Other Carbon-Centered Radicals

Absolute rate constants for the attack of aryl radicals on a variety of substrates have been reported by Scaiano and Stewart (Ph ) 7 and Citterio at al. (/j-CIPh-).379,384 The reactions are extremely facile in comparison with additions of other carbon-centered radicals [e.g. jfc(S) = 1.1x10s M"1 s"1 at 25 °C].3,7 Relative reactivities are available for a wider range of monomers and other substrates (Tabic 3.b). Phenyl radicals do not show clear cut electrophilic or... 

Most of the living radical polymerizations using organic radicals as regulating persistent species involved nitroxides. Exceptions are triphenylmethyl and other carbon-centered radicals in the early work of Otsu and Braun.24,25 More recently, Chung showed that borinate radicals 10 formed by the thermal cleavage of in situ generated alkyl boryl peroxides (Scheme 31) can be employed to control methacrylate... 

Some examples of free-radicals generated by the bioactivation of drug molecules are shown in Fig. 10.31. Isoniazid is acetylated to its major metabolite acetylisoniazid, which is hydrolyzed to acetylhydrazine and isonicotinic acid (Fig. 10.31-1). Acetylhydrazine is further metabolized by the CYP2E1 to an N-hydroxy intermediate that hydrates into an acetyl radical, which can then initiate the process that leads to hepatic necrosis. Other carbon-centered radicals are formed from hydrazines such as the antihypertensive hydralazine, and thio-radicals from the ACE inhibitor captopril (Fig. 10.31-2 and -3). 

Morita Y, Nishida S (2010) Phenalenyls, cyclopentadienyls, and other carbon-centered radicals. In Hicks RG (ed) Stable radicals fundamentals and applied aspects of odd-electron compounds. Wiley, Chichester... 

There are large differences in the reactivity of the free radicals generated from type I photoinitiators towards unsaturated monomers. For example, the rate constants for the addition of diphenylphosphinoyl radicals to vinyl monomers are of the order of 10 to 10 mol 1 s -that is, several orders of magnitude larger than those for benzoyl or other carbon-centered radicals [18b]. 

The hydrogen abstraction addition ratio is generally greater in reactions of heteroatom-centered radicals than it is with carbon-centered radicals. One factor is the relative strengths of the bonds being formed and broken in the two reactions (Table 1.6). The difference in exothermicity (A) between abstraction and addition reactions is much greater for heteroatom-centered radicals than it is for carbon-centered radicals. For example, for an alkoxy as opposed to an alkyl radical, abstraction is favored over addition by ca 30 kJ mol"1. The extent to which this is reflected in the rates of addition and abstraction will, however, depend on the particular substrate and the other influences discussed above. 

The combination of carbon-centered radicals usually involves head-to-head (a,a ) coupling. Exceptions to this general rule occur where the free spin can be delocalized into a n-system. The classic example involves the triphenylmethyl radical (13) which combines to give exclusively the a-para coupling product (26), Scheme I.8).27 This chemistry is also seen in cross reactions of 13 with other tertiary radicals.146... 

Aliphatic acyloxy radicals undergo facile fragmentation with loss of carbon dioxide (Scheme 3,69) and, with few exceptions,428 do not have sufficient lifetime to enable direct reaction with monomers or other substrates. The rate constants for decarboxylation of aliphatic acyloxy radicals are in the range l 10xl09 M 1 s at 20 °C.429 lister end groups in polymers produced with aliphatic diacyl peroxides as initiators most likely arise by transfer to initiator (see 3.3.2.1,4). The chemistry of the carbon-centered radicals formed by (3-scission of acyloxy radicals is discussed above (see 3.4.1). 

The reaction between nitroxides and carbon-centered radicals occurs at near (but not at) diffusion controlled rates. Rate constants and Arrhenius parameters for coupling of nitroxides and various carbon-centered radicals have been determined.508 311 The rate constants (20 °C) for the reaction of TEMPO with primary, secondary and tertiary alkyl and benzyl radicals are 1.2, 1.0, 0.8 and 0.5x109 M 1 s 1 respectively. The corresponding rate constants for reaction of 115 are slightly higher. If due allowance is made for the afore-mentioned sensitivity to radical structure510 and some dependence on reaction conditions,511 the reaction can be applied as a clock reaction to estimate rate constants for reactions between carbon-centered radicals and monomers504 506"07312 or other substrates.20... 

The role of oxygen in radical and other polymerizations has been reviewed by Rhanu and Kishore.187 Rate constants for the reaction of carbon-centered radicals with oxygen are extremely fast, generally - 1 d" M 1 s 1.181,188 The initially formed... 

The absolute rate constants for attack of carbon-centered radicals on p-benzoquinone (38) and other quinones have been determined to be in the range I0M08 M 1 s 1.1 -04 This rate shows a strong dependence on the electrophilicity of the attacking radical and there is some correlation between the efficiency of various quinones as inhibitors of polymerization and the redox potential of the quinone. The complexity of the mechanism means that the stoichiometry of inhibition by these compounds is often not straightforward. Measurements of moles of inhibitor consumed for each chain terminated for common inhibitors of this class give values in the range 0.05-2.0.176... 

The hindered carbon-centered radicals are most suited as mediators in the polymerization of 1,1-disubstituled monomers e.g. MMA,78,95 other methacrylates and MAA,06 and AMS97). Polymerizations of monosubstituted monomers are not thought to be living. Dead end polymerization is observed with S at polymerization temperatures <100°C.98 Monosubstituted monomers may be used in the second stage of AB block copolymer synthesis (formation of the B block).95 However the non-living nature of the polymerization limits the length of the B block that can be formed. Low dispersities are generally not achieved. 

Reactions such as these are of interest in themselves. Beyond that, one can use the pulse radiolysis experiment as a preparative technique for other species. Thus, the reactions of numerous aliphatic, carbon-centered radicals have been evaluated.22 If one employs a reasonably high concentration of solute, say 0.1-1 M CH3OH, the formation of CH2OH is complete within the electron pulse. Following that, reactions such as the following can be studied ... 

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. "... 

The spin density of tocopheroxyl radical 2, a classical phenoxyl radical, is mainly concentrated at oxygen 0-6, which is the major position for coupling with other C-centered radicals, leading to chromanyl ethers 5. These products are found in the typical lipid peroxidation scenarios. Also at ortho- and para-positions of the aromatic ring, the spin density is increased. At these carbon atoms, coupling with other radicals, especially O-centered ones, proceeds. Mainly the para-position (C-8a) is involved (Fig. 6.3), leading to differently 8a-substituted chromanones 6. 

The nitroxide radical (from processes 5 and 6 and attack by other radicals on the parent piperidine) is found in photo-oxidizing PPH samples in concentrations of M. x 10 M (initial piperidine level 5 x 10-3M) up to the embrittlement point of the PPH film (7.). Nitroxides are well known to scavenge carbon centered radicals (but not peroxyl radicals) in both polymers and liquid alkanes (reaction 7) (10, 8). In the liquid phase k7 is... 

Abstract Diazonium salts have been previously used to cleave DNA via generation of carbon centered radicals and cations. Efforts have been made in the past decade or so to develop diazo compounds and a-diazoketones for physiologically relevant DNA cleavage. These efforts, coupled with their relevance to the mechanism of action of kinamycin and lomaiviticin antibiotics and other naturally occurring diazo compounds, will be discussed. 

In recent years, interest in radical-based transformations of allenes has been renewed for two major reasons. First, a number of useful intramolecular additions of carbon-centered radicals to 1,2-dienes have been reported, which allowed syntheses of complex natural product-derived target molecules to be accomplished in instances where other methods have failed to provide similar selectivities. Further, a large body of kinetic and thermochemical data has become accessible from results of experimental and theoretical investigations in order to predict selectivities in addition reactions to allenes more precisely. Such contributions originated predominantly from (i) studies directed towards an understanding of the incineration process,... 

Table VIII contains rate constants for reactions of tin hydrides with carbon-centered radicals. A striking feature of Table VIII in comparison to other tables in this work is the high percentage of reactions for which Arrhenius parameters were determined by direct LFP or the LFP-clock method. These results are expected to be among the most accurate listed in this work. Scores of radical clocks have been studied with Bu3SnH, but the objectives of those studies were to determine rate constants for the clocks using tin hydride trapping as the calibrated basis reaction.

Cyclobutanes which are suitably substituted to form carbon-centered radicals in the a-, / - or y-position of the side chain undergo ring enlargement to cyclohexanones on treatment with tributyltin hydride and a radical initiator in refluxing benzene.115-119 A cascade of different ring closures and bond breakages follows the initial radical formation. To assure the necessary low concentration it is important to add the tributyltin hydride slowly over several hours to the refluxing mixture of the other components. 

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