Stability kinetic anion

As a result, these anions can gain exceptional stability and their carbon acids are unusually acidic. Because the above resonance forms probably play very important roles in stabilizing the anion, these species are not purely carbanions. However, thermodynamics favors reactions (protonation or alkylation) at carbon so their behavior is generally characteristic of carbanions. It also should be noted that kinetics often favor reaction at the heteroatom in these anions though leading to the less stable product. ... 

Thus it is evident that the modified polyethylenimines provide a matrix for achieving homogeneous catalysis of decarboxylation of activated anionic substrates in an aqueous environment. Clearly, the modified polyethylenimines provide solvation features that stabilize the anionic transition structure in the state with particularly sensitive bonds. Large solvent effects have been observed in kinetic studies of many reactions involving anions.46 47,50,51 Suitable derivatives of polyethylenimine should manifest interesting effects in many of these reactions also. 

When i,2-dimethylimidazole is lithiated both 2- and 5-isomers are formed, the former under kinetic control. Transmetallation at temperatures above — 100°C gives the 5-lithio derivative. Because the 5-anion is harder than the 2-methylene anion, harder Lewis acids (e.g., D2O, R3SnCl, MejSiCI) react better at the 5-position <83JCS(pi)271>. Sometimes a particular regioisomer of a lithioimidazole can be stabilized by coordination. Thus, some 1-alkoxymethyl groups are able to stabilize an anion at C-5 <90JCR(S)58>. 

The high values of the pK s of carbon acid substrates and the associated instability of enolate anion intermediates in nonenzymatic reactions first led to the expectation that these intermediates could not be rendered kinetically competent in enzymatic reactions [3]. As a result, the expectation was that these reactions must be concerted, thereby avoiding the problem of how an active site might provide sufficient, significant stabilization of the intermediates. However, the weight of the experimental evidence now is that enzymes that abstract protons from carbon acids are able to sufficiently stabilize enolate anion intermediates so that they can be kinetically competent. 

By comparison, decarboxylation is largely a kinetic problem. Enzymes have developed a variety of strategies for stabilizing the anionic intermediate that is produced in the decarboxylation step. Metal ion stabilization of enolates is a common theme, particularly for decarboxylation of /8-keto acids. The most elegant solutions are perhaps the extensive electron delocalizations seen in pyri-doxal phosphate and thiamin pyrophosphate. 

Summary In contrast to carbon chemistry, compounds with two or three H-acidic electronegative groups like NH2 or OH at one silicon atom can be stabilized kinetically [1 - 5]. In the 1980s the first aminosilanol [6], (Me3C)2Si(NH2)OH, was prepared. Its alkaline metal derivatives form an aminosilanolate anion that is 15.9 kcal/mol more stable than the isoelectronic amidosilanol anion [7]. 

In some cases donation of nonbonded electrons by atoms )3 to the carbene carbon can stabilize singlet carbenes dramatically. Arduengo and co-workers reported that treating 1,3-di-l-adamantylimidazolium chloride (34) with the anion CH3SOCH2 in THF produced the singlet carbene 1,3-di-l-adamantyl-inudazol-2-ylidene (35, equation 5.29). This carbene is stabilized thermodynamically by electron donation from the two amino groups, and it is stabilized kinetically by bulky adamantyl substituents that make it less likely to react intermolecularly. As a result, 35 is stable in the presence of air and moisture, and the crystals of 35 melt at 240-241°C. 

FIGURE 17.18 Pigment particles stabilized with anionic surfactants adsorbed on their surfaces. The kinetic stability is ensured by the barrier of energy xKT. If the particles cross over the barrier, they fall into the unstable zone from where, to redisperse them, one would need (x + y)KT of energy. PE = potential energy. 

Removal of the middle proton leads to a resonance-stabilized enolate anion with three resonance structures, as shown. Removal of the methyl proton leads to an enolate anion with only two resonance contributors. The middle proton is significantly more acidic and will deprotonate to give the enolate shown. If LDA is used, the kinetic enolate is the one derived from removal of the more acidic proton, which is the same enolate anion. 

The energy of the bottom of the conduction band is one factor influencing electron kinetics. Anions formed by attachment are stabilized in solution, and volume changes are manifestations of the free energy... 

Other studies have demonstrated the importance that droplet charge has on lipid oxidation kinetics in oil-in-water emulsions (McClements and Decker, 2000 Hu et al., 2003 Klinkesom et al., 2005 Mei et al., 1998a, 1998b). In a similar study. Boon and co-workers (2008) showed that the oxidative stability of lycopene emulsions is higher when stabilized with cationic and non-ionic surfactants than in emulsions stabilized with anionic surfactants. 

Hthiated 4-substituted-2-methylthia2oles (171) at -78 C (Scheme 80). Crossover experiments at—78 and 25°C using thiazoles bearing different substituents (R = Me, Ph) proved that at low temperature the lithioderivatives (172 and 173) do not exchange H/Li and that the product ratios (175/176) observed are the result of independent metala-tion of the 2-methyl and the C-5 positions in a kinetically controlled process (444). At elevated temperatures the thermodynamic acidities prevail and the resonance stabilized benzyl-type anion (Scheme 81) becomes more abundant, so that in fine the kinetic lithio derivative is 173, whereas the thermodynamic derivative is 172. 

For the deprotonation of less acidic precursors, which do not lead to mesomerically stabilized anions, butyllithium/TMEDA in THF or diethyl ether, or the more reactive, but more expensive,. seobutyllithium under these conditions usually are the most promising bases. Het-eroatomic substitution on the allylic substrate, which docs not contribute to the mesomeric or inductive stabilization often facilitates lithiation dramatically 58. In lithiations, in contrast to most other metalations, the kinetic acidity, caused by complexing heteroatom substituents, may override the thermodynamic acidity, which is estimated from the stabilization of the competing anions. These directed lithiations59 should be performed in the least polar solvent possible, e.g.. diethyl ether, toluene, or even hexane. 

Bancroft and Gesser [870] conclude that kinetic factors are predominant in determining whether decomposition of a metal bromate yields residual bromide or oxide. The thermal stabilities of the lanthanide bromates [877] and iodates [877,878] decrease with increase in cationic charge density, presumably as a consequence of increased anionic polarization. Other reports in the literature concern the reactions of bromates of Ag, Ni and Zn [870] and iodates of Cd, Co, Mn, Hg, Zn [871], Co and Ni [872], Ag [864], Cu [867], Fe [879], Pb [880] andTl [874]. 

The kinetic stability of 17 increases on deprotonation. The half-life times of 17 and its anion N 19 have been estimated [104] from the observed [105, 106] and computed free energy to be only 10 min and 2.2 days, respectively. The high kinetic stability of the anion 19 can be understood in terms of enhanced pentgon stability and aromaticity. The deprotonation raises the energy of lone pair orbitals and promotes cyclic delocalization of o- and rr-electrons. 

The fundamental aspects of the structure and stability of carbanions were discussed in Chapter 6 of Part A. In the present chapter we relate the properties and reactivity of carbanions stabilized by carbonyl and other EWG substituents to their application as nucleophiles in synthesis. As discussed in Section 6.3 of Part A, there is a fundamental relationship between the stabilizing functional group and the acidity of the C-H groups, as illustrated by the pK data summarized in Table 6.7 in Part A. These pK data provide a basis for assessing the stability and reactivity of carbanions. The acidity of the reactant determines which bases can be used for generation of the anion. Another crucial factor is the distinction between kinetic or thermodynamic control of enolate formation by deprotonation (Part A, Section 6.3), which determines the enolate composition. Fundamental mechanisms of Sw2 alkylation reactions of carbanions are discussed in Section 6.5 of Part A. A review of this material may prove helpful. 

Among Michael acceptors that have been shown to react with ketone and ester enolates under kinetic conditions are methyl a-trimethylsilylvinyl ketone,295 methyl a-methylthioacrylate,296 methyl methylthiovinyl sulfoxide,297 and ethyl a-cyanoacrylate.298 Each of these acceptors benefits from a second anion-stabilizing substituent. The latter class of acceptors has been found to be capable of generating contiguous quaternary carbon centers. 

The composition of the electrolyte is quite important in controlling the electrolytic deposition of the pertinent metal, the chemical interaction of the deposit with the electrolyte, and the electrical conductivity of the electrolyte. In the case of molten salts, the solvent cations and the solvent anions influence the electrodeposition process through the formation of complexes. The stability of these complexes determines the extent of the reversibility of the overall electroreduction process and, hence, the type of the deposit formed. By selecting a suitable mixture of solvent cations to produce a chemically stable solution with strong solute cation-anion interactions, it is possible to optimize the stability of the complexes so as to obtain the best deposition kinetics. In the case of refractory and reactive metals, the presence of a reasonably stable complex is necessary in order to yield a coherent deposition rather than a dendritic type of deposition. 

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