Lewis base-Br0nsted acid

Isatins (e.g., 136) are considerably less reactive than simple ketones, and a successful enantioselective cyanoethoxycarbonylation protocol gave poor results for such substratesA Lewis base-Br0nsted acid cooperative enantioselective catalyst now allows reaction at ambient temperature with yields of adducts (137) and ees in the... 

Dixon and co-workers reported on the total synthesis of (i )-rolipram 48 by applying the Michael reaction of dimethyl malonate 44 with nitroolefin 45 in the presence of Lewis-base-Br0nsted-acid bifunctional organocatalyst 46 derived from epicinchonine (Scheme 27.8). The Michael adduct of 47 was obtained in 96% yield with 94% ee. With a single recrystallization from ether-petroleum ether, the enantiomeric purity was increased to 99%. Total synthesis of (7 )-rolipram 48 was achieved in three steps from 47. 

LEWIS ACID LEWIS BASE BR0NSTED THEORY Acid anhydrides,... 

It is a weak base, so no protons are abstracted and actual hydroxyl groups can be observed. It also allows the investigation of both Lewis and Br0nsted acidities [15]. 

For a long period, chiral Lewis adds were the most studied catalysts. Only recently have chiral bases, Br0nsted acids, and N-heterocycUc carbenes been utilized as effective organocatalysts for the oxa-hetero-Diels-Alder reaction. 

Polar Interactions Lewis and Br0nsted Acid-Base Approaches... 

Organocatalysts in principle can be Lewis and Bronsted bases, as well as Lewis and Br0nsted acids however, most of the reported organocatalysts are involved in Lewis base... 

Clearly the two reactions are analogous and demonstrate that the reaction between hydroxide ion and hydrogen bromide is simultaneously a Brpnsted acid-base reaction and a Lewis acid Lewis base reaction Br0nsted acid-base reactions constitute a sub category of Lewis acid Lewis base reactions... 

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... 

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. 

There is no fundamental difference between trimethylamine acting as a Br0nsted base or as a Lewis base, except that in the Br0nsted concept it donates its electrons to a proton electrophile, whereas as a Lewis base it donates its electrons to a Lewis acid electrophile. 

A substance that liberates protons as a consequence of its dissolution or dissociation. See Br0nsted Theory Lewis Acid Lewis Base... 

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). 

Even more general is the Lewis concept of acids and bases a Lewis base has a lone pair available for formation of a coordinate bond, and a Lewis acid has a vacant acceptor orbital handy. This concept is applicable to reactions in the gas phase or in inert solvents (as discussed in the previous section) as well as to complex formation in solution and the acid/ base phenomena studied by Arrhenius, Br0nsted and Lowry. 

Brpnsted-Lowry acid Br0nsted-Lowry base conjugate acid conjugate base acidity constant Ka pKa organic acid organic base Lewis acid Lewis base... 

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