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A practical method for N-methylation of NH-containing heterocycles using an environmentally safe and less toxic methylating reagent, dimethyl carbonate, has been developed. N,N,N',N'-tetramethylethylenediamine (TMEDA), an extremely active organocatalyst for the methylation of imides, indoles, benzimidazoles, and piperazines in conjunction with dimethyl carbonate, can lead to N-methylation products with high conversion. The reaction sequence consists of competing alkylation and acylation pathways and involves TMEDA as a nucleophilic catalyst. A possible reaction mechanism is discussed based on the reaction results. [image omitted] [ABSTRACT FROM AUTHOR]