Iron(II) Complexes Containing Unsymmetrical P–N–P′ Pincer Ligands for the Catalytic Asymmetric Hydrogenation of Ketones and Imines

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• 作者：Paraskevi O. Lagaditis ; Peter E. Sues ; Jessica F. Sonnenberg ; Kai Yang Wan ; Alan J. Lough ; Robert H. Morris
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：January 29, 2014
• 年：2014
• 卷：136
• 期：4
• 页码：1367-1380
• 全文大小：653K
• ISSN：1520-5126

文摘

After their treatment with LiAlH₄ and then alcohol, new iron dicarbonyl complexes mer-trans-[Fe(Br)(CO)₂(P鈥揅H鈺怤鈥揚鈥?][BF₄] (where P鈥揅H鈺怤鈥揚鈥?= R₂PCH₂CH鈺怤CH₂CH₂PPh₂ and R = Cy or iPr or P鈥揅H鈺怤鈥揚鈥?= (S,S)- Cy₂PCH₂CH鈺怤CH(Me)CH(Ph)PPh₂) are catalysts for the hydrogenation of ketones in THF solvent with added KOtBu at 50 掳C and 5 atm H₂. Complexes with R = Ph are not active. With the enantiopure complex, alcohols are produced with an enantiomeric excess of up to 85% (S) at TOF up to 2000 h^鈥?, TON of up to 5000, for a range of ketones. An activated imine is hydrogenated to the amine in 90% ee at a TOF 20 h^鈥?and TON 99. This is a significant advance in asymmetric pressure hydrogenation using iron. The complexes are prepared in two steps: (1) a one-pot reaction of phosphonium dimers ([cyclo-(PR₂CH₂CH(OH)^{鈭?/sup>)₂][Br]₂), KOtBu, FeBr₂, and Ph₂PCH₂CH₂NH₂ (or (S,S)-Ph₂PCH(Ph)CH(Me)NH₂ for the enantiopure complex) in THF under a CO atmosphere to produce the complexes cis- and trans-[Fe(Br)₂(CO)(P鈥揅H鈺怤鈥揚鈥?]; (2) the reaction of these with AgBF₄ under CO(g) to afford the dicarbonyl complexes in high yield (50鈥?0%). NMR and DFT studies of the process of precatalyst activation show that the dicarbonyl complexes are converted first to hydride鈥揳luminum hydride complexes where the imine of the P鈥揅H鈺怤鈥揚鈥?ligand is reduced to an amide [P鈥揅H₂N鈥揚鈥瞉鈭?/sup> with aluminum hydrides still bound to the nitrogen. These hydride species react with alcohol to give monohydride amine iron compounds FeH(OR鈥?(CO)(P鈥揅H₂NH鈥揚鈥?, R鈥?= Me, CMe₂Et as well as the iron(0) complex Fe(CO)₂(P鈥揅H₂NH鈥揚鈥? under certain conditions.}