Carbanionic transition states generalization

Comparison of the data for methoxide with those for t-butoxide in Table 6.4 illustrates a second general trend Stronger bases favor formation of the less substituted alkene. " A stronger base leads to an increase in the carbanion character at the transition state and thus shifts the transition state in the Elcb direction. A linear correlation between the strength of the base and the difference in AG for the formation of 1-butene versus 2-butene has been established. Some of the data are given in Table 6.5. 

Amination of the deactivated carbanion of 4-benzylpyridine formed with excess sodamide presumably proceeds because the strong indirect deactivation is overcome by electrophilic attack by Na+ at the partially anionic azine-nitrogen and by concerted nucleophilic attack by H2N at the 2-position via a 6-membered cyclic transition state (75). However, in simple nucleophilic displacement a carbanion will be more deactivating than the corresponding alkyl group, as is true in general for anionic substituents and their non-ionic counterparts. 

The main aim of this review is to survey the reactions by which the Co—C bond is made, broken, or modified,.and which may be used for preparative purposes or be involved in catalytic reactions. Sufficient evidence is now available to show that there exists a general pattern of reactions by which the Co—C bond can be made or broken and in which the transition state may correspond to Co(III) and a carbanion (R ), Co(II) and a radical (R-), Co(I) and a carbonium ion (R ), or a cobalt hydride (Co—H) and an olefin. Reactions are also known in which the organo ligand (R) may be reversibly or irreversibly modified (to R ) without cleavage of the Co—C bond, or in which insertion occurs into the Co—C bond (to give Co—X—R). These reactions can be shown schematically as follows ... 

Kinetic Acidities in the Condensed Phase. For very weak acids, it is not always possible to establish proton-transfer equilibria in solution because the carbanions are too basic to be stable in the solvent system or the rate of establishing the equilibrium is too slow. In these cases, workers have turned to kinetic methods that rely on the assumption of a Brpnsted correlation between the rate of proton transfer and the acidity of the hydrocarbon. In other words, log k for isotope exchange is linearly related to the pK of the hydrocarbon (Eq. 13). The a value takes into account the fact that factors that stabilize a carbanion generally are only partially realized at the transition state for proton transfer (there is only partial charge development at that point) so the rate is less sensitive to structural effects than the pAT. As a result, a values are expected to be between zero and one. Once the correlation in Eq. 13 is established for species of known pK, the relationship can be used with kinetic data to extrapolate to values for species of unknown pAT. 

In general, in cases where the carbanion can be stabilized by catalytic intervention, as in the initial conversion of a 2-ketoacid to a cyanohydrin, the transition state leading to its formation will be stabilized. In addition, the stability of the carbanion generated by loss of carbon dioxide also depends on its molecular environment. The rate of decarboxylation of pyridine-2-carboxylic acid is enhanced in a nonpolar environment as the zwitterionic ground state is destabilized and the less polar transition state is stabilized.5... 

The solvent effects on the absorption spectra of ion pairs were studied by many authors and the direction of the observed shift depends on the change (increase or decrease) of dipole moment upon the electronic transition [25]. Generally a bathochromic shift is observed with an increase of solvent polarity. When going from a polar solvent to a less polar one, the association in the ground state increases more strongly than in the excited state this may be understood if the ion pair switches progressively from SSIP to CIP status. Observations of this type were often made, together with cation effects, as for instance in the case of alkali phenolates and enolates [7], fluorenyl and other carbanion salts [22] or even for aromatic radical anions [26, 27],... 

Recent work [log] casts some doubt on the generality of this interpretation. Evidence from deuterium-exchange experiments on butan-2-one indicates that the transition state for enolisation has little if any carbanion character that weak bases, like acids, favour formation of the more substituted enol, and that even the fairly strong base DO" caused deute-... 

Reaction of the a-carbanion of an alkyl aryl sulfoxide (RCH2SOAr) with aldehydes may give four dia-stereomers. In general, the reaction is highly diastereoselective with respect to the a-sulEnyl carbon, but poorly diastereofacially selective with respect to attack on the carbonyl component. In fact, the a-carb-anion (31) of benzyl t-butyl sulfoxide adds to an aldehyde to produce only two diastereomers (32a) and (32b). As shown in Scheme 10, the selectivity increases when the counterion is A transition state structure (33) is proposed to account for the anti stereoselection. Addition of the dianion of (/ )-3-(p-to-lylsulfinyOptopionic acid (34) to aldehydes affords two main diasteieoisomeric 3 (p-tolylsulfinyl)-y-lac-tones (35 R = Ph and Bu. ca. 60 40). These isomers (35) were separated by chromatography, and their... 

D.A. Jencks and W.P. Jencks, On the Characterisation of Transition States by Structure-Reactivity Coefficients, J. Am. Chem. Soc., 1977, 99, 7948. W.P. Jencks, When is an Intermediate Not an Intermediate Enforced Mechanisms of General Acid-Base Catalysed, Carbocation, Carbanion, and Ligand Exchange Reactions, Aces. Chem. Res., 1980, 13, 161. 

The energy required to proceed from reactants to products is AG, the free energy of activation, which is the energy at the transition state relative to the reactants. We develop the theoretical foundation for these ideas about reaction rates in Section 3.2. We first focus attention on the methods for evaluating the inherent thermodynamic stability of representative molecules. In Section 3.3, we consider general concepts that interrelate the thermodynamic and kinetic aspects of reactivity. In Section 3.4, we consider how substituents affect the stability of important intermediates, such as carbocations, carbanions, radicals, and carbonyl addition (tetrahedral) intermediates. In Section 3.5, we examine quantitative treatments of substituent effects. In the final sections of the chapter we consider catalysis and the effect of the solvent medium on reaction rates and mechanisms. 

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