Alkali metal dihydropyridines in transfer hydrogenation catalysis of imines: amide basicity versus hydride surrogacy

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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-tBuC5H5NM, M(tBuDHP), where M = Li-Cs. Reactions have been monitored in C6D6 and tetrahydrofuran-d8 (THFd8). 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.
Original languageEnglish
Article numbere202304966
Number of pages10
JournalAngewandte Chemie International Edition
Volume62
Issue number27
Early online date3 May 2023
DOIs
Publication statusPublished - 3 Jul 2023

Keywords

  • alkali metal
  • caesium
  • catalysis
  • dihydropyridine
  • hydrogenation
  • catalytic reduction
  • experimental study

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