Acid-catalyzed Transformations Research Articles

Preparation and applications of novel fluoroalkyl end-capped sulfonic acid oligomers–silica gel polymer hybrids

Fluoroalkyl end-capped 2-methacryloxyethanesulfonic acid homo-oligomer [RF–(MES)n–RF] and 2-methacryloxyethanesulfonic acid–N,N-dimethylacrylamide co-oligomers [RF–(MES)x–(DMAA)y–RF] reacted with tetraethoxysilane (TEOS) under acidic conditions to afford RF–(MES)n–RF homo-oligomer–SiO2 polymer hybrid and RF–(MES)x–(DMAA)y–RF co-oligomer–SiO2 polymer hybrid, respectively. Thermogravimetric–mass spectra showed that the thermal stability of RF–(MES)n–RF homo-oligomer–SiO2 polymer hybrid was superior to that of traditionally well-known perfluorinated ion exchange polymers such as Nafion 112 (TR). The sol solutions of the fluorinated co-oligomer–SiO2 polymer hybrid were applied to the surface modification of glass to exhibit not only a strong oleophobicity imparted by fluorine but also a good hydrophilicity on the glass surface. On the other hand, RF–(MES)x–(DMAA)y–RF co-oligomer reacted with TEOS in the presence of a variety of silica nanoparticles (mean diameters: 11–95 nm) under alkaline conditions to afford fluoroalkyl end-capped oligomers–silica nanoparticles (mean diameters: 32–173 nm) with a good dispersibility and stability in methanol. Similarly, a variety of fluorinated oligomers containing sulfo groups–silica nanoparticles were prepared by the homo- and co-oligomerizations of fluoroalkanoyl peroxides with 2-methacryloxyethane sulfonic acid (MES) and comonomers such as N,N-dimethylacrylamide (DMAA) and acryloylmorpholine (ACMO) in the presence of silica nanoparticles. Interestingly, these isolated fluorinated particle powders were found to afford nanometer size-controlled colloidal particles with a good redispersibility and stability in aqueous and organic media such as methanol. These fluorinated nanoparticles containing sulfo groups were also applied to an excellent heterogeneous catalyst for Bronsted acid-catalyzed transformations. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 110–117, 2007

Influence of substitution on kinetics and mechanism of ring transformation of substituted S-[1-phenylpyrrolidin-2-on-3-yl]isothiuronium salts.

Twelve new substituted S-(1-phenylpyrrolidin-2-on-3-yl)isothiuronium bromides and twelve corresponding 2-imino-5-(2-phenylaminoethyl)thiazolidin-4-ones have been prepared and characterised. Kinetics and mechanism of transformation reaction of S-[1-(4-methoxyphenyl)pyrrolidin-2-on-3-yl]isothiuronium bromide and its N,N-dimethyl derivative 5a into corresponding substituted thiazolidin-4-ones 2a and 6a have been studied in aqueous solutions of amine buffers (pH 8.1-11.5) and sodium hydroxide solutions (0.005-0.5 mol l(-1)) at 25 degrees C and at I= 1 mol l(-1) under pseudo-first-order reaction conditions. The kinetics observed show that the transformation reaction is subject to general acid-base, and hydroxide ion catalyses. Acid catalysis does not operate in the transformation of 1a; the rate-limiting step of the base-catalysed transformation is the decomposition of bicyclic tetrahedral intermediate In(+/-) and the Brønsted dependence is non-linear (pK(a) approximately 9.8). In the case of derivative 5a both base and acid catalyses make themselves felt. In the base catalysis, the rate-limiting step consists of the decomposition of bicyclic intermediate In, and the Brønsted dependence is linear (beta = 0.9; pK(a) > 11.5). The acid-catalysed transformation of 5a also proceeds via the intermediate In, and the reaction is controlled by diffusion (alpha approximately equal to 0). With compound 5a in triethylamine and butylamine buffers, the general base catalysis changes into specific base catalysis. The effect of substitution in aromatic moiety of compounds 1a-h and 3a-h on the course of the transformation reaction has been studied in solutions of sodium hydroxide (0.005-0.5 mol l(-1)) at 25 degrees C by the stopped-flow method. The electron-acceptor substituents 4-NO(2) and 4-CN do not obey the Hammett correlation, which is due to a suppression of cross-conjugation in the ring-closure step of the transformation reaction.

Acid-Catalyzed Intra- and Intermolecular Acyl Exchabge in Mono- and Diglycerides

Acid-catalyzed transformation of 1-monolauroylglycerine (1-MLG) was investigated. It has been demonstrated that 1-MLG disproportionates readily and quickly into a mixture of 1,2- and 1,3-diacylglycerines and glycerine. The transformation of 1,3-diglycerides into 1,2-diglycerides was studied.

Acid-catalyzed transformation of rubrene to indenonaphthacene and its paired interacting orbital (PIO) analysis

Treatment of rubrene (5,6,11,12-tetraphenylnaphthacene) with trifluoroacetic acid in dichloromethane gives 4b,9,10-triphenyl-4b,9-dihydroindeno[1,2,3- fg]naphthacene ( 1 ) as the sole isolable product. Paired interacting orbital (PIO) analysis indicates that the reaction is initiated by preferential attack of H + at C(11) position of rubrene.

Solvation as a Main Factor That Determines the Strength of Liquid Superacids and the Selectivity of the Acid‐Catalyzed Reactions of Olefins

AbstractFor Abstract see ChemInform Abstract in Full Text.

Acid-Catalyzed Trimerization of Acetaldehyde: A Highly Selective and Reversible Transformation at Ambient Temperature in a Zeolitic Solid

Acetaldehyde is shown to undergo a reversible Brønsted acid-catalyzed cyclotrimerization reaction, with 100% selectivity, at ambient temperature within the zeolite host material ferrierite. It is shown by in situ solid-state 13C NMR spectroscopy and other techniques that the cyclic trimer is the only product formed in this reaction. The equilibrium proportions of acetaldehyde and the cyclic trimer at ambient temperature correspond to a conversion greater than 90%. In contrast, in the corresponding acid-catalyzed transformation of acetaldehyde in the liquid state, a broad distribution of products is obtained. The reversibility of the cyclotrimerization reaction in ferrierite is confirmed from the fact that, on adsorption of a pure sample of the cyclic trimer within ferrierite, a reaction occurs to produce acetaldehyde as the only product with the same equilibrium distribution of the cyclic trimer and acetaldehyde as that obtained from the reaction of acetaldehyde in ferrierite. The role of Brønsted acid catalysis in the transformation between acetaldehyde and the cyclic trimer in ferrierite is confirmed from the fact that no reaction occurs on adsorption of acetaldehyde within sodium-exchanged ferrierite.

Solvation as a main factor that determines the strength of liquid superacids and the selectivity of the acid-catalyzed reactions of olefins

Analysis of literature and ab initio quantum chemical calculations indicates that the solvation of protons in anhydrous H 2SO 4 or HF is considerably weaker than in aqueous solutions. This results in very low constants of autoprotolysis of anhydrous superacids. In contrast, hydration of protons in moderately concentrated acids is much stronger, resulting in higher dissociation constants. Therefore, the strength of superacids is connected to the high chemical activity of the weakly solvated protons, rather than to the amount of protons. In a similar way, solvation of protonated active intermediates of the acid-catalyzed transformations of olefins in anhydrous superacids is also weaker than in aqueous solutions. This results in the real carbenium ion mechanisms of the isoparaffin–olefin alkylation or “conjunct oligomerization” of olefins. In contrast, solvation with water in moderately concentrated or dilute sulfuric acid transforms alkyl carbenium ions into oxonium ions. Therefore, cationic polymerization of olefins in moderately concentrated sulfuric acid is not a real carbenium ion reaction. Similar to the acid-catalyzed transformations of olefins on zeolites, it involves oxonium ions as the stable active intermediates and alkyl carbenium ions as transition states.

SOLVATION EFFECTS IN CATALYTIC TRANSFORMATIONS OF OLEFINS IN SULFURIC ACID

Analysis of literature and ab initio quantum chemical calculations indicate that the properties of anhydrous H2SO4 as a superacid are determined not by the extent of dissociation of this acid but by the unusually high chemical activity of the weakly solvated protons. In a similar way, the selectivity of the acid-catalyzed transformations of olefins is also determined by solvation of the active intermediates. Depending on the concentration of sulfuric acid, the properties of the active species are changing from almost free carbenium ions in the anhydrous acid to the hydrated oxonium ions in the moderately dilute sulfuric acid. In the later case, similar to zeolites, the carbenium-ion-like alkyl species are formed only as transition states. The subsequent hydrolysis of oxonium ions upon stronger dilution of the acid results in the formation of alcohols. These conclusions are supported by ab initio quantum chemical calculations and by in situ 13C-NMR of protonated active intermediates.

FTIRS study of the nature and reactivity of the products of nitromethane transformation on the surface of copper- and cobalt-exchanged ZSM-5 zeolites as related to the mechanism of NO x CH 4-SCR

For Cu- and Co-ZSM-5, at temperatures around 300°C, the adsorption of nitromethane followed by its acid-catalyzed transformation into isocyanates and melamine as well as the reactivity of these species with respect to oxygen and NO were studied by FTIRS. Both species easily interact with molecular oxygen, while no reaction with NO was observed. In the course of melamine oxidation, partial oxidation products including isocyanates appear to be produced. The reactivity of melamine and isocyanate depends upon the temperature and nature of transition metal cation, being reasonably high to suggest their role as intermediates in the NO x CH 4-SCR.

Adsorbed carbocations as transition states in heterogeneous acid catalyzed transformations of hydrocarbons

Ab initio quantum chemical calculations indicated that adsorbed carbenium and carbonium ion active intermediates of acid-catalyzed transformations of hydrocarbons on zeolites are not the really existing highly reactive species but the transition states of the corresponding elementary steps. Adsorbed carbenium ion-like activated complexes can be formed both via proton addition to the double bonds of olefins or as energetically excited unstable ion pairs resulting from partial dissociation of the carbonyl bonds in more stable alkoxy species. In contrast, the highly energetically excited adsorbed carbonium ion-like transition states result only from proton attack at the C–C or C–H bonds of paraffins. The quantum chemical calculations provided the information on geometry and electronic structure of these activated complexes which depend on the elementary reactions in which these transition states are involved. The calculated heat effects and activation energies for the main elementary steps in acid catalyzed transformations of hydrocarbons on zeolites, i.e. of double bond shift, skeletal isomerization and cracking of olefins or protolytic dehydrogenation, protolytic cracking of paraffins and hydride transfer from isoparaffins to carbenium ions are in a reasonable agreement with the experiment.

Preparation, Characterization, and Acidity of a Silica Gel/Tungsten Oxide Solid Acid

Solid acidity is measured by simultaneously solving data from a calorimetric titration and an adsorption isotherm measurement to provide the amount, equilibrium constant, and enthalpy for each acid site. The utility of this procedure, referred to as cal-ad, in the synthesis of solid acids is illustrared with the preparation and characterization of a new hydrated tungsten oxide−silica-supported acid catalyst, referred to as SG-W. Cal-ad shows that SG-W, prepared from WCl6, is a stronger acid with more Bronsted sites per gram than tungsten oxide, tungstic acid, or silica-supported WO3 prepared by thermal decomposition of ammonium tungstate (SG-WO3). Surface studies show a much better dispersion of tungsten in SG-W than in SG-WO3. SG-W adds to a growing series of reproducible, characterized solid acids of varying strengths. A quantitative cal-ad series of solid acid strengths is given to provide a new scale for correlating acidity with catalytic activity and to establish acidity thresholds for different acid-catalyzed transformations. Solid acids with varying acidities are compared as catalysts for alcohol dehydration, light paraffin isomerization, and the alkylation of phenol with methanol to illustrate this approach.

Intramolecular Photochemical Reactions of Bichromophoric 3-(Alkenyloxy)phenols and 1-(Alkenyloxy)-3-(alkyloxy)benzene Derivatives. Acid-Catalyzed Transformations of the Primary Cycloadducts

Irradiation of 3-(alkenyloxy)phenols and 1-(alkenyloxy)-3-(alkyloxy)benzene derivatives, at λ = 254 nm in acidic media, yields benzocyclobutenes, 3-alkylphenols, 3-alkylanisols, and 4-alkyl-1,2-dialkyloxybenzenes depending on the substitution pattern of the aromatic ring and the olefinic side chain. The final products are derived from an intramolecular [2 + 2] photocycloaddition and acidic rearrangements from a common intermediate.

The characterization of the surface properties of VPO-based catalysts by probe molecules

The surface properties of the VPO-based catalysts are discussed referring to their reactivity towards different organic substrates. Various saturated and unsaturated hydrocarbons were fed under aerobic and anaerobic conditions, and the results were discussed in terms of the main surface characteristics of the active component, the vanadyl pyrophosphate (VO) 2P 2O 7. The combination of acid and oxygen-insertion properties of the vanadyl pyrophosphate makes several contemporaneous reactions possible, and the final products distribution is a function of (i) the reaction conditions, which also affect the average oxidation state of vanadium, (ii) the extent of the interaction between the hydrocarbon and the surface, and (iii) the tendency of the activated hydrocarbon to undergo acid-catalysed transformations rather than oxidative ones.

Zeolite catalysis studied by radiolysis/EPR. Transformations of cyclic olefins

The Bronsted acid-catalyzed transformations of cyclic olefins in zeolites HZSM5 and H-mordenite were elucidated by the radiolysis/EPR method. Products of catalytic reactions were spin- labeled by radiolytic means and identified by using EPR spectroscopy. Comparison of reactions on HZSM5 and H-mordenite reveals strong shape selectivity on reactions in these catalysts. The radiolysis/EPR method is a new in situ spectroscopy with excellent sensitivity and structural specificity for the study of reaction mechanisms in zeolite catalysts.

Zinc(II)-catalysed transformation of epoxides to aziridines

The Lewis acid-catalysed transformation of epoxides to aziridines with iminophosphoranes as the nitrogen-fragment donor has been investigated. Of the Lewis acids tested, zinc(II) complexes had the best catalytic properties. The method works best for terminal and cyclic epoxides, internal epoxides being less reactive. Of the various iminophosphoranes employed N-(triphenylphosphoranylidene)-aniline and -isopropylamine were the most successful. The zinc(II)-catalysed reaction has been studied for chiral styrene oxides for which the enantiomeric excess of the aziridine produced is dependent on the reaction time. The reaction of achiral and chiral styrene oxides and N-(triphenylphosphoranylidene)aniline in the presence of a zinc(II) complex having a chiral ligand has been investigated as has the reaction for cis-deuteriostyrene oxide in order to obtain information about the stereochemical outcome of the reaction. A mechanism for the title reaction is discussed on the basis of the experimental results.

Kinetic scheme of homogeneous acid-catalyzed transformation of isopentenols

The reactions occurring in an equilibrium mixture of 3-methyl-1-buten-3-ol and 3-methyl-2-buten-1-ol in 24–49 % aqueous solutions of H2SO4 yield isoprene, 3-methyl-3-buten-1-ol, isobutylene, formaldehyde, 3-methylbutane-1,3-diol. Isobutylene is rapidly hydrated to give 2-methylpropan-2-ol. The presence of formaldehyde in the reaction mixture indicates that the transformations involve the reverse Prins reaction. On the basis of experimental and literature data, two most probable reaction schemes were suggested.

Acid-catalysed transformation of oleic acid, and 12-hydroxystearic acid, into γ-octadecanolactone. Evidence for the occurrence of a novel side reaction

γ-Octadecanolactone is produced in the reaction between oleic acid (cis-9-octadecenoic acid) and concentrated sulfuric acid at 80 °C. To account for its formation, two types of mechanism have been considered. Both begin with protonation at C-9/C-10 to give a carbocation, following which the positive charge is transmitted to C-4, where nucleophilic attack by the carboxyl group is facilitated. One of them invokes a "zipperlike" succession of 1,2-shifts of intervening methylenic protons, whereas the other amounts to a stepwise migration of the alkene function along the carbon chain. Results obtained through the use of deuterated sulfuric acid to catalyse formation of the lactone allow for the existence of both types. However, each is characteristic of a different region of the C18 structure. Of the total isotope incorporated (individual molecules contained from zero to nine atoms of deuterium), >85% had been distributed relatively uniformly over the chain segment from C-7 to C-17, according to 2H nuclear magnetic resonance spectra recorded in the presence of the shift reagent Yb(fod)3, and 13C nuclear magnetic resonance data. This evidence is consistent with the occurrence of a side reaction that creates many protonation sites over that specific region of the lactone's precursors. By contrast, as very little of the isotope had reached the proximity of the lactone ring, the latter is more likely to have been formed via the 1,2 shift type of mechanism. These events can also be initiated at other sites, as shown by the preparation of γ-octadecanolactone, and a comparably deuterated form of it, from 12-hydroxystearic acid. Reactions of these three stearic acid derivatives with HI, HBr, and HCl are described, by way of comparison.

Cyclization and Rearrangements of Farnesol and Nerolidol Stereoisomers in Superacids

Acid-catalyzed cyclization of 2,3-trans- and 2,3-cis-farensol proceeds regioselectively and stereospecifically, yielding drimenol and epi-drimenol, respectively. The trans and cis isomers of nerolidol undergo carbo- and heterocyclization reactions. The trans isomer gives tricyclic caged hydrocarbons with new skeletal types, while the rearrangements of the cis isomer produce derivatives of 2-oxabicyclo[4.4.0]decane. The ICAR computer program was used to derive reasonable mechanisms for the acid-catalyzed transformations of nerolidol, and the probability of the mechanisms was evaluated by the molecular mechanics method

Acid-catalysed transformation of α-trifluoromethanesulfonates of γ- and δ-lactones into 2,5-disubstituted homochiral tetrahydrofurans

Excellent yields of methyl tetrahydrofuran-α-carboxylates are obtained by treatment of 2-O-trifluoromethanesulfonates (triflates) of either 1,4-or 1,5-lactones with acid in methanol; in some cases, very different tetrahydrofurans may result from acid or base treatment of a methanolic solution of the same starting material.

(H+,K+)-ATPase inhibiting 2-[(2-pyridylmethyl)sulfinyl]benzimidazoles. 4. A novel series of dimethoxypyridyl-substituted inhibitors with enhanced selectivity. The selection of pantoprazole as a clinical candidate.

[(Pyridylmethyl)sulfinyl]benzimidazoles 1 (PSBs) are a class of highly potent antisecretory (H+,K+)-ATPase inhibitors which need to be activated by acid to form their active principle, the cyclic sulfenamide 4. Selective inhibitors of the (H+,K+)-ATPase in vivo give rise to the nonselective thiophile 4 solely at low pH, thus avoiding interaction with other thiol groups in the body. The propensity to undergo the acid-catalyzed transformation is dependent on the nucleophilic/electrophilic properties of the functional groups involved in the formation of 2 since this step is both rate-determining and pH-dependent. The aim of this study was to identify compounds with high (H+,K+)-ATPase inhibitory activity in stimulated gastric glands possessing acidic pH, but low reactivity (high chemical stability) at neutral pH as reflected by in vitro (Na+,K+)-ATPase inhibitory activity. The critical influence of substituents flanking the pyridine 4-methoxy substituent present in all derivatives was carefully studied. The introduction of a 3-methoxy group gave inhibitors possessing a combination of high potency, similar to omeprazole and lansoprazole, but increased stability. As a result of these studies, compound 1a (INN pantoprazole) was selected as a candidate drug and is currently undergoing phase III clinical studies.