Radical arene complexes

The I9e electron-reservoir complexes Fe Cp(arene) can give an electron to a large number of substrates and several such cases have been used for activation. After ET, the [FenCp(arene)]+ cation left has 18 valence electrons and thus cannot react in a radical-type way in the cage as was the case for 20e Fe°(arene)2 species. Thus the 19e Fe Cp(arene) complexes react with the organic halide RX to give the coupled product and the [FeCp(arene)]+ cation. Only half of the starting complex is used e.g., the theoretical yield is limited to 50% [48] (Scheme VI) contrary to the reaction with Fe°(arene)2 above. 

A stoichiometric example of the combination of two reagents has recently been reported by O Connor in the context of the Bergman cycloaromatization. The generation of the aromatic diradical can be induced by [Cp Ru(CH3CN)3]PF6. Radical reduction occurs through HAT from CpW(CO)3H to provide a cationic Ru-arene complex as shown in Scheme 15 [35]. 

If this radical is not at the HRP compound 1 level of oxidation, then the catalysis observed by Traylor and co-workers may require a different mechanism, possibly an iodosyl arene complex of the iron N-alkylporphyrin. No reactions were observed between iodosylbenzene and (N-MeTTP)FenCl. The lack of reactions does not preclude reactions under the reaction conditions used in the previous works. 

In a series of reports, Fischer and Muller (71-76) described a method for the synthesis of olefin and arene complexes through photolysis of [Fe((- 3117)3]. No information was given concerning the photochemical steps, but the overall reaction appears to be replacement of alkyl radicals by olefin or aromatic molecules [Eq. (84)]. The equimolar product mixture of... 

Alkali metal arene radical anion complexes are useful sources of solvated electrons for reductive desulfonylation reactions.14 Aromatic compounds such... 

Use of Lithium Naphthalenide. Lithium arene radical anion complexes are mild and highly effective reagents for the reductive desulfonylation process of functionalized sulfones. These reagents have only rarely been used with vinylic and allylic sulfones. In addition to high yields and their operational simplicity, metal arene radical anion complexes demonstrate high chemoselectivity (Eq. 67).123... 

From the experimental point of view, reductive desulfonylations with alkali metal arene radical anion complexes require a large excess of the radical anion, very short reaction times at low temperatures, and must be run under an inert atmosphere. Sodium or lithium naphthalenides in tetrahydrofuran at —78° or lower temperatures are typical reaction conditions. Tetrahydrofuran solutions of lithium naphthalenide are dark green. This color is lost when the substrate is added and restored once the reaction is finished. Upon completion, the excess reagent is quenched with a saturated aqueous solution of ammonium chloride or low molecular alcohols such as methanol or ethanol. 

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 are known to exhibit extremely high photoactivity as initiators. Quantum efficiencies have been found to be greater than I in the photopolymerisation of dicyanate esters. Phenylglycine derivatives have been found to be excellent co-synergists for the iron-arene complexes when used in conjunction with dyes and amines. Complexes of various types have also been proposed. Maleic anhydride-THF complexes have been used for the photopolymerisation of oligourethane acrylates while metal-ion complexes of spiropyran copolymers undergo reversible polymer precipitation. Azo and polyazo initiators have been used to make butadiene-isoprene block copolymers while charge-transfer complexes of morpholine-chlorine induce the radical polymerisation of methyl methacrylate. The presence of zinc chloride enhances the... 

CYP450 TT-complex, the one-electron transfer yields a radical o-complex which can either collapse to arene or alkene epoxide (steps a or d. Fig. 10.6), undergo a 1,2-group migration to a carbonyl product (steps a and b. Fig. 10.6), or give a vinyl hydroxylated product (step c. Fig. 10.6), or a o-complex which can break down to a phenol (step e, Fig. 10.6). The presence of an oxygen radical in the porphyrin ring... 

Photoinitiator systems from hydrogen abstraction or electron transfer usually contain two components a photoinitiator (typically an aromatic ketone) and a co-initiator with a weak covalent bond. Some examples for this system include benzophenones (1), thioxanthones (2), benzyls (3), camphor-quinones (CQs) (4), and ketocoumarins (5), which can be used in the presence of H donors (alcohols, THE, and thiols) or electron donors (such as amines) (Table 2). One of the drawbacks of photoinitiation requiring electron transfer is back electron transfer that limits their practical utility. For this reason, the acidity of the C-H bond of the co-initiator is of great importance. A few initiators are also able to undergo cationic and radical photoinitiation such as iodium and sodium salts and arene complexes. ... 

There are only a few reports of antimicrobial activity of Ru(II) arene complexes. Tocher et al. [73] found that coordination of metronidazole in [(r -benzene)-RuCl2(metronidazole)j (21, Fig. 2.9) did not alter the electron affinity of metronidazole, but decreased the lifetime of the one-electron reduction product (the nitro radical anion) and increased the differential cytotoxicity towards E. coli grown under hypoxic conditions. 

Over the last decade, our group has shown in multiple instances that monometallic Ru catalysts active in olefin metathesis could also be turned into efficient promoters for atom transfer radical reactions with only minor ligand tuning. Such a dual activity was evidenced, among others, for Grubbs catalyst Ru-arene complex 3, Herrmann bis-NHC or mixed-... 

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