Nitrogen compounds imine-based ligands

Several pyridine containing ligands of this type have been reported by Toftlund [2]. A combination of two aliphatic and four imine nitrogen functions seems to provide a ligand field at the crossover point. A versatile class of ligands of this type is based on aliphatic diamines substituted with four alkylpyridine groups. The simplest compound of this type is tetrakis(2-pyri-dylmethyl)-l,2-ethanediamine (tpen) (18). 

For the reaction of the 3-pyridine aldimine 16e, the first equivalent of zinc chloride is coordinated to the more nucleophilic pyridine nitrogen and thus is inactivated. Only the second equivalent activates the imine. However, the zinc chloride coordinated to the more nucleophilic pyridine nitrogen, logically exposes more nucleophilic chlorine ligands for the interaction with the silyl dienol ether 19. Therefore, in this case the nucleophile 19 is introduced from the free back side of 16e (Formula C) and furnishes selectively the Mannich base 28 with opposite stereochemistry compared to the compounds 27. 

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