Lewis base-Br0nsted acid system

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). 

Br0nsted-Lowery acids are H+ donors and bases are H+ acceptors. Strong acids dissociate completely in water. Weak acids only partially dissociate, establishing an equilibrium system. Weak acid and base dissociation constants (Ka and Kb) describe these equilibrium systems. Water is amphoteric, acting as both an acid or a base. We describe water s equilibrium by the Kw expression. A pH value is a way of representing a solution s acidity. Some salts and oxides have acid-base properties. A Lewis acid is an electron pair acceptor while a Lewis base is an electron pair donor. 

Use the Lewis definition of acids and bases to explain the example. given for the Br0nsted-Lowry, Lux-Flood, and. solvent system definitions (Eqs. 9.1-9.4, 9.5-9.8. 9.9-9.36). 

Enantioenriched indolines are very important compounds, which exist extensively in natural alkaloids or unnatural bioactive compounds. The development of facile methods toward chiral indolines continues to be a challenge for organic chemists. Unprotected indole 106 could be considered as a special enamine in an aromatic system. Inspired by the possible shift between enamine and imine of indole in the presence of strong acid. Sun and Chen et al. envisioned that a direct reduction of indole 106 to indoline 107 by a combined catalysis of Lewis base and Br0nsted acid would be highly possible (Scheme 32.24). Indeed, using the strategy... 

Another catalytic system that allows for a direct aldol reaction contains a soft Lewis acid, hard Br0nsted base, and hard Lewis base. This ternary catalyst permits enantioselective aldol reaction between thioamides and enolizable aldehydes. A representative example of this approach is shown in Scheme 10.12. ... 

In Sect. 2.1, heterobimetallic transition metal (Cu)/rare earth metal (Sm) system with Lewis acid/Brpnsted basic properties was introduced. By suitably selecting metal combinations depending on the targeted reactions, variety of chiral bimetallic Lewis acid/Br0nsted base bifunctional catalysts could be created, such as aPd/La/la... 

It is seen that metal and metal oxide catalysts have significant roles in the catalyHc process for the production of fuels and chemicals. There are several catalysts that are used commercially in the biorefinery system to produce fuels and chemicals. Metal oxides are composed of cations possessing Lewis acid sites and anions wifh Br0nsted base sites. They are classified into single metal oxides and mixed metal oxides. Transition metal oxides have catalytic activity for cellulose hydrolysis, and when used as solid acid catalysts, they are reusable and thus may be easily separated from the reaction mixture. 

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