Enolate vinylation

Frequently, when the enol content is high, both forms can be isolated. The pure keto form of acetoacetic ester melts at — 39°C, while the enol is a liquid even at — 78°C. Each can be kept at room temperature for days if catalysts such as acids or bases are rigorously excluded.Even the simplest enol, vinyl alcohol (CH2= CHOH), has been prepared in the gas phase at room temperature, where it has a half-life of 30min. " The enol Me2C=CCHOH is indefinitely stable in the solid state at —78°C and has a half-life of 24h in the liquid state at 25°C. When both forms cannot be isolated, the extent of enolization is often measured by NMR. 

The iminium ion then reacts with an enol (vinyl alcohol) compound (such as the side chain of tyrosine) to form a cross-link (reaction 15.6). 

Mechanism. Addition of HgS04 generates a cyclic mercurinium ion, which is attacked by a nucleophilic water molecule on the more substituted carbon. Oxygen loses a proton to form a mercuric enol, which under work-up produces enol (vinyl alcohol). The enol is rapidly converted to 2- butanone. 

We have established that the formation of enols is catalysed by acids and bases. The reverse of this reaction—the formation of ketone, from enol—must therefore also be catalysed by the same acids and bases. If you prepare simple enols in the strict absence of acid or base they have a reasonable lifetime. A famous example is the preparation of the simplest enol, vinyl alcohol, by heating ethane-1,2-diol (glycol—antifreeze) to very high temperatures (900 °C) at low pressure. Water is lost and the enol of acetaldehyde is formed. It survives long enough for its proton NMR spectrum to be run, but gives acetaldehyde slowly. 

In the gas phase the free energy of protonation of A,N-dimethylvinylamine (equation 48) is — 220.0 kcal mol-1 64. As discussed in Section II.A.4, protonation occurs at Cp, as shown506 112,113. The free energies of protonation of methyl vinyl ether and methyl vinyl sulfide are —198.8 and —198.6 kcal mol-1, respectively these substances also are protonated at C/14. Far more basic than any of these is the enolate of acetaldehyde (vinyloxide-) for which C-protonation is accompanied by AG°(g) = —359 kcal mol-1 115. Thus, the gas-phase reactivity order is enolates > enamines en-olethers enols vinyl sulfides. (For the present purpose, we ignore the real, but smaller, differences in reactivity between enols, their ethers and vinyl sulfides.)... 

The Tebbe reagent (4.85) converts esters to enol (vinyl) ethers. 

Aryl and vinyl sulfonate esters are reactive toward oxidative addition, and the perfluoroaUcyl versions are useful substrates in the Heck reaction. Conditions can be mild, comparable to those for vinyl iodide reactions. The enol (vinyl) triflates are particularly attractive, since they are prepared directly from the corresponding ketone (equation 21). ... 

Recently, it has become possible to generate the simplest enol, vinyl alcohol (hydroxyethene), in such a fashion that the acid or base catalysts of the previous reactions are absent (Fig. 19.16). As long as it is protected from acids or bases, the enol... 

FIGURE 19.16 The parent of all enols, vinyl alcohol, is stable as long as it is protected from acid or base catalysts. 

The MacMillan group developed an impressive array and complexity of highly enantioselective SOMO transformations with the addition of enamine-derived radical cations to electron-rich olefins, allowing the aUylation, enolation, vinylation, styrenation, chlorination, polyene cydization, arylation, benzylation, and alkylation of a range of aldehydes (Scheme 39.3) [le, 5a, 5b, 6,8c, 13]. A common mechanistic... 

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