At a high temperature (>200°C), the reaction with alicyclic ketones is a very attractive application of DMC as a methoxycarbonylating agent. In particular, cyclopentanone and cyclohexanone, whose reactions with DMC (or b = diethylaminoethyl cholride hydrochloride (DEC)] and a base (K2CO3) yield adipic and pimelic dimethyl (or diethyl) esters, respectively (Scheme 4.18).37
Industry, in fact, has a major interest in these diesters as building blocks for nylon 6,6 and nylon 7,7 in the production of polyesters and polyamides.38 However, their present synthesis raises an environmental concern. For instance, the oxidation of cyclohexanone by nitric acid (for the preparation of adipic acid), accounts for more than 10% of the total yearly release of N2O, which is among the main gases responsible for the greenhouse effect. The reaction of Scheme 4.14 represents an eco-friendly alternative synthesis of a,v-diesters which uses green reagents and, relevantly, has a 100% atom economy.3 The overall process is mechanistically described as a retro-Claisen condensation.
The reaction is even more general and applicable to ketones bearing an a-methylene. Yields up to 90% of the corresponding dimethyl esters are obtained in the case of benzylic substrates (Scheme 4.19).
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