Alkali Metal Dihydropyridines in Transfer Hydrogenation Catalysis of Imines: Amide Basicity versus Hydride Surrogacy

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-tBuC(5)H(5)NM, M(tBuDHP), where M=Li-Cs. Reactions have been monitored in C6D6 and tetrahydrofuran-d(8) (THF-d(8)). A definite trend is observed in catalyst efficiency with the heavier alkali metal tBuDHPs outperforming the lighter congeners. In general, Cs(tBuDHP) 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.

Automated summary

The catalytic reduction of aldimines and ketimines to amines using 1,4-dicyclohexadiene as the transfer hydrogenation agent has been studied. s-block pre-catalysts, specifically 1-metallo-2-tert-butyl-1,2-dihydropyridines (M(tBuDHP)), where M=Li-Cs, have been employed for this purpose. The efficiency of the catalysts shows a clear trend with the heavier alkali metal tBuDHPs demonstrating superior performance. Among the various catalysts, Cs(tBuDHP) is found to be the optimal pre-catalyst, yielding quantitative amine products within minutes at room temperature using only 5 mol % catalyst. Density Functional Theory (DFT) calculations support the experimental findings and suggest that Cs has a lower rate determining step compared to Li in its pathway.