Delocalized radicals

Other limitations of the reaction are related to the regioselectivity of the aryl radical addition to double bond, which is mainly determined by steric and radical delocalization effects. Thus, methyl vinyl ketone gives the best results, and lower yields are observed when bulky substituents are present in the e-position of the alkene. However, the method represents complete positional selectivity because only the g-adduct radicals give reductive arylation products whereas the a-adduct radicals add to diazonium salts, because of the different nucleophilic character of the alkyl radical adduct. ... 

Onto which atoms (carbon, nitrogen or both) is the unpaired electron in tricyanomethyl radical delocalized Rationalize your result by drawing resonance contributors. 

Figure 8.9 TbPc2-based single ion magnets (SIMs) [90] (a) Schematic representation molecular localization of the three spin-systems characteristics of the [TbPc2]° complex J = 6, the uniaxial magnetic moment of the 4f configuration / = 3/2 nuclear spin of the only stable and naturally occurring 159Tb isotope S = 1/2 organic radical delocalized over the two Pc ligands. The radical...

In the phosphoryl fullerenyl radical the unpaired electron is - similar to the alkylfullerenyl radical - delocalized over two six-membered rings adjacent to the C-P bond [10]. The rotation barrier for the radical C5oP(0)(0 Pr)2 was determined to be 4.8 kcal mol Another phosphorus-containing radical fullerenyl adduct, determined via ESR spectroscopy, is PFjjO BujCgQ. It can be obtained by photolytic reaction of Cgo with HPF4 and tert-butylperoxide in tert-butylbenzene [43]. 

Synthetic melanic compounds can also be prepared by peroxidase oxidation of dihydroxyindoles. Similarly to phenol oxidation, the reaction proceeds through radical coupling, and the product distribution agrees with the radical delocalization in the semiquinone [47] (Fig. 6.3d). 

Fig. 12. Hypothetical high-valent hydroxylating species for different monooxygenases. For the pterin monooxygenases, the N represents histidine residues coordinated to the catalyrtically active Fe in tyrosine, phenylalanine, and tryptophan hydroxylases (.140, 141). The two squares for cyttochrome F450 represent the porphyrin ring, and the + represents a rr-radical delocalized over this ring.

Radical Delocalization. If the two radical centers on D and A are extensively delocalized, coupling is inhibited and a SET pathway is encouraged. This actually represents a special case of D-A bond strength, because the coupling of delocalized radicals leads to weak bonds. Numerous examples of all of these predictions exist (2) these examples make the foregoing analysis a most useful one. We see therefore that only by viewing the polar process as a single-electron shift does the relationship between the polar and SET pathways become clear. 

Delocalization of the unpaired electron accounts for the greater stability of the allyl radical when compared to primary, secondary, and tertiary radicals. Although some delocalization occurs in primary, secondary, and tertiary radicals, delocalization is not as effective because it occurs only through hyperconjugation (Section 6.1 IB) with tr bonds. 

Despite the great variety of chemical transformations catalyzed by cytochromes P450, the vast majority of them are imdoubtedly driven by Compoimd I. This ferryl-oxo intermediate with a 7C-cation radical delocalized on the porphyrin is a very reactive species. All attempts to observe this species in a P450 system using atmospheric dioxygen have so far failed. However, important spectroscopic characterization and reactivity measirrements have been obtained by using the... 

The choice of solvents for PVA-based FRRPP synthesis has been narrowed by looking into the various chain transfer constants (Brandrup and Immergut, 1999). High rates of chain transfer are detrimental to polymerization control as they can lead to radical delocalization and relatively quick termination. 

CONTEXT The allyl radical delocalizes three electrons over three carbon atoms, resulting in a relatively stable intermediate in many chemical reactions. For example, it takes about 44 kj mol less energy to break the C — H bond in propene (which leaves an allyl radical) than in propane. This allows techniques in organic synthesis that take advantage of radical intermediates, but the allyl radical also appears in biochemical systems. Lipoxygenases and cyclooxygenases form peroxides from fatty acids, with the remarkable control over rate and stereochemistry that makes enzymes the most highly skilled chemists on Earth. The mechanism for these reactions is not firmly established, but an allyl radical has been proposed as one of the key intermediates. 

In our own work we were interested in the possibility that radicals delocalized in ir-systems might interact with adjacent peroxy groups, for excunple in the systems 1 and 2 (1). Induced decomposition paths... 

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