Gomberg radical

In view of the subsequent work, the content of the actual monomeric diradicals in Schlenk hydrocarbons was rather miniscule. For Gomberg radical, the percentage of the monomeric radical in dilute solutions is relatively small (K msoc 10-4 M-1 at 293 K) 31 thus, the content of the monomer for the corresponding diradicals is likely to be much smaller that less than few percent for triphenylmethyl in dilute solutions. Also, the reported violet color of 8 is unlikely to correspond to monomeric diradical.63... 

Most speculative radicals of the nineteenth century, like benzoyl, cacodyl, ethyl or acetyl (some of which later turned into formal-descriptive rests or functional groups ) had no chance to become real entities, or better, to be given an empirically adequate description. The Gomberg radical, in contrast, became more than just some strange movement behind the curtain. 

Von Baeyer (Nobel Prize, 1905) should be credited for having recognized in 1902 the saltlike character of the compounds formed. He then suggested a correlation between the appearance of color and salt formation—the so-called halochromy. Gomberg (who had just shortly before discovered the related stable triphenylmethyl radical), as well as Walden, contributed to the evolving understanding of the structure of related cationic dyes such as malachite green. 

Pyridine has been phenylated with the following free-radical sources benzenediazonium chloride with aluminum trichloride the Gomberg reaction " phenylhydrazine and metal oxides A -nitroso-acetanilide dibenzoyl peroxide phenylazotriphenylmethane di-phenyliodonium hydroxide and electrolysis of benzoic acid. ° Although 2-phenylpyridine usually accounts for over 50% of the total phenylated product, each of the three phenyl derivatives can be obtained from the reaction by fractional recrystallization of the... 

In a search for new spasmolytics, Heilbron and Hey and their co-workers synthesized a number of pyridylquinolines by coupling diazotized arainoquinolines with pyridine. Pyridine has also been substituted by the 3-pyridyl radical in the Gomberg reaction, the 2-and 3-substituted products being obtained in 55 and 5% yield, respectively, together with a third product obtained in 20% yield w hich w as tentatively formulated as the 4-isomer. The same radical w as coupled with 4-methyl- and 4-ethyl-pyridine and gave in each case mixtures of the twm possible substitution products in which the radical had coupled predominantly with the carbon atom of pyridine adjacent to the alkyl substituent, as showm in Eq. (19). ... 

There is an early report that thiophene reacts at the 3-position in phenylation with benzenediazonium chloride and aluminum trichloride, but in the Gomberg reaction thiophene has been found to substitute mainly at the 2-position both with p-tolyl and with p-chloro-phenyl radicals.Bcnzothiazole is phenylated at the 2-position in low yield by dibenzoyl peroxide a small quantity of the 4-isomcr is also obtained. ... 

A similar case is the catalysis of Gomberg-Bachmann arylations by A,A-diphenyl-hydroxylamine, which was discovered by Cooper and Perkins (1969). As Scheme 8-46 shows, the covalent adduct cation 8.62 first loses a proton. This facilitates the homolytic dissociation, as a stable radical, A/,A-diphenylnitroxide (8.63), is formed. This... 

Dimethyl-I,l -biphenyl has been prepared by a wide variety of procedures, but few of these are of any practical synthetic utility Classical radical biarjl syntheses such as the Gomberg reaction or the thermal decomposition of diaroyl peroxides give complex mixtures of products m which 4,4 dimethyl-l.l -biphenyl is a minor constituent A radical process maj also be involved in the formation of 4,4 dimethyl-1, l -biphenyl (13%) by treatment of 4-bromotoluene with hydrazine hydrate 5 4,4 -Dimethyl-l,l -biphenyl has been obtained in moderate to good yield (68-89%) by treatment of either dichlorobis(4-methyl phenyl)tellurium or l,l -tellurobis(4-methylbenzene) with degassed Raney nickel in 2 methoxyethyl ether 6... 

For a review, see Sholle, V.D. Rozantsev, E.G. Russ. Chem. Rev., 1973, 42, 1011. Gomberg, M. J. Am. Chem. Soc., 1900, 22, 757, Ber., 1900, 33, 3150. Hexaphenylethane has still not been prepared, but substituted compounds [hexakis(3,5-di-ferf-butyl-4-biphenylyl)ethane and hexakis(3,5-di-tert-butylphenyl)ethane] have been shown by X-ray crystallography to be nonbridged hexaarylethanes in the solid state Stein, M. Winter, W. Rieker, A. Angew. Chem. Int. Ed. Engl., 1978,17, 692 Yannoni, N. Kahr, B. Mislow, K. J. Am. Chem. Soc., 1988,110, 6670. In solution, both dissociate into free radicals. 

The Pschorr reaction can take place by two different mechanisms, depending on conditions (1) attack by an aryl radical (as in the Gomberg-Bachmann reaction) or (2) attack by an aryl cation (similar to the SnI mechanism discussed on p. 853). Under certain conditions the ordinary Gomberg-Bachmann reaction can also involve attack by aryl cations. 

Free-radical arylation by diazonium salts (Gomberg-Bachmann,... 

The radical dissociation of the Gomberg dimer , [3-(diphenyl-methylidene)-6-(triphenylmethyl)-l,4-cyclohexadiene] [48], is familiar to organic chemists as the original observation of carbon-carbon a bond dissociation in a solution (Gomberg, 1900 Lankamp et al., 1968). The yellow colour of the triphenylmethyl radical in the benzene solution should have been an observation convincing synthetic organic chemists of the stable existence of the triphenylmethyl radical [8-j. 

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... 

Sodium triphenylmethane treatment of triphenylmethyl chloride with an excess of sodium in benzene leads, via the triphenylmethyl radical (Gomberg, 1900), to triphenylmethyl sodium ... 

Steric effects have been discussed in free radical chemistry ever since the discovery of the first free radical, triphenylmethyl 1 by M. Gomberg in 19001. To what extent is the dissociation of its dimer, which was believed to be hexaphenylethane 23 till 19682 determined by electronic stabilization of triphenylmethyl 1 or by steric strain in its dimer ... 

Synthesis of the first stable free radical PhsC M. Gomberg [47]... 

Free radicals are classified from their lifetimes into long-lived (stable) and short-lived (intermediate) radicals. The most famous and first example of the former is triphenylmethyl (1), which was discovered just one hundred years ago by Gomberg [37]. 

In the paper published in 1900, he reported that hexaphenylethane (2) existed in an equilibrium mixture with 1. In 1968, the structure of the dimer of 1 was corrected to be l-diphenylmethylene-4-triphenylmethyl-2,5-cyclohexadiene 3, not 2 [38]. Since Gomberg s discovery, a number of stable radicals have been synthesized and characterized, e.g., triarylmethyls, phenoxyls, diphenylpicryl-hydrazyl and its analogs, and nitroxides [39-43]. The radical 1 is stable, if oxygen, iodine, and other materials which react easily with it are absent. Such stable radicals scarcely initiate vinyl polymerization, but they easily combine with reactive (short-lived) propagating radicals to form non-paramagnetic compounds. Thus, these stable radicals have been used as radical scavengers or polymerization inhibitors in radical polymerization. 

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. 

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-... 

Free Radicals and Moses Gomberg s Contribution. Pure and Applied Chem., 15, 1-13 (1967) also in Free Radicals in Solution, Butterworth1s, London, 1967, pp. 1-13. Condensed version in Chem. Engr. News, 44, no. 41, 90-92 (Oct. 3, 1966). 

By single electron transfer from an electron donor, e.g. a transition metal ion, a trivalent phosphorous derivative or a base, followed by dissociation of the intermediate diazenyl radical in an aryl radical and dinitrogen. The aryl radical reacts with the solvent or with added reagents in various ways, as shown by the relatively large number of classical named reactions (Sandmeyer, Pschorr, Gomberg-Bachmann, Meerwein reactions). 

Gaseous carbonium ions from the decay of tritiated molecules, 8, 79 General base and nucleophilic catalysis of ester hydrolysis and related reactions, 5, The Gomberg Century Free Radicals 1900-2000, 36, 1 Gomberg and the Nobel Prize, 36, 59... 

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