Mono-N-methylation of primary amines with alkyl methyl carbonates over Y faujasites. 2. Kinetics and selectivity

In the presence of a Na-exchanged Y faujasite, the reaction of primary aromatic amines 1 with 2-(2-methoxyethoxy)methylethyl carbonate [MeO(CH2)(2)O(CH2)(2)OCO2Me, 2a] yields the corresponding mono-N-methyl derivatives ArNHMe with selectivity up to 95%, at substantially quantitative conversions. At 130 degreesC, the reaction can be run under diffusion-free conditions and is strongly affected by the solvent polarity: for instance, in going from xylene (is an element of(r) = 2.40) to triglyme (is an element of(r) = 7.62) as the solvent, the pseudo-first-order rate constant for the aniline (1a) disappearance shows a 5-fold decrease. In DMF (is an element of(r) = 38.25), the same reaction does not occur at all. Competitive adsorption of the solvent and the substrate onto the catalytic sites accounts for this result. The behavior of alkyl-substituted anilines ZC(6)H(4)NH(2) [Z = p-Me, p-Et, p-Pr, 1p-(n-Bu) (1b-e) ; Z = 3, 5-di-tert-butyl- and 2,4,6-tri-tert-butylanilines (1f,g)] and p-alkoxyanilines p-ZC(6)H(4)NH(2) [Z = OMe, OEt, OPr, O-n-Bu (1b'-e')] clearly indicates a steric effect of ring substituents: as diffusion of the amine into the catalytic pores is hindered, the reaction hardly proceeds and the mono-N-methyl selectivity (S-M/D) drops as well. Moreover, the strength of adsorption of the amine onto the catalyst influences the rate and the selectivity as well: the reaction of p-anisidine and p-toluidine-despite the higher nucleophilicity of these compounds-is slower and even less selective with respect to aniline. From a mechanistic viewpoint, the intermediacy of carbamates ArN(Me)CO2R [R = MeO(CH2)(2)O(CH2)(2)] is suggested. At 90 degreesC, the reaction of benzylamine (7)-a model for aliphatic amines with dimethyl carbonate shows that the reaction outcome can be improved by tuning the amphoteric properties of the catalyst: in going from CsY to the more acidic LiY zeolite, methylation is not only more selective (S-M/D ratio increases from 77% to 84%) but even much faster (CsY, conversion of 36% after 22 h; LiY, conversion of 43% after 7 h).