Asymmetric hydrogenation, transfer hydrogenation and hydrosilylation of ketones catalyzed by iron complexes.
ABSTRACT The conventional homogeneous catalysts for the enantioselective hydrogenation or transfer hydrogenation of ketones are based on platinum metals and, in particular, ruthenium. This method provides valuable enantiopure alcohols for the fine chemical industries. This tutorial review summarizes recent successes in replacing expensive and toxic ruthenium in these catalysts with "greener" iron substitutes including my lab's recent progress in this area using iron complexes containing readily-prepared tetradentate ligands. It will enlighten chemists interested in homogeneous catalysis and asymmetric synthesis.
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ABSTRACT: A series of iron dicarbonyl complexes with bipyridine-based PNN pincer ligands were synthesized and characterized by multinuclear NMR spectroscopy (1H, 13C, 15N, 31P), IR spectroscopy, cyclic voltammetry, 57Fe Mössbauer spectroscopy, XPS spectroscopy, and single-crystal X-ray diffraction. The complexes with the general formula [(R-PNN)Fe(CO)2] (5: R-PNN=tBu-PNN=6-[(di-tert-butylphosphino)methyl]-2,2′-bipyridine, 6: R-PNN=iPr-PNN=6-[(diisopropylphosphino)methyl]-2,2′-bipyridine, and 7: R-PNN=Ph-PNN=6-[(diphenylphosphino)methyl]-2,2′-bipyridine) feature differently P-substituted PNN pincer ligands. Complexes 5 and 6 were obtained by reduction of the corresponding dihalide complexes [(R-PNN)Fe(X)2] (1: R=tBu, X=Cl; 2: R=tBu, X=Br; 3: R=iPr, X=Cl; 4: R=iPr, X=Br) in the presence of CO. The analogous Ph-substituted complex 7 was synthesized by a reaction of the free ligand with iron pentacarbonyl. The low-spin complexes 5–7 (S=0) are diamagnetic and have distorted trigonal bipyramidal structures in solution, whereas in the solid state the geometries around the iron are best described as distorted square pyramidal. Compared to other structurally characterized complexes with these PNN ligands, shortened interpyridine CC bonds of about 1.43 Å were measured. A comparison with known examples, theoretically described as metal complexes bearing bipyridine π-radical anions (bpy.−), suggests that the complexes can be described as FeI complexes with one electron antiferromagnetically coupled to the ligand-based radical anions. However, computational studies, at the NEVPT2/CASSCF level of theory, reveal that the shortening of the CC bond is a result of extensive π-backbonding of the iron center into the antibonding orbital of the bpy unit. Hence, the description of the complexes as Fe0 complexes with neutral bipyridine units is the favorable one.Chemistry 01/2014; · 5.93 Impact Factor
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ABSTRACT: Hydrogen liberation: An attractive approach to reversible hydrogen storage applications is based on the decomposition of formic acid. The efficient and selective hydrogen liberation from formic acid is catalyzed by an iron-pincer complex in the presence of trialkylamine. Turnover frequencies up to 836 h(-1) and turnover numbers up to 100 000 were achieved at 40 °C. A mechanism including well-defined intermediates is suggested on the basis of experimental and computational data.Chemistry 05/2013; · 5.93 Impact Factor
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ABSTRACT: Ir complexes of chiral phosphine-diamine ligands catalyse the hydrogenation and transfer hydrogenation of aryl-piperidin-4-yl methanones, and ketones bearing both an aryl group and secondary alkyl substituent with up to 98% e.e., and with substrate to catalyst ratios of up to 4000.Chemical Communications 09/2013; · 6.38 Impact Factor