Diethylmalonate enolate

When the nucleophile is a stabilized carbanion such as the enolate of acetylacetone, 1-benzoylacetophenone, diethylmalonate, or ethyl acetatoacetone, the reaction proceeds similarly. The monosubstituted complex is isolated as long as it contains an acidic hydrogen in the benzylic position. In addition, for the case of diketones CH2(COR)2 (R = Me, Ph, OEt), a deacetylation is observed in an acidic medium [92,93]. These features are the same as described above in the case of the substitution of Cl by stabilized carbanions in monochloroaromatics (the second chlorine being an inert arene substituent [99] Scheme XVII, Eq. (31) and Tables 10 and 11. 

A sub case in this category is the benzylation of enolates through the Srn 1 process. In this way, both a-nitro- and a-chloro-4-nitrocumene are alkylated by the enolates of 2-nitropropane, diethylmalonate, or diethyl 2-butylmalonate. A particular case of benzylic C-C fragmentation is the electron transfer photosensitized Cope rearrangement of 2,5-diphenyl-1,5-hexadienes [218,219]. 

The use of catalysts that are supported on a solid phase is becoming increasingly popular. Such procedures allow for catalyst recycling and also reduce the levels of palla-dium/phosphine contamination in the final product. The use of palladium on graphite or alumina has been reported to be effective for catalyzing the reaction between allyl acetate and the enolate derived from diethylmalonate. ... 

Solvents and temperature also affect the proportions of enol. Thus pentane-2,4-dione contains 80 % enol as a neat liquid, but in water, it is only 15 % enolized. The equilibrium is shifted because water hydrogen bonds so strongly to the keto form. An increase in temperature also favors the keto form, but this is not a large effect at the temperatures at which ordinary organic reactions are carried out. The rate of ketone/enol interconversion depends strongly on pH, and the individual tautomers of pentane-2,4-dione have been isolated at low temperature, under strictly neutral conditions. Other carbonyl compounds, including esters and amides, can and do enolize, but with much more difficulty. For example, neat diethylmalonate (diethyl propanedioate, 17.13) contains only 0.01 % enol at equilibrium, in contrast to the 80 % for pentane-2,4-dione. 

Enolate anions, whether stabilized (like the anions of diethylmalonate or ethyl acetoacetate) or not, add invariably and predictably at the conjugate position of unsaturated carbonyl compounds. An example is shown in Figure 17.67. The anion of diethylmalonate adds 1,4 to the enone, and the intermediate is reprotonated. The diester can be hydrolyzed and decarboxylated to give the final product. The key feature of this reaction, which is very extensively used in synthesis, is that we have synthesized a 1,5-dicarbonyl compound—and this is the standard synthesis of molecules with this pattern of substitution. 

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