Lewis/Bronsted acid catalysis

Greeves e( al. reported that lanthanide triflates were effective for allylation of carbonyl compounds (20). Five mole percent of Yb(OTf)3 catalyzed the allylation of a series of aldehydes in good yield (Scheme 13.9) [20a]. Later they found a significant rate of acceleration by the Bronsted add in allylation of aldehydes with allyltributyltin catalyzed by ytterbium triflate (Table 13.9) [20b]. Recently, the concept of combined Lewis acid-Bronsted catalysis has attracted much attention from the chemical community because it often gives rise to significant improvement... 

CHIRAL BRONSTED BASE-LEWIS ACID BIFUNCTIONAL CATALYSIS... 

Keywords Aldol, Direct, Ketone, Asymmetric catalysis, Enantioselective reaction, Diastereo-selectivity, 1,2-Diol, Aldehyde, Enamine, Lewis acid, Bronsted base, Organocatalysis, Bimetal-... 

Mixtures of polyphosphoric acid (PPA) or K-10 clay and Bi(OTf)3 xH20 have been reported to be efficient systems for the rearrangement of indanone oximes (Equation 36) [73]. The apparent synergy between PPA and Bi(OTf)3 xH20 might be rationalized as Bronsted acid assisted Lewis acid (BLA) catalysis. 

Yang T, Ferrali A, Sladojevich F, Campbell L, Dixon DJ (2009) Bronsted base/lewis acid cooperative catalysis in the enantioselective conia-ene reaction. J Am Chem Soc 131 (26) 9140-9141... 

The homolytic decomposition of hydroperoxides was proved to be catalyzed by Bronsted as well as Lewis acids (for example, BF3, A1C13, SbCls) [230]. Experimental data on acid catalysis of the homolytic decomposition of hydroperoxides are collected in Table 10.9. 

Abstract In the first part of this mini review a variety of efficient asymmetric catalysis using heterobime-tallic complexes is discussed. Since these complexes function at the same time as both a Lewis acid and a Bronsted base, similar to enzymes, they make possible many catalytic asymmetric reactions such as nitroal-dol, aldol, Michael, Michael-aldol, hydrophosphonyla-tion, hydrophosphination, protonation, epoxide opening, Diels-Alder and epoxi-dation reaction of a, 3-unsaturated ketones. In the second part catalytic asymmetric reactions such as cya-nosilylations of aldehydes... 

The development of catalytic asymmetric reactions is one of the major areas of research in the field of organic chemistry. So far, a number of chiral catalysts have been reported, and some of them have exhibited a much higher catalytic efficiency than enzymes, which are natural catalysts.111 Most of the synthetic asymmetric catalysts, however, show limited activity in terms of either enantioselectivity or chemical yields. The major difference between synthetic asymmetric catalysts and enzymes is that the former activate only one side of the substrate in an intermolecular reaction, whereas the latter can not only activate both sides of the substrate but can also control the orientation of the substrate. If this kind of synergistic cooperation can be realized in synthetic asymmetric catalysis, the concept will open up a new field in asymmetric synthesis, and a wide range of applications may well ensure. In this review we would like to discuss two types of asymmetric two-center catalysis promoted by complexes showing Lewis acidity and Bronsted basicity and/or Lewis acidity and Lewis basicity.121... 

Homogeneous Chemical Catalysis of the Reduction of Carbon Dioxide. Synergistic Effect of Bronsted and Lewis Acids... 

The direct electrochemical reduction of carbon dioxide requires very negative potentials, more negative than —2V vs. SCE. Redox catalysis, which implies the intermediacy of C02 (E° = —2.2 V vs. SCE), is accordingly rather inefficient.3 With aromatic anion radicals, catalysis is hampered in most cases by a two-electron carboxylation of the aromatic ring. Spectacular chemical catalysis is obtained with electrochemically generated iron(0) porphyrins, but the help of a synergistic effect of Bronsted and Lewis acids is required.4... 

Trost s group reported direct catalytic enantioselective aldol reaction of unmodified ketones using dinuclear Zn complex 21 [Eq. (13.10)]. This reaction is noteworthy because products from linear aliphatic aldehydes were also obtained in reasonable chemical yields and enantioselectivity, in addition to secondary and tertiary alkyl-substituted aldehydes. Primary alkyl-substituted aldehydes are normally problematic substrates for direct aldol reaction because self-aldol condensation of the aldehydes complicates the reaction. Bifunctional Zn catalysis 22 was proposed, in which one Zn atom acts as a Lewis acid to activate an aldehyde and the other Zn-alkoxide acts as a Bronsted base to generate a Zn-enolate. The... 

Chiral base catalysis was classified into five sections and reviewed. Although the reactions described herein are promoted by Bronsted or Lewis bases, the Lewis acidic characteristics of metals play important roles in both substrate activation and enantioselection. Compared with chiral Lewis acid-catalyzed reactions,... 

An area of broad interest in catalysis is the search for viable replacements for the widely used Bronsted liquid acids such as HF and H2SO4 and solid Lewis acids such as AICI3 and MgCl2. The liquid acids are corrosive and also costly, because of the need to work up the products by neutralization and repeated washing. In many cases, the contamination of the products by these acids induces degradation over time and limits the application of the products. The cost of multi-step washing can be quite high. Acidic ionic liquids therefore offer potential alternatives for such reactions (//). 

Some ionic liquids have tunable Lewis acidities and basicities. The tuning can be achieved simply by varying the anion fraction in the overall ionic liquid composition. In some cases, Bronsted acidity can also be introduced into stable ionic liquids. Many publications show the broad applicability of acidic or basic ionic liquid media in catalysis replacing corrosive liquids and solid catalysts. 

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). 

Lewis-Acid-Assisted Bronsted Acid Catalysis [124]... 

Cince the catalytic activity of synthetic zeolites was first revealed (1, 2), catalytic properties of zeolites have received increasing attention. The role of zeolites as catalysts, together with their catalytic polyfunctionality, results from specific properties of the individual catalytic reaction and of the individual zeolite. These circumstances as well as the different experimental conditions under which they have been studied make it difficult to generalize on the experimental data from zeolite catalysis. As new data have accumulated, new theories about the nature of the catalytic activity of zeolites have evolved (8-9). The most common theories correlate zeolite catalytic activity with their proton-donating and electron-deficient functions. As proton-donating sites or Bronsted acid sites one considers hydroxyl groups of decationized zeolites these are formed by direct substitution of part of the cations for protons on decomposition of NH4+ cations or as a result of hydrolysis after substitution of alkali cations for rare earth cations. As electron-deficient sites or Lewis acid sites one considers usually three-coordinated aluminum atoms, formed as a result of dehydroxylation of H-zeolites by calcination (8,10-13). 

Catalytic and Electron Transfer Properties. The isomerization of cyclopropane on HY zeolites activated at temperatures less than 600° C is attributed to catalysis by Bronsted acid sites (12, 13), and the activation temperature for maximum activity was in the range 300°-400°C (13). On the other hand, rearrangement of protoadamantane to adamantane proceeds by hydride ion abstraction at Lewis acid sites (lfy. Materials B, therefore, appear to have good Bronsted activity (Figure 5) and in view... 

Studies of catalytic asymmetric Mukaiyama aldol reactions were initiated in the early 1990s. Until recently, however, there have been few reports of direct catalytic asymmetric aldol reactions [1]. Several groups have reported metallic and non-metallic catalysts for direct aldol reactions. In general, a metallic catalysis involves a synergistic function of the Bronsted basic and the Lewis acidic moieties in the catalyst (Scheme 2). The Bronsted basic moiety abstracts an a-pro-ton of the ketone to generate an enolate (6), and the Lewis acidic moiety activates the aldehyde (3). 

Amines usually react with epoxides at the less substituted carbon atom (Scheme4.73) [329, 330], With sterically demanding reaction partners these reactions will often proceed slowly or, as with tetraalkyl epoxides, not at all [252, 331]. Higher reaction rates can be achieved by increasing the concentration of the reactants, by using lithium amides as nucleophiles [332], or by catalysis with Lewis acids [252, 333] or Bronsted acids [334]. Ammonia can also be alkylated by 2,3-dialkyl epoxides (80 °C, 15-60 h [335]). Hydroxymethyl epoxides (but not alkoxymethyl epoxides) can be activated toward nucleophilic attack by amines by use of stoichiometric amounts of Ti(OiPr)4 [336] (third example, Scheme4.73). 

---