Radical-Site Rearrangements

Part of the driving force for the first step (rH) is provided by the formation of the extremely strong O—H bond which makes the distonic intermediate more stable than the original ketone ion. Part of the driving force for the second step is the resonance stabilization of the radical site in the product ion, which is isoelectronic with the allyl radical (see Heinrich 1986). Note that this requires -bond cleavage in the second reaction step, and thus necessitates y-H transfer to produce the reactive intermediate. 

Unknown 4.15. What is the mass of the characteristic OE ion (or ions) which will be formed by this six-membered-ring H rearrangement from each of the following  

Draw out the mechanism, complete with fishhooks, for each. 

Unknowns 4.16 and 4.17. These are the spectra of 3- and 4-methyl-2-pentanone which is which  

Unknown 4.18. Does an important odd-electron ion aid in the solution of this unknown  

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Aryl migrations are promoted by steric crowding in the initial radical site. This trend is illustrated by data from the thermal decomposition of a series of diacyl peroxides. The amount of product derived from rearrangement increases with the size and number of substituents ... 

Because the polymerization with the thermal iniferters previously described was performed at a high temperature, some side reactions might be unavoidable, e.g., ordinary bimolecular termination between polymer radicals, disproportionation between a polymer radical and a small radical leading to deactivation of the iniferter site, initiation by the radical generated from the iniferter sites, rearrangements of the structure of the iniferter sites, and spontaneous initiation of polymerization. 

As hydrogen rearrangements prior to dissociation are prevalent in alkenes, the radical site migrates along the chain, thereby obscuring the location of the double bond. [60]... 

The chemistry of radical sites adjacent to phosphatoxy centers elicited interest because of the involvement of such species in DNA degradation processes. These species can give rise to rearrangement, elimination, and substitution products, and for some time concerted eliminations and migrations as well as heterolysis to a radical cation and a phosphate anion were considered to be involved (Scheme 2). Recently, experimental studies of the l,2-dibenzyl-2-(diphenylphosphatoxy)-2-phenylethyl radical and complementary theoretical studies of l,l-dimethyl-2-(dimethylphosphatoxy)ethyl radical have been interpreted as indicating that a radical cation/anion pathway with initial formation of 49 is favored. ... 

Rearrangement of Cl, Br, or I to an adjacent radical site has been proposed to account for a number of results. Chlorine migration occurs in reaction sequences such as that shown in Scheme 14.189 Generation of a radical center adjacent to a carbon bearing a bromine substituent has been postulated to be accompanied by bridging of the bromine to form the radical analog of the bromonium ion (Equation 9.108).190-193... 

Recently, Cooks (1969) collated a comprehensive range of skeletal rearrangements in which great stress was laid on the presumed importance of charge and radical sites. 

The rearrangement-elimination reaction (120) is one example of the so-called McLalferty rearrangement (McLafferty, 1959) commonly encountered in mass spectrometry. The first step in McLafferty s scheme (120) is regarded as a bond-forming reaction of the radical site... 

The free-radical activity is conceived as being generated by an "internal redox" reaction, in which the central metal ion is reduced to a lower valency state (in this case cobalt(II)) by an electron transfer from the ligand, the latter then acquiring a free-radical site by electronic rearrangement. Polymerisation then proceeds by propagation from the free-radical site so generated. 

The term sigmatropic rearrangement is used here only in a formal sense, i.e., for a process described by the formulae above, without implication of reaction mechanism. Therefore, thermal, metal-catalyzed and even radical-induced rearrangements, which fulfill this requirement, are included in this section. However, with respect to nomenclature, sigmatropic rearrangements are described in the way introduced by Woodward and Hoffmann, i.e., by stating the number of atoms from the cleavage site to the centers where the new n-bond will be formed. 

The breakage of the polymer chain is the most probable at the allyl position to double bonds, forming primary macroradicals in unsaturated hydrocarbon chain polymers. The deplacement of the primary radical site is carried on through the rearrangement of the C=C double bonds, producing monomer or cyclic dimer and reproducing the primary macroradicals (4 and 5) according to Scheme 12.4. 

The presence of heteroatoms with non-bonding n electrons favors the localization of charges. When the radical and/or the charge are localized, this influences the fragmentation and the reaction mechanisms can be classified as sigma electron ionizations, localized charge initiations, radical site initiations, and rearrangements. 

That a biradical is a likely intermediate follows from the fact that the activation energy for the rearrangement is at least equal to, if not higher, than a thermochemical estimate of the enthalpy difference between the starting material and a biradical having no interaction between the radical sites. 

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