Abstract
Pyridyl-substituted mesoionic triazolylidene ruthenium cymene complexes catalyze the oxidation of both aromatic and aliphatic amines to nitriles with high activity and selectivity under benign conditions using dioxygen as the terminal oxidant. Modification on the pyridyl moiety of the ligand scaffold has negligible effect on the catalytic performance, while substituents on the triazolylidene directly affect the catalytic fitness of the metal center, leading to distinct catalytic profiles. Pre-dissociation of the cymene ligand and formation of a solvento analogue further enhances the catalytic activity towards nitrile formation. Variation of reaction conditions provided valuable mechanistic insights and resulted in a highly efficient protocol for nitrile formation with maximum turnover numbers around 10 000. The turnover frequency reaches up to 400 h-1, providing one of the fastest catalytic systems known to date for this transformation.
Highlights
Modification on the pyridyl moiety of the ligand scaffold has negligible effect on the catalytic performance, while substituents on the triazolylidene directly affect the catalytic fitness of the metal center, leading to distinct catalytic profiles
Nitriles are valuable synthetic building blocks in the synthesis of fine chemicals.[3,4]. They are prepared by conventional methods such as dehydration of amides/aldoximes,[5,6] through cyanation of alkyl or aryl halides,[7] or by the Sandmeyer and Schmidt-type reactions,[8,9] among many others.[10,11]. These traditional synthetic routes to prepare nitriles proceed with low atom economy, produce stoichiometric waste, require toxic reagents like HCN, have limited selectivity and often require harsh reaction conditions.[12]
A substantial amount of research has been directed towards the development of direct oxidative dehydrogenation (ODH) of primary amines to nitriles by molecular oxygen in the presence of transition metal catalysts, a pathway that starts from abundant low-value precursors and avoids toxic reagents and harsh reaction conditions.[13,14]
Summary
Nitriles are a prominent class of organic molecules which serve as versatile intermediates in organic transformations and are included in a wide variety of natural products and biologically active compounds.[1,2] nitriles are valuable synthetic building blocks in the synthesis of fine chemicals.[3,4] They are prepared by conventional methods such as dehydration of amides/aldoximes,[5,6] through cyanation of alkyl or aryl halides,[7] or by the Sandmeyer and Schmidt-type reactions,[8,9] among many others.[10,11] These traditional synthetic routes to prepare nitriles proceed with low atom economy, produce stoichiometric waste, require toxic reagents like HCN, have limited selectivity and often require harsh reaction conditions.[12]. Pyridyl-substituted mesoionic triazolylidene ruthenium cymene complexes catalyze the oxidation of both aromatic and aliphatic amines to nitriles with high activity and selectivity under benign conditions using dioxygen as the terminal oxidant.
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