Cation Equivalents Research Articles

Umpolung Approach to Aldol Products via Isoxazoline N-Oxides as Intermediates.

A two-step umpolung approach to the diastereoselective synthesis of aldols was developed, in which a conjugated nitroalkene is used as the synthetic equivalent of the enolonium cation, while a sulfur ylide acts as the equivalent of the α-carbinol anion. The resulting isoxazoline N-oxides undergo catalytic reductive cleavage to aldols under mild conditions at room temperature and under 1 atm hydrogen pressure. The efficiency of the method was demonstrated by the synthesis of a series of polysubstituted β-hydroxyketones that are difficult to synthesize using the classical aldol reaction. The developed approach allows for the regiodivergent assembly of aldols by selecting a nitroalkene isomer with the appropriate position of the C,C double bond.

Accessing elusive σ-type cyclopropenium cation equivalents through redox gold catalysis

Cyclopropenes are the smallest unsaturated carbocycles. Removing one substituent from cyclopropenes leads to cyclopropenium cations (C3+ systems, CPCs). Stable aromatic π-type CPCs were discovered by Breslow in 1957 by removing a substituent on the aliphatic position. In contrast, σ-type CPCs—formally accessed by removing one substituent on the alkene—are unstable and relatively unexplored. Here we introduce electrophilic cyclopropenyl-gold(III) species as equivalents of σ-type CPCs, which can then react with terminal alkynes and vinylboronic acids. With catalyst loadings as low as 2 mol%, the synthesis of highly functionalized alkynyl- or alkenyl-cyclopropenes proceeded under mild conditions. A class of hypervalent iodine reagents—the cyclopropenyl benziodoxoles (CpBXs)—enabled the direct oxidation of gold(I) to gold(III) with concomitant transfer of a cyclopropenyl group. This protocol was general, tolerant to numerous functional groups and could be used for the late-stage modification of complex natural products, bioactive molecules and pharmaceuticals.

Colorimetric Chemosensor for Cu2+ and Fe3+ Based on a meso-Triphenylamine-BODIPY Derivative.

Optical chemosensors are a practical tool for the detection and quantification of important analytes in biological and environmental fields, such as Cu2+ and Fe3+. To the best of our knowledge, a BODIPY derivative capable of detecting Cu2+ and Fe3+ simultaneously through a colorimetric response has not yet been described in the literature. In this work, a meso-triphenylamine-BODIPY derivative is reported for the highly selective detection of Cu2+ and Fe3+. In the preliminary chemosensing study, this compound showed a significant color change from yellow to blue-green in the presence of Cu2+ and Fe3+. With only one equivalent of cation, a change in the absorption band of the compound and the appearance of a new band around 700 nm were observed. Furthermore, only 10 equivalents of Cu2+/Fe3+ were needed to reach the absorption plateau in the UV-visible titrations. Compound 1 showed excellent sensitivity toward Cu2+ and Fe3+ detection, with LODs of 0.63 µM and 1.06 µM, respectively. The binding constant calculation indicated a strong complexation between compound 1 and Cu2+/Fe3+ ions. The 1H and 19F NMR titrations showed that an increasing concentration of cations induced a broadening and shifting of the aromatic region peaks, as well as the disappearance of the original fluorine peaks of the BODIPY core, which suggests that the ligand-metal (1:2) interaction may occur through the triphenylamino group and the BODIPY core.

Synthesis of Unnatural Amino Acids from in situ Generated Glycine Cation Equivalents

Synthesis of Unnatural Amino Acids from in situ Generated Glycine Cation Equivalents

Diversification of Indoles and Pyrroles by Molecular Editing: New Frontiers in Heterocycle-to-Heterocycle Transmutation.

Skeletal editing via single-atom insertion reactions involving nitrogen heterocycles have been reported by two innovative and complementary methods for the conversion of pyrroles and indoles to pyridines, quinolines and quinazolines. The use of electrophilic carbonyl cation equivalents and in situ generated nitrenes enables molecular editing to transform heterocycles forming the foundation of best-selling pharmaceuticals. Considering the importance of heterocycles in medicinal chemistry, biology and natural products, these methods offer innovative approach to complex molecular structures by heterocycle diversification and peripheral editing.

Intermolecular Hydroalkoxylation and Hydrocarboxylation of 2-Azadienes with High Efficiency.

Described here is a method for intermolecular hydroalkoxylation and hydrocarboxylation of 2-azadienes through cobalt-catalyzed hydrogen atom transfer and oxidation. This protocol provides a source of 2-azaallyl cation equivalents under mild conditions, is chemoselective in the presence of other C═C double bonds, and requires no excess amount of added alcohol or oxidant. Mechanistic studies suggest that the selectivity arises from lowering the transition state that leads to the highly stabilized 2-azaallyl radical.

Design, development and applications of copper-catalyzed regioselective (4 + 2) annulations between diaryliodonium salts and alkynes

Diaryliodonium salts have been extensively applied in organic synthesis as aryl cation equivalents. However, in the electrophilic reactions with alkenes or alkynes, only the electrophilic carbon of the diaryliodonium salts was involved while the other part of the aryl ring was not utilized. Herein, a reaction pattern of diaryliodonium was reported as oxa-1,4-dipoles to undergo (4 + 2) cycloaddition reactions with alkynes. Broad spectrum of the two reaction partners could be utilized in this protocol, enabling an operationally simple, high yielding, and regioselective synthetic approach to isocoumarins. Particularly, good to excellent regioselectivities were achieved for the sterically unbiased unsymmetrical diaryl acetylenes, which was challenging for other transition metal-catalyzed processes. The reaction could be scaled up with the ideal 1:1 stoichiometry and the isocoumarin type natural products Oospolactone and Thunberginol A could be obtained in one or three steps through this methodology.

Chemoselective and Diastereoselective Intramolecular (3+2) Cycloadditions of Epoxy and Aziridinyl Enolsilanes.

Epoxy and aziridinyl enolsilanes react as oxyallylic cation equivalents in highly chemo- and diastereoselective intramolecular (3+2) cycloadditions with a range of dienes and olefins. With acyclic dienes, the (3+2) cycloaddition outcompetes the (4+3) pathway traditionally observed in this kind of system almost exclusively. With both conjugated dienes and isolated olefins, excellent diastereoselectivities are observed, and cycloadducts can be obtained in optically-enriched forms. Computational studies indicate that the stepwise (3+2) cycloaddition involves an activated epoxy/aziridinyl intermediate and the conformational flexibility of the intermediate determines the preference for (3+2) cycloadduct formation. Further transformations of the (3+2) cycloadducts produce densely functionalized trans-hydrindane scaffolds.

Chemoselective and Diastereoselective Intramolecular (3+2) Cycloadditions of Epoxy and Aziridinyl Enolsilanes

AbstractEpoxy and aziridinyl enolsilanes react as oxyallylic cation equivalents in highly chemo‐ and diastereoselective intramolecular (3+2) cycloadditions with a range of dienes and olefins. With acyclic dienes, the (3+2) cycloaddition outcompetes the (4+3) pathway traditionally observed in this kind of system almost exclusively. With both conjugated dienes and isolated olefins, excellent diastereoselectivities are observed, and cycloadducts can be obtained in optically‐enriched forms. Computational studies indicate that the stepwise (3+2) cycloaddition involves an activated epoxy/aziridinyl intermediate and the conformational flexibility of the intermediate determines the preference for (3+2) cycloadduct formation. Further transformations of the (3+2) cycloadducts produce densely functionalized trans‐hydrindane scaffolds.

Organometallic and Organic Dimers: Moderately Air-Stable, Yet Highly Reducing, n-Dopants.

ConspectusElectrical doping using redox-active molecules can increase the conductivity of organic semiconductors and lower charge-carrier injection and extraction barriers; it has application in devices such as organic and perovskite light-emitting diodes, organic and perovskite photovoltaic cells, field-effect transistors, and thermoelectric devices. Simple one-electron reductants that can act as n-dopants for a wide range of useful semiconductors must necessarily have low ionization energies and are, thus, highly sensitive toward ambient conditions, leading to challenges in their storage and handling. A number of approaches to this challenge have been developed, in which the highly reducing species is generated from a precursor or in which electron transfer is coupled in some way to a chemical reaction. Many of these approaches are relatively limited in applicability because of processing constraints, limited dopant strength, or the formation of side products.This Account discusses our work to develop relatively stable, yet highly reducing, n-dopants based on the dimers formed by some 19-electron organometallic complexes and by some organic radicals. These dimers are sufficiently inert that they can be briefly handled as solids in air but react with acceptors to release two electrons and to form two equivalents of stable monomeric cations, without formation of unwanted side products. We first discuss syntheses of such dimers, both previously reported and our own. We next turn to discuss their thermodynamic redox potentials, which depend on both the oxidation potential of the highly reducing odd-electron monomers and on the free energies of dissociation of the dimers; because trends in both these quantities depend on the monomer stability, they often more-or-less cancel, resulting in effective redox potentials for a number of the organometallic dimers that are approximately -2.0 V vs ferrocenium/ferrocene. However, variations in the dimer oxidation potential and the dissociation energies determine the mechanism through which a dimer reacts with a given acceptor in solution: in all cases dimer-to-acceptor electron transfer is followed by dimer cation cleavage and a subsequent second electron transfer from the neutral monomer to the acceptor, but examples with weak central bonds can also react through endergonic cleavage of the neutral dimer, followed by electron-transfer reactions between the resulting monomers and the acceptor. We, then, discuss the use of these dimers to dope a wide range of semiconductors through both vacuum and solution processing. In particular, we highlight the role of photoactivation in extending the reach of one of these dopants, enabling successful doping of a low-electron-affinity electron-transport material in an organic light-emitting diode. Finally, we suggest future directions for research using dimeric dopants.

Hydrogen-Bridged Oligosilanylsilyl Mono- and Oligosilanylsilyl Dications.

Hydrogen‐bridged oligosilanylsilyl borates 8 [B(C6F5)4], 9[B(C6F5)4] and diborates 10 [B(C6F5)4]2 have been prepared by hydride transfer between α‐ω‐dihydrido‐ (11) and branched tetrahydrido‐oligosilanes (13) and trityl cation. The obtained cyclic intramolecularly stabilized silylium ions 8, 9 and bissilylium ion 10 were characterized by low temperature NMR spectroscopy supported by the results of density functional calculations. The branched Si−H−Si monocation 9 undergoes at low temperatures a fast degenerate rearrangement, which exchanges the Si−H groups with a barrier of 31 kJ mol−1 via an antarafacial transition state. Reaction of the branched monocation 9 with a second equivalent of trityl cation or of the branched oligosilane 13 with two equivalents of trityl cation, gives at −80 °C the corresponding bissilylium ion 10, an example for a new class of highly reactive poly‐Lewis acids.

Synthesis of a Nickel(I) alkoxide and related cation equivalents

An (N-heterocyclic carbene)nickel(I) hexamethyldisilazide deprotonates neopentanol (NpOH) to form the corresponding nickel(I) neopentoxide, which crystallizes as an alkoxide-bridged dimer. Reaction of the dimer with one equivalent of tert-butyldimethylsilyl trifluoromethanesulfonate gives an alkoxide-bridged dinickel(I) complex bridged by the CF3SO3 (OTf) anion. In both [LNiONp]2 and [(LNi)2(μ-ONp)(μ-OTf)], Ni•••Ni distances are consistent with the absence of metal–metal bonding, and Evans NMR data are consistent with the S = 1 spin state. Treatment of the dimer with two equivalents of silyl triflate results instead in complete abstraction of alkoxide. Addition of diethyl ether forms the monomeric d9 complex LNi(OEt2)OTf.

Carbon Atom Insertion into Pyrroles and Indoles Promoted by Chlorodiazirines.

Herein, we report a reaction that selectively generates 3-arylpyridine and quinoline motifs by inserting aryl carbynyl cation equivalents into pyrrole and indole cores, respectively. By employing α-chlorodiazirines as thermal precursors to the corresponding chlorocarbenes, the traditional haloform-based protocol central to the parent Ciamician-Dennstedt rearrangement can be modified to directly afford 3-(hetero)arylpyridines and quinolines. Chlorodiazirines are conveniently prepared in a single step by oxidation of commercially available amidinium salts. Selectivity as a function of pyrrole substitution pattern was examined, and a predictive model based on steric effects is put forward, with DFT calculations supporting a selectivity-determining cyclopropanation step. Computations surprisingly indicate that the stereochemistry of cyclopropanation is of little consequence to the subsequent electrocyclic ring opening that forges the pyridine core, due to a compensatory homoaromatic stabilization that counterbalances orbital-controlled torquoselectivity effects. The utility of this skeletal transform is further demonstrated through the preparation of quinolinophanes and the skeletal editing of pharmaceutically relevant pyrroles.

Lewis Acid‐Mediated Decarboxylative Allylation of Enol Carbonates

AbstractA homoallylic ketone can be transformed and functionalized by various synthetic reactions, and thus, is regarded as one of the representative building blocks in organic chemistry. An additional route to access homoallylic ketones, namely, a Lewis acid‐mediated decarboxylative allylation of cyclic enol carbonates, prepared by fixation of carbon dioxide onto propargyl alcohols, was developed in this work. The treatment of a cyclic enol carbonate with a Lewis acid in the presence of an allylsilane resulted in the formation of a homoallylic ketone. It was found that the title reaction proceeded well by the combined use of zirconium tetrachloride with allyltrimethylsilane. The allylation occurred with high regioselectivity and the corresponding homoallylic ketones were obtained in good‐to‐high yields. A reaction mechanism involving the decarboxylative formation of an oxyallyl cation equivalent is proposed.

Use of ANFIS/Genetic Algorithm and Neural Network to Predict Inorganic Indicators of Water Quality

The present research used novel hybrid computational intelligence (CI) models to predict inorganic indicators of water quality. Two CI models i.e. artificial neural network (ANN) and a hybrid adaptive neuro-fuzzy inference system (ANFIS) trained by genetic algorithm (GA) were used to predict inorganic indicators of water quality including total dissolved solids (TDS), total hardness (TH), total alkalinity (TAlk), and electrical conductivity (σ). The study was conducted on samples collected from water wells of Kermanshah province through analyzing water parameters including pH, temperature (T), and the sum of mill equivalents of cations (SC) and anions (SA). A multilayer perceptron (MLP) structure was used to forecast inorganic indicators of water quality using the ANN approach. A MATLAB code was used for the proposed ANFIS model to adjust and optimize the ANFIS parameters during the training process using GA. The accuracy of the generated models was described using various evaluation techniques such as mean absolute error (MAE), correlation factor (R), and mean relative error percentage (MRE%). The results showed that both methods were suitable for predicting inorganic indicators of water quality. Moreover, the comparison of the two methods showed that the predicted values obtained from the ANFIS/GA model were better than those obtained from the ANN approach.

Access to Vinyl Ethers and Ketones with Hypervalent Iodine Reagents as Oxy-Allyl Cation Synthetic Equivalents.

We report an Umpolung strategy of enol ethers to generate oxy-allyl cation equivalents based on the use of hypervalent iodine reagents. Under mild basic conditions, the addition of nucleophiles to aryloxy-substituted vinylbenziodoxolone (VBX) reagents, easily available in two steps from silyl alkynes, resulted in the stereoselective formation of substituted aryl enol ethers. The reaction was most efficient with phenols as nucleophiles, but preliminary results were also achieved for C- and N- nucleophiles. In absence of external nucleophiles, the 2-iodobenzoate group of the reagent was transferred. The obtained aryl enol ethers could then be transformed into α-difunctionalized ketones by oxidation. The described "allyl cation"-like reactivity contrast with the well-established "vinyl-cation" behavior of alkenyl iodonium salts.

A trigold carbide cation stabilized as a labile pyridine adduct

Auration of trimethylsilyldiazomethane by an (N-heterocyclic carbene)gold alkoxide affords a stable (silyldiazoalkyl)gold complex. The reaction of this species with a gold(I) fluoride, more efficient in the presence of an added gold cation equivalent, results in auration, desilylation, and N2 loss. The putative trigold carbide cation was isolated as its pyridine adduct, a stable N-(triauromethyl)pyridinium ion. Reaction of the cationic pyridine adduct with carbon monoxide breaks the CN bond, forming the stable trigold ketenylidene cation. DFT studies suggest substantial negative charge at the bridging carbon in the methylpyridinium analogue and in two possible intermediates, the trigold carbide cation and the pyridine adduct of a neutral digold carbide.

PM2.5 Associated Phenols, Phthalates, and Water Soluble Ions from Five Stationary Combustion Sources

ABSTRACT Phenols and phthalates (PAEs) are always linked with the formation of secondary organic aerosols (SOA), while the water soluble ions (WSIs) are connected to the formation of secondary inorganic aerosols (SIA). A total of PM2.5 associated 20 phenols, 6 phthalates, and 9 WSIs were detected using GC-MS, ICS-1100, ICP-OES, and UV-VIS spectrophotometer for 5 stationary incineration sources including the domestic garbage (DG), garbage-fired power plant (GFPP), workshop of cable combustion for metal reclamation (WCC), peanut straw (PS), and wheat straw (WS). The anion equivalent (AE) and cation equivalent (CE) concentrations indicated that the emitted PM2.5 was alkaline for all the 5 combustion sources. Cl– possessed high contents among all the 5 burning sources and the highest value occurred at WCC due to the high Cl content in PVC. The WSI profiles were different from each other for the 5 incineration sources on the basis of high coefficients of divergence (CDs). The mass contributions of 9 WSIs in PM2.5 from 5 sources were far lower than those of atmospheric PM2.5. DEHP and DBP dominated in PM2.5 from 4 sources, while WCC possessed high levels of DEHP, DNOP, and DBP. WCC possessed the highest daily intakes of PAEs due to its highest ∑6PAEs of 32000 ng g–1 resulted from the high usage of plasticizers in PVC. The PAE profile similarities were found for both GFPP vs. DG and PS vs. WS based on low CDs. Only 11 phenols were detected for the 5 sources and WCC possessed the highest level of phenols although only phenol was detected. WS had the high levels of phenols due to the using of phenolic pesticides during wheat growth process.

Diurnal Variations and Source Analysis of Water-soluble Compounds in PM2.5 During the Winter in Liaocheng City

To investigate the diurnal variations and sources of water-soluble compounds in Liaocheng City, PM2.5 samples were collected between January and February 2017. The PM2.5 samples were analyzed for the compositions, concentrations, and sources of water-soluble inorganic ions, oxalic acid, and levoglucosan. The sources of these chemical compound were investigated using principal component analysis (PCA) and multiple linear regression (MLR) modeling. The results showed that the mass concentrations of PM2.5during the nighttime were higher than those during the daytime, and the average concentrations exceeded the National Ambient Air Quality Standard (GB 3095-2012) by more than 1.8 times. Moreover, atmospheric pollution was worse during the day than during the night. SNA (SO42-, NO3-, and NH4+) were the dominant species among the inorganic ions, the relative abundance of which with respect to the total concentrations of inorganic ions was 73.4% and 77.1% during the daytime and nighttime, respectively. The ratios of anion to cation equivalents (AE/CE) were less than one, suggesting that the PM2.5 was slightly alkaline, and the degree of acidity at night was stronger than during the day. The results of the correlation analyses suggested that aqueous-phase oxidation was the major formation pathway of oxalic acid, which is driven by acid-catalyzed oxidation. The oxalic acid was mainly influenced by biomass burning during the winter in Liaocheng City. The results of the PCA-MLR model suggested that water-soluble compounds in Liaocheng City were mostly from vehicular emissions and secondary oxidation, biomass burning, while the impacts of mineral dust and coal burning were relatively minor.

Diaryl-λ3-chloranes: Versatile Synthesis and Unique Reactivity as Aryl Cation Equivalent.

We have developed a versatile, high-yield synthesis of diarylchloroniums/λ3-chloranes through the reaction of various chloroarenes with readily prepared mesityldiazonium tetrakis(pentafluorophenyl)borate under mild conditions. The scope of the reaction is broad, including ArCl, ArBr, and ArI. The diarylchloroniums/λ3-chloranes prepared here show unique reactivity in various respects, enabling intermolecular electrophilic arylation reaction of weak nucleophiles, and chlorane-halogane exchange reaction.