Lewis acids proton

Aldehydes can be converted to acylals by treatment with an anhydride in the presence of BF3, other Lewis acids, proton acids, or PCI3. The reaction cannot normally be applied to ketones, though an exception has been reported when the reagent is trichloroacetic anhydride, which gives acylals with ketones without a catalyst. ... 

Because they are good Lewis acids, protons are particularly well suited for detection by Scheme 12. Grigg has modified tert-butyl calix[4]arene with one and two [Ru(bpy)3]2+ subunits and examined the chemosensing properties of 50 through 52 [386]. At pH > 8-9, formation of the phenolate anion... 

The oxygen atom is basic and reacts with electrophiles, such as the proton and Lewis acids. Protonation of the oxygen increases the electron deficiency of the carbon atom of the carbonyl group. Hence many addition reactions of the carbonyl group are acid catalysed (Scheme. 3.34). 

The first step of the mechanism is the coordination of the Lewis acid (proton in the scheme) to the ring oxygen atom of the A/-glycoside. Subsequently the ring is opened, and the loss of a proton gives rise to an enolic intermediate, which in turn undergoes tautomerization to the corresponding 1-amino-1-deoxyketose. 

Lewis acid-proton acid combination such as BF3 0Et2-CF3C02H was found to be highly effective for the activation of imines as heterodienophiles in Diels-Alder cycloaddition reactions, and under these conditions compound (189 X = O) was obtained in excellent yield <85JMC1756, 87TL2103>. 

Halogens Lewis acids Protonic acids Transition-metal halides... 

An alternate mechanism, suggested by Ogata and Okano, has precedence in the absence of a Lewis acid. Protonation of formaldehyde activates it for nucleophilic attack by formation of cation 5. Attack of 5 by benzene once more results in the formation of resonance stabilized cation 6. Loss of a proton regenerates the aromatic ring, forming benzyl alcohol (7),... 

FIGURE 19.68 Two reactions of the weakly nucleophilic enol with Lewis acids. In the first case, it is protonated to regenerate acetaldehyde in the second, the enol adds to the strongly Lewis acidic protonated carbonyl group to give aldol. 

Boron trifluoride and other Lewis acid protonic acids have also been found to function as effective accelerators for epoxy-anhydride systems. Their effectiveness has been attributed to the ease of proton formation according to reaction (74), which then facilitates anhydride rind-opening according to reaction (75) in Scheme 25. ... 

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... 

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. 

Turning the argument around reactions that do not involve proton transfer steps will only experience a significant effect of the Lewis acids if a direct interaction exists between catalyst and reactant. The conventional Diels-Alder reaction is a representative of this class of reactions. As long as monodentate reactants are used, the effects of Lewis acids on this reaction do not exceed the magnitude expected for simple salt effects, i.e. there are no indications for a direct interaction between Lewis-acid and substrate. 

A regioselective aldol condensation described by Biichi succeeds for sterical reasons (G. Biichi, 1968). If one treats the diaidehyde given below with acid, both possible enols are probably formed in a reversible reaaion. Only compound A, however, is found as a product, since in B the interaction between the enol and ester groups which are in the same plane hinders the cyclization. BOchi used acid catalysis instead of the usual base catalysis. This is often advisable, when sterical hindrance may be important. It works, because the addition of a proton or a Lewis acid to a carbonyl oxygen acidifies the neighbouring CH-bonds. 

Another category Ic indole synthesis involves cyclization of a-anilino aldehydes or ketones under the influence of protonic or Lewis acids. This corresponds to retro.synthetic path d in Scheme 4.1. Considerable work on such reactions was done in the early 1960s by Julia and co-workers. The most successful examples involved alkylation of anilines with y-haloacetoacetic esters or amides. For example, heating IV-substituted anilines with ethyl 4-bromoacetoacetate followed by cyclization w ith ZnClj gave indole-3-acetate esterfi]. Additional examples are given in Table 4.3. 

The Lewis definitions of acids and bases provide for a more general view of acid-base reactions than either the Arrhenius or Br0nsted-Lowry pic ture A Lewis acid is an electron pair acceptor A Lewis base is an electron pair donor The Lewis approach incorporates the Br0nsted-Lowry approach as a subcategory m which the atom that accepts the electron pair m the Lewis acid is a proton... 

The catalysts for cationic polymerization are either protonic acids or Lewis acids, such as H2SO4 and HCIO4 or BF3, AICI3, and TiCl4 ... 

The Lewis acids must be used with a protonic cocatalyst such as water or methanol which generates protons through the following kinds of equilibria ... 

The cyanoacryhc esters are prepared via the Knoevenagel condensation reaction (5), in which the corresponding alkyl cyanoacetate reacts with formaldehyde in the presence of a basic catalyst to form a low molecular weight polymer. The polymer slurry is acidified and the water is removed. Subsequendy, the polymer is cracked and redistilled at a high temperature onto a suitable stabilizer combination to prevent premature repolymerization. Strong protonic or Lewis acids are normally used in combination with small amounts of a free-radical stabilizer. 

Lewis acid catalysts, such as AlCl or BF, coordinate strongly with non-bonded electron pairs but they iateract only weakly with bonded electron pairs. Therefore, n-donon reagents, such as alkyl haUdes, can react with Lewis acid catalysts even under complete exclusion of moisture or any other proton source ... 

Electrophile Addition Reactions. The addition of electrophilic (acidic) reagents HZ to propylene involves two steps. The first is the slow transfer of the hydrogen ion (proton) from one base to another, ie, from Z to the propylene double bond, to form a carbocation. The second is a rapid combination of the carbocation with the base, Z . The electrophile is not necessarily limited to a Lowry-Briiinsted acid, which has a proton to transfer, but can be any electron-deficient molecule (Lewis acid). 

According to Figure 3, hydroperoxides are reduced to alcohols, and the sulfide group is oxidized to protonic and Lewis acids by a series of stoichiometric reactions. The sulfinic acid (21), sulfonic acid (23), sulfur trioxide, and sulfuric acid are capable of catalyzing the decomposition of hydroperoxides to nonradical species. 

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