Studies on the catalytic ability of palladium wire, foil and sponge in the Suzuki-Miyaura cross-coupling

Different forms of metallic palladium (wire, foil and sponge) have been tested as potential catalysts in the Suzuki-Miyaura cross-coupling. All samples showed to be catalytically active for both electron-poor and electron-rich aryl bromides combined with a variety of arylboronic acids. Palladium wire has been recycled six times without decrease of activity. A series of poisoning experiments demonstrated that the true catalyst is a soluble form of palladium arising from a leaching process. Interestingly, metal leaching from palladium foil has been clearly evidenced by SEM.     

Application of palladium(II) complex with bidentate phosphine sulfide ligands to palladium-catalyzed C-C coupling reaction

A phosphine sulfide Pd(II) complex, [Pd(p₂S₂)₂](BF₄)₂ (1) (p₂S₂ = 1,2-bis(diphenylphosphino)ethane disulfide), was synthesized and characterized by an X-ray crystal structure analysis and ³¹P NMR spectroscopy. The p₂S₂ ligand exchange rate of 1 with free p₂S₂ in chloroform was revealed to be comparable to the general solvent exchange rate on Pd(II). The catalytic activity of 1 was evaluated by carrying out the Heck reaction. The diminishing of the induction period and acceleration of the reaction were observed for 1 by comparing the phosphine Pd(II) complexes with a leaving chloro ligand, [PdCl(p₃)]Cl (p₃ = bis[2-(diphenylphosphino)ethyl]phenylphosphine) and [PdCl(pp₃)]Cl (pp₃ = tris[2-(diphenylphosphino)ethyl]phosphine), and the catalytic activity was comparable to that of the phosphine Pd(0) complex, [Pd(PPh₃)₄]. Such a high catalytic activity of 1 is attributed to the π-accepting ability of the phosphine sulfide S atom which stabilizes the catalytically active Pd(0) species electronically and weak σ-donation of the S atom which does not block the formation and a subsequent reaction of the Pd(II) substrate adduct in the catalytic cycle.     

Palladium complexes with hydrophosphorane ligands (HP∼O and HP∼N), catalysts for Heck cross-coupling reactions

The synthesis and structural characterization of palladium complexes 1-3 incorporating H-spirophosphorane HP(OCMe₂CMe₂O)₂ and H-phosphonate congener HP(O)(OCMe₂CMe₂O) ligands are reported. The chemical composition of the complexes [PdCl(μ-Cl){P(OCMe₂CMe₂O)OCMe₂CMe₂OH}]₂ (1), [Pd{P(O)(OCMe₂CMe₂O)}₂]_n (2), and [Pd(μ-Cl){P(OCMe₂CMe₂O)OH}{P(OCMe₂CMe₂O)O}]₂ (3) was established by means of NMR, IR, ESI-MS spectroscopic methods. The relative stereochemistry of 1 and 3 was unambiguously determined by single X-ray diffraction studies. It was proved that complexes 1-3, and complex [PdCl₂P(OCH₂CMe₂NH)OCH₂CMe₂NH₂] (4) previously described in the literature, are very efficient catalysts for the Heck cross-coupling of bromobenzene with n-butyl acrylate. Moreover, they were also found to be effective catalysts in the stereoselective synthesis of trans-stilbenes.     

Influence of reaction conditions on the enantioselectivity of biocatalyzed C-C bond formations under high pressure conditions

Benzoylformate decarboxylase (BFD, EC 4.1.1.7) is a homotetrameric thiamine diphosphate (ThDP)-dependent enzyme which catalyzes the synthesis of chiral 2-hydroxyketones accepting a broad range of aldehydes as substrates. In this study the synthesis of 2-hydroxypropiophenone (2-HPP) from benzaldehyde and acetaldehyde was catalyzed by three BFD variants namely BFD F464I, BFD A460I and BFD A460I-F464I. This paper reports the effect of hydrostatic pressure up to 290 MPa when the reactions were carried out at different benzaldehyde concentrations (5-40 mM) as well as at different pH values (7.0-8.5). Acetaldehyde concentration was fixed at 400 mM in all biotransformations. Reactions performed at high benzaldehyde concentrations and at high hydrostatic pressures showed an increase in (R)-2-HPP formation catalyzed by all BFD variants. For BFD A460I-F464I we observed an increase in the ee of (R)-2-HPP up to 80%, whereas at atmospheric conditions this variant synthesizes (R)-2-HPP with an ee of only 50%. Alkaline conditions (up to pH 8.5) and high hydrostatic pressures resulted in an increase of (R)-2-HPP synthesis, especially in the case of BFD A460I and BFD F464I.     

Anchoring of palladium(II) in chemically modified mesoporous silica: An efficient heterogeneous catalyst for Suzuki cross-coupling reaction

The synthesis and characterization of a highly efficient and reusable catalyst, Pd(II) immobilized in mesoporous silica MCM-41, are described. Pd(II) Schiff-base moiety has been anchored onto mesoporous silica surface via silicon alkoxide chemistry. The catalyst has been characterized by small-angle X-ray diffraction (SAX), FTIR and electronic spectroscopy as well as elemental analysis. The catalyst is used in Suzuki cross-coupling reaction of various aryl halides, including less reactive chlorobenzene, and phenylboronic acid to give biaryls in excellent yields without any additive or ligand. High selectivity for the bi-aryl products containing both electron-donating and electron-withdrawing substituents, mild reaction conditions and possibility of easy recycle makes the catalyst highly desirable to address the industrial needs and environmental concerns.     

Application of the Suzuki-Miyaura cross-coupling reaction for the modification of phenylalanine peptides

We have demonstrated an exceptionally simple and a useful methodology for modification of unusual phenylalanine peptides by adapting the building block approach using the Suzuki-Miyaura cross-coupling reaction as a key step. This strategy gave a good overall yield of various modified tri- and pentapeptides that may be useful to prepare various biologically active peptides in a short period of time.     

The effect of heating by microwave irradiation and by conventional heating on the aldehyde concentration in aqueous glutaraldehyde solutions

Summary The effect of short time heating of aqueous solutions of glutaraldehyde (GA) on relative aldehyde concentration was determined using spectrophotometric analysis. Because free monomeric GA absorbs U. V. light at 280 nm, whereas the alpha, beta polymeric forms absorb at 235 nm, the purity of GA solutions can be expressed as the ratio: A 235 nm/A 280 nm (purification index, P.I.).Heating of 4 ml aliquots of 0.85% distilled aqueous GA solution resulted in an increase of the absorption at 280 nm which is correlated positively with temperature. No increase of absorption at 235 nm was found when solutions were kept at 40°C for several hours. The increase of absorption at 280 nm is caused by a rapid decyclization of hemiacetals producing an increase in free aldehyde concentration.No major differences in absorption were found between the solutions heated by microwave and by conventional heating. However, because microwave irradiation is known to produce an homogeneous rise in temperature, especially in bulky samples, it is expected that the results of fixation procedures will improve by the combined effect of higher temperature and enhanced diffusion rates of the fixating species.     

Solvent-free synthesis of chiral Schiff-base ligands based on ferrocene under microwave irradiation and application to enantioselective nitroaldol (Henry) reaction

Chiral Schiff-bases 3a-f based on ferrocene were designed and synthesized using solvent-free methods by mixing ferrocene carbaldehyde 1 with amino alcohols and amines 2a-f under microwave irradiation and classical method for the enantioselective nitroaldol (Henry) reaction. The Schiff-bases were obtained in shorter reaction times and improved yield under microwave irradiation method over classical method. The highest enantioselectivity was observed in ligand 3e (95% ee) when CH(2)Cl(2) was used as solvent.     

Selective enzymic esterification of free fatty acids with n-butanol under microwave irradiation and under classical heating

Selective enzymic esterification of free fatty acids, obtained from blackcurrant oil by chemical saponification, with n-butanol using four immobilized lipases under microwave irradiation and under classical heating was studied. A positive effect of microwave irradiation on chemical yields of the products of the enzymic reactions and specificity of lipases were observed in comparison with a controlled heating in an incubator equipped with shaking (classical heating) applied during the identical enzyme-mediated processes. The maximum quantity of -linolenic acid (30%) was obtained with Lipozyme used as biocatalyst of the reaction under microwave irradiation. The maximum quantity of butyl -linolenate (20%) was obtained by a Pseudomonas cepacia lipase catalyzed esterification under classical heating.     

Cyclodiphosphazane appended with thioether functionality: Synthesis, transition metal chemistry and catalytic application in Suzuki-Miyaura cross-coupling reactions

The coordination chemistry of thioether functionalized cyclodiphosphazane ligand, cis-{^tBuNP(OCH₂CH₂SCH₃)}₂ (1) is described. The reactions of 1 with [Pd (COD)Cl₂] in 1:1, 1:2 and 2:1 M ratios afforded cis-[PdCl₂{^tBuNP(OCH₂CH₂SCH₃)}₂] (2), cis-[{PdCl₂}₂{^tBuNP(OCH₂CH₂SCH₃)}₂] (3) and trans-[PdCl₂{(^tBuNP(OCH₂CH₂SCH₃))₂}₂] (4), respectively. Treatment of 1 with [Pd(PEt₃)Cl₂]₂ or [PdCl(η³-C₃H₅)]₂ in appropriate molar ratios produce the mono- and binuclear complexes [PdCl₂(PEt₃{^tBuNP(OCH₂CH₂SCH₃)}₂] (5) and [{PdCl(η³-C₃H₅)}₂{^tBuNP(OCH₂CH₂SCH₃)}₂] (6) in good yield. The reaction of 1 with [{Ru(p-cymene)Cl₂}₂] afforded the mononuclear cationic complex, [{(p-cymene)RuCl{^tBuNP(OCH₂CH₂SCH₃)}₂]Cl (7), whereas the reactions of [Rh(COD)Cl]₂, [Pt(COD)Cl₂] and [Au(SMe₂)Cl] with 1 yielded the corresponding P-coordinated neutral complexes, [RhCl(COD){^tBuNP(OCH₂CH₂SCH₃)}₂] (8)cis-[PtCl₂{^tBuNP(OCH₂CH₂SCH₃)}₂] (9), respectively. The binuclear palladium(II) complex 3 was found to be an effective catalyst for the Suzuki-Miyaura cross-coupling reactions.     

Degradation of hyaluronan by ultrasonication in comparison to microwave and conventional heating

Hyaluronan (hyaluronic acid, HA) was depolymerised by ultrasonication (US), microwave irradiation (MW) and conventional heating (CH), and the effect of pH and oxidants was investigated. The degradation was followed by viscometry and size exclusion chromatography coupled with low-angle light scattering. The results demonstrated that depolymerisation of HA by US leveled off to a limiting molecular mass, and the degradation was significantly enhanced by acidic and alkaline pH only in the presence of oxidants. In contrast to US, the course of depolymerisation by MW was strongly pH-dependent, and the degradation rate increased with decreasing pH. The expected enhancement of depolymerisation by MW in comparison to CH was marked only at very short heating time at pH <4. The NMR and FTIR spectral analyses indicated that HA in the whole M_w-range studied retained almost the backbone of the parent polysaccharide independently on the degradation method used. At harsh degradation conditions (long-term treatments, particularly at acidic pH or alkaline pH and in presence of oxidants) the depolymerisation was accompanied by destruction of both constituent sugar residues and formation of unsaturated structures detectable by UV-absorption at 230–240 and 260–270 nm. US-assisted oxidative degradation under mild reaction conditions was shown to be the most appropriate procedure to reduce the molecular mass of HA to 100 kDa without significant chemical modification of the polysaccharide.     

Survival of enterobacteria in liquid cultures during microwave radiation and conventional heating

Bacteria in food have been reported to survive in larger numbers after processing by microwave radiation than after conventional processing. The bactericidal effect of a domestic microwave oven (SHARP R-7280) on certain pathogenic enterobacteria species was investigated in vitro, in comparison with conventional heating (boiling). The death rates of different nosocomial strains of Escherichia coli, Salmonella sofia, Salmonella enteritidis, Proteus mirabilis and Pseudomonas aeruginosa were tested. The microwave oven and the conventional heating system used were both calibrated in order to calculate temperatures from exposure times. For each strain duplicate samples of 25 ml of pure culture with concentrations at least 10(6) cfu/ml were exposed to microwave radiation. An equal number of samples of the same volume and concentration were exposed to conventional heating. Subsequently all samples were examined qualitatively and quantitatively following standard microbiological procedures. The results indicate that microwaves have an efficient bactericidal effect on the enterobacteria in liquid cultures.     

Comparison of bacterial growth to high-intensity microwave exposure and conventional heating

Escherichia coli pol A⁺ and pol A^? strains were exposed to 8.8-GHz microwaves pulsed at 1,000 Hz (1-μs pulse width) and an SAR of 40 W/kg, which increased the temperature of the cell culture by 7 °C. Two-way analysis of variance showed no significant difference between the growth rates of microwave irradiated and thermally exposed cells.     

Synthesis and characterization of sterically hindered thiolate Pd(II) complexes of the type [Pd(SR)2(TMEDA)]: Examination of their catalytic properties in phosphane-free Suzuki-Miyaura cross couplings

The sterically hindered thiolate complexes [Pd(SR)₂(TMEDA)] SR = SC₆F₅ (2), SC₆F₄-4-H (3), SC₆H₄-2-SiPh₃ (4) were easily synthesized by metathetical reactions of the corresponding palladium chloro compound [Pd(Cl)₂(TMEDA)] (1) and the lead salt of the corresponding thiol. The identity of the three species being unequivocally determined by single crystal X-ray diffraction techniques. The catalytic activity of the palladium species [Pd(SR)₂(TMEDA)] was explored in the Suzuki-Miyaura cross coupling reactions of different p-substituted bromobenzenes.     

Chiral recyclable dimeric and polymeric Cr(III) salen complexes catalyzed aminolytic kinetic resolution of trans-aromatic epoxides under microwave irradiation

Aminolytic kinetic resolution (AKR) of trans-stilbene oxide and trans-beta-methyl styrene oxide proceeded smoothly under microwave irradiation using chiral dimeric and polymeric Cr(III) salen complexes as efficient catalysts, giving regio-, diastereo-, and enantioselective anti-beta-amino alcohols in high yields (49%) and chiral purity (ee up to 94%) in case of 4-methylaniline within 2 min. The kinetic resolution system is approximately five times faster than traditional oil bath heating at 70 degrees C and 420 times faster than the reaction conducted at room temperature with concomitant recovery of respective chirally enriched epoxides (ee, 92%) in excellent yields (up to 48%). The catalyst 1 worked well in terms of enantioselectivity than the catalyst 2, but both the catalysts were easily recovered and reused five times with the retention of its efficiency.     

Decarboxylative aldol reaction catalysed by lipases and a protease in organic co-solvent mixtures and nearly anhydrous organic solvent media

Abstract

The effects of the choice of lipase, reaction medium, immobilization, presence of additives and temperature on conversion and stereoselectivity during a lipase catalysed decarboxylative aldol reaction were examined. It was shown that Candida antarctica lipase B (CALB) catalysed a decarboxylative aldol reaction between 4-nitrobenzaldehyde and ethyl acetoacetate in a 60% acetonitrile–40% aqueous buffer co-solvent mixture. Interestingly, free and immobilized forms of CALB showed opposite enantioselectivity in this media. The addition of 30 mol% imidazole increased the reaction rate from 8.5 to 55.7 μM min^??1 mg^??1. A 98% conversion could be achieved in 14 h (instead of 168 h) by adding imidazole. Other lipases also catalysed this reaction in different reaction media to a varying extent. With Mucor javanicus lipase in 30% DMSO, 20% enantiomeric excess (ee) of the (R)-product was observed. CALB also catalysed this reaction in nearly anhydrous acetonitrile. In the presence of cross-linked protein coated microcrystals of CALB, 90% conversion was obtained in this media in 24 h. A commercially available protease, alcalase, was also found to catalyse this reaction. While low water media gave poor conversion, the reaction in aqueous–60% acetonitrile co-solvent mixture gave 99% conversion in 72 h, provided imidazole was used as an additive.     

A convenient synthesis of phosphine-functionalized N-heterocyclic carbene ligand precursors, structural characterization of their palladium complexes and catalytic application in Suzuki coupling reaction

A series of imidazolium chlorides as ligand precursors, L · HCl (L = (1-R)-(3-diphenylphosphanylethyl)-imidazol-2-ylidene; R = aryl, benzyl, naphthylmethyl), for the phosphine-functionalized N-heterocyclic carbene (NHC), L, were prepared by a convenient synthetic procedure of reacting 1,2-dichloroethane with appropriate N-substituted imidazoles to give (β-chloroethyl)imidazolium chlorides, which were subsequently reacted with HPPh₂ producing L · HCl in good yield. Palladium complexes of L, PdLCl₂ (4), were prepared by a one pot reaction of PdCl₂, sodium acetate, and L · HCl in DMSO. Complexes 4b (R = naphthylmethyl) and 4e (R = m-methoxybenzyl) were characterized by X-ray crystallography. Catalytic studies have shown that the palladium complexes are efficient in Suzuki coupling reactions of aryl bromides with phenylboronic acid.     

Activation and deactivation of a carbene containing non-classical ruthenium hydride complex in catalytic processes involving C-H bond cleavage

The carbene-containing non-classical ruthenium hydride complex [(PCy₃)Ru(H)₂(H₂)₂(IMes)] 1 (IMes=1,3-Bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene) is an active catalyst for H/D exchange in aromatic ketones. It is also capable of combining sp² C-H bond activation with C-C bond formation. Comparing the chemo- and regio-selectivities of the H/D exchange process and the C-C bond formation clearly indicates that different intermediates are involved in the two processes. High pressure NMR studies provide strong evidence that the key intermediate for the C-C coupling reaction is analogous to that for other ruthenium catalysts reported previously. Catalytic turnover is limited by the instability of this intermediate in the presence of the olefinic coupling partner.     

Synthesis of aryl pyrazole via Suzuki coupling reaction,in vitro mushroom tyrosinase enzyme inhibition assay and in silico comparative molecular docking analysis with Kojic acid

Aryl pyrazoles are well recognized class of heterocyclic compounds found in several commercially available drugs. Owing to their significance in medicinal chemistry, in this current account we have synthesized a series of suitably substituted aryl pyrazole by employing Suzuki cross-coupling reaction. All compounds were evaluated for inhibition of mushroom tyrosinase enzyme both in vitro and in silico. Compound 3f (IC₅₀ = 1.568 ± 0.01 µM) showed relatively better potential compared to reference kojic acid (IC₅₀ = 16.051 ± 1.27 µM). A comparative docking studies showed that compound 3f have maximum binding affinity against mushroom tyrosinase (PDBID: 2Y9X) with binding energy value (−6.90 kcal/mol) as compared to Kojic acid. The 4-methoxy group in compound 3f shows 100% interaction with Cu. Compound 3f displayed hydrogen binding interaction with His61 and His94 at distance of 1.71 and 1.74 Å which might be responsible for higher activity compared to Kojic acid.