Brpnsted base catalysis compounds

With 1-alkoxyallenes as proelectrophiles, the palladium-catalysed asymmetric allylic alkylation proceeds with 1,3-dicarbonyl compounds as pronucleophiles with excellent regioselectivity good enantioselectivity (82-99% ee) was obtained with the Trost lig- and. The pH of the medium proved crucial for the reactivity and selectivity. By using the more acidic Meldrum s acids, the reactions required a co-catalytic amount of a Brpnsted acid, such as CF3CO2H. On the other hand, the less acidic 1,3-diketones failed to react under these conditions but the reaction proved to occur in the presence of the weaker benzoic acid, suggesting the need for general base catalysis. Indeed, a mixture of Et3N and PhCOiH proved to be optimal (93-99% ee). A mechanistic model to rationalize these results has been developed.88... 

The compounds considered as bases according to the Lewis classification are practically the same as those by the Brpnsted definition Species, which are able to add a proton, possess a pair of electrons and, naturally, they react with electron-pair acceptors. In contrast, there are substantial differences when acids are considered. First of all, Brpnsted acids are able to split off proton, whereas Lewis acids do not necessarily contain a hydrogen atom. Acid-base reactions, correspondingly, are much simpler in the Br0nsted sense, since they involve proton transfer, that is, the transfer of a nucleus without electrons. Furthermore, the reaction of any Brpnsted acid with a base B produces the same species, the conjugate acid BH+ irrespective of the acid. That is why the behavior of Brpnsted acids is similar toward all bases and indicators, and their catalytic effect is the same in specific acid catalysis. The products of the reaction of Lewis acids with a base, in turn, are different with each acid-base complex having specific properties. The behavior of Lewis acids toward bases and indicators, and their catalytic effect, therefore, may be substantially different. It is quite interesting to point out that an acid is a proton donor in the Brpnsted sense, but the proton itself is the acid in the Lewis picture due to its vacant orbital. 

The aldol reaction is an important carbon-carbon bond-forming method for constructing p-hydroxy carbonyl compounds in which new stereogenic centers are created. Especially, regio- and stereoselective aldol reactions are the most useful for organic synthesis of complex molecular skeletons [11-15]. From a viewpoint of atom economy, an aldol reaction via direct formation of an enolate with a catalytic amoimt of base is highly desired, and high Brpnsted basicity of the alkaline-earth metal compounds is suitable for this purpose. In recent researches on chiral alkaline-earth metal catalysis, direct-type asymmetric aldol and related reactions have been developed. 

The Mukaiyama aldol reaction is beyond doubt a brilliant triumph of modem synthetic organic chemistry however, the reaction products are contaminated with pre-activated silyl enol ethers derived from the carbonyl compounds with stoichiometric amounts of silylation agent and base. In addition, silylated wastes are inherently formed. Circumventing the pre-activation process improves atom efH-ciency in this case, the carbonyl nucleophiles react directly with the carbonyl electrophiles in the presence of catalyst. The first Bronsted acid-catalyzed direct aldol reactions have been achieved using chiral Hg-BINOL-derived phosphoric acid 96 (Scheme 28.12) [66], The aldol products (127) have syn-configurations and, thus, this reaction is complementary to (S)-proHne catalysis in Brpnsted acids, which in general yields the anti configuration [11]. 

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