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Light-Induced Enantioselective Hydrogenation Using Chiral Derivatives of Casey’s Iron–Cyclopentadienone Catalyst
journal contribution
posted on 2011-07-25, 00:00 authored by Albrecht Berkessel, Sebastian Reichau, Adrian von der Höh, Nicolas Leconte, Jörg-M. NeudörflWe herein report the first example of an asymmetric ketone hydrogenation
using chirally modified derivatives of the homogeneous iron(II)–cyclopentadienone–tricarbonyl
system, known as Casey’s catalyst. For the synthesis of the
chirally modified catalysts, one of three carbonyl ligands was exchanged
for a chiral phosphoramidite. To this end, either oxidative decarbonylation
using trimethylamine-N-oxide or photolysis was applied.
Photolysis was also used to convert the tricarbonyl iron precatalyst
(and, analogously, the dicarbonyl phosphoramidite complexes) to the
coordinatively unsaturated dicarbonyl (monocarbonyl, respectively)
complexes, which are intermediates in the catalytic cycle of ketone
hydrogenation. Hydrogen uptake by the latter species affords the “loaded”
hydride, as evidenced by 1H NMR spectroscopy. Thus, the
preparation of sensitive iron hydrides by the typically low-yielding
Hieber reaction could be avoided. Instead, the catalytic cycle is
accessed from air-stable carbonyl precursors. In the hydridic form
of the phosphoramidite–carbonyl catalysts, the iron atom itself
becomes a stereocenter. NMR spectroscopy confirmed the generation
of two hydride diastereomers. With the MonoPhos iron dicarbonyl complex,
moderate enantioselectivity (up to 31% ee) was achieved in the hydrogenation
of acetophenone.