Selective mono-N-methylation of primary aromatic amines by dimethyl carbonate over faujasite X- and Y-type zeolites

Abstract

The reaction of dimethyl carbonate (DMC) with different primary aromatic amines has been investigated under batch conditions (autoclave) in the presence of Y- and X-type zeolites. Operating at 120–150 °C, highly selective mono-N-methylations are observed for anilines even when they are deactivated by either electronic effects or steric hindrance (G–C₆H₄NH₂, G = p-NO₂, p-CN, o-CO₂CH₃ and 2,6-dimethylaniline); typical selectivities for the formation of the corresponding mono-N-methyl derivatives [ArNH(CH₃)] are in the range 92–98%, at a substrate conversion of 72–93%. A synergic effect between the reactivity of DMC (acting both as a methylating and as a reversible methoxycarbonylating agent) and the dual acid–base properties of zeolites is considered to be responsible for the unusually high selectivity observed; accordingly, a reaction mechanism is discussed, involving carbamates (ArNHCO₂CH₃) and N-methylcarbamates [ArN(CH₃)CO₂CH₃] as intermediates. The reaction is an example of a synthesis with low environmental impact: it couples the use of a non-toxic methylating agent (DMC, in place of the highly toxic methyl halides or dimethyl sulfate) with eco-friendly catalysts (zeolites) in a waste-free process.