Acid-catalyzed Condensation Research Articles

Syntheses of Biodiesel Precursors: Sulfonic Acid Catalysts for Condensation of Biomass‐Derived Platform Molecules

Synthesis of transportation fuel from lignocellulosic biomass is an attractive solution to the green alternative-energy problem. The production of biodiesel, in particular, involves the process of upgrading biomass-derived small molecules to diesel precursors containing a specific carbon range (C11 -C23). Herein, a carbon-upgrading process utilizing an acid-catalyzed condensation of furanic platform molecules from biomass is described. Various types of sulfonic acid catalysts have been evaluated for this process, including biphasic and solid supported catalysts. A silica-bound alkyl sulfonic acid catalyst has been developed for promoting carbon-carbon bond formation of biomass-derived carbonyl compounds with 2-methylfuran. This hydrophobic solid acid catalyst exhibits activity and selectivity that are comparable to those of a soluble acid catalyst. The catalyst can be readily recovered and recycled, possesses appreciable hydrolytic stability in the presence of water, and retains its acidity over multiple reaction cycles. Application of this catalyst to biomass-derived platform molecules led to the synthesis of a variety of furanic compounds, which are potential biodiesel precursors.

ChemInform Abstract: An Efficient Synthesis of Quinalone‐3‐(N‐phenylpyrazoles) and Quinalone‐3‐cyclohexadienone Derivatives

Abstractbased on acid‐catalyzed condensation reactions of chalcone derivatives (I) with phenylhydrazine and acetoacetanilide (V), respectively

Direct Formation of S-Nitroso Silica Nanoparticles from a Single Silica Source

Nitric oxide (NO) is a ubiquitous molecule in the body. Because of its multiple pathophysiologic roles, the potential for treating various diseases by the exogenous administration of NO has been under intensive investigation. However, the unstable, radical nature of NO poses a major challenge to the effective delivery of NO. Previously, silica nanoparticles synthesized by the traditional method have been developed into NO-carrying systems. In the present study, for the first time NO-carrying silica nanoparticles were prepared from a single silica precursor using a simple nanoprecipitation method. (3-Mercaptopropyl)-trimethoxysilane (MPTMS) was used as the sole silane source, which was subjected to acid-catalyzed S-nitrosation and condensation reactions in a one-pot organic phase. S-Nitroso silica nanoparticles (SNO-SiNPs) were then produced by injecting a smaller quantity of the organic phase into a larger amount of water without surfactants. Various preparation parameters were tested to obtain optimized conditions. Moreover, a phase diagram demonstrating the ouzo effect was constructed. The prepared SNO-SiNPs were spherical particles with a tunable size in the range of 100-400 nm. The nanoparticles in aqueous dispersions exhibited high colloid stability, possibly resulting from highly negatively charged surfaces. The result of solid-state (29)Si NMR shows the predominance of T(2) and T(3) silicon structures, suggesting that nanoparticles were formed from polycondensed silica species. In conclusion, NO-loaded silica nanoparticles have been directly prepared from a single silane precursor using a surfactant-free, low-energy, one-step nanoprecipitation approach. The method precludes the need for the initial formation of bare particles and subsequent functionalization steps.

Mechanism of intra- and inter-molecular C[dbnd]C bond formation of propanal on Brønsted acid sites contained within MFI zeolites

Kinetic and chemical titration studies are used to unravel the reaction pathways and catalytic requirements for propanal deoxygenation over Brønsted acid sites contained within MFI zeolites. Propanal deoxygenation in the absence of external hydrogen source is initiated via primary and competitive pathways of inter- and intra-molecular CC bond formation that involve bimolecular coupling of propanal and uni-molecular deoxygenation steps, respectively. The inter-molecular CC bond formation proceeds via mechanistic steps resembled the acid-catalyzed aldol condensation reactions in the homogeneous phase, and its reactive collision frequencies increase with increasing propanal pressure. The reaction is initiated by keto–enol tautomerization of propanal to form small concentrations of propenol. The propenol undergoes kinetically-relevant nucleophilic attack to protonated propanal, the most abundant surface intermediates, to create the inter-molecular CC bond. The competitive uni-molecular deoxygenation step involves kinetically-relevant hydrogen transfer from hydrogen-donating agents and occurs at rates that remain invariance with propanal pressure. Hydrogen-donating agents are aliphatic rings produced from consecutive inter-molecular CC bond formation and ring closure events and donate hydrogen via dehydrogenation steps to increase their extent of unsaturation. Hydrogen-donating events must kinetically couple with the direct hydrogen insertion step on propanal to satisfy the deoxygenation stoichiometry and form propanol, which upon dehydration evolves predominantly propene, thus preserving the carbon backbone. Water as a by-product prevents binding of larger, inactive carbonaceous species on acid sites and inhibits the inter-molecular CC bond formation step by increasing the reverse rate of this step. Water, however, does not alter the net rate for intra-molecular CC bond formation, because of its irreversible nature. An increase in the rate ratio for intra- over inter-molecular CC bond formation upon the addition of 3-methyl-1-pentene, an effective hydrogen-donating agent, confirms the kinetic relevance of the hydrogen transfer step for propene formation. These findings on the different kinetic dependencies for the competitive reactions and their mechanistic interpretations provide the operating strategies to tune the reaction pathways, manipulate the extent of hydrogen transfer, and tailor the distributions of larger oxygenates and alkenes during propanal deoxygenation reactions.

Mechanically induced silyl ester cleavage under acidic conditions investigated by AFM-based single-molecule force spectroscopy in the force-ramp mode

AFM-based dynamic single-molecule force spectroscopy was used to stretch carboxymethylated amylose (CMA) polymers, which have been covalently tethered between a silanized glass substrate and a silanized AFM tip via acid-catalyzed ester condensation at pH 2.0. Rupture forces were measured as a function of temperature and force loading rate in the force-ramp mode. The data exhibit significant statistical scattering, which is fitted with a maximum likelihood estimation (MLE) algorithm. Bond rupture is described with a Morse potential based Arrhenius kinetics model. The fit yields a bond dissociation energy De = 35 kJ mol(-1) and an Arrhenius pre-factor A = 6.6 × 10(4) s(-1). The bond dissociation energy is consistent with previous experiments under identical conditions, where the force-clamp mode was employed. However, the bi-exponential decay kinetics, which the force-clamp results unambiguously revealed, are not evident in the force-ramp data. While it is possible to fit the force-ramp data with a bi-exponential model, the fit parameters differ from the force-clamp experiments. Overall, single-molecule force spectroscopy in the force-ramp mode yields data whose information content is more limited than force-clamp data. It may, however, still be necessary and advantageous to perform force-ramp experiments. The number of successful events is often higher in the force-ramp mode, and competing reaction pathways may make force-clamp experiments impossible.

The two-step mechanochemical synthesis of porphyrins.

Porphyrin synthesis under solvent-free conditions represents the "greening" of a traditional synthesis that normally requires large amounts of organic solvent, and has hindered the industrial-scale synthesis of this useful class of molecules. We have found that the four-fold acid-catalysed condensation of aldehyde and pyrrole to yield a tetra-substituted porphyrin is possible through mechanochemical techniques, without a solvent present. This represents one of the still-rare examples of carbon-carbon bond formation by mechanochemistry. Specifically, upon grinding equimolar amounts of pyrrole and benzaldehyde in the presence of an acid catalyst, cyclization takes place to give reduced porphyrin precursors (reversible), which upon oxidation form tetraphenylporphyrin (TPP). The approach has been found to be suitable for the synthesis of a variety of meso-tetrasubstituted porphyrins. Oxidation can occur either by using an oxidizing agent in solution, to give yields comparable to those published for traditional methods of porphyrin synthesis, or through mechanochemical means resulting in a two-step mechanochemical synthesis to give slightly lower yields that are still being optimized. We are also working on "green" methods of porphyrin isolation, including entrainment sublimation, which would hopefully further reduce the need for large amounts of organic solvent. These results hold promise for the development of mechanochemical synthetic protocols for porphyrins and related classes of compounds.

Core‐Modified Analogues of Expanded Porphyrins Containing Rigid β,β′‐Linked o‐Phenylene Bridges

Structural modifications that lead to the creation of π-extended aromatic macrocyles involving a heterocyclic ring other than pyrrole and rigid β-β' linkages have not been well studied up to date. The rigidity caused by the conformational restriction would change the spectroscopic properties of the system as compared with those of the normal congeners. With these considerations, we have synthesized and fully characterized π-extended, core modified expanded porphyrins bearing rigid bipyrrole units. Core-modified naphthorubyrins were synthesized by the Lewis acid-catalyzed condensation of naphthobipyrrole with thiophene/furan diols, whereas naphthosapphyrins were obtained by reacting 2,9-diformyl-naphthobipyrrole with 16-thia/oxatripyrranes under mild reaction conditions. The core-modified analogues of both naphthorubyrin and naphthosapphyrin displayed the aromatic character. The dithiarubyrin analogues showed a lack of conformational change as expected and displayed well-resolved (1) H NMR resonances at room temperature. On the other hand, the oxasapphyrin analogue adopts a furan-inverted geometry, and the ring inversion is independent of the protonation state. The oxanaphthosapphyrin also exhibited a weak fluorescence emission at 613 nm.

Synthesis of 6-Substituted 3-(Alkoxycarbonyl)-5-aryl-α-pyrones

An efficient synthesis of 6-substituted 3-(alkoxycarbonyl)-5-aryl-α-pyrones is reported. This methodology consists of the successive manipulation of an addition–elimination reaction between benzyl ketone derivatives and dimethyl methoxymethylenemalonate, and an acid-catalyzed condensation reaction. The synthesis is applicable to various 5-<i>p</i>-substituted-aryl 6-substituted α-pyrones and good to excellent yields were obtained over two steps from benzyl ketone derivatives that were easily prepared from phenylacetic acid by the Negishi coupling or the Claisen decarboxylation reaction.

Conformational Change from a Twisted Figure‐Eight to an Open‐Extended Structure in Doubly Fused 36π Core‐Modified Octaphyrins Triggered by Protonation: Implication on Photodynamics and Aromaticity

Two examples of core-modified 36π doubly fused octaphyrins that undergo a conformational change from a twisted figure-eight to an open-extended structure induced by protonation are reported. Syntheses of the two octaphyrins (in which Ar=mesityl or tolyl) were achieved by a simple acid-catalyzed condensation of dipyrrane unit containing an electron-rich, rigid dithienothiophene (DTT) core with pentafluorobenzaldehyde followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The single-crystal X-ray structure of the octaphyrin (in which Ar=mesityl) shows a figure-eight twisted conformation of the expanded porphyrin skeleton with two DTT moieties oriented in a staggered conformation with a π-cloud distance of 3.7 Å. Spectroscopic and quantum mechanical calculations reveal that both octaphyrins conform to a [4n]π nonaromatic electronic structure. Protonation of the pyrrole nitrogen atoms of the octaphyrins results in dramatic structural change, which led to 1) a large redshift and sharpening of absorption bands in electronic absorption spectrum, 2) a large change in chemical shift of pyrrole β-CH and -NH protons in the (1)H NMR spectrum, 3) a small increase in singlet lifetimes, and 4) a moderate increase in two-photon absorption cross-section values. Furthermore, nucleus-independent chemical shift (NICS) values calculated at various geometrical positions show positive values and anisotropy-induced current density (AICD) plots indicate paratropic ring-currents for the diprotonated form of the octaphyrin (in which Ar=tolyl); the single-crystal X-ray structure of the diprotonated form of the octaphyrin shows an extended structure in which one of the pyrrole ring of each dipyrrin subunit undergoes a 180° ring-flip. Four trifluoroacetic acid (TFA) molecules are bound above and below the molecular plane defined by meso-carbon atoms and are held by N-H···O, N-H···F, and C-H···F intermolecular hydrogen-bonding interactions. The extended-open structure upon protonation allows π-delocalization and the electronic structure conforms to a [4n]π Hückel antiaromatic in the diprotonated state.

Synthesis, Spectral, and Structural Studies of Porphyrins Having Sterically Hindered [η5-CpCo(η4-C4Ph4)] Cobalt Sandwich Units at the Meso Positions

Synthesis, spectral, and structural studies of the first examples of porphyrins substituted at the meso positions with sterically hindered η(5)-CpCo(η(4)-C4Ph4) cobalt sandwich units are described. The novel dipyrromethane derived cobalt sandwich compound {η(5)-[(C4H4N)2CH]C5H4}Co(η(4)-C4Ph4) 1, as well as its parent aldehyde, η(5)-[C5H4(CHO)]Co(η(4)-C4Ph4), were used in the synthesis of porphyrins having one or two η(5)-CpCo(η(4)-C4Ph4) groups at their meso positions. 1,9-Diformyldipyrromethane derived η(5)-CpCo(η(4)-C4Ph4) 2 was synthesized using dipyrromethane 1 under Vilsmeier conditions. A reaction of 2 with unsubstituted dipyrromethane under basic conditions in the presence of Pd(C6H5CN)2Cl2 yielded an A-type palladium coordinated porphyrin 3 [where A = η(5)-CpCo(η(4)-C4Ph4)]. A similar reaction of 2 with meso aryl and ferrocenyl-substituted dipyrromethanes yielded trans-AB type palladium coordinated porphyrins 4-6 [where A = η(5)-CpCo(η(4)-C4Ph4) and B = 4-tert-butylphenyl 4, ferrocenyl 5, and pentafluorophenyl 6]. Reactions of 2 with 5-ferrocenyl dipyrromethane under the same reaction conditions in the presence of Ni(acac)2 and Zn(OAc)2 gave the trimetallic nickel(II) and zinc(II) complexed trans-AB type porphyrins 7 and 8 having both cobalt and iron sandwich units at the meso positions. Crystal structure of the Pd(II) porphyrin 5 and nickel(II) porphyrin 7 showed nonplanar structures having distinct ruffle type distortion of the porphyrin ring. Demetalation of the zinc(II) trans-AB type porphyrin 8 in the presence of trifluoroacetic acid gave the metal free base porphyrin 9. Reactions of the cobalt sandwich aldehyde [(η(5)-C5H4(CHO)]Co(η(4)-C4Ph4) with sterically hindered dipyrromethane derivatives under acid-catalyzed condensation reactions gave trans-A2B2 type porphyrins [where A = η(5)-CpCo(η(4)-C4Ph4) and B = pentafluorophenyl, 10 mesityl 11]. In contrast, reactions of [η(5)-C5H4(CHO)]Co(η(4)-C4Ph4) with sterically unhindered meso-4-tert-butylphenyl dipyrromethane resulted in both AB3 12 and cis-A2B2 13 type porphyrins [where A = η(5)-CpCo(η(4)-C4Ph4) and B = (4-tert-butylphenyl] as a result of scrambling. The new porphyrin derivatives have been structurally characterized, and their spectral and electrochemical features were determined.

Synthesis and antimicrobial evaluation of 1,3,4-oxadiazole-based chalcone derivatives

A series of chalcones-bearing 1,3,4-oxadiazole derivatives was synthesized as novel bio-active antimicrobial agents against multidrug-resistant bacteria and fungi. The lead compounds (Z)-2-(5-(3-nitrophenyl)-1,3,4-oxadiazol-2-ylthio)-N-(4-(3-(aryl)acryloyl)phenyl)acetamides 5a–n were synthesized via acid-catalyzed aldol condensation (SOCl2) by reacting N-(4-acetylphenyl)-2-(5-(3-nitrophenyl)-1,3,4-oxadiazol-2-ylthio)acetamide (4) with differently substituted aldehydes. Compound (4) was obtained by reacting 5-(3-nitrophenyl)-1,3,4-oxadiazole-2-thiol (2) with N-(4-acetylphenyl)-2-chloroacetamide (3) in the presence of K2CO3. The intermediates (2) and (3) were synthesized simultaneously from 3-nitrobenzohydrazide (1) and 4-aminoacetophenone, respectively. The formation of intermediates and targeted compounds were confirmed for their structure by means of various spectral–analytical techniques like IR, 1H NMR, 13C NMR, elemental analysis, and mass spectra. Antimicrobial properties of all the synthesized compounds have been evaluated against broad panel of bacteria and fungi.

Synthesis of Benziporphyrins and Heterobenziporphyrins and an Assessment of the Diatropic Characteristics of the Protonated Species

Benzitripyrranes were prepared by reacting diphenyl-substituted benzenedicarbinols with excess pyrrole in the presence of BF3·Et2O. These dipyrrolic compounds underwent acid-catalyzed condensations with a pyrrole dialdehyde to afford good yields of diphenylbenziporphyrins, and further reaction with palladium(II) acetate gave stable organometallic derivatives. The X-ray crystal structure of a palladium(II) benziporphyrin showed that the system deviates significantly from planarity. Although the benzitripyrranes failed to give stable macrocyclic products with furan or thiophene dialdehydes, they afforded tetraphenyl heterobenziporphyrins upon reaction with diphenyl-substituted furan- or thiophenedicarbinols and BF3·Et2O. Benziporphyrins and their heteroanalogues showed no indication of a diamagnetic ring current by proton NMR spectroscopy, but addition of TFA gave rise to the formation of weakly diatropic dications.

Chemo‐ and Regioselective Homogeneous Rhodium‐Catalyzed Hydroamidomethylation of Terminal Alkenes to N‐Alkylamides

A rhodium/xantphos homogeneous catalyst system has been developed for direct chemo- and regioselective mono-N-alkylation of primary amides with 1-alkenes and syngas through catalytic hydroamidomethylation with 1-pentene and acetamide as model substrates. For appropriate catalyst performance, it appears to be essential that catalytic amounts of a strong acid promoter, such as p-toluenesulfonic acid (HOTs), as well as larger amounts of a weakly acidic protic promoter, particularly hexafluoroisopropyl alcohol (HOR(F) ) are applied. Apart from the product N-1-hexylacetamide, the isomeric unsaturated intermediates, hexanol and higher mass byproducts, as well as the corresponding isomeric branched products, can be formed. Under optimized conditions, almost full alkene conversion can be achieved with more than 80% selectivity to the product N-1-hexylamide. Interestingly, in the presence of a relatively high concentration of HOR(F) , the same catalyst system shows a remarkably high selectivity for the formation of hexanol from 1-pentene with syngas, thus presenting a unique example of a selective rhodium-catalyzed hydroformylation-hydrogenation tandem reaction under mild conditions. Time-dependent product formation during hydroamidomethylation batch experiments provides evidence for aldehyde and unsaturated intermediates; this clearly indicates the three-step hydroformylation/condensation/hydrogenation reaction sequence that takes place in hydroamidomethylation. One likely role of the weakly acidic protic promoter, HOR(F) , in combination with the strong acid HOTs, is to establish a dual-functionality rhodium catalyst system comprised of a neutral rhodium(I) hydroformylation catalyst species and a cationic rhodium(III) complex capable of selectively reducing the imide and/or ene-amide intermediates that are in a dynamic, acid-catalyzed condensation equilibrium with the aldehyde and amide in a syngas environment.

Synthesis of a phloroacetophenon[4]arene consisting of four isomers via the one-pot, acid-catalysed condensation of phloroacetophenone with benzaldehyde

A phloroacetophenon[4]arene (3) consisting of four isomers was synthesised for the first time via the condensation of phloroacetophenone and benzaldehyde in a one-pot reaction with refluxing toluene in the presence of catalytic amounts of TsOH·H2O for a maximum yield of 64%. The conformational elucidation of four isomers (3a–d) by 1H NMR spectroscopy and X-ray crystal structure analysis showed them to be cone, partial cone, 1,3-alternate and 1,2-alternate types of conformations, respectively.

Silica Sulfuric Acid-Catalyzed One-Pot Synthesis, Mechanism, and Fluorescence Properties of 2-(2-arylquinolin-4-(1H)-ylidene)malononitriles

A mild, one-pot synthesis of 2-(2-arylquinolin-4-(1H)-ylidene)malononitriles is developed via the silica sulfuric acid-catalyzed tandem condensation of 2-aminoacetophenone with malononitrile and aromatic aldehyde in ionic liquid. It is proposed that malononitrile acted as not only a reactant but also a promoter in the interesting process. The fluorescence properties screening showed a new compound has high fluorescence quantum yield.

Synthesis and Crystal Structure of 4,4′-(Methylenediimino)bis-1,2,5-Oxadiazole-3-carboxylic Acid and Carboxamide

4,4′-(Methylenediimino)bis-1,2,5-oxadiazole-3-carboxylic acid and 4,4′-(methylenediimino)bis-1,2,5-oxadiazole-3-carboxamide have been synthesized by the acid-catalyzed condensation of 4-amino-1,2,5-oxadiazole-3-carboxylic acid and 4-amino-1,2,5-oxadiazole-3-carboxamide with formaldehyde. The crystal and molecular structures of the compounds have been determined by X-ray crystallography. 4,4′-(Methylenediimino)bis-1,2,5-oxadiazole-3-carboxylic acid crystallizes in space group C2/c, and its measured density is 1.800 g/mL, significantly above the calculated value of 1.68 g/mL. 4,4′-(Methylenediimino)bis-1,2,5-oxadiazole-3-carboxamide crystallizes in space group P21/c, and its measured density is 1.623 g/mL, in close agreement with the calculated value of 1.64 g/mL. The structure of the starting amide 4-amino-1,2,5-oxadiazole-3-carboxamide has also been determined. These data, combined with literature data, suggest that ortho-aminocarboxylic acids have unusually high densities, but the reasons for this are unclear.

Synthetic Ion Channels: From Pores to Biological Applications

In this Account, we describe the development of several diverse families of synthetic, membrane-active amphiphiles that form pores and facilitate transport within membrane bilayers. For the most part, the compounds are amphiphiles that insert into the bilayer and form pores either on their own or by self-assembly. The first family of synthetic ion channels prepared in our lab, the hydraphiles, used crown ethers as head groups and as a polar central element. In a range of biophysical studies, we showed that the hydraphiles formed unimolecular pores that spanned the bilayer. They mediated the transport of Na(+) and K(+) but were blocked by Ag(+). The hydraphiles are nonrectifying and disrupt ion homeostasis. As a result, these synthetic ion channels are toxic to various bacteria and yeast, a feature that has been used therapeutically in direct injection chemotherapy. We also developed a family of amphiphilic heptapeptide ion transporters that selected Cl(-) >10-fold over K(+) and showed voltage dependent gating. The formed pores were approximately dimeric, and variations in the N- and C-terminal anchor chains and the acids affected transport rates. Surprisingly, the longer N-terminal anchor chains led to less transport but greater Cl(-) selectivity. A proline residue, which is present in the ClC protein channel's conductance pore, proved to be critical for Cl(-) transport selectivity. Pyrogallol[4]arenes are macrocycles formed by acid-catalyzed condensation of four 1,2,3- trihydroxybenzenes with four aldehydes. The combination of 12 hydroxyl groups on one face of the macrocycle and four pendant alkyl chains conferred considerable amphiphilicity to these compounds. The pyrogallol[4]arenes inserted into bilayer membranes and conducted ions. Based on our experimental evidence, the ions passed through a self-assembled pore comprising four or five amphiphiles rather than passing through the central opening of a single macrocycle. Pyrogallol[4]arenes constructed with branched chains are also amphiphilic and active in membranes. The pyrogallol[4]arene with 3-pentyl sidechains formed a unique nanotube assembly and functioned as an ion channel in bilayer membranes. Finally, we showed that dianilides of either isophthalic or dipicolinic acids, compounds which have been extensively studied as anion binders, can self-assemble to form pores within bilayers. We called these dianilides tris-arenes and have shown that they readily bind to phosphate anions. These structures also mediated the transport of DNA plasmids through vital bilayer membranes in the bacterium Escherichia coli and in the yeast Saccharomyces cerevisiae . This transformation or transfection process occurred readily and without any apparent toxicity or mutagenicity.

Novel silica membrane material for molecular sieve applications

Development of new silica membranes properties, e.g., molecular sieving properties, has been increasingly gaining importance in the last few years. A novel unsupported silica membrane, referred to as hydrophobic metal-doped silica, was developed by cobalt-doping within the organic templated silica matrix. The novel material was prepared by the acid-catalyzed hydrolysis and condensation process of tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES), which is the precursor for methyl ligand covalently bounded to the silica matrix. The synthesis and surface properties of the novel unsupported silica membrane as well as the unsupported blank silica and modified silica membranes were revealed by surface and microstructural techniques, such as water contact angle measurement, FTIR, X-ray, Solid-state 29Si MAS NMR, TGA and N2 and CO2 adsorption measurements. The results showed that the thermal stability of the organic templated silica matrix was enhanced by cobalt-doping process. A hydrophobic microporous silica membrane material with high thermal stability up to ∼560°C in oxidizing atmosphere and a narrow pore size distribution centered at 1.1nm was obtained. Therefore, a novel precursor material for molecular sieve silica membranes applications has been achieved and developed.

Synthesis, structure, and spectroscopy of two benzil-based α-diimine ligands and their palladium(II) complexes

Two α-diimine ligands were prepared in 60–70% yield via p-toluenesulfonic acid-catalyzed condensation reactions from benzil with 4-bromoaniline and with p-anisidine. Palladium(II) complexes were prepared from both ligands in 70–80% yield. X-ray structures were obtained for the ligand prepared from p-anisidine and its palladium(II) complex. A notable feature observed in the former was its unconjugated C–N double bonds, both in the (E)-configuration. The latter structure possessed two molecules of the metal complex in its unit cell, both of which have diimine cores with a degree of conjugation and a nonideal square-planar geometry around palladium caused by the small bite angles (79.61(3) and 79.15(3)°) of the diimine ligands. Solution-phase electronic absorption spectra of the ligands in chloroform have two bands from π→π ∗ and n→π ∗ transitions at 269–345 nm. Absorption spectra of the complexes in chloroform exhibited bands attributed to ligand-centered transitions that were red-shifted as compared to free ligands. Only the spectrum obtained from a chloroform solution of the palladium(II) complex with the diimine ligand prepared from p-anisidine featured a band at approximately 520 nm, which was assigned to a combination of d π(Pd)→π ∗ and n(Cl)→π ∗ transitions.

The Combined Role of Catalysis and Solvent Effects on the Biginelli Reaction: Improving Efficiency and Sustainability

The traditional Biginelli reaction is a three-component condensation between urea, benzaldehyde and an acetoacetate ester to give a dihydropyrimidinone. An investigation into catalytic and solvent effects has returned the conclusion that the diketo-enol tautomerisation equilibrium of the dicarbonyl reactant dictates the yield of the reaction. Whereas the solvent is responsible for the tautomerisation equilibrium position, the catalyst only serves to eliminate kinetic control from the reaction. Generally, to preserve reaction efficiency and improve sustainability, bio-derivable p-cymene was found to be a useful solvent. The metal-enolate intermediate that results from the application of a Lewis acidic catalyst often cited as promoting the reaction appears to hinder the reaction. In this instance, a Brønsted acidic solvent can be used to return greater reactivity to the dicarbonyl reagent.