Radicals, and rearrangements

W. J. Bailey s(52,53) work with ketene acetals deserves mention as potentially a route to biodegradable addition polymers. Its novelty resides in the instability of the vinyl radical and rearrangement to introduce a polyester linkage into a radically produced polymer. As we shall see in the next section, polyesters are biodegradable hence, their Introduction into a polymer with a C-C backbone produces weak links which fracture the polymer into oligomers which we have seen are biodegradable. This chemistry is exemplified schematically, below. 

Tetramethylpiperidin-l-oxyl (8) has also been used as a very fast trap for cyclopropyl-alkyl radicals, and rearranged species generated from peroxides.Peroxide decomposition is induced by 8 which then combines with the transient radicals. This work included a study of the decomposition of bis[(bicyclo[2.1.0]pentan-2-yl)carbonyl]peroxide (7) in chlorobenzene and 2,2,4-trimethylpentane good yields of trialkylhydroxylamines 9 and 10 were reported. The rate of the trapping reaction was solvent dependent and the rate constant of the extremely fast clock rearrangement of the bieyclo[2.1.0]pentan-2-yl radical was determined from the yields of the two products. 

Organosilicon peroxides (including cyclic peroxides) radicals and rearrangements 07T10385. 

Radicals react with alkenes to form a new radical that can react further to give addition reaction products. The initial reaction will generate the more stable radical, and rearrangement is not observed. In the presence of peroxides, alkenes react with HBr to give the alkyl bromide having Br on the less substituted carbon. This is called anti-Markovnikov addition. 

Because many organic peroxides undergo thermolysis to form useful free radicals, they are used commercially as initiators for free-radical reactions. Many organic peroxides also undergo reactions in which free radicals are not involved, eg, heterolyses, hydrolyses, reductions, and rearrangements. Numerous reviews of the chemistry and appHcations of organic peroxides have been pubHshed (11,13—41). 

The reactions of copper salts with diacyl peroxides have been investigated quite thoroughly, and the mechanistic studies indicate that both radicals and carbocations are involved as intermediates. The radicals are oxidized to carbocations by Cu(II), and the final products can be recognized as having arisen from carbocations because characteristic patterns of substitution, elimination, and rearrangement can be discerned " ... 

The resistance of the unsubstituted system to the di-7i-methane rearrangement probably occurs at the second step of the rearrangement. If the central carbon is unsubstituted, this step results in the formation of a primary radical and would be energetically unfavorable. 

The resulting radical and/or cationic species (11) and (12) can then lead to rearrangements which have found interesting applications in the steroid field. 

Only the positively charged species are accelerated out of the ionization region neutral radicals—e.g., CULE in Equation 2, and molecules— e.g.y ME in Equation 3, produced by fragmentation and rearrangement, and un-ionized sample are pumped away. 

In addition to fragmentation by the McLafferty rearrangement, aldehydes and ketones also undergo cleavage of the bond between the carbonyl group and the a carbon, a so-called a cleavage. Alpha cleavage yields a neutral radical and a resonance-stabilized acyl cation. 

Many radicals undergo fragmentation or rearrangement in competition with reaction with monomer. For example, f-butoxy radicals undergo p-scission to form methyl radicals and acetone (Scheme 3.6). 

Other radicals undergo rearrangement in competition with bimolecular processes. An example is the 5-hexenyl radical (5). The 6-heptenoyloxy radical (4) undergoes sequential fragmentation and cyclization (Scheme 3.8).1S... 

Ideally all reactions should result from unimolecular homolysis of the relatively weak 0-0 bond. However, unimolecular rearrangement and various forms of induced and non-radical decomposition complicate the kinetics of radical generation and reduce the initiator efficiency.46 Peroxide decomposition induced by radicals and redox chemistry is covered in Sections 3.3.2.1.4 and 3.3.2.1.5 respectively. 

Recombination of the ion radicals within the cage is thought of as forming the path to rearrangement whilst escape of the radicals and subsequent reaction with the hydrazo compound leads to the formation of disproportionation products often observed. The theory is mainly directed at the two-proton mechanism and does not accommodate well the one-proton mechanism, since this requires the formation of a cation and a neutral radical, viz. 

In Volume 13 reactions of aromatic compounds, excluding homolytic processes due to attack of atoms and radicals (treated in a later volume), are covered. The first chapter on electrophilic substitution (nitration, sulphonation, halogenation, hydrogen exchange, etc.) constitutes the bulk of the text, and in the other two chapters nucleophilic substitution and rearrangement reactions are considered. 

Experimental conversion-time data, obtained from the literature, on the bulk free radical polymerization of MMA initiated by AIBN at several temperatures and initiator concentrations, were described by the model. However, the expressions for the rate of conversion and gel effect index were first simplified and rearranged. ... 

Kolbe electrolysis is a powerful method of generating radicals for synthetic applications. These radicals can combine to symmetrical dimers (chap 4), to unsymmetrical coupling products (chap 5), or can be added to double bonds (chap 6) (Eq. 1, path a). The reaction is performed in the laboratory and in the technical scale. Depending on the reaction conditions (electrode material, pH of the electrolyte, current density, additives) and structural parameters of the carboxylates, the intermediate radical can be further oxidized to a carbocation (Eq. 1, path b). The cation can rearrange, undergo fragmentation and subsequently solvolyse or eliminate to products. This path is frequently called non-Kolbe electrolysis. In this way radical and carbenium-ion derived products can be obtained from a wide variety of carboxylic acids. 

In this chapter, we discuss free-radical substitution reactions. Free-radical additions to unsaturated compounds and rearrangements are discussed in Chapters 15 and 18, respectively. In addition, many of the oxidation-reduction reactions considered in Chapter 19 involve free-radical mechanisms. Several important types of free-radical reactions do not usually lead to reasonable yields of pure products and are not generally treated in this book. Among these are polymerizations and high-temperature pyrolyses. 

---