The First Radical

The first organic free radical to be discovered was triphenylmethyl, the result of the effort of Gomberg to prepare hexaphenylethane.6 In geographical exploration, isolated white spaces on the map rarely contain anything strikingly different from the neighboring explored regions chemical exploration leads to all sorts of surprises. 

Treatment of triphenylmethyl chloride with silver gave not the expected hydrocarbon but an oxygen-containing compound later found to be the peroxide. The reaction run in an inert atmosphere did give a hydrocarbon, but one with unusual properties. It reacted rapidly with oxygen, chlorine, and bromine, and was quite different from tetra-phenylmethane or what was expected of hexaphenylethane. Gomberg 

The reaction with oxygen has been used to measure the rate of dissociation of the ethane.7 

The dissociation is the slow step that fixes the rate of the over-all oxidation reaction, but only if an inhibitor is added to remove the intermediate triphenylmethylperoxy radicals. Otherwise they contribute to the disappearance of the ethane by attacking it directly rather than waiting for additional free triphenylmethyl radicals. 

The initial product of the inhibition step is not known in this case and may be a molecular complex.8 The direct reaction of the ethane with the peroxy radical is an example of a covalent compound giving a reaction resembling that of a related free radical. The molecular weight determination by Gomberg was therefore a necessary part of the proof that he was dealing with radicals and not merely an unusually reactive hydrocarbon. The presence of free radicals has since been confirmed by measurements of the paramagnetic susceptibility and the paramagnetic resonance absorption.9-10 The latter evidence also rules out an alter- 

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The first landmark syntheses of persistent radicals of the heavier group 14 elements were reported nearly 30 years ago by Lappert s group. ° The first radical of this series, [(Me3Si)2CH]3Sn , was prepared by photolysis of the stable stannylene [(Me3Si)2CH]2Sn with visible light in benzene (Scheme 2.26). ° =... 

Carbon-centered organic radicals are highly reactive trivalent species with only one nonbonding electron. While most known radicals have their unpaired electron in a pure p- or a delocalized Ji-orbital, there are also examples of radicals centered in s/t" hybrid o-orbitals, such as the well known phenyl and cyclopropyl radicals. The first radical reported in the literature is credited to Gomberg s landmark paper in 1900 when he postulated the formation of triphenylmethyl radical 36, also known as tri-fyj 99,100 jj-jjyj j-adical is an example of a persistent radical that exists in equilibrium... 

The first radicals to be studied were, hardly surprisingly, those that were somewhat less reactive, and thus capable of rather longer independent existence. The first such radical to be detected unequivocally was Ph3C- (4), obtained in 1900 on reacting Ph3CCl with finely divided silver (cf. p. 43). The radical reacted with halogens to reform the triphenylmethyl halide (5), or with oxygen from the air to form (6), a peroxide (all radicals react readily with 02 from the air) ... 

Clive and coworkers have developed a new domino radical cyclization, by making use of a silicon radical as an intermediate to prepare silicon-containing bicyclic or polycyclic compounds such as 3-271 and 3-272 (Scheme 3.69) [109], After formation of the first radical 3-267 from 3-266, a 5-exo-dig cyclization takes place followed by an intramolecular 1,5-transfer of hydrogen from silicon to carbon, providing a silicon-centered radical 3-269 via 3-268. Once formed, this has the option to undergo another cyclization to afford the radical 3-270, which can yield a stable product either by a reductive interception with the present organotin hydride species to obtain compounds of type 3-271. On the other hand, when the terminal alkyne carries a trimethylstannyl group, expulsion of a trimethylstannyl radical takes place to afford vinyl silanes such as 3-272. 

TTF CH=CH py)2] (PF6)- 2CH2C12 was the first radical cation salt of a paramagnetic transition metal complex containing TTF CH=CH py as ligand. 

Because of the apparent importance of tin hydrides in synthetic sequences, reactions of R3SnH with alkyl radicals were among the first radical... 

The first radical source we shall consider is thermal homolysis. At sufficiently high temperatures, all chemical bonds will break to form radicals, but in the temperature range of ordinary solution chemistry, below 200°C, the bonds that will do so at reasonable rates are limited to a few types, the most common of which are the peroxy bond and the azo linkage.42 Substances that produce radicals easily in a thermal process are designated initiators. Equations 9.10-9.14 illustrate a few typical examples with activation parameters. 

This simple species is of particular interest since it is the first radical studied by electron spin resonance in which the isotropic coupling to an a-proton is positive and large. The structural implications of this large coupling are briefly considered in the next section but first we give evidence supporting the identification. 

There is a great diversity of initiating and propagation steps for radical substitution reactions. Bond homolyses, fragmentations, atom abstraction reactions, and addition reactions to C=C double bonds are among the possibilities. All of these reactions can be observed with substituted alkylmercury(II) hydrides as starting materials. For this reason, we will examine these reactions as the first radical reactions in Section 1.6. 

According to the spectroscopic and other qualitative proposals, the first radical anion formed in the case of aryl halides is of % nature. This radical anion gives, by an intramolecular ET to the C—X bond, a a radical anion, which dissociates into the aryl radical and the anion of the leaving group. The C—X bond strength is decreased once the radical anion is formed, as has been shown by thermodynamic cycles40. 

Always bear this in mind bond strength is only a guide to selectivity in radical reactions. As we shall see shortly, it s not the only factor involved. Indeed, you ve already seen steric effects in action when the Br radical added to the less hindered end of the alkene in the first radical reaction of this chapter, and you will later see how front/erorfj/fa/ effects can ope rate too. 

The first radical is an allylic radical, stabilized by conjugation with the remaining alkene in the old butadiene molecule. Addition could now occur to another butadiene or to styrene. 

Laurent was the first to react a carbonyl compound, benzil, with an alkali metal. These attempts were more eventful than successful instead of the first radical anion he observed spontaneous ignition [34], Some fifty years later, Beckmann and Paul were more successful, when they reacted aromatic ketones (benzo-phenone) and diketones (benzil) with sodium [35]. They noted the sensitivity of the solutions and of the colored solid products to air and moisture and worked in a hydrogen atmosphere. Although they demonstrated admirable laboratory skills, (cf. Fig. 3), they did not understand the nature of the colored intermediates. 

Abstraction of hydrogen by Br can produce either of two allylic radicals. The first radical, resulting from abstraction of a secondary hydrogen, is more likely to be formed. 

FIGURE 9.2 Vector models (projections) illustrating pair substitution. The labels 1 and 2 denote the electron spin of the first and the second radical of the pairs. The observed proton is contained in the first radical. Its spin state, la) or IP), is displayed at the respective leftmost projection. The radical pairs are bom in the triplet state, and the product is formed from the singlet state c gives the singlet character. First radical pair RPi, positive g-value difference, zero hyperfine coupling constant second radical pair RP2, equal g values, positive hyperfine coupling constant. For the situations without pair substitution, the spin evolutions under the influence of the Zeeman and the hyperfine interaction have been separated for clarity. Further explanation, see text. 

The first radical containing S-N linkages, PhSNSPh (12), was produced in 1925. It was 50 years later when the ESR spectra of (12) and related radicals were reported. Some of the more important S-N radicals include the Vtt-electron system S3N2+ (13) and the isoelectronic neutral radicals (14) and (15). 

There are two different sites in ethylbenzene from which a hydrogen atom can be abstracted by a chlorine radical, leading to two possible radicals. The first radical 7 (Schenie 3.6) is stabilized by resonance interaction with the benzene ring. This is not possible with the second radical 8 and hence this mode of attack is not... 

Additional evidence comes from the study of the radical cations of trans- and cis-methyl-2-phenycyclopentyl ether and trans- and di -l-methyl-2-phenylcyclopentane, where only the first radical cation undergoes C-C bond cleavage since in this case in addition to the overlap between the scissible bond and the Tt-system, the conformation of the methoxy group allows interaction of a nonbonding electron pair on the oxygen with the a orbital of this bond [183, 184]. 

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