Iron catalysis complex

In this chapter we described [2 + 2 + 2] and related cycloaddition reactions using palladium, iron, manganese, rhenium, and other transition metals. Palladium complexes are able to catalyze [2 + 2 + 2] and related cycloaddition reactions, which proceed via cascade-type mechanism or metallacycle intermediates. It is worthy of note that arynes are suitable substrates for this palladium catalysis. Iron complexes are promising catalysts for practical [2 + 2 + 2] cycloaddition reactions, owing to their low cost and nontoxicity, although both catalytic activity and substrate scope are not satisfactory. Manganese and rhenium complexes allow the use of 3-keto esters as a cycloaddition partner. To realize the practical process and broaden the product scope, further development of new transition-metal catalysts is expected in this research field. 

Because there exist a number of reviews which deals with the structural and mechanistic aspects of high-valent iron-oxo and peroxo complexes [6,7], we focus in this report on the application and catalysis of iron complexes in selected important oxidation reactions. When appropriate we will discuss the involvement and characterization of Fe-oxo intermediates in these reactions. 

Apart from catalysis with well-defined iron complexes a variety of efficient catalytic transformations using cheap and easily available Fe(+2) or Fe(+3) salts or Fe(0)-carbonyls as precatalysts have been pubhshed. These reactions may on first sight not be catalyzed by ferrate complexes (cf. Sect. 1), but as they are performed under reducing conditions ferrate intermediates as catalytically active species cannot be excluded. Although the exact nature of the low-valent catalytic species remains unclear, some of these interesting transformations are discussed in this section. 

Catalysis has not been established with these DNICs until now nevertheless, these complexes are interesting low-valent iron complexes that lit into the class of ferrates from which one can learn a lot about reactivity, stability, and electronic properties. 

Co-oxidation of indene and thiophenol in benzene solution is a free-radical chain reaction involving a three-step propagation cycle. Autocatalysis is associated with decomposition of the primary hydroperoxide product, but the system exhibits extreme sensitivity to catalysis by impurities, particularly iron. The powerful catalytic activity of N,N -di-sec-butyl-p-phenylenediamine is attributed on ESR evidence to the production of radicals, probably >NO-, and replacement of the three-step propagation by a faster four-step cycle involving R-, RCV, >NO, and RS- radicals. Added iron complexes produce various effects depending on their composition. Some cause a fast initial reaction followed by a strong retardation, then re-acceleration and final decay as reactants are consumed. Kinetic schemes that demonstrate this behavior but are not entirely satisfactory in detail are discussed. 

Iron as a cofactor in catalysis is receiving increasing attention. The most common oxidation states of iron are Fe2+ and Fe3+. Iron complexes are nearly all octahedral, and practically all are paramagnetic (as a result of unpaired electrons in the 3d orbital). The most common form of iron in biological systems is heme. Heme groups (Fe2+) and hema-tin (Fe3+) most frequently involve a complex with protoporphyrin IX (fig. 10.19). They are the coenzymes (prosthetic... 

A titanium complex derived from chiral /V-arencsulfonyl-2-amino-1 -indanol [20], a cationic chiral iron complex [21], and a chiral oxo(salen)manganese(V) complex [22] have been developed for the asymmetric Diels-Alder reaction of oc,P-unsaturated aldehydes with high asymmetric induction (Eq. 8A.11). In addition, a stable, chiral diaquo titanocene complex is utilized for the enantioselective Diels-Alder reaction of cyclopentadiene and a series of a.P Unsaturated aldehydes at low temperature, where catalysis occurs at the metal center rather than through activation of the dienophile by protonation. The high endo/exo selectivity is observed for a-substituted aldehydes, but the asymmetric induction is only moderate [23] (Eq. 8A. 12). 

This section provides only a brief insight into iron-catalyzed reactions. Iron complexes as catalytically active species undergo typical steps of transition metal catalysis... 

Roelfes et al. prepared a non-heme iron(II) complex 26 from pentadentate ligand N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine 25 (Scheme 3.32) [126]. In the presence of H202, complex 26 reacted to a low-spin Fe(III)OOH intermediate, which was cleaved homolytically to an oxo Fe(IV) species and a hydroxy radical. Both species are capable of oxidizing various organic substrates via a radical pathway (Scheme 3.32). Under the catalysis of complex 26, cyclohexene la was oxidized with excess H202 to a mixture of products 2a, 3a and 4a. The TON was found to be solvent dependent, with higher TON in acetonitrile than in acetone (Scheme 3.32). In no case were isolated yields given and, furthermore, the allylic oxidation is limited to cyclohexene la. 

With respect to the mechanism of the iron catalysis, the activity of FeCl3 -6H20 is closely related to its ability to give dionato chelate complexes 3 with [i-dicarbonyl compounds. Without prior deprotonation - even in Bronsted acidic media - these deeply colored iron complexes are instantly formed. With this property, Fe(III) is unique among all other transition metals, which require a stoichiometric amount of base for dionato complex formation. Known for over 100 years, the significant color of the complexes has been utilized for the detection of [i-oxo esters and [i-di ketones. 

Due this recent revival, there is the need for an authoritative review of this important chemistry. It is the purpose of this book not only to introduce the chemistry community to the most recent achievements in the field of catalysis, but also to create a deeper understanding of the underlying fundamentals in the organometallic chemistry of iron complexes. 

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