An Experimental-Theoretical Study of the Factors That Affect the Switch between Ruthenium-Catalyzed Dehydrogenative Amide Formation versus Amine Alkylation

A ruthenium(II) diamine complex can catalyze the intramolecular cyclization of amino alcohols H2N(CH2)(n)OH via two pathways: (i) one yields the cyclic secondary amine by a redox-neutral hydrogen-borrowing route with loss of water; and (ii) the second gives the corresponding cyclic amide by a net oxidation involving loss of H-2. The reaction is most efficient in cases where the product has a six-membered ring. The amide and amine pathways are closely related: DFT calculations show that both amine and amide formations start with the oxidation of the amino alcohol, 5-amino-1-pentanol, to the corresponding amino aldehyde, accompanied by reduction of the catalyst. The intramolecular condensation of the amino aldehyde takes place either in the coordination sphere of the metal (path I) or after dissociation from the metal (path II). Path I yields the Ru-bound zwitterionic form of the hemiaminal protonated at nitrogen, which eliminates H-2, forming the amide product. In path I I, the free hemiaminal dehydrates, giving an imine, which yields the amine product by hydrogenation with the reduced form of the catalyst generated in the initial amino alcohol oxidation. For amide to be formed, the hemiaminal must remain metal-bound in the key intermediate and the elimination of H-2 must occur from the same intermediate to provide a vacant site for beta-elimination. The elimination of H-2 is affected by an intramolecular H-bond in the key intermediate. For amine to be formed, the hemiaminal must be liberated for dehydration to imine and the H-2 must be retained on the metal for reduction of the imine intermediate.