ARTICLE pubs.acs.org/Organometallics
Light-Induced Enantioselective Hydrogenation Using Chiral Derivatives of Casey’s IronCyclopentadienone Catalyst Albrecht Berkessel,* Sebastian Reichau, Adrian von der H€oh, Nicolas Leconte, and J€org-M. Neud€orfl Department of Chemistry, University of Cologne, Greinstrasse 4, 50939 Cologne, Germany
bS Supporting Information ABSTRACT: We herein report the first example of an asymmetric ketone hydrogenation using chirally modified derivatives of the homogeneous iron(II)cyclopentadienonetricarbonyl 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 phosphoramiditecarbonyl 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.
’ INTRODUCTION As highlighted by the Nobel Prize in Chemistry awarded to Knowles and Noyori in 2001, catalytic asymmetric hydrogenation is a reaction of utmost importance for the synthesis of chiral molecules in enantiopure form.1 Most hydrogenation reactions rely on the use of noble metals as the catalytically active center. Replacing noble metal catalysts by iron-based systems is expected to lead to more cost-effective and environmentally benign processes since iron is cheap and abundant.2 In nature, hydrogenases provide biological precedent for iron-catalyzed hydrogenation reactions.3 Over the last years, an increasing number of researchers have focused on the application of iron catalysts to a variety of organic transformations.2 Very recently, the two first reports of asymmetric iron-catalyzed transfer hydrogenations of ketones and imines were published. Morris et al. applied catalyst 1 in the transfer hydrogenation of ketones,4 while Beller et al. generated a chiral catalyst for the transfer hydrogenation of imines in situ by reaction of the precatalyst 2 with the chiral ligand 3 (Figure 1).5 The first iron catalyst containing a phosphoramidite ligand 4 was recently described by Bauer et al., but applied as an oxidation catalyst rather than as a hydrogenation catalyst (Figure 1).6 The iron(II) hydride complex 5 was originally described by Kn€olker et al. in 1992 (Figure 2).7a It is a bifunctional system with a hydride on iron and a protic hydrogen atom in the hydoxyl group of the cyclopentadienyl ligand. It was originally synthesized by means of the Hieber reaction (i.e., treatment of the iron tricarbonyl complex with aqueous NaOH and subsequent r 2011 American Chemical Society
Figure 1. Catalyst 1 used by Morris,4 precatalyst 2 and the ligand 3 described by Beller,5 and catalyst 4 reported by Bauer.6
protonation with acid). The isolated yield of the thereby formed iron hydride 5 is relatively low (in our hands
