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 ЊC, highly selective mono-N-methylations are observed for anilines even when they are deactivated by either electronic effects or steric hindrance (G᎐C 6 H 4 NH 2 , G = p-NO 2 , p-CN, o-CO 2 CH 3 and 2,6-dimethylaniline); typical selectivities for the formation of the corresponding mono-N-methyl derivatives [ArNH(CH 3 )] 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 acidbase properties of zeolites is considered to be responsible for the unusually high selectivity observed; accordingly, a reaction mechanism is discussed, involving carbamates (ArNHCO 2 CH 3 ) and N-methylcarbamates [ArN(CH 3 )CO 2 CH 3 ] 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.

New developments in dimethyl carbonate chemistry

“…In the presence of suitable zeolites, and at atmospheric pressure, the same amines yield the corresponding mono-N-methyl derivatives [ArNH(CH 3 )] with selectivities >90%, at conversions up to 95% (Scheme 3) [12].…”

Full N,N-Methylation of 4,4′-Methylenedianiline with Dimethyl Carbonate: A Feasible Access to 4,4′-Methylene bis(N,N-Dimethylaniline)

“…And then, the amino group of this intermediate further attacks the methyl of DMC, obtaining highly selective mono-N-methylated products. It should be noted that the selective mono-N-methylation reaction occurs in the octahedral zeolite cages of NaY in pathway (ii) [16,18], making the particular pore structures of NaY to be a decisive factor in the selective synthesis of mono-N-methylated derivatives.…”

“…However, this cannot guarantee a directional selectivity to a particular product, which was closely related to its particular pore structures. Generally, the N-methylation reactions of aromatic amines with DMC may follow two possible pathways [16,18,27]: (i) the amino group of aromatic amine directly attacks the methoxy group of DMC. (ii) The amino group of aromatic amine firstly attacks the carbonyl group of DMC, producing N-methoxycarbonylation intermediate.…”

“…As the zeolites exhibited effective catalytic activity to the methylation reaction of some aromatic amines [16,17,[22][23][24], several different zeolites such as H , H-ZSM-5, NaX, NaY, and MCM-41 were chosen to catalyze the reaction of MDA with DMC in order to obtain MBDMA, as shown in Table 1. Disparate catalytic performances were observed.…”

“…The N-methylation of primary amines with DMC has been extensively investigated [14][15][16][17][18][19][20]; in particular, a general protocol for the reductive N-methylation of primary and secondary amines using dimethyl carbonate and molecular hydrogen was developed recently [21]. It is worth noting that the mono-N-methylation of primary aromatic amines with dimethyl carbonate could occur in high selectivity [15,16]. However, the N,N-methylation or mono-N-methylation of diamines with DMC has not been described.…”