Perfluorinated Trialkoxysilanol with Dramatically Increased Brønsted Acidity

The Brønsted acidity of the perfluorinated trialkoxysilanol {(F₃C)₃CO}₃SiOH is more than 13 orders of magnitude higher than that of orthosilicic acid, Si(OH)₄, and even more for most previously known silanols. It is easily deprotonated by simple amines and pyridines to give the conjugate silanolates [OSi{OC(CF₃)₃}₃]⁻, which possess extremely short Si−O bonds, comparable to those of silanones.     

IR Spectroscopic Study of Alkane Interaction with the Brønsted Acid Sites of Hydrogen-Exchanged Zeolites

Diffuse-reflectance IR spectroscopy is used to study the interaction of C₃ and C₆ alkanes (propane, n-hexane, 3-methylpentane, and cyclohexane) with the Brønsted acid sites of hydrogen-exchanged mordenite, ferrierite, ZSM-5, and faujasite. It is found that a shift of the absorption band of the stretching vibrations of acidic Si(OH)Al groups toward lower frequencies (OH) due to the formation of a hydrogen bond with adsorbed alkanes increases in the following series: OH(propane) < OH(n-hexane) = OH(3-methylpentane). The accessibility of Si(OH)Al groups to alkane molecules is determined by the dimension of rings through which molecules enter zeolite channels and cavities. It follows from the measured OH values that the strength of Brønsted acid sites decreases in the following series: HZSM-5 > H-mordenite H-ferrierite HY. The difference between the three high-silica zeolites is not great. The results obtained are compared with the published IR data on Si(OH)Al groups of zeolites with adsorbed alkanes and other weak bases.     

Steam-Induced Coarsening of Single-Unit-Cell MFI Zeolite Nanosheets and Its Effect on External Surface Brønsted Acid Catalysis

Commonly used methods to assess crystallinity, micro-/mesoporosity, Brønsted acid site density and distribution (in micro- vs. mesopores), and catalytic activity suggest nearly invariant structure and function for aluminosilicate zeolite MFI two-dimensional nanosheets before and after superheated steam treatment. Yet, pronounced reaction rate decrease for benzyl alcohol alkylation with mesitylene, a reaction that cannot take place in the zeolite micropores, is observed. Transmission electron microscopy images reveal pronounced changes in nanosheet thickness, aspect ratio and roughness indicating that nanosheet coarsening and the associated changes in the external (mesoporous) surface structure are responsible for the changes in the external surface catalytic activity. Superheated steam treatment of hierarchical zeolites can be used to alter nanosheet morphology and regulate external surface catalytic activity while preserving micro- and mesoporosity, and micropore reaction rates.     

Brønsted Acid-Catalyzed Enantioselective Cycloisomerization of Arylalkynes

The first example of an enantioselective carbocyclization of an alkyne-containing substrate catalyzed by chiral Brønsted acids was achieved. The use of the 2-hydroxynaphthyl substituent on the alkyne as a directing group constituted the key parameter enabling both efficient regioselective protonation of the carbon–carbon triple bond and chiral induction. The key cationic intermediate could be depicted either as a cationic vinylidene ortho-quinone methide or a stabilized vinyl cation. Atropoisomeric phenanthrenes derivatives were produced in high yields and good enantioselectivities under mild, metal-free reaction conditions in the presence of chiral N-triflylphosphoramide catalysts. The carbenic nature of the cationic intermediate was also exploited to describe an example of alkyne/alkane cycloisomerization.     

Brønsted acid-catalyzed imine amidation

A new method for the Br?nsted acid-catalyzed addition of amide nucleophiles to imines to produce protected aminal products is described. Simple Br?nsted acids (phenyl phosphinic acid and trifluoromethanesulfonimide) were shown to be excellent catalysts, providing high yields of the aminal product. A catalytic asymmetric imine amidation using sulfonamides as nucleophiles was successful when a hindered biaryl phosphoric acid catalyst derived from 2,2'-diphenyl-[3,3'-biphenanthrene]-4,4'-diol (VAPOL) was used. Excellent yields and enantioselectivities were found in these additions (up to 99% ee).     

Super Brønsted acid catalysis

Br?nsted acid catalysis has emerged as a new class of catalysis in modern organic synthesis. However, in order to make the utility of the Br?nsted acid catalysis as broad as the well-developed Lewis acid catalysis, it is desirable to develop Br?nsted acids demonstrating both high reactivities and selectivities. In this feature article, we will present our achievement in the design and development of strong Br?nsted acids and their application to organic reactions. Furthermore, we will describe the Tf(2)NH-catalyzed Mukaiyama aldol reaction of super silyl enol ethers. We also will highlight the differences in reactivity and chemo- and stereo-selectivity between Br?nsted and Lewis acid catalysis.     

Brønsted acid-catalyzed benzylation of 1,3-dicarbonyl derivatives

The direct alkylation of 1,3-dicarbonyl compounds with benzylic alcohols is shown to be efficiently catalyzed by simple Br?nsted acids such as triflic acid (TfOH) and p-toluenesulfonic acid (PTS) to give rise to monoalkylated dicarbonyl derivatives in high yields. In the absence of the nucleophile, substituted alkenes, generated through a formal dimerization reaction, are obtained. The reactions are carried out in air using undried solvents, with water being the only side product of the process.     

Brønsted Acid-Catalysed Dehydrative Substitution Reactions of Alcohols

The direct, catalytic dehydrative substitution of alcohols is a challenging, yet highly desirable process in the development of more sustainable approaches to organic chemistry. This review outlines recent advances in Brønsted acid-catalysed dehydrative substitution reactions for C−C, C−O, C−N and C−S bond formation. The wide range of processes that are now accessible using simple alcohols as the formal electrophile are highlighted, while current limitations and therefore possible future directions for research are also discussed.     

Relationship between the Brønsted Coefficients of the Steps of a Heterogeneous Catalytic Reaction

Kinetics and Catalysis - The thermodynamics of the catalytic cycle under the influence of the reaction medium on the catalyst was considered. The change in the activation energy of the reaction...     

Dynamical Equilibrium between Brønsted and Lewis Sites in Zeolites: Framework-Associated Octahedral Aluminum

While the structures of Brønsted acid sites (BAS) in zeolites are well understood, those of Lewis acid sites (LAS) remain an active area of investigation. Under hydrated conditions, the reversible formation of framework-associated octahedral aluminum has been observed in zeolites in the acidic form. However, the structure and formation mechanisms are currently unknown. In this work, combined experimental ²⁷Al NMR spectroscopy and computational data reveal for the first time the details of the zeolite framework-associated octahedral aluminium. The octahedral LAS site becomes kinetically allowed and thermodynamically stable under wet conditions in the presence of multiple nearby BAS sites. The critical condition for the existence of such octahedral LAS appears to be the availability of three protons: at lower proton concentration, either by increasing the Si/Al or by ion-exchange to non-acidic form, the tetrahedral BAS becomes thermodynamically more stable. This work resolves the question about the nature and reversibility of framework-associated octahedral aluminium in zeolites.     

Brønsted acid catalyzed enantioselective α-amidoalkylation in the synthesis of isoindoloisoquinolines

The Parham cyclization-intermolecular α-amidoalkylation sequence results in the facile enantioselective synthesis of 12b-substituted isoindoloisoquinolines (ee up to 95%) using BINOL-derived Br?nsted acids. α-Amidoalkylation of indole occurs through the formation of a chiral conjugate base/bicyclic quaternary N-acyliminium ion pair.     

Chiral Brønsted acid catalyzed enantioselective allenylation of aldehydes

A versatile and highly enantioselective chiral Br?nsted acid-catalyzed allenylation of aldehydes with propargyl borolane is reported. The reaction is shown to be practical and quite general with a broad substrate scope covering aryl, heteroaryl, α,β-unsaturated, and aliphatic aldehydes.     

Generalized principle of designing neutral superstrong Brønsted acids

A generalized principle of designing superstrong Br?nsted acids is suggested according to the following scheme: M=O --> M=Z(X)(n). It consists of the formal replacement of =O fragment in carbonyl, sulfonyl, etc. groups in various acidic systems (e.g., CH(3)CHO, FSO(3)H, where M is the CH(3)CH= or FSO(2)H=fragment, respectively) by =NSO(2)F, =NCN, =C(CN)(2), =P(SO(2)F)(3), =S(CN)(4), or any other formally bivalent group =Z(X)(n) (where the formal valency of the central atom Z is n + 2), leading to highly acidic systems (e.g., HC(=P(CN)(3))NH(2), FS(=C(CN)(2))(2)OH, etc.). It is demonstrated that in several cases the introduction of the double-bonded substituent at the central atom (e.g., N, C, P, S, Cl) that carries the potentially acidic proton or the acidity site (e.g., OH, NH(2), CH(3), etc. groups) will lead to the enormous (up to ca. 120 kcal/mol or 88 pK(a) units!) increase of the intrinsic acidity of the respective parent acid. The acidity of the resulting acids and the scope and limitations of the principle are explored using density functional theory calculations at B3LYP 6-311+G level. Some of the resulting acids (or their anions) were found to undergo fragmentation in the course of the geometry optimization. The general trend that follows from the results of the calculations is that the stability of the resulting compounds is influenced by both the M and the Z. If M is a first row element (carbon or nitrogen), then stable species are produced with almost any Z. If M is a second row element (sulfur or phosphorus), then the species with first row Z are mostly predicted to be stable, but most of the species with second row Z are expected to undergo fragmentation during the geometry optimization. The Z = N and Z = C derivatives (e.g., =NSO(2)CF(3), =C(CN)(2), =C(SO(2)CF(3))(2), etc.) are predicted to be the most stable. However, they have relatively modest electron-accepting power as compared to their penta-, hexa-, and heptavalent counterparts. The acidifying effects of the =Z(X)(n)() groups with the same X increase with increasing n: =NCN < =C(CN)(2) < =P(CN)(3) < =S(CN)(4) and =NSO(2)F < =C(SO(2)F)(2) < =P(SO(2)F)(3). Also, the acidifying effect of a fluorosulfonyl-substituted substituent is higher than that of the corresponding cyano-substituted substituent.     

Brønsted Acid Catalyzed Asymmetric Synthesis of cis-Tetrahydrocannabinoids**

We report herein the catalytic asymmetric cyclization of 1-aryl terpenols to afford enantiomerically highly enriched Δ⁹-cis-tetrahydrocannabinoid scaffolds in a single step. As powerful chiral catalysts strongly acidic imidodiphosphorimidates (IDPis) have been identified which furnish the products with good yields and excellent enantioselectivity. Upon MOM-deprotection some naturally occurring cannabimimetica such as (−)-cis-Δ⁹-tetrahydrocannabinol and (−)-perrottetinene as well as some unnatural analogues were made accessible along a merely 3-step biomimetic sequence (MOM=methoxymethyl).     

Brønsted acid-promoted cyclizations of 1-siloxy-1,5-diynes

We have developed the first HNTf2-promoted 5-endo-dig cyclizations of 1-siloxy-1,5-diynes, which proceed with concomitant formation of C-Hal bonds as a result of halide abstraction from a halocarbon by the intermediate alkenyl cation. This process is enabled by a chemoselective activation of the more electron-rich siloxy alkyne moiety of the diyne cyclization precursor and represents an efficient and highly diastereoselective method for assembly of a range of beta-halo enones.     

New insights in the search for chiral Brønsted bases

The concept of using chiral bases in asymmetric synthesis appeared with the emergence of the chemistry of chiral lithium amides. In recent years, new classes of chiral bases, such as chiral magnesium bisamides and chiral alkali alkoxides have proven to be highly efficient and easy to handle. This paper highlights recent advances and new concepts in the chemistry of this second generation of chiral bases.     

Natural Abundance 17O DNP NMR Provides Precise O−H Distances and Insights into the Brønsted Acidity of Heterogeneous Catalysts

Heterogeneous Brønsted acid catalysts are tremendously important in industry, particularly in catalytic cracking processes. Here we show that these Brønsted acid sites can be directly observed at natural abundance by ¹⁷O DNP surface-enhanced NMR spectroscopy (SENS). We additionally show that the O−H bond length in these catalysts can be measured with sub-picometer precision, to enable a direct structural gauge of the lability of protons in a given material, which is correlated with the pH of the zero point of charge of the material. Experiments performed on materials impregnated with pyridine also allow for the direct detection of intermolecular hydrogen bonding interactions through the lengthening of O−H bonds.