Stabilized carbanions

Carbanions stabilized by phosphorus and acyl substituents have also been frequently used in sophisticated cyclization reactions under mild reaction conditions. Perhaps the most spectacular case is the formation of an ylide from the >S-lactam given below using polymeric Hflnig base (diisopropylaminomethylated polystyrene) for removal of protons. The phosphorus ylide in hot toluene then underwent an intramolecular Wlttig reaction with an acetyl-thio group to yield the extremely acid-sensitive penicillin analogue (a penem I. Ernest, 1979). 

Table 1. Relationships Between Monomer Reactivity, Carbanion Stability, and Suitable Initiators...

The most useful reactions combine carbanion nucleophiles with activated aziridines. For example, the ring expansion which occurs on treatment of aziridines (219) with malonate salts typifies the heterocyclic synthesis possible. The conversion is quite general since many analogous transformations have been observed in which different carbanion stabilizing substituents were employed (73S546). 

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. 

Table 7.6. Carbanion Stabilization Iq Substituent Groups (kcal/mol)...

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... 

Alkenyllithium derivatives, carrying carbanion-stabilizing substituents, which facilitate the formation of solvent-separated ion pairs, can also exhibit preparatively useful configurational stability in respect to the double bond of the precursor. 

Freparatively useful induced diastereoselectivities have been reported mainly for 1,1-di-substituted allyllithium derivatives which bear carbanion-stabilizing substituents. l-[Methyl-thio-l-(trimethylsilyl)-2-propenyl]lithium106 and the appropriate 1-phenylthio107 derivative, generated from the allylic sulfide with sec-butyllithium, in the reaction with tetrahydropyranyl-protected pregnolone, furnish a single diastereomer. 

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. 

A different approach to the problem of hydrocarbon acidity, and hence carbanion stability, is that of Shatenshtein and Shapiro, who treated hydrocarbons with deuterated potassium amide and measured the rates of hydrogen exchange. The experiments did not measure thermodynamic acidity, since rates were measured, not positions of equilibria. They measured kinetic acidity, that is, which compounds... 

Stabilization by Sulfur or phosphorus. Attachment to the carbanionic carbon of a sulfur or phosphorus atom causes an increase in carbanion stability, though the reasons for this are in dispute. One theory is that there is overlap of the unshared pair with an empty d orbital" (pn-dn bonding, see p. 45). For example, a carbanion containing the SO2R group would be written... 

However, there is evidence against d-orbital overlap and the stabilizing effects have been attributed to other causes. In the case of a PhS substituent, carbanion stabilization is thought to be due to a combination of the inductive and polarizability effects of the group, and d-pn resonance and... 

Stabilization by a Nonadjacent n Bond. In contrast to the situation with carbocations (see pp. 408-411), there have been fewer reports of carbanions stabilized by interaction with a nonadjacent 7t bond. One that may be mentioned is 13, formed when optically active camphenilone (11) was treated with a strong base (potassium terf-butoxide). ° That 13 was truly formed was shown by the following facts (1) A proton was abstracted ordinary... 

Ellison, G.B. Engelking, PC. Lineberger, W.C. J. Am. Chem. Soc., 1978, 100, 2556. Retention of configuration has never been observed with simple carbanions. Cram has obtained retention with carbanions stabilized by resonance. However, these carbanions are known to be planar or nearly planar, and retention was caused by asymmetric solvation of the planar carbanions (see p. 764). 

P-Keto esters have been prepared in moderate to high yields by treatment of aldehydes with diethyl diazoacetate in the presence of a catalytic amount of a Lewis acid such as SnCL, BF3, or GeCL. The reaction was successful for both aliphatic and aromatic aldehydes, but the former react more rapidly than the latter, and the difference is great enough to allow selective reactivity. In a similar process, aldehydes react with certain carbanions stabilized by boron, in the presence of (F3CC0)20 or NCS, to give ketones. 

However, attack at the 3 position has been reported when the 4 position contains one or two carbanion-stabilizing groups such as SiMe3 Klumpp, G.W. Mierop, A.J.C. Vrielink, J.J. Brugman, A. Schakel, M. J. Am. Chem. Soc., 1985, 107, 6740. 

The intermediate corresponding to 64 in the case of 58 is a symmetrical compound, and the three-membered ring can be opened with equal probability on either side of the carbonyl, accounting for the results with In the general case, 64 is not symmetrical and should open on the side that gives the more stable carbanion. This accounts for the fact that 60 and 61 give the same product. The intermediate in both cases is 63, which always opens to give the carbanion stabilized by resonance. The cyciopropanone intermediate (64) has been isolated in the case where R2=R5 f.Bu and R =R =H,... 

Structural factors necessary for carbocation and carbanion stability are discussed briefly. The mechanistic problems of dissociation of the carbon-carbon a bond are then discussed, and finally the physicochemical nature of hydrocarbon salts and the related novel hydrocarbons. 

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