Kinetics and mechanism of the protonation of 6-(triphenylphosphonio-3′-cyclopentadienyl)-2,3,5-trihalocyclohexa-2,5-diene-1,4-dione by trifluoroacetic acid in aprotic media

6-(Triphenylphosphonio-3′-cyclopentadienyl)-2,3,5-trihalocyclohexa-2,5-diene-1,4-diones (2) are a new class of phosphorus containing dyes that behave as weak bases. Mono-protonation occurs at the cyclopentadiene ring leading to three isomers as assessed by NMR. The kinetics of the protonation were studied by the stopped-flow technique in aprotic media. The activation energies and deuterium isotope effects suggest that the rate-limiting step is the addition of trifluoroacetic acid to the quinone ring rather than direct protonation of the cyclopentadiene ring. © 1997 John Wiley & Sons, Inc.     

Two-dimensional infrared study of 3-azidopyridine as a potential spectroscopic reporter of protonation state

The lack of general spectroscopic probes that can be used in a range of systems to probe kinetics and dynamics is a major obstacle to the widespread application of two-dimensional infrared (2D IR) spectroscopy. We have studied 3-azidopyridine to characterize its potential as a probe of the protonation state of the pyridine ring. We find that the azido-stretching vibration is split by accidental Fermi resonance interactions with one or more overtones and combination states. Using 2D IR spectroscopy, we determine the state structure of the resulting eigenstates for complexes of 3-azidopyridine with formic acid and trifluoroacetic acid in which the pyridine ring is unprotonated and protonated, respectively. Based on the measurements, we develop a two-oscillator depurturbation model to determine the energies and couplings of the zeroth-order azido-stretching state and the perturbing dark state that couples to it. Based on these results, we conclude that the azido-stretching vibration is, in fact, sensitive to the protonation state of the pyridine shifting up in frequency by 8?cm(-1) in the complex with trifluoroacetic acid relative to the formic acid complex. These results suggest that, although 3-azidopyridine is not suitable as a spectroscopic probe, the approach of employing an organic azide as a remote probe of protonation state holds significant promise.     

Protonation of metal carbonyl complexes : III. Cyclopentadienyl carbonyl complexes of manganese

Using IR spectroscopy, the phosphine derivatives of cyclopentadienylmanganese tricarbonyl have been shown to undergo protonation in solutions of trifluoroacetic acid and in mixtures of this solvent with methylene chloride, protonation at the metal atom being the most probable. Ease of protonation increases with increasing electron-releasing properties of both the π-ring substituents and the phosphine ligands attached to the manganese atom.     

Synthesis of Novel Scolopendra-type Polydodecyloxybenzoyl[1,5]-diazocine as New Material for Optical Sensor

A new scolopendra-type polymer ofpolydodecyloxybenzoyl[1,5]-diazocine (PDBD) was designed and prepared using 2,5-bis(4-(dodecyloxy)-benzoyl)terephthaloyl azide with trifluoroacetic acid (TFA) via one-pot reaction in good yields.The structure of polymer was characterized using 1H-NMR,13C-NMR and MALDI-TOF spectra.The polymer PDBD exhibits good thermal stability as measured by TGA and DSC,and can be dissolved well in common organic solvents such as chloroform and tetrahydrofuran.In addition,UV-Vis spectral studies indicate that the polymer PDBD shows unique optical property changes (protonation/deprotonation) in the different trifluoroacetic acid environments.The new polymer is expected to be utilized as an optical functional material for fabricating optical sensors in environmental and biological fields.     

Acylation of alkll 1,4-dimethoxybenzenes by substituted benzoic acids in trifluoroacetic acid

A clean, convenient and highly selective acylation of alkyl 1,4-dimethoxy-benzenes, e.g., 2-methyl-1,4-dimethoxybenzene, by using substituted benzoic acids in dry trifluoroacetic acid has been achieved. The substituted benzophenones were found to be the sole product and their formation could be much reduced or even completely inhibited by adding a small amount of water. Kinetic studies show that the reaction obeys second-order kinetics with rate = k₂.[substrate]-[ArCO₂H]. The very negative reaction constant (p = -6.43) derived from the excellent Hammett correlation (r = 0.99) reveals that electron-donating substituents in benzoic acid strongly favour the protonation of the acid by trifluoroacetic acid, increasing the concentration of the acylating agents, ArtC⁺=0, and thus lead to the effective aromatic acylation.     

Carbon-13 n.m.r. studies of substituted bisalkylamino-s-triazines in acidic media

The carbon-13 n.m.r. spectra of six substituted bisalkylamino-s-triazines are reported. The site of protonation of 2-methoxy-s-triazines is shown to be at N-1 or N-3. The effect of trifluoroacetic acid on 2-chloro-s-triazines is also discussed.     

Investigation of the protonation of 5-azaindole derivatives by pmr spectroscopy

The protonation of some 5-azaindoles and 5-azaindolines by trifluoroacetic acid in media with different dielectric constants was studied by PMR spectroscopy. Protonation occurs at the nitrogen atom of the pyridine ring. The structures of the monocations of 5-azaindole, 5-azaindoline, and their 1-phenyl derivatives correspond to a considerable contribution of the quinoid structure with transfer of positive charge to the nitrogen atom of the pyrrole fragment of the molecule. On the basis of an investigation of the chemical shifts of the protons of 1-phenyl-5-azaindole and 1-phenyl-5-azaindoline on the trifluoroacetic acid concentration in methylene chloride, acetonitrile, and deuteroacetone, a protonation mechanism in which transfer of a proton from the donor to the acceptor in slightly polar media occurs through the formation of a hydrogen-bonded complex of the base with the acid is proposed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 767–772, June, 1973.     

Discovery of cationic domino reaction under the NMR experiment conditions: diastereoselective dimerization of 2-arylmethylideneindolin-3-ones to pentacyclic 3-hydroxyindolenine system

NMR spectroscopy (1D and 2D) showed that the protonation of the capto-dative heterocyclic α-amino enones, viz., (Z)-2-arylmethylideneindolin-3-ones, with strong acids led to stable (aryl)(3-hydroxyindol-2-yl)methyl cations. These substrates in trifluoroacetic acid underwent cationic domino reaction, which resulted in polynuclear pyrido[1,2-a: 5,6-b′]-[1,2′]biindoles.     

Indole chemistry

Judging from the PMR spectra, the protonation of various 3-alkyl- and 3-arylindoles in sulfuric and trifluoroacetic acids proceeds primarily at the 3 position. Prototropic shift of a radical to form 2-substituted indoles occurs on heating with these acids, and better results are obtained with polyphosphoric acid. Ideas regarding the mechanism of the process are presented.See [1] for communication XXVIII.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 55–60, January, 1972.     

Diprotonated [28]Hexaphyrins(1.1.1.1.1.1): Triangular Antiaromatic Macrocycles

Protonation of meso‐aryl [28]hexaphyrins(1.1.1.1.1.1) triggered conformational changes. Whereas protonation with trifluoroacetic acid led to the formation of monoprotonated Möbius aromatic species, protonation with methanesulfonic acid led to the formation of diprotonated triangular antiaromatic species. A peripherally hexaphenylated [28]hexaphyrin was rationally designed and prepared to undergo diprotonation to favorably afford a triangular‐shaped antiaromatic species.     

Acid base interactions of octaphenyltetrapyrazinoporphyrazine and its complex with lutetium(III) in proton-donor media

Acid base interactions of octaphenyltetrapyrazinoporphyrazine and its complex with lutetium(III) in acetic and trifluoroacetic acid media were studied, and parameters characterizing equilibria between the acid and base forms were determined. Acid-base interactions of octaphenyltetrapyrazinoporphyrazine involve primarily nitrogen atoms in the pyrazine fragments, while its lutetium(III) complex undergoes protonation at the meso-nitrogen atoms.     

The effect of medium acidity on the rate of protonation of 6-dimethylaminofulventricarbonyl-chromium, -molybdenum, -tungsten and comparison of their basicity

Protonation of 6-dimethylaminofulvenic complexes of chromium (I), molybdenum (II) and tunsten (III) carbonyls in trifluoroacetic and acetic acid solutions of differing acidity in methylene chloride has been studied using PMR and IR spectroscopy. It has been established that the complexes are protonated at the metal atom and that the protonation rate is linearly dependent on the acidity of the medium. Comparison of the basicities of complexes I–III has shown that their basicity ratio is I/II/III ? 1/150/1800.     

Protonation, deprotonation, and protonation: conjugated photochemical reactions of ferrocenylazophenol

4-Ferrocenylazophenol (FAP) and 8-(4-ferrocenylazophenoxy)octanyl-1-bromide (FAOB) have been synthesized and their protonation and photochemical behavior have been investigated. FAP showed reversible protonation and deprotonation behavior in the presence of trifluoroacetic acid and potassium tert-butylate, respectively. FAP and FAOB exhibit high photoreactivity, which was induced through excitation of not only the pi-pi* band by UV light but also the d-pi* band by green light. The photoreaction of FAP and FAOB in ethanol or acetonitrile with a trace amount of water afforded phenylhydrazonocyclopentadiene (PHP) derivatives, indicating that hydrolysis of the photoexcited species occurs.     

Complexation of crown-containing butadienyl dyes with alkali and alkaline earth metal cations in the ground and excited electron states

Spectrophotometry and steady-state fluorimetry were used for the study of complexation of crown-containing butadienyl dyes with alkali and alkaline earth metal cations in acetonitrile, as well as protonation of these dyes with trifluoroacetic acid. Complexation of the compounds studied with the metal cations leads to the 1: 1 product, whereas protonation with trifluoroacetic acid affords the 1: 2 product containing 2 moles of the acid per 1 mole of the dye. Stability constants of the complexes are varied from 10 to 10⁶ L mol⁻¹, basicity of the crown-containing dyes in the reaction with trifluoroacetic acid increases with the increase in the macrocycle size. On complexation, the fluorescence spectra are shifted considerably less than the absorption spectra. This indicates that photorecoordination of the cation in the complex excited molecules occurred. Based on the correlation of the spectral shifts with the metal cation charge density, three types of complexes differing in the extent of influence of the cation charge on the spectral shifts can be singled out: “tight”, “loose”, and “solvent-separated”.     

Protonation of 2-quinolylthiazoles in the ground and excited states in ethanol

The protonation of some 2-quinolylthiazoles by trichloroacetic, trifluoroacetic, and sulfuric acids in the ground and excited states was studied in ethanol. The formation of the cations was found in the reaction with sulfuric acid. The formation of a mixture of cations and ionic pairs was found in the reactions with trichloroacetic and trifluoroacetic acids. The spectral-luminescence parameters of the reaction products obtained and the kinetic characteristics of the processes were determined.     

Enantioselective hydrogenation over cinchona-modified Pt: the special role of carboxylic acids

The influence of acetic acid (AcOH) and trifluoroacetic acid (TFA) on the hydrogenation of ethyl-4,4,4-trifluoroacetoacetate has been investigated by using Pt/Al(2)O(3) modified by cinchonidine and O-methylcinchonidine. We have shown that the sometimes dramatic changes in enantioselectivity and rate cannot simply be interpreted by protonation of the alkaloid modifier. We propose a new three-step reaction pathway, involving interaction of the carboxylic acid with the reactant and the chiral modifier. The mechanism is supported by IR spectroscopic identification of cyclic TFA-modifier ion pairs. This new approach can rationalise the poorly understood role of acids in the enantioselective hydrogenation of activated ketones over cinchona-modified platinum metals.     

Trifluoromethylation of enamines under acidic conditions

A method for the trifluoromethylation of enamines using Me₃SiCF₃ leading to α-CF₃-substituted amines is described. The reaction is promoted by hydrofluoric acid generated from KHF₂ and either trifluoroacetic or triflic acid, and involves protonation of the enamine followed by transfer of the CF₃-carbanion from the silicon reagent to the cationic electrophile.     

The 31P NMR spectra of ArCr(CO)2P(C6H5)3 compounds in neutral and acidic media

The ³¹P NMR spectra of C₆H₅XCr(CO)₂P(C₆H₅)₃ (X = H, CH₃, OCH₃, N(CH₃)₂, COOCH₃) (I), p-C₆H₄X₂Cr(CO)₂P(C₆H₅)₃ (X = COOCH₃)(II) and C₆H₃X₃Cr(CO)₂P(C₆H₅)₃ (X = CH₃) (III) complexes in neutral and acidic media were investigated. The protonation of complexes I and III in trifluoroacetic acid results in the greater upfield shielding of ³¹P{¹H} signal. In this case the complexes I (X = H, CH₃, OCH₃) are completely protonated at the metal, complex I (X = COOCH₃)is partially protonated, while no protonation occurs in the case of complex II.Temperature-dependence of the ³¹P{¹H} NMR spectra was investigated for complexes I (X = H, OCH₃) in a 1/10 mixture of trifluoroacetic acid and toluene and for complexes I (X = COOCH₃) and II in trifluoroacetic acid. The degree of protonation was found to increase with decreasing temperature.     

Synthesis of (E)- and (Z)-alpha-alkylidene-gamma-aryl-gamma-butyrolactones via alkenylalumination of oxiranes

Alkenylalumination of substituted styrene oxides with [alpha-(ethoxycarbonyl)alkenyl]diisobutylaluminum, in the presence of BF(3).Et(2)O, affords the corresponding (Z)-alpha-alkylidene-gamma-aryl-gamma-hydroxy esters in 81-100% Z-selectivity. Chromatographic separation of isomers, followed by lactonization with trifluoroacetic acid, provides isomerically pure (Z)-alpha-alkylidene-gamma-aryl-gamma-butyrolactones in 53-78% overall yield. Isomerization of the (Z)-alkylidene hydroxyl esters using LDA, followed by protonation using a bulky proton source, such as BHT, provides a simple route to the corresponding alpha-(E)-alkylidene-gamma-phenyl-gamma-hydroxy esters in 72-78% yield, which were cyclized to obtain the corresponding (E)-butyrolactones in 78-85% yield.     

Acid-catalyzed isomerization of pivalaldehyde to methyl isopropyl ketone via a reactive protosolvated carboxonium ion intermediate.

Quantitative rearrangement of pivalaldehyde to methyl isopropyl ketone is observed in acids such as trifluoromethanesulfonic acid, anhydrous HF, and trifluoroethyl alcohol-BF3 but not in trifluoroacetic acid. Studies in a mixture of trifluoroacetic acid and trifluoromethanesulfonic acid show that acids with H(o) < or = -11 are able to carry out complete isomerization. These results and density functional theory calculations at the B3LYP/6-31G level suggest that protonated pivalaldehyde undergoes further protosolvation at higher acidities to a reactive superelectrophilic species resulting in rearrangement. A mechanism for the pivalaldehyde rearrangement to methyl isopropyl ketone in strong protic acids involving a reactive protosolvated superelectrophilic intermediate is suggested. Aspects of the related mechanism of the reaction with isobutane with CO in HF/BF3 medium leading to methyl isopropyl ketone are also discussed.