Ammonia IRMS-TPD measurements on Brønsted acidity of proton-formed SAPO-34

By utilizing the advantages of a combined method of IRMS-TPD of ammonia and DFT calculations, the solid acidity of HSAPO-34 was studied. The number, strength and structure of the Br?nsted OH were measured experimentally. The quantitative measurements and DFT calculations supported the identification of Br?nsted OH to account for the generation model of the Br?nsted OH primarily located in the edge of the Si domain (island). The acid strength of SAPO-34 was slightly weaker than that of chabazite, a zeolite with the same structure. Thus, some important insights were obtained to understand the acid site generation of SAPO-34.     

Activation of H2O2 by chiral confined Brønsted acids: a highly enantioselective catalytic sulfoxidation

Confined chiral Br?nsted acids are shown to catalyze asymmetric oxidations of sulfides to sulfoxides with hydrogen peroxide. The wide generality and high enantioselectivity of the developed method compare even to the best metal-based systems and suggest utility in other asymmetric oxidations.     

Interpretation of Brønsted acidity by triadic paradigm: a G3 study of mineral acids

Deprotonation enthalpies and the gas-phase acidities of 24 inorganic acids are calculated by using composite G3 and G2 methodologies. The computed values are in very good accordance with available measured data. It is found that the experimental DeltaH(acid) values of the FSO(3)H and CF(3)SO(3)H are too high by some 6 and 7 kcal mol(-1), respectively. Furthermore, a new DeltaH(acid) value for HClO(4) of 300 kcal mol(-1) is recommended and suggested as a threshold of superacidicity in the gas phase. The calculated deprotonation enthalpies are interpreted by employing the trichotomy paradigm. Taking into account that the deprotonation enthalpy is a measure of acidity, it can be safely stated that the pronounced acidities of mineral acids are to a very large extent determined by Koopmans' term with very few exceptions, one of them being H(2)S. To put it in another way, acidities are predominantly a consequence of the ability of the conjugate bases to accommodate the excess electron charge, since Koopmans' term in trichotomy analysis is related to conjugate base anion. The final state is decisive in particular for superacids like ClSO(3)H, CF(3)SO(3)H, HClO(4), HBF(4), HPF(6), HAlCl(4), and HAlBr(4). However, in the latter two molecules the bond dissociation energy of the halogen-H bond substantially contributes to their high acidity too. Therefore, acidity of these two most powerful superacids studied here is determined by cooperative influence of both initial and final state effects. It should be emphasized that acidity of hydrogen halides HCl and HBr is a result of concerted action of all three terms included in triadic analysis. A byproduct of the triadic analysis are the first adiabatic ionization energies of the anionic conjugate bases. They are in fair to good agreement with the experimental data, which are unfortunately sparse. A fairly good qualitative correlation is found between the gas-phase deprotonation enthalpies of six mineral O-H acids and available Hammett-Taft sigma(p)- constants of the corresponding substituent groups.     

A comprehensive self-consistent spectrophotometric acidity scale of neutral Brønsted acids in acetonitrile

For the first time, the self-consistent spectrophotometric acidity scale of neutral Br?nsted acids in acetonitrile (AN) spanning 24 orders of magnitude of acidities is reported. The scale ranges from pK(a) 3.7 to 28.1 in AN. The scale includes 93 acids that are interconnected by 203 relative acidity measurements (DeltapK(a) measurements) and contains compounds with gradually changing acidities, including representatives from all of the conventional families of OH (alcohols, phenols, carboxylic acids, sulfonic acids), NH (anilines, diphenylamines, disulfonimides), and CH acids (fluorenes, diphenylacetonitriles, phenylmalononitriles). The CH acids were particularly useful in constructing the scale because they do not undergo homo- or heteroconjugation processes and their acidities are rather insensitive to traces of water in the medium. The scale has been fully cross-validated: the relative acidity of any two acids on the scale can be found by combining at least two independent sets of DeltapK(a) measurements. The consistency standard deviation of the scale is 0.03 pK(a) units. Comparison of acidities in many different media has been carried out, and the structure-acidity relations are discussed. The large variety of the acids on the scale, its wide span, and the quality of the data make the scale a useful tool for further acidity studies in acetonitrile.     

Direct thioesterification from carboxylic acids and thiols catalyzed by a Brønsted acid

In the presence of a catalytic amount of trifluoromethanesulfonic acid, free carboxylic acids reacted with free thiols directly to afford the corresponding thioesters in high yields.     

Ring-opening polymerisation of ɛ-caprolactone catalysed by Brønsted acids

Two new Brønsted acids [2,2′-ethylidene-bis (4,6-di-tert-butylphenol)] phosphoric acid (EDBPPOOH) and (3,3′,5,5′-tetra-tert-butylbiphenyl-2,2′-diol) phosphoric acid (TBPO-POOH) were synthesised and fully characterised by ¹H NMR and ¹³C NMR spectra and mass spectra. The ringopening polymerisation (ROP) of ?-caprolactone (?-CL) catalysed by the two Brønsted acids proceeded at 110°C without a solvent or at ambient temperature in toluene. Experimental results indicated that the two Brønsted acids were efficient catalysts for the ROP of ?-CL with moderate number-average molar mass (Mn) and narrow polydispersity indices (PDI). The catalytic activity of TBPO-POOH is higher than EDBP-POOH in the ROP of ?-CL. After benzyl alcohol was added, it was able to accelerate the polymerisation process. The polymerisation can also occur with the addition of water with a monomer/catalyst/initiator mole ratio of 100: 1: 1. The living polymerisation was ascertained by the linear relationships of the Mn vs. monomer conversion, then it was further confirmed by a second-feed experiment of a double monomer producing double Mn. A kinetic study of the relationships between monomer concentration and time revealed a first-order dependence on monomer concentration in the polymerisation. End-group analysis of ¹H NMR spectra and electrospray-ionisation mass spectra suggests that the two Brønsted acids are capable of catalysing and initiating the ROP of ?-CL.     

Mutable Lewis and Brønsted acidity of aluminated SBA-15 as revealed by NMR of adsorbed pyridine-15N

(1)H and variable-temperature (15)N NMR techniques have been used to study the effect of the gradual alumination of SBA-15 on the structure and adsorption properties of this mesoporous material. The interpretation of experimental spectra suggests that aluminum chlorhydrol most effectively reacts with silica surfaces in the confinement of the cavities of rough mesopore walls, instead of forming a homogeneous aluminum film. This first leads to a gradual filling of the cavities and finally results in aluminum islands on the inner surfaces of mesopores. In the sample with a Si/Al atomic ratio of 4.1, up to half of the inner surface area of the mesopores is covered with aluminum. The alumination produces Br?nsted acid sites attributed to silanol groups interacting with aluminum but does not affect the proton-donating ability of isolated silanol groups. At high Si/Al ratios, the surface contains only one type of Lewis site attributed to tetracoordinated aluminum. At lower Si/Al ratios, Lewis acid sites with a lower electron-accepting ability appear, as attributed to pentacoordinated aluminum. The numerical values of the surface densities of all chemically active sites have been estimated after annealing at 420 and 700 K. We were surprised to observe that gaseous nitrogen can occupy Lewis acid sites and hinder the interaction of the aluminum with any other electron donor. As a result, aluminated surfaces saturated with nitrogen do not exhibit any Br?nsted or Lewis acidity. At room temperature, it takes days before pyridine replaces nitrogen at the Lewis acid sites.     

Staining of fluid-catalytic-cracking catalysts: localising Brønsted acidity within a single catalyst particle

A time‐resolved in situ micro‐spectroscopic approach has been used to investigate the Brønsted acidic properties of fluid‐catalytic‐cracking (FCC) catalysts at the single particle level by applying the acid‐catalysed styrene oligomerisation probe reaction. The reactivity of individual FCC components (zeolite, clay, alumina and silica) was monitored by UV/Vis micro‐spectroscopy and showed that only clay and zeolites (Y and ZSM‐5) contain Brønsted acid sites that are strong enough to catalyse the conversion of 4‐fluorostyrene into carbocationic species. By applying the same approach to complete FCC catalyst particles, it has been found that the fingerprint of the zeolitic UV/Vis spectra is clearly recognisable. This almost exclusive zeolitic activity is confirmed by the fact that hardly any reactivity is observed for FCC particles that contain no zeolite. Confocal fluorescence microscopy images of FCC catalyst particles reveal inhomogeneously distributed micron‐sized zeolite domains with a highly fluorescent signal upon reaction. By examining laboratory deactivated FCC catalyst particles in a statistical approach, a clear trend of decreasing fluorescence intensity, and thus Brønsted acidity, of the zeolite domains is observed with increasing severity of the deactivation method. By comparing the average fluorescence intensities obtained with two styrenes that differ in reactivity, it has been found that the Brønsted acid site strength within FCC catalyst particles containing ZSM‐5 is more uniform than within those containing zeolite Y, as confirmed with temperature‐programmed desorption of ammonia.     

Direct enantioselective Brønsted acid catalyzed N-acyliminium cyclization cascades of tryptamines and ketoacids

A direct enantio- and diastereoselective N-acyliminium cyclization cascade through chiral phosphoric acid catalyzed condensation of tryptamines with γ- and δ-ketoacid derivatives to provide architecturally complex heterocycles has been developed. The reaction is technically simple to perform, atom-efficient, and broad in scope. Employing 10 mol % of (R)-BINOL derived chiral phosphoric acids in refluxing toluene allowed the polycyclic product materials to be generated in good yields (53-99%) and moderate to high enantioselectivities (68-98% ee).     

Dynamic kinetic resolution of azlactones catalyzed by chiral Brønsted acids

Chiral Br?nsted acids have been shown for the first time to catalyze the dynamic kinetic resolution of azlactones. 3,3'-Bis-(9-anthryl)-BINOL phosphoric acid 3c is particularly effective in the case of 4-aryl-substituted substrates, producing 85-92% ee's.     

Asymmetric dearomatization enabled by chiral Brønsted acid activation of ynamides

Science China Chemistry -     

Dual Brønsted acid/nucleophilic activation of carbonylimidazole derivatives

Carbonylimidazole derivatives have been found to be highly active acylation reagents for esterification and amidation in the presence of pyridinium salts. These reactions are thought to involve both Br?nsted acid and nucleophilic catalysis. This mode of activation has been applied to the synthesis of difficult to access oxazolidinones, as well as esters and amides. Finally, the use of pyridinium salts has been shown to accelerate the esterification of carboxylic acids with imidazole carbamates.     

Regioselective Hydration of Alkynes by Iron(III) Lewis/Brønsted Catalysis

The triflimide iron(III) salt [Fe(NTf(2) )(3) ] promotes the direct hydration of terminal and internal alkynes with very good Markovnikov regioselectivities and high yields. The enhanced carbophilic Lewis acidity of the Fe(III) cation mediated by the weakly-coordinating triflimide anion is crucial for the catalytic activity. The iron(III) metal salt can be recycled in the form of the OPPh(3) /[Fe(NTf(2) )(3) ] system with similar activity and selectivity. However, spectroscopic and kinetic studies show that [Fe(NTf(2) )(3) ] hydrolyzes under the reaction conditions and that catalytically less active Br?nsted species are formed, which points to a Lewis/Br?nsted co-catalysis. This triflimide-based catalytic system is regioselective for the hydration of internal aryl-alkynes and opens the door to a new synthetic route to alkyl ketophenones. As a proof of concept, the synthesis of two antipsychotics Haloperidol and Melperone, with general butyrophenone-like structure, is shown.     

Direct catalytic asymmetric aminoallylation of aldehydes: synergism of chiral and nonchiral Brønsted acids

The development of a catalytic asymmetric method for the direct aminoallylation of aldehydes is described that gives high asymmetric inductions for a broad range of substrates including both aromatic and aliphatic aldehydes. This method allows for direct isolation of unprotected analytically pure homoallylic amines without chromatography. The unique catalyst system developed for this process involves the synergistic interaction between a chiral and a nonchiral Br?nsted acid.     

Enantioselective oxysulfenylation and oxyselenenylation of olefins catalyzed by chiral Brønsted acids

This paper describes Brønsted acid catalyzed enantioselective oxysulfenylation and oxyselenenylation of olefins. Enantiomerically enriched tetrahydrofurans are formed with up to 63% ee with dibenzoyl-d-tartaric acid and its derivatives as catalyst. Chiral β-carboxyl sulfides and selenides have also been obtained with up to 50% and 84% ee, respectively, via enantioselective desymmetrization of thiiranium and seleniranium ions in the presence of catalytic amounts of chiral binaphthol derived N-triflyl phosphoramide.