1.7 Product Class 7: Organometallic Complexes of Iron

G. R. Stephenson

General Introduction

This chapter supersedes the original ▶ Section 1.7 of Science of Synthesis, taking into account the literature published since 2000. Previously published information regarding this product class can be found in Houben–Weyl, Vol. 13/9a, pp 175–523. In ▶ Section 1.7, the chemistry of organometallic complexes of iron, with emphasis on their use as intermediates in organic synthesis, is surveyed.

The complexation of ligands, typically unsaturated hydrocarbons, dramatically alters the chemistry of the organic moiety, stabilizing structures that would otherwise be impossible to isolate, and, conversely, imparting reactivity on structures that are normally relatively unreactive. Often the reactivity of the metal complex is different from, and complementary to, the chemistry of the free ligand. Iron complexes of ligands ranging from η6 to η1 now have varied uses as intermediates in synthesis, and the reactivity patterns in the iron series follow those of complexes of other transition metals discussed elsewhere in Science of Synthesis. Because iron has the ability to stabilize a positive charge in π-complexes, reactions of π-bound ligands with nucleophiles make up the most generally used class of reactions in organoiron chemistry, providing valuable bond-forming procedures across the series η6 to η2, and typically forming products with hapticity one less than the starting material, as cationic electrophilic π-complexes tend to react at the terminus of the metal-bound portion of the ligand. (In Science of Synthesis, reactions are classified according to the products formed, therefore these reactions will be found in the sections describing the preparation of η5- to η1-structures.) Reactions with nucleophiles at internal positions produce structures with lower hapticity. In common with other organotransition-metal structures, organoiron complexes with η1-ligands exhibit σ-bond migrations to π-bound ligands and pairs of π-bound ligands can combine within the coordination sphere of the metal. These processes are typical of neutral structures of relatively low hapticity.

Although organoiron (typically carbonyliron) complexes can be used as catalysts, these procedures do not constitute major methods of synthesis, and in Science of Synthesis they are more properly classified in the sections dealing with each individual organic product class. Thus, in ▶ Section 1.7, the focus of attention is on stoichiometric transformations of the iron complexes, and transformations that afford an organic product by detachment of iron from its ligand. Two particular classes of organoiron complexes, ferrocenes [Fe(Cp)2 and the Cp* analogue] and iron acyl structures, [Fe(COR1)Cp(CO)(L)], have synthetic applications in which the metal complex plays a crucial structural role, but the bond-forming chemistry is more closely related to the organic chemistry of the ligands. Ferrocene affords an architecture in which substituents on the two coplanar cyclopentadienyl ligands are aligned in a unique fashion, affording the basis for important series of chiral auxiliaries, in cases where there are stereogenic centers in substituents or sufficient substituents on the cyclopentadienyl ring to introduce planar chirality. In the acyl complexes, the chirality of the pseudotetrahedral iron atom (typically L = PPh3) strongly influences the reactivity of the acyl structure, which, from the standpoint of asymmetric synthesis, behaves essentially in the fashion of a chiral ketone equivalent. These aspects of the chemistry of iron are discussed in more detail in Sections 1.7.8 and 1.7.8.17 (ferrocenes) and within the sections in Science of Synthesis dealing with the synthesis of aldehydes, ketones, acids, and esters with chirality in the hydrocarbon chain near the organic carbonyl group; transformations of the η5- and η1-ligands of these complexes are, however, discussed in ▶ Section 1.7 to relate them to the parallel examples with ligands of other hapticities.

The subject area of ▶ Section 1.7 is similar to that of two far more extensive reviews of mononuclear iron complexes with hydrocarbon ligands, covering the periods through to 1981,[1–3] and between 1982 and 1994,[4] and citing over 2000 references; these have been followed by surveys of uses of iron complexes in synthesis in 1999[5,6] and 2000[7] for stoichiometric methods, and 2000[8] and 2004[9] for catalytic procedures. Information on specific compounds [e.g., pentacarbonyliron,[10] nonacarbonyldiiron,[11] (η4-benzylideneacetone) tricarbonyliron,[12] tricarbonyl(cyclobutadiene)iron,[13] tricarbonyl(cyclohexadiene)iron,[14] and tricarbonyl(cyclohexadienyl)iron tetrafluoroborate[15]] is also available in the Encyclopedia of Reagents for Organic Synthesis.

The history of the discovery of ferrocene has been reviewed,[16] and the first 50 years of the development of ferrocene chemistry was celebrated in a special article in 2002.[17] Ferrocene chemistry is described comprehensively in a classic series of annual surveys by Rockett and Marr up to 1989,[18] and in 2008 in a compilation edited by Štěpnička.[19] Iron fullerene complexes were reviewed in 1999.[20] There are also more recent surveys of specific classes of ferrocene-containing compounds, e.g. ferrocenophanes,[21] ferrocenyl heterocycles,[22] ferrocenes as rotary modules for molecular machines,[23] and polymers[24] (including liquid-crystalline polymers,[25] dendrimers,[26,27] and multichannel molecular receptors for cations and anions[28]) (see ▶ Sections 1.7.8.17.2.14 and 1.7.8.17.2.15). There is also a detailed Encyclopedia of Reagents for Organic Synthesis entry for ferrocene, as well as many ferrocenyl derivatives, by Pauson.[29]

Work on ferrocene synthesis directed toward the preparation of new classes of chiral amine and phosphine ligand structures, as well as molecules designed for the needs of materials science, in which ferrocene is incorporated in the final structure for functional effect, has also been reviewed.[30,31] The key aspect of enantioselective synthesis of multiply substituted ferrocenes has grown in importance over the years, and was reviewed in 2008,[32] 2009,[33,34] 2010,[35] and 2013.[36] Ligand classes with mixed planar and atom-centered chirality[37] and biferrocene-based ligands[38] have also been surveyed. The applications of chiral ferrocenyl ligands have been summarized.[39–42] There is also a more recent review (2010) on metallocyclic ferrocenyl ligands.[43]

▶ Section 1.7.8.17.2.12 gives examples of the uses of ferrocenyl compounds in biology. This relatively new topic has already been the subject of several reviews, including those on antitumor ferrocenes,[44] ferrocenyl conjugates of amino acids, peptides, and nucleic acids,[45–47] modulation of estrogen receptors,[48] and antimalarials.[49,50] The bioorganometallic chemistry of ferrocene was described in Chemical Reviews in 2004,[51] and the medicinal chemistry of ferrocenes was discussed in 2007.[52] The more general use of ferrocene derivatives in analytical chemistry was surveyed in 2008.[53] ▶ Sections 1.7.8.17.2.4, 1.7.8.17.2.12, and 1.7.8.17.2.15 give examples of “click” chemistry involving ferrocene-containing components. The importance of this convenient strategy was reviewed in 2011.[54]

The tricarbonyliron chemistry of cyclic ligands was reviewed by Pearson[55] and Knölker,[56] and that of the acyclic ligands was covered by Grée[57,58] and Donaldson,[59,60] and also by Ong,[61] Thomas,[62] and Danks,[63] who reviewed the chemistry of tricarbonyliron triene complexes, vinylketene, vinylketenimine, and vinylallene complexes, and azadiene complexes, respectively. Enders and co-workers have reviewed iron-mediated allylic substitution.[64] Ley has surveyed ferralactone chemistry,[65] and Knölker has reviewed the use of azadiene complexes to prepare non-racemic tricarbonyliron diene complexes.[66] The photochemistry of iron sandwich complexes, discussed in ▶ Section 1.7.1.4 as a method to remove η6-ligands, has been reviewed by Astruc.[67]

Iron acyl chemistry has been reviewed by Davies.[68] New organic synthetic methods using iron carbonyl reagents were reviewed in 2000,[8] updating a summary of applications of...