Iron-arene salts

Recently, Meier and Zweifel have described that iron arene salts jjavi g... 

Iron arene salts are generally prepared from ferrocene according to the method reported by Nesmeyanov . Various types of such complexes described by the following general formula are listed in the literature... 

Fig. 8. Formation of a Lewis acid by irradiation of an iron arene salt...

Iron arene photoinitiators have excellent light absorption properties in the ultraviolet and visible parts of the spectrum. As shown in Fig. 18, the absorption can be varied over a wide range by structural changes in the ligands. Iron arene salts can be sensitized, for example with anthracene derivatives... 

Due to the chemical structure and the UV absorption ranges of the iron arene salts (the optical density in the UV and visible part of the spectrum decreases during irradiation), even thick films up to several 100 micrometers can be cross-linked. 

Besides the most important area of surface coatings, the use of photopolymers as photoresists in the manufacture of printed circuits is well established. Photoimaging with aryldiazonium salt photoinitiators and multifunctional cresol-novolac epoxides was first described by Schlesinger Crivello has mentioned several new photoresists based on the photopolymerization of epoxides with onium initiators Meier and Zweifel have shown that iron arene salts in combination with multifunctional cresol-novolac epoxides yield photoresists with high resolution and contrast. Dual functional epoxides (cf. Sect. 5) containing chalcone groups as light-sensitive units have been described as suitable photoresists especially... 

Iron-arene salts may also exhibit an appreciable absorption in the visible or near-UV part of the spectrum. The photochemistry of these compounds as well as their use as photoinitiators has recently been described by Lohse and Zweifel In certain applications the presence of strong acid m the photopolymer is desirable since the reaction continues after exposure. 

Complex iron(III) salts are frequently used in oxidative arene coupling reactions and quinone formation and tetra-n-butylammonium hexacyanoferrate(III) has several advantages in it use over more conventional oxidative procedures. When used as the dihydrogen salt, Bu4N[H2Fe(CN)6], it oxidizes 2,6-di-z-buty 1-4-methylphenol (1) to the coupled diarylethane (2), or aryl ethers (3) and (4) (Scheme 10.4), depending on the solvent. It is noteworthy that no oxidation occurs even after two days with the tris-ammonium salt. 

Photolysis of the arene complexes in the presence of monodentate ligands, e.g. carbon monoxide, leads to new complexes of the type CpFe(L) whereas in pure aprotic solvents, ferrocene and iron salts are formed Investigation of the photo-lytic reaction of an iron arene complex with excess ethylene oxide in methylene chloride solution (Meier and Rhis ) showed that a crystalline crown ether complex (structure shown in Fig. 9) was obtained in high yield. Only traces of dioxane could be detected. 

Photoinitiated cationic polymerization has been the subject of numerous reviews. Cationic polymerization initiated by photolysis of diaryliodonium and triarylsulfonium salts was reviewed by Crivello [25] in 1984. The same author also reviewed cationic photopolymerization, including mechanisms, in 1984 [115]. Lohse et al. [116], reviewed the use of aryldiazonium, diphenyliodonium, and triarylsufonium salts as well as iron arene complexes as photoinitiators for cationic ring opening polymerization of epoxides. Yagci and Schnabel [117] reviewed mechanistic studies of the photoinitiation of cationic polymerization by diaryliodonium and triarylsulfonium salts in 1988. Use of diaryliodonium and sulfonium salts as the photoinitiators of cationic polymerization and depolymerization was again reviewed by Crivello [118] in 1989 and by Timpe [10b] in 1990. 

In the (keto)coumarin/amine/ferrocenium salt system, the ferrocenium salt plays a crucial role that is rather complex. In a three-component photoinitiator system [238,239] consisting of a coumarin, an iron arene complex such as CpFe +Ar and a phenylglycine derivative as an amine, the first step of the photoreaction occurs between the dye and the complex according to an electron process. The amine reacts with the radical (created on the complex) through hydrogen abstraction. Therefore, no detrimental ketyl radicals are formed. 

Iron Arene Complexes-Based Photoinitiators Iron arene complexes or ferrocenium salts are attractive photoinitiators for cationic polymerization of... 

Like cerium(IV), iron(III) salts can oxidize electron-rich centers by single-electron transfer to form radicals [1]. Early applications were developed for the oxidation of aromatic compounds, which undergo C-C bond formation to dimeric products. Because of their electronic properties, methoxy substituted arenes 25 are most reactive. Iron(III) chloride supported on silica gel was the reagent of choice, since inter-and intramolecular coupling products 26 are obtained in excellent yields (Scheme 8)... 

Ionization potential. See Ionization energy a- and p-lonone, 1049 lonophore, 624,1023 Iron, reduction of nitroarenes by, 878 Iron(III) salts as catalysts in halogenation of arenes, 446, 448—450 Isoamyl acetate, in bananas, 85, 788 Isobutane, 57. See also 2-Methylpropane Isobutene. See 2-Methylpropene... 

Roberts have made a detailed study of the synthesis of (i -arene)( -cyclopentadienyl)iron(ll) salts using... 

Organometallic compounds which can act as highly efficient photoinitiators for epoxide monomers have been described in the literature (25-27,69-71). The most commonly used organometallic compounds are the photoinitiators based on iron arene complex developed by the investigators at the Ciba-Geigy Corp. (37,69-71). These salts are generally prepared from ferrocene, as reviewed by Lohse and Zweifel (37). The molecular structure of a typical iron-arene complex is shown as (3). 

In a similar manner, certain bis(arene)iron(II) salts react easily with nucleophiles and may be used in organic synthesis. For example, (mesityIene)2Fe(PF6)2 reacts with phenyl, terf-butyl, and vinyllithium to give either the compound XLVIII or the pseudo-ferrocene derivative XLIX, according to the stoichiometry of the reaction (Helling and Braitsch, 1970). 

The mechanism involves the formation of a nucleophilic ferrate species from the iron(IIl) salt and ter/-butylmagnesium chloride. It undergoes oxidative addition to the aryl halide. Transmetalation with the Grignard reagent leads to an alkyliron compound. The latter is not preferentially prone to reductive elimination to form alkylated arenes. Instead, P-hydride elimination and subsequent reductive elimination of the hydrido-iron complex reforms the iron(-II) catalyst releasing isobutene and the defiinctionalized arene (Scheme 4-226). ... 

Azaferrocene reacts with aromatic hydrocarbons in the presence of aluminium chloride, giving rise to the cationic complexes of the type (Ti -arene)(Ti -cyclopenta-dienyl)iron(l+) isolated as BF4 salts [87JOM(333)71]. The complex 28 is obtained by reaction of the sulfane compound [Cp(SMc2)3Fe]BF4 with pentamethyl-pyrrole [88AG(E)579 88AG(E)1468 90ICA(170)155]. The metallic site in this center reveals expressed Lewis acidity (89CB1891). 

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