Brpnsted acids Lewis base-, system

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

The real promise of this catalytic reaction is the eventual development of an efficient enantioselective allylboration catalyzed by chiral Lewis acids. A stereoselective reaction using a substoichiometric amount of a chiral director has been reported, but only modest levels of stereo-induction were achieved with an aluminum-BINOL catalyst system (Eq. 19)P Recently, a chiral Brpnsted acid catalyzed system has been devised based on a diol-tin(IV) complex (Eq. 80). In this approach, aliphatic aldehydes provide enantioselectivities (up to 80% ee) higher than those of aromatic aldehydes when using the optimal complex 114. Although the levels of absolute stereoselectivity of this method remain too low for practical uses, promising applications are possible in double diastereoselection (see section on Double Diastereoselection ). 

This classification has been broadened39,40 by replacing the Brpnsted acid (proton donor) with a Lewis acid (an electron acceptor) and the Brpnsted base with a Lewis base (an electron donor). (A Brpnsted acid is a Lewis acid but not necessarily vice versa.) Solvent-proton interactions are therefore included as one subdivision of this classification, but many solvation reactions of cations with solvents also will be included as reactions of Lewis acid-base systems. This approach still does not solve the problem of fitting specific solvation interactions into the classification scheme. For example, acetonitrile behaves as a good Lewis base toward silver ion, but a poor one toward hydronium ion. The broader scheme also does not specifically take into account hydrogenbonding effects in hydroxylic and other solvents, which affect both the dielectric... 

Lewis defined a base as an electron-pair donor and an acid as an electron-pair acceptor. This definition further expands the list to include metal ions and other electron pair acceptors as acids and provides a handy framework for nonaqueous reactions. Most of the acid-base descriptions in this book will use the Lewis definition, which encompasses the Brpnsted-Lowry and solvent system definitions. In addition to all the reactions discussed previously, the Lewis definition includes reactions such as... 

For each of the following reactions identify the acid and the base. Also indicate which acid-base definition (Lewis, solvent system, Brpnsted) applies. In some cases, more than one definition may apply. 

Zirconia and titania both contain Lewis-acid and Lewis-base sites, with the latter having stronger adsorption properties. The titania phase also has strong Brpnsted acidic sites. Basic compounds are less retained on zirconia and titania phases, due to their basic nature. Neutral compounds such as poly aromatic hydrocarbons (PAH), due to their rr-electron system, behave as Lewis bases and the interactions with Lewis acid sites on the zirconia and titania packing materials become dominant for retention. 

E. Concepts of Acids and Bases — Brpnsted-Lowry approaches, Lewis theory, solvent system approaches... 

The semi-empirical models nicely demonstrate the differences between various different oxide surfaces. Systems such as MgO in which differences in the energies of adsorption sites are dominated by electrostatic potential differences were explicitly distinguished from more covalent oxide systems such as alumina or silica surfaces. In addition, the models help to understand why surface reconstruction is often important when Lewis acid and base centers are converted into their corresp>onding Brpnsted base and acid sites. 

Use the Lewis definition of acids and bases to explain the examples given for the Brpnsted-Lowry. Lux-Rood, and solvent system delinilioas (Cqs. 9.I-9.4. 9.S-9.S. 9.9-9.36). 

In Sect. 2.1, heterobimetallic transition metal (Cu)/rare earth metal (Sm) system with Lewis acid/Brpnsted basic properties was introduced. By suitably selecting metal combinations depending on the targeted reactions, variety of chiral bimetallic Lewis acid/Br0nsted base bifunctional catalysts could be created, such as aPd/La/la... 

Hydrogen bonding. Interaction between molecules or portions of a molecule resulting from the Lewis acid or base properties of the molecular units. Most commonly applied to water or hydroxyl containing systems (e.g., alcohols) in the sense of Brpnsted-Lowry acid-base theory, but also found in molecules having hydrogen bound to nitrogen (amines and amides). 

The authors also investigated the mode of activation of these BINOL-derived catalysts. They proposed an oligomeric structure, in which one Ln-BINOL moiety acts as a Brpnsted base, that deprotonates the hydroperoxide and the other moiety acts as Lewis acid, which activates the enone and controls its orientation towards the oxidant . This model explains the observed chiral amplification effect, that is the ee of the epoxide product exceeds the ee of the catalyst. The stereoselective synthesis of cw-epoxyketones from acyclic cw-enones is difficult due to the tendency of the cw-enones to isomerize to the more stable fraw5-derivatives during the oxidation. In 1998, Shibasaki and coworkers reported that the ytterbium-(f )-3-hydroxymethyl-BINOL system also showed catalytic activity for the oxidation of aliphatic (Z)-enones 129 to cw-epoxides 130 with good yields... 

More highly functionalized products can be obtained, often in better yield, by the ene reaction of a-substituted acrylate esters. EtAlCb is a more effective catalyst than AlCb for these reactions because it is a Brpnsted base as well as a Lewis acid. The EtAlCb catdyzed reactions of methyl a-bromo- and a-chloro-acrylate with trans-1,2-disubstituted and trisubstituted alkenes are both regio- and stereo-specific. - The major product (13a 85-95%) is formed via transition state (12a) in which the methoxycar-bonyl group is endo. The stereochemistry of the major adduct was established by the stereospecific conversion of (13a) to both diastereomers of ( )-2-amino-4-methyl-5-hexenoic acid and confirmed by X-ray crystallographic analysis in a related system. ... 

In some ways, this acid-base terminology is unfortunate, since there are other, more common descriptors (e.g. Brpnsted, Lewis, hard and soft). However, the terminology has been helpful in suggesting lines of research for the study of non-aqueous systems, and its use will probably continue. 

On the basis of ESl-MS observation as well as positive nmilinear effects of this system, we assumed that p-oxo-p-aiyloxy-trimer complex is the most enantiose-lective active species (Fig. 3). Therefore, Sm50(0-/Pr)i3 with a well-ordered structure would have beneficial effects for the formation of desired trimer species. Postulated catalytic cycle of the reaction based on the initial rate kinetic studies and kinetic isotope effect studies is shown in Fig. 4. In this catalyst system, both Cu and Sm are essential. We assume that the cooperative dual activation of nitroalkanes and imines with Cu and Sm is important to realize the syn-selective catalytic asymmetric nitro-Mannich-type reaction. The Sm-aryloxide moiety in the catalyst would act as a Brpnsted base to generate Sm-nitronate. On the other hand, Cu(ll) would act as a Lewis acid to control the position of iV-Boc-imine. Among possible transition states, the sterically less hindered TS-1 would be more favorable. Thus, the stereoselective C-C bond formation via TS-1 followed by protonation with phenolic proton affords syn product and regenerates the catalyst. 

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