Catalytic Reductive Alkylation of Secondary Amine with Aldehyde and Silane by an Iridium Compound

An efficient methodology for the reductive alkylation of secondary amine with aldehyde and Et3SiH using an iridium complex as a catalyst has been developed. For example, treatment of dibutylamine with butyraldehyde and Et3SiH (a 1:1:1 molar amount of amine, aldehyde, and silane) in 1,4-dioxane at 75 °C under the influence of a catalytic amount of [IrCl(cod)]2 gave tributylamine in quantitative yield. In this reaction, no reduction of aldehyde took place. It was found that IrCl3, which is a starting material for preparation of iridium complexes such as [IrCl(cod)]2, acts as an efficient catalyst for the present reductive alkylation of amine. In addition, a cheaper, easy-to-handle, and environmentally friendly reducing reagent such as polymethylhydrosiloxane (PMHS) in place of Et3SiH was also useful. Thus, a variety of secondary amines could be alkylated by allowing them to react with aldehydes and PMHS in the presence of an iridium catalyst to afford the corresponding tertiary amines in good to excellent yields. From the deuterium label experiments, it was revealed that silane and water, generated during the formation of enamine by the reaction of amine and aldehyde, seem to behave as a hydrogen source. The catalytic cycle was discussed.