Radicals selectivity

In contrast to the direct reduction as described above, the indirect electrochemical reduction of perfluoroalkyl halides is a versatile and novel method for generating perfluoroalkyl radicals selectively. Saveant et al. have demonstrated many successful examples. Using terephthalonitrile as a mediator, the indirect reduction of CF3Br in the presence of styrene leads to the dimer of the radical adduct obtained by the attack of CF on styrene. On the other hand, in the presence of butyl vinyl ether, the mediator reacts with the radical adduct obtained by the attack of CF3. on the olefin (Scheme 3.4) [14]. 

The exceptionally stoically demanding trimethylsilyl radical selectively removes an equatorial chlorine atom. Attack on axial chlorine atoms is severely limited here by the concave molecular geometry. Tnalkyi- and triarylstannane were also tried, but they failed to distinguish between the chlorines and instead produced a 1 1 epi-meric mixture at C-7. 

Chatgilialoglu C, Guerra M, Mulazzani QG (2003) Model studies of DNA C5 radical. Selective generation and reactivity of 2 -deoxyadenosine-5 -yl radical. J Am Chem Soc 125 3839-3848... 

In this section results for a variety of organic radicals selected from current literature are discussed. 

However, Jacobsen was able to show that, after addition of a pyridine JV-oxide derivative, trisubstituted alkenes are in fact excellent substrates. The dissymmetry of the chiral salen ligand can effectively orient the radical selectivity ... 

Scheme 23 Hydroxyl radical-selective trapping reaction with y - locopheryl - iso-propyl ether (56) and y-tocopheryl ether derivatives (58, 59) showing different reactivity towards radicals...

The idea of radicals selectively abstracting hydrogen atoms from C—H bonds weakened by this n-a interaction was also used by Deslongchamps et al. (1972a) in their initial treatment of the ozonolysis of [1] and [2], Although the balance of evidence now points to this reaction being one of hydride abstraction, at the time a hydrogen atom abstraction mechanism was entirely reasonable. 

Although the ability of zirconium complexes to generate radicals selectively was noted early [165, 166], not many radical reactions mediated by zirconium complexes are known [167]. Oshima et al. reported the only examples of zirconium-catalyzed radical reactions. 

Nucleophilic and electrophilic radicals abstract different hydrogen atoms from butyrolactone 7.10 the ferf-butoxy radical selectively abstracts a hydrogen atom from the methylene group adjacent to the oxygen atom, whereas a boryl radical abstracts a hydrogen atom from the methylene group a to the carbonyl group. 

The decomposition by the iron(II) salt of hydroxylamine-O-sulfonic acid in the presence of formamide, alkylformamides, or A -alkylacetamides provides the amino radical cation +NH3 which, by hydrogen abstraction, generates carbamoyl and a-Wamidoalkyl radicals. Both kinds of radicals selectively attack protonated quinoxaline. 

These results suggest that of the two copolymerization parameters, and e, only the e values are aflFected appreciably by an increase in pressure. Calculations have been made obtaining relative Q and e values which bear out this thesis. The decreasing radical selectivity found in these experiments implies an increased radical reactivity. This offers further support to Walling s suggestion that the ally acetate radical increases in reactivity with increasing pressure. 

Figure 16 Schematic representation of the reaction pattern for the oxidation of organic compounds with simultaneous oxygen evolution at metal-oxide anodes reactions (a), (b), (c), d) as in Figure 11 (e) combustion of the organic compound R via electrochemical oxidation mediated by physisorbed hydroxyl radicals (/) selective chemical oxidation of the organic compound at the higher metal oxide surface sites. (From Ref. 15. Copyright 1994, Pergamon Press Ltd. Reprinted with permission.)...

The reaction of nucleophilic radicals, under acidic conditions, with heterocycies containing an imine unit is by far the most important and synthetically useful radical substitution of heterocyclic compounds. Pyri-dines, quinolines, diazines, imidazoles, benzothiazoles and purines are amongst the systems that have been shown to react with a wide range of nucleophilic radicals, selectively at positions a and y to the nitrogen, with replacement of hydrogen. Acidic conditions are essential because A-protonation of the heterocycle... 

Selectivity Affected by the Nature of the Radical. Selectivity is also seen in which atom is abstracted when there is more than one to choose from, as we have seen already in the reactions of the lactone 7.11 and propionic acid 7.14. When the tributyltin radical has a choice of a C—S, a C—Se, or a C halogen bond,... 

The 2AP radical cations, 2AP +, formed together with hydrated electrons by the photoionization process, rapidly deprotonate to the neutral radicals, 2AP(-H)- [31]. The latter radicals selectively oxidize nearby G bases to form G(-H)" radicals - a process that is completed within 100gs after the actinic laser flash. Combination of the G(-H)" and "N02 radicals occurs on millisecond time scales with similar rate... 

Keywords Samarium diiodide, Electron transfer, Organosamarium, Radicals, Selectivity... 

A is a bivalent radical selected from (1), (2), and (3) and R1 and R3 are the same or different and each is a substituent selected from H, Cl, Q- alkyl, methoxy, ethoxy, and sulphonic acid radical or a salt thereof, sulphonamide radical, and Q- alkyl-substituted sulphonamide radical. 

The artemisinins appear to kill the parasite by a free radical mechanism—not by the generation of ROS but, rather, by virtue of a free radical associated with the endoperoxide, possibly involving a carbon radical. Evidence points toward activation of the endoperoxide via an iron-dependent mechanism. The resulting free radical selectively targets sarcoplasmic/endoplasmic reticulum Ca " -ATPase of the Plasmodium falciparum (PfATP6), altering calcium stores (49). The artemisinins actually may form covalent adducts to specific membrane-associated proteins after concentrating in infected erythrocytes. 

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