As far as the mechanism of DMC-mediated mono-N-methylation reactions is concerned, adsorption phenomena of reagents over zeolite catalysts have to be examined. Model cases of aniline and DMC are suitable to this investigation.

According to Czjiek et al.,29 the steric requisites of aniline allows it to diffuse into the pores of NaY (supercages), where two sites of adsorption have been identified through diffraction techniques. Figure 4.6 describes the case of deuter-ated aniline. The first site (I) is in the proximity of a cation Na+, which forms a p complex with PhND2. The ion is perfectly centered in the aromatic ring of the aniline. In the second site (II), the amine is held by the interactions with the 12 oxygen atoms that form the supercavity of the zeolite.

Also, the adsorption of DMC on faujasites, has been described through two modes of interaction:30, 31 IR experiments indicates that DMC acts as a base to form acid-base complexes with the Lewis acidic sites of the catalyst (Scheme 4.12).

It should be noted that the formation of both complexes (III and IV) implies a lengthening of the O—CH3 bond of DMC, meaning that DMC undergoes an elec-trophilic activation within the pores of the solid.

Based on Figure 4.6 and Scheme 4.12, the proposed mechanism for the mono-N-methyl of aniline with DMC is described pictorially on Scheme 4.13. Once the reagents (amine and DMC) diffuse into the supercages of NaY, they can approach each other only according to the steric requisites of their adsorption

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