Direct oxidation of amines to nitriles in the presence of ruthenium-terpyridyl complex immobilized on ILs/SILP

Abstract

The immobilization of a ruthenium complex (Ru₂Cl₄(az-tpy)₂) within a range of supported ionic liquids ([C₄C₁im]Cl, [C₄C₁im][NTf₂], [C₆C₁im]Cl, [C₄C₁pyrr]Br, [C₄C₁im]Br, [C₄C₁pyrr]Cl) dispersed silica (SILP) operates as an efficient heterogeneous catalyst in oxidation of long chain linear primary amines to corresponding nitriles. This reaction follows a “green” route using a cheap and easy to handles oxidant (oxygen or air). The conversion was found to be strongly influenced by the alkyl chain length of the amine substrate and the choice of oxidant. No condensation reaction was observed between the starting amines and the selectivity to nitrile is 100%. Moving from a composition of 20 atm N₂/5 atm O₂ to 5 atm N₂/20 atm O₂ led to enhancements in the conversion (n-alkylamines) and selectivity (benzonitrile) which have been correlated with an increase of the solubilized oxygen. This was further supported by using different inert gas (nitrogen, helium, argon)/oxygen mixtures indicating that the O₂ solubility in the SILP system, has an important effect on conversions and TON in this reaction using SILP catalysts. Experiments performed in the presence of CO₂ led to a different behaviour due to the formation of amine-CO₂ adducts. The application of the Weisz–Prater criterion confirmed the absence of any diffusional constraints.