Sustainable Metal Catalysis with Iron: From Rust to a Rising Star?

Abstract

Dedicated to Professor Wolfgang A. Herrmann on the occasion of his 60th birthday coupling reactions · homogeneous catalysis · iron · oxidation · reductionThe development of sustainable, more efficient, and selective organic synthesis is one of the fundamental research goals in chemistry. In this respect, catalysis is a key technology, since approximately 80 % of all chemical and pharmaceutical products on an industrial scale are made by catalysts-even more in the case of modern processes (ca. 90 %). In particular, organometallic compounds have become an established synthetic tool for both fine and bulk chemicals and several hundreds of molecular, defined pre-catalysts are commercially available for chemists around the world. The reactivity and selectivity of the active catalyst are widely influenced by the choice of the central metal and by the design of surrounded ligands. During the last decades, manifold transition-metal catalysts especially based on precious metals such as palladium, rhodium, iridium, and ruthenium have been proven to be efficient for a large number of applications. However, the limited availability of these metals as well as their high price ( Figure 1) and significant toxicity makes it desirable to search for more economical and environmentally friendly alternatives. A possible solution of this problem could be the increased use of catalysts based on first row transition metals, such as iron, copper, zinc, and manganese. Especially iron offers significant advantages compared with precious metals, since it is the second most abundant metal in the earth crust (4.7 wt %). Various iron salts and iron complexes are commercially accessible on a large scale or easy to synthesize. Furthermore, iron compounds are relatively nontoxic. In contrast to man-made precious-metal catalysts, iron takes part in various biological systems as essential key element, for example, in metalloproteins for the transport or metabolism of small molecules (oxygen, nitrogen, methane, etc.) and electron-transfer reactions (Figure 2). Thanks to the facile change of oxidation state and the distinct Lewis acid character, iron catalysts allow in principle a broad range of synthetic transformations, for example,

Iron catalysed direct alkylation of amines with alcohols

“…Iron is considered to be the ultimate, sustainable alternative for ruthenium 18 ; however, no unequivocal reductive amination via a borrowing hydrogen mechanism has been reported 19 .…”

Recent advances in the iron-catalyzed cycloaddition reactions

“…Among all the available transition metals, iron is generally regarded as one of the most inexpensive, abundant, benign and relatively non-toxic metals [2]. Many transformations have been effectively promoted by iron catalysts, and excellent reviews on the iron-catalyzed reactions are available [3][4][5][6][7][8][9][10][11][12][13][14][15]. Bolm and co-workers [3] summarized the most important transformations with the aid of iron catalysts in 2004, including addition, substitution, cycloaddition, reduction and other reactions.…”

Iron Catalyzed Highly Enantioselective Epoxidation of Cyclic Aliphatic Enones with Aqueous H2O2

“…[1][2][3][4] Catalytic epoxidation methodologies based on iron complexes and peroxides (especially H 2 O 2 ), which can be considered as biologically inspired, are interesting because of the availability and low environmental impact of these reagents. [5][6][7][8][9][10][11][12][13][14][15]16 Despite appealing, the approach is challenging because it requires the design of iron coordination complexes that can activate the O-O bond of peroxides to create selective metal based oxidants, and avoid the often facile production of hydroxyl radicals via the Fenton reaction. 10,11,17,18 Recent reports have disclosed successful examples where asymmetric epoxidation is accomplished, in some cases producing high levels of stereoselectivity ( Figure 1).…”

“…In general, substitutions at the olefinic side ( and β) decrease ee's, while substitutions at the opposite side (' and β') lead to important improvement. For example, alkyl substitution at the  position of 5 and 6-member ring enones caused a significant decrease in ee's (62-65% ee, entries [5][6], that could be partially rescued by employing 2 as catalyst (for S5 75% yield and 76 % ee, and for S6 81 % yield and 75% ee). Also, for tert-butyl group in  position the reaction didn't take place (entry 7, S7).Although the current ee values leave room for improvement, it should be stated that the current catalysts constitute the first ones that provide good enantioselectivities for these substrates.…”