Rethinking the Dehydrogenative Amide Synthesis

This paper is concerned with the mechanism of the catalytic dehydrogenative coupling of primary alcohols with amines; it addresses the question on what happens to the aldehyde produced in the catalytic solution upon dehydrogenation of an alcohol substrate. Here we demonstrate a rapid catalytic reaction of acetaldehyde with primary amines, leading to acetamides. The facile amide bond formation is a low-energy, outer-sphere catalytic process elucidated with the help of DFT calculations. Overall, the dehydrogenative amide synthesis comprises two metal-catalyzed cycles: the first producing the aldehyde and H2, and the second where the hemiaminal is formed and is dehydrogenated. The results call into question the existing mechanistic ideas (reviewed by Li and Hall) that invoke the uncatalyzed formation of a free hemiaminal intermediate and assume that the hemiaminal dehydrogenation requires a 16-electron catalyst.