Chiral Iron(II) NPPN Complexes: Synthesis and Application in the Asymmetric Strecker Reaction of Azomethine Imines

The open-chain NPPN ligand (1S,1′S)-1,1′-((ethane-1,2-diyl­bis­(phenyl­phosphane­diyl))­bis­(2,1-phenyl­ene))­bis­(N-cyclo­hexyl­methan­imine) (1) was prepared by condensation of cyclo­hexyl­amine with
enantio­merically pure (1S,1′S)-2,2′-(ethane-1,2-diyl­bis­(phenyl­phosphane­diyl))­dibenz­aldehyde
((S,S)-6). Ligand 1 coordinates to [Fe­(OH2)6]­(BF4)2 or [Fe­(MeCN)6]­(SbF6)2 in aceto­nitrile to give the dicationic complex [Fe­(MeCN)2­(1)]­(X)2 (2) (X
= BF4– or SbF6–). The corresponding carbonyl (3), bromo­carbonyl
(4), and bis­(tert-butyl­iso­nitrile)
(5) derivatives were prepared and fully characterized.
Complex 2 reacts with Me3SiCN to give the
corresponding tri­methyl­silyl iso­cyanide derivative 18 featuring a Fe–CNSiMe3 linkage. The X-ray
structures of 2, 3, 5, and 18 show that ligand 1 assumes the Λ-cis-α geometry, which allows comparing the trans influence of these ligands. Complexes 2, 3, 5, and 18 were applied
in the asymmetric addition of tri­methyl­silyl cyanide to
azo­methine imines (Strecker reaction), whose enantio­selectivity
reached 22% ee. The low enantio­selectivity can be explained
on the basis of the Me3SiCN/Me3SiNC isomerization
and of the reaction product partially displacing the NPPN ligand from
iron.