Electron transfer-catalyzed

Khramov, M.I. and Parmon, V.N. (1993) Synthesis of ultrafine particles of transition-metal sulfides in the cavities of lipid vesicles and the light-stimulated transmembrane electron-transfer catalyzed by these particles. Journal of Photochemistry and Photobiology A-Chemistry, 71, 279-284. 

However, the most severe criticism of the CIEEL hypothesis relates to the chemiexcita-tion efficiency experimentally obtained for the standard CIEEL systems, diphenoyl peroxide (4) and 1,2-dioxetanone (2) . In a study on the electron transfer catalyzed decomposition of l,4-dimethoxy-9,10-diphenylanthracence peroxide (21), Catalan and Wilson obtained very low chemiexcitation quantum yields with various commonly utilized activators (4>s =2 10 EmoH ) and reinvestigated the CL of diphenoyl peroxide (4), determining quantum yields in the same order of magnitude (4>s = (2 1)10 Emol ) as those obtained by 21 (Table 1). We have more recently determined the quantum yields in the rubrene-catalyzed decomposition of dimethyl-1,2-dioxetanone (9) and also found a much lower value than the one initially reported (Table 1) °. Since the diphenoyl peroxide and the 1,2-dioxetanone systems are the two prototype CIEEL systems, the validity of this hypothesis itself might be questioned due to its low efficiency in excited-state formation. ... 

Electron Transfer-induced and Electron Transfer-catalyzed Ligand... 

Thus, one-electron reduction of (bpy)Cr(C0)4 results in facilitation of the CO-PPh3 replacement in an electron-transfer-catalyzed reaction (Miholova Vlcek 1985). Sometimes, the catalytic cycles in Scheme 1-49 become available (Kochi 1986). 

Amatore C, Thiebault A, Verpeaux J-N (1989) Unexpected single electron-transfer catalyzed cyclization of prenyl sufore dimer - evidence for radical-anion coupling in the outer-sphere oxidation of prenyl sulfone carbanion. J Chem Soc Chem Commun 20 1543-1545... 

Finally, several cases of electron-transfer catalyzed 2n + 2n cyclodimerizations have been reported and suggested to have chain mechanisms (Ledwith, 1972 Bard et al., 1976 Cedheim and Eberson, 1976). 

The cluster does not undergo electron transfer-catalyzed CO substitution reactions because the lifetimes of the intermediate radical anions are too short (see below). 

Electron transfer catalyzed cycloadditions via radical cations show remarkable selectivity that could be exploited for expanded synthetic methodology. As a complement to the neutral Diels-Alder reaction, ET catalysis hlls the void of the electron-rich diene/electron-rich dienophile cyclizations. In attempt to understand the intricate details of the reaction, experimentalists and theorists have uncovered a range of novel factors to control and manipulate these high-energy reactive intermediates. As exemplihed by the cases discussed in this contribution, the charged character of the intermediates and the presence of back electron transfer leading to the biradical reaction manifold opens new pathways to control the chemo-, peri-, and stereochemical patterns in these dynamic species. 

Masters, B. S., Bilimoria, M. H., Kamin, H., and Gibson, Q. H., 1991, The mechanism of 1- and 2-electron transfers catalyzed by reduced triphosphopyridine nucleotide-cytochrome c reductase. Nature 351 714n718. 

A better approach to substitution at (1) is the use of catalytic routes. Induction with Me3NO has proved successful for (1) in some cases, but of particular value for a clean reaction with (1) was electron-transfer catalyzed substitution using Ph2CONa in low concentration as a reductive activator. The likely intermediate is the 49-electron species [Ru3(CO)i2], which quickly reacts to form [Ru3(CO)iiL]". Electron-transfer occurs again to form Ru3(CO)iiL and to regenerate [Ru3(CO)i2] to repeat the cycle. The same type of reaction cycle can be induced with [PPN]X, where X = acetate, cyanide, or halides. The [PPN][Ru3(CO)i2-nX] (n = 1, 2, 3) species is the intermediate, and X is displaced by the appropriate ligand to obtain the substitution product. 

The real utility of this electron-transfer catalyzed reaction lies in the enormous enhancement of reactivity toward substitution in the anion radicals. For example, thermal substitution of 51 (R = CF3) by phosphines gives 10-60% product yields after several hours in boiling benzene. By contrast, the monosubstituted complexes 52 (n = 1) were produced in greater than 90% yield, in less than 1 minute at room temperature, by adding a catalytic amount of the reductant [Ph2CO] to a 1 1 mixture of 51 and L (727). 

An unusually efficient example of an electron transfer catalyzed electrocyclic reaction proceeding via anion radicals has also been established [124]. The conversion of the bis(allene) shown in Scheme 78 to the corresponding cyclobutene derivative... 

In case 1.3 the pro-catalyst interacts with the substrate (for example by photosubstitution) to generate the actual catalyst. Such a case is germane to photo-induced electron transfer catalyzed reactions [7] shown in case 1.4. Here, an excited state of a transition metal complex (designated as I) could interact with the substrate by a positive or negative electron transfer. Because of the activation induced by electron transfer [13] S would be transformed into S, whose fate is to be regenerated one or several steps after yielding P. 

One may note that these radical reactions involve catalytic amounts of organo-metallic and catalytic amounts of light (0 > 1) but, as is the case for some photo-induced electron transfer catalyzed reactions (cf. Section 3.2.9.1), they could be classified as chain processes rather as catalytic processes [70]. 

The following results support the stereoisomerization as an electron transfer catalyzed reaction. 

In the much faster electron transfer catalyzed rearrangement a new stereoisomer, r-l-phenyl-c-2,t-3-dimethylcyclopropane cis,trans-90 shows up in the equilibrium with cis,cis-88 and trans,trans-89. The formation of cis,trans-90 requires that in the course of the equilibration a cyclopropane bond (instead of... 

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