Substoichiometric Amounts Research Articles

Gold(III) Bis(di-N-heterocyclic carbene) Square Planar Trication with Axial Ligand Interactions with Bromides from Ag/Br Counteranion Assemblies

The mononuclear tricationic bis(di-N-heterocyclic carbene) gold(III) complex 13+ of formula [Au(MeImCH2ImMe)2]3+ (Im = imidazol-2-ylidene) was successfully synthesized by transmetalation of the di(N-heterocyclic carbene) ligand from the corresponding silver(I) complex to KAuBr4. The counteranion of the gold(III) cationic complex depends on the synthetic procedure. The crude product, isolated by the transmetalation reaction, consists of infinite (Ag4Br73–)n ribbons. By adding a stoichiometric amount of AgPF6, the complete precipitation of AgBr is achieved, and the counteranions are PF6–. If substoichiometric amounts of silver salts are added, then the (Ag4Br73–)n ribbon breaks, and silverbromides anionic aggregates of lower nuclearity are obtained, for example, Ag2Br64–. The X-ray crystal structures of 1-Ag4Br7, 1-PF6, and 1-Br,(Ag2Br6)0.5 were determined. The complexes with any type of bromide present interactions between the Au center and two bromides of the counteranions so that the geometry around gold...

Trace water amounts can increase benzene H/D exchange rates in an acidic zeolite

Reactions between benzene and the zeolite HZSM-5, carried out at low pressure and room temperature, revealed that controlled addition of sub-stoichiometric amounts of water increases the rate of isotopic 1H/2H exchange between benzene-d6 and the surface acid site by almost an order of magnitude relative to the control case in which no water is added. Single-ring aromatics like benzene have been identified as an active form of the hydrocarbon pool in methanol-to-hydrocarbon chemistries, but their role as reactive centers for alkylation and dealkylation chemistry is general to all of solid-acid catalysis. Maximum reaction rates were observed for water loadings of ca. 0.1 equivalents for catalysts with Si/Al equal to 15. No beneficial water effect was observed in benzene/zeolite reactions using a much lower acid-density catalyst HZSM-5 with Si/Al equal to 40. In-situ experiments strongly suggest that the origin of the water enhancement effect for the high-acid density catalyst arises from an increase in the “vehicle hopping” proton-transfer mechanism, which cannot occur when acid sites are sufficiently far apart as in the Si/Al equal to 40 catalyst. When the water loading is too high, water’s competitive adsorption and excluded volume reduces the benzene adsorption probability due to the increased proton affinity of a water cluster.

Readily Accessible 1,2-Amino Ether Ligands for Enantioselective Intramolecular Carbolithiation

A new class of chiral 1,2-amino ether ligands, readily accessible from naturally occurring α-amino- or α-hydroxy acids, was found to provide high levels of both conversion and stereocontrol (up to 95:5 er) in intramolecular carbolithiation reactions, outperforming the benchmark ligand (-)-sparteine. The ligand could be used in a substoichiometric amount (0.25 equiv) without significant loss of enantioselectivity.

Unexpected H-Bond Dimers in Organic Electrochemistry: The Case of Phenylenediamines and Pyridines in Acetonitrile

Oxidation or reduction of organic redox couples typically leads to large changes in acidity or basicity, with the result that proton transfer often accompanies electron transfer, particularly in aqueous solution. In less polar organic solvents, H-bonding also can play an important role. For one thing, oxidation and reduction can strengthen or weaken H-bonding interactions in the absence of proton transfer. It can also result in strong interactions between the products of the electron transfer/proton transfer, which can have a significant effect on the mechanism of the overall reaction. A case in point is the electrochemistry of p-phenylenediamines in acetonitrile. In general two reversible or quasi-reversible voltammetric waves are observed that correspond to the one electron oxidation to a stable radical cation, followed at more positive potential by the removal of a second electron to give the highly acidic quinoidal dication. However, as we showed a number of years ago (J. Phys. Chem C 2010, 114, 8938-49), the actual mechanism if NH’s are present is far more complex. The second CV wave decreases substantially in size relative to the first as the phenylenediamine concentration is decreased and/or the scan rate increased. In fact, at low enough concentration and fast enough scan rate, it is not present at all. The best explanation that we could come up with was that at low concentration the current was dominated by a surface process in which the effective high concentrations lead to dimerization of the initially formed radical cations, followed by proton transfer and disproportionation to give a mixed-valent, H-bonded dimer that can not be further oxidized. This dimer is stable at the high concentrations on the surface, but not at the low concentrations in solution. So once desorbed, it breaks apart with reverse electron transfer/proton transfer to give the radical cation, which is then oxidized to the quinoidal dication in the second voltammetric wave. New evidence supporting this hypothesis comes from close examination of the electrochemistry of the phenylenediamines in the presence of pyridines. Previously we had observed that addition of 1 equivalent of pyridine leads to a large negative shift in the potential of the second oxidation with maintenance of reversibility, followed by much smaller shifts in potential upon further additions. Unpublished UV-vis spectroelectrochemical data indicate the formation of pyridinium, suggesting that the large shift in potential upon addition of one equivalent is due to proton transfer. The fact that the wave stays quasi-reversible, and does not lead to an immediate jump in height of the first wave (as would be expected if the uncharged radical was formed) further suggests that the pyridinium remains H-bonded to the oxidized phenylenediamine. In new work, CV’s with sub-stoichiometric amounts of pyridine have been examined. These studies show that the negatively shifted second oxidation wave is actually fully formed with only half equivalent of pyridine added. This lends strong support to the dimer hypothesis, since the only simple explanation of this stoichiometry is that pyridine removes one proton per two phenylenediamines.

One-pot efficient pseudo-five-components synthesis of 4,4′-(arylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ols) at room temperature assisted by K2CO3

ABSTRACTA convenient approach for the synthesis of 4,4′-(arylmethylene)-bis(3-methyl-1-phenyl-1H-pyrazol-5-ols) by an efficient one-pot reaction of aromatic aldehydes, phenylhydrazine, and ethyl acetoacetate by using substoichiometric amount of K2CO3 in acetonitrile solvent at room temperature is described. This protocol of one-pot synthesis produced a library of bis-pyrazolylmethane derivatives, a potential bioactive compound in very good to excellent yields.

Mechanistic Insights into the B(C6F5)3-Initiated Aldehyde–Aniline–Alkyne Reaction To Form Substituted Quinolines

A substoichiometric quantity of the Lewis acid B(C6F5)3 is sufficient to initiate the aldehyde–amine–alkyne reaction, in a one-pot methodology that enables the synthesis of a range of functionalized quinolines. Optimization studies revealed that key requirements for the high-yielding tricomponent reaction initiated by B(C6F5)3 at raised temperatures include an excess of the in situ generated imine (which acts as a hydrogen acceptor) and an alkyne substituent able to stabilize positive charge buildup during the cyclization. Mechanistic experiments revealed that under these conditions B(C6F5)3 is acting as a Lewis acid-assisted Brønsted acid, with H2O–B(C6F5)3 being the key species enabling catalytic quinoline formation. This was indicated by deuterium labeling studies and the observation that the cyclization of N-(3-phenylpropargyl)aniline using B(C6F5)3 under anhydrous conditions afforded the zwitterion [(N-H-3-B(C6F5)3-4-Ph-quinolinium], which does not undergo protodeboronation to release B(C6F5)3 and the quinoline product under a range of conditions. Finally, a brief substrate scope exploration demonstrated that this is an operationally simple and effective methodology for the production of functionalized quinolines.

Water-Soluble Chlorophyll Protein (WSCP) Stably Binds Two or Four Chlorophylls.

Water-soluble chlorophyll proteins (WSCPs) of class IIa from Brassicaceae form tetrameric complexes containing one chlorophyll (Chl) per apoprotein but no carotenoids. The complexes are remarkably stable toward dissociation and protein denaturation even at 100 °C and extreme pH values, and the Chls are partially protected against photooxidation. There are several hypotheses that explain the biological role of WSCPs, one of them proposing that they function as a scavenger of Chls set free upon plant senescence or pathogen attack. The biochemical properties of WSCP described in this paper are consistent with the protein acting as an efficient and flexible Chl scavenger. At limiting Chl concentrations, the recombinant WSCP apoprotein binds substoichiometric amounts of Chl (two Chls per tetramer) to form complexes that are as stable toward thermal dissociation, denaturation, and photodamage as the fully pigmented ones. If more Chl is added, these two-Chl complexes can bind another two Chls to reach the fully pigmented state. The protection of WSCP Chls against photodamage has been attributed to the apoprotein serving as a diffusion barrier for oxygen, preventing its access to triplet excited Chls and, thus, the formation of singlet oxygen. By contrast, the sequential binding of Chls by WSCP suggests a partially open or at least flexible structure, raising the question of how WSCP photoprotects its Chls without the help of carotenoids.

Reversible Redox Cycling of Well-Defined, Ultrasmall Cu/Cu2O Nanoparticles.

Exceptionally small and well-defined copper (Cu) and cuprite (Cu2O) nanoparticles (NPs) are synthesized by the reaction of mesitylcopper(I) with either H2 or air, respectively. In the presence of substoichiometric quantities of ligands, namely, stearic or di(octyl)phosphinic acid (0.1-0.2 equiv vs Cu), ultrasmall nanoparticles are prepared with diameters as low as ∼2 nm, soluble in a range of solvents. The solutions of Cu NPs undergo quantitative oxidation, on exposure to air, to form Cu2O NPs. The Cu2O NPs can be reduced back to Cu(0) NPs using accessible temperatures and low pressures of hydrogen (135 °C, 3 bar H2). This striking reversible redox cycling of the discrete, solubilized Cu/Cu(I) colloids was successfully repeated over 10 cycles, representing 19 separate reactions. The ligands influence the evolution of both composition and size of the nanoparticles, during synthesis and redox cycling, as explored in detail using vacuum-transfer aberration-corrected transmission electron microscopy, X-ray photoelectron spectroscopy, and visible spectroscopy.

Visible-Light-Promoted Asymmetric Cross-Dehydrogenative Coupling of Tertiary Amines to Ketones by Synergistic Multiple Catalysis.

Reported herein is an unprecedented photocatalytic asymmetric cross-dehydrogenative coupling reaction between tertiary amines and simple ketones, and it proceeds by synergistic multiple catalysis with substoichiometric amounts of a hydrogen acceptor. This process is enabled by a simple chiral primary amine catalyst through the coupling of a catalytic enamine intermediate and an iminium cation intermediate in situ generated from tetrahydroisoquinoline derivatives by coupled Ru/Co catalysis.

Visible‐Light‐Promoted Asymmetric Cross‐Dehydrogenative Coupling of Tertiary Amines to Ketones by Synergistic Multiple Catalysis

AbstractReported herein is an unprecedented photocatalytic asymmetric cross‐dehydrogenative coupling reaction between tertiary amines and simple ketones, and it proceeds by synergistic multiple catalysis with substoichiometric amounts of a hydrogen acceptor. This process is enabled by a simple chiral primary amine catalyst through the coupling of a catalytic enamine intermediate and an iminium cation intermediate in situ generated from tetrahydroisoquinoline derivatives by coupled Ru/Co catalysis.

High Purity Oligomeric Oxymethylene Ethers as Diesel Fuels

AbstractOligomeric oxymethylene dimethyl ethers (OMEn, CH3(OCH2)nOCH3, n = 1 – 5) are promising diesel fuels for the reduction of harmful emissions. If OMEs are produced from dimethoxymethane and trioxane, the resulting OME mixtures usually contain residual trioxane which appears, after rectification, in the OME2 fraction. To circumvent this obstacle, substoichiometric amounts of trioxane have been employed in OME synthesis. Thus, OME2 samples with high purity could be prepared. Physicochemical and fuel data of high purity OME2 and higher OMEs have been determined to supplement previously reported data.

Diisobutylaluminum Hydride Promoted Cyclization of 1-Hydrosilyl-4-silyl-1,3-enynes to Polysubstituted Siloles.

An efficient method for preparing unsymmetrically multisubstituted siloles is described. The reaction of 1-hydrosilyl-4-silyl-1,3-enynes with diisobutylaluminum hydride (DIBAL-H) gave multisubstituted siloles in good to high yields. This method could be applied to the synthesis of benzosiloles using 2-hydrosilyl-1-(silylethynyl)benzenes as substrates. The silole formation was also promoted even by a substoichiometric amount of DIBAL-H. The reaction provides a straightforward method to prepare siloles and benzosiloles.

Dual-gold(I)-generated trifluoromethylation of terminal alkynes with Togni’s reagent

The interaction of a Au(I) catalyst (JohnPhosAu(I)-MeCN/SbF6) and the Togni’s reagent 1, as a source of electrophilic trifluoromethyl group, has been studied in order to develop gold-catalysed alkyne trifluoromethylation reactions. Alkyne-CF3 products were prepared in moderate yields (up to 46%) by electrophilic trifluoromethylation of terminal arylalkynes with Togni’s reagent 1 in the presence of sub-stoichiometric amounts of gold catalyst (25%). The proposed addition-elimination reaction mechanism proceeds through a Au-Togni Reagent complex with a linear Togni Reagent-O-Au(I)-P-(phosphane) coordination mode (X-ray analysis). Alkyne deprotonation gives rapid formation of protonated Togni Reagent and a σ,π-acetylide dual-Au complex, confirmed by X-ray analysis. It was shown that the σ,π-dual-Au complex activates for trifluoromethylation, most likely by transfer of a [LAu]+ fragment to the alkyne substrate. The resulting reactive π-Au+-alkyne intermediate probably undergoes O-/CF3-addition of Togni Reagent, and final elimination of Togni alcohol gives the alkyne-CF3 product.

Mapping of Complete Set of Ribose and Base Modifications of Yeast rRNA by RP-HPLC and Mung Bean Nuclease Assay.

Ribosomes are large ribonucleoprotein complexes that are fundamental for protein synthesis. Ribosomes are ribozymes because their catalytic functions such as peptidyl transferase and peptidyl-tRNA hydrolysis depend on the rRNA. rRNA is a heterogeneous biopolymer comprising of at least 112 chemically modified residues that are believed to expand its topological potential. In the present study, we established a comprehensive modification profile of Saccharomyces cerevisiae’s 18S and 25S rRNA using a high resolution Reversed-Phase High Performance Liquid Chromatography (RP-HPLC). A combination of mung bean nuclease assay, rDNA point mutants and snoRNA deletions allowed us to systematically map all ribose and base modifications on both rRNAs to a single nucleotide resolution. We also calculated approximate molar levels for each modification using their UV (254nm) molar response factors, showing sub-stoichiometric amount of modifications at certain residues. The chemical nature, their precise location and identification of partial modification will facilitate understanding the precise role of these chemical modifications, and provide further evidence for ribosome heterogeneity in eukaryotes.

Synthesis of 3-Fluoroindoles via Photoredox Catalysis.

A method for the synthesis of 3-fluoroindoles from N-arylamines substituted with the CF2I group is described. The reaction is mediated by a ruthenium photocatalyst in the presence of a substoichiometric amount of triphenylphosphine upon irradiation with blue light. The starting N-arylamines are readily obtained by nucleophilic iododifluoromethylation of iminium ions.

Electrocatalytic Z → E Isomerization of Azobenzenes.

A variety of azobenzenes were synthesized to study the behavior of their E and Z isomers upon electrochemical reduction. Our results show that the radical anion of the Z isomer is able to rapidly isomerize to the corresponding E configured counterpart with a dramatically enhanced rate as compared to the neutral species. Due to a subsequent electron transfer from the formed E radical anion to the neutral Z starting material the overall transformation is catalytic in electrons; i.e., a substoichiometric amount of reduced species can isomerize the entire mixture. This pathway greatly increases the efficiency of (photo)switching while also allowing one to reach photostationary state compositions that are not restricted to the spectral separation of the individual azobenzene isomers and their quantum yields. In addition, activating this radical isomerization pathway with photoelectron transfer agents allows us to override the intrinsic properties of an azobenzene species by triggering the reverse isomerization direction (Z → E) by the same wavelength of light, which normally triggers E → Z isomerization. The behavior we report appears to be general, implying that the metastable isomer of a photoswitch can be isomerized to the more stable one catalytically upon reduction, permitting the optimization of azobenzene switching in new as well as indirect ways.

Effect of seven Indian plant extracts on Fenton reaction-mediated damage to DNA constituents

The influences of substoichiometric amounts of seven plant extracts in the Fenton reaction-mediated damage to deoxynucleosides, deoxynucleoside monophosphates, deoxynucleoside triphosphates, and supercoiled plasmid DNA were studied to rationalize anticancer properties reported in some of these extracts. Extracts from Acacia catechu, Emblica officinalis, Spondias dulcis, Terminalia belerica, Terminalia chebula, as well as gallic acid, epicatechin, chebulagic acid and chebulinic acid enhance the extent of damage in Fenton reactions with all monomeric substrates but protect supercoiled plasmid DNA, compared to standard Fenton reactions. The damage to pyrimidine nucleosides/nucleotides is enhanced by these extracts and compounds to a greater extent than for purine ones in a concentration dependent manner. Dolichos biflorus and Hemidesmus indicus extracts generally do not show this enhancement for the monomeric substrates though they protect plasmid DNA. Compared to standard Fenton reactions for deoxynucleosides with ethanol, the presence of these five plant extracts render ethanol scavenging less effective as the radical is generated in the vicinity of the target. Since substoichiometric amounts of these extracts and the four compounds produce this effect, a catalytic mechanism involving the presence of a ternary complex of the nucleoside/nucleotide substrate, a plant compound and the hydroxyl radical is proposed. Such a mechanism cannot operate for plasmid DNA as the planar rings in the extract compounds cannot stack with the duplex DNA bases. These plant extracts, by enhancing Fenton reaction-mediated damage to deoxynucleoside triphosphates, slow down DNA replication in rapidly dividing cancer cells, thus contributing to their anticancer properties.

A general Brønsted acid-catalyzed allylation of benzhydryl alcohols

Efficient Brønsted acid-catalyzed allylation of benzhydryl alcohols has been developed. The reaction occurs within 5min in the presence of sub-stoichiometric amounts of HBF4·OEt2 to afford the desired products in good to excellent yields. A variety of functional groups were tolerated under the developed conditions including amides, aldehydes, hydroxyl and carboxylic acid.

Inhibitory Effect of Multivalent Rhamnobiosides on Recombinant Horseshoe Crab Plasma Lectin Interactions with Pseudomonas aeruginosa PAO1.

To evaluate the molecular interaction of recombinant horseshoe crab plasma lectin (rHPL) with Pseudomonas aeruginosa PAO1, multivalent rhamnobioside derivatives were designed. Eight rhamnoclusters with three or four α(1-3)-rhamnobiosides attached to different central cores, such as methyl gallate, pentaerythritol, and N-Boc Tris, through either an ethylene glycol or a tetraethylene glycol linker, were assembled in two consecutive azide-alkyne cycloaddition click reactions. The synthetic method embraced the preparation of two α(1-3)-rhamnobiosides with different linker arms and their conjugation, in stoichiometric or substoichiometric amounts, to propargyl ether-functionalized tri- or tetravalent scaffolds. A divalent derivative and two self-assembling rhamnobiosides were also prepared. The different architectures and valences of the rhamnoclusters provided an opportunity to evaluate the impact of topology and valency on the binding properties toward rHPL. Inhibitory ELISA data showed that all covalently linked rhamnoclusters could inhibit P. aeruginosa PAO1 recognition activity of rHPL with high efficacy. Trivalent rhamnobiosides showed a stronger inhibitory effect on P. aeruginosa PAO1 binding, and the more flexible clusters on a pentaerythritol or a Tris core were superior to the less flexible methyl gallate-based clusters. Interestingly, the length of the linker arms had a very low impact on the binding ability of the rhamnoclusters. Herein, the two trivalent derivatives on an N-Boc protected Tris central core were the best inhibitors. The self-assembling amphiphilic rhamnobioside derivatives were found to display no multivalent effect.

ChemInform Abstract: Efficient O‐Acylation of Alcohols and Phenol Using Cp2TiCl as a Reaction Promoter.

A method has been developed for the conversion of primary, secondary, and tertiary alcohols, and phenol, into the corresponding esters at room temperature. The method uses a titanium(III) species generated from a substoichiometric amount of titanocene dichloride together with manganese(0) as a reductant, as well as methylene diiodide. It involves a transesterification from an ethyl ester, or a reaction with an acyl chloride. A radical mechanism is proposed for these transformations.