Vinyl anions, configurational stability

The question of configurational stability has been investigated first for vinylidene carbenoids and, more recently, for alkylcarbenoids. Vinyl anions are usually considered to be configurationally stable" ° the calculated inversion barrier of the ethenyl anion 10 (R = H) is about 35 kcal mol (equation 4)" . Concerning lithioalkenes, this configurational stability has been confirmed experimentally for a-hydrogen, a-alkyl and a-aryl substituted derivatives . The inversion of vinylidene lithium carbenoids was already... 

The anti stereochemistry of the sodium-ammonia reduction appears to result from the greater stability of the vinyl radical in the trans configuration, where the alkyl groups are farther apart. An electron is added to the trans radical to give a trans vinyl anion, which is quickly protonated to the trans alkene. 

There are two other reactive species whose cis-trans isomerism can be important to the distribution of products, organometallics derived from vinyl halides, and enolate anions. The configurational stability of these species will be discussed in detail in Sections 8.4.F, 8.5.D and in Sections 9.2.C, 9.5, respectively. The... 

Stereoelectronic features of vinyl anions are associated with their bent geometries and usual configurational stabilities in solution. Theory predicts a sizable inversion barrier for the parent vinyl anion (28.7kcal/ mol at the MP2/6-31+G(d)//HF/6-31 -nG(d) level). The two conformers of 1-propenyl anion can be formed stereospecifically in the gas phase (via desilylation of E- and Z-l-trimethylsilylpropene) and show different reactivity patterns. ... 

Both the radical anion and the vinylic anion can have either the cis or the trans configuration. The cis and trans radical anions are in equilibrium, and so are the cis and trans vinylic anions. In each case, the equilibrium favors the trans isomer. The greater stability of the trans isomers is what causes the product to be a trans alkene (Figure 7.5). 

The chiral cyanide (618) has been synthesized to study the stereochemical stability of the vinyl anion in protonic solvents. At 90 in methanol using sodium methoxide as a base, the vinyl anion is formed and exhibits a very high degree of retention of configuration (>99%) on reprotonation. ... 

The ether-catalyst complex (II) splits into a complex anion (III) and a carbonium ion (IV), which rearranges to the configuration of maximum stability (V). This carbonium ion (V) could itself initiate polymerisation, but it is more likely that it attacks the double bond of the closely associated anion (III), giving the double ion (VI) in equilibrium with the aldehyde (VII). Rearrangements of the type (I)-(VII) have been observed for vinyl ethers [7], and a closely parallel isomerisation is that of isobutyl phenyl ether into para-tertiary butyl phenol under the influence of A1C13 [8]. It is unlikely that the steps from (II) to (VI) take place in a well defined succession. The process probably proceeds by a single intramolecular transformation. 

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