Cyclopalladation of mono-, di- and tribenzylamine by palladium(II) acetate: Influence of bulkiness around the nitrogen atom of benzylamine upon internal metallation

Cyclopalladation of mono-, di- and tribenzylamine has been investigated by reacting the corresponding amines with an equimolar amount of palladium(II) acetate (reaction i), or by heating the corresponding bis-amine complexes [Pd(O₂CMe)₂{(PhCH₂)_nNH_3−n}₂] (n=1, 2) (reaction ii). By the reaction i, all the three amines undergo cyclopalladation. However, in the case of the reaction ii, only the dibenzylamine complex [Pd(O₂CMe)₂{(PhCH₂)₂NH}₂] has been converted into a cyclopalladated complex. The reactivity of the three benzylamines towards cyclopalladation has been discussed in terms of the co-ordinating ability influenced by the bulkiness around the nitrogen atom. Temperature-dependent ¹H-NMR spectra are observed for mononuclear cyclopalladated complexes [Pd(O₂CMe){C₆H₄CH₂N(CH₂Ph)₂–C¹, N}L] (L=PPh₃, AsPh₃) and are attributed to the dissociation of the nitrogen atom in the cyclopalladated chelate ring. A heteroleptic bis-cyclopalladated complex [Pd[C₆H₄CH₂N(CH₂Ph)₂–C¹, N](C₆H₄CH₂NMe₂–C¹, N)] has also been prepared. X-ray crystallographic studies on [{Pd(O₂CMe)[C₆H₄CH₂N(CH₂Ph)₂–C¹, N]}₂] and [Pd[C₆H₄CH₂N(CH₂Ph)₂–C¹, N](C₆H₄CH₂NMe₂–C¹, N)] have been reported.     

Synthesis of non-symmetrical alkynylpyridines by chemoselective Sonogashira cross-coupling reactions

4-Bromo-2,3,5-trichloro-6-iodopyridine was studied as a new substrate in chemoselective Sonogashira reactions. This approach provides an efficient access to non-symmetrical mono-, di-, tri- and pentaalkynylated pyridines in good yields. Selected pentaalkynylated pyridines were studied with regard to their UV/Vis- and emission properties showing moderate to high fluorescence quantum yields.     

Multi-step reactions of N-monosubstituted (polyfluoroalkane)thioamides with alkyllithium reagents

Reactions of N-alkyl- or N-aryl(perfluoroalkyl)thiocarboxamides with alkyl lithium reagents are described. Trifluorothioacetamides are converted into the corresponding lithium salts. Compounds bearing a long polyfluorinated chain terminated by a CHF₂ group and compounds containing an N-alkyl substituent with a proton adjacent to nitrogen react further via a multi-step reaction sequence involving HF elimination and then vinylic fluorine substitution and/or S_N′ type fluorine substitution. These transformations led to unsaturated N-monosubstituted polyfluorinated thioamides.     

Pd(OAc)2/dppf as an efficient and highly active catalyst for the allylation of amines, alcohols and carboxylic acids with 1-phenyl-1-propyne

Pd(OAc)₂/1,1′-bis(diphenylphosphino)ferrocene as an efficient, highly active catalyst for the allylation of amines, alcohols and carboxylic acids with 1-phenyl-1-propyne has been developed. The effect of various reaction parameters, such as ligand, time, solvent, temperature, metal: ligand ratio and catalyst concentration on yields of the product were investigated. The optimized procedure works well under mild operating conditions and permits rapid generation of a library for various allylated products.     

Solvent-modulated Pd/C-catalyzed deprotection of silyl ethers and chemoselective hydrogenation

Recently we have reported undesirable and frequent deprotection of the TBDMS protective group of a variety of hydroxyl functions occurred under neutral and mild hydrogenation conditions using 10% Pd/C in MeOH. The deprotection of silyl ethers is susceptible to significant solvent effect. TBDMS and TES protecting groups were selectively cleaved in the presence of acid-sensitive functional groups such as TIPS ether, TBDPS ether and dimethyl acetal under hydrogenation condition using 10% Pd/C in MeOH. In contrast, chemoselective hydrogenation of reducible functional groups such as acetylene, olefin and benzyl ether, proceeds in the presence of TBDMS or TES ethers in AcOEt or MeCN.     

A short and efficient synthesis of N-aryl- and N-heteroaryl-N-(arylalkyl)piperazines

A new synthesis of N-aryl- and N-heteroaryl-N^′-(arylalkyl)piperazines using palladium-catalyzed amination of aryl bromides and heteroaryl chlorides with mono N-benzyl- or N-(arylethyl)piperazines is reported. Most coupling processes proceed in high yield and good selectivity using either diadamantyl-n-butylphosphine (1), 2-(dicyclohexylphosphino)-2^′-(N,N-dimethylamino)biphenyl (2), or 2-(di-tert-butylphosphino)biphenyl (3) as ligand. Applying an automated parallel synthesizer the preparation of a small library of potentially bioactive compounds is easily achieved.     

Synthesis of N,N,N′,N′-tetrasubstituted 1,3-bis(4-aminophenyl)azulenes and their application to a hole-injecting material in organic electroluminescent devices

After a preliminary search of the reaction conditions for the Suzuki-Miyaura cross-coupling of haloazulenes with arylboronic acids, the title compounds were synthesized either by the direct coupling reaction between 1,3-dihaloazulene and the corresponding N,N-disubstituted 4-aminophenylboronic acids or by a two-step sequence involving the cross-coupling with 4-bromophenylboronic acid and subsequent Pd-catalyzed amination. Application of the title diamines to a hole-injecting material in organic electroluminescent devices was carried out to provide their prominent characteristics as a novel durable, non-cyanine and non-polyamine substance without color fade. The diamine derivatives, extended by an ethynyl unit between the azulenyl core and the 4-aminophenyl moiety, were also synthesized and found, unfortunately, unsuitable for vacuum deposition in preparing a multilayer composite.     

Well-defined NHC-Pd(II)-Im (NHC=N-heterocyclic carbene; Im=1-methylimidazole) complexes catalyzed amination of aryl chlorides

A new class of well-defined NHC-Pd(II)-Im complexes was synthesized and was found to be an efficient catalyst for the amination reactions of aryl chlorides. Under the optimal reaction conditions, a range of amines can be coupled to give the amination products in good to high yields.     

A homogeneous catalyst made of poly(4-vinylpyridine-co-N-vinylpyrrolidone)-Pd(0) complex for hydrogenation of aromatic nitro compounds

Poly(4-vinylpyridine-co-N-vinylpyrrolidone)(VPy-co-NVP) and its palladium complex (VPy–NVP–Pd) were prepared. The palladium complex was used as catalyst for the hydrogenation of some nitroaromatics. The molar content of VPy units in VPy-co-NVP was determined as 31.25% by ¹H NMR. VPy–NVP–Pd can be easily resolved in ethanol forming a homogeneous catalytic hydrogenation system together with substrates. The optimum catalytic activity for hydrogenation of nitrobenzene appeared when VPy/Pd molar ratio was 2. The catalytic behavior of the catalyst was found to be greatly affected by the type and concentration of added alkalies. The highest hydrogenation rate for nitrobenzene was found in a 0.1 mol/l ethanol solution of potassium hydroxide. The catalytic stability was examined by using nitrobenzene and 4-nitroanisole as substrates.     

Graphene oxide (GO)-catalyzed chemoselective thioacetalization of aldehydes under solvent-free conditions

An efficient method for the synthesis of open chain, cyclic, and unsymmetrical dithioacetals from aryl/hetero-aryl/aliphatic aldehydes is described. The reaction is performed using graphene oxide (GO) as the catalyst under solvent-free and aerobic conditions. High chemoselectivity is observed in the reaction as aryl/alkyl ketones do not give thioketals under the condition.     

N-Iodosuccinimide (NIS) as a mild and highly chemoselective catalyst for deprotection of tert-butyldimethylsilyl ethers

A variety of alcoholic TBDMS (t-butyldimethylsilyl) ethers are easily removed in excellent yields by treatment with a catalytic amount of N-iodosuccinimide (NIS, 5 mol %) in methanol. This method is able to deprotect TBDMS ethers of alcohols in the presence of TBDMS ethers of phenols.     

Effects of solvent and base on the palladium-catalyzed amination: PdCl2(Ph3P)2/Ph3P-catalyzed selective arylation of primary anilines with aryl bromides

A readily accessible catalytic system, PdCl₂(Ph₃P)₂/Ph₃P, was developed for the selective arylation of primary anilines with aryl bromides. The strong influence of solvents and bases on the catalytic activity was observed. In refluxing o-xylene, triphenylphosphine shows high efficiency for Pd-catalyzed intermolecular amination reactions. By changing the bases, mono- and diarylation of primary amines could be selectively achieved in high yields. Moreover, the catalytic system showed good toleration for the steric hindrance of anilines. A series of N,N,N′,N′-tetraaryl-1,1′-biphenyl-4,4′-diamines, important intermediates of OLED hole transport materials, were synthesized facilely via coupling reactions between 4,4′-diaminobiphenyls and aryl bromides.     

Synthesis and properties of N,N,N′,N′-tetrasubstituted 1,3-bis(5-aminothien-2-yl)azulenes and their application as a hole-injecting material in organic light-emitting devices

Two title compounds, N,N,N′,N′-tetraphenyl-1,3-bis(5-aminothien-2-yl)azulene (3a) and 1,3-bis{5-(9-carbazolyl)thien-2-yl}azulene (3b), were synthesized from 1,3-di(2-thienyl)azulene (4) by a two-step sequence involving bromination and subsequent Pd-catalyzed amination. These compounds were characterized by spectroscopic analyses and the structure of 3a was determined by X-ray crystallographic analysis. Their HOMO energy levels were estimated using their electrochemical oxidation potentials, and these compounds were used as a hole-injecting material in organic light-emitting devices. The device with 3a showed greater durability than that with copper phthalocyanine.     

Stereoselective synthesis of N-protected pyrrolidines via Pd-catalyzed reactions of γ-(N-acylamino) alkenes and γ-(N-Boc-amino) alkenes with aryl bromides

The stereoselective synthesis of N-acyl- and N-Boc-protected pyrrolidines via Pd-catalyzed reactions of γ-(N-acylamino) alkenes and γ-(N-Boc-amino) alkenes with aryl bromides is described. These reactions effect formation of two bonds in a single operation and proceed with generally high levels of diastereoselectivity. In contrast to previously described reactions of γ-(N-arylamino) alkenes, these transformations proceed in high yield and high regioselectivity with both electron-rich and electron-deficient aryl bromides as well as vinyl bromide substrates.     

Pd2dba3/P(i-BuNCH2CH2)3N: a highly efficient catalyst for the one-pot synthesis of trans-4-N,N-diarylaminostilbenes and N,N-diarylaminostyrenes

A Pd₂dba₃/P(i-BuNCH₂CH₂)₃N catalyzed one-pot synthesis of unsymmetrically substituted trans-4-N,N-diarylaminostilbenes and both symmetrically and unsymmetrically substituted N,N-diarylaminostyrene derivatives is reported. The procedure involves two or more palladium catalyzed sequential coupling reactions (an amination and an inter-molecular Heck reaction) in one-pot using the same catalyst system with two different aryl halides, including aryl chlorides and hetero aryl halides as the coupling partners.     

STUDIES ON THE HYDROGENATION OF METHYL ACRYLATE CATALYZED BY Pd/DVB CROSSLINKED POLY-(N-VINYL PYRROLIDONE)

Many kinds of dispersed Pd (0) supported on cross-linked poly (N-vinyl pyrrolidone) were prepared and characterized by IR and X-ray diffraction. Their hydrogenation properties of methyl acrylate have been examined. Experimental results indicate that the hydrogenation rate will reach a maximum as the degree of crosslinking increases to a certain point. The catalytic activity of the catalysts is closely related to the preparation condition and the particle size of Pd of the catalysts. Some other factors which affect the catalytic properties have also been studied.     

Structural diversity of a series of chlorocadmate(II) and chlorocuprate(II) complexes based on benzylamine and its N-methylated derivatives

Protonated benzylamine and its N-methylated derivatives, [C₆H₅CH₂NH_3?n(CH₃)_n]⁺ (n?=?0?3), have been adopted as cations in chlorocadmate(II) and chlorocuprate(II) complexes, showing inorganic–organic hybrid architectures. For the Cd(II) compounds, the anionic structures vary from perovskite-type layers (n?=?0) to chains (n?=?1–3). For Cu(II) compounds, the anionic structures range from perovskite-type layers (n?=?0), chains (n?=?1) to mononuclear species (n?=?2–3). Coordination geometries of the metal ions and intermolecular interactions have been analyzed. Their dielectric properties have been measured.     

Sequential regio and chemoselective cross-coupling reactions by means of O-tri-isopropylsulfonate of 4-bromo-pyridazine 3,6-dione

Regioselective desymmetrization of 4-substituted pyridazin-3,6-diones using sterically hindered 2,4,6-triisopropylphenyl-sulfonylchloride allowed efficient sequential palladium cross-coupling reactions.     

Synthesis of 7H-indolo[2,3-c]quinolines: study of the Pd-catalyzed intramolecular arylation of 3-(2-bromophenylamino)quinolines under microwave irradiation

D-ring substituted 5-methyl-5H-indolo[2,3-c]quinolines (4) have been synthesized in three steps starting from commercially available 3-bromoquinoline (5) and 2-bromoanilines (6). The methodology consists of two consecutive palladium-catalyzed reactions: a selective Buchwald-Hartwig amination followed by a regioselective intramolecular Heck-type reaction. The latter step has been investigated under microwave irradiation. Heating at 180 °C allows to seriously reduce the catalyst loading and get a full conversion to reaction product in 10 min. In addition, the former simplifies the purification.     

Efficient access to (1H)-isoindolin-1-one-3-carboxylic acid derivatives by orthopalladation and carbonylation of methyl arylglycinate substrates

The orthopalladation of methyl arylglycinate derivatives has been studied. The reaction proceeds efficiently for different electron-withdrawing and electron-releasing substituents at the aryl ring. The carbonylation of the orthopalladated complexes affords, in a single step, substituted (1H)-isoindolin-1-one-3-carboxylates. These compounds constitute valuable synthetic intermediates and can be transformed diastereoselectively into octahydroisoindole-1-carboxylic acid derivatives, an important scaffold in the synthesis of many biologically active compounds.