Unprecedented synergistic effects between weak Lewis and Brønsted acids in Prins cyclization

Novel synergistic effects between Lewis and Br?nsted acids in Prins cyclization are reported. Non-reactive Lewis acids and non-reactive Br?nsted acids, which failed to perform Prins cyclization when used alone, have shown remarkable synergistic effects when used in combination to perform the reaction successfully.     

Identifying the Role of Brønsted and Lewis Acid Sites in the Diels-Alder Cycloaddition of 2,5-DMF and Ethylene

The role of Lewis and Brønsted acid sites in the Diels-Alder cycloaddition (DAC) of ethylene to 2,5-dimethylfuran (2,5-DMF) to p-xylene was investigated. Amorphous silica catalysts containing Al³⁺ (ASA), Ga³⁺ (ASG), and In³⁺ (ASI) were prepared via homogeneous deposition-precipitation. Silica modified with Zr⁴⁺ (ASZ) was prepared by impregnation. Their acidic properties were characterized by various IR and NMR spectroscopic techniques. Measurements using pyridine as a probe molecule highlighted the presence of mostly Lewis acid sites (LAS) in all materials. Using CO as a probe, in contrast, demonstrated the existence of Brønsted acid sites (BAS) in ASA and ASG, which were nearly absent in ASI and ASZ. Differences in basic strength can explain the contrast in results observed between the two probe molecules. The highest p-xylene yield (~20 %) in the DAC reaction, could be achieved with ASA and ASG. The lack of BAS in ASI and ASZ resulted in inferior performance in the DAC, with p-xylene yields below 5 %. These results indicate the importance of BAS for the DAC reaction. Several other heterogeneous and homogeneous catalysts were explored for the DAC reaction to show the generality of our conclusion that BAS play a critical role in obtaining p-xylene from 2,5-DMF and ethylene.     

Complex containing a Lewis acid and Brønsted acid for the catalytic reactions of aza-Michael addition

The novel efficient complex catalyst containing a Lewis acid and a Brønsted acid have been prepared by the reaction of proline ion liquid and cuprous iodide. The catalyst was characterized by FT-IR techniques using pyridine as probe molecule. A fast, mild, and quantitative procedure for aza-Michael addition reactions between various amines and α,β-unsaturated carbonyl compounds and nitriles has been developed using the novel complex catalyst. The results showed that the novel catalyst owned high activities for the reactions with excellent yields within 1 min.     

Lewis Acid Assisted Brønsted Acid Catalysed Decarbonylation of Isocyanates: A Combined DFT and Experimental Study

An efficient and mild reaction protocol for the decarbonylation of isocyanates has been developed using catalytic amounts of Lewis acidic boranes. The electronic nature (electron withdrawing, electron neutral, and electron donating) and the position of the substituents (ortho/meta/para) bound to isocyanate controls the chain length and composition of the products formed in the reaction. Detailed DFT studies were undertaken to account for the formation of the mono/di-carboxamidation products and benzoxazolone compounds.     

Lewis and Brønsted multifunctionality: an unusual heterocycle from the reaction of bis(pentafluorophenyl)borinic acid with nitriles

The combination of Lewis and Br?nsted acidity as well as Lewis basicity in (C6F5)2BOH results in a remarkable reactivity towards organonitriles to give novel heterocyclic compounds containing a BOBOCN six-membered ring.     

Influence of the structure of polyfluorinated alcohols on Brønsted acidity/hydrogen-bond donor ability and consequences on the promoter effect

The influence of substituents on the properties of tri- and hexafluorinated alcohols derived from 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) was examined. Measurements of specific solvent-solute interactions revealed that H-bond donation (HBD) of fluorinated alcohols is sensitive to the steric hindrance of the OH group, whereas their Br?nsted acidity is dependent only on the number of fluorine atoms. For hexafluorinated alcohols (HFAs), their association with amines characterized by X-ray diffraction showed that the balance between HBD and acidity is influenced by their structure. Moreover, the ability of HFAs to donate H-bonds is exerted in synclinal (sc), synperiplanar (sp), and also antiperiplanar (ap) conformations along the C-O bond. Comparison of the effects of fluorinated alcohols as promoting solvents in three reactions is reported. The positive correlation between rate constants and H-bonding donation ability for sulfide oxidation and imino Diels-Alder reaction brings to light the role of this property, while acidity might have a minor influence. In the third reaction, epoxide opening by piperidine, none of these properties can clearly be put forward at this stage.     

The structure of the strongest Brønsted acid: the carborane acid H(CHB11Cl11)

The strongest and most robust carborane acid, H(CHB11Cl11), has a monomeric structure in the gas phase. IR spectra show two nuH-Cl bands at 2357(br) and 2066(br) cm-1 which, together with DFT calculations, indicate the coexistence of at least two isomers. The acidic proton bridges adjacent chlorine atoms with asymmetric Cl-H...Cl hydrogen bonding. The 12,7 isomer is more stable than the 7,8 isomer. These monomers can be condensed into an amorphous solid phase but are metastable. They quickly decay, first to an amorphous dimeric structure, then to a crystalline polymeric phase that has been characterized by X-ray crystallography. In the polymeric structure, the acidic proton bridges chlorine atoms from the 7-11 positions of carborane anions in linear chains. The dimeric phase (nuCl-H...Cl = 1100-2200 cm-1) and polymeric phase (nuasClHCl ca. 1100 cm-1, v broad) have more nearly symmetrical, low-barrier H-bonding. These findings have implications for the dependency of acid strength upon phase.     

On the nature and accessibility of the Brønsted-base sites in activated hydrotalcite catalysts

Base catalysis is of importance for organic synthesis in general and fine chemicals manufacture in particular. Activated hydrotalcites have recently received a great deal of attention as solid base catalysts; however, no systematic work on the nature of their active sites has been published up till now. In this work two different methods have been applied to activate Mg-Al hydrotalcites to obtain Br?nsted-base catalysts for liquid-phase condensation reactions. Activation via thermal treatment followed by rehydration (HT-reh) resulted in irregularly stacked platelets ( approximately 60 nm), whereas the sample activated via aqueous ion-exchange (HT-exc) preserved its original hexagonal hydrotalcite platelets ( approximately 100 nm). The specific activity for the self-condensation of acetone of HT-reh was over 10 times that of HT-exc. The enthalpy of CO2 adsorption on the activated hydrotalcites determined with calorimetry to gain insight into the strength of the basic sites showed very similar values. IR spectra of adsorbed CDCl3 as probe molecule on the differently activated samples revealed large differences in adsorbed amounts, but again the strength of the basic sites appeared to be the same. These results point to steric hindrance for the substrate molecules as the main factor determining differences in catalytic activity. The high accessibility of Br?nsted-base sites in HT-reh is proposed to involve a distorted edge structure of the platelets. The edge structure of exchanged samples could be distorted too, either by exchange under reflux conditions or under ultrasonic treatment. In line with the proposed model, the distorted exchanged samples displayed a much higher catalytic activity than HT-exc.     

Synthesis of a “Masked” Terminal Zinc Sulfide and Its Reactivity with Brønsted and Lewis Acids

The “masked” terminal Zn sulfide, [K(2.2.2-cryptand)][^MeLZn(S)] ( 2 ) (^MeL={(2,6-ⁱPr₂C₆H₃)NC(Me)}₂CH), was isolated via reaction of [^MeLZnSCPh₃] ( 1 ) with 2.3 equivalents of KC₈ in THF, in the presence of 2.2.2-cryptand, at −78 °C. Complex 2 reacts readily with PhCCH and N₂O to form [K(2.2.2-cryptand)][^MeLZn(SH)(CCPh)] ( 4 ) and [K(2.2.2-cryptand)][^MeLZn(SNNO)] ( 5 ), respectively, displaying both Brønsted and Lewis basicity. In addition, the electronic structure of 2 was examined computationally and compared with the previously reported Ni congener, [K(2.2.2-cryptand)][^tBuLNi(S)] (^tBuL={(2,6-ⁱPr₂C₆H₃)NC(^tBu)}₂CH).     

Highly efficient Brønsted acid and Lewis acid catalysis systems for the Friedländer Quinoline synthesis

The efficient and green Brønsted acid or Lewis acid catalysis systems for the Friedländer synthesis of 2,3,4-trisubstituted quinolines from the condensation of 2-aminoarylketones and β-ketoesters/ketones had been developed. The results confirmed that 4-toluenesulfonic acid, magnesium chloride, and cupric nitrate were the desired catalyst independently. This protocol had the advantages of mild conditions, operational simplicity, and excellent yields.     

Determination of the pKa of cyclobutanone: Brønsted correlation of the general base-catalyzed enolization in aqueous solution and the effect of ring strain

The induction of strain in carbocycles, thereby increasing the amount of s-character in the C-H bonds and the acidity of these protons, has been probed with regard to its effect on the rate constants for the enolization of cyclobutanone. The second-order rate constants for the general base-catalyzed enolization of cyclobutanone have been determined for a series of 3-substituted quinuclidine buffers in D(2)O at 25 °C, I = 1.0 M (KCl). The rate constants for enolization were determined by following the extent of deuterium incorporation (up to ～30% of the first α-proton) into the α-position, as a function of time. The observed pseudo-first-order rate constants correlated to the [basic form] of the buffer and yielded the second-order rate constants for the general base-catalyzed enolization of cyclobutanone for four tertiary amine buffers. A Br?nsted β-value of 0.59 was determined from the second-order rate constants determined. Comparison of the results for cyclobutanone to those previously reported for acetone and a 1-phenylacetone derivative, under similar conditions, indicated that the ring strain of the carbocycle appeared to have only a small effect on the general base-catalyzed rate constants for enolization. The similarity of the rate constants for the general base-catalyzed enolization of cyclobutanone to those determined for acetone allowed for an estimation of the limits of the rate constant for protonation of the enolate intermediate of cyclobutanone by the conjugate acid of 3-quinuclidinone (k(BH) = 5 × 10(8) - 2 × 10(9) M(-1) s(-1)). Combining the rate constants for deprotonation of cyclobutanone (k(B)) and protonation of the enolate of cyclobutanone (k(BH)) by 3-quinuclidinone and its conjugate acid, the pK(a) of the α-protons of cyclobutanone has been estimated to be pK(a) = 19.7-20.2.     

Lewis base assisted Brønsted base catalysis: direct regioselective asymmetric vinylogous alkylation of allylic sulfones

A Lewis base assisted Brønsted base catalysis (LBABB) strategy is applied for direct asymmetric vinylogous alkylation of allylic sulfones with Morita–Baylis–Hillman (MBH) carbonates, in which a strong Brønsted base, tert‐butoxy anion, generated in situ from a tertiary amine catalyst and MBH carbonate, is crucial in activating unstabilized nucleophiles. The γ‐regioselective alkylation products were obtained with good to excellent enantiomeric excess values when catalyzed by a modified cinchona alkaloid.     

Highly efficient synthesis of dicoumarols and xanthene derivatives in presence of Brønsted–Lewis acidic ionic liquids catalyst

A series of metal chloride-based acidic ionic liquids have been prepared and used as an efficient catalyst in one-pot multicomponent synthesis of biscoumarins and substituted xanthenes derivatives under solvent-free conditions. Among the acidic ionic liquids, N-methylpyrrolidonium zinc chloride (Hnmp/ZnCl₃)-based Brønsted–Lewis acidic ionic liquids were found to be an effective and recyclable catalyst for a one-pot synthesis of biscoumarins through the domino Knoevenagel–Michael reaction of a variety of aldehydes with 4-hydroxycoumarin in short reaction times. The reactions which occur under relatively mild conditions afforded the biscoumarin derivatives employing a very low loading of catalyst in satisfactory isolated yields and high purity after simple work-up. The Brønsted–Lewis acidic ionic liquid catalyst was reused four times without any variation in yield.     

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.     

The crystallographic structure of a Lewis acid-assisted chiral Brønsted acid as an enantioselective protonation reagent for silyl enol ethers

It is difficult to control the enantioselectivity in the protonation of silyl enol ethers with simple chiral Br?nsted acids, mainly due to bond flexibility between the proton and its chiral counterion, the orientational flexibility of the proton, and the fact that the proton sources available are limited to acidic compounds such as chiral carboxylic acids. To overcome these difficulties, we have developed a Lewis acid-assisted chiral Br?nsted acid (LBA) system. The coordination of Lewis acids with Br?nsted acids restricts the orientation of protons and increases their acidity. Optically active binaphthol (BINOL) derivative.SnCl4 complexes are very effective as enantioselective protonation reagents for silyl enol ethers. However, their exact structures have not yet been determined. We describe here optically active 1,2-diarylethane-1,2-diol derivative.SnCl4 as a new type of LBA for the enantioselective protonation as well as its crystallographic structure. A variety of optically active 1,2-diarylethane-1,2-diols could be readily prepared by asymmetric syn-dihydroxylation. This is a great advantage over BINOL for the flexible design of a new LBA. The most significant finding is that we were able to specify the conformational direction of the H-O bond of LBA, which has some asymmetric inductivity, by X-ray diffraction analysis. The stereochemical course in the enantioselective protonation of silyl enol ethers using LBA would be controlled by a linear OH/pi interaction with an initial step. The absolute stereopreference in enantioselective reactions using BINOL.SnCl4 can also be explained in terms of this uniformly mechanistic interpretation.