Triphenylmethyl reaction with methyl radicals

Romanian scientists compared one-electron transfer reactions from triphenylmethyl or 2-methyl benzoyl chloride to nitrobenzene in thermal (210°C) conditions and on ultrasonic stimulation at 50°C (lancu et al. 1992, Vinatoru et al. 1994, Chivu et al. 2006). In the first step, the chloride cation-radical and the nitrobenzene anion-radicals are formed. In the thermal and acoustic variants, the reactions lead to the same set of products with one important exception The thermal reaction results in the formation of HCl, whereas ultrasonic stimulation results in CI2 evolution. At present, it is difficult to elucidate the mechanisms behind these two reactions. As an important conclusion, the sonochemical process goes through the inner-sphere electron transfer. The outer-sphere electron transfer mechanism is operative in the thermally induced process. 

The reactivity of a free radical can be defined by the rate constants of its reactions with other molecules and other free radicals. In general this reactivity depends on the extent of localization of the unpaired electron. When it is highly localized on a single atom, as in the methyl radical for example, this site is highly reactive. However, delocalization of the unpaired electron over aromatic rings reduces the reactivity to the point where some free radicals can be kept virtually for ever in the form of stable, unreactive samples. The triphenylmethyl radical is the best known example. 

In several ways this reaction resembles the molecular rearrangements involving transitory carbonium ions. The reaction is acid-catalyzed60 (see the Schmidt reaction). In the presence of triphenylmethyl free radicals no mixed products are formed.61 The migrating group never leaves the field of the electronically deficient atoms concerned since the Curtius reaction with (—)o-(2-methyl-6-nitrophenyl)-benzoic acid (LXXV) produces an optically active amine62 (LXXVI). 

Photooxidation of the triphenylmethyl cation (as its tetrafluoroborate salt) in the presence of an aromatic donor was similarly found to afford bis(triphenyl-methyl) peroxide [31]. The mechanism was proposed to proceed through initial electron transfer from the aromatic donor to the singlet-excited triphenylmethyl cation to give the triphenylmethyl radical. Reaction of the radical with triplet oxygen, and subsequent coupling with another triphenylmethyl radical gave the observed peroxide. It was noted that electron transfer from the tetrafluoroborate counterion to the excited state cation could not be completely excluded, because the cation was slowly photooxygenated in the absence of an electron donor. 

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

Although it might be expected that reactions which employed triarylmethyl halides would occur very readily, such reactions are rendered potentially more complex by the known nature of the halides and their propensity for involvement in free radical reactions. Whereas normal alkylation proceeds between sodium diethyl phosphite and diphenyl-methyl halides, success, or otherwise, in the use of the triphenylmethyl halides depends to some extent on the individual halide and on the metal in the phosphite salt. Thus, in an early study (in 1939), Arbuzov found that in reactions between silver dialkyl phosphites and triphenylmethyl bromide, dialkyl triphenylmethylphosphonates were indeed formed, but the use of the corresponding alkyl chloride provided the phosphite triester instead (metal dialkyl phosphites possess ambident anions ). A later study confirmed the behaviour of the silver salts towards the chloride, but also showed that, whereas dialkyl phophites with primary alkyl groups yielded phosphonic diesters (as had already been found), those with secondary alkyl groups afforded phosphite triesters moreover, the presence and nature of aromatic substituents were also able to control the course of the reaction. Reactions which involve triarylmethyl halides and sodium dialkyl phosphites may well be of a free radical nature since repeated studies have demonstrated the forma-... 

Gomberg was the first to characterize a free radical when, in 1900, he generated triphenylmethyl radical 5 by reacting chlorotriphenylmethane (4) with zinc metal.l Triphenylmethyl radical 5 is unusual in that it is quite stable and its formation is probably the first experimental verification of a free radical. Frankland, however, may have been the first to generate transient methyl and ethyl radicals in the reaction of iodomethane and iodoethane with zinc, in 1849.2 In the last 30 years, attention has been focused on radicals, their reactivity, and their applications to organic synthesis. Excellent monographs by Davies and Parrott, Lazar et al., Hay, ... 

The idea that such carbon atoms with seven valence electrons could be involved in organic reactions took firm hold in the 1930 s. Two experimental studies stand out with historical significance in the development of the concept of free-radical chemistry. The work of Gomberg around 1900 provided evidence that when triphenyl-methyl chloride was treated with silver metal, the resulting solution contained PhsC in equilibrium with a less reactive dimeric molecule. It was generally assumed that the triphenylmethyl radical was in equilibrium with hexaphenylethane. Only... 

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