Acid chlorides, reaction with malonate enolates

Magnesium enolates play an important role in C-acylation reactions. The magnesium enolate of diethyl malonate, for example, can be prepared by reaction with magnesium metal in ethanol. It is soluble in ether and undergoes C-acylation by acid anhydrides and acyl chlorides (entries 1 and 3 in Scheme 2.14). Monoalkyl esters of malonic acid react with Grignard reagents to give a chelated enolate of the malonate monoanion. 

One preparative procedure for achieving this reaction involves the acylation of the magnesium enolate of diethyl malonate with an acid chloride in benzene solution (cf. Expt 5.96), and the resulting aclymalonic ester is then heated to 200 °C with an arylsulphonic acid to effect the decarbethoxylation step. An illustrative example is the preparation of ethyl 3-oxopentanoate (ethyl propionyl-acetate, Expt 5.177). 

Fig. 12.11. Mechanism of the tert-butylation of malonic acid diethyl ester. From the point of view of tert-butyl chloride it is an SN1 reaction with the malonic ester enol (B) acting as the nucleophile.

The preparation of diethyl benzoylmalonate (entry 12) represents the use of an acid anhydride, a function in which it is much more reactive than an ester, as the acylating agent. The reaction must be carried out in nonnucleophilic solvents to prevent solvolysis of the anhydride from competing with the desired reaction. Other limitations on the use of highly reactive acylating agents, such as acid anhydrides and acid chlorides, in reactions with enolates derive from the fact that O-acylation may be the dominant reaction. The magnesium salt of diethyl malonate (entries 12 and... 

A reaction similar to the above involves the acylation of malonic ester through its magnesium enolate. Thus, the reaction of propionyl chloride with the ester enolate leads to diethyl propionylmalonate. Thermal decomposition of this compound with /3-naphthalenesulfonic acid yields ethyl propionylacetate (57%). This modification appears to be general in that it has been extended to the use of aliphatic, aromatic, and car-balkoxy acyl chlorides. ... 

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