Eike B. Bauer

Iron Catalysis: Historic Overview and Current Trends

Iron catalysis is a growing area of research, as seen by an exponential increase in the publication activities on the topic. This introductory chapter provides a historic overview of the development of iron catalysis including some notable milestones. The advantages of iron, i.e., its abundance, low price, and relative nontoxicity, are discussed, and an overview of the main type of reactions catalyzed by iron is outlined. The advances of heterogeneous iron catalysis (which is not covered in this volume) are exemplified with a few notable cases. Finally, the potential impact of metal impurities in iron sources on the catalytic activity is discussed.

New Chiral Phosphoramidite Complexes of Iron as Catalytic Precursors in the Oxidation of Activated Methylene Groups

New phosphoramidite complexes of iron were synthesized and structurally characterized. Reaction of the known chiral phosphoramidites (RO)2PNR’2 (R = binaphthyl, R’ = CH3, 1a; R = binaphthyl, R’ = benzyl, 1b) with [FeBr(Cp)(CO)2] afforded the title compounds [FeBr(Cp)(CO)(1a,b)] (4a,b) in 34 and 65 % isolated yields as mixtures of diastereomers, since both the metal and the ligand are stereogenic. Similarly, reaction of 1b with [Fe(Cp)I(CO)2] in the presence of catalytic [Fe(Cp)(CO)2]2 afforded [Fe(Cp)I(CO)(1b)] (5b) in 81% yield as a mixture of diastereomers. The molecular structures of 4a, 4b and 5 were determined, revealing a pseudo octahedral coordination geometry about the iron center. The new metal complexes are catalytically active in the oxidation of benzylic methylene groups to the corresponding ketones, utilizing t-BuOOH as oxidant (2 mol% catalyst, 36 h, room temperature, 31−80% yield).

New five-coordinate Ru(II) phosphoramidite complexes and their catalytic activity in propargylic amination reactions

The first five-coordinate, square-pyramidal ruthenium complexes of the general formula [RuCl2(PPh3)2L] have been prepared, where L is a phosphoramidite ligand. The new complexes were employed as catalysts for the amination reactions of propargylic esters (18 h, at room temperature or 45 °C, Cs2CO3) to give propargylic amines in isolated yields up to 94%.

Iron Catalysis II

Iron catalysis is a growing area of research, as seen by an exponential increase in the publication activities on the topic. This introductory chapter provides a historic overview of the development of iron catalysis including some notable milestones. The advantages of iron, i.e., its abundance, low price, and relative nontoxicity, are discussed, and an overview of the main type of reactions catalyzed by iron is outlined. The advances of heterogeneous iron catalysis (which is not covered in this volume) are exemplified with a few notable cases. Finally, the potential impact of metal impurities in iron sources on the catalytic activity is discussed.

Spectroscopic investigation and direct comparison of the reactivities of iron pyridyl oxidation catalysts

The growing interest in green chemistry has fueled attention to the development and characterization of effective iron complex oxidation catalysts. A number of iron complexes are known to catalyze the oxidation of organic substrates utilizing peroxides as the oxidant. Their development is complicated by a lack of direct comparison of the reactivities of the iron complexes. To begin to correlate reactivity with structural elements, we compare the reactivities of a series of iron pyridyl complexes toward a single dye substrate, malachite green (MG), for which colorless oxidation products are established. Complexes with tetradentate, nitrogen-based ligands with cis open coordination sites were found to be the most reactive. While some complexes reflect sensitivity to different peroxides, others are similarly reactive with either H2O2 or tBuOOH, which suggests some mechanistic distinctions. [Fe(S,S-PDP)(CH3CN)2](SbF6)2 and [Fe(OTf)2(tpa)] transition under the oxidative reaction conditions to a single intermediate at a rate that exceeds dye degradation (PDP=bis(pyridin-2-ylmethyl) bipyrrolidine; tpa=tris(2-pyridylmethyl)amine). For the less reactive [Fe(OTf)2(dpa)] (dpa=dipicolylamine), this reaction occurs on a timescale similar to that of MG oxidation. Thus, the spectroscopic method presented herein provides information about the efficiency and mechanism of iron catalyzed oxidation reactions as well as about potential oxidative catalyst decomposition and chemical changes of the catalyst before or during the oxidation reaction.