Magic Acid Research Articles

Carbonylation of Polyfluorinated 1-Arylalkan-1-ols and Diols in Superacids.

We describe the carbonylation of a series of mono and dihydroxy derivatives of polyfluorinated alkylbenzenes and benzocycloalkenes with OH groups at benzylic positions using carbon monoxide in the presence of a superacid (TfOH, a TfOH-SbF5 mixture, or a FSO3H-SbF5 mixture). It was shown that the superacid-catalyzed addition of CO to various primary and secondary polyfluorinated alcohols and diols gives the corresponding mono- and dicarboxylic acids or lactones. The efficiency of various superacids depending on alcohol structure was evaluated, and FSO3H-SbF5 yielded the best results in most transformations. The addition of CO to secondary 1-arylalkan-1-ols containing vicinal fluorine atoms was found to be accompanied by elimination of HF with the formation of α,β-unsaturated aryl-carboxylic acids. In contrast to primary and secondary alcohols, conversion of tertiary perfluoro-1,1-diarylalkan-1-ols into carbonylation products is not complete, and the resulting carboxylic acids are easily decarboxylated after water treatment of the reaction mixture.

Influence of Positive Charge on the NMR Parameters of Mono- and Diprotonated Forms of 4-Dimethylaminopyridine

By the example of 4-dimethylaminopyridine and some of its derivatives taken as model nitrogen heterocycles with several basic centers, the correlation of the parameters of the NMR spectra with the direction (regioselectivity) and multiplicity of protonation of these compounds in CF3COOH–CD2Cl2, TfOH–CD2Cl2, and FSO3H–SbF5 acid systems was studied. The established dependence of the spectral characteristics on the value of the introduced positive charge will be useful in studying the reactivity of similar heterocyclic systems in the presence of acidic agents.

Methylarene-Based PAH Synthesis via Domino Cyclization of 1,1-Difluoro-1-alkenes

Abstract Polycyclic aromatic hydrocarbons (PAHs) containing 4–7 benzene rings were synthesized via a methylarene-based protocol. Trimethyl[2-(trifluoromethyl)allyl]silane was electrophilically benzylated with Ar1CH2Br (prepared from Ar1CH3) to afford 2-trifluoromethyl-1-alkenes that were in turn nucleophilically benzylated with Ar2CH2Li (prepared from Ar2CH3) through an SN2′-type reaction to produce 1,1-difluoroethylenes, which are cyclization precursors bearing two 2-arylethyl groups. Magic acid efficiently promoted the domino Friedel–Crafts-type cyclization of these precursors, followed by dehydrogenation that enabled the connection among two aryl groups (Ar1 and Ar2) by forming two benzene rings between them, facilitating the synthesis of the desired higher-order PAHs. With the proposed protocol, the combination of even a limited number of methylarenes can yield a variety of PAHs in diverse configurations.

Electrochemical Oxidation of Methane in Superacid

Methane is abundant in nature including approximately ~6,000,000 Tg located undersea or underground and 5000 Tg (~1%) in the atmosphere. In the last 200 years, atmospheric methane concentrations have doubled from 0.8 ppm to 1.6 ppm. The fifth report of the Intergovernmental Panel on Climate Change (IPCC) claims that its global warming potential is 25~30 times greater than CO2, which contributes more than one-third of today’s anthropogenic greenhouse global warming. Capturing and reusing methane offers a unique opportunity to reduce its anthropogenic and natural emissions and simultaneously bring economic benefit. Unfortunately, conventional Fischer-Tropsch route via synthesis gas is operated at extremely high temperature and pressure (700 ~1000 oC, 15~40 atm) in expensive reactors with heat transfer and coke management challenge. A more efficient pathway is therefore highly desired to directly convert methane to valued products. A prototypical system of particular interest has been superacid-catalytic model proposed by Olah using “magic acid” (HSO3F-Sb5F) as reaction media, which could directly convert methane into higher hydrocarbons under mild conditions.1-2 In addition, electrocatalysis has recently emerged as an alternative pathway for C-H activation with a lower cost and more energy efficient process.3 In this work, we explore a viable electro-oxidation process for coupling of methane in superacid in a simple Teflon made two-electrode cell, with Pt wire as the working electrode and graphite rod as both counter electrode and reference electrode. The acidity of superacid also plays a critical role during methane conversion process and only strong acid with hammett acid function (H0 ) in excess of approximately -12 is capable of methane conversion. Through employing appropriate superacid system as electrolyte, methane could be directly converted into ethane accompanied with certain amount of ethylene at room temperature. Methane oxidation could be modulated through adjusting potential in ternary acid (HF/H2SO4/HSO3F), and the Faradaic efficiency (FE) reaches to 17 % for ethane generation with a high yield of 194 mmol m-2 h-1 at 1.0 V (vs. RHE). Moreover, the cationic Pt is proposed to work as active sites for methane conversion based on anodic potential scan and X-ray photoelectron spectroscopy (XPS) analysis. This study provides a new insight for methane electro-oxidation process and has a great impact on mitigating methane emissions as well.

Revisiting the Perfluorinated Trityl Cation.

Although ultimately not isolable for X-ray structural characterization, the free perfluorinated trityl cation was shown to be observable in neat triflic acid, which represents milder conditions than previous reports of this cation in "magic acid" or oleum. A triflate-bound species could be generated in organic solvents using stoichiometric amounts of triflic acid and was shown to be synthetically viable for hydride abstraction from Et3 SiH. It was demonstrated that the para-position on the -C6 F5 rings is the primary point of attack for decomposition of the cation.

George Olah dies at 89

George A. Olah, the Donald P. and Katherine B. Loker Distinguished Professor of Organic Chemistry at the University of Southern California and the recipient of the 1994 Nobel Prize in Chemistry, has died. He was 89. Olah was a towering figure, physically and scientifically, who earned international chemistry fame 40 years ago for his novel use of “magic acid,” a concoction of antimony pentafluoride and fluorosulfonic acid that is billions of times as strong as sulfuric acid, to prepare long-lived carbocations. By extending the lifetimes of these fleeting species, Olah was able to probe them directly via spectroscopy methods. That work rapidly advanced and greatly popularized the study of reactive intermediates and organic reaction mechanisms. It ultimately led to Olah’s receipt of the Nobel Prize. In addition to research in fluorine chemistry, Olah and longtime USC colleague and scientific collaborator G. K. Surya Prakash recently focused on the chemical transformations

Crystal Structure Determination of the Pentagonal-Pyramidal Hexamethylbenzene Dication C6 (CH3 )62.

In contrast to the well-known 2-norbornyl cation, the structure of which was a matter of long debate until its pentacoordinated nature was recently proven by an X-ray structure, the pentagonal-pyramidal dication of hexamethylbenzene has received considerably less attention. This species was first prepared by Hogeveen in 1973 at low temperatures in magic acid (HSO3 F/SbF5 ), for which he proposed a non-classical structure (containing a hexacoordinated carbon) based on NMR spectroscopy and reactivity studies, but no X-ray crystal structure has been reported. C6 (CH3 )62+ can be obtained through the dissolution of hexamethyl Dewar benzene epoxide in HSO3 F/SbF5 and crystallized as the SbF6- salt upon addition of excess anhydrous hydrogen fluoride. The crystal structure of C6 (CH3 )62+ (SbF6- )2 ⋅HSO3 F confirms the pentagonal pyramidal structure of the dication. The apical carbon is bound to one methyl group (distance 1.479(3) Å) and to the five basal carbon atoms (distances 1.694(2)-1.715(3) Å).

Experimental NMR and DFT Studies of Persistent Carbocations Derived from Hetero-Polycyclic Aromatic Hydrocarbons Containing Oxygen Atom: Dibenzo[b,d]furan, Benzo[b]naphtho[1,2-d]furan, Benzo[b]naphtho[2,3-d]furan, Benzo[b]naphtho[2,1-d]furan, and Dinaphtho[2,1-b:1′,2′-d]furan

Abstract Persistent protonation carbocations generated from hetero-PAHs containing oxygen atoms in their aromatic rings, dibenzo[b,d]furan (5), benzo[b]naphtho[1,2-d]furan (6), benzo[b]naphtho[2,3-d]furan (7), benzo[b]naphtho[2,1-d]furan (8), and dinaphtho[2,1-b:1′,2′-d]furan (9), were directly observed by NMR measurements in superacid. Compound 5 was protonated mainly at C(2) in FSO3H–SbF5 (1:1) or (4:1)/SO2ClF, and 6, 8, and 9 were protonated exclusively at C(5) in CF3SO3H or FSO3H/SO2ClF, whereas 7 was protonated at C(6) and C(11) to give two species in FSO3H/SO2ClF. Surprisingly, compound 5 resists protonation in FSO3H/SO2ClF to show NMR spectra corresponding to that of the intact material. Positive charge delocalization mapping for carbocations based on experimental Δδ13C values indicates limited delocalization in these systems. The chemical shifts and charge delocalization modes derived by DFT calculations agreed with the experimental results.

Activation of Electrophilicity of Stable Y-Delocalized Carbamate Cations in Intramolecular Aromatic Substitution Reaction: Evidence for Formation of Diprotonated Carbamates Leading to Generation of Isocyanates

Although cations with three heteroatoms, such as monoprotonated guanidine and urea, are stabilized by Y-shaped conjugation and such Y-conjugated cations are sufficiently basic to be further protonated (or protosolvated) to dications in strongly acid media, only O-monoprotonated species have been detected in the case of carbamates even in magic acid. We found that the trifluoromethanesulfonic acid-catalyzed cyclization of arylethylcarbamates proceeds to afford dihydroisoquinolones in high yield. In strong acids, methyl carbamates are fully O-monoprotonated, and these monocations do not undergo cyclization even under heating. But, as the acidity of the reaction medium is further increased, the cyclization reaction of methyl phenethylcarbamates starts to proceed as a first-order reaction, with a linear relationship between rate and acidity. The sign and magnitude of the entropy of activation ΔS(‡) were found to be similar to those of other A(Ac)1 reactions. These results strongly support the idea that further protonation of the O-protonated carbamates is involved in the cyclization, but the concentration of the dications is very low and suggests that the rate-determining step is dissociation of methanol from the diprotonated carbamate to generate protonated isocyanate, which reacts with the aromatic ring. Therefore, O-protonated carbamates are weak bases in sharp contrast to other Y-shaped monocations.

Magic Acid Free Generation of Antiaromatic Dications at Room Temperature

A new method for the generation of dicationic species via ionization of diols is described. The method makes use of milder reagents at room temperatures, an advantage over use of Magic Acid at -78 °C. A series of mono- and dications were synthesized successfully, including previously unattainable species.

Acid strength of solids probed by catalytic isobutane conversion

The literature is controversial on whether sulfated zirconia (SZ) is “just” as strong acid as H-zeolites, or a strong superacid. Spectroscopic studies of adsorbed probe molecules concluded that SZ is not a superacid, whereas acidity measurements based on Hammett indicators, alkane transformations and the 1/2% isobutane conversion test [B. Umansky, J. Engelhardt, W.K. Hall, J. Catal. 127 (1991) 128; D. Fraenkel, Chem. Lett. (1999) 917] indicated that SZ is superacidic. We for the first time applied direct acidity measurements under comparative conditions for SZ and common H-zeolites in a single series and found that the apparent acid strength order is SZ ( H 0 ∼ −18) ≫ HM (−14) > HZSM-5 (−10) ≈ HY (−9), in agreement with previous studies employing the zeolites and SZ separately. Product distribution at 1/2% isobutane conversion is in full agreement with the acid ranking and strength; SZ gives the typical pattern of known superacids (Magic Acid®, HCl–AlCl 3), whereas the weaker acid zeolites’ pattern is consistent with that of other weaker acids.

Dynamic 13C NMR studies of ligand exchange in linear (d10) silver(I) and gold(I) and square‐planar (d8) rhodium(I) homoleptic metal carbonyl cations in superacidic media

The dynamic CO exchange of the monovalent metal carbonyl cations [Ag(13CO)]+, [Au(13CO)2]+-Au(13CO) SO3F and [Rh(12CO)4-x(13CO)x]+ (x < or = 1) in superacidic solutions was studied by variable-temperature 13C NMR methods. The exchange rates are strongly dependent on the acidity of the solvent, the concentration of metal carbonyl cations and temperature. Whereas a suitable exchange rate of the Ag(I) system is only accessible in magic acid (HSO3F-SbF5), the more stable Au(I) and Rh(I) systems were studied in the less acidic fluorosulfuric acid. Selected solutions of Ag(I), Rh(I) and Au(I) yielded activation barriers deltaG* of 42.7, 43.5, and 56.2 kJ mol(-1) respectively.

H/D isotope exchange between methane and magic acid (HSO3F–SbF5): an in situ NMR study

The kinetics of hydron exchange between methane and a series of DSO3F–SbF5 superacids were measured by in situ2H decoupled 1H NMR spectroscopy. The rates of exchange showed a strong dependence on antimony pentafluoride concentration, with the free energy of activation ΔG# (30 °C) decreasing from 97 to 84 kJ mol−1 over the range of concentration 19 to 49 mol % SbF5. The constant free enthalpy of activation ΔH# (ca. 65 kJ mol−1) and the decreasing entropy of activation ΔS# seem to indicate that an increase in acidity of the superacid system does not substantially change the nature of the transition state but rather acts on its solvation.

Reactive, Dicationic Electrophiles: Electrophilic Activation Involving the Phosphonium Group.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A 19F nuclear magnetic resonance study of the conjugate Brønsted–Lewis superacid HSO3F–SbF5. Part II.

Solutions of SbF5 in HSO3F with xSbF5 = 0.012 to 0.405 are studied by 500 MHz 1H NMR (299 K) and 471 MHz 19F NMR (213–250 K), using NMR tubes fitted with fluoropolymer lining. The initial process during dissolution is the fast solvolysis of monomeric SbF5 in HSO3F according to SbF5 + nHSO3F [Formula: see text] SbF5 – n(SO3F)n + nHF (n = 1, 2). All HF formed during solvolysis will no longer be removed by reaction with glass, but will remain in the superacid system. Besides participation in the fast formation of various fluoro-fluorosulfato anions [SbF6 – n(SO3F)n]– (n = 0, 1, 2) and acidium ions [H2X]+(solv.) (X= F, SO3F), HF is involved in slow-exchange reactions of the type [SbF6 – n(SO3F)n]–(solv.) + HF [Formula: see text] [SbF7 – n(SO3F)n– 1]–(solv.) + HSO3F (n = 1, 2) detected because of a delay of 3 months between sample preparation and measurements and confirmed by repeating theses measurements after a further 3 months. There are three notable differences to our earlier study, affecting concentrations of the fluoro-fluorosulfato antimonate anions observed: (i) in dilute solutions [SbF6]– is formed in high concentrations (34.7–76.1%), with [Sb2F11]– now clearly detected at intermediate to high SbF5 concentrations (up to 5.8%); (ii) bis-fluorosulfato anions (cis-, trans-[SbF4(SO3F)2]–) are found in much lower concentrations only, which decrease further with time, while tris-fluorosulfato anions ([SbF3(SO3F)3]–) are now no longer observed; (iii) these reduced concentrations of poly-fluorosulfato anions in dilute solutions are responsible for the formation of fewer µ-SO3F-oligomers at lower concentrations, when more SbF5 is added. As a consequence, the HSO3F–SbF5 magic acid system is now less complex than found previously and only seven anionic species are clearly observed. Key words: superacids, antimony(V) fluoroanions, 1H NMR, 19F NMR, solvolysis.

Formation of M(CO) 4+ (M=Co, Rh) by dissolution of neutral Co and Rh carbonyl clusters in strong acids under CO atmosphere

The dissolution of the neutral Co carbonyl cluster, Co 2(CO) 8, in HSO 3F or magic acid (HSO 3F · SbF 5) under a CO atmosphere results in the formation of the Co(I) tetracarbonyl cation, [Co(CO) 4] + (solv), and H 2. The IR and Raman spectra for [Co(CO) 4] + (solv) suggest a trigonal bipyramidal structure in which two CO ligands together with a solvent ligand are in the equatorial plane, and the two axial CO ligands are in a nearly linear alignment. No cobalt carbonyl complexes are formed in less acidic solution such as 96% H 2SO 4. The dissolution of Rh 4(CO) 12 in concentrated H 2SO 4, HSO 3F or magic acid (HSO 3F · SbF 5) under a CO atmosphere leads to the formation of the square-planar Rh(I) tetracarbonyl cation, [Rh(CO) 4] +, and H 2, where Rh(0) is unusually oxidized to Rh(I) by H +.

Syntheses, molecular structures, and vibrational spectra of chloropentacarbonylrhodium(III) and -iridium(III) undecafluorodiantimonate(V), [Rh(CO)5Cl][Sb2F11]2 and [Ir(CO)5Cl][Sb2F11]2: an experimental and density functional study.

The reactions of either bis(mu-chloro)tetracarbonyldirhodium(I), [Rh(CO)2(mu-Cl)]2, or chlorotricarbonyliridium(I), [Ir(CO)3Cl]n, in the conjugate Brønsted-Lewis superacid HF-SbF5 and in a CO atmosphere, produce [Rh(CO)5Cl][Sb2F11]2 or [Ir(CO)5Cl][Sb2F11]2, respectively. In these oxidative carbonylation reactions, antimony(V) fluoride functions as an oxidizing agent. The reduced product is identified as 6SbF3.5SbF5. [Rh(CO)5Cl][Sb2F11]2 is obtained in the form of single crystals. Crystal data: monoclinic, space group P2(1) (No. 4); a = 9.721(1), b = 12.602(1), c = 10.538(1) A; beta = 106.51(1) degrees; V = 1237.7(2) A3; Z = 2; T = 300 K; R1 [I > 3 sigma (I)] = 0.0367, wR2 = 0.0739. Single crystals of [Ir(CO)5Cl][Sb2F11]2 are produced in small amounts from a solution of mer-Ir(CO)3(SO3F)3 in magic acid, HSO3F-SbF5. The possible source of chlorine will be discussed. Crystal data for [Ir-(CO)5Cl][Sb2F11]2: monoclinic, space group P2(1) (No. 4); a = 9.686(2), b = 12.585(2), c = 10.499(2) A; beta = 106.59(2) degrees; V = 1226.5(4) A3; Z = 2; T = 294 K; R1[I > 3 sigma (I)] = 0.032, Rw = 0.031. The bond lengths and bond angles are nearly identical in the two isostructural salts; however, the cell volume of [Ir(CO)5Cl][Sb2F11]2 is slightly smaller than that of [Rh(CO)5Cl][Sb2F11]2. The cations (point group C4v) feature unusually long M-C bonds (M = Rh, Ir) and correspondingly short CO bonds, as well as high CO stretching wavenumbers and high CO stretching force constants. The [Sb2F11]- anions are not symmetry related, and their dihedral and bridge angles differ slightly in both salts. There are significant interionic contacts in [Ir(CO)5Cl][Sb2F11]2 exclusively of the C-F type (about 2 for each C atom of the five carbonyl groups) resulting in extended structures. The vibrational spectra for both [M(CO)5Cl]2+ cations (M = Rh, Ir) are assigned with the help of density functional calculations, which also provide intensities for IR and Raman bands. While [Rh(CO)5Cl]2+ is the first cationic carbonyl derivative of Rh(III), the vibrational and structural parameters for [Ir(CO)5Cl]2+ are compared to data for [Ir(CO)6]3+ and mer-Ir(CO)3(SO3F)3.

A 19F nuclear magnetic resonance study of the conjugate Brönsted-Lewis superacid HSO3F-SbF5. Part 1

The Brönsted-Lewis superacid HSO3F-SbF5 or "magic acid" is re-investigated by modern 19F NMR methods over a wide concentration range. The system is found to be considerably more complex than had been assumed previously. A total of 13 different anions are identified of which only five have previously been identified in magic acid. With increasing SbF5 contents the concentration of monomeric anions like [SbF6]-, [SbF5(SO3F)]-, cis- and trans-[SbF4(SO3F)2]-, and mer-[SbF3(SO3F)3]- gradually decreases. Except for [Sb2F11]-, which is present in very small concentrations only, the formation of oligomers involves exclusively μ-fluorosulfato bridges. In addition to donor (SO3F)- and acceptor (SbF5) complex formation to give [SbF5(SO3F)]- and possibly ligand redistribution, the solvolysis of SbF5 or SbF4(SO3F) in HSO3F appears to be the principal formation reaction for polyfluorosulfatofluoroantimonate(V) anions. In glass (NMR tubes) the solvolysis product HF is converted to the oxonium ion [H3O]+, which has previously been identified by 1H NMR and structurally characterized as [H3O][Sb2F11] by us.Key words: magic acid, conjugate superacid, fluorosulfuric acid, 19F NMR spectra.

Preparation, 1H and 13C NMR, and ab initio/IGLO study of mono-O-protonated deltic acid and theoretical investigation of di-, tri-, and tetra-O-protonated deltic acids

The hitherto elusive mono-O-protonated deltic acid C3O3H3+ was prepared by protolysis of di-tert-butoxy deltate in FSO3H-SO2ClF and in FSO3H:SbF5 (Magic Acid; 1:1 molar solution) in SO2ClF as solvent at -78°C and was characterized by 1H and 13C NMR spectroscopy. The structure and NMR chemical shifts were also calculated by the ab initio/IGLO method. No NMR evidence was found for persistent di-O-protonated deltic acid under these conditions, although a limited equilibrium with the mono-O-protonated species can be involved. Di-, tri-, and tetra-O-protonated deltic acids were also studied by ab initio/IGLO method.Key words: protonated deltic acid, aromaticity, superacids, NMR spectroscopy, ab initio and IGLO calculations.

Deuteration of hexane by [formula omitted] in supercritical deuterium oxide

Hexane is shown to undergo isotopic hydrogen exchange with 2 HCl in supercritical deuterium oxide at 380 and 400°C. The deuteration rate follows pseudo first order kinetics at both temperatures with the methylene reaction rate being about 1.6 times that of methyl. The isotopic exchange reaction is analyzed as a two step acid/base mechanism, with hexane acting as a base analogous to its behavior in `magic acid' solution. Measured K b's for the methyl group are 3.5×10 −28 and 9.2×10 −28, while the methylene groups have K b's of 6.0×10 −28 and 1.5×10 −27 at 380 and 400°C, respectively. No evidence is seen for hydride abstraction, such as formation of carbocation rearrangement species or hydrogen gas evolution as in `magic acid'. Hydride abstraction to form carbocations either does not occur or occurs at a rate too slow to be observed in the time scale of the experiments reported here.