Voigt Reaction

Abstract The preparation of a secondary or tertiary α‐aminoketone by the condensation of a primary amine (or a secondary amine) with a benzoin in the presence of phosphorus pentoxide or hydrochloric acid is generally known as the Voigt reaction, or Voigt condensation. The study finds that due to the effect of steric hindrance, this reaction generally works well for the primary amines, and phosphorus pentoxide is used as the condensation reagent in most cases. However, certain cyclic secondary amines such as pyrrolidine and piperidine do react readily with benzoins to give the corresponding tertiary aminoketones in fairly good yields. Furthermore, the Voigt condensation is also affected by electron density on both benzoins and amines, where an electron‐donating group on the benzoins impedes the reaction. In addition, this reaction requires an α‐proton and in the absence of such α‐proton, the reaction becomes the direct replacement of the hydroxyl group by the amino group, which proceeds with difficulty. This reaction has importance for the preparation of α‐aminoketones and α‐amino alcohols with two phenyl groups.