Carbanions stability order

Carbanion-stabilizing effects have been calculated at several levels of theory. Table 7.6 gives some gas-phase data. The AMI and PM3 semiempirical calculations have also been done in water. The order NO2 > CH=0 > CN > Ph > CH2=CH is in accord with the experimental trends and reflects charge delocalization. The electronegative substituents F, OH, and NH2 are stabilizing by virtue of polar effects. The small stabilization provided by CH3 is presumabfy a polarization effect. 

The effects of fluonnation on carbanion stability are largely deduced from C-H acidity data (p. 988) [64], a-Halogens stabilize carbanions in the order Br > Cl > F, which IS opposite the inductive electron-withdrawing order and reflects the... 

A study of the rates of water-catalyzed detritiation of six disulfonyl-activated carbon acids contains results of interest in connection with electronic effects192. Thus the carbanion stabilizing abilities of several groups as measured kinetically lie in the order SOzPh > SOzEt > S02Me. 

However, experiments in the gas phase gave different results. In reactions of OH with alkyltrimethylsilanes, it is possible for either R or Me to cleave. Since the R or Me comes off as a carbanion or incipient carbanion, the product ratio RH/MeH can be used to establish the relative stabilities of various R groups. From these experiments, a stability order of neopentyl > cyclopropyl > rcrt-butyl > n-pro-pyl > methyl > isopropyl > ethyl was found. On the other hand, in a different kind of gas-phase experiment, Graul and Squires were able to observe CHi ions, but not the ethyl, isopropyl, or (ert-butyl ions. 

Continuing to assume that Gibbs energies and enthalpies are essentially equal, the above reaction is endothermic by 14 kJ moP for R = vinyl. However, it is exothermic for the other R groups cyclopropyl, —6 ethyl, —20 propyl, —22 isobutyl, —26 neopentyl, —31 cyclobutyl, —35 and cyclopentyl, —39 kJmoP. The values are in the order expected for carbanion stability. 

In order to avoid polymerization and to achieve better stereocontrol by quasi-intramolecular addition, a carbanion-stabilizing group and a complexing substituent for capturing alkyllithium/(—)-sparteine in the substrate are useful. This carbolithiation protocol was realized with great success by Marek, Normant and coworkers (equation 125) Addition of n-BuLi/(—)-sparteine (11) onto the lithium alcoholate derived from ( )-cinnamyl alcohol (457) in cumene at 0°C afforded the addition product with 82% yield and 80% ee. 

Isomerization of jS-isophorone to a-isophorone has been represented as a model reaction for the characterization of solid bases 106,107). The reaction involves the loss of a hydrogen atom from the position a to the carbonyl group, giving an allylic carbanion stabilized by conjugation, which can isomerize to a species corresponding to the carbanion of a-isophorone (Scheme 9). In this reaction, zero-order kinetics has been observed at 308 K for many bases, and consequently the initial rate of the reaction is equal to the rate constant. The rate of isomerization has been used to measure the total number of active sites on a series of solid bases. Figueras et al. (106,107) showed that the number of basic sites determined by CO2 adsorption on various calcined double-layered hydroxides was proportional to the rate constants for S-isophorone isomerization (Fig. 3), confirming that the reaction can be used as a useful tool for the determination of acid-base characteristics of oxide catalysts. 

A different technique to measure the relative stabilities of heterocyclyl carbanions is to determine their efficacy as leaving groups. In the hydrolysis of phosphonium salts, the rate-determining step is (iii). By comparing the rates of hydrolysis of 2-thienyl, phenyl and benzyl substituted phosphonium salts, the following order of carbanion stability has been arrived at 2-thienyl>benzyl>phenyl>alkyl (70JCS(B)1490). 

RLi vs C6H5I (9). These K values represent a first-order sequence of carbanion stabilities and relate directly to the parent weak acid RH (M-SAD Series) (71). 

Like other strong bases, carbanions are unlikely to be found in acidic solutions. The stability order of carbanions reflects their high electron density. Alkyl groups and other electron-donating groups slightly destabilize a carbanion. The order of stability is usually the opposite of that for carbocations and free radicals. 

On the other hand, there is the well-known fact that sulfur is an excellent carbanion stabilizer. But not for those reasons popularly ascribed to dir-pir backbonding of the carbanion lone pair into the vacant 3d orbital of sulfur, as suggested earlier. Recent molecular orbital calculations appear to challenge the validity of this model. These calculations, for example, predict the order of gas phase (no solvent effects to worry about) carbanion stabilization as sulfur > oxygen > carbon whether or not the 3d orbitals of sulfur participate in the calculations. Besides, these theoretical considerations surprisingly show that the C-S bond in the thiomethylene carbanion is longer than in the neutral thio-methane parent. Until recently, the prevailing mechanism of stabilization was centered around the classical concept of polarizability of the electron cloud around the sulfur nucleus. 

In the case of unsymmetrically substituted cyclopropanones, one would expect two possibilities for the ring cleavage, reflecting the relative stabilities of the anions formed according to Scheme 14. While this accounts for the facts in most cases, there are examples where the product distribution does not parallel the expected order of carbanion stability. Thus, in the reaction of 2,2-di-t-butylcyclopropanone , the main product is not derived from the primary carbanion but rather from the tertiary carbanion, as shown in Scheme 15. Here, the propensity of the C(l)-C(2) bond to cleave appears to be enhanced by steric considerations involving the presence of two bulky t-butyl substituents at... 

Ab initio MO calculations on the hypothetical anion CH2S(0)H suggest a nonplanar geometry to be preferable at the anionic center, and the order of carbanion stability is shown in Figure 2. ... 

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