Bronsted bases carbon nucleophile

The mechanism operating in enantioselective Michael reactions under PTC conditions involves the formation of a chiral ion-pair between the nucleophile and the catalyst as the key phenomenon operating in the stereocontrolled formation of the new stereogenic center. The reaction typically incorporates an acidic pro-nucleophile (in almost all cases an enolizable carbonyl compound), the Michael acceptor, the catalyst and a Bronsted base, which is typically an inorganic salt such as a hydroxide or a carbonate (Figure 5.1). It starts with the... 

Because all nucleophiles are Bronsted bases as well (see Chapter 4), we also study correlations between nucleophilicity and basicity. Basidty and nudeophiUdty are often related because they both involve a lone pair of electrons making a bond to another atom. In the case of a base, the lone pair makes a bond to a proton, while with a nudeophile, the lone pair most commonly aeates a bond to an electrophilic carbon. In general, sterically unhindered strong bases are good nucleophiles. 

Michael donors and acceptors are common components in Bronsted base-mediated catalysis. Such transformations offer an uncomplicated route towards all-carbon quaternary stereocenters. In the most basic form, a, 5-unsaturated aldehydes are highly reactive templates towards nucleophilic reactions. Under such conditions, mechanistic studies show no polymerization of the unsaturated aldehydes under cinchona alkaloid catalysis [10]. This absence of polymerization is a key mechanistic indicator that the quinucUdine nitrogen of the catalyst does not act as a nucleophilic promoter. Rather, the quinucUdine nitrogen acts, as predicted, in a Bronsted basic deprotonation-activation of various cycUc and acyclic... 

Other examples are known in which the rate-determining reaction step is a nucleophilic attack of the base on a carbon atom rather than a proton abstraction. They belong to quite a different category, called nucleophilic catalysis. If the logarithms of the second-order rate coefficients for nucleophilic catalysis are plotted against the pK values of the bases Bronsted slopes much larger than 1 (ca. 1.5—2) may be obtained [29]. In some cases, the data do not follow the Bronsted relationship. As it may be... 

The most commonly used type of catalyst is a relatively small, bifunctional molecule that contains both a Lewis base and a Bronsted acid center, the catalytic properties being based on the activation of both the donor and the acceptor of the substrates. The majority of organocatalysts are chiral amines, e.g. amino acids or peptides. The acceleration of the reaction is either based on a charge-activated reaction (formation of an imminium ion 4), or involves the generalized enamine catalytic cycle (formation of an enamine 5). In an imminium ion, the electrophilicity compared to a keton or an oxo-Michael system is increased. If the imminium ion is deprotonated to form an enamine species, the nucleophilicity of the a-carbon is increased by the electron-donating properties of the nitrogen. ... 

Many mechanisms in organic chemistry start with an acid/base reaction. This may be just a simple Bronsted-Lowry protonation of a hydroxyl group, which results in the activation of a C-OH bond or it may be a Lewis acid/base reaction as, for example, when aluminium trichloride complexes with a halogenoalkane in the first step of the Friedel-Crafts reaction. In each case, the initial intermediate usually reacts further and leads to the desired product. In inorganic chemistry, the acid/base reaction may be all that is of interest, e.g. the treatment of a carbonate with an acid to liberate carbon dioxide. However, it is unusual in organic chemistry for the acid/base reaction to be an end in itself. It is for this reason that acid/base characteristics are normally considered as a property of the molecule, similar to the nucleophilic and electrophilic properties to which they are closely related, rather than as a fundamental reaction type as is the case in inorganic chemistry. 

The hydroxide anion may act as a base when it reacts with a proton to form a water molecule. This is a typical acid/base reaction under the Bronsted-Lowry definition. However, it may also act as a nucleophile when it is attracted towards a positive carbon centre. There is, however, an important difference between the positive centre of the proton and that of the carbonium ion. The difference lies in their relative charge densities has the proton or the carbonium ion the greater charge density ... 

Although carbonyls react as a Bronsted-Lowry base or a Lewis base similarly to the chemistry of an alkene, there is a major difference in their reactivity. The C=0 group is polarized such that the oxygen is 6- and the carbon is 6+ (see 2). This polarization, combined with the presence of a weak n-bond, leads to a reaction for carbonyls that is not observed with the C=C unit of alkenes. A nucleophile reacts with the positive carbon of a carbonyl to form a new a-covalent bond to carbon, with cleavage of the x-bond and transfer of those two electrons to the oxygen. 

Nucleophilicity, Lewis basicity, is a measure of how well a nucleophile competes for an empty carbon 2p orbital, and Bronsted basicity is a measure of how well a base competes for an empty hydrogen It orbital. One would expect there to be a general correlation of nucleophilicity with base strength although the two are not exactly the same thing, because and 2p orbitals are different in energy and shape. Nevertheless, basicity and nucleophilicity are related phenomena (Fig. 7.31). 

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