The versatile role of norbornene in C-H functionalization processes: concise synthesis of tetracyclic fused pyrroles via a threefold domino reaction

The synthesis of novel tetracyclic fused pyrroles from 1-(2-iodophenyl)-1H-pyrrole and various bromoalkyl-aryl alkynes via a palladium(0)-catalyzed and norbornene-mediated threefold domino reaction is reported. PdCl₂ and tri-2-furylphosphine (TFP) in the presence of norbornene and Cs₂CO₃ in CH₃CN at 90 °C gave a variety of tetracyclic fused pyrroles in usually high yields. In the described reaction sequence two of the three carbon-carbon bonds are formed by functionalization of an unactivated aryl C-H bond.     

Synthesis of 1-vinylpyrrole-2-carbonitriles

A new highly synthetically potent series of bifunctional pyrroles, 1-vinylpyrrole-2-carbonitriles, were synthesized from readily available 1-vinylpyrrole-2-carbaldehyde oximes by two methods: (1) reaction with acetylene (KOH/DMSO, 70 °C, 10 min, yields 58-67%) and (2) reaction with acetic anhydride (90-100 °C, 5 h, yields 83-93%). Starting from 2-phenyl-1-vinylpyrrole, the one-pot synthesis of the corresponding 1-vinyl-2-carbonitrile was accomplished directly by successive treatment with a DMF/(COCl)₂ complex, NH₂OH·HCl/NaOAc, and acetic anhydride (yield 58%).     

Easy α- to β-migration of an enol moiety on a pyrrole ring

Functionalized pyrrolic enols, 2-(2,2-dicyano-1-hydroxyethenyl)-1-methylpyrroles, synthesized from 2-ethenylpyrroles by a nucleophilic SEt-OH exchange, upon heating (75-142 °C) are readily rearranged to their 3-isomers in near to quantitative yield. The inter or intramolecular auto-protonation of a pyrrole ring by the acidic enol hydroxyl to form a mesomeric pyrrolium cation or zwitterion is suggested to be a key step in the rearrangement.     

Ruthenium-catalyzed cyclization of 3-en-1-ynyl imines with nucleophiles via tandem 5-exo-dig cyclization and nucleophilic addition

Treatment of 3-en-1-ynyl imines with TpRuPPh₃(CH₃CN)₂PF₆ catalyst (1 mol %) in DCE (50 °C, 6 h) effected catalytic cyclization with suitable nucleophiles and gave functionalized pyrroles in good yields. The reaction mechanism is proposed to proceed via (2-pyrrolyl)carbenoid intermediates derived from 5-exo-dig cyclization. This catalytic reaction works well with various nucleophiles, including water, alcohols and anilines.     

A simple and convenient synthesis of substituted furans and pyrroles by CuCl2-catalyzed heterocyclodehydration of 3-yne-1,2-diols and N-Boc- or N-tosyl-1-amino-3-yn-2-ols

A simple and economical synthesis of substituted furans and pyrroles, by ligand-free CuCl₂-catalyzed heterocyclodehydration of readily available 3-yne-1,2-diols and N-Boc- or N-tosyl-1-amino-3-yn-2-ols, respectively, is presented. Reactions are carried out in MeOH at 80-100 °C for 1-24 h and afford the corresponding heterocyclic derivatives in 53-99% isolated yields.     

An approach to the Paal-Knorr pyrroles synthesis catalyzed by Sc(OTf)3 under solvent-free conditions

A facile synthesis of N-substituted pyrroles by the Paal-Knorr condensation has been accomplished using a simple procedure. Among different metal triflates screened, 1 mol % Sc(OTf)₃ efficiently promoted the reaction to give excellent yield (89-98%) under mild reaction conditions. Additionally, Sc(OTf)₃ could be recovered easily after the reactions and reused without evident loss in activity.     

1,4-Carbonylative addition of arylboronic acids to methyl vinyl ketone: a new synthetic tool for rapid furan and pyrrole synthesis

The rhodium catalysed 1,4-carbonylative addition of arylboronic acids to methyl vinyl ketone under carbon monoxide pressure was studied. High yields of 1,4-diketones were obtained using a catalytic system formed from Rh(COD)₂BF₄ (COD=1,5-cyclooctadiene) and triphenylphosphine even at very low catalyst loading (0.02 mol %). A short synthetic procedure combining this carbonylation reaction with a subsequent cyclisation step affords pyrroles or furans.     

One-pot assembly of 4-methylene-3-oxa-1-azabicyclo[3.1.0]hexanes from alkyl aryl(hetaryl) ketoximes, acetylene, and aliphatic ketones: a new three-component reaction

A new three-component reaction between alkyl aryl(hetaryl)ketoximes, acetylene, and aliphatic ketones in the superbasic systems KOH/DMSO and LiOH/CsF/DMSO (70-90 °C, initial acetylene pressure 13-15 atm, 5-60 min) affords novel 4-methylene-3-oxa-1-azabicyclo[3.1.0]hexanes in yields of up to 75%. Using KOH/DMSO, the side products of the reaction are O-vinylketoximes and 2-aryl(hetaryl)pyrroles, while with LiOH/CsF/DMSO, the reaction proves to be selective, only minor amounts of the corresponding alkyl aryl(hetaryl) ketones being detectable.     

Greenish metal-lustrous organic crystals formed from 1-aryl-2-(2-thienyl)-5-(5-tricyanoethenyl-2-furyl)pyrroles

On the treatment of 1-aryl-2-(2-furyl)-5-(2-thienyl)pyrroles with tetracyanoethylene, 1-aryl-2-(2-thienyl)-5-(5-tricyanoethenyl-2-furyl)pyrroles were produced. These compounds formed crystals with greenish metallic luster. In their solid-state UV-vis-NIR diffuse reflection-absorption spectra, absorption band corresponding to metallic reflection spreads in the range of 550-900 nm. Furthermore, strong absorption appeared below 520-540 nm. This absorption results in the appearance of green color. Single-crystal X-ray crystallographic analysis revealed the crystal structure of 1-(4-methoxyphenyl)-2-(2-thienyl)-5-(5-tricyanoethenyl-2-furyl)pyrrole (2c). The distinct features of the crystal structure are as follows: (1) the thiophene-pyrrole-furan-tricyanoethenyl π-system is approximately flat; (2) the conformational relation between the pyrrole ring and the furan ring is anti, that is, these rings are pointing in opposite directions and the dihedral angle of N-C-C-O=180°; (3) as a result, the tricyanoethenyl group is far from the 4-methoxyphenyl group; (4) the molecules of 2c are arranged in a ribbon structure; (5) the ribbons are assembled side-by-side to form a terraced layer; (6) the layers stack so that the π-orbitals of 2c become close to each other.     

Carboxylation of indoles and pyrroles with CO2 in the presence of dialkylaluminum halides

The Lewis acid-mediated carboxylation of arenes with CO₂ has been successfully applied to 1-substituted indoles and pyrroles by using dialkylaluminum chlorides instead of aluminum trihalides. Thus, the carboxylation of 1-methylindoles, 1-benzyl-, and 1-phenylpyrroles proceeds regioselectively with the aid of an equimolar amount of Me₂AlCl under CO₂ pressure (3.0 MPa) at room temperature to afford the corresponding indole-3-carboxylic acids and pyrrole-2-carboxylic acids in 61-85% yields, while the same treatment of 1,2,5-trimethylpyrrole affords the 3-carboxylic acid in 52% yield.     

Porphyrins with exocyclic rings. Part 25: synthesis of porphyrins with a fused cyclic ether subunit from tetrahydro-4H-pyan-4-one

A novel porphyrin system with a fused dihydropyran ring has been synthesized from commercially available 4-oxotetrahydropyran. The cyclic ketone reacted with oximes derived from acetoacetate esters in the presence of zinc dust, sodium propionate, and propionic acid at 150-155 °C to give good yields of 5-oxatetrahydroindoles. A low yield of a benzo-fused analog was also obtained using 4-oxochromanone as the starting reagent. Reaction of the b-annelated pyrroles with N-chlorosuccinimide gave the 7-chloro-derivatives and these underwent an acid catalyzed condensation with α-unsubstituted pyrroles to give dipyrrolic products linked by a dihydropyran moiety. Hydrogenolysis of a dipyrrole dibenzyl ester gave the corresponding dicarboxylic acid and this was converted into a dialdehyde by treatment with TFA-CH(OMe)₃. MacDonald ‘2+2’ condensation with two different dipyrrylmethanes under optimized conditions afforded the porphyrin ethers in 30-36% yield. This approach provides a pilot study for the synthesis of porphyrin crown ether structures. In addition, insightful results on the conformations of dihydropyran-linked dipyrroles were obtained by careful examination of their proton NMR spectra in CDCl₃ and DMSO-d₆.     

ZrCl4 catalyzed highly selective and efficient Michael addition of heterocyclic enamines with α,β-unsaturated olefins

Highly selective and efficient Michael additions of heterocyclic enamines, viz. indoles, pyrroles, and pyrazoles with α,β-unsaturated olefins using 2 mol % of ZrCl₄ has been achieved.     

Ruthenium vinylidene and carbyne complexes containing a multifunctional tridentate ligand with a PNN donor set

The neutral, octahedral ruthenium vinylidene complexes mer,trans-[(PNN)Cl₂Ru(CCHR)] (PNN = N-(2-diphenylphosphinobenzylidene)-2-(2-pyridyl)ethylamine; R = Ph, 1a; R = ^tBu, 1b) are reported. An X-ray crystallographic study of 1a confirms the tridentate, meridional coordination mode of the PNN ligand. Compounds 1a and 1b undergo regioselective electrophilic addition with HBF₄ · Et₂O at C_β of the vinylidene ligand at low temperatures, and are cleanly and quantitatively converted to the ruthenium carbynes mer,trans-[(PNN)Cl₂Ru(CCH₂R)][BF₄] (R = Ph, 2a; R = ^tBu, 2b). Carbynes 2a and 2b are stable only at low temperatures (<−50 °C). Complex 1a undergoes ligand substitution with L to yield mer,trans-[(PNN)Cl₂Ru(L)] (L = MeCN, 3a; L = CO, 3b).     

Tandem catalysis in ionic liquids: a recyclable catalytic synthesis of benzofuran derivatives

A convenient, recyclable catalytic synthesis of benzofuran-2-acetic esters 2 by sequential Pd(0)-catalyzed deallylation—Pd(II)-catalyzed carbonylative heterocyclization of 1-(2-allyloxyphenyl)-2-yn-1-ols 1 in ionic liquids is presented. Reactions were typically carried out in BmimBF₄ as the solvent at 100 °C and under 30 atm of CO, in the presence of catalytic amounts (1 mol %) of PdI₂ in conjunction with KI (1 equiv), PPh₃ (4 mol %), MeOH (28 equiv), and H₂O (2 equiv). The solvent-catalyst system could be recycled several times without appreciable loss of catalytic activity.     

Syntheses, crystal structures of a series of copper(II) complexes and their catalytic activities in the green oxidative coupling of 2,6-dimethylphenol

Three mixed-ligand Cu^II complexes bearing iminodiacetato (ida) and N-heterocyclic ligands, namely, [Cu₂(ida)₂(bbbm)(H₂O)₂] · H₂O (1), [Cu₂(ida)₂(btx)(H₂O)₂] · 2H₂O (2) and [Cu₂(ida)₂(pbbm)(H₂O)₂] · H₂O · 3CH₃OH (3) (bbbm = 1,1^′-(1,4-butanediyl)bis-1H-benzimidazole, btx = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene, pbbm = 1,1^′-(1,3-propanediyl)bis-1H-benzimidazole), in addition to three fcz-based Cu^II complexes, namely, {[Cu(fcz)₂(H₂O)₂] · 2NO₃}_n (4), {[Cu(fcz)₂(H₂O)] · SO₄ · DMF · 2CH₃OH · 2H₂O}_n (5) and {[Cu(fcz)₂Cl₂] · 2CH₃OH}_n (6) (fcz = 1-(2,4-difluorophenyl)-1,1-bis[(1H-1,2,4-triazol-l-yl) methyl]ethanol) have been prepared according to appropriate synthetic strategies with the aim of exploiting new and potent catalysts. Single crystal X-ray diffraction shows that 1 and 2 possess similar binuclear structures, 3 features a 2D pleated network, and 4 exhibits a 1D polymeric double-chain structure. Complexes 1-6 are tested as catalysts in the green catalysis process of the oxidative coupling of 2,6-dimethylphenol (DMP). Under the optimized reaction conditions, these complexes are catalytically active by showing high conversion of DMP and high selectivity of PPE. The preliminary study of the catalytic-structural correlations suggests that the coordination environment of the copper center have important influences on their catalytic activities.     

Thermal degradation of keratin waste

Thermal degradation of sheep wool, human hair and chicken feathers was studied by TG-MSD/FTIR and by pyrolysis followed by GC-MSD analysis in order to identify the degradation compounds and the temperature range in which they are formed. Only small differences were found between the studied keratin samples. They consist mainly in shift of characteristic temperatures of degradation and in relative amounts of compounds in degradation products, especially in aqueous phase. Degradation started with formation of ammonia and CO₂ (from 167 and 197 °C respectively and with maximum evolution at 273 and 287 °C respectively), continues with formation of sulphur-containing inorganic compounds (SCS, SCO, H₂S and SO₂ at 240, 248, 255 and 253-260 °C respectively) and of water (255 °C). Thiols are formed in two stages (257 and 320 °C) while the evolution of nitriles is maximum around 340 °C and continues up to about 480 °C. Phenol and 4-methylphenol are the most important degradation compounds, formed at 370 and 400 °C respectively. Nitrogen was present mainly in aliphatic/aromatic nitriles, pyrroles, pyridines and amides while sulphur was found mainly as sulphides, thiols, thiazoles and thiophenes.     

pH-dependent self-assembly of copper(II) complexes with a new imidazole-containing polyamine ligand: Synthesis,structure and magnetic property

Four copper(II) complexes were synthesized by reactions of new imidazole-containing polyamine ligand N¹-(2-aminoethyl)-N¹-(1H-imidazol-4-ylmethyl)-ethane-1,2-diamine (HL) with Cu(ClO₄)₂ · 6H₂O under different pH and their structures were characterized by X-ray crystallography. Interestingly, the complexes have diverse structures from protonated ligand [H₃(HL)][CuCl₄] · Cl (1), dinuclear [Cu₂(HL)₂Cl](ClO₄)₃ · H₂O (2), one-dimensional chain polynuclear {[Cu(L)](ClO₄)}_n (3) to cyclic-tetranuclear [Cu₄(L)₄](ClO₄)₄ · 3CH₃CN (4) coordination compounds by varying reaction pH from acidic to basic. The results indicate that the reaction pH has great impact on the formation and structure of the complexes. The magnetic measurements show that there are antiferromagnetic interactions between the Cu(II) centers with g = 2.09, J = −39.0 cm⁻¹ and g = 2.17, J = −36.8 cm⁻¹ for 3 and 4, respectively.     

Nanocrystalline F-doped tin dioxide materials: texture, morphology and photosensitization with a perylene-substituted organotin

The controlled hydrolysis of fluoro(2-methylbutan-2-oxy)di(pentan-2,4-dionato)tin followed by annealing at 400-550 °C gave conductive mesoporous F-doped SnO₂ materials. The materials consist of a porous network of aggregated nanoparticles, the mesoporosity observed corresponding to the interparticle space. Tuning of the annealing temperature enabled us to prepare materials with surface areas ranging from 70 to 150 m² g⁻¹, with an average pore size comprised between 50 and 100 Å and with a mean particle diameter ranging from 50 to 120 Å. Resistivities as low as 1-2 Ω cm were measured for the sample treated at 550 °C which contained 2-3 at.% of fluorine. This powder reacted with 3-(6-trihex-1-ynylstannylhexyl)perylene to furnish a dye-sensitized F-doped SnO₂ mesoporous materials. An intensity-dependent photocurrent was produced under blue light illumination using the cavity microelectrode (CME) technique. With an aqueous NaBr solution, the photopotential reaches 700 mV, a value slightly higher than that found for an undoped sensitized SnO₂ powder (∼600 mV).     

New N-heterocyclic carbene silver(I) and mercury(II) 2-D supramolecular layers by the π-π stacking interactions

The precursor 1-(9-anthracenylmethyl)-3-alkylbenzimidazolium chlorides (1a, alkyl = C₄H₉, 1b, alkyl = C₆H₁₃) and their three new NHC silver(I) and mercury(II) complexes {[1-(9-anthracylmethyl)-3-alkylbimy]MCl}₂ (2a, alkyl = C₄H₉, M = Ag; 2b, alkyl = C₆H₁₃, M = Ag; 3a, alkyl = C₄H₉, M = Hg; bimy = benzimidazol-2-ylidene) have been prepared and characterized. The crystal structures of 2a, 2b and 3a showed that 2-D supramolecular layers are formed by both benzimidazole ring head to tail π-π stacking interactions and anthracene ring face-to-face π-π stacking interactions.     

Microwave-assisted zinc chloride-catalyzed synthesis of substituted pyrroles from homopropargyl azides

Ligand-free 5-endo-dig cyclization of 1,4- and 1,2,4-substituted but-3-yn-1-yl (homopropargyl) azides in the presence of zinc chloride (usually 20 mol %) in dichloroethane at elevated temperature provides 2,5-di- and 2,3,5-trisubstituted pyrroles in high to moderate yields (91-41%). Both conventional and microwave protocols furnished comparable results. A structure of 2-(4-fluorophenyl)-5-(4-methylphenyl)-1H-pyrrole was confirmed by X-ray crystallography.