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The Leuckart Reaction

A useful procedure for the reductive alkylation of ammonia, 1°-, & 2°-amines, in which formic acid or a derivative thereof serves as the reducing agent, is known as the Leuckart Reaction. Some examples of this reaction are shown below.

Three Leuckart reactions with HCO2H reductant: acetophenone plus ethylamine, naphthaldehyde plus morpholine, and an amine dimethylated by formaldehyde

Hydride transfer from formate to an iminium ion in the Leuckart reduction The manner in which a hydride moiety is transferred from formate to an iminium intermediate is a matter for speculation, but may be summarized roughly as shown on the right. Both aldehydes and ketones may be used as the carbonyl reactant. By using ammonia as a reactant, this procedure may be used to prepare 1°-amines; however, care must be taken to avoid further alkylation to 2° & 3°-amines. Polyalkylation is sometimes desired, as in example #3 where dimethylation is accomplished with formaldehyde. This is sometimes referred to as the Eschweiler-Clarke procedure, and it has proven to be a useful method for converting 1°-amines to precursors for Hofmann or Cope elimination reactions.