Bronsted acidity, and Lewis

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. 

This section deals with Bronsted acid and Lewis acid catalyzed reactions, excluding Friedel-Crafts reactions, but including reactions such as nitrations, halogenations, and Claisen rearrangements. Friedel-Crafts reactions are discussed in the subsequent Sections 5.1.2.2 and 5.1.2.3. 

Adsorption of water is thought to occur mainly at steps and defects and is very common on polycrystalline surfaces, and hence the metal oxides are frequently covered with hydroxyl groups. On prolonged exposure, hydroxide formation may proceed into the bulk of the solid in certain cases as with very basic oxides such as BaO. The adsorption of water may either be a dissociative or nondissociative process and has been investigated on surfaces such as MgO, CaO, TiOz, and SrTi03.16 These studies illustrate the fact that water molecules react dissociatively with defect sites at very low water-vapor pressures (< 10 9 torr) and then with terrace sites at water-vapor pressures that exceed a threshold pressure. Hydroxyl groups will be further discussed in the context of Bronsted acids and Lewis bases. 

Hydroxyl Groups as Bronsted Acids and Lewis Bases... 

In contrast to some related reviews, which use reaction class or electrophiles as organizational elements, this chapter is divided into three main sections according to catalyst class (i) Bronsted acid catalysis by phosphoric acid and phosphoramide derivatives, (ii) N—H hydrogen bond catalysis by organic base and ammonium systems, and (iii) combined acid catalysis including Bronsted-acid-assisted Bronsted acid, Lewis-acid-assisted Bronsted acid, and Lewis-acid-assisted Br0nsted acid systems (Figure 5.1). 

Figure 8. Bronsted acidity ( ) and Lewis acidity (O) of La zeolites pretreated at 900° C and rehydrated, against cation contents/unit celt. Bronsted aridity (c) and Lewis aridity ( ) of the Na-8.7 zeolite pretreated at 800°C and rehydrated are also plotted...

A procedure for the formation of enamines by condensation of the parent ketone and secondary amines in the presence of molecular sieves was studied. The role of the molecular sieve is to trap the water formed. The enamine-forming reaction is acid catalyzed and both Bronsted acids and Lewis acids can be used. 

More recently, Hattori and Shiba (26) and Ward (68) studied the acidity of X zeolites. For Mg, Mn, and ZnX, Hattori and Shiba (26) report a small amount of Bronsted acidity and Lewis acidity which is too weak to be converted into Bronsted acidity by water. On Ca and SrX, they reported strong Lewis acidity which could be converted into Bronsted acidity. In contrast, Ward (68) observed Bronsted acidity on all Group II A zeolites, the concentration of sites increasing with decreasing cation radius or increasing field. The concentration of Bronsted acid sites was increased by hydration. No Lewis acid sites were detected, although pyridine interaction with the cations was observed. Ignat eva et al. (32) reported the presence of Bronsted acid sites on CaY but not on NaY. Bronsted acidity but no Lewis acidity was observed on the transition metal ions, Mn, Co, Zn, Ag, Cd, but not on Cu (68). The concentration was increased in all cases by hydration. There appeared to be no relationship between the concentration of acid sites and the physical properties of the zeolites. Studies of the same series of transition metal ion Y zeolites yielded similar results (69). 

Other suitable molecules include trialkyl phosphines and trialkyl phosphine oxides 134-36]. Phosphines bound to Bronsted acid and Lewis acid sites and physisorbed molecules yield distinguishable resonances, although the latter two are poorly resolved from each other. There is no indication of exchange broadening in these cases. 

A number of BINOL-based bifunctional organocatalysts, for example (7.171-7.173), containing both Bronsted acidic and Lewis basic sites have been used to good effect in the asymmetric MBH reaction. The amine-thiourea (7.171) promotes the MBH reaction of aliphatic aldehydes with 2-cyclohexenone with ees ranging from 80 to 94% while both the (pyridinylaminomethyl)BINOL (7.172) and phosphine (7.173) catalyse the aza-Bayhs-Hilhnan reaction of simple a,p-carbonyls such as MVK and phenyl acrylate with N-tosyl arylaldmines with similar levels of enantioselectivity. 

Ionic disproportionation of NO can be promoted in non-polar media by the addition of Bronsted acids and Lewis acids [10]. The photochemical activation of the nitrosonium donor-acceptor complex via irradiation of the charge-transfer absorption band produces the aromatic radical cation. The most direct pathway to aromatic nitration proceeds via homolytic coupling of the aromatic radical cation with NO [16] because the intermediate subsequently undergoes very rapid deprotonation ... 

The methods for determining the strength and amount of acid described in the foregoing sections (2.1.1.A, B, and C) do not distinguish between Bronsted acid sites and Lewis acid sites. The acid amount which is measured is the sum of the amounts of Bronsted acid and Lewis acid at a certain acid strength. In order to elucidate the catalytic actions of solid acids, it is often necessary to distinguish between Brensted acids and Lewis acids. 

The same authors proposed the origin of this high selectivity by a computational method, namely, ONIOM (B3LYP/6-31G HF/3-21G) calculations. In the TS, chiral phosphoric acid simultaneously activated both carbonyl and enol moieties with Bronsted acidic and Lewis basic sites, respectively. Owing to the steric repulsion between the aryl moiety of the substrate and the triisopropylphenyl group at 3,3 -positions, the attack of the TS from the Re-face was 1.3 kcalmoT more unstable than attack from the Si-face, giving rise to the preferential formation of the (S)-enantiomer. 

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