Acetaldehyde enolization

The Hydrate and Enol Form. In aqueous solutions, acetaldehyde exists in equihbrium with the acetaldehyde hydrate [4433-56-17, (CH2CH(0H)2). The degree of hydration can be computed from an equation derived by BeU and Clunie (31). Hydration, the mean heat of which is —21.34 kJ/mol (—89.29 kcal/mol), has been attributed to hyperconjugation (32). The enol form, vinyl alcohol [557-75-5] (CH2=CHOH) exists in equihbrium with acetaldehyde to the extent of approximately 1 molecule per 30,000. Acetaldehyde enol has been acetylated with ketene [463-51-4] to form vinyl acetate [108-05-4] (33). 

N-MorphoIinoearbonyI-aniIino)-2-phenyI-acetaldehyd (Enol-Form) liefert mit Lithiumaianat unter Ab-spaltung der Morpholinocarbonyl-Oruppe und C=C-Reduktion 2-Anilino-2-phenyl-dlhanol, mit Natriumboranat wird dagegen 2-Oxo-3,4-diphenyl-1,3-oxazolidin (55%, d.Th.) gebildet4 ... 

The chemistry of the thiazoles does, of course, bear some similarity to that of the oxazoles. Metallated thiazoles and thiazole derivatives have found use, for example, both as carbonyl equivalents and masked acetaldehyde enolates. Additionally, as will be exemplified below, thiazole derivatives have served as catalysts for benzoin-type condensations, and they have provided access to a unique class of activated acid derivatives. 

The arylation of acetaldehyde enolate by iodobenzene17-128 or p-iodoaniline73 failed, and the reduced products, benzene or aniline were obtained. In the reaction of aldehyde enolates with o-substituted derivatives, however, arylation does occur and in these cases subsequent cyclization reactions usually take place (see Section 2.2.4.2). The special effect of the o-substituents in facilitating reactions with aldehyde enolates is shown by the rare example of simple substitution (i.e. without concomitant cyclization) in equation (34).71... 

Ring closure reactions can be induced electrochemically in liquid ammonia. For instance, the reaction of o-iodoaniline with acetaldehyde enolate ion gives 95% yield of indole, and with acetone enolate ion 87% of 2-methylindole321. 

SCHEME 51. Lithium coordination modes in acetaldehyde enolate and in lithium ester silenolate201... 

Monomeric Vinoli was reconsidered later by Schleyer and coworkers204 205. Using more sophisticated theoretical approaches led to similar conclusions concerning the rj3-C,0 form, found to be more stable by 5.1 to 1.2 kcal mol-1. A full optimization performed on the threshold conformers furnished all the geometrical characteristics as well as the natural charges (Scheme 52). The charge borne by the lithium of acetaldehyde enolate was... 

Propen-2-ol (acetone enol) and ethenol (acetaldehyde enol) were some of the first reactive molecules that were generated and characterized by NRMS [65]. Neutral enols are typically less stable than their oxo tautomers and undergo facile isomerization by acid or base catalyzed proton transfer in solution [66] or... 

TABLE 13. Computed reaction and activation energies (kcalmoD ) and corresponding kinetic (KIE) and equilibrium (ElE) isotope effects a reaction sequence between lithium acetaldehyde enolate (Lien) and formaldehyde... 

Fig. 6.25. 3-21G optimized geometries of three transition-state structures for the reaction of acetaldehyde enolate with formaldehyde... 

Fig. 6.33. Views of the 3-2IG transition-state structure for the aldol reaction of lithium acetaldehyde enolate with formaldehyde...

The vinyl tin derivatives (20) are readily prepared, and are converted into the equivalent of the acetaldehyde enolate (21) using n-butyl-lithium. Reaction of (21) with aldehydes, ketones, or a/S-unsaturated ketones leads to the formation of a/S-unsaturated aldehydes by a two-carbon homologation (Scheme 49). ... 

---