An efficient aerobic oxidative aromatization of Hantzsch 1,4-dihydropyridines and 1,3,5-trisubstituted pyrazolines

4-Substituted Hantzsch 1,4-dihydropyridines and 1,3,5-trisubstituted pyrazolines were oxidized to the corresponding pyridines and pyrazoles, respectively, in high yields by molecular oxygen in the presence of catalytic amount of N-hydroxyphthalimide (NHPI) and Co(OAc)₂ in acetonitrile at room temperature.     

3,4-Dihydro-2H-pyran promoted aerobic oxidative aromatization of 1,3,5-trisubstituted pyrazolines and Hantzsch 1,4-dihydropyridines

An unprecedented facile oxidation of 1,3,5-trisubstituted pyrazolines and Hantzsch 1,4-dihydropyridines (DHPs) to the corresponding pyrazoles and pyridines was observed, mediated by 3,4-dihydro-2H-pyran in air. The reaction showed excellent reactivity, functional group tolerance, and high yield without using any metal and/or halogen based oxidizing agents.     

Oxidative aromatization of Hantzsch 1,4-dihydropyridines by aqueous hydrogen peroxide-acetic acid

A simple method for the oxidative aromatization of Hantzsch 1,4-dihydropyridines to the corresponding pyridines is achieved by using hydrogen peroxide as green oxidant and acetic acid as catalyst in aqueous solution.     

Catalytic aromatization of Hantzsch 1,4-dihydropyridines by ferric perchlorate in acetic acid

Catalytic oxidation of Hantzsch 1,4-dihydropyridines is described using a catalytic amount of ferric perchlorate in acetic acid at room temperature.     

Oxidative aromatization of Hantzsch 1,4-dihydropyridines by sodium chlorite

Hantzsch 1,4-dihydropyridines were converted to the corresponding pyridines efficiently by sodium chlorite under mild conditions in excellent yields.     

Oxidative aromatization of Hantzsch 1,4-dihydropyridines by cupric bromide under mild heterogeneous condition

A mild heterogeneous oxidizing agent, cupric bromide has been utilized for the oxidative aromatization of Hantzsch 1,4-dihydropyridines to the corresponding pyridine derivatives in excellent yields with the isolation of products by simple work-up procedure.     

TBPA+·mediated aromatization of 1,3,5-trisubstituted pyrazolines

1,3,5-Trisubstituted pyrazolines were oxidized to the corresponding pyrazoles in high yield with tris(4-bromophenyl)aminium (TBPA ·) hexachloroantimonate in chloroform at room temperature.     

Aromatization of Hantzsch 1,4-dihydropyridines and 1,3,5-trisubstituted pyrazolines with HIO3 and I2O5 in water

Hantzsch 1,4-dihydropyridines and 1,3,5-trisubstituted pyrazolines were converted to the corresponding pyridines and pyrazoles efficiently by the treatment of a catalytic amount of HIO₃ or I₂O₅ in water.     

Visible light mediated metal-free oxidative aromatization of 1,3,5-trisubstituted pyrazolines

The visible light mediated oxidation of 1,3,5-trisubstituted pyrazolines under metal-free conditions was developed. Various substituted pyrazolines were oxidized to pyrazoles by irradiation with visible light/sunlight. A plausible mechanism was proposed for the light mediated oxidation proceeding via formation of intermediates with electron rich C-3 positions and electron deficient C-5 positions. Correlation between the electronic effect induced by the aryl substituents and the rate of oxidation was studied by UV–vis spectroscopy. This correlation study supported the proposed mechanism. Electron withdrawing substituents at the 3-phenyl group and electron releasing substituents at the 5-phenyl group enhanced the rate of oxidation; whereas, electron donating substituents at the 3-phenyl group and electron withdrawing substituents at the 5-phenyl group reduced the rate.     

N-Nitroso-2-aryl-1,3-oxazolidines catalyzed aromatization of Hantzsch 1,4-dihydropyridines

A catalytic amount of N-nitroso-2-aryl-1,3-oxazolidines leading to the aromatization of Hantzsch 1,4-dihydropyridines (DHPs) was successfully achieved. A catalytic mechanism for the reaction is proposed.     

n-Butyltriphenylphosphonium peroxodisulfate: A cheap and efficient reagent for the aromatization of Hantzsch 1,4-dihydropyridines

Different 4-substituted 1,4-dihydropyridines have been oxidized to pyridine derivatives by n-butyltriphenyl phosphonium peroxodisulfate in acetonitrile solution at reflux condition.Formation of two kinds of products has been observed depending on the type of 4-substituent.     

Photochemistry of Hantzsch 1,4-dihydropyridines and pyridines

The photochemistry of some Hantzsch 4-phenyl-1,4-dihydropyridines bearing a substituent on the phenyl ring (the three isomeric chloro derivatives and the 4′-nitro derivative) has been studied. All of these compounds underwent inefficient aromatization to the corresponding pyridines (quantum yield <10⁻⁴ at 366 nm, <10⁻² at 254 nm). This process is scarcely affected by molecular oxygen and is initiated by proton transfer (from C₄-H), probably to the solvent, from the excited singlet. In turn, the thus formed pyridines were photoreactive with comparable or higher efficiency. Thus, the 4-(3′-chlorophenyl) and 4-(4′-chlorophenyl) Hantzsch pyridines underwent positional rearrangement to form two isomers each. The reaction occurs via Dewar benzene--prismane path. In the case of the minor isomer a further 1,3-shift take place at the Dewar benzene level. The 4-(2′-chlorophenyl) derivative underwent C-Cl bond homolysis, which led to cyclization of the phenyl group onto one of the ester groups forming a pyrane ring.     

1,4-二氢吡啶衍生物合成方法的改进和芳构化研究

改进了Hantzsch合成法,通过一锅煮合成1,4-二氢吡啶衍生物,大大缩短了反应时间,操作简便,发现氯化铁是一个较好的1,4-二氢吡啶衍生物芳构化的氧化剂。     

Synthesis and characterization of Si-Zr-Mo nanocomposite as a rapid and efficient catalyst for aromatization of Hantzsch 1,4-dihydropyridines

Nanocomposite of Silica-zirconia-molybdate designated as Si-Zr-Mo was prepared via the reaction of the in situ generated zirconium-tetra octanoxide [Zr(Oct)₄] through condensation of zirconium-tetra-n-butoxide and 1-octanol in a sol–gel method with sulfuric acid and tetraethylorthosilicate (TEOS) followed by grafting of MoO₄^2? on modified silico zirconia nanocomposite under reflux conditions. The prepared nanocomposite was characterized using inductively coupled plasma (ICP), N₂ sorption isotherms, transmission electron microscopy (TEM), and FT-IR spectroscopy. The as prepared nanocomposite had a surface area and pore dimension of 140 m²/g and 1.48 nm, respectively. The morphology of sulphated silico zirconia nanocomposite after immobilization MoO₄^2? has been changed from nanoparticles to nanaorods. It was found that the synthesized nanocomposite successfully catalyze the oxidative dehydrogenation of 1,4-dihydropyridines (1,4-DHPs) with 92–100% conversion and 80–100% selectivity toward the desired products.     

Rapid and efficient aromatization of Hantzsch 1,4-dihydropyridines with Potassium peroxomonosulfate catalyzed by manganese(III) Schiff base complexes

Rapid and efficient oxidation of Hantzsch 1,4-dihydropyridine with Potassium peroxomonosulfate is reported. The Mn(III)-salophen/monopersulfate catalytic system converts 1,4-dihydropyridines to their corresponding pyridine derivatives at room temperature in a 1:1, CH₃CN/H₂O mixture. The ability of various Schiff base complexes in the oxidation of 1,4-dihydropyridine was also investigated.     

NaI readily mediated oxidative aromatization of Hantzsch 1,4-dihydropyridines with hydrogen peroxide at room temperature: A green procedure

The oxidative conversion of 1,4-dihydropyridines to give the corresponding pyridine derivatives in excellent yields was easily effected using the catalytic amount of NaI in combination with H₂O₂ (30%) as a green external oxidant. The process is highly green wherein the solid products are easily filtered out after the addition of ice-water to the reaction mixture. This non-transition metal catalyst is cost-effective, affords simple work-up and easy separation of the product.     

Silica-supported 1,3-dichloro-5,5-dimethylhydantoin(DCH) as a useful reagent for microwave-assisted aromatization of 1,3,5-trisubstituted pyrazolines under solvent-free conditions

1,3,5-Trisubstituted pyrazolines are rapidly and conveniently oxidized to their corresponding pyrazoles by 1,3-dichloro-5,5- dimethylhydantoin(DCH) in solution and solvent-free conditions under microwave irradiation.The presence of silica gel as a supporting agent is shown to be effective in reducing the reaction times and increasing the yields.     

Catalytic aromatization of 1,4-dihydropyridines by radical cation salt prompted aerobic oxidation

Aromatization of Hantzch 1,4-dihydropyridines was achieved under radical cation salt induced conditions, in which triarylamine radical cation acts as an efficient catalyst to prompt the aerobic oxidation of 1,4-DHPs in a catalytic way.     

Oxidative diphosphonylation of 1,4-dihydropyridines and pyridinium salts

An oxidative double phosphonylation of dihydropyridines 1 and pyridinium salts 2 is achieved through the use of dialkyl phosphites, DDQ, and triethylamine. Acceptable to good yields of 2, 6-diphosphonylated-1,2-dihydropyridines 3 are obtained in a one-pot reaction involving tandem nucleophilic addition/oxidation processes. Isomerization of 3 to the more stable 2,4-diphosphonylated-1, 4-dihydropyridine 4 was observed in some cases.