-stable vinyl benzotriazoles can serve as valuable alternative radical acceptors during the synthesis of phenanthridines.

Different Lewis Acid Promotor-Steered Highly Regioselective Phosphorylation of Tertiary Enamides

Different Lewis acid promotor-steered highly regioselective phosphorylation of tertiary enamides with diverse H-phosphonates or H-phosphine oxides was developed. Under the catalysis of iron salt, the phosphonyl group was introduced into the α-position of tertiary enamides, affording various α-phosphorylated amides in high efficiency. On the other hand, the β-phosphorylated tertiary enamides were efficiently obtained as the products in the presence of manganese(III) acetylacetonate.

Synthesis of quinazolinone and quinazoline derivatives using green chemistry approach

Green chemistry has been most compelling area of research. Green chemistry is vital to long-term sustainability, not only because of its fundamental notion of reducing the use and manufacture of hazardous materials, but also because of its broad applicability as one of the most efficient and problem-solving pathways for the synthesis of new materials. Various chemists have studied a plethora of strategies to lessen the release of hazardous chemical waste, waste material recyclization and reuse. New techniques have been created based on a green chemistry strategy that includes the utilization of catalysts, nanosized materials and composites, such as metal and non-metal nanoparticles, their oxides and salts, and different heterocyclic rings. Quinazolines and quinazolinones are biologically significant heterocyclic rings with a wide range of characteristics. In a summary, this chapter focuses on recent novel synthesis methods for quinazoline and quinazolinone derivatives, which are vital to humanity.

Visible-light-initiated regio- and stereoselective C(sp2)–H phosphorylation of enamides under transition-metal-free conditions

A visible-light-induced stereo- and regioselective phosphorylation of enamides with phosphine oxides under transition-metal-free conditions has been disclosed.

Recent Advances on Photoinduced Cascade Strategies for the Synthesis of N-Heterocycles

Over the last decade, visible-light photocatalysis has proved to be a powerful tool for the construction of N-heterocyclic frameworks, important constituents of natural products, insecticides, pharmacologically relevant therapeutic agents and catalysts. This account highlights recent developments and established methods towards the photocatalytic cascades for preparation of different classes of N-heterocycles, giving emphasis on our contribution to the field.

Generation of aryl radicals by redox processes. Recent progress in the arylation methodology

Arylation methods based on the generation and use of aryl radicals have been a rapidly growing field of research in recent years and currently represent a powerful strategy for carbon – carbon and carbon – heteroatom bond formation. The progress in this field is related to advances in the methods for generation of aryl radicals. The currently used aryl radical precursors include aryl halides, aryldiazonium and diaryliodonium salts, arylcarboxylic acids and their derivatives, arylboronic acids, arylhydrazines, organosulfur(II, VI) compounds and some other compounds. Aryl radicals are generated under mild conditions by single electron reduction or oxidation of precursors induced by conventional reagents, visible light or electric current. A crucial role in the development of the radical arylation methodology belongs to photoredox processes either catalyzed by transition metal complexes or organic dyes or proceeding without catalysts. Unlike the conventional transition metal-catalyzed arylation methods, radical arylation reactions proceed very often at room temperature and have high functional group tolerance. Without claiming to be exhaustive, this review covers the most important advances of the current decade in the generation and synthetic applications of (het)aryl radicals. Examples of reactions are given and mechanistic insights are highlighted.

The bibliography includes 341 references.

Redox-Neutral P(O)-N Coupling between P(O)-H Compounds and Azides via Dual Copper and Photoredox Catalysis

We report a redox-neutral P(O)-N coupling reaction of P(O)-H compounds with azides via photoredox and copper catalysis, providing new access to useful phosphinamides, phosphonamides, and phosphoramides. This transformation tolerates a wide range of nucleophilic functionalities including alcohol and amine nucleophiles, which makes up for the deficiency of classical nitrogen nucleophilic substitution reactions. As a demonstration of the broad potential applications of this new methodology, late-stage functionalization of a diverse array of azido-bearing natural products and drug molecules, a preliminary asymmetric reaction, and a continuous visible-light photoflow process have been developed.

A scalable electrochemical dehydrogenative cross-coupling of P(O)H compounds with RSH/ROH

A practical, scalable electrochemical dehydrogenative cross-coupling of P(O)H compounds with thiols, phenols and alcohols in both an undivided cell and a continuous-flow setup is disclosed. Its broad substrate scope (>50 examples), good functional-group tolerance and scalability (>10 g) show potential for practical synthesis. A preliminary mechanistic study suggests that the phosphorus radicals are involved in the catalytic cycle.

Radical Cyanotrifluoromethylation of Isocyanides: Step-Economical Access to CF3-Containing Nitriles, Amines, and Imines

A novel copper-catalyzed radical cyanotrifluoromethylation has been achieved through a multicomponent reaction of isocyanides, Togni's reagent, and trimethylsilyl cyanides, affording trifluoroacetimidoyl nitriles in good yields. This reaction demonstrates a unique feature of merging two valuable functional groups-trifluoromethyl (CF₃) and cyan (CN)-onto the same C atom. The transformation proceeds by the initial addition of the CF₃ radical to isocyanide and the subsequent intermolecular C-CN formation. The products can be successfully transformed to a series of CF₃-containing amines and imines that may serve in the synthesis of valuable pharmaceuticals and agrochemicals.

Selective P-C(sp3 ) Bond Cleavage and Radical Alkynylation of α-Phosphorus Alcohols by Photoredox Catalysis

Herein the first P-C(sp³ ) bond cleavage and radical alkynylation of α-phosphorus alcohols to construct phosphonoalkynes is reported. The phosphorus radical is generated upon P-C bond cleavage reaction via the alkoxyl radical through photoredox catalysis with cyclic iodine(III) reagents. Various arylphosphinoyl-, alkylphosphinoyl-, phosphonate-, and phosphonic amide alcohols serve as radical phosphorus precursors to construct phosphonoalkynes for the first time.

Electrooxidative CH/PH functionalization as a novel way to synthesize benzo[b]phosphole oxides mediated by catalytic amounts of silver acetate

Electrochemical oxidation synthesis of benzo[b]phosphole oxides has been performed at room temperature with catalytic amounts of AgOAc.

Crystal Structure of Bis(2,4,6-trimethylphenyl)-phosphine Oxide

The single crystal structure of bis(2,4,6-trimethylphenyl)phosphine oxide has been determined. All interatomic distances and angles can be considered normal. The aryl substituents adopt an intermediate configuration when compared to both sterically unhindered (e.g., diphenylphosphine oxide) and congested (e.g., bis(2,4,6-tri-tert-butylphenyl)phosphine oxide) secondary phosphine oxides, illustrating the influence of steric congestion on the molecular structure.

Transition-Metal-Free Direct C-H Arylation of Quinoxalin-2(1H)-ones with Diaryliodonium Salts at Room Temperature

A method of synthesizing 3-arylquinoxalin-2(1H)-ones using diaryliodonium tetrafluoroborates under mild conditions is described. This protocol has a wide substrate scope and enables direct C-H functionalization. The synthetic potential of this coupling was explored using a range of readily accessible diaryliodonium salts and quinoxalin-2(1H)-ones.

Silver-Catalyzed Radical-Involved Cascade Cyclization of Diphenylphosphine with Cinnamamides: Access to 2-Phosphinoyl-3H-pyrrolo[1,2-a]indoles

A Ag-catalyzed phosphorus radical promoted cyclization of cinnamamides to give exclusively 2-phosphinoyl-3H-pyrrolo[1,2-a]indoles derivatives via a 5-endo-trig manner is described. This novel reaction leads to the simultaneous formation of C-C and C-P bonds in one step with excellent chemo- (only 5-endo) and diastereoselectivity (dr >20:1) under mild conditions in moderate to good yields.

Transition metal-free stereospecific access to (E)-(1-fluoro-2-arylvinyl)phosphine borane complexes

Synthesis of (E)-(1-fluoro-2-arylvinyl)phosphine boranes has been achieved under metal-free conditions.

Active methylene compounds (AMCs) controlled facile synthesis of acridine and phenanthridine from morita Baylis–Hillman acetate

We carried out simple and facile syntheses of acridines and phenanthridines from MBH acetates of 2-chloro-quinoline-3-carbaldehyde with active methylene compounds (AMCs).

Microwave-assisted synthesis of hydroxyl-containing isoquinolines by metal-free radical cyclization of vinyl isocyanides with alcohols

A convenient microwave-assisted protocol for the synthesis of hydroxyl-containing isoquinolines from a metal-free radical cyclization reaction of vinyl isonitriles with alcohols was developed with moderate-to-excellent yields.