Synthesis and Structure of the Small Superelectrophile [C2(OH)2Me2]2.

The acid-activation of 1,2-dicarbonyl compounds plays a key role in a variety of electrophilic reactions, some of which are only accessible in superacidic media when a superelectrophilic dication is formed. To obtain structural and electronic information about these elusive species, the vicinal dication [C2(OH)2Me2]2+ is synthesized and characterized by Raman spectroscopy and X-ray diffraction. Since this superelectrophile could not be stabilized in convenient superacids, the usage of liquid SO2 turned out to be crucial. The experimental data are discussed together with quantum-chemical calculations on the B3LYP/aug-cc-pVTZ level of theory. Natural Bond Orbital (NBO) analyses quantify the superelectrophilic interactions found in the solid state.

Synthesis and Structure of Protonated Propiolic Acid.

Propiolic acid was investigated in the superacidic system XF/SbF5 (X = H, D). The salts of the monoprotonated species of propiolic acid were characterized by vibrational and NMR spectroscopy as well as single-crystal analyses. The rotational conformers of the protonated species can be distinguished by NMR spectroscopy via the temperature-dependent rotational barrier. In the solid state, they can be detected by H/D exchange and packing effects due to different anions. The experimental results are discussed together with an IRC calculation of the rotational barrier. After acetic acid and formic acid, this is the third protonated carboxylic acid for which the energy differences between the conformers have been determined.

Taming the Lewis Superacidity of Non-Planar Boranes: C-H Bond Activation and Non-Classical Binding Modes at Boron.

The rational design of a geometrically constrained boron Lewis superacid featuring exceptional structure and reactivity is disclosed. It enabled the formation of non-classical electron deficient B-H-B type of bonding, which was supported by spectroscopic and structural parameters as well as computational studies. Taming the pyramidal Lewis acid electrophilicity through weak coordinating anion dissociation enabled a series of highly challenging chemical transformations, such as Csp2 -H and Csp3 -H activation under a frustrated Lewis pair regime and the cleavage of Csp3 -Si bonds. The demonstration of such rich chemical behaviour and flexibility on a single molecular compound makes it a unique mediator of chemical transformations generally restricted to transition metals.

Isolation and Structural Characterization of Eightfold Protonated Octacyanometalates [M(CNH)8 ]4+ (M=MoIV , WIV ) from Superacids.

Octacyanometalates K4 [Mo(CN)8 ] and K4 [W(CN)8 ] are completely protonated in superacidic mixtures of anhydrous hydrogen fluoride and antimony pentafluoride. The resulting hydrogen isocyanide complexes [Mo(CNH)8 ]4+ [SbF6 ]- 4 and [W(CNH)8 ]4+ [SbF6 ]- 4 are the first examples of eight-coordinate homoleptic metal complexes containing hydrogen isocyanide (CNH) ligands. The complexes were crystallographically characterized, revealing hydrogen-bonded networks with short N⋅⋅⋅H⋅⋅⋅F contacts. Low-temperature NMR measurements in HF confirmed rapid proton exchange even at -40 °C. Upon protonation, ν(C≡N) increases of about 50 cm-1 which is in agreement with DFT calculations.

Investigations on Squaric Acid in Superacidic Media.

The syntheses of [OC(COX)3 ][MF6 ] and [(COX)4 ][MF6 ]2 ⋅2 HF were carried out in superacidic media XF/MF5 (M=As, Sb; X=H, D). The degree of protonation is highly dependent on the stoichiometric ratio of the Lewis acid with regard to squaric acid. The salts of diprotonated squaric acid were characterized by Raman spectroscopy and, in the case of [(COH)4 ][MF6 ]2 ⋅2 HF (M=As, Sb), by single-crystal X-ray structure analyses. [(COH)4 ][AsF6 ] crystallizes in the monoclinic space group P21 /n with two formula units per unit cell. Analysis of the vibrational spectra was achieved with the support of quantum chemical calculations of the cation [(COH)4 ]2+ ⋅4 HF on the PBE1PBE/6-311G++(3df,3pd) level of theory. Furthermore, a salt of monoprotonated squaric acid, [OC(COH)3 ][AsF6 ], was characterized by a single-crystal X-ray structure analysis. It crystallizes in the monoclinic space group P21 /n with four formula units per unit cell. The protonation of squaric acid leads to a change of the carbon skeleton, which is discussed for the entire series, starting with the dianion of squaric acid and ending with the tetrahydroxy dication.

Diprotonation of Guanidine in Superacidic Solutions.

Guanidinium chloride reacts with the superacidic solutions HF/MF5 (M=As, Sb) at a molar ratio of 1:2 under formation of the diprotonated guanidinium salts [C(NH2 )2 (NH3 )][AsF6 ]2 and [C(NH2 )2 (NH3 )][SbF6 ]2 . The compounds were characterized by using infrared and Raman spectroscopy. Furthermore, single-crystal X-ray structure analysis of the guanidinium(2+) salts [C(NH2 )2 (NH3 )][SbF6 ]2 ⋅HF, [C(NH2 )2 (NH3 )]2 [Ge3 F16 ]⋅HF, and [C(NH2 )2 (NH3 )]2 [Ge3 F16 ]⋅2 HF and the guanidinium(1+) salt [C(NH2 )3 ][SbF6 ] is reported. The discussion of the experimental data is supported by quantum-chemical calculations of the [C(NH2 )2 (NH3 )]2+ and [C(NH2 )3 ]+ ions to investigate the modification of the resonance stabilization during the protonation process at the PBE1PBE/6-311G++(3df,3pd) level of theory. The planar CN3 skeleton of the guanidinium(2+) ion has two carbon-nitrogen bonds in the range 1.286(4)-1.293(4) Šand one carbon-nitrogen bond of 1.453(4) Å, which can be explained with a decreased resonance stabilization relative to the guanidinium(1+) ion.

Superacid-Catalyzed Trifluoromethylthiolation of Aromatic Amines.

Upon activation under superacid conditions, functionalized tailor-made N-SCF3 sulfenamides served as reagents for the trifluoromethylthiolation of aromatic amines. This method has a broad substrate scope and can be used for the late-stage functionalization of complex molecules such as alkaloids or steroids. Mechanistic studies based on in situ low-temperature NMR spectroscopy revealed the involvement of dicationic superelectrophilic intermediates.

Solid-State Structure of Protonated Ketones and Aldehydes.

Protonated carbonyl compounds have been invoked as intermediates in many acid-catalyzed organic reactions. To gain key structural and electronic data about such intermediates, oxonium salts derived from five representative examples of ketones and aldehydes are synthesized in the solid state, and characterized by X-ray crystallography and Raman spectroscopy for the first time. DFT calculations were carried out on the cations in the gas phase. Whereas an equimolar reaction of the carbonyl compounds, acetone, cyclopentanone, adamantanone, and acetaldehyde, with SbF5 in anhydrous HF yielded mononuclear oxonium cations, the same stoichiometry in a reaction with benzaldehyde resulted in formation of a hemiprotonated, hydrogen-bridged dimeric cation. Hemiprotonated acetaldehyde was obtained when a 2:1 ratio of aldehyde and SbF5 was used. Experimental and NBO analyses quantify the significant increase in electrophilicity of the oxonium cations compared to that of the parent ketones/aldehydes.

Stabilizing Superacid Anions: The New [H(S4 O13 )2 ]3- Anion in the Crystal Structure of Li3 [H(S4 O13 )2 ].

The unique hydrogenium-bis-tetrasulfate anion [H(S4 O13 )2 ]3- in the crystal structure of Li3 [H(S4 O13 )2 ] (monoclinic, P21 /n (No. 14), Z=2, a=552.46(4) pm, b=939.70(6) pm, c=1876.6(1) pm, ß=97.492(3)°, V=965.9(1)⋅106  pm3 ) is the longest protonated polysulfate chain ever observed. Very strong symmetrical hydrogen bonds are a bold feature of the crystal structure. The protonation of a very weak base such as [S4 O13 ]2- and accordingly the stabilization of the first base of the superacid H2 S4 O13 is a significant success towards the still elusive polysulfuric acids.

Enolizable Carbonyls and N,O-Acetals: A Rational Approach for Room-Temperature Lewis Superacid-Catalyzed Direct α-Amidoalkylation of Ketones and Aldehydes.

An efficient catalytic room-temperature direct α-amidoalkylation of carbonyl donors, that is, ketones and aldehydes with unbiased N,O-acetals, is described. Sn(NTf2 )4 is an optimal catalyst to promote this challenging transformation at low loading and the reaction shows promising scope. A comprehensive and rational evaluation of this reaction has led to the establishment of an empirical scale of nucleophilic reactivity for a broad set of ketones that should be helpful in the synthetic design and development of carbonyl α-functionalization methods.

Hexafluoroantimonic acid catalysis: formal [3+2+2] cycloaddition of aziridines with two alkynes.

A practical method for the synthesis of azepine derivatives, a typical seven-membered heterocyclic ring system, was developed and involves the use of hexafluoroantimonic acid to catalyze a formal [3+2+2] cycloaddition of aziridines with two alkynes. This method was applicable to two of the same or different terminal alkynes for the [3+2+2] cycloaddition with unactivated aziridines, and furnished the corresponding azepine derivatives in good yields with good levels of chemo- and regioselectivity. The mechanism was also discussed according to the results of the in situ HRMS and (1)H NMR analysis.

Can conventional bases and unsaturated hydrocarbons be converted into gas-phase superacids that are stronger than most of the known oxyacids? The role of beryllium bonds.

The association of BeX2 (X: H, F, Cl) derivatives with azoles leads to a dramatic increase of their intrinsic acidity. Hence, whereas 1H-tetrazole can be considered as a typical N base in the gas phase, the complex 1H-tetrazole-BeCl2 is predicted to be, through the use of high-level G4 ab initio calculations, a nitrogen acid stronger than perchloric acid. This acidity enhancement is due to a more favorable stabilization of the deprotonated species after the beryllium bond is formed, because the deprotonated anion is a much better electron donor than the neutral species. Consequently, this is a general phenomenon that should be observed for any Lewis base, including those in which the basic site is a hydroxy group, an amino group, a carbonyl group, an aromatic N atom, a second-row atom, or the π system of unsaturated hydrocarbons. The consequence is that typical bases like aniline or formamide lead to BeX2 complexes that are stronger acids than phosphoric or chloric acids. Similarly, water, methanol, and SH2 become stronger acids than sulfuric acid, pyridine becomes a C acid almost as strong as acetic acid, and unsaturated hydrocarbons such as ethylene and acetylene become acids as strong as nitric and sulfuric acids, respectively.

Protonation of γ-Butyrolactone and γ-Butyrolactam.

γ-Butyrolactone and γ-butyrolactam were reacted in the superacidic systems XF/MF5 (X=H, D; M=As, Sb). Salts of the monoprotonated species of γ-butyrolactone were obtained in terms of [(CH2 )3 OCOH]+ [AsF6 ]- , [(CH2 )3 OCOH]+ [SbF6 ]- and [(CH2 )3 OCOD]+ [AsF6 ]- and the analogous lactam salts in terms of [(CH2 )3 NHCOH]+ [AsF6 ]- , [(CH2 )3 NHCOH]+ [SbF6 ]- and [(CH2 )3 NDCOD]+ [AsF6 ]- . The salts were characterized by low temperature Raman and infrared spectroscopy and for both protonated hexafluoridoarsenates, [(CH2 )3 OCOH]+ [AsF6 ]- and [(CH2 )3 NHCOH]+ [AsF6 ]- , single-crystal X-ray structure analyses were conducted. In addition to the experimental results, quantum chemical calculations were performed on the B3LYP/aug-cc-pVTZ level of theory. As in both crystal structures C⋅⋅⋅F contacts were observed, the nature of these contacts is discussed with Mapped Electrostatic Potential as a rate of strength.

New Developments in Superacid Chemistry: Characterization of HC+ =O and FC+ =O Cations.

A high gas pressure and use of the superacid HF/SbF5 (1/1) were required for direct spectroscopic characterization of the HC+ =O cation, which was until recently discussed as the short-lived intermediate of carbonylation reactions. The fluorocarbonyl cation FC+ =O, which was equally difficult to characterize, was also directly observed with NMR spectroscopy as the product of protolytic cleavage of tert-butyl fluoroformate [Eq. (a)].

σ-Bishomoconjugation (σ-Bishomoaromaticity) in 4C/3(2)e Cations-Scope and Limitations.

For the seco-dodecahedradiene 1, in which the distance dππ between the carbon atoms of the two double bonds ranges from 2.90 to 3.20 Å and pyramidalization angles Φ range from 14.9 to 35.5°, the geometrical prerequisites for in-plane (σ)-homoconjugative (σ-bishomoaromatic) electron delocalization in 4C/3(2)e cations could be better defined. Whilst the 4C/3e radical cation is persistent in a matrix, the 4C/2e dication in superacid medium is, at best, an intermediate en route to the stable bisallylic dication.

Polystyrene-Bound Tetrafluorophenylbis(triflyl)methane as an Organic-Solvent-Swellable and Strong Brønsted Acid Catalyst.

Several advantages over inorganic solid acids such as zeolites and perfluororesinsulfonic acids such as Nafion are offered by the new reusable polystyrene-bound catalyst 1: a broader range of applications, improved yields, improved selectivity, and milder reaction conditions. Tf = F3 CSO2 .

Trihydroxycarbenium Hexafluorometalates: Salts of Protonated Carbonic Acid.

Stabilization by salt formation: carbonic acid, which is not available as a pure substance, was isolated in the form of its trihydroxycarbenium salts, C(OH)3+ MF6- (M=As, Sb), from the reaction of carbonic acid bis(trimethylsilyl) ester in the superacids HF/MF5 . The structure of the cation is shown in the picture. The ion was also characterized by vibrational spectroscopic studies.

Formaldehyde in Super Acids: A Succession of Products from Carbenium through Oxonium Ion to Hydroxymethyl(methylidene)oxonium Salts.

An oxonium ion with hydroxymethyl and methylidene substituents is found in the salts 1-MF6 (M=As, Sb), which can be isolated from solutions of formaldehyde in superacids HF/MF5 . Their formation was at first surprising, since NMR spectroscopy indicates that protonated monofluoromethanol is present in solution.

The Crystal Structures of Peroxonium Hexafluoroantimonate H3 O2 SbF6 and Bis(dihydrogenperoxo)hydrogen Hexafluoroantimonate H5 O4 SbF6.

Protonated hydrogen peroxide is produced from the reaction of antimony pentafluoride with bis(trimethylsilyl)peroxide in the presence of hydrogen fluoride. Depending on the stoichiometry of the reaction mixture, the compounds H3 O2 SbF6 and H5 O4 SbF6 are formed, which are stable up to room temperature and have been characterized by X-ray crystallography. The structure of the H3 O2+ ion is shown on the right.

A sulfated ZrO2 hollow nanostructure as an acid catalyst in the dehydration of fructose to 5-hydroxymethylfurfural.

Mesoporous hollow colloidal particles with well-defined characteristics have potential use in many applications. In liquid-phase catalysis, in particular, they can provide a large active surface area, reduced diffusion resistance, improved accessibility to reactants, and excellent dispersity in reaction media. Herein, we report the tailored synthesis of sulfated ZrO2 hollow nanostructures and their catalytic applications in the dehydration of fructose. ZrO2 hollow nanoshells with controllable thickness were first synthesized through a robust sol-gel process. Acidic functional groups were further introduced to the surface of hollow ZrO2 shells by sulfuric acid treatment followed by calcination. The resulting sulfated ZrO2 hollow particles showed advantageous properties for liquid-phase catalysis, such as well-maintained structural integrity, good dispersity, favorable mesoporosity, and a strongly acidic surface. By controlling the synthesis and calcination conditions and optimizing the properties of sulfated ZrO2 hollow shells, we have been able to design superacid catalysts with superior performance in the dehydration of fructose to 5-hydroxymethyfurfural than the solid sulfated ZrO2 nanocatalyst.