Iron low valent

Fiirstner A, Majima K, Martin R, Krause H, Kattnig E, Goddard R, Lehmann CW (2008) A cheap metal for a Noble task preparative and mechanistic aspects of cycloisomerization and cycloaddition reactions catalyzed by low-valent iron complexes. J Am Chem Soc 130 1992-2004... 

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. 

Low-valent iron in the formal oxidation states (I) and (0) is mostly found in iron-carbonyl and metal-organic compounds which have been intensively studied since... 

Table 8.7 Isomer shift and quadrupole splitting of some low-valent iron complexes...

Three- and Four-Coordinate Low-Valent Iron Compounds... 

Within group 8, a bis-dinitrogen complex of an iron(O) tridentate pyridinediimine structure has also recently been shown to catalyze the hydrosilylation of alkyne.60 This discovery is a new example of the utility of low-valent iron in catalysis.61... 

An example of how cyclic voltammetry can be used to apply the kinetic advantage method is summarized in Figure 3.29. The goal is to investigate the reaction of low-valent iron porphyrins, Fe1 and Fe°, with an alkyl halide. 

An interesting alternative to the use of chromium(VI) oxidants for the conversion of 1 to 2 involves the use of a low-valent iron reagent prepared in situ by the action of hydrogen peroxide on an iron(II) complex of 1 (73). Vinblastine (as the free base) is treated with 2 equiv of perchloric acid in acetonitrile at -20°C. Ferrous perchlorate is then added, followed by the addition of excess 30% hydrogen peroxide. Work-up of the reaction mixture with a saturated solution of ammonium hydroxide gives 2 in yields of 35-50% after chromatography. 

Scheme 4.17 Hydrogenation with low-valent iron complexes (cod = 1,5-cyclooctadiene coe = cyclooctene).

The Alder-ene reaction is an atom-economic reaction which forms a new carbon carbon-bond from two double bond systems (alkenes, carbonyl groups, etc.) with double bond migration [5]. This reaction follows the Woodward-Hoffmann rules if the reaction is performed under thermal conditions. However, when transition metal catalysts are involved, thermally forbidden Alder-ene reactions can also be realized (Scheme 9.1). Examples of such processes are the formal [4 + 4]-Alder-ene reaction catalyzed by low-valent iron catalysts. 

On the other hand, Takacs and coworkers added organometallic reducing agents to the reaction mixture and promoted the formation of low-valent iron(O) bipyridine complexes. The mechanism of the low-valent iron-catalyzed Alder-ene reaction involves coordination of the two starting materials within the ligand sphere of the iron, which makes the Woodward-Hoffmann rules for such reactions obsolete [11]. Thereby, the scope of the reactions was broadened so that alkenes and 1,3-dienes could also be used as educts in a formal [4 + 4]-cycloisomerization (Scheme 9.3) [12]. Intriguingly, the diastereoselectivity of the cydopentane products can be influenced by either the application of the 2Z-isomer 3 or the 2 E-isomer 4. Especially the E-isomers 4 gave almost exclusive cis selectivity [13]. 

Scheme 9.3 Alder-ene reaction with low-valent iron catalysts.

The application of this chemistry to the synthesis ofterpene natural products such as (—)-mitsugashiwalactone and (+)-isoiridomyrmecin [15] (Scheme 9.5) involved low-valent iron-catalyzed Alder-ene reactions in key steps in these synthesis. 

The generation of six-membered ring systems by means of cycloaddition reactions can be divided into two main approaches. The first is the cyclotrimerizationofalkynes utilizing low-valent iron catalyst systems, whereas the second approach is the Diels-Alder (DA) reaction of a diene and a dienophile. The latter reaction can itself be divided into three subclasses DA reactions with normal, neutral and inverse electron demand are known. The electronic structure of the educts dictates the oxidation state of the catalyst system required to perform the diverse classes of DA reactions. Nevertheless, for each subclass examples can be found. 

Okamoto and coworkers recently described the iron-catalyzed cyclotrimerization of alkynes utilizing a low-valent iron-diimine complex that was generated in situ upon reduction with zinc dust (Scheme 9.34) [92]. 

Other methods leading to ring enlargement utilize low-valent iron complexes such as Fe(CO)5, as was shown by Taber and coworkers in the ring expansion of vinylcyclopropane derivatives 55 under carbonylating conditions to give the cyclo-hexenone derivatives (Scheme 9.43) as a 5.9 1 mixture of the 2,5- (56) and the constitutional isomer, the 2,6-substituted cydohexenones [101, 102],... 

Another approach for the ring expansion of epoxides uses low-valent iron complexes which open epoxides under reductive conditions, as reported by Hilt et al. [106]. The iron complexes are reduced and after coordination of the epoxide to the iron center an electron transfer initiates the radical-type ring opening of the epoxide. Under formal insertion of an alkene, regioselective formation of tetrahy-drofurans was observed (Scheme 9.46). The reaction is applicable to a broad range of acceptor-substituted alkenes bearing another double or triple bond system in conjugation with the inserted carbon-carbon double bond. 

The most prominent reactions catalyzed by low-valent iron species involving radical intermediates are cross-coupling reactions of alkyl halides (recent reviews [32-35]) and atom transfer radical reactions. In cross-coupling reactions the oxidation state of the catalytically active species can vary significantly depending on the reaction conditions very often it is not known exactly. To facilitate a summary, all iron-catalyzed cross-coupling reactions are treated together and involved oxidation states, where known, are mentioned at the example. In contrast, iron-catalyzed Kharasch reactions will be treated at the oxidation state of the iron precursors. 

Low-Valent Iron Catalysis 1,5-Hydrogen Transfer/Kumada Coupling... 

Low-Valent Iron Catalysis Radical Cyclizations and Tandem Processes... 

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