Visible light-driven photocatalytic sulfonative oxidation of benzyl secondary amines

A method for the α-oxidation and sulfonation of benzyl secondary amines was developed utilizing Ir(III) or Eosin Y as the photocatalyst in the presence of O2 as a green oxidant. Using commercial substrates, 37 products from cyclic and acyclic benzylamines were achieved with good functional group compatibility in 48-87% yields. Furthermore, tetrahydroisoquinoline protected by an Ac or a Boc group was oxidized under standard conditions.

Cobalt-Catalyzed Chemodivergent Synthesis of Cyclic Amines and Lactams from Ketoacids and Anilines Using Hydrosilylation

Here, commercially available Co2(CO)8 was utilized as an efficient catalyst for chemodivergent synthesis of pyrrolidines and pyrrolidones from levulinic acid and aromatic amines under slightly different hydrosilylation conditions. 1.5 and 3 equiv of phenylsilane selectively yielded pyrrolidone and pyrrolidine, respectively. Various ketoacids and amines were successfully tested. Plausible mechanism involves the condensation of levulinic acid and amine to form an imine, which cyclizes to 3-pyrrolidin-2-one followed by reduction to pyrrolidone. The final reduction of pyrrolidone gave pyrrolidine.

Cp2Fe-Mediated Electrochemical Synthesis of Phosphorylated Oxindoles and Indolo[2,1-a]isoquinolin-6(5H)-ones

An efficient and environmentally friendly electrochemical synthesis of phosphorylated oxindoles and indolo[2,1-a]isoquinolin-6(5H)-ones mediated by readily available Cp2Fe has been developed, which illustrated a broad substrate scope and diverse functional group compatibility. This protocol featured an external oxidant-free process and was at room temperature, which was proposed to be driven by the anodic oxidation of Cp2Fe.

One-Step Synthesis of 7-Bromobenzo[c]chromeno[4,3,2-gh]phenanthridines through a Sequential Bromination/Cyclization/Aromatization Reaction Using N-Bromosuccinimide (NBS)

Herein, metal- and oxidant-free synthesis of 7-bromobenzo[c]chromeno[4,3,2-gh]phenanthridines is reported using N-bromosuccinimide. Sequential regioselective bromination, intramolecular ring cyclization, and aromatization reactions occur in a single step through a successive radical-catalyzed pathway. The mechanistic pathway for the cyclization is supported by a DFT study. Selective bromination in the fully aromatic skeleton is accomplished without involving additional aromatic electrophilic ring bromination. As a synthetic application, the Suzuki coupling reaction of compound 5a with boronic acid is reported to get compound 8a. Aggregation-induced emission of one of the synthesized compounds (5h) is also investigated in THF/hexane solvent along with concentration-dependent emission spectroscopy.

Blue Light Irradiated Metal-, Oxidant-, and Base-Free Cross-Dehydrogenative Coupling of C(sp2)-H and N-H Bonds: Amination of Naphthoquinones with Amines

Herein, we report a blue-light-driven amination of C(sp²)-H bond of naphthoquinones and quinones with the N-H bond of primary and secondary amines for the synthesis of 2-amino-naphthoquinones and 2-amino-quinones. The coupling of naphthoquinones with a wide array of aliphatic, aromatic, chiral, primary, and secondary amines having electron donating (-CH₃, -OCH₃, -SCH₃), withdrawing (-F, -Cl, -Br, -I), and CO₂H, -OH, -NH₂ groups with acidic protons selectively occurred to afford C-N coupled 2-amino-naphthoquinones in 60-99% yields and hydrogen gas as a byproduct in methanol solvent without using any additional reagents, additives, and oxidant under the blue light irradiation. Mechanistic insight by DFT computation, controlled experiments, kinetic isotopic effect, and substitution effect of the substrates suggest that the reaction proceeds by radical pathway in which naphthoquinone forms a highly oxidizing naphthoquinonyl biradical upon irradiation of blue light (457 nm). Consequently, electron transfer from electron-rich amine to an oxidizing naphthoquinonyl biradical leads to a naphthoquinonyl radical anion and aminyl radical cation, followed by proton transfer and delocalization leading to a carbon-centered naphthoquinonyl radical. The cross-coupling of naphthoquinonyl carbon-centered and aminyl nitrogen radicals forms a C-N bond, with subsequent elimination of hydrogen gas (which was also confirmed by GC-TCD), affording 2-amino-1,4-naphthoquinone under metal-, reagent-, base-, and oxidant-free conditions.

Aniline assisted dimerization of phenylalanines: convenient synthesis of 2-aroyl-3-arylquinoline in an I2-DMSO system

We herein report an efficient synthesis of 2-aroyl-3-arylquinolines from phenylalanines and anilines. The mechanism involves I₂-mediated Strecker degradation enabled catabolism and reconstruction of amino acids and a cascade aniline-assisted annulation. Both DMSO and water act as oxygen sources in this convenient protocol.

Tetravalent Spiroselenurane Catalysts: Intramolecular Se···N Chalcogen Bond-Driven Catalytic Disproportionation of H2O2 to H2O and O2 and Activation of I2 and NBS

Chalcogen-bonding interactions have recently gained considerable attention in the field of synthetic chemistry, structure, and bonding. Here, three organo-spiroselenuranes, having a Se(IV) center with a strong intramolecular Se···N chalcogen-bonded interaction, have been isolated by the oxidation of the respective bis(2-benzamide) selenides derived from an 8-aminoquinoline ligand. Further, the synthesized spiroselenuranes, when assayed for their antioxidant activity, show disproportionation of hydrogen peroxide into H₂O and O₂ with first-order kinetics with respect to H₂O₂ for the first time by any organoselenium molecules as monitored by ¹H NMR spectroscopy. Electron-donating 5-methylthio-benzamide ring-substituted spiroselenurane disproportionates hydrogen peroxide at a high rate of 15.6 ± 0.4 × 10³ μM min^-1 with a rate constant of 8.57 ± 0.50 × 10^-3 s^-1, whereas 5-methoxy and unsubstituted-benzamide spiroselenuranes catalyzed the disproportionation of H₂O₂ at rates of 7.9 ± 0.3 × 10³ and 2.9 ± 0.3 × 10³ μM min^-1 with rate constants of 1.16 ± 0.02 × 10^-3 and 0.325 ± 0.025 × 10^-3 s^-1, respectively. The evolved oxygen gas from the spiroselenurane-catalyzed disproportion of H₂O₂ has also been confirmed by a gas chromatograph-thermal conductivity detector (GCTCD) and a portable digital polarographic dissolved O₂ probe. Additionally, the synthesized spiroselenuranes exhibit thiol peroxidase antioxidant activities for the reduction of H₂O₂ by a benzenethiol co-reductant monitored by UV-visible spectroscopy. Next, the Se···N bonded spiroselenuranes have been explored as catalysts in synthetic oxidation iodolactonization and bromination of arenes. The synthesized spiroselenurane has activated I₂ toward the iodolactonization of alkenoic acids under base-free conditions. Similarly, efficient chemo- and regioselective monobromination of various arenes with NBS catalyzed by chalcogen-bonded synthesized spiroselenuranes has been achieved. Mechanistic insight into the spiroselenuranes in oxidation reactions has been gained by ⁷⁷Se NMR, mass spectrometry, UV-visible spectroscopy, single-crystal X-ray structure, and theoretical (DFT, NBO, and AIM) studies. It seems that the highly electrophilic nature of the selenium center is attributed to the presence of an intramolecular Se···N interaction and a vacant coordination site in spiroselenuranes is crucial for the activation of H₂O₂, I₂, and NBS. The reaction of H₂O₂, I₂, and NBS with tetravalent spiroselenurane would lead to an octahedral-Se(VI) intermediate, which is reduced back to Se(IV) due to thermodynamic instability of selenium in its highest oxidation state and the presence of a strong intramolecular N-donor atom.

Transition metal-free advanced synthetic approaches for isoindolinones and their fused analogues

The transition metal-free synthetic protocols of isoindolinone and its fused analogues are highlighted in this review since 2007 to 2021.

Ligand Enabled Pd(II)-Catalyzed γ-C(sp3)-H Lactamization of Native Amides

γ-Lactams form important structural cores of a range of medicinally relevant natural products and clinical drugs, principal examples being the new generation of immunomodulatory imide drugs (IMiDs) and the brivaracetam family. Compared to conventional multistep synthesis, an intramolecular γ-C-H amination of aliphatic amides would allow for the direct construction of valuable γ-lactam motifs from abundant amino acid precursors. Herein we report a novel 2-pyridone ligand enabled Pd(II)-catalyzed γ-C(sp³)-H lactamization of amino acid derived native amides, providing the convenient synthesis of γ-lactams, isoindolinones, and 2-imidazolidinones. C6-Substitution of the 2-pyridone ligand is crucial for the lactam formation. This protocol features the use of N-acyl amino acids, which serve as both the directing group and cyclization partner, practical and environmentally benign tert-butyl hydrogen peroxide (TBHP) as the sole bystanding oxidant, and a broad substrate scope. The utility of this protocol was demonstrated through the two-step syntheses of a lenalidomide analog and brivaracetam from readily available carboxylic acids and amino acids.

Regioselective Synthesis of 2° Amides Using Visible-Light-Induced Photoredox-Catalyzed Nonaqueous Oxidative C-N Cleavage of N,N-Dibenzylanilines

A visible-light-driven photoredox-catalyzed nonaqueous oxidative C-N cleavage of N,N-dibenzylanilines to 2° amides is reported. Further, we have applied this protocol on 2-(dibenzylamino)benzamide to afford quinazolinones with (NH₄)₂S₂O₈ as an additive. Mechanistic studies imply that the reaction might undergo in situ generation of α-amino radical to imine by C-N bond cleavage followed by the addition of superoxide ion to form amides.

Copper-catalyzed aerobic double functionalization of benzylic C(sp3)-H bonds for the synthesis of 3-hydroxyisoindolinones

A copper-catalyzed aerobic 3-hydroxyisoindolinone synthesis was developed via the benzylic double C(sp³)-H functionalization of 2-alkylbenzamides. In this reaction, molecular oxygen was used as both an oxidant for C(sp³)-H functionalization and an oxygen source. Our method can be extended to diverse benzylic C(sp³)-H bonds and shows excellent functional group tolerance.

Graphene oxide: a green oxidant-acid bifunctional carbon material for the synthesis of functionalized isoindolin-1-ones via formal amide insertion and substitution

A formal amide insertion and substitution reaction using graphene oxide as an oxidant-acid bifunctional carbon material

8-Aminoquinoline-Assisted Synthesis and Crystal Structure Studies of Ferrocenyl Aryl Sulfones

A copper-catalyzed 8-aminoquinoline-directed oxidative cross-coupling of the C-H bond of ferrocene with sodium arylsulfinates has been achieved. The robust copper catalyst tolerates a range of methyl, tert-butyl, bromo, chloro, iodo and nitro functional groups in the phenyl ring, and set the stage for the synthesis of substituted ferrocene sulfones. Furthermore, X-ray crystal structure study on several ferrocenyl sulfones reveals the tetrahedral geometry around sulfur; interestingly, the O-S-O angle is larger than the electropositive substituent C-S-C angle which could be explained by Bent's rule. Further, unusual intramolecular O(S)⋅⋅⋅N(amide) short contacts (2.925-3) and O(S)⋅⋅⋅C=O were also noticed in ferrocenyl sulfones.

Visible-light-induced intramolecular sp3 C-H oxidation of 2-alkyl-substituted benzamides for the synthesis of functionalized iminoisobenzofurans

We have developed a protocol for the synthesis of functionalized iminoisobenzofurans by means of visible-light-induced intramolecular cyclization reactions of 2-alkyl-substituted benzamides. This one step-economical protocol, which involves intramolecular sp3 C-O bond formation, features mild reaction conditions, exclusive chemoselectivity, and high yields.

Transition metal-catalyzed sp3 C-H activation and intramolecular C-N coupling to construct nitrogen heterocyclic scaffolds

Nitrogen heterocycles are of great medicinal importance, and the construction of nitrogen heterocyclic scaffolds has been one of the focuses in synthetic organic chemistry. Recently, the strategy of transition metal-catalyzed sp3 C-H activation and intramolecular C-N coupling to construct nitrogen heterocyclic scaffolds has been well developed. Palladium, copper, silver, nickel, cobalt, ruthenium and rhodium catalysis were successfully used for the construction of nitrogen heterocyclic scaffolds, aziridines, azetidines, pyrrolidines, pyrrolidine-2,5-diones, indolines, isoindolines, isoindolinones, tetrahydropyridines, oxazolidinones, oxazinanones, β-lactams, γ-lactams etc., which have been synthesized by the sp3 C-H activation strategy. Here, we summarize the progress of transition metal-catalyzed sp3 C-H activation/intramolecular C-N bond formation, and introduce both the reaction development and mechanisms in numerous synthetically useful intramolecular sp3 C-H catalytic aminations/amidations.

Multicomponent reactions (MCRs): a useful access to the synthesis of benzo-fused γ-lactams

Benzo-fused γ-lactam rings such as isoindolin-2-ones and 2-oxindoles are part of the structure of many pharmaceutically active molecules. They can be often synthesized by means of multicomponent approaches and recent contributions in this field are summarized in this review. Clear advantages of these methods include the efficiency in saving raw materials and working time. However, there is still a need of new catalytic systems to allow the enantioselective preparation of these heterocycles by multicomponent reactions.

Merging Photochemistry with Electrochemistry: Functional-Group Tolerant Electrochemical Amination of C(sp3 )-H Bonds

Direct amination of C(sp³ )-H bonds is of broad interest in the realm of C-H functionalization because of the prevalence of nitrogen heterocycles and amines in pharmaceuticals and natural products. Reported here is a combined electrochemical/photochemical method for dehydrogenative C(sp³ )-H/N-H coupling that exhibits good reactivity with both sp² and sp³ N-H bonds. The results show how use of iodide as an electrochemical mediator, in combination with light-induced cleavage of intermediate N-I bonds, enables the electrochemical process to proceed at low electrode potentials. This approach significantly improves the functional-group compatibility of electrochemical C-H amination, for example, tolerating electron-rich aromatic groups that undergo deleterious side reactions in the presence of high electrode potentials.

Mechanism of Rhodium(III)-Catalyzed C-H Activation/Annulation of Aromatic Amide with α-Allenol: A Computational Study

With the help of DFT calculations, the reaction mechanisms of the rhodium(III)-catalyzed C-H activation/annulation between aromatic amide and α-allenol leading to the formation of isoindolinone have been theoretically investigated. Our calculated results show that the catalytic cycle consists of four stages: N-H deprotonation and C-H activation (Stage I), allene insertion, rearrangement and isomerization (Stage II), β-H elimination and enol-keto tautomerism (Stage III), and catalyst regeneration resulting in the five-membered ring product (Stage IV). For stage IV, besides the reaction paths proposed by the experimentalists, i.e., the insertion and reductive elimination (labeled as path a) and the reductive elimination and hydroamination (labeled as path b), an alternative path which involves C-N and C-H reductive eliminations (labeled as path c) was proposed and examined. The computational results show that the newly established path c is more energetically favorable than the reaction paths proposed by the experimentalists (paths a and b). The allene (non-terminal double bond) insertion step contributes to the rate-determining step with an overall activation free energy of 24.6 kcal/mol. Our study is beneficial for a better comprehension of the reaction mechanisms and provides a significant suggestion for further development of similar reactions.

Copper-catalyzed remote C(sp3)-H azidation and oxidative trifluoromethylation of benzohydrazides

The Hofmann-Löffler-Freytag (HLF) reaction is a prototypical example of radical-based remote functionalization of unactivated C(sp3)-H bond. While 1,5-hydrogen atom transfer (1,5-HAT) of the amidyl radical is thermodynamically favorable and is well-established, the method for the subsequent functionalization of the translocated carbon radical is still limited. We report herein two catalytic remote C(sp3)-H functionalization protocols. Cu(MeCN)4PF6-catalyzed reaction of 2-alkyl benzohydrazides 3 with TMSN3 in the presence of MeCO2OtBu affords the γ-azido amides 4, while CuCl-catalyzed reaction of 3 with Togni's reagent provides 2-(β-trifluoromethylvinyl)benzamides 5 via an oxidative δ-trifluoromethylation of the alkyl group. Mechanistic studies suggest that the γ-azidation of benzohydrazides 3 goes through 1,5-HAT followed by a Cu-mediated azido transfer cascade, while the oxidative δ-trifluoromethylation of 3 proceeds via, after 1,5-HAT process, a radical-polar crossover mechanism.

Ru(ii)-Catalyzed spirocyclization of aryl N-sulfonyl ketimines with aryl isocyanates through an aromatic C-H bond activation

A novel one-pot strategy has been developed for the construction of spiro-isoindolinone-benzosultams through a Ru(ii)-catalyzed sequential C-C and C-N bond formation. This method is highly atom-economical and compatible with a wide range of substituents. This is the first report on the spirocyclization of 3-aryl N-sulfonyl ketimines with aryl isocyanates to produce highly rigid and pharmaceutically relevant spirocycles through a C-H bond activation.

Synthesis of polysubstituted pyridines from oxime acetates using NH4I as a dual-function promoter

An NH₄I dual-function promoter enables pyridine synthesis through oxime N–O bond reduction and subsequent condensation reactions.

Direct functionalization of benzylic and non-benzylic C(sp3)–H bonds via keteniminium ion initiated cascade [1,5]-hydrogen transfer/cyclization

Keteniminium intermediates generated in situ initiate the cascade [1,5]-hydride transfer/cyclization for the direct functionalization of benzylic and non-benzylic C(sp³)–H bonds.