Efficient synthesis of 2-(2′-hydroxyphenyl)benzoxazole by palladium(II)-catalyzed oxidative cyclization

An efficient method for the synthesis of 2-(2′-hydroxyphenyl)benzoxazole has been developed by using palladium-mediated oxidative cyclization.     

Protonation of palladium(II)-allyl and palladium(0)-olefin complexes containing 2-pyridyldiphenylphosphine

The pendant nitrogen atom of the Ph₂PPy ligand in the Pd(II)-allyl complexes [PdCl(η³-2-CH₃-C₃H₄)(Ph₂PPy)] (1) and [Pd(η³-2-CH₃-C₃H₄)(Ph₂PPy)₂]BF₄ (3) has been protonated with methanesulfonic acid to afford the corresponding pyridinium salts [PdCl(η³-2-CH₃-C₃H₄)(Ph₂PPyH)](CH₃SO₃) (1a) and [Pd(η³-2-CH₃-C₃H₄)(Ph₂PPyH)₂](CH₃SO₃)₂(BF₄) (3a).Protonation strongly influences the ¹H and ¹³C NMR spectral parameters of the allyl moieties of 1a and 3a whose signals resonate at lower fields with respect to the parent species indicating that upon protonation Ph₂PPy becomes a weaker σ-donor and a stronger Π-acceptor. The allyl moiety, which in 1 is static, becomes dynamic in 1a, the observed syn-syn and anti-anti exchange being due to deligation of the protonated phosphine from the metal centre. Treatment of complex 3 with diethylamine in the presence of fumaronitrile gives the new Pd(0)-olefin complex [Pd(η²-fumaronitrile)(PPh₂Py)₂] (4) which has been characterized by elemental analysis and NMR spectroscopy. Low temperature protonation of 4 with methanesulfonic acid leads to the bis-protonated species [Pd(η²-fumaronitrile)(Ph₂PPyH)₂](CH₃SO₃)₂ (4a) which is stable only at temperatures <0 °C.     

A novel multicomponent reaction: easy access to ferrocenyl (alkylimino)-1,4-dihydroquinolines

A facile and rapid synthetic approach of ferrocenyl (alkylimino)-1,4-dihydroquinolines through a novel multicomponent reaction is described. This strategy is high yielding (60-90%). The easy removal of the ferrocenyl entity led to formation of substituted quinolines by microwave method.     

Efficient palladium(II)-catalyzed homocoupling of thiazole-4-carboxylic or oxazole-4-carboxylic derivatives

An efficient Pd(OAc)₂-catalyzed homocoupling of thiazole-4-carboxylic or oxazole-4-carboxylic derivatives is described. It represents a facile and practical methodology to prepare bis-5,5′-thiazole (oxazole)-4,4′-dicarboxylic derivatives in good to excellent yields. This protocol tolerates a series of substitutions on the thiazole (oxazole) rings, including alkyl, carbonyl, and electron-withdrawing/donating group substituted phenyl groups.     

Synthesis of alkenylsilanes via palladium(0)-catalyzed silylation of alkenyl iodides with hydrosilane

Various alkenyl iodides were silylated with hydrosilanes in the presence of KOAc and a catalytic amount of Pd₂(dba)₃·CHCl₃ to afford the corresponding regio- and stereodefined (E)-alkenylsilanes in high yields.     

Bromination and Azidation Reactions of 2-Styrylchromones. New Syntheses of 4(5)-Aryl-5(4)-(2-chromonyl)-1,2,3-triazoles

Summary. The bromination of 2-styrylchromones, bearing electron neutral substituents, with two molar equivalents of piridinium tribromide gave 2-(2-aryl-1,2-dibromoethyl)chromones and 3-bromo-2-(2-aryl-1,2-dibromoethyl)chromones. The presence of electron-donating substituents on their B ring led to a mixture of compounds due to the higher reactivity of their C(2)=C(3) and C=C double bonds, whereas the strongly electron-withdrawing group hindered the bromination. The dehydrobromination of 2-(2-aryl-1,2-dibromoethyl)chromones with triethylamine gave a diastereomeric mixture of (E)- and (Z)-2-(-bromostyryl)chromones. Some novel 4(5)-aryl-5(4)-(2-chromonyl)-1,2,3-triazoles have been obtained from the reactions of 2-(2-aryl-1,2-dibromoethyl)chromones, 2-(-bromostyryl)chromones, and 2-styrylchromones with sodium azide. The reactions of 2-styrylchromones with sodium azide are more efficient, general, and constitute a one-pot synthetic method of 4(5)-aryl-5(4)-(2-chromonyl)-1,2,3-triazoles allowing the preparation of 1,2,3-triazoles bearing either electron-donating or electron-withdrawing substituents in their aryl ring. The structure of all new compounds was established by extensive NMR spectroscopic studies.     

Pd(0)-catalyzed intramolecular Heck reaction: a versatile route for the synthesis of dibenzoazocinone derivatives

Two efficacious synthetic methodologies for the construction of medium-sized nitrogen heterocycles dibenzoazocinones have been developed. The phosphine-assisted protocol afforded better yields of the dibenzoazocinones (73-82%).     

Enantioselective synthesis of 2-amino-4-(nitromethyl)-4H-chromene-3-carbonitriles from 2-iminochromenes

A series of medicinally important 2-amino-4-(nitromethyl)-4H-chromene-3-carbonitriles were synthesized using enantioselective conjugate addition of nitromethane to 2-iminochromenes. The reactions were performed using L8-Cu (II) catalytic system and moderate to good enantioselectivities (62–88%) and yields (66–96%) were obtained.     

Amine- and phosphine-free palladium(II)-catalyzed homocoupling reaction of terminal alkynes

An efficient, amine- and phosphine-free palladium(II)-catalyzed homocoupling of terminal alkynes has been developed. In the presence of PdCl₂, CuI, Me₃NO, and NaOAc, homocoupling of various terminal alkynes underwent smoothly to afford the corresponding diynes in moderate to high yields without any phosphine ligands. In contrast, the presence of a phosphine ligand (PPh₃) disfavored this palladium-catalyzed homocoupling procedure. Bases, solvents, and CuI have fundamental influence on the palladium-catalyzed homocoupling of terminal alkynes.     

Molecular sieves-supported palladium(II) catalyst: Suzuki coupling of chloroarenes and an easy access to useful intermediates for the synthesis of irbesartan, losartan and boscalid

Palladium(II) chloride supported on 4 Å molecular sieves efficiently catalyzes the Suzuki coupling reactions of chlorobenzenes in presence of tetrabutylammonium bromide without any ligand. The useful intermediates for the synthesis of bioactive compounds such as irbesartan, and losartan have been prepared in one step following this reaction. The preparation of this catalyst is very simple. The FE-SEM image shows a cube shape ordered structure. The catalyst does not exhibit any nanoparticles as indicated by TEM. EDS and XPS demonstrate anchoring of Pd on molecular sieves in +2 oxidation state. This heterogeneous catalyst is stable, non-air sensitive and recyclable.     

4-(2-噻唑偶氮)-苯二甘氨酸的合成及其与钯显色反应的研究

报道了水溶性 4 ( 2 噻唑偶氮 ) 苯二甘氨酸 (TAPC)新试剂的合成。该试剂对钯具有较高灵敏度和选择性。在 0 .5mol/LHNO3介质中 ,TAPC与Pd(Ⅱ )形成 2∶1蓝色配合物 ,最大吸收波长 632nm。钯浓度在 0 μg/mL～ 1 .6μg/mL范围内符合比耳定律 ,表观摩尔吸光系数ε632 =5× 1 0 4 L·mol- 1 ·cm- 1 。可允许许多离子共存 ,可直接用于含钯分子筛样中微量钯的测定     

A new synthesis for thermolabile low-valent palladium complexes by electron transfer reactions from nickel(0) to palladium(II) compounds

The nickel(0) complex [Ni(bpy)(cod)] (bpy: 2,2′-bipyridine, cod: cycloocta-1,5-diene) was used as a mild reducing reagent for the synthesis of the extremely reactive low-valent palladium complexes [Pd₂X₂(cod)₂] (1: X = Cl, 2: X = Br), Pd(cod)₂ (3) and Pd(norbornene)₃ (4). The X-ray analysis of 1 showed that the two [Pd(cod)(Cl)] moieties are only connected by a short Pd(I)-Pd(I) bond (bond length: 2.5379(4) Å) with the chloride ions as monodentate ligands. The X-ray structure of 3 which is also known to be an extremely reactive compound could be determined by X-ray diffraction. As expected, the Pd(0) centre is surrounded by the two cod ligands to form a PdC₄ tetrahedron with typical Pd-C bond lengths. The crystal structure of 3 shows it to be very similar to the closely related complexes M(cod)₂ (M: Ni, Pt). The X-ray structure of 4 displays that the Pd(0) centre is in a trigonal planar environment of the three olefin groups. According to ¹H NMR measurements the complexes have the same structure in solution as found in the solid state.     

Highly substituted cyclohexanes: strong proximity effects influence synthetic access to 1,3,5-tris(bromomethyl)-1,3,5-trialkylcyclohexanes (alkyl = methyl, n-propyl)

1,3,5-Tris(bromomethyl)-1,3,5-trialkylcyclohexanes (alkyl = methyl, n-propyl) were prepared. These are the first examples of 1,3,5-tris(halomethyl)-1,3,5-trialkylcyclohexanes. One synthetic method directly converted the corresponding triols with PPh₃Br₂, where an excess of the bromination reagent and high temperature (175 °C) were required. Stoichiometric use of PPh₃Br₂ under mild conditions, successfully employed for the synthesis of the parent tris(bromomethyl)cyclohexane, did not lead to the desired tribromides but rather to cyclic ethers. Proximity effects triggered by the 1,3,5-alkyl groups strongly influence the reactivity of such highly substituted cyclohexanes. An alternative synthetic access to the tris(bromomethyl) compounds was also developed, using 1,3,5-tris(triflatomethyl)-1,3,5-trialkylcyclohexanes (triflato = F₃CSO₃) as synthetic intermediates. An X-ray crystal structure of 1,3,5-tris(bromomethyl)-1,3,5-trimethylcyclohexane was obtained.     

Pd(OAc)2-catalyzed orthogonal synthesis of 2-hydroxybenzoates and substituted cyclohexanones from acyclic unsaturated 1,3-carbonyl compounds

A Pd-catalyzed orthogonal synthesis of substituted 2-hydroxybenzoates and substituted cyclohexanones was developed for the first time. The substituted 2-hydroxybenzoates were obtained from acyclic unsaturated 1,3-carbonyl compounds using a combination of catalytic Pd(OAc)₂ and Cu(OAc)₂. On the other hand, the substituted cyclohexanones were produced from similar substrates via catalytic Pd(OAc)₂ and hydrogen chloride. Each transformation was clean, easy to work up, provided the desired compounds in good purities, and did not require column chromatography purification.     

The use of iminopyridines as efficient ligands in the palladium(II)-catalyzed cyclization of (Z)-4′-acetoxy-2′-butenyl 2-alkynoates

Iminopyridines were found to be a sort of efficient bidentate ligands for the palladium(II)-catalyzed cyclization of (Z)-4′-acetoxy-2′-butenyl 2-alkynoates in acetic acid to afford the α-(Z)-acetoxyalkylidene-β-vinyl-γ-butyrolactones. The iminopyridine ligands could not only inhibit β-hydride elimination but also stabilize the vinyl-palladium intermediate in acetic acid in the reaction.     

1,3-Cycloaddition of nitrones in ionic liquids catalyzed by Er(III): an easy access to isoxazolidines

1,3-Dipolar cycloadditions of nitrones with alkenes afforded the corresponding isoxazolidines in ionic liquids in the presence of Er(OTf)₃. The ionic liquid and the catalyst are recycled up to five times without any specific treatment or loss of activity. Extension of the procedure to the synthesis of isoxazolidinyl nucleosides has been investigated.     

Modification of a phase-inhomogeneous alumina support of a palladium catalyst. Part II: the effect of palladium dispersion on the formation of hydride forms,electronic state,and catalytic performance in the reaction of partial hydrogenation of unsaturated hydrocarbons

It was shown for the first time that amorphous phase in an alumina support promotes the formation of palladium particles in a wide size range. This catalyst has a low selectivity to butenes in the 1,3-butadiene hydrogenation. It was suggested that surface palladium aluminates contribute to an increase in butene selectivity up to 99.5% at a hydrogenation temperature of not more than 65 °C. At higher reaction temperatures, the catalyst based on phase-homogeneous γ-Al₂O₃ has the highest activity and butene selectivity. This catalyst was obtained by the traditional impregnation method and contains highly dispersed palladium particles with a sufficiently high electron density. It was shown that the formation of hydride forms on palladium particles with a size of less than 1 nm was detected by temperature-programmed reduction with hydrogen.     

Totally regio- and stereoselective synthesis of (E)-3-arylidene-3,4-dihydro-2H-1,4-benzoxazines under palladium catalyst

A new, one-pot palladium catalyzed reaction has been developed for the general synthesis of (E)-3-arylidene-3,4-dihydro-2H-1,4-benzoxazines at room temperature. The reaction procedure tolerates various functional groups. The method is characterized by regio- and stereoselectivity, operational simplicity, mild reaction conditions, and short reaction time.     

Palladium(0)-catalyzed amination of allylic natural terpenic functionalized olefins

The palladium(0) catalyzed amination of allylic acetates and carbonates derivatives from terpenic olefins was carried out under mild conditions. The reaction offers a very good method for the preparation of allylic amines and thus to provide a useful entry to new functionalized terpenic olefin products. The mechanism involving a formation of p-allyl-palladium intermediate complex is in good agreement with the results obtained with the optically active substrates, as well as via an analysis of the observed regio-and stereoselectivity.      

Copper(I)-catalyzed [3+1] cycloaddition of alkenyldiazoacetates and iminoiodinanes: easy access to substituted 2-azetines

The copper(I)-catalyzed reaction of alkenyldiazoacetates and iminoiodinanes affords functionalized azetine derivatives. This process is consistent with the formation of an aziridinyldiazoacetate intermediate, which gives rise to the four-membered heterocycles by metal-catalyzed ring expansion. The resulting azetine structure is a direct precursor of azeditine-2-carboxylic acid derivatives (EWG = electron-withdrawing group).