Other Important Radical Reactions

Radical mechanisms are important in understanding many other organic reactions. We shall see other examples in later chapters, but let us examine a few important radicals and radical reactions here oxygen and superoxide, the combustion of alkanes, DNA cleavage, autox-idation, antioxidants, and some reactions of chlorofluoromethanes that have threatened the protective layer of ozone in the stratosphere. 

Nitric oxide, synthesized in the body from the amino acid arginine, serves as a chemical messenger in a variety of biological processes, including blood pressure regulation and the immune response. Its role in relaxation of smooth muscle in vascular tissues is shown in Fig. 10.7. 

Hydrolysis of oGMP allows smooth muscle contraction (decreasing blood flow) 

Levitra , and Viagra , prolonging smooth muscle relaxation by maintaining oGMP concentrations 

One product of the second chain-propagating step is R—OOH, called an alkyl hydroperoxide. The oxygen-oxygen bond of an alkyl hydroperoxide is quite weak, and it can break and produce radicals that can initiate other chains  

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Other Important Radical Reactions 487 THE CHEMISTRY OF... Antioxidants 489 THE CHEMISTRY OF... Ozone Depletion and Chlorofluorocarbons (CFCs) 490... 

Other Important Radical Chain Reactions 10.11 A Molecular Oxygen and Superoxide... 

Another important rearrangement is that of cyclopropylmethyl radicals to the corresponding homoallyl radicals. This is an exceptionally fast reaction (t1/2 10 8) and has been used as a radical clock to determine the rates of other free-radical reactions.95 Cyclopropylcarbene also undergoes rearrangement, leading to cyclobutene.96... 

The simplest example of radical recycling is given by the atmospheric oxidation of carbon monoxide (there are no other important CO reactions in the air) ks 34 = 2.2 10 ... 

Tocotrienols differ from tocopherols by the presence of three isolated double bonds in the branched alkyl side chain. Oxidation of tocopherol leads to ring opening and the formation of tocoquinones that show an intense red color. This species is a significant contributor to color quaUty problems in oils that have been abused. Tocopherols function as natural antioxidants (qv). An important factor in their activity is their slow reaction rate with oxygen relative to combination with other free radicals (11). 

Two other important commercial uses of initiators are in polymer cross-linking and polymer degradation. In a cross-linking reaction, atom abstraction, usually a hydrogen abstraction, occurs, followed by termination by coupling of two polymer radicals to form a covalent cross-link ... 

Homolytic cleavage of dlazonlum salts to produce aryl radicals is induced by titan1um(III) salt, which is also effective in reducing the a-carbonylalkyl radical adduct to olefins, telotnerization of methyl vinyl ketone, and dimerization of the adduct radicals. The reaction can be used with other electron-deficient olefins, but telomerization or dimerization are important side reactions. 

The initial discussion in this chapter will focus on addition reactions. The discussion is restricted to reactions that involve polar or ionic mechanisms. There are other important classes of addition reactions which are discussed elsewhere these include concerted addition reactions proceeding through nonpolar transition states (Chapter 11), radical additions (Chapter 12), photochemical additions (Chapter 13), and nucleophilic addition to electrophilic alkenes (Part B, Chi iter 1, Section 1.10). 

Part D of Table 12.2 gives rates of some other important kinds of radical reactions, including reaction with O2 (entries 36 and 37), decarbonylation (entries 38—41), and group transfer for the phenylselenenyl group (entries 44 and 45). 

The majority of sequential radical reactions deal with cyclizations as the key steps. The constructions of carbocycles, oxygen, and nitrogen heterocycles using (TMSlsSiH as a mediator are many and represents the expansion and importance of these synthetic approaches. For example, Nicolaou and coworkers found that (TMSlsSiH serves as a superior reagent in the radical-based approach toward the synthesis of azadirachtin, an antifeedant agent currently used as an insecticide, and in other related systems. ° ° Here below we collected a number of reactions mostly from the recent work in the area of intramolecular reactions. 

Carotenoid radicals — Many of the important oxidations are free-radical reactions, so a consideration of the generation and properties of carotenoid radicals and of carbon-centered radicals derived from carotenoids by addition of other species is relevant. The carotenoid radicals are very short-lived species. Some information has been obtained about them by the application of radiation techniques, particularly pulse radiolysis. Carotenoid radicals can be generated in different ways. "... 

This reaction has been the subject of a great deal of theoretical and mechanistic study, largely because of the commercial importance of the polymers to which it can give rise. Like the other radical reactions we have discussed, it can be said to involve three stages—(a) initiation, (i>) propagation, and (c) termination ... 

Since operation in an autothermal mode implies a feedback of energy to preheat the feed, provision must be made for ignition of the reactor in order to attain steady-state operation. The ordinary gas burner and many other rapid combustion reactions are examples of autothermal reactions in which the reactants are preheated to the reaction temperature by thermal conduction and radiation. (Back diffusion of free radicals also plays an important role in many combustion processes.)... 

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