And kinetic acidity

There have been numerous studies of the rates of deprotonation of carbonyl compounds. These data are of interest not only because they define the relationship between thermodynamic and kinetic acidity for these compounds, but also because they are necessary for understanding mechanisms of reactions in which enolates are involved as intermediates. Rates of enolate formation can be measured conveniently by following isotopic exchange using either deuterium or tritium ... 

Table 8.1 is a thermodynamic acidity scale and applies only to positions of equilibria. For the distinction between thermodynamic and kinetic acidity, see p. 228. 

Systematic studies of the thermodynamic and kinetic acidity of metal hydrides in acetonitrile were carried out by Norton et al. [10, 11]. A review of the acidity of metal hydrides presents extensive tabulations of pKa data [12] only a few of the trends will be mentioned here. Metal hydrides span a wide range of pKa values considering only metal carbonyl hydrides shown in Table 7.1, the range exceeds 20 pfCa units. As expected, a substantial decrease in acidity is... 

Table I. Thermodynamic and Kinetic Acidities of Metal Hydrides in Acetonitrile...

It has been shown, by comparison of AGT j (gas phase) and kinetic acidities measured in MeONa-MeOH, that proton transfer to form a hydrogen-bonded carbanion and the subsequent breaking of that weak bond to form a free carbanion in MeOH may differ greatly even for compounds of comparable acidity, such as 9-phenylfluorene... 

Even though conventions of nomenclature designate all metal complexes with a bond to hydrogen as metal hydrides, many metal hydrides can exhibit acidic as well as hydridic behavior. Norton and coworkers studied extensively the thermodynamic and kinetic acidities of a wide series of metal carbonyl hydrides in acetonitrile [7]. Table 3.1 shows a few of the plQ values reported to document the range of pK, values and to illustrate some of the trends more extensive compilations are available [7]. The acidity of (CO)4CoH, the metal hydride involved in catalytic hydroformylations, is 8.3, which is similar to the acidity of HCl in CH3CN. Clearly, some metal hydrides can exhibit quite acidic behavior Morris... 

A study of the thermodynamic and kinetic acidities of (CO)5Cr=C(OMe)CH2R (R = H or Ph) indicates that the resulting anions are significantly resonance stabilised. Changing the solvent from water to a 1 1 mixture of MeCN and water can induce a 2-5 fold increase in the rate of... 

Kinetic acidity is defined as the relative rate of proton abstraction by a base and is often measured by hydrogen isotope exchange. The linear free energy relationship often found between equilibrium and kinetic acidities for related groups of compounds is usually referred to as a Br nsted relationship (equation 4). 

By comparing the measured pfC, with the respective chemical structure (Scheme 8.3), it is easy to understand the influence of substituents on the thermodynamic stability of the enolates, and it is hence possible to appreciate which substrates are suitable to be racemized in situ. Indeed a Broensted plot, correlating thermodynamic and kinetic acidities of the analyzed substrates, shows a straight line (Figure 8.1), indicating that there is a direct relation between the enolate stabihty and the deprotonation rate the more stable the enolate, the faster the anion is formed and, consequently, the faster the racemization at the a-carbon occurs. 

Solubility, thermodynamic and kinetic acidities of nitroalkanes are of great importance in the course of the reaction, but the hydrophobic effect (Lubineau et al., 1994a) could be involved, since additives such as glucose or saccharose accelerate the reaction even more (Lubineau and Aug6, 1992). Amphiphilic molecules can also influence the hydrophobic interactions. In fact, cetyltrimethylammonium chloride (CTACl) as cationic surfactant promoted the Michael reaction of various nitroalkanes with conjugated enones in dilute aqueous solutions of sodium hydroxide (Ballini and Bosica, 1996) ... 

Norton and coworkers examined the thermodynamic and kinetic acidity of a series of transition metal complexes [128-133]. They found that the rates of H transfer from transition metal hydrides can be quite slow when compared to organic and mineral acids of similar pfCa s [134]. Walker et al. attribute the slow rates of transfer from hydride ligands to the following factors (illustrated in... 

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