Radicals electron transfer

Electron spin resonance (esr) 10 2 to 1 Excitation of unpaired electron-spin orientations in a magnetic field Electron distribution in radicals, electron-transfer reactions (Section 27-9)... 

Recently, the distinction between electrophilic and ion radical (electron-transfer) mechanisms of addition reactions to the vinyl double bond of aryl vinyl sulfides and ethers has been achieved by studying substituent effects (Aplin Bauld 1997). Specifically, the effects of meta and para substituents on the rates of electrophilic addition correlate with Hammett cr values, while ionization of the substrates to the corresponding cation radicals correlates with cr+. The significance of the respective correlations was confirmed by statistical tests. The application of this criterion to the reaction of aryl vinyl sulfides and ethers with tetracyanoethylene revealed that formation of cyclobutanes occurs via direct electrophilic addition to the electron-rich alkene and not via an electron-transfer mechanism. 

Keywords Catalysis Transition metals Radicals Electron transfer Crosscoupling Addition Cyclization... 

Formation of cycloadducts can be completely quenched by conducting the experiment in a nucleophilic solvent. This intercepts radical cations so rapidly that they cannot react with the olefins to yield adducts. In Scheme 54 the regiochemistry of solvent addition to I-phenylcyclohexene is seen to depend on the oxidizability or reducibiiity of the electron-transfer sensitizer. With ]-cyanonaphthalene the radical cation of the olefin is generated, and nucleophilic capture then occurs at position 2 to afford the more stable radical. Electron transfer from excited 1,4-dimethoxynaphthalene, however, generates a radical anion. Its protonation in position 2 gives a radical that is oxidized by back electron transfer to the sensitizer radical before being attacked by the nucleophilic solvent in position 1. Thus, by judicious choice of the electron-transfer sensitizer, it is possible to direct the photochemical addition in either a Markovnikov (157) or anti-Markovnikov (158) fashion (Maroulis and Arnold, 1979). 

Further studies of the SET-induced photoaddition of amines to a/3-unsaturated ketones have been reported. Such reactions have been shown to proceed by way of conjugate addition of free a-amino radicals.Electron-transfer pathways are also involved in the conversion of benzotriazole into 1-arylbenzotriazoles,a transformation which can be achieved by irradiation in the presence of aromatic hydrocarbons and 9,10-dicyanoanthracene, and in the photoreactions of some 2(4),5-dihydro-l,2,4-triazines. 

Nuclear tunneling is potentially a significant consideration in outer-sphere radical electron transfer reactions. The case of reduction of NO2 to NO2 is notable in that nuclear tunneling is predicted to increase the self-exchange rate constant by a factor of 79 relative to the classical value.75 Kinetic isotope effect measurements could provide experimental evidence for nuclear tunneling. 180/160 KIE measurements have indeed provided evidence for nuclear tunneling in reactions involving the O2/O2 redox couple.76... 

TABLE 9.9 Some Reversible Dissociative/Associative Radical Electron Transfer Reactions... 

The Pm state has a very high midpoint potential and it is readily reduced. Transfer of an electron into the catalytic site in state Pm, provided from cytochrome r—> Cua—> heme a, probably results in reduction of the Tyr288 radical. Electron transfer to the catalytic site is coupled to a series of proton transfers, which is thought to be the same every time an electron is transferred to the catalytic site in the reaction cycle two protons are taken up from the N-side of the protein and one is released from the P-side. One of the protons taken up goes to the catalytic site (substrate proton) and the other proton is pumped. 

Aminium radicals add readily to olefins (p. 245). With other cation radicals electron transfer followed by dimerization (170) often... 

Ferrous ions are similarly employed in the production of radicals from N-chloroamines and hydroperoxides. The transfer of one electron to or from a species containing only paired electrons results in the formation of free radicals. Electron transfer can occur both from charged and neutral molecules (Scheme 4.12). 

For species that are known to ionize, such as HI and HBr (and less obvious examples, such as SnC and acyl halides), the ionic mechanism is the most probable and will be especially favored in more polar solvents. The nucleophilic mechanism occurs with many organic halides and requires the availability of an unshared electron pair on the (L) M species. The free radical electron-transfer mechanism is an obvious candidate when XY is an oxidizing agent, such as Cl and Brj. The other possible reactions, noted in Scheme 5.17 for the radical path, u e account of the fact that the concentration conditions are such as to favor reaction of the radicals with reagents rather than with other radicals, unless the latter are reasonably persistent. 

Dunster, C., and Willson, R. L., 1990, Thiyl free radicals Electron transfer, addition or hydrogen abstraction reactions in chemistry and biology, and the catalytic role of sulphur compounds, in Sulphur-centred Reactive Intermediates in Chemistry and Biology (C. Chatgilialoglu and K. D. Asmus, eds.), pp. 377-387, NATO-ASI Series, Life Sciences, Plenum Press, New York. 

Fig. 4.16. Hypothetical enzyme mechanism of the 5-lipoxygenase reaction. In the resting state, the enzyme contains reduced iron (Fe ). Enzyme activation occurs when iron is oxidized from Fe to Fe by hydroperoxides present at the reaction site or formed in situ. The Fe enzyme oxidizes arachidonic acid, forming the pentadienyl radical and a proton, while Fe is reduced to An oxygen molecule attacks stereoselectively this radical, producing a 5(S)-hydro-peroxy radical. Electron transfer and protonation produce 5-hydroper-oxyeicosatetraenoicacid(5-HPETE), on the one hand, and Fe on the other. Dehydration of 5-HPETE yields leukotriene A4 (LTA4).

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