Electron transfer fragmentation reactions

Chen, L., Lucia, L., and Whitten, D.G., Cooperative electron transfer fragmentation reactions. Amplification of a photoreaction through a tandem chain fragmentation of acceptor and donor... 

The electron transfer induced reaction of this diene system results in rapid [4 + 2]dimerization conversely, the dimer rapidly undergoes cycloreversion upon electron transfer. Both reactions result in strong CIDNP effects. The monomer polarization supports a radical cation with a spin density distribution like those of the butadiene or fulvene radical cations. The dimer polarization identifies a dimer radical cation with appreciable spin density only on two carbons of the dienophile fragment this species can only be the doubly linked radical cation D [135, 136], Significantly, a second dimer radical cation is implicated in a pulsed... 

Among the electron transfer induced reactions of cyclobutane systems, cycloreversions are the most prominent. These reactions are the reverse of the cycloadditions discussed in Sect. 4.1. The reactivity of the corresponding radical cations depends on their substitution pattern. We have mentioned the fast two-bond cycloreversion of quadicyclane radical cation as well as the ready ring closure of a tetracyclic system (3, Sect. 4.1). A related fragmentation of cis-, trans-, cis-1,2,3,4-tetraphenylcyclobutane (84) can be induced by pulse radiolysis of 1,2-dichloro-ethane solutions. This reaction produces the known spectrum of trans-stilbene radical cation (85) without a detectable intermediate and with a high degree of... 

In turn, photosensitized electron transfer fragmentation of a benzylic bond has been among the first reaction reported when this branch of photochemistry began to take shape in the seventies, with the decarboxylation of phenylacetic acids reported by Libman [23] and the C-C cleavage in phenylethyl methyl ethers reported by Arnold [24]. 

The fragmentation of radical anions and the reverse reaction, the addition of anions to radicals, are the critical steps of SRN1 reactions [110] which constitute perhaps the largest class of fragmentation reactions initiated by photoinduced electron transfer. These reactions are chain processes and photoinduced ET is involved only in the initiation step, which is usually poorly defined. The reactions may also be initiated by other means, not involving absorption of a photon. The SRN1 reactions and related redox-activation processes have been recently extensively reviewed [72a, 110,127] and will not be discussed here. 

Single-electron transfer from a borate anion particle to the excited polymethine cation generates a dye radical and an aLkylphenylbotanyl radical. The aLkylphenylbotanyl radical fragments to form an active alkyl radical. It is the alkyl radical particles that initiate the polymerization reactions (101). 

The possible mechanism of ionization, fragmentation of studied compound as well as their desoi ption by laser radiation is discussed. It is shown that the formation of analyte ions is a result of a multi stage complex process included surface activation by laser irradiation, the adsoi ption of neutral analyte and proton donor molecules, the chemical reaction on the surface with proton or electron transfer, production of charged complexes bonded with the surface and finally laser desoi ption of such preformed molecules. 

Another useful generalization is the principle of maximum hardness. This states that molecular arrangements that maximize hardness are preferred. Electronegativity and hardness detennine the extent of electron transfer between two molecular fragments in a reaction. This can be approximated numerically by the expression... 

The use of direct electrochemical methods (cyclic voltammetry Pig. 17) has enabled us to measure the thermodynamic parameters of isolated water-soluble fragments of the Rieske proteins of various bci complexes (Table XII)). (55, 92). The values determined for the standard reaction entropy, AS°, for both the mitochondrial and the bacterial Rieske fragments are similar to values obtained for water-soluble cytochromes they are more negative than values measured for other electron transfer proteins (93). Large negative values of AS° have been correlated with a less exposed metal site (93). However, this is opposite to what is observed in Rieske proteins, since the cluster appears to be less exposed in Rieske-type ferredoxins that show less negative values of AS° (see Section V,B). 

An important synthetic application of this reaction is in dehalogenation of dichloro- and dibromocyclopropanes. The dihalocyclopropanes are accessible via carbene addition reactions (see Section 10.2.3). Reductive dehalogenation can also be used to introduce deuterium at a specific site. The mechanism of the reaction involves electron transfer to form a radical anion, which then fragments with loss of a halide ion. The resulting radical is reduced to a carbanion by a second electron transfer and subsequently protonated. 

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