Electron Lewis acid

The catalyst acid sites are both Bronsted and Lewis type. The catalyst can have either strong or weak Bronsted sites or, strong i)i weak Lewis sites. A Bronsted-type acid is a substance capable of donating a proton. Hydrochloric and sulfuric acids are typical Bronsted acids. A Lewis-type acid is a substance that accepts a pair of electrons. Lewis acids may not have hydrogen in them but they are still acids. Aluminum chloride is the classic example of a Lewis acid. Dissolved in water, it will react with hydroxyl, causing a drop in solution pH. 

G.N. Lewis extended and generalized the acid-base concept to nonprotonic systems.5,6 He defined an acid as a substance that can accept electrons and defined a base as a substance that can donate electrons. Lewis acids are electron-deficient molecules or ions such as BF3 or carbocations, whereas Lewis bases are molecules that contain readily available nonbonded electron pairs (as in ethers, amines, etc.) [Eq. (1.6)]. 

The ionic oxidant (KrF+) gives an ionic mechanism, whereas the Lewis acid in association with F2 or PtF6, which are radical oxidants, results in a radical mechanism. In all the systems the one-electron (Lewis acid-F2 or PtF6) or two-electron (KrF+) oxidizer reacts with the substrate (NF3). This leads to an electron transfer to the oxidant. Either simultaneously (for KrF+) or subsequently (for Lewis acid-F2 or PtF6), the intermediate radical cation (NF3+) is fluorinated to give NF4+. 

But there is another important type of acid the Lewis acid. These acids don t donate protons—indeed they usually have no protons to donate. Instead they accept electrons. It is indeed a more general definition of acids to say that they accept electrons and of bases that they donate electrons. Lewis acids are usually halides of the higher oxidation states of metals, such as BF3, AICI3, ZnCl2, SbFj, and TiCl4. By removing electrons from organic compounds, Lewis acids act as important catalysts in important reactions such as the Friedel-Crafts alkylation and acylation of benzene (Chapter 21), the S l substitution reaction (Chapter 15), and the Diels-Alder reaction (Chapter 34). 

Boron is the first member of Group 13 elements. It is a non-metal and forms only covalent compounds. It exhibits an oxidation state +3 in all its compounds. The electron configuration of boron is ns np and boron is said to form three covalent bonds using sp hybrid orbitals. The compounds of boron are electron deficient and accept a pair of electrons (Lewis acids). The bonding in certain boron compounds is of considerable theoretical interest. 

Strategy Determine which species in each reaction accepts a pair of electrons (Lewis acid) and which species donates a pair of electrons (Lewis base). 

The utilization of copper(I) catalysis in asymmetric transformations is universal due to the special valence electron, Lewis acidity, and coordination characteristic of the metal. Copper salts are easily available, cost-efficient, and nontoxic. Copper(l)-catalyzed asymmetric cycloaddition and cascade addition-cyclization reactions are straightforward methodologies for the stereoselective construction of various biologically and medicinally important heterocyclic compounds. In the past 5 years, main endeavors have been paid into catalytic asymmetric [3+2] cycloadditions other types of cycloaddition protocols are relatively less developed. The examples described in this chapter clearly demonstrate the potential of chiral Cu(I) complexes in the synthesis of enantioenriched heterocycles. Further studies may lie in the diversification of catalytic system, reaction type, and catalysis mode. Research in this field is still challenging and highly desirable, and it would be expected that more discoveries will come in the near future. 

The first method, described as the chemical method, consists of identifying the active sites on the surface of the solid and expressing a bond between the adsoibate and the active site. This bond is a coordinate bond, obtained by pooling of doublets of electrons (Lewis acids and bases) either from the gas molecule or from a surface species of the solid this bond is more or less ionic. This is a concept of loealized adsorption. 

The term acid was extended by Lewis to include substances which are electron acceptors. Thus AICI3 can accept electrons from a chloride ion forming the [AICI4] ion and is a Lewis acid. 

Many solids have foreign atoms or molecular groupings on their surfaces that are so tightly held that they do not really enter into adsorption-desorption equilibrium and so can be regarded as part of the surface structure. The partial surface oxidation of carbon blacks has been mentioned as having an important influence on their adsorptive behavior (Section X-3A) depending on conditions, the oxidized surface may be acidic or basic (see Ref. 61), and the surface pattern of the carbon rings may be affected [62]. As one other example, the chemical nature of the acidic sites of silica-alumina catalysts has been a subject of much discussion. The main question has been whether the sites represented Brpnsted (proton donor) or Lewis (electron-acceptor) acids. Hall... 

We have seen that a base can be defined as combining with a proton and, therefore, requires at least one lone pair of electrons. A more general definition of acids and bases, due to G. N. Lewis, describes a base as any species (atom, ion or molecule) which can donate an electron pair, and an acid as any species which can accept an electron pair— more simply, a base is an electron-pair donor, an acid an electron-pair acceptor. Some examples of Lewis acids and bases are ... 

Boron trioxide is not particularly soluble in water but it slowly dissolves to form both dioxo(HB02)(meta) and trioxo(H3B03) (ortho) boric acids. It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours—a fact utilised in analysis as the "borax bead test , cf alumina p. 150. Boric acid. H3BO3. properly called trioxoboric acid, may be prepared by adding excess hydrochloric or sulphuric acid to a hot saturated solution of borax, sodium heptaoxotetraborate, Na2B407, when the only moderately soluble boric acid separates as white flaky crystals on cooling. Boric acid is a very weak monobasic acid it is, in fact, a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of Lowry-Bronsted acids, i.e. 

The BF4 ion has a regular tetrahedral configuration. The most important property of boron trifluoride is its great capacity to act as an electron pair acceptor (Lewis acid). Some examples of adducts are ... 

Desimoni et al. initially advocated the Acceptor Number (AN) as the dominant solvent parameter" The AN describes the ease with which a solvent can act as an electron pair acceptor (Lewis acid) and... 

The second important influence of the solvent on Lewis acid - Lewis base equilibria concerns the interactions with the Lewis base. Consequently the Lewis addity and, for hard Lewis bases, especially the hydrogen bond donor capacity of tire solvent are important parameters. The electron pair acceptor capacities, quantified by the acceptor number AN, together with the hydrogen bond donor addities. O, of some selected solvents are listed in Table 1.5. Water is among the solvents with the highest AN and, accordingly, interacts strongly witli Lewis bases. This seriously hampers die efficiency of Lewis-acid catalysis in water. 

Finally, the solvent also interacts with sites of the Lewis acid and the Lewis base that are not directly involved in mutual coordination, thereby altering the electronic properties of the complex. For example, delocalisation of charges into the surrounding solvent molecules causes ions in solution to be softer than in the gas phase . Again, water is particularly effective since it can act as an efficient electron pair acceptor as well as a donor. 

Surprisingly, the highest catalytic activity is observed in TFE. One mi t envisage this to be a result of the poor interaction between TFE and the copper(II) cation, so that the cation will retain most of its Lewis-acidity. In the other solvents the interaction between their electron-rich hetero atoms and the cation is likely to be stronger, thus diminishing the efficiency of the Lewis-acid catalysis. The observation that Cu(N03)2 is only poorly soluble in TFE and much better in the other solvents used, is in line with this reasoning. 

As anticipated, the complexation is characterised by negative p-values, indicating that the binding process is favoured by electron donating substituents. The order of the p-values for complexation of the different Lewis-acids again follows the Irving-Williams series. 

The effect of substituents on the rate of the reaction catalysed by different metal ions has also been studied Correlation with resulted in perfectly linear Hammett plots. Now the p-values for the four Lewis-acids are of comparable magnitude and do not follow the Irving-Williams order. Note tlrat the substituents have opposing effects on complexation, which is favoured by electron donating substituents, and reactivity, which is increased by electron withdrawirg substituents. The effect on the reactivity is clearly more pronounced than the effect on the complexation equilibrium. 

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

An unshared pair of electrons from the Lewis base is used to form a covalent bond between the Lewis acid and the Lewis base The Lewis acid and the Lewis base are shown as ions m the equation but they need not be If both are neutral molecules the analogous equation becomes... 

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