Brdnsted acid

Shatenshtein et al., and Satchell517, therefore attribute the increased reactivity in the presence of a catalyst to greater polarisation of the acid by the catalyst, but Satchell and Comyns et al.516 seemed to favour reaction of this upon a complex formed between the aromatic and the catalyst. It is difficult, however, to envisage how this latter complex would have enhanced reactivity, since the catalysts are electron acceptors and it is possible that the catalyst enhances the reactivity of the Brdnsted acid and lowers the reactivity of the aromatic, the former effect being... 

In this equation, r) the absolute hardness, is one-half the difference between /, the ionization potential, and A, the electron affinity. The softness, a, is the reciprocal of T]. Values of t) for some molecules and ions are given in Table 8.4. Note that the proton, which is involved in all Brdnsted acid-base reactions, is the hardest acid listed, with t — c (it has no ionization potential). The above equation cannot be applied to anions, because electron affinities cannot be measured for them. Instead, the assumption is made that t) for an anion X is the same as that for the radical Other methods are also needed to apply the treatment to polyatomic... 

In general, two classes of acids have to be discussed Lewis and Brdnsted acids. In macrocycles, only a few Lewis acid centres have been incorporated, e.g. tin (Newcomb et al., 1987 Newcomb and Blanda, 1988 Blanda and Newcomb, 1989 Blanda et al., 1989) and boron (Reetz et al., 1991). Here we will discuss Brpnsted acids, starting with carboxylic acids. 

R, 25) -2- [(ethoxycarbonyl) amino] -1 -phenyl-1 -propanol [Brdnsted acid promoted reduction of c/-amino ketone to erythro a-hydroxy amine], 124-125... 

B2) Metathetical exchange of a nickel(II(-bonded anionic ligand by an anion of a stronger Brdnsted acid. The nickel II) component can be either an organonickel complex or a nickel hydride. 

It is generally assumed that the electron donor can have one or several of the following functions to increase the stability of the catalytically active species to increase the catalytic activity of the catalyst to allow a control over the selectivity of the catalytic reaction or to increase the solubility of the catalyst in organic media. The main effect of Lewis or Br0nsted acids is to increase the catalyst activity, but their influence on selectivity control is not considered to be of great significance (see, however, Sections IV,D,2 and IV,F). The increase in activity of the catalyst (see below) on the addition of Lewis or Brdnsted acids is frequently accompanied by a decrease in stability of the system. 

The formation of cationic nickel hydride complexes by the oxidative addition of Brdnsted acids (HY) to zero-valent nickel phosphine or phosphite complexes (method C,) has already been discussed in Section II. Interesting in this connection is a recent H NMR study of the reaction of bis[tri(o-tolyl)phosphite]nickelethylene and trifluoroacetic acid which leads to the formation of a square-planar bis[tri(o-tolyl)phosphite] hydridonickel trifluoroacetate (30) (see below) having a cis arrangement of the phosphite ligands (82). 

Many of the boron hydrides have been shown to function as Brdnsted acids (See 34 for references). The bridging hydrogens are acidic. Pentaborane(9) is a monoprotic acid in the presence of a variety of bases (35,36,37). An example of such a reaction (38) is given in Scheme I, Reaction (4). By removing the bridging proton to give BsHs , a boron-boron bond is formed which is susceptible to insertion of electrophillic agents. 

Finally in Chapters 11-13, some of the more recent discoveries that have led to a renaissance in the field of organocatalysis are described. Included in this section are the development of chiral Brdnsted acids and Lewis acidic metals bearing the conjugate base of the Bronsted acids as the ligands and the chiral bifunctional acid-base catalysts. 

The conditions favoring cracking by the monomolecular path are high temperature and low olefin concentrations, i.e. low paraffin partial pressure and/or low conversion. The proposed reaction intermediate is formed by protonation of the paraffin feed by a Brdnsted acid site of the catalyst. We may compare this with similar paraffin protonation by CH5 in chemical ionizations occurring in an ion cyclotron resonance mass spectrometer [10], The C0H15 ion produced collapses to the same products as we have observed with zeolites HZ as the proton source (Fig.1). This is surprising, since the... 

Fig. 13. Dehydration of methanol on a Brdnsted acid site (a) shows the side-on complex, (b) the transition state, and (c) the dissociative complex of surface methoxy and water. Reprinted with permission from Ref. 221. Copyright 1995 American Chemical Society.

Boron heterocycles bearing a hydroxy group on the boron atom are acidic. Boric, boronic and borinic acids are Lewis acids towards hydroxide ion. It was suggested that six-membered 5-hydroxy compounds behaved as Brdnsted acids. The experimental evidence for this was the similarity between the UV spectra of the hydroxy compounds in neutral and basic solution. The rationale for this fact was the maintained aromaticity which was supposed to favour the formation of, for example, ion (163) over ion (164). This suggestion was at first supported by nB NMR spectroscopy, which differentiates between anions of Lewis-acidic... 

There are thus two classes of acids on surfaces of metal oxides Lewis acids and Brdnsted acids (which are also termed proton acids). The weight of evidence (1-8) shows that strong Brpnsted acids are the primary seat of catalytic activity for skeletal transformations of hydrocarbons. In the solids under review, they consist of protons associated with surface anions. 

Rates of model reactions are more commonly used to determine relative rather than absolute surface acidities and a variety of acid-catalyzed reactions have been used for this purpose (1-3). Xylene isomerization is a particularly well-substantiated model reaction, thanks to work by Ward and Hansford (43). They demonstrated that the conversion of o-xylene to p- and /n-xylenes over a series of synthetic silica-alumina catalysts increases as the alumina content is increased from 1 to 7%. The number of strong Brdnsted acids in each member of the catalyst series was measured by means of infrared spectroscopy. Since conversion of o-xylene was found to be a straight-line function of the number of Br0nsted acids (see Fig. 9), rate of xylene isomerization appears to be a valid index of the amount of surface acidity for this catalyst series. This correlation also indicates that the acid strengths of these silica-alumina preparations are roughly equivalent. 

Fluoride addition promotes the cracking (108-110) and isomerization (108, 111) activity of alumina, presumably, because of the formation of Brdnsted acid sites. In a comprehensive study of fluorided aluminas, Antipina et al. (110) demonstrated that there is a close parallelism between generation of cumene cracking activity at 430°C and surface acidity when fluoride content is increased from 0 to 7% wt (see Fig. 15). Acidity was measured by n-butylamine titration endpoints were determined by means of an arylcarbinol indicator that detected Br0nsted acids stronger than those corresponding to a pKa of — 13.3. In a separate article (112), Anti-... 

Goble and Lawrence attributed the high isomerization activity of chlorinated platinum-alumina catalyst to the creation of a localized dual site comprising a Lewis acid site and an adjacent platinum site. However, as has since been pointed out by Asselin et al. (88), carbonium ion intermediates over low-temperature isomerization catalysts are probably created by the same process as that observed for Friedel-CrEifts catalyst abstraction of hydride ion from the paraffin by a strong Brdnsted acid according to the equation... 

Sugioka and Aomura (133) provide kinetic evidence indicating that the rate-determining step in the hydrocracking of aliphatic sulfur compounds over silica-alumina is catalyzed by Brdnsted acid sites. Conversions of reactants were measured by use of a pulse reactor hydrogen was used as the carrier gas. They found that reactivities of mercaptans are in the following order ... 

These results strongly pointed toward the involvement of the acidic hydroxyl groups in the catalytic reaction as suggested by Benesi (157), since the maximum activity was obtained when the zeolite was completely deammoniated. In addition, catalysts which had been dehydroxylated by high-temperature calcination demonstrated low activity. Thus, Benesi proposed that the Brtfnsted acid sites rather than the Lewis acids were the seat of activity for toluene disproportionation. This conclusion was supported by the enhancement in toluene disproportionation activity observed when the dehydroxylated (Lewis acid) Y zeolite was exposed to small quantities of water. As discussed previously, Ward s IR studies (156) indicated a substantial increase in Brdnsted acidity upon rehydration of dehydroxylated Y sieve. 

The observed catalytic behavior in the case of l-methyl-2-ethylbenzene isomerization (158) was not so straightforwardly related to the Brdnsted site concentration. The maximum activity was observed with samples activated at temperatures where significant dehydroxylation had occurred. This reaction occurs more readily over acid catalysts than toluene disproportionation or xylene isomerization and may require fewer Brdnsted acid sites, or the reaction mechanism may involve Lewis sites. 

M0O3) has been investigated for samples in both original and rehydroxylated form. The spectra are shown in Figure 3. It appears that Brdnsted acid sites, characterized by the 1636 and 1540 cm l bands are observed only, when the discs are rehydroxylated in wet air before the calcination under high vacuum in the IR cell takes place (Figure 3b). By consequence all spectra have been recorded for such rehydroxylated samples. Only one Lewis band is observed for the molybdenum-alumina sample, opposite to the observations of Kiviat and Petrakis (JL9), who have observed two Lewis bands for their samples. 

Spectra of adsorbed pyridine have been recorded for the MoCo-124 catalysts, for which the final calcination temperature after the cobalt impregnation has been varied. It turns out that the 400 and 500°C calcined samples and the 650 and 700°C calcined samples show very similar spectra. Therefore we show only the spectra of the 400°C (low calcined) and the 650°C (high calcined) samples. Figure 4 shows spectra after desorption at 150 and 250°C. Few Brdnsted acid sites are observed in the low calcined MoCo-124 samples. The reflection spectra (Figure 1) indicate for these low calcined samples the presence of cobalt on the catalyst surface, because no cobalt aluminate phase could be detected. The high calcined samples do show the presence of Brdnsted acid sites the presence of a cobalt aluminate phase is concluded from the reflection spectra (Figure 1) for these samples. 

These experiments indicate that at low calcination temperatures the cobalt ions are present on the catalyst surface and neutralize the Brdnsted acid sites of the molybdate surface layer. At the higher calcination temperatures, the cobalt ions move into the alumina lattice. The BrGnsted acid sites reappear, indicating that the situation on the molybdate surface is restored. 

The spectra of adsorbed pyridine for MoCo-153 and MoNi-153 are compared in Figure 9. Two final calcination temperatures have been applied, 480 and 650°C. The spectra of the 480°C samples (Figure 9a and 9b) are nearly identical. The BrtSnsted acid bands are weak, while the 1612 cm l Lewis bands are strong. The intensity of the Brdnsted acid bands increases for both 650°C calcined samples (Figure 9c and 9d). The Lewis acid bands show a marked difference now. The 1612 band remains high in intensity for the MoCo-153 catalyst, but this Lewis band decreases appreciably in intensity for the MoNi-153 catalyst. 

The molybdate surface layer in the molybdenum-alumina samples is characterized by the presence of BrGnsted acid sites ( 1545 cm- ) and one type of strong Lewis acid sites (1622 cm l). Cobalt or nickel ions are brought on this surface on impregnation of the promotor. The absence of BrtSnsted acid sites is observed for both cobalt and nickel impregnated catalysts, calcined at the lower temperatures (400-500°C). Also a second Lewis band is observed at 1612 cnrl.The reflection spectra of these catalysts indicate that no cobalt or nickel aluminate phase has been formed at these temperatures. This indicates that the cobalt and nickel ions are still present on the catalyst surface and neutralize the Brdnsted acid sites of the molybdate layer. These configurations will be called "cobalt molybdate" and "nickel molybdate" and are shown schematically in Figure 11a. 

The reappearance of Brdnsted acid sites has been observed for the high calcined nickel-molybdenum-alumina catalysts. The presence of a nickel aluminate phase has been concluded from the reflectance spectra. The second Lewis band (1612 cm l) has a very low intensity, in comparison with the cobalt containing catalysts of a same composition and after the same calcination conditions. 

Kiviat and Petrakis (19) have shown that the cobalt and nickel ions influence the spectra of pyridine adsorbed on molybdenum-alumina. They have concluded that the introduction of these ions results in a change of the ratio of the intensities of the bands of the Lewis and Brdnsted acid sites. Our observations show that this... 

A picture has been formed of the way in which the promotor ions are built in the M0O3-AI2O3 system. The neutralization of the Brdnsted acid sites, as originally present in M0O3-AI2O3 systems by the cobalt ions for the catalysts calcined at low temperatures ( 500°C) indicates that the cobalt ions are present on the catalyst surface. The liberation of these sites in catalysts calcined at high temperatures ( 650°C) and the observation of the characteristic reflectance spectrum of C0AI2O4 show that the cobalt ions enter the alumina lattice. However the interaction between cobalt and molybdenum, as indicated by the second Lewis band remains present. This leads to the conclusion that the cobalt ions are present in the surface layers of the alumina lattice. 

Inspired by the recent observation that imines are reduced with Hantzsch esters in the presence of achiral Lewis or Brpnsted acid catalysts (Itoh et al. 2004), we envisioned a catalytic cycle for the reductive amination of ketones which is initiated by protonation of the in situ generated ketimine 10 from a chiral Brdnsted acid catalyst (Scheme 13). The resulting iminium ion pair, which may be stabilized by hydrogen bonding, is chiral and its reaction with the Hantzsch dihydropyridine 11 could give an enantiomerically enriched amine 12 and pyridine 13. 

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