Synthesis and Asymmetric Alkene Hydrogenation Activity of C2-Symmetric Enantioenriched Pyridine Dicarbene Iron Dialkyl Complexes

Peter Viereck, Stephan M. Rummelt, Natalia A. Soja, Tyler P. Pabst, Paul J. Chirik

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Abstract

Enantioenriched N-alkyl-imidazole-substituted pyridine dicarbene iron dialkyl complexes have been synthesized and characterized by 1H NMR and zero-field 57Fe Mössbauer spectroscopies as well as single-crystal X-ray diffraction. In benzene-d6, reversible coordination of N2 was observed establishing an equilibrium between a five-coordinate, S = 1 iron dialkyl derivative and the corresponding six-coordinate, diamagnetic dinitrogen complex. A modest enantioselectivity of 45% enantiomeric excess (ee) was observed for the catalytic asymmetric hydrogenation of 1-isopropyl-1-phenyl ethylene at 4 atm of H2 using 10 mol % of an enantioenriched iron dialkyl precatalyst, (ACNC)Fe(CH2SiMe3)2 ((ACNC) = bis(alkylimidazol-2-ylidene)pyridine). Decreasing the H2 pressure to 1 atm increased the ee to 70%. Incubation experiments established that the reaction of the iron dialkyl precatalysts with H2 initiates a background reaction leading to the generation of a less selective catalyst; suppressing this pathway is crucial for obtaining high enantioselectivity. The attempted hydrogenation of methyl-2-acetamidoacrylate identified a deactivation pathway where N-H bond activation generated an iron alkyl κ2-amidate alkyl. For productive catalytic reactions, deuterium labeling studies are consistent with a pathway for hydrogenation involving fast, reversible [2,1]-alkene insertion and a slow, enantiodetermining [1,2]-insertion. Monitoring the catalytic alkene hydrogenation reaction by NMR spectroscopy supports a homogeneous active catalyst that also undergoes C-H activation of the ACNC ligand backbone as a competing reaction.

Original languageEnglish (US)
Pages (from-to)1053-1061
Number of pages9
JournalOrganometallics
Volume40
Issue number8
DOIs
StatePublished - Apr 26 2021

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry

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