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

详细信息    查看全文

• 作者：Raffael Huber ; Raphael Bigler ; Antonio Mezzetti
• 刊名：Organometallics
• 出版年：2015
• 出版时间：July 13, 2015
• 年：2015
• 卷：34
• 期：13
• 页码：3374-3384
• 全文大小：559K
• ISSN：1520-6041

文摘

The open-chain NPPN ligand (1S,1鈥?i>S)-1,1鈥?((ethane-1,2-diylbis(phenylphosphanediyl))bis(2,1-phenylene))bis(N-cyclohexylmethanimine) (1) was prepared by condensation of cyclohexylamine with enantiomerically pure (1S,1鈥?i>S)-2,2鈥?(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^{鈥?/sup> or SbF₆鈥?/sup>). 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鈥揅NSiMe₃ linkage. The X-ray structures of 2, 3, 5, and 18 show that ligand 1 assumes the 螞-cis-伪 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.}