Basicity properties, carbanions

Among much less nucleophilic substrates, alcoholates are very interesting. They are often very inexpensive, they allow the formation of carbanions, they generally do not compete with carbanions in condensation reactions, they lead to complex bases able to give elimination reactions (vide infra) and finally they allow a large modulation of the basic properties. For these reasons we studied and we are continuing to study a lot of them. 

The poor solvating property of t-butyl alcohol coupled with the strong basic properties of /-butoxide encourage a transition state with well-developed double-bond character and transition state I and to a lesser extent II are favoured. In the more solvating media, C -X bond breaking is depressed and a shift towards a carbanion-like E2 transition state is encouraged. Conforma-... 

To evaluate properties of basic catalysts, the Knoevenagel condensation over aluminophosphate oxynitrides was investigated [13]. In this reaction usually catalysed by amines, the solid catalysts function by abstraction of a proton from an acid methylene group, which is followed by nucleophilic attack on the carbonyl by the resultant carbanion, re-protonation of oxygen and elimination of water. The condensation between benzaldehyde and malononitrile is presented below. 

The most widely studied physical property of carbanions is their basicity, which of course is a direct measure of the acidity of the parent carbon acid. Carbon acidity measurements date back to the early part of the twentieth century and a myriad of techniques have been employed for the measurements. Although early measurements were only able to provide semiquantitative data, more recent ones have resulted in accurate acidity measurements across a vast range of effective acid dissociation constants, Ka values. This section will begin with a brief description of definitions and methodologies followed by representative data as well as applications of those data. 

As noted in Section 4.2.1, the gas phase has proven to be a useful medium for probing the physical properties of carbanions, specifically, their basicity. In addition, the gas phase allows chemists to study organic reaction mechanisms in the absence of solvation and ion-pairing effects. This environment provides valuable data on the intrinsic, or baseline, reactivity of these systems and gives useful clues as to the roles that solvent and counterions play in the mechanisms. Although a variety of carbanion reactions have been explored in the gas phase, two will be considered here (1) Sn2 substitutions and (2) nucleophilic acyl substitutions. Both of these reactions highlight some of the characteristic features of gas-phase carbanion chemistry. 

Figure 3.9a may also represent the interaction of a nonbonded ( lone-pair ) orbital with an adjacent polar n or a bond [67]. If a polar n bond, one can explain stabilization of a carbanionic center by an electron-withdrawing substituent (C=0), or the special properties of the amide group. If a polar a bond, we have the origin of the anomeric effect. The interaction is accompanied by charge transfer from to A, an increase in the ionization potential, and a decreased Lewis basicity and acidity. These consequences of the two-electron, two-orbital interaction are discussed in greater detail in subsequent chapters. 

Pak has shown, by a correlation between O—H stretching frequency and the strength of the M—OH bond, that the Bronsted acidity of ZnO is similar to that of MgO and NiO and is much less than that of y-Al20a. A similar conclusion was reached from studies of benzene adsorption and of its effect on hydroxyl stretching frequencies, a for ZnO of 10.5 being reported. In general then we may expect ZnO to display basic/Lewis acid properties in catalytic reactions, as shown by MgO, on which carbanions are readily produced from both alkenes... 

The chemical reactivities of the alkali metal organometallic compounds (RM) vary widely depending on metal M, basicity of the solvent systems used, and steric and electronic properties of the organic group R. In many reactions an important factor is the stabilization resulting from formation of a delocalized carbanion system as in the polymerization of dienes or aromatic substituted ethylenes, and in Reactions 3, 4, 5, and 10 in Table I. It is primarily with these delocalized carbanion systems that this review is concerned although saturated organolithium compounds are discussed briefly. 

In Fig. 2 we have represented both the r Acetone values and the total site density (nj) as a function of catalyst composition. Qualitatively, the variation of r Acetone with increasing A1 content is similar to that followed by nj thereby suggesting that acetone conversion depends on both acid and base sites. Pure MgO was the most active catalyst whereas AI2O3 showed the lowest activity. This is because Al-0 pairs are much less active than Mg-0 pairs for promoting the proton abstraction and carbanion stabilization steps involved in aldol condensation reactions. We have showed [1] that the acetone aldolization rate is controlled on basic catalysts by the number of metal cation-oxygen anion surface pairs. Mg-rich Mg AlOx oxides are less active than MgO because they exhibit a lower base site density and also poor acidic properties. In contrast, Al-rich Mg AlOx oxides are more active than AI2O3 due to a proper combination of acid and base sites. 

Carbanions as intermediates have been suggested in the base-catalyzed isomerization of imsaturated nitriles. As long as zeolites with anion-exchange properties are not available, basic centers must be introduced by exchange of the nonframework ions by alkali metal ions or of silicon by germanium, by precipitation... 

Many important organic reactions involve nucleophilic carbon species (car-banions). The properties of carbanions will be discussed in detail in Chapter 7 and in Part B, Chapters 1 and 2. Most C—H bonds are very weakly acidic and have no tendency to ionize spontaneously to form carbanions. Reactions that involve car-banion intermediates are therefore usually carried out by reaction of the neutral organic molecule and the electrophile in the presence of a base that can generate the more reactive carbanion intermediate. Base-catalyzed condensation reactions of carbonyl compounds provide many examples of this type of reaction. The reaction between acetophenone and benzaldehyde which was considered in Section 4.2, for example, requires a basic catalyst to proceed, and the kinetics of the reaction show that the rate is proportional to the catalyst concentration. This is because the neutral acetophenone molecule is not nucleophilic and does not react with benzaldehyde. The enolate (carbanion) formed by deprotonation is much more nucleophilic. 

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