Chiral Iron(II) NPPN Complexes: Synthesis and Application in the Asymmetric Strecker Reaction of Azomethine Imines
datasetposted on 13.07.2015, 00:00 by Raffael Huber, Raphael Bigler, Antonio Mezzetti
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.