- Electrochemistry of Carbonium Ions in Acidic Media. 1. Triphenylmethyl Ion in Aluminum Chloride Containing Melts
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The electrochemistry of the triphenylmethyl carbonium ion (1) has been studied in a room-temperature aluminum chloride containing melt.Formation of 1 is dependent on the acidity or pCl of a molten aluminum chloride/n-butylpyridinium chloride (1:1) binary mixture.The equilibrium constant for reaction 1 is 20.4 +/- 0.3 mol L-1 at 40 deg C as determined by spectroscopic techniques.Formation of 1 from triphenylmethyl chloride is too slow to quantitatively determine a rate constant (kf) by conventional electrochemical techniques.Cyclic voltammetric and controlled-potential coulometric data reveal that 1 is reduced in an one-electron process to a triphenylmethyl radical (2).Formation of 2 in the AlCl3 melt is verified by ESR spectroscopy.Dimerization of 2 produces an electrochemically oxidizable form of 1-(diphenylmethylene)-4-(triphenylmethyl)-2,5-cyclohexadiene (3); the rate constant for the dimerization of 2 is 1.33 +/- 0.08 * 103 1 mol-1s-1 at 40 deg C.Workup of an exhaustively electrolyzed solution of 1 in the melt produces triphenylmethane, which is the result of proton isomerization of 3.
- Luer, Garth D.,Bartak, Duane E.
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- Kinetic, thermodynamic and mechanistic studies on the reduction of carbenium ions by NAD(P)H analogues
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Hydride transfer mechanisms of the reductions of xanthylium ion by NAD(P)H analogues (i.e. BNAH, HEH and AcrH2) were investigated. Both the kinetic observations and an analysis of thermodynamic driving forces for each mechanistic step in all the possible mechanisms indicate that the reductions are initiated by a rate-determining electron transfer, followed by a fast hydrogen atom abstraction. The mechanism of the reductions of 9-phenylxanthylium and triphenylmethylium ions by BNAH were also investigated and are similarly discussed.
- Cheng, Jin-Pei,Lu, Yun
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- Rates of C-S bond cleavage in tert-alkyl phenyl sulfide radical cations
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Radical cations of tert-alkyl phenyl sulfides 1-4 have been generated photochemically in MeCN in the presence of the N-methoxyphenanthridinium cation (MeOP+), and the rates of C-S bond cleavage have been determined by laser flash photolysis.
- Baciocchi, Enrico,Del Giacco, Tiziana,Gerini, Maria Francesca,Lanzalunga, Osvaldo
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- 1H and 13C NMR detection of the carbocations or zwitterions from Rhodamine B base, a fluoran-based black color former, trityl benzoate, and methoxy-substituted trityl chlorides in the presence of alkali metal or alkaline earth metal
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The cleavage of C-O bonds in two fluoran-based dyes by the addition of alkali metal (M+ = Li+, Na+) and alkaline earth metal (M2+ = Mg2+, Ba2+) perchlorates in acetonitrile solution was exa
- Hojo, Masashi,Ueda, Tadaharu,Yamasaki, Masanori,Inoue, Akihiko,Tokita, Sumio,Yanagita, Mitsuhiro
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- Straightforward formation of carbocations from tertiary carboxylic acids: Via CO release at room temperature
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We report an unprecedented mode of reactivity of carboxylic acids. A series of tertiary carboxylic acids, containing at least one phenyl α-substituent, undergo loss of carbon monoxide at room temperature (295 K), by a one pot reaction with 0.5-1 molar equivalents of WCl6 in dichloromethane. A plausible mechanism for the Ph3CCO2H/WCl6 reaction, leading to [CPh3][WOCl5] and Ph3CCl, is proposed on the basis of DFT calculations. The analogous reactions involving CEt(Ph)2CO2H, CMe(Ph)2CO2H and CMe2(Ph)CO2H selectively afforded stable hydrocarbons (alkene or indene, depending on the case), apparently resulting from the rearrangement of elusive tertiary carbocations.
- Bartalucci, Niccolò,Pampaloni, Guido,Marchetti, Fabio,Bortoluzzi, Marco,Zacchini, Stefano
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supporting information
p. 1574 - 1577
(2019/04/02)
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- "naked" Lithium Cation: Strongly Activated Metal Cations Facilitated by Carborane Anions
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Experimental and spectroscopic studies revealed unprecedented reactivity of a "naked" lithium cation with very weakly coordinating anions, including carborane anions. The superactivated lithium cation has greatly enhanced Lewis acidic character and mediates various organic reactions such as carbonyl-ene reaction, NBS-bromination of unactivated aromatics, and Friedel-Crafts alkylation, which are not promoted by conventional lithium salts. Chemical robustness of the counteranion also plays an important role in the chemistry of the strongly activated lithium cation.
- Kitazawa, Yu,Takita, Ryo,Yoshida, Kengo,Muranaka, Atsuya,Matsubara, Seijiro,Uchiyama, Masanobu
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p. 1931 - 1935
(2017/02/26)
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- The Tris(2-thienyl)methyl Cation Problem. A Nuclear Magnetic Resonance Spectroscopic Study
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A series of homologues of the trityl cation obtained by total or partial substitution of the phenyl rings for 2- or 3-thienyl groups has been studied by 1H and 13C NMR.In general, the spectral parameters found follow the expected trends.However, a most unexpected effect is found in the transformation of tris(2-thienyl)carbinol (1d) into tris(2-thienyl)methyl cation (2d) , since the increase of the total chemical shift (ΔΣδ) associated with the ionization is as low as 105 ppm.To account for this observation, several hypotheses are discussed including the possibility of through-space charge delocalization by the three sulfur atoms and the interaction of the thiophene rings in an alternate conformation.
- Abarca, Belen,Asensio, Gregorio,Ballesteros, Rafael,Varea, Teresa
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p. 3224 - 3229
(2007/10/02)
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- Heterolysis and homolysis energies for some carbon-oxygen bonds
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Methods described previously for obtaining heterolysis (ΔHhet) and homolysis (ΔHhomo) enthalpies for bonds that can be cleaved to produce resonance-stabilized carbenium ions, anions, and radicals are extended to the study of carbon-oxygen bonds through the reactions of resonance-stabilized carbenium ions with substituted phenoxide ions. Titration calorimetry was used to obtain the heat of heterolysis, and the second-harmonic ac voltammetry (SHACV) method was used to obtain reversible oxidation potentials for the anions. In several cases, the electrode reactions were so fast that reversible potentials were obtained only with the greatest difficulty. Nonetheless, there is remarkably good agreement between these oxidation potentials for phenoxide ions obtained by electrochemical methods in sulfolane solution and those reported by others using entirely different techniques in different media. Such agreement provides unprecedented evidence for the soundness of the various methods used to study redox potentials of organic ions and radicals. As before, a wide variety of correlations was tested between ΔHhet and ΔHhomo. These two properties showed little correlation with each other, but ΔHhet gave good correlations between many properties for which neutral species are converted into ions or vice versa, such as redox potentials of both types of ions, the pKas of the anions, or the free energies of electron transfer. In contrast to the earlier study of cleavage to carbanions and carbenium ions, the present ΔHhet values are predicted well by a general equation that employs the pKR+ of the carbenium ion (without modification) and the pKa of the phenol. The improvement is consistent with the fact that the cleavage of carbon-oxygen bonds of the triarylcarbinols used to establish the pKR+ stability scale is a more appropriate model for the heterolysis of carbon-oxygen bonds in sulfolane at 25°C than it is for the cleavage of carbon-carbon bonds under the same conditions.
- Arnett, Edward M.,Amarnath, Kalyani,Harvey, Noel G.,Venimadhavan, Sampath
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p. 7346 - 7353
(2007/10/02)
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- Mechanism of reduction of trityl halides by lithium dialkylamide bases
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Trityl chloride (TCl) and bromide are reduced by hindered lithium dialkylamide bases in THF to give predominantly triphenylmethane and a small amount of trityl dimer. Rate constants for the reduction of TCl by lithium diisopropylamide and lithium tert-butylethylamide in THF at -78 ?C have been measured; the reactions are first order in monomeric base and in trityl chloride. Inter- and intramolecular kinetic isotope effect studies employing β-deuterium substituted bases and substituent effect studies coupled with other kinetic information were used to formulate a scheme for the reactions. The reactions proceed by a rapid predissociation of the trityl halide to form an ion pair containing the trityl-THF oxonium cation followed by diffusion controlled electron transfer (ET) from the monomeric form of the base to the trityl-THF oxonium ion. The radical pair thus formed reacts by fast, highly regioselective β-hydrogen atom transfer from the aminyl radical to the methine carbon of the trityl radical to give triphenylmethane. Radical escape from the cage is a minor competing process. An outer-sphere ET process is energetically acceptable, but an inner-sphere process appears to be more likely.
- Newcomb, Martin,Varick, Thomas R.,Goh, Swee-Hock
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p. 5186 - 5193
(2007/10/02)
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- Kinetic method for the measurement of the pKR value of the triphenylmethyl cation in the aqueous standard state
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A kinetic method is presented for the measurement directly in the aqueous standard state of the equilibrium constant KR for the reaction Ph3C+ + H2O = Ph3COH + H+. The basis of the procedure is direct measurement of the rate constant kf for the reaction proceeding in the forward direction with the technique of laser flash photolysis. The rate constant kr for the reverse reaction is measured by following oxygen exchange in a labeled alcohol in a weakly acidic (0.005 M HCl) solution. At 25 °C in water, kf = (1.68 ± 0.06) × 105 s-1, kr = (5.79 ± 0.29) × 10-2 M-1 s-1, and the value for pKR calculated from their ratio is -6.46 ± 0.03. The value for kr is based on a slight extrapolation from 5% acetonitrile in water. The value for kf is measured in water. This directly measured value for pKR provides a test of the validity of the acidity function and excess acidity methods commonly employed to determine pK values for weak bases where the equilibrium with the conjugate acid form can only be studied in strong acids. Values of pKR previously reported for Ph3C+ based upon these approaches vary from -5.85 to -6.89, although the range is smaller, -6.44 to -6.89, if data obtained in concentrated HCl are omitted. The conclusion is that these methods do lead to pKR values in reasonable agreement with the directly measured number.
- Mathivanan,McClelland, Robert A.,Steenken, Steen
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p. 8454 - 8457
(2007/10/02)
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- Stabilisation of Carbenium Ions by Acidic Salt Melts - an NMR Study
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The acidic salt melt N-butylpyridinium chloride/AlCl3 is a convenient medium for the dissociation of trityl chlorides.This reaction was followed by NMR spectroscopic labels, the p-substituents OCH3 and CF3.An NMR-line shape analysis yields for 4-(trifluoromethyl)triphenylmethyl chloride (2) an activation enthalpy ΔH(excit.) of 86 +/- 10 kJ/mol.The transition state seems to be close to the products as can be deduced by comparison with thermodynamic data.The bulky ions formed from trityl chloride (3) and AlCl3 give a salt with a melting point of 50 deg C even in the absence of butylpyridinium chloride. - Key Words: Carbenium ions/ Salt melts
- Bertlein, Gerhard,Dickert, Franz,Feigl, Matthias
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p. 1661 - 1664
(2007/10/02)
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- Flash-Photolysis Generation and Reactivities of Triarylmethyl and Diarylmethyl Cations in Aqueous Solutions
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A series of 18 triarylmethyl cations and 10 diarylmethyl cations have been generated by nanosecond laser flash photolysis of cyanide, 4-cyanophenyl ether, and acetate precursors in acetonitrile/water (AN/W) solutions and first-order rate constants for their reaction with the solvent (ks) have been directly measured following the decay in their optical density.In the standard solvent employed, 1:2 AN/W, the triarylmethyl cations which were studied had ks values at 20 deg C ranging from 1E1 s-1 (for the 4,4',4''-(MeO)3-substituted ion) to 9*1E6 s-1 (4,4'-(CF3)2), while diarylmethyl cations had ks values ranging from 1E5 s-1 (4,4'-(MeO)2) to 3*1E7 s-1 (4,4'-Me2).The parent diphenylmethyl cation and its derivative with one 4-methyl substituent were too short-lived (s of varying the amounts of acetonitrile were investigated for several cations.As water is added to 100percent acetonitrile, ks increases significantly, but at around 15percent by volume water, there is a leveling and from that point to 100percent water, ks is almost unchanged, decreasing by about 20percent.A plot of log ks versus ?+ constructed for the triarylmethyl cations shows significant deviations from linearity for the points for the ? donors, in the direction which indicates that ?+ is underestimating the stabilizing effect of these substituents for a fully formed cation.A plot versus ?C+, a parameter obtained from the analysis of NMR spectra of solutions of carbocations, is reasonably linear.A two-parameter correlation indicates that polar andresonance interactions of substituents do not proceed in parallel along the reaction coordinate, the addition of water to cation resulting at the transition state in the loss of 73percent of the equilibrium resonance effect but only a 33percent loss of the polar effect.A rate-equilibrium plot (log ks versus pKR) was constructed which covers 23 pKR units.A single line of slope 0.64 can be drawn to include the entire set of data for both triarylmethyl and diarylmethyl cations.From a small extrapolation the ks value for the tert-butyl cation in water is obtained as 1E10.5 s-1.
- McClelland, Robert A.,Kanagasabapathy, V. M.,Banait, Narinder S.,Steenken, Steen
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p. 3966 - 3972
(2007/10/02)
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- ON THE 1H-NMR SPECTRA OF HIGHLY CHARGED MULTI-TRIPHENYLMETHYLIUM IONS
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Relative to their neutral precursors, highly charged multi-tritylium ions show typical downfield 1H-NMR shifts, whereas the corresponding phenylfluorenylium ions show upfield shifts.A Hueckel based model is presented that accounts for this contrasting behavior in terms of antiaromatic ring current in the five-membered ring of the fluorenyl system with essentially null ring currents in the two six-membered rings.
- Wilcox, C.F.,Hellwinkel, D.,Stahl, H.,Gaa, H.G.,Doerner, M.
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p. 5501 - 5504
(2007/10/02)
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- Electrophilic Reactivity of the Triphenylmethyl Carbocation in Aqueous Solutions
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The triphenylmethyl (trityl) carbocation has been generated as a transient intermediate by laser flash photolysis of 1:2 (v/v) acetonitrile:water solutions of trityl acetate and trityl 4-cyanophenyl ether.Identification of the transient as the free carbocation in the ground state was based on its characteristic absorption spectrum and upon conductivity changes.Rate constants have been measured for the reaction of the cation in this solvent with a series of ionic and neutral nucleophiles.The solvent rate constant at 20 deg C is 1.5 x 105 s-1.Azide ions reacts at 4.1 x 109 M-1s-1; the directly measured azide:water ratio is compared to literature values determined by product analysis.Chloride ion reacts at 2 x 106 M-1 s-1; with bromide the equilibrium addition can be observed with k(comb) = 5 x 106 M-1 s-1 and k(ion) for Ph3CBr = 8 x 105 s-1.Rate constants do not adhere to the N+ relationship.This predicts a slope of unity in a plot of log k(Ph3C+) vs.N+, with the better nucleophiles reacting at the 1010 encounter-controlled limit.Azide is the only nucleophile of those studied to approach this.Sulfite and thiolate ions, which are better N+ nucleophiles, react at 2-3 x 108 M-1s-1, while amines react in the 106-107 M-1s-1 range.The plot vs.N+ has a slope of 0.3-0.4.One explanation is that rate constants for the better nucleophiles do level, but this occurs considerably below the 1010 limit.Alternatively, the less than unit slope is real and this more reactive cation, in contrast to more stable analogues, is exhibiting selectivity.
- McClelland, Robert A.,Banait, Narinder,Steenken, Steen
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p. 7023 - 7027
(2007/10/02)
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- The Thermodynamic and Kinetic Behaviour of the Four-Coordinate Carbocation (C6H5)3C(DMF)+ - a NMR and Relaxation Study
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Trityl ions form an 1:1 adduct with dimethylformamide according to titration curves via the NMR chemical shift.This four-coordinated carbocation ((C6H5)3C(DMF)+) further reacts with the carbenium ion ((C6H5)3C+) in a consecutive step.In dichloromethane the rate constants of formation and dissociation of (C6H5)3C(DMF)+ could be determined by NMR line shape analysis (k12(298K)=1.5E8 M-1s-1; k21(298K)=4.2E7 s-1).From the rate constant of the reaction between the carbocation and the CF3COO- anion (measured with pressure jump experiments) and the formation equilibrium of the carbocation the recombination rate of the carbenium ion with the trifluoroacetate anion is calculated.The very high value of k(298K)=2.5E9 M-1s-1 indicates that CF3COO- in dimethylformamide reacts as a naked anion. - Keywords: Kinetics / NMR spectroscopy / Carbocations
- Blumenstock, H.,Dickert, F. L.,Hammerschmidt, A.
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p. 123 - 132
(2007/10/02)
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- Stabilities of Carbocations in Solution. 14. An Extended Thermochemical Scale of Carbocation Stabilities in a Common Superacid
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Until now, thermodynamic stabilities of carbocations have been limited to (1) relatively stable resonance delocalized ions which are ranked on the pKR scale and (2) relatively unstable aliphatic and alicyclic ions which have been compared in the gas phase by ion cyclotron resonance or by calorimetry in SbF5/SO2ClF superacid at -50 to -120 deg C in our laboratory.The present paper will present an extensive series of new measurements which is designed to close the gap between the stable triarylmethyl cations and the unstable ions so as to put them all on a common energy scale.Carbinols were used as precursors in SbF5/HSO3F/SO2ClF at -40 deg C.As we reported recently (J.Am.Chem.Soc., 104, 3522(1982)), these conditions are necessary to avoid complications which appear to be introduced by ion-pairing when the alcohols are treated with the SbF5/SO2ClF system which was used previously to ionize alkyl chlorides.The results from the present work place 39 typical carbocations representing saturated, secondary, and tertiary, and aliphatic, bicyclic, and substituted cumyl, benzhydryl, and trityl systems on a common scale.Correlations and interpolation equations for relating other measurements in the gas phase and solution will be presented.The results provide useful comparisons of the ?+ and ?C+ scales for correlating carbocation stabilities and provide new data for several classic questions in the field such as the ranking of methyl, phenyl, and cyclopropyl groups for stabilizing ions and also the reactions of carbocations with water.
- Arnett, Edward M.,Hofelich, Thomas C.
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p. 2889 - 2895
(2007/10/02)
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- The Effect of Both ?-Donor Substituents and Acid Media on the Formation of Trityl Cations, Studied by Relaxation Methods and NMR-Spectroscopy
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Trityl trifluoroacetates were taken to study why ion formation from ionogens is favoured by acid media.From relaxation measurements, NMR-spectroscopy and conductivity studies it was found that this effect is due to anion solvation by acid, namely a specific solvation of the free anions and a statistical solvation of the anions bound in the ionogen.This leads to a drastically reduced ion recombination rate constant even at low concentrations of acid and an increased dissociation rate constant at rather high concentrations of acid. -.Keywords: Kinetics/ Ion solvation/ Carbenium ions/ NMR-spectroscopy
- Blumenstock, H.,Dickert, F.,Fackler, H.,Hammerschmidt, A.
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p. 157 - 170
(2007/10/02)
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