Carbon-centered radicals from

Hargreaves has suggested that the insolubilization of some closely related polymers is due to photolytic homolysis of the endoperoxide 0-0 bond and subsequent generation of carbon-centered radicals from the O radicals (19). There are several facts that make this an extremely unlikely explanation for the data described here these include the quantitative insufficiency of the maximum amount of endoperoxide reaction obtainable with a few hundred mJ/cm2 dose (homolysis quantum yield <0.5 (46), and extinction coefficient 1 (M cm)-1 (47)), and the synthetic utility of such homolysis reactions in related molecules in the presence of good hydrogen atom donors (implying facile epoxide formation) (48). Clearly the crosslinking observed under N2 is not accounted for by this mechanism. 

Although the vast majority of publications involving Barton esters have involved the generation of carbon-centered radicals from carboxylic acids, the 0-acylthiohy-droxamate decarboxylative protocol has also been extended to other substrates. 

Free radical cycUzations. Cobaloxime in combination with a sacrificial Zn foil anode has been used to effect the reductive generation of carbon-centered radicals from bromoacetals in electrochemical proces.ses. When an unsaturated side chain is present in such acetals, cyclization may occur. 

Organic carbon-centered radicals from alcohols seem to react mostly with thiol functions in amino-acids (cysteine) or small peptides such as glutathione (121). The reaction is the so-called "repciir process"... 

Newcomb has measured the rate of H transfer to carbon-centered radicals from the water and methanol complexes of Cp2TiCl [34]. The rate constant (IkH) for HAT to the radical below (determined by the radical clock method. Scheme 1.3) in THF at room temperature is 1.0 x 10 M s . 

TABLE 9.1 Rate Constants for H-Atom Abstraction of Different Types of Carbon-Centered Radicals from (TMS) SiH and Bu SnH [38]... 

Carbon-centered radicals generally react very rapidly with oxygen to generate peroxy radicals (eq. 2). The peroxy radicals can abstract hydrogen from a hydrocarbon molecule to yield a hydroperoxide and a new radical (eq. 3). This new radical can participate in reaction 2 and continue the chain. Reactions 2 and 3 are the propagation steps. Except under oxygen starved conditions, reaction 3 is rate limiting. 

Cycloalkoxy radical intermediates are readily generated from a parent alcohol by various methods (e.g., nitrite ester photolysis, hypohalite thermolysis, lead tetraacetate oxidation) (83MI1). Once formed, reactive cycloalkoxy radicals undergo /3-scission to produce a carbonyl compound and a new carbon-centered radical. 

The success of intramolecular conjugate additions of carbon-centered radicals in multifunctional contexts is noteworthy. Compound 57 (see Scheme 10), prepared by an interesting sequence starting from meto-toluic acid (54) (see 54 > 55 > 56 > 57), can be converted to the highly functionalized perhydroindane 58 through an intramolecular conjugate addition of a hindered secondary radical.21-22 This radical cyclization actually furnishes a 6 1 mixture of perhydroindane diastereoisomers, epimeric at C-7, in favor of 58 (96 % total yield). It should be noted that a substantially less strained cis-fused bicyclo[4.3.0] substructure is formed in this cyclization. 

Carbon-centered radicals have been shown to undergo addition reactions with azirine-3-carboxylates. Methyl 2-(2,6-dichlorophenyl)azirine-2-carboxylate thus reacts with alkyl and aryl iodides in the presence of triethylborane to give aziridines in good yields. The radical approaches from the opposite face to the aryl substituent, giving the cis products as single diastereoisomers (Scheme 4.43) [63],... 

While nitroxides give overwhelmingly combination in their reaction with carbon-centered radicals, the amount of disproportionation is finite (Scheme 9.24). Disproportionation cannot always be rigorously distinguished from elimination and it is possible that both reactions occur. The combinatiomdisproportionation ratio (or extent of elimination) depends on the nitroxide and radical structure and within a scries of structurally related systems appears to increase as... 

The carbon-centered radical R, resulting from the initial atom (or group) removal by a silyl radical or by addition of a silyl radical to an unsaturated bond, can be designed to undergo a number of consecutive reactions prior to H-atom transfer. The key step in these consecutive reactions generally involves the intra-or inter-molecular addition of R to a multiple-bonded carbon acceptor. As an example, the propagation steps for the reductive alkylation of alkenes by (TMSfsSiH are shown in Scheme 6. 

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 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... 

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