Zeolites dimethyl carbonate reactions

Cesium-exchanged zeolite X was used as a solid base catalyst in the Knoevenagel condensation of benzaldehyde or benzyl acetone with ethyl cyanoacetate [121]. The latter reaction is a key step in the synthesis of the fragrance molecule, citronitrile (see Fig. 2.37). However, reactivities were substantially lower than those observed with the more strongly basic hydrotalcite (see earlier). Similarly, Na-Y and Na-Beta catalyzed a variety of Michael additions [122] and K-Y and Cs-X were effective catalysts for the methylation of aniline and phenylaceto-nitrile with dimethyl carbonate or methanol, respectively (Fig. 2.37) [123]. These procedures constitute interesting green alternatives to classical alkylations using methyl halides or dimethyl sulfate in the presence of stoichiometric quantities of conventional bases such as caustic soda. 

The Williamson reaction, discovered in 1850, is still the best general method for the preparation of unsymmetrical or symmetrical ethers.The reaction can also be carried out with aromatic R, although C-alkylation is sometimes a side reaction (see p. 515). The normal method involves treatment of the halide with alkoxide or aroxide ion prepared from an alcohol or phenol, although methylation using dimethyl carbonate has been reported. It is also possible to mix the halide and alcohol or phenol directly with CS2CO3 in acetonitrile, or with solid KOH in Me2SO. The reaction can also be carried out in a dry medium,on zeolite-or neat or in solvents using microwave irradiation. Williamson ether synthesis in ionic liquids has also been reported. The reaction is not successful for tertiary R (because of elimination), and low yields are often obtained with secondary R. Mono-ethers can be formed from diols and alkyl halides. Many other... 

Alkylation of catechol with dimethyl carbonate in the presence of Li0H/Al203 at 553 K gave the monoether with 84% selectivity at 100% conversion. Some 3-methylcate-chol also formed in the process.37 When the reaction of aniline with dimethyl carbonate is carried out in the gas phase with zeolite KY, the reaction was 93.5% selective for A-... 

Alkali-exchanged zeolites can be used for selective condensation reactions [32-37], double bond isomerization, alkylation of toluene by methanol, mono-methylation of phenylacetonitrile with methanol and dimethyl carbonate (DMC) [38], and selective N-alkylation with methanol, ethanol, and 2-propanol as alkylating agents [39-44]. However, the basicity of alkaline exchanged zeolites is rather low. 

Richter, Zeolite and Other Heterogeneous Catalysts for The Transesterijication Reaction of Dimethyl Carbonate with Ethanol, Catal. Today, 2006,114, 226-230. 

But the series of reactions needed to bring about the conversion is much more complex. Indeed, the mechanisms are not yet agreed for all the stages in the process. The first stage is the dehydration of methanol to methoxymethane (dimethyl ether), but there is much controversy over the mechanism of the initial carbon-carbon bond formation. However, once ethene has been formed (presumably via a slow step), well-known fast reactions take over, analogous to those seen when zeolites are used in petroleum refining. 

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