Reductive amination of aldehydes using a lithium-arene(cat.) reducing system. A simple one-pot procedure for the synthesis of secondary amines.

Amines are present in many naturally occurring bioactive molecules showing interesting herbicidal and fungicidal activities, and they are also utilised as versatile intermediates for the synthesis of pharmaceuticals, agrochemicals, dyes, and as valuable building blocks in synthetic organic chemistry. Amines can be synthesized by the reaction of aldehydes or ketones with ammonia, primary or secondary amines in presence of different reducing agents. This is the so-called reductive amination reaction which is described as direct when the carbonyl compound and the amine are mixed together with a suitable reducing agent in a single operation, without preformation of an imine or an iminium salt. On the other hand, the reducing systems based on the use of alkali-metals in combination with arenes in aprotic media, with the arene acting as electron carrier have received much attention. In the last years, we have been working on the preparation of transition metal nanoparticles by fast reduction of the corresponding metal chlorides with lithium and a catalytic amount of an arene [naphthalene, 4,4´-di-tert-butylbiphenyl (DTBB)] as electron carrier. In this work we want to introduce a simple and efficient procedure for the direct reductive amination of aldehydes for the one-pot synthesis of secondary amines, using primary amines as starting materials in the presence of lithium powder and a catalytic amount of DTBB or a polymer supported naphthalene as electron carrier. The direct reductive amination of a variety of aldehydes was achieved simply by adding a mixture of the corresponding carbonyl compound and the amine, over a solution of the lithium arenide in THF at room temperature. For most of the substrates tested the main reaction products were the secondary amines along with variable amounts of the corresponding alcohol and imine products. DFT methods have been applied in order to explain the differences in reactivity observed.