The secondary radicals

At higher temperatures, ionic radicals are not expected to be stable, but rather these species protonate or deprotonate to form neutral, secondary radical products. As mentioned, the first of these products, T(C6H), was identified as evolving from the thymine anion in early ESR studies. Later, the decay of the guanine cation was predicted to be related to the growth of G(N1) [73,74]. T(CH2) has also been observed in highly hydrated DNA samples [73]. Evidence exists that the cytosine anion is stabilized by protonation at N3 at 77 K [75]. Additionally, in thymine deuterated DNA samples, a deuteron has been determined to add to the C6 position of the cytosine anion [73]. Despite the fact that the types of products observed are diverse, these products were each observed in different samples. 

Advances have been made in the past few years to identify more than two or three products in one DNA sample. The most promising results were obtained by Hiitteimaim and cowoikers, in both oriented fibers [76] and in randomly 

The role of sugar radicals in DNA radiation damage is uncertain since no sugar radicals were identified in preliminary studies on fiill DNA samples [42]. 

From these studies it is clear that damage to DNA is broader than initially expected from the two-component model since products on aU four bases and the sugar moiety have been proposed. These proposals include sugar and phosphate radicals despite early failures to detect radicals in the backbone of the DNA double helix. More work is requited in order to determine the exact identity of the radical products since structural information is difficult to obtain through the methods implemented thus far. 

Besides direct damage of the DNA strand, it is also possible for the surrounding water molecules to be involved in radiation damage mechanisms. The hydration layer of DNA consists of a primary layer (approximately 20 or 21 water molecules per nucleotide), which possesses properties different from crystalline ice upon freezing, and a secondary layer, which cannot be distinguished from bulk water upon crystallization. Upon irradiation of water, many different products can be formed  

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Because the starting material (propane) and one of the products (H ) are the same m both processes the difference m bond dissociation energies is equal to the energy dif ference between an n propyl radical (primary) and an isopropyl radical (secondary) As depicted m Figure 4 20 the secondary radical is 13 kJ/mol (3 kcal/mol) more stable than the primary radical... 

FIGURE 4 20 The bond dis sociation energies of methy lene and methyl C—H bonds in propane reveal difference in stabilities between two isomeric free radicals The secondary radical is more stable than the primary... 

In the mass spectrum (Figure 8) of the corresponding ketal of 5-deoxy-D-xt/Zo-hexose, 5-deoxy-l,2-0-isopropylidene-D- rt/Zo-hexofuranose (11), the peak from C-4-C-5 cleavage, m/e 159, is of minor relative intensity. Since the ions at m/e 159 are the same from both isomers, 10 and 11, the intensity difference must be attributable to the lower stability of the primary radical formed from C-5 of 11 compared with the secondary radical from 10 ... 

In the stepwise decomposition of azo-compounds such as 4, products can arise from reactions within the primary diazenyl-alkyl radical pair or from the secondary radical pair produced by loss of nitrogen from the... 

The observation (Porter ef a ., 1972) that added BrCCla almost completely suppresses the polarization of the olefin, while leaving the polarization of trans-4 unalfected, points to the secondary radical pair as the principal immediate precursor of a-methylstyrene. A rate constant for the decomposition of thediazenyl radical of 10 -10 sec has been estimated. Cage collapse and free-radical formation are also thought to occur and appropriately polarized products have been identified (see above). 

The reactions can also be effected by phenyliodonium diacetate.377 A mechanistic prototype can be found in the conversion of pentanol to 2-methyltetrahydrofuran. The secondary radical is most likely captured by iodine or oxidized to the carbocation prior to cyclization.378... 

One way of stabihzing the initial radical or anion radical is therefore the addition of an acid. Expulsion of a base should produce a similar effect. This is indeed the case (Scheme 2.21), and the secondary radical thus formed is similarly easier to reduce than the starting molecule in most cases. RX is a molecule containing a low-lying orbital able to accommodate the incoming electron, thus leading to the primary radical, RX -, before the nucleophile X- is expelled. We consider here the case of a stepwise process in which the reaction pathway involves the intermediacy of the primary radical rather than a... 

If the provoked or spontaneous acid-base reactions overcome the radical reactions of the primary radical, the secondary radical is easier to reduce, or to oxidize, than the substrate in most cases. Exceptions to this rule are scarce, but exist. They involve substrates that are particularly easy to reduce thanks to the presence of a strongly electron-withdrawing substituent (for reductions, electron-donating for oxidation), which is expelled upon electron transfer, thus producing a radical that lacks the same activation. Alkyl iodides and aryl diazonium cations are typical examples of such systems. 

To confirm the trends observed with 10, we also investigated the behavior of epoxide 17 under ET conditions. Here, a tertiary radical would be formed after reductive opening that is more persistent than the secondary radical obtained from 10, as depicted in Scheme 8. The results of the opening reactions are summarized in Table 3. 

The rate constants for reaction of Bu3SnH with the primary a-alkoxy radical 24 and the secondary ce-alkoxy radical 29 are in reasonably good agreement. However, one would not expect the primary radical to react less rapidly than the secondary radical. The kinetic ESR method used to calibrate 24 involved a competition method wherein the cyclization reactions competed with diffusion-controlled radical termination reactions, and diffusional rate constants were determined to obtain the absolute rate constants for the clock reactions.88 The LFP calibrations of radical clocks... 

In the formation of 1-chlorobutane, an intermediate primary radical is involved, and there are no stereochemical consequences. However, the secondary radical involved in 2-chlorobutane formation is planar, and when it abstracts a chlorine atom from a chlorine molecule it can do so from either side with equal probability. The result is formation of a racemic product, ( )-2-chlorobutane. 

The very high value of Cm for vinyl chloride is attributed to a reaction sequence involving the propagating center XVIII formed by head-to-head addition [Hjertberg and Sorvik, 1983 Llauro-Darricades et al., 1989 Starnes, 1985 Starnes et al., 1983 Tornell, 1988]. Intramolecular migration of a chlorine atom (Eq. 3-114) yields the secondary radical XIX that subsequently transfers the chlorine atom to monomer (Eq. 3-115) to yield poly(vinyl chloride)... 

In order to determine the significance of the 1,5-H shift and the secondary radical species for antimalarial activity, the trioxanes 22a-c were synthesized and tested . The diastereomeric trioxanes 22a and 22b possessed very different antimalarial activity against both chloroqnine-resistant and chloroqnine-sensitive strains of the parasite the A-fi isomer was approximately twice as active as artemisinin while the A-a isomer and the disubsti-tnted trioxane were more than sixty times less potent. The anthors proposed that the a-snbstitnent prevented the snprafacial 1,5-H shift and therefore snppressed the activity of these componnds. 

As we have seen in Sections 1 and 4, the principal primary products of the radiolysis of water are powerful oxidizing and reducing radicals in approximately equal yields. For water radiolysis to be a useful tool in general chemistry, it is desirable to convert the primary radicals to a single kind of secondary radical to achieve either totally oxidizing or reducing conditions. Moreover, there is the possibility of designing the system to have the required redox properties by suitable selection of the secondary radicals. Some useful systems that meet these requirements are described below. 

Simple alcohols are often used as the source of the secondary radicals because they also react rapidly with both OH and H (e.g., 2-propanol in reactions (73) and (74)) ... 

In the case of unsymmetrical alkenes, the OH radical can add to either end of the double bond. There is evidence that, as expected, it preferentially adds to form the secondary radical. For example, for the propene reaction (Cvetanovic, 1976), 65% of the adducts formed correspond to (34a) and 35% to (34b) ... 

Another relevant example is the pyrolysis of cnt/ -2,7,7-trimethylbicyclo[3.1.1]hept-2-en-6-ol (21) at 430 =C, which produces a mixture of several products. Gas chromatographic separation gives, among many other compounds, 3,7-dimethylocta-3,6-dienal (23) and 3,7-dimethylocta-2,6-dienal (24) in 13 and 6% yield, respectively.107 As can be seen in the diradical 22, the C — C double bond is able to offer 71-stabilization to the secondary radical. For this reason, the 1,4-diradical is generated exclusively.107... 

Spin adducts with nitrosobenzenes differ in the g factor and can be distinguished (Ter-abe Konaka 1971 Terabe and co-authors 1973). The primary alkyl radicals, aryl and arylthio radicals, form spin adducts with a free valence at nitrogen the tertiary radicals produce spin adducts with a free valence at oxygen and the secondary radicals give both types of adducts. 

In poly(acrylic acid), two radicals are also formed upon -OH-attack. Again, the secondary radical undergoes intramolecular H-abstraction, leading to the tertiary radical [reaction (4) Ulanski et al. 1996c],... 

Fig. 1.34. Chlorination of adamantane to furnish tert-adamantyl chloride (A) and sec-adamantyl chloride (B), using differents reagents. In the reaction with the A B selectivity amounts to 68 32 in carbon disulphide and 39 61 in carbon tetrachloride (each at 25°C). In the reaction with S02CI, in sulfolan the A B selectivity is > 97.5 2.5 (at 60°C). The formation of sec-adamantyl chloride (B) proceeds via the secondary radical C, tert-adamantyl chloride (A) is formed via the tertiary radical D.

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