Carbon-centered radicals consequences

The last comprehensive review of reactions between carbon-centered radicals appeared in 1973.142 Rate constants for radical-radical reactions in the liquid phase have been tabulated by Griller.14 The area has also been reviewed by Alfassi114 and Moad and Solomon.145 Radical-radical reactions arc, in general, very exothermic and activation barriers are extremely small even for highly resonance-stabilized radicals. As a consequence, reaction rate constants often approach the diffusion-controlled limit (typically -109 M 1 s"1). 

The reaction enthalpy and thus the RSE will be negative for all radicals, which are more stable than the methyl radical. Equation 1 describes nothing else but the difference in the bond dissociation energies (BDE) of CH3 - H and R - H, but avoids most of the technical complications involved in the determination of absolute BDEs. It can thus be expected that even moderately accurate theoretical methods give reasonable RSE values, while this is not so for the prediction of absolute BDEs. In principle, the isodesmic reaction described in Eq. 1 lends itself to all types of carbon-centered radicals. However, the error compensation responsible for the success of isodesmic equations becomes less effective with increasingly different electronic characteristics of the C - H bond in methane and the R - H bond. As a consequence the stability of a-radicals located at sp2 hybridized carbon atoms may best be described relative to the vinyl radical 3 and ethylene 4 ... 

Nitrogen-centered radicals (R2N ) or oxygen-centered radicals (RO) are less stable than carbon-centered radicals R3C. They are higher in energy because of the higher electronegativity of these elements relative to carbon. Such radicals are consequently less common than analogous carbon radicals, but are by no means unheard of. 

Thus, for being thermodynamically stable, carbon-centered radicals should correspond to a very small BDE(C—C) [AG° = 8.0 kcal mol-1 if BDE C—C) = 4.0 kcal mol-1] or to a very large value of 2SE°(R ) -SE°(R—R). It can be anticipated that very few species fulfill this condition. Consequently, practically no free radical should be thermodynamically stable. In other words, each time the recombination occurs it should quantitatively give the dimer. 

One possible way of overcoming this problem is to introduce into the reaction mixture a compound that decomposes at a constant rate to free radicals (X ) capable of extracting a hydrogen atom from the PUFAs (RH) and consequently initiating the autoxidation process. The compounds most frequently used for this are the so-called azo-initiators (X-N=N=X), which thermally decompose to highly reactive carbon-centered radicals. ... 

The most notable addition reaction is that of ground state oxygen with carbon centered radicals. This leads to peroxy radicals that can abstract hydrogens from other molecules regenerating carbon-centered radicals. Additionally, the peroxide so formed can then undergo homolysis to yield alkoxy and hydroxy radicals. Consequently, this homolytic initiation leads to additional radicals that propagate and accentuate the autocatalytic nature of the reaction. 

The previous chapter covered radical cation cyclization reactions that were a consequence of single-electron oxidation. In the following section, radical anion cyclization reactions arising from single-electron reduction will be discussed. In contrast to the well documented cyclization reactions via carbon-centered free radicals [3, 4], the use of radical anions has received limited attention. There are only a few examples in the literature of intramolecular reductive cyclization reactions via radical anions other than ketyl. Photochemi-cally, electrochemically or chemically generated ketyl radical anions tethered to a multiple bond at a suitable distance, have been recognized as a promising entry for the formation of carbon-carbon bonds. 

The initiation reaction is the hemolytic abstraction of hydrogen to form a carbon-centered alkyl radical in the presence of an initiator. Under normal oxygen pressure, the alkyl radical reacts rapidly with oxygen to form the peroxy radical, which in turn reacts with more unsaturated lipids to form hydroperoxides. The lipid-free radical thus formed can further react with oxygen to form a peroxy radical. Hence, the autoxidation is a free radical chain reaction. Because the rate of reaction between the alkyl radical and oxygen is fast, most of the free radicals are in the form of the peroxy radical. Consequently, the major termination takes place via the interaction between two peroxy radicals. 

The reduction of an electron-deficient alkene renders a formally electropositive carbon center nucleophilic. The )6-carbon of the radical anion derived from an Q , -unsaturated ester, for example, displays nucleophilic rather than electrophilic character. Realization of this factor uncovered a very rich and fertile territory for the development of new reactions, the exploration of mechanism, and the application of the methodology to the synthesis of materials. The fields of electrohydrodimerization and electrohydrocylization are clear and direct consequences of this realization. In the sections to follow, we describe a variety of... 

It should be noted that the surface curvature of the carbon network exerts a profound impact on the reactivity of the fullerene core. The most striking consequence is the pyramidalization of the individual carbon atoms. Influenced by the curvature, the sp hybrids, which exist in truly 2-dimensional planar carbon networks, adopt a sp hybridization with p-orbitals that posses a s-character of 0.085 (19). Accordingly, the exterior surface is much more reactive than planar analogues, and becomes comparable to those of electron deficient polyolefines. This, in turn, rationalizes the high reactivity of the fullerene core towards many photolytically generated carbon- and heteroatomic-centered radicals (20). 

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