Intramolecular Ullmann-type C−N coupling for the synthesis of substituted benzo[4,5]imidazo[1,2-a]pyrrolo[3,4-c]pyridines

An efficient CuI catalyzed intramolecular Ullmann-type C−N coupling reaction is described here. Newly substituted 1H-benzo[4,5]imidazo[1,2-a]pyrrolo[3,4-c]pyridine-1,3,5(2H, 11H)-trione has been easily synthesized from ortho halogenated N-substituted pyrrolo[3,4-c]pyridine-1,3,6(5H)-triones in presence of CuI catalyst and K₂CO₃ base without any ligand. This procedure is based on intramolecular Ullmann-type reaction and provides a proficient method of making fused tetracyclic heterocycles.     

Reaction of Imidazo[1,2‐a]pyridines with Aliphatic Aldehydes: A Simple and Efficient Method for the Preparation of Novel C‐3 Vinylic Derivativesof Imidazo[1,2‐a]pyridines

A simple and efficient method for the preparation of novel C‐3 vinylic derivatives of imidazo[1,2‐a]pyridines has been developed by the reaction of imidazo [1,2‐a]pyridines with appropriate aliphatic aldehydes in acetic acid in a sealed tube.     

Sulfonylation of 1,4-Diazabicyclo[2.2.2]octane: Charge-Transfer Complex Triggered C−N Bond Cleavage

A novel charge-transfer complex triggered sulfonylation of 1,4-diazabicyclo[2.2.2]octane (DABCO) with mild reaction conditions has been developed. The formation of a charge-transfer complex between electron-withdrawing (hetero)aryl sulfonyl chloride and DABCO allows the synthesis of N-ethylated piperazine sulfonamide in good yields. The reaction has a high functional group tolerance. Spectroscopic studies confirmed the charge-transfer complex formation between sulfonyl chlorides and DABCO, which facilitates the C−N bond cleavage of DABCO.     

Synthesis of Substituted Pyrido[1,2-a]benzimidazoles by Ruthenium-Catalyzed C−H Bond Activation and Tandem Cyclization of 2-Arylimidazo[1,2-a]pyridines with Iodonium Ylides

Ruthenium-catalyzed C−H bond activation and tandem cyclization of 2-arylimidazo[1,2-a]pyridines with iodonium ylides proceed efficiently. For the first time, the easily available iodonium ylides and inexpensive ruthenium complex were employed to synthesize pyrido[1,2-a]benzimidazole derivatives in good yields under simple and easy-to-operate conditions. Several primary mechanism investigations and synthetic applications involving gram-scale preparation, derivatization reactions and the transformation of iodonium ylide generated in situ have also been conducted.     

Synthesis of 2-substituted 6,8-dinitro[1,2,4]triazolo[1,5-a]pyridines and the formation of the related zwitterionic σ-adducts

A method for the synthesis of 2-substituted 6,8-dinitro[1,2,4]triazolo[1,5-a]pyridines is proposed. The method includes the reaction of 2-chloro-3,5-dinitropyridine with the corresponding 5-substituted tetrazoles. The resulting compounds react with anhydro bases of α- and γ-methylazinium salts to give zwitterionic σ-adducts. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1405–1407, July, 1999.     

Microwave-prompted rapid and efficient synthesis of 3-alkyl substituted imidazo[1,5-a]pyridines

Under regular heating and microwave irradiation, 3-alkyl substituted imidazo[1,5-a] pyridines were synthesized from 2,2＇- pyridil, di-2-pyridyl ketone and aliphatic aldehydes in the presence of ammonium acetate and acetic acid. Compared to the traditional heating condition, the reaction time under microwave irradiation was shorter and 3-alkyl imidazo[1,5-a]pyddines were given in higher yield.     

Intramolecular Nitrene Insertion into Saturated C−H-Bond-Mediated C−N Bond Cleavage of a Coordinated NHC Ligand

Ruthenium(II) complexes bearing a tridentate bis(N-heterocyclic carbene) ligand reacted with iminoiodanes (PhI=NR) resulting in the formation of isolable ruthenium(III)–amido intermediates, which underwent cleavage of a C−N bond of the tridentate ligand and formation of an N-substituted imine group. The Ru^III–amido intermediates have been characterized by ¹H NMR, UV/Vis, ESI-MS, and X-ray crystallography. DFT calculations were performed to provide insight into the reaction mechanism.     

Construction and regulation of imidazo[1,5-a]pyridines with AIE characteristics via iodine mediated Csp2−H or Csp−H amination

The widespread applications of aggregation-induced emission luminogens (AIEgens) inspire the creation of AIEgens with novel structures and functionalities. In this work, we focused on the direct and efficient synthesis of a new type of AIEgens, imidazo[1,5-a]pyridicne derivatives, via iodine mediated cascade oxidative Csp²–H or Csp–H amination route from phenylacetylene or styrenes under mild conditions. The resulted compounds showed excellent AIE characteristics with tunable maximum emissions, attractive bioimaging performance, and potential anti-inflammatory activity, which exert broad application prospects in material, biology, medicine, and other relevant areas.     

The Influence of Secondary Interactions on the [N−I−N]+ Halogen Bond

[Bis(pyridine)iodine(I)]⁺ complexes offer controlled access to halonium ions under mild conditions. The reactivity of such stabilized halonium ions is primarily determined by their three-center, four-electron [N−I−N]⁺ halogen bond. We studied the importance of chelation, strain, steric hindrance and electrostatic interaction for the structure and reactivity of halogen bonded halonium ions by acquiring their ¹⁵N NMR coordination shifts and measuring their iodenium release rates, and interpreted the data with the support of DFT computations. A bidentate ligand stabilizes the [N−I−N]⁺ halogen bond, decreasing the halenium transfer rate. Strain weakens the bond and accordingly increases the release rate. Remote modifications in the backbone do not influence the stability as long as the effect is entirely steric. Incorporating an electron-rich moiety close by the [N−I−N]⁺ motif increases the iodenium release rate. The analysis of the iodine(I) transfer mechanism highlights the impact of secondary interactions, and may provide a handle on the induction of stereoselectivity in electrophilic halogenations.     

Novel syntheses of hexahydro-1H-pyrrolo[1,2-a]ĭmidazoles and Octahydroimidazo[1,2-a]pyridines

1-Phenyl-5-(benzotriazol-1-yl)hexahydro-1H-pyrrolo[1,2-a]?midazole (18) and 1-phenyl-5-benzotriazolyloctahydroimidazo[1,2-a]pyridine (27) were readily prepared from succindialdehyde or glutaraldehyde, benzotriazole, and N-phenylethylenediamine. Synthons 18 and 27 reacted with Grignard reagents, allylsilanes, silyl ethers, and triethyl phosphite to produce 1-phenyl-5-substituted-hexahydro-1H-pyrrolo[1,2-a]?midazoles 20a-f, 22, 24a,b, and 25 and 1-phenyl-5-substituted-octahydroimidazo[1, 2-a]pyridines 28a-e, 32, 33a,b, and 34 in good to excellent yields. The configurations of 20, 22, 24, and 25 were determined to be cis isomers by NOE experiment, while the configurations and conformations of 28a-e, 32, 33a,b, and 34 were elucidated by (1)H-(1)H COSY and (1)H-(13)C COSY.     

Re-Catalyzed Annulations of Weakly Coordinating N-Carbamoyl Indoles/Indolines with Alkynes via C−H/C−N Bond Cleavage

Described herein are rhenium-catalyzed [3+2] annulations of N-carbamoyl indoles with alkynes via C−H/C−N bond cleavage, which provide rapid access to fused-ring pyrroloindolone derivatives. For the first time, the weakly coordinating O-directing group was successfully employed in rhenium-catalyzed C−H activation reactions, enabled by the unique catalytic trio of Re₂(CO)₁₀, Me₂Zn and ZnCl₂. Mechanistic studies revealed that aminozinc species plays an important role in the reaction. Based on the mechanistic understanding, a more powerful catalytic trio of Re₂(CO)₁₀, [MeZnNPh₂]₂ and Zn(OTf)₂ was devised and applied successfully in the [4+2] annulations of indolines and alkynes affording pyrroloquinolinone derivatives.     

Thioethers as Dichotomous Electrophiles for Site-Selective Silylation via C−S Bond Cleavage

The development of aryl alkyl sulfides as dichotomous electrophiles for site-selective silylation via C−S bond cleavage has been achieved. Iron-catalyzed selective cleavage of C(aryl)−S bonds can occur in the presence of β-diketimine ligands, and the cleavage of C(alkyl)−S bonds can be achieved by t-BuONa without the use of transition metals, resulting in the corresponding silylated products in moderate to excellent yields. Mechanistic studies suggest that Fe−Si species may undergo metathesis reactions during the cleavage of C(aryl)−S bonds, while silyl radicals are involved during the cleavage of C(alkyl)−S bonds.     

Synthesis of New Pyrido[2′,3′:3,4]pyrazolo[1,5-a]pyrimidines and Their Use in the Preparation of Tetraheterocyclic Systems

8,10-Dimethyl-3-(unsubstituted, methyl, ethyl, n-butyl, phenyl)-4-hydroxypyrido[2′,3′:3,4]pyrazolo[1,5-a]pyrimidin-2(1H)-ones and 3-(2-hydroxyethyl)-2,8,10-trimethylpyrido[2′,3′:3,4]pyrazolo[1,5-a]pyrimidin-4-ol were synthesized by cyclocondensation of 3-amine-4,6-dimethyl-1H-pyrazolo[3,4-b]pyridine with ethyl malonates and α-acetyl-γ-butyrlolactone. Dichloro- and diazido- derivatives were obtained from the reaction of pyridopyrazlopyrimidine derivatives with POCl₃ followed by NaN₃. The tetrahetrocyclic systems were formed by cyclization of 4-chloro-3-(2-chloroethyl)-2,8,10-trimethylpyrido[2′,3′:3,4]pyrazolo[1,5-a]pyrimidine with the appropriate primary amines. The structures of all compounds were established by NMR and mass spectra.     

Redox-Induced Oxidative C−C Bond Cleavage of 2,2′-Pyridil in Diruthenium Complexes

In the recent years, there has been an emerging research interest in the domain of C−C bond-cleavage reactions. The present contribution deals with the redox-mediated dioxygen activation and C−C bond cleavage in a diruthenium complex [(acac)₂Ru^II(μ-L1)Ru^II(acac)₂], 1 (acac=acetylacetonate) incorporating 2,2′-pyridil (L1) as the bridging ligand. The above process leads to a C−C-cleaved monomeric product [(acac)₂Ru^III(pic⁻)], 2 (pic⁻=piconilate). Intriguingly, similar diastereomeric complexes [(acac)₂Ru^II(μ-L2)Ru^II(acac)₂], meso (ΔΛ): 3 a and rac (ΔΔ/ΛΛ): 3 b , involving an analogous diimine bridge (L2=N1,N2-diphenyl-1,2-di(pyridin-2-yl)ethane-1,2-diimine), were stable towards such oxidative transformations. Electrochemical and spectroelectrochemical studies, in combination, establish the potential non-innocent feature of the 2,2′-Pyridil (L1) and its derivative (L2) both in oxidation and reduction processes. Additionally, theoretical calculations have been employed to verify the redox states and their behavior. Furthermore, transition state (TS) calculations at the M06L/6-31G*/LANL2DZ level of theory together with detailed kinetic studies outline a putative mechanism for the selective transformation of 1 → 2 involving the formation of an intermediate bearing peroxide linkage to complex 1 .     

One-pot Copper Nanoparticle-catalyzed Synthesis of Imidazo[1,2-α]pyridines Under Solvent-free Conditions

A direct and efficient approach to synthesize imidazo[1,2-α]pyridines through three-component one-pot reaction of 2-aminopyridine,aldehyde and terminal alkyne catalyzed by Cu-nanoparticles under solvent-free conditions has been developed.This method provides a rapid access to substituted imidazo[1,2-α]pyridines with good yields（up to 85％）.     

Metal-Free,Visible-Light-Induced Selective C−C Bond Cleavage of Cycloalkanones with Molecular Oxygen

A metal-free, visible-light-induced oxidative C−C bond cleavage of cycloketones with molecular oxygen is described. Cooperative Brønsted-acid catalysis and photocatalysis enabled selective C−C bond cleavage of cycloketones to generate an array of γ-, δ- and ϵ-keto esters under very mild conditions. Mechanistic studies indicate that singlet molecular oxygen (¹O₂) is responsible for this transformation.     

Fluoroalkyl-containing lithium β-diketonates in the synthesis of 1,2,4-triazolo[1,5-a]pyrimidines

Cyclocondensation of fluorine-containing lithium -diketonates with 3-amino-1,2,4-triazoles afforded 7-fluoromethyl-1,2,4-triazolo[1,5-a]pyrimidines.     

Carbon Monoxide Activation by a Molecular Aluminium Imide: C−O Bond Cleavage and C−C Bond Formation

Anionic molecular imide complexes of aluminium are accessible via a rational synthetic approach involving the reactions of organo azides with a potassium aluminyl reagent. In the case of K₂[( NON )Al(NDipp)]₂ ( NON =4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethyl-xanthene; Dipp=2,6-diisopropylphenyl) structural characterization by X-ray crystallography reveals a short Al−N distance, which is thought primarily to be due to the low coordinate nature of the nitrogen centre. The Al−N unit is highly polar, and capable of the activation of relatively inert chemical bonds, such as those found in dihydrogen and carbon monoxide. In the case of CO, uptake of two molecules of the substrate leads to C−C coupling and C≡O bond cleavage. Thermodynamically, this is driven, at least in part, by Al−O bond formation. Mechanistically, a combination of quantum chemical and experimental observations suggests that the reaction proceeds via exchange of the NR and O substituents through intermediates featuring an aluminium-bound isocyanate fragment.