Anion vinylic

Synthesis The vinyl anion synthon can either be the vinyl Grignard reagent or the acetylide arrion, in which case the synthesis becomes ... 

The vinyl anion synthon is best represented by an acetylide ion (frame 33). Synthesis ... 

Several intermediates are involved in the latter reaction. The first is a radical anion resulting from electron transfer from sodium to the alkyne. This then deprotonates ammonia leading to a vinyl radical. The process repeats (electron transfer and deprotonation), and involves a vinyl anion intermediate. 

The vinylic radica1 accepts another electron from a second lithium atom to produce a vinylic anion. . . ... 

Trans stereochemistry of the alkene product is established during the second reduction step when the less hindered trans vinylic anion is formed from the vinylic radical. Vinylic radicals undergo rapid cis-trans equilibration, but vinylic anions equilibrate much less rapidly. Thus, the more stable trans vinylic anion is formed rather than the less stable cis anion and is then protonated without equilibration. 

A reasonable idea of the stability of the stereoisomeric trigonal vinyl cations can be gained from the behavior of vinyl anions and radicals. It is known that the interconversion between stereoisomeric vinyl anions is fairly slow, with an activation energy of the order of 18-24 kcal/mole (171). On the other hand, inversion of stereoisomeric vinyl radicals is reasonably rapid, even at fairly low temperatures, with an activation energy of the order of 2-8 kcal/mole (172). Hence, extrapolating from the electron-rich vinyl anion through the neutral vinyl radical to the electron-deficient vinyl cation, one would expect rapid interconversion between stereoisomeric vinyl cations and only a small amount (if any) of stereospecificity. To put it differently, the vinyl cation should be mostly linear with an empty p orbital and very little trigonal character. 

The anion from vinyl ether (20) (this vinyl anion... 

Since vinyl anions generally retain configuration 39> while isomeric vinyl radicals rapidly interconvert 40) these results constitute evidence that reductions of alkyl iodides do proceed via radical intermediates. Isomerization of stereoisomeric vinyl anions is ruled out by the lack of effect of phenol on the stereochemistry of the products (Scheme III). Since cis and trans-3-hexene are formed in differing proportions from the two halides, it may be concluded that the stereoisomeric vinyl radicals are being intercepted by electron trans-... 

The energy barrier calculated for inversion of the vinyl anion (2---> 21) by changing... 

TABLE 3. Bond distances (in A) and angles (in degrees) in ethylene, 1, and vinyl anion, 2°... 

TABLE 4. SCF and MP2 energies for ethylene and vinyl anion and the deprotonation energy (A/7lcid) for ethylene0,6... 

The formation of the acetal 271 is evidence for a polar nature of 258 in terms of the zwitterions 258-Zi, whose pyrylium-ion character explains the attack of the tert-butoxide ion to give the vinyl anion 265 (Scheme 6.58). In contrast to 260/260-Zi, 258 does prefer cycloadditions if it is generated in the presence of an activated... 

This is manifest in the reactivity of 180/180-Z1 which was generated from 3-bromo-41-f-pyran (283) by /3-elimination of hydrogen bromide with KOtBu (Scheme 6.61). Whether or not this reaction was conducted in the presence of styrene or furan, the only product identified was tert-butyl 4H-pyran-4-yl ether (284). This is in line with the relationship of the intermediate to a pyrylium ion. Thus, the addition of the tert-butoxide ion to 180/180-Zj has to be expected at the 4-position with formation of the vinyl anion 285, which is then protonated to give 284. Likewise, the attack of the nucleophile is predicted at C2and C6 leading to the vinyl anions 286, which... 

The Michael-type conjugate addition of an alkoxide such as methoxide to an a,p-unsaturated nitrile or aldehyde proceeds in solution quite readily, being complete in 5—10 minutes. In the gas phase however, it was found that reaction (6a) does not occur there is no evidence of the addition product even at the longest trapping times, nor in the unquenched mode. Rather, the sole product observed is due to proton loss from the nitrile, to form a nominal vinyl anion, reaction (6b). 

Unfortunately, the vinyl anion undergoes protonation under these reaction conditions and leads to the simple aUcene 92 as the major product. It has been suggested that the proton source is actually the tosylhydrazone monoanion 88 , and that the basicity of 91 is strong enough to abstract an a proton from the unreacted tosylhydrazone monoanion 88 or even from 89 by ortijo-metalation of the tosyl ring. Thus, in order to avoid this protonation in the reaction, an excess of base (>3 equivalents) was required to obtain good conversion into alkenyl derivatives. 

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... 

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