It is noteworthy that in the case of methyl sulfones (ArSO2Me), the reaction proceeds with the homologation of the methyl to an ¿-propyl group: that is, the methylation is still highly selective toward the substitution of only two of the three methyl protons. Aside from the synthetic results, this observation is relevant from the mechanistic viewpoint, as will be clarified in the discussion of Scheme 4.8.
Under the same conditions (batch or GL-PTC) discussed for CH2-acidic compounds, primary aromatic amines also react with DMC. In this case, although the reaction yields selectively the mono-N-methylated amines with no dimethylated by-products, sizable amounts of methyl carbamates (ArNHCO2Me) are formed.10,17c Much better results can be gathered in the presence of zeolites, particularly alkali metal exchanged Y and X faujasites. These aluminosilicates posses pseudospheri-cal cavities (supercavities) of 11-8 A in diameter, which can be accessed through channels whose size is 7.4 A.22
In the presence of these solid catalysts, different anilines—even deactivated by both electronic and steric effects—yield the corresponding mono-N-methyl derivatives [ArNHMe] with selectivities of 93-98%, at conversions up to 95% (Scheme 4.8).23
A further added-value of the combined use of DMC and zeolites, is that deri-vatization (protection/deprotection) sequences may be avoided. In particular, in
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