Gomberg dimer

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. 

However, with the exception of small ring compounds, much too little is known about the range of these dimensions in different carbon structures and even less about the factors responsible for observed variations. The dimension of this question is recognized immediately when the C—C bond strength in ethane (88.2 kcal mol-1)11 is compared with that of the central bond in the Gomberg dimer 1 (12 kcal mol-1)2,3). 

Triphenylmethyl radicals couple to the Gomberg dimer 2.53, rather than the hexaphenylethane, PhsC-CPhs (2.54), as Gomberg originally proposed. The reason is that it is energetically more favourable for the dimeric compound to lose aromatic stabilization from one ring than to form the sterically strained 2.54. 

Diols (pinacols) can be synthesized by reduction of aldehydes and ketones with active metals such as sodium, magnesium, or aluminum. Aromatic ketones give better yields than aliphatic ones. The use of a Mg—Mgl2 mixture has been called the Gomberg-Bachmann pinacol synthesis. As with a number of other reactions involving sodium, there is a direct electron transfer here, converting the ketone or aldehyde to a ketyl, which dimerizes. 

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 ... 

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 most serious obstacle for the free radical nature of triphenylmethyl was the series of experiments carried out to determine the molecular weight of triphenylmethyl . Cryoscopy was performed in a range of solvents and invariably showed that the molecular weight corresponded to that of the dimer, 486. Only in naphthalene, which in admixture with triphenylmethyl froze at about 80°C, was a lower value obtained, 410. This was a serious dilemma, but Gomberg in 1904 had a reasonable suggestion involving an equilibrium between a dimer and the free triphenylmethyl. 

However, do these conclusions really express doubt about the existence of the free radical triphenylmethyl Or is it the nomenclature that is ambiguous With the correct answer at hand, one cannot state today that the chemical reactivity of a solution of ca. 2% trityl radical and 98% dimer 1 is entirely determined by the chemistry of the radical. Maybe Gomberg was talking about triphenylmethyl ... 

Historically, the triphenylmethyl radical (1), studied by Gomberg in 1987, is the first organic free radical. The triphenylmethyl radical can be obtained by the reaction of triphenylmethyl halide with metal Ag as shown in eq. 1.1. This radical (1) and the dimerized compound (2) are in a state of equilibrium. Free radical (1) is observed by electron spin resonance (ESR) and its spectrum shows beautiful hyperfine spin couplings. The spin density in each carbon atom can be obtained by the analysis of these hyperfine spin coupling constants as well as information on the structure of the free radical. 

Today, we recognize that the ability of triphenylmethyl radicals to exist free in solution is due to two factors. First, the radical has considerable resonance stabilization. Second, and more important, there is considerable steric hindrance to the dimerization of the radical due to the three bulky phenyl groups. In fact, it has recently been shown that Gomberg s hydrocarbon is not hexaphenylethane but actually results from one... 

For a long time after Gomberg first carried out this reaction in 1900 it was assumed the radical dimerized to form hexaphenylethane ... 

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