Alkali Metal Dihydropyridines in Transfer Hydrogenation Catalysis of Imines: Amide Basicity versus Hydride Surrogacy
Peter A. Macdonald, Sumanta Banerjee, Alan R. Kennedy, Alexander van Teijlingen, Stuart D. Robertson, Tell Tuttle, Robert E. Mulvey
Abstract
Abstract Catalytic reduction of a representative set of imines, both aldimines and ketimines, to amines has been studied using transfer hydrogenation from 1,4‐dicyclohexadiene. Unusually, this has been achieved using s‐block pre‐catalysts, namely 1‐metallo‐2‐ tert ‐butyl‐1,2‐dihydropyridines, 2‐ t BuC 5 H 5 NM, M( t BuDHP), where M=Li–Cs. Reactions have been monitored in C 6 D 6 and tetrahydrofuran‐d 8 (THF‐d 8 ). A definite trend is observed in catalyst efficiency with the heavier alkali metal t BuDHPs outperforming the lighter congeners. In general, Cs( t BuDHP) is the optimal pre‐catalyst with, in the best cases, reactions producing quantitative yields of amines in minutes at room temperature using 5 mol % catalyst. Supporting the experimental study, Density Functional Theory (DFT) calculations have also been carried out which reveal that Cs has a pathway with a significantly lower rate determining step than the Li congener. In the postulated initiation pathways DHP can act as either a base or as a surrogate hydride.