Carbon-centered radicals diradicals

A one-electron reduction of the bond between an aliphatic carbon and a halogen leads to a halogen anion and a carbon-free radical. A good example is the reduction of carbon tetrachloride as discussed earlier in this chapter. The first product in the reduction is the trichloromethyl-free radical. Carbon-centered radicals are not very reactive with biological molecules, but they react very rapidly with molecular oxygen (a diradical) to form a peroxy-free radical (Fig. 5.15), which is quite toxic (10). 

This chapter begins with an introduction to the basic principles that are required to apply radical reactions in synthesis, with references to more detailed treatments. After a discussion of the effect of substituents on the rates of radical addition reactions, a new method to notate radical reactions in retrosynthetic analysis will be introduced. A summary of synthetically useful radical addition reactions will then follow. Emphasis will be placed on how the selection of an available method, either chain or non-chain, may affect the outcome of an addition reaction. The addition reactions of carbon radicals to multiple bonds and aromatic rings will be the major focus of the presentation, with a shorter section on the addition reactions of heteroatom-centered radicals. Intramolecular addition reactions, that is radical cyclizations, will be covered in the following chapter with a similar organizational pattern. This second chapter will also cover the use of sequential radical reactions. Reactions of diradicals (and related reactive intermediates) will not be discussed in either chapter. Photochemical [2 + 2] cycloadditions are covered in Volume 5, Chapter 3.1 and diyl cycloadditions are covered in Volume 5, Chapter 3.1. Related functional group transformations of radicals (that do not involve ir-bond additions) are treated in Volume 8, Chapter 4.2. 

With anionic precursors, the neutralization step entails electron removal, i.e. an oxidation reaction, as does reionization of a neutral to cations. Therefore, O2 which is the best reionization target (see above) also is the target of choice for the neutralization of anions. Conversely, cation neutralization targets, which effect a gas-phase reduction, are most suitable for reionization to anions. Widely used targets for this purpose have been xenon and trimethyl-amine. Reionization to anions suffers from substantially poorer yields than reionization to cations, often by ca. 10 times or more therefore, it has been employed to a much lesser extent. However, in certain cases it can provide superior structural information for example, the unequivocal identification of oxygen-centered radicals, such as the carbonate radical CO3 and the diradical CH2CH20 relied on their reionization to anions. 

As noted in the introduction, nonconjugated diradicals are those in which the partially filled orbitals reside on two different carbons that are connected by one or more saturated carbons or by unsaturated carbons in which the it bonds do not overlap the two radical centers. Two types are discussed here 1,3-diradicals, in which the radical centers are separated by a single carbon, and 1.4-diradicals, in which two carbons separate the radical centers. The archetypal 1,3-diradical is trimethylene (TM), the diradical formed by cleavage of a C-C bond in cyclopropane and tetramethylene, the diradical formed by breaUng a C-C bond in cyclobutane, is the archetypal 1,4-diradical. 

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