Insights into Dehydrogenative Coupling of Alcohols and Amines Catalyzed by a (PNN)–Ru(II) Hydride Complex: Unusual Metal–Ligand Cooperation

• Published in: Inorganic chemistry Vol. 50; no. 21; pp. 10572 - 10580
• Main Authors: Zeng, Guixiang, Li, Shuhua
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 07.11.2011
• DOI: 10.1021/ic200205e
• ISSN: 0020-1669; 1520-510X

Density functional theory calculations were performed to elucidate the mechanism of dehydrogenative coupling of primary alcohols and amines mediated by a PNN–Ru(II) hydride complex (PNN = (2-(di-tert-butylphosphinomethyl)-6-(diethylaminomethyl)pyridine)). A plausible reaction pathway was proposed which contains three stages: (1) The alcohol dehydrogenation reaction to generate the aldehyde and H2; (2) The aldehyde-amine condensation reaction to form the hemiaminal intermediate; (3) The dehydrogenation process of the hemiaminal intermediate to yield the final amide product with the liberation of H2. The first and third stages occur via a similar pathway: (a) Proton transfer from the substrate to the PNN ligand; (b) Intramolecular rearrangement of the deprotonated substrate to form an anagostic complex; (c) Hydride transfer from the deprotonated substrate to the Ru center to yield the trans-dihydride intermediate and the aldehyde (or amide); (d) Benzylic proton migration from the PNN ligand to the metal center forming a dihydrogen complex and subsequent H2 liberation to regenerate the catalyst. In all these steps, the metal–ligand cooperation plays an essential role. In proton transfer steps (a) and (d), the metal–ligand cooperation is achieved through the aromatization/dearomatization processes of the PNN ligand. While in steps (b) and (c), their collaboration are demonstrated by the formation of an anagostic interaction between Ru and the C–H bond and two ionic hydrogen bonds supported by the PNN ligand.