Primary and secondary amines are N-methylated by a mixture of paraformaldehyde and oxalic acid dihydrate in good to excellent yields. The reaction proceeds without involvement of organic solvents and toxic formalin. Reaction temperatures of 100C are required for the decomposition of oxalic acid into the intermediate formic acid which acts as the actual reductant. The reaction conditions have been optimized, and the mechanism has been elucidated by means of deuteration experiments.
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Rosenau et al. (2002) studied this question.