Radicals abstraction selectivity

Thus, even if /-amyloxy radicals (101) show similar specificity for addition 1 5-abstraction to /-butoxy radicals, abstraction will be of lesser importance.42"42 The reason is that most /-amyloxy radicals do not react directly with monomer. They undergo [3-scission and initiation is mainly by ethyl radicals. Ethyl radicals are much more selective and give addition rather than abstraction. This behavior has led to /-amyl peroxides and peroxyesters being promoted as superior to the corresponding /-butyl derivatives as polymerization initiators.423... 

An interesting example shows the stability of a 1,2,4-trioxolane ring hydrogen to radical abstraction relative to that on a 1,3-dioxolane. Reaction of (117) with A-bromosuccinimide under radical conditions gave selective formation of the bromoethyl ester (118) (Equation (21)) <94TL1743>. 

Pogocki D, Schoneich C (2001) Thiyl radicals abstract hydrogen atoms from carbohydrates reactivity and selectivity. Free Rad Biol Med 31 98-107... 

The propagation step, Eq. (4), is much slower than Eq. (3) as an example, its rate constant kp is 0.18 M 1 sec-1 for cumene at 303K. Values of kp can vary considerably for different substrates, as shown by the oxidation rates of substituted toluenes (8). With respect to toluene, taken as 1.0, the reactivity of 4-nitrotoluene toward ROO is 0.33 and that of / -xylene is 1.6. A homolytic process like the fission of the C-H bond should be essentially apolar, but data for substituted toluenes correctly suggest that the hydrogen radical abstraction is favored by electron-donor substituents and that in the transition state the carbon atom involved has a partial positive charge. The difference in kp between different molecules or different groups of the same molecule is the reason of the selectivity of autoxidation. 

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 3° allylic H is abstracted selectively (faster than any other type in the molecule), forming an intermediate represented by two non-equivalent resonance forms. Partial radical character on two different carbons of the allylic radical leads to two different products. 

Alkylperoxy radicals are selective, and abstraction of a secondary H-atom is favored over abstraction of a primary H-atom. Alkoxy radicals, on the other hand, are less selective and can abstract primary hydrogens ... 

Another example of this type of selectivity, more muted but still easily measurable, is the different selectivities shown by methyl radicals and chlorine atoms for the methylene and methyl groups of propionic acid 7.14. Methyl radicals abstract the hydrogen atoms on C-2 5.2 times faster than the hydrogen atoms on the methyl group C-3. However, chlorine atoms abstract the hydrogen atoms on the methyl group 50 times faster than the hydrogen atoms on C-2.973... 

The utility of SmT-generated ketyl radicals in selective organic synthesis can be ascribed in part to their persistence. Thus ketyl radical anions have been postulated to form reversibly in the presence of Sml2 [9]. With the possible exception of aromatic carbonyls, the radical anions are not further reduced under the reaction conditions to the corresponding dianions. In addition, ketyls are not particularly prone to quenching by hydrogen atom abstraction from the solvent, from disproportionation, or related processes [52],... 

Unlike the 4-alkenyl/CO system, the following two cyclization systems, based on C-N double bonds, are completely selective and favor a five-membered ring (Scheme 8). Fallis and Brinza who used a diphenylhydrazone derivative as an acyl radical trap [32] reported the first example in this series. As shown in the first two examples in Table 3, radical carbonylation gave 2-hydrazinocyclopentanones in good yields. Ryu, Komatsu and coworkers reported acyl radical cyclization onto N C double bond, which proceeds exclusively in a 5-exo manner to give pyrrolidi-nones in good yield (runs 3-5) [33]. For an aromatic substrate, it is necessary to use a ketimine rather than an aldimine, since aromatic radical abstracts an imine hy-... 

Direct strand breaks or alkali-labile lesions are not observed from 76,80 or 83 under anaerobic conditions. (Reminder Radicals are referred to using the same descriptor whether they are nucleosides or in biopolymers.) Small amounts of direct strand breaks are detected at the 5 -adjacent nucleotide when the formal C6-hydrogen atom adduct of thymidine (80) is produced under aerobic conditions. " Strand scission is reduced approximately fourfold when the 5 -adjacent nucleotide is deuterated at the Cl -position. " Deuteration of the d -, or C4 -positions has no effect suggesting that per-oxyl radical 86 selectively abstracted the Cl -hydrogen atom from the 5 -adjacent nucleotide (Scheme 36). Additional support for intemucleotidyl Cl -hydrogen atom abstraction was obtained when the tandem lesion containing 2-deoxyribonolactone (15) was observed in low yield when 80... 

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