Carboxonium Ions Research Articles

Carboxonium derivatives based on closo-dodecaborate anions [1,2-B12H10O2CR]−: synthesis and molecular orbital analysis

Borylated carboxonium ions based on closo-dodecaborate anions were obtained. The nature of the interaction between the cluster cage and the organic moiety was analysed theoretically.

IRMPD Spectra of Protonated Hydroxybenzaldehydes: Evidence of Torsional Barriers in Carboxonium Ions

Protonation at the formyl oxygen atom of benzaldehydes leading to the formation of carboxonium ions yields two distinct isomers, depending on the relative orientation of the proton either cis or trans with respect to the hydrogen atom on the adjacent carbon. In this context, the IR multiple photon dissociation (IRMPD) spectra of protonated ortho, meta, and para-hydroxybenzaldehydes (OH-BZH+ ), delivered into the gas phase by electrospray ionization of hydro-alcoholic solutions, are reported in the 3200-3700 cm-1 spectral range. This range is characteristic of O-H stretching modes and thus able to differentiate cis and trans carboxonium isomers. Comparison between IRMPD spectra and DFT calculations at the B3LYP/6-311++G(2df2p) level suggests that for both p-OH-BZH+ and m-OH-BZH+ only cis conformers are present in the ion population analyzed. For o-OH-BZH+ , IRMPD spectroscopy points to a mixture comprising one trans and more than one cis conformers. The energy barrier for cis-trans isomerization calculated for each OH-BZH+ isomer is a measure of the degree of π-electron delocalization. Furthermore, IRMPD spectra of p-OH-BZH+ , m-OH-BZH+ and protonated phenol (this last used as reference) were recorded also in the fingerprint range. Both the observed C-O and O-H stretching vibrations appear to be a measure of π-electron delocalization in the ions.

Nanosecond-Scale Isomerization of the 4'-Carboxonium Cation Oxidatively Produced in Pyrimidine Units of DNA.

The long-standing puzzle of the chemistry producing the Stubbe-Kozarich abasic site, which is the minor product in the oxidation of 2'-deoxycytidine units of DNA by Fe(II)-bleomycin, has been computationally solved in this study. Scrupulous DFT-based calculations that included extensive screening of the potential energy surface of model-solvated nucleotides and the elucidation of the chemical structure of the located nucleotide cations via natural bond orbital analysis demonstrated that the 2'-deoxycytidine unit bearing the 2'-deoxyribose ring 2e-oxidized at the 4'-position undergoes carboxonium ion- iminium ion (C═O+-C → C═N+) isomerization. This 1,2-elimination of the carbonyl group 4'-C═O from the carboxonium cation fragment is associated with minimal spatial reorganization of the molecule and appears to be an ultrafast reaction. The calculated barrier Δ G0# of 2.7 kcal mol-1 for this isomerization is lower than that reported for the addition of water to oxocarbenium ions. Thus, this unusual nucleotide transformation is the key chemical reaction that yields the Stubbe-Kozarich product. Such a product cannot be formed for purine nucleotide units in DNA. The isomerization of 4'-dehydro-2'-deoxyribose-4'-carboxonium cations formed in these DNA units is slower because it destroys the purine aromaticity, and the cations are intercepted by water molecules before they isomerize.

Effect of heat in chitosan dissolution and addition of KI on the enhancement of ionic conductivity

This work focuses on the effect of chitosan-based carbohydrate polymer electrolytes (CHPE) that were prepared by the application of heat in chitosan dissolution. Furthermore, various wt% of potassium iodide (KI) that was incorporated into this heat-applied chitosan (KI-HAC) were investigated. Electrochemical impedance spectroscopy (EIS) shows that the pure HAC has the room temperature ionic conductivity of 5.57 × 10−4 S/cm. The conductivity enhanced from 5.57 × 10−4 S/cm to 6.39 × 10−4 S/cm on the addition of 0.3 wt% of KI and further enhanced to 2.07 × 10−2 S/cm on the increment of KI from 0.3 to 1.8 wt%. The conductivity enhancement in chitosan was found due to the formation of amino glucosyl carboxonium ions (AGCI) and is due to the effect of heat in chitosan dissolution. XRD affirms the reduction in crystallinity. SEM analysis shows the spherical morphology for pure chitosan, porous morphology for HAC, and KI-HAC. FTIR confirms the degradation of chitosan and the formation of AGCI. Dielectric study, dielectric tan loss study, and dielectric modulus study confirm the results obtained by EIS. The results are authenticating that the conductivity enhancement in CHPE is not only due the addition of inorganic salt but also due to the chitosan which plays the role in conductivity enhancement. Dye-sensitized solar cell using HAC and KI-HAC shows efficiency of 1.25 and 1.62% at 1 sun condition.

Ionic Conductivity and Power Conversion Efficiency Study of KI Incorporated Glucosyl Carboxonium Ion-based Biopolymer Crust Electrolyte

ABSTRACTThe impedance of well-characterized KI-incorporated glucosyl carboxonium ion-based biopolymer crust electrolytes up to a maximum 2.7 wt% was measured using electrical impedance spectroscopy. Enhanced ionic conductivity of 2.3657 × 10−2 S cm−1 on the addition of 2.7 wt% of KI was observed in contrast to earlier reported value for pure GCI of 4.5278 × 10−4 S cm−1. This is attributed due to the increased concentration of KI in the system and is corroborated with increased ion density (n), mobility (µ), and diffusion coefficients (D). Dielectric and modulus study shows the capacitive nature of electrolyte. Fabricated dye-sensitized solar cell using pure glucosyl carboxonium ion crust and KI-incorporated glucosyl carboxonium ion crust shows the efficiency of 1.19% for pure and shows the efficiency of 2.14% for 2.6 wt% of KI in glucosyl carboxonium ion at 1 sun condition.

Environmental Polymer Degradation: Using the Distonic Radical Ion Approach to Study the Gas-Phase Reactions of Model Polyester Radicals.

A novel precursor to the distonic O- and C-centered radical cations Oxo+O• and Oxo+C• was designed and synthesized, which represents model systems for radicals produced during polyester degradation. The precursor is equipped with a nitrate functional group, which serves as a masked site for these alkoxyl and carbon radicals that are unleashed through collision-induced dissociation (CID). Oxo+O• and Oxo+C• feature a cyclic carboxonium ion as permanent charge tag to enable monitoring their ion-molecule reactions on the millisecond to second time scale in the ion trap of the mass spectrometer. The reactions of Oxo+O• and Oxo+C• with cyclohexene, cyclohexadiene, ethyl acetate, 1,1-dimethoxyethane, and 1,2-dimethoxyethane, which exhibit structural features present in both intact and defective polyesters, were explored through product and kinetic studies to identify "hot spots" for radical-induced damage in polyesters. All reactions with Oxo+O• were extremely fast and proceeded predominantly through HAT. Oxo+C• was about two orders of magnitude less reactive and did not noticeably damage aliphatic ester moieties through hydrogen abstraction on the time scale of our experiments. Radical addition to alkene π systems was identified as an important pathway for C-radicals, which needs to be included in polymer degradation mechanisms.

Effect of Prolonged Duration of Gelatinization in Starch and Incorporation with Potassium Iodide on the Enhancement of Ionic Conductivity

ABSTRACTThree systems of starch-based crust electrolytes were prepared using various gelatinization times, various weight percentages (wt%) of starch, and various wt% of starch incorporated into potassium iodide. All the samples were subjected to electrochemical impedance spectroscopy, X-ray diffraction spectroscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, and transference number measurements. Electrochemical impedance spectroscopy shows that 1.7 wt% of starch has maximized the room temperature conductivity of the electrolyte to 1.4587 × 10−4 S cm−1. The conductivity was enhanced to 4.5278 × 10−4 S cm−1 on the increment of starch’s wt% from 1.7 to 3.2. This conductivity was further enhanced to 3.4609 × 10−3 S cm−1 on the addition of 0.3 wt% of potassium iodide. The conductivity enhancement was found due to the formation of glucosyl carboxonium ions. The effect of longer heating time in gelatinization is attributed to the formation of glucosyl carboxonium ions. X-ray diffraction spectroscopy affirms the reduction in crystallinity of starch. Scanning electron microscopy analysis shows the porous morphology of starch electrolyte, and addition of potassium iodide shows the morphology of bean nuts like particles seated on the pores. Fourier transform infrared confirms the degradation of starch. Transference number measurements of the crust electrolyte shows that ions are the dominant conducting species. All the results are authenticating that the conductivity enhancement in starch-based crust electrolyte is due to starch and the addition of inorganic salts.

Substituent Effects on the pH Sensitivity of Acetals and Ketals and Their Correlation with Encapsulation Stability in Polymeric Nanogels.

The effect of structural variations in acetal- and ketal-based linkers upon their degradation kinetics is studied through the design, synthesis, and study of six series of molecules, comprising a total of 18 different molecules. Through this systematic study, we show that the structural fine-tuning of the linkers allows access to variations in kinetics of degradation of more than 6 orders of magnitude. Hammett correlations show that the ρ value for the hydrolysis of benzylidene acetals is about -4.06, which is comparable to an SN1-like process. This shows that there is a strong, developing positive charge at the benzylic position in the transition state during the degradation of acetals. This positively charged transition state is consistent with the relative degradation rates of acetals vs ketals (correlated to stabilities of 1°, 2°, and 3° carboxonium ion type intermediates) and the observed effect of proximal electron-withdrawing groups upon the degradation rates. Following this, we studied whether the degradation kinetics study correlates with pH-sensitive variations in the host-guest characteristics of polymeric nanogels that contains these acetal or ketal moieties as cross-linking functionalities. Indeed, the trends observed in the small molecule degradation have clear correlations with the encapsulation stability of guest molecules within these polymeric nanogels. The implications of this fundamental study extend to a broad range of applications, well beyond the polymeric nanogel examples studied here.

Superacid-promoted synthesis of polychlorinated dibenzofurans

Polychlorinated dibenzofurans have been prepared in one step by the condensations of o-chloranil with arenes in triflic acid. A mechanism is proposed involving formation of a monoprotonated quinone (carboxonium ion), electrophilic attack at the arene, and cyclization of a carbocation intermediate. The chemistry is further examined by spectroscopic and theoretical studies.

Exploring the intrinsic polar [4 + 2+] cycloaddition reactivity of gaseous carbosulfonium and carboxonium ions

Gas-phase reactions of model carbosulfonium ions (CH(3)-S(+)=CH(2;) CH(3)CH(2)-S(+)=CH(2) and Ph-S(+)=CH(2)) and an O-analogue carboxonium ion (CH(3)-O(+)=CH(2)) with acyclic (isoprene, 1,3-butadiene, methyl vinyl ketone) and cyclic (1,3-cyclohexadiene, thiophene, furan) conjugated dienes were systematically investigated by pentaquadrupole mass spectrometry. As corroborated by B3LYP/6-311 G(d,p) calculations, the carbosulfonium ions first react at large extents with the dienes forming adducts via simple addition. The nascent adducts, depending on their stability and internal energy, react further via two competitive channels: (1) in reactions with acyclic dienes via cyclization that yields formally [4+2(+)] cycloadducts, or (2) in reactions with the cyclic dienes via dissociation by HSR loss that yields methylenation (net CH(+) transfer) products. In great contrast to its S-analogues, CH(3)-O(+)=CH(2) (as well as C(2)H(5)-O(+)=CH(2) and Ph-O(+)=CH(2) in reactions with isoprene) forms little or no adduct and proton transfer is the dominant reaction channel. Isomerization to more acidic protonated aldehydes in the course of reaction seems to be the most plausible cause of the contrasting reactivity of carboxonium ions. The CH(2)=CH-O(+)=CH(2) ion forms an abundant [4+2(+)] cycloadduct with isoprene, but similar to the behavior of such α,β-unsaturated carboxonium ions in solution, seems to occur across the C=C bond.

ChemInform Abstract: Stable Ion Study of Protonated Cyclopenta[a]phenanthrenes. Structure-Reactivity Relationships and Charge Delocalization in the Carbocations.

Protonation studies are reported for a series of cyclopenta[a]phenanthrenes Cp[a]P in superacid media. Hydrocarbons 1, 4, 7, are ring protonated in FSO3H–SO2ClF to form monoarenium ions. The Δcompounds 3, 6 are protonated at the D-ring double-bond to form stable α-phenanthrene-substituted carbocations. The 17-keto derivatives 2, 5, 8, 9, 19, 20 are CO-protonated in FSO3H–SO2ClF to form carboxonium ions. Carboxonium ions derived from 8 and 20 undergo ring fluorosulfonation in the biologically important A-ring under thermodynamic control (higher temperatures and prolonged reaction times). Low temperature protonation of 8 and 9 with FSO3H·SbF5 (4∶1)–SO2ClF gives their corresponding carboxonium-arenium dications (protonation of 2 with FSO3H·SbF5 (1∶1)–SO2ClF gave a mixture of mono- and dications), where ring protonation sites are controlled by the position of the methyl group and occur in the A-ring for the A-ring methylated derivatives (8, 9). Whereas the 11-methoxy derivative (16) forms a carboxonium ion in FSO3H–SO2ClF analogous to the 11-Me derivative (5), the 11-phenol derivative (15), the ethoxy (17) and propoxy (18) derivatives are more reactive, forming a mixture of mono- and dication (with 15 and 17) or give mostly a carboxonium-arenium dication (with 18). Substituent effects observed under stable ion conditions emphasize relative carbocation stability and relief of peri-strain. Under thermodynamic control, carboxonium ions undergo fluorosulfonation in the biologically important A-ring. Charge delocalizations in the resulting mono- and dications (deduced primarily based on magnitude of Δδ13C) are discussed and compared. In an effort to further enhance the NMR assignments and for comparison, mono-arenium ions 1H++, 4H++, 6H++, 7H++ and their neutral precursors were calculated at the B3LYP/6-31G(d,p) level of ab initio theory; their 1H and 13C NMR chemical shifts were computed by the GIAO method and their overall charge delocalization paths were deduced via differences in the NPA charges (cation minus neutral). The results are compared and discussed. Stable ion studies of CP[a]P provide useful insights into the contrasting regioselectivities observed in chemical and biological activiation. p

The reactivity of 1,1-dialkoxyalkanes with niobium and tantalum pentahalides. Formation of coordination compounds, C–H and C–C bond activation and the X-ray structure of the stable carboxonium species [Me2CCHC(OMe)Me][NbCl5(OMe)

The reactions of the pentahalides NbX5 (X = Cl, Br) with the 1,1-dialkoxyalkanes CHR'(OEt)2 or 1,3-dioxolane yield the coordination adducts NbX5[kappa1-(OEt)CHR'(OEt)] (X = Cl, R' = H, 2a; X = Br, R' = H, 2b; X = Cl, R' = Me, 2c; X = Br, R' = Me, 2d) or NbCl5(kappa1-right angle OCH2OCH2C right angle H2), 3, respectively. Compounds 2a-c and 3 are stable at room temperature, while 2d slowly converts into the alkoxide NbBr4[OCH(Me)OEt], 4. Room-temperature fragmentations are observed upon reacting CH2(OMe)2, CHMe(OMe)2 and CMe2(OMe)2 with MX5 (M = Nb, Ta; X = Cl, Br). The complexes MX5[O(Me)(CH2X)], 5, and [MX3(OMe)2]2, 6, form selectively from MX5/CH2(OMe)2 (M = Nb, Ta; X = Cl, Br), while mixtures of [NbX4(OMe)]2 (X = Cl, 8a; X = Br, 8b) and organic halides are obtained from NbX5/CHMe(OMe)2. Instead, the reaction of NbCl5 with CMe2(OMe)2 affords the stable carboxonium species [Me2C=CHC(=OMe)Me][NbCl5(OMe)], 7, as prevalent product. Furthermore, 1,1-dialkoxyalkanes are generally activated by MF5 (M = Nb, Ta) at room temperature: according to the cases, alcohols, ethers, esters and ketones have been detected in the reaction mixtures after hydrolysis. In agreement with NMR data, the carboxonium ion [Me2C=CHC(=OMe)Me]+ is produced from NbF5/CMe2(OMe)2. The complexes NbF5[OEt(Me)], 9a, and NbF5[O=CH(OEt)], 9b, obtained by addition of CH2(OEt)2 to NbF5, have been characterised by NMR spectroscopy. Trimethyl formate, CH(OMe)3, reacts with NbX5 (X = Cl, Br) with formation of the alkoxides 6 and 8, in admixture with NbX5[O=CH(OMe)] and MeX; otherwise, MF5[O=CH(OMe)] and MF5(OMe2) have been synthesised from MF5/CH(OMe)3 (M = Nb, Ta). Complete screening of the organic fragments produced in the distinct reactions has been carried out by both GC-MS and NMR analyses on the reaction mixtures, after treatment with water. The solid state structures of the mixed halo-alkoxy complexes [TaX3(OMe)(mu-OMe)]2 (X = Cl, 6c; X = Br, 6d) and of the stable alkylated ketone containing species 7 have been ascertained by X-ray diffraction studies.

Synthesis and Stable‐Ion Studies of Regioisomeric Acetylnitropyrenes and Nitropyrenyl Carbinols and GIAO‐DFT Study of Nitro Substituent Effects on α‐Pyrenyl Carbocations

AbstractSeveral regioisomeric acetylnitropyrenes were synthesized from isomeric acetylpyrenes by mild protic nitration. Nitration of 1‐acetylpyrene gave the 3‐, 6‐, and 8‐nitro derivatives (with 8‐nitro as the major isomer), from which the corresponding carbinols [NO2‐Py‐CH(OH)CH3; Py = pyrene] were synthesized. Isomeric 4‐acetylnitropyrenes and their corresponding carbinols were synthesized by starting from hexahydropyrene through nitration/aromatization/reduction or aromatization/nitration/reduction sequences. The molecular structures of 4‐acetyl‐3‐nitropyrene and 1‐(6‐nitropyren‐1‐yl)ethanol were established by X‐ray analysis. Tetrahydropyrene was the starting point for the synthesis of isomeric nitro‐2‐acetylpyrenes. Low‐temperature protonation of 1‐acetyl‐8‐nitropyrene, 4‐acetyl‐3‐nitropyrene, and 2‐acetyl‐6‐nitropyrene in FSO3H/SO2ClF or in FSO3H/SbF5 (1:1)/SO2ClF resulted in the formation of onium dications (by C=O and NO2 protonation). Charge delocalization (pyrenium ion character) in the carboxonium ions is strongly influenced by the position of the carboxonium group, with the 4‐acetyl‐3‐nitropyrene dication being the most delocalized. Superacid protonation of 1‐(3‐nitropyren‐4‐yl)ethanol gave a persistent onium dication rather than an α‐pyrenyl carbocation. With all other isolated nitropyrenyl carbinol isomers, low‐temperature protonation (with FSO3H/SO2ClF) led to polymerization within 5 min standing at dry‐ice–acetone temperature. For these cases, nitro substituent effects on the α‐pyrenyl carbocations were gauged by DFT and GIAO‐DFT studies. An interesting relationship between the computed nitro tilt angles and the GIAO‐derived charge delocalization modes was observed.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Stable Ion and Electrophilic Chemistry of the Sterically Crowded Stilbene 1,1′-Bi(benzocyclobutenylidene) and Its Derivatives

Generation and NMR studies of novel carbocations and carboxonium ions are reported from sterically hindered stilbene 1,1'-bi(benzocyclobutenylidene) 1, its dimethoxy derivative 5, and from their skeletally rearranged derivatives, namely, the spirocyclic ketone 6, diastereomeric alcohols 7 and isomeric diols 8. Quenching experiments on the carbocations under various conditions resulted in the formation/isolation of several novel covalent adducts. Acid-catalyzed isomerization of the diols 8 produced a remarkable dimeric molecule, whose structure was confirmed by X-ray analysis. Reactions of hindered stilbenes 1 and 5 with Br 2/CDCl 3 were examined via NMR experiments. The experimentally observed carbocations were also studied computationally by GIAO-DFT and by NICS.

Stable Ion NMR and GIAO-DFT Study of Novel Cations from 8,16-Dicyano[2.2]metacyclophanedienes and from Strategically Substituted/Benzannelated Dihydropyrenes: Charge-Induced Tropicity Modulation and π-Switching

The dicyanometacyclophanediene 1 is diprotonated at the cyano groups (1H2 2+) in various superacid media. Upon quenching, intact 1 and the ring-closed CPD 2 were obtained in a 3:2 or 3:1 ratio, depending on the superacid system. Compound 2 undergoes ring opening in the superacid to give the ipso-monoprotonated 2H+, which on quenching furnishes 1-cyanopyrene as a major product together with 2 and 1. The dication 3 2+, with strongly deshielded internal methyls, was generated from the epoxyannulene 3. Ketones 4-6 and ester 7 are O/C diprotonated to give paratropic carboxonium-annulenium dications (4H2 2+, 5H2 2+, 6H2 2+, and 7H2 2+, respectively). Ester 8 gives a trication by two-electron oxidation and O-protonation. Conjugated carboxylic acid 9 gives a mixture of two dications by CO and ring protonation. The dibromo derivatives 10 and 11 form carboxonium ions, whereas the monobromo derivative 12 is O/C diprotonated to give an oxonium-annulenium dication. Charge delocalization modes and tropicity in the resulting species are evaluated by NMR and GIAO-DFT. Facile formation of 2 from 1 in quenching experiments indicates that thermal closing can be achieved with the diprotonated dinitrile, without imposing skeletal rearrangement.

Reactions of Acenaphthenequinone and Aceanthrenequinone with Arenes in Superacid.

The hydroxyalkylation reactions of aceanthrenequinone (6) and acenapthenequinone (7) with a series of arenes have been studied. In reactions with the Brønsted superacid CF(3)SO(3)H (triflic acid), the condensation products are formed in good yields (58-99%, 10 examples) with high regioselectivity. Computational studies were also done to examine the structures and energies of mono- and diprotonated species from 6 and 7. The results from the condensation reactions are consistent with the formation of superelectrophilic species involving protosolvation of carboxonium ion intermediates.

Rearrangement and Fluorination of Quinidinone in Superacid.

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Rearrangement and fluorination of quinidinone in superacid

In HF/SbF 5 at −78 °C, quinidinone 1 yields fluoroketone 3 (50% yield). The reaction implies a cyclic carboxonium ion as an intermediate, which reacts through a concerted rearrangement and fluorination to yield ketone 3.

Trifluoroperacetic Acid in Consecutive Criegee Rearrangement and Carboxonium Ions Generation

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Trifuoroperacetic acid in consecutive Criegee rearrangement and carboxonium ions generation

Recent achievements in the use of trifluoroperacetic acid (TFPAA) in consecutive Criegee rearrangement are covered in this review. Consecutive Criegee rearrangement promoted by TFPAA represents a new approach to this classic method in organic chemistry. The concept of selective double and triple O-insertion is described through the analysis of kinetic data, the overview of synthetic examples and studies of the reaction mechanisms. TFPAA in trifluoroacetic acid (TFAA) media is introduced as a specific reagent for consecutive Criegee rearrangement and for stable carboxonium ion generation. Heterolytic decarboxylation and bisperfluoroacyl peroxide synthesis are discussed as miscellaneous and useful reactions.