Radicals dimerisation

The mechanisms of all three oxidations involve initial slow one-equivalent oxidation to a radical (NH20-, -NHOCH3 and N02-) followed either by rapid secondary oxidation (of NH20- or N02 ) or by radical dimerisation (-NHOCHs). 

Some observations point toward the key step of the reacton, the radical dimerisation [Eq. (45c)], to be a heterogeneous reaction (230) ... 

The particular enhancement of a radical dimerisation reaction, Eqs. (45a) and (45b), is observed on hydrophobic metals like Hg, Pb, and Sn at potentials close to their respective point of zero charge where electrosorption is strongest and diminishes at more negative and positive potential. If the reduction potential of the oxo-compound, which in accordance with the equilibrium reaction (45b) is pH dependent (60 mV per pH unit), is shifted by more than 100 mV away from the point of zero charge, then the ketyl radicals are no longer adsorbed strongly enough and they are displaced from the... 

The acyloin reaction is a similar radical dimerisation but at the ester oxidation level.11 At first it looks just like a pinacol electrons are added to the carbonyl groups to give 47, the radicals combine to give a new C-C bond and the ethoxide groups are lost 48 to give the 1,2-diketone 49. 

Dimer 32 presumably originates by abstraction of hydrogen from the solvent by radicals formed in the reaction, giving 33, which undergoes radical dimerisation. Nonetheless, evidence that radicals are present in the reaction mixture is not evidence that they are an intermediate in the reaction pathway. 

Peroxy radicals dimerise at diffusion rates to tetroxides, which in turn, above 150 K, decompose into alkoxy radicals and oxygen. Low-temperature ESR has established the linear tetroxides as real molecules, with mM to pM dissociation constants at 110-150 The combination is subject to a very large magnetic isotope effect, caused by the coupling to O to the electronic spin, which relaxes some of the spin-symmetry prohibitions on radical recombination a value of knjki of 1.8 is observed for combination of with of R- 0- 0. If, however, the peroxy radical is a to a hydroxyl... 

Radical reactions are used industrially in many C-C bond-forming reactions, particularly the polymerisation of olefins. Such methods are beyond the scope of this book. Some simpler molecules are made by radical dimerisation. Diene (21), used in the manufacture of pyrethroid insecticides, is a dimer of the stable allylic radical (22) and is made industrially at ICI from allylic halide (23). 

This section is concerned with radical dimerisations of this kind (22-23), mostly to make 1,2-difunctionalised compounds. Thus a 1,2-diol (24) could be made from two radicals (25) and this is indeed how it is made. The radicals (25) are generated by metal catalysed reduction of acetone. The product (24) is known as pinacol and the reaction is sometimes called pinacol reduction. The reaction works for most ketones, including enolisable ketones. 

The phenoxy radical, derived from an H-abstraction reaction by NO, couples with another -NO molecule to give 4-nitrocyclohexa-2,5-dienone, which readily rearranges, after a keto-enol tautomerism to 2,6-di- -butyl-4-nitrophenol. 2,4-di-t-butylphenol and 4-r-butylphenol react in the same way, whereas the presence of three substituents on the aromatic ring of 2,4,6-tri-r-butylphenol and 2,6-di- butyl-4-methylphenol does not allow keto-enol tautomerism in these cases 2,4,6-tri- -butyl-4-nitrocyclohexa-2,5-dienone and 2,6-di-r-butyl-4-methylnitrocyclohexa-2,5-dienone were the sole nitro compounds obtained. The reaction between NO and 2,6-di-t-butylphenol or 2,4-di- -butylphenol in methanol resulted in phenoxyl radical dimerisation together with nitration. As shown in (Scheme 5.100), phenoxyl radical dimerises to give an... 

Rhodocene is even more reactive than cobaltocene, and can be isolated only in the absence of solvents, by reduction of the [( r-C5H5)2Rh) cation with sodium vapour, followed by vacuum sublimation on to a cold finger. It behaves as a radical, dimerising rapidly at room temperature to give 7.22, in which the rhodium atom has attaiBed the 18-electron configuration. 

A particular case of a [3C+2S] cycloaddition is that described by Sierra et al. related to the tail-to-tail dimerisation of alkynylcarbenes by reaction of these complexes with C8K (potassium graphite) at low temperature and further acid hydrolysis [69] (Scheme 24). In fact, this process should be considered as a [3C+2C] cycloaddition as two molecules of the carbene complex are involved in the reaction. Remarkable features of this reaction are (i) the formation of radical anion complexes by one-electron transfer from the potassium to the carbene complex, (ii) the tail-to-tail dimerisation to form a biscarbene anion intermediate and finally (iii) the protonation with a strong acid to produce the... 

The presence of the denominator term in the rate equation (17) suggests that the equilibrium (18) precedes the oxidation step. Two sequences of reactions are proposed (see below), depending on whether the sulphite radical ion dimerises (20) or attacks further acid chromate ion (21). It should be noted that of the species prevalent in dilute aqueous chromic acid, namely CrOj , Cr207, HCrO and H2Cr04, only the last is regarded as possessing oxidising powers. This fact, noted by Westheimer , is tacitly assumed in all recent discussion of... 

As is quite often the case, the hydrolysed form of the oxidant is more reactive than the hexa-aquo species. The stoichiometries suggest dimerisation, rather than secondary oxidation, is the normal fate of the hydrazoyl radical. The variations in ky and k suggest oxidation occurs at the non-protonated nitrogen atom. 

Simple electron transfer followed by rapid protonation would give the ketyl radical which dimerises very rapidly. 

In general, the rate ratio of the disproportionation k and dimerisation k. increases vith the bulk or size of the radicals concerned117. For simple alkyl radicals even a... 

The resultant radical (86) can, in turn, be reduced back to the carbanion by shaking with sodium amalgam. In suitable cases, e.g. (87), the oxidation of carbanions with one-electron oxidising agents, usually iodine, can be useful synthetically for forming a carbon-carbon bond, through dimerisation (— 88) of the resultant radical (89) ... 

Almost certainly, the acetylide anion—formed in the basic solution—is oxidised by Cu(u) (another one-electron oxidising agent) to the corresponding radical, which then undergoes dimerisation. 

In the absence of other species with which a radical can react (e.g. abstraction of H from a suitable solvent), their life is terminated largely by dimerisation,... 

The mixture is known as Fenton s reagent, and the effective oxidising agent in the system is the hydroxyl radical, HO. This is particularly good as an abstractor of H, and can be used either to generate the resultant radical, e.g. (30), for further study, or, in some cases prepara-tively through the latter s dimerisation, e.g. (31) ... 

Radicals, (34), that subsequently dimerise, are also obtained through the anodic oxidation of carboxylate anions, RCO20, in the Kolbe electrolytic synthesis of hydrocarbons ... 

Conversely, electrolysis of ketones, (35), results in their cathodic reduction to radical anions (36), which dimerise to the dianions of pinacols (37) ... 

If each aromatic nucleus in the radical has a bulky p-substituent, e.g. (50), then, irrespective of any substitution at the o-positions, dimerisation will be greatly inhibited, or even prevented [cf. (7), p. 301] ... 

It is important to emphasise that in any reaction of a radical with a neutral molecule a further radical will be formed (cf. p. 309), thus establishing a chain reaction that does not require further input of initiator radicals to sustain it. Such a chain reaction is normally terminated by the relatively rare reaction of two radicals with each other (radicals are present in only very low ambient concentration) resulting in dimerisation or disproportionation (cf. p. 305), with no new radical now being produced. 

Attack of, for example, Ph on aromatic species such as benzene is found to lead to products other than the one arising from overall substitution (107, Ra — Ph). This is because the intermediate radical (106), as well as undergoing H-abstraction to (107), can also dimerise to (108) and/or disproportionate to (107) + (109) ... 

We might well expect the resultant phenoxy radical to attack— through the unpaired electron on its O, or on its o- or p-C, atom—a further molecule of phenol or phenoxide anion. Such homolytic substitution on a non-radical aromatic substrate has been observed where the overall reaction is intramolecular (all within the single molecule of a complex phenol), but it is usually found to involve dimerisation (coupling) through attack on another phenoxy radical ... 

This is frequently, but not necessarily, a "dimerisation" as in 16]. The tendency to react in this way is very marked and constitute chemically important reaction pathways in many cases. A biologically important example is [17], and it is noteworthy that this radical is converted from an electron-acceptor (RS ) into an electron-donor (RS-SR) . 

The participation of surface radical species is, then, strongly suspected in the Kotbe reaction, but there are other radical reactions, such as the reductive dimerisation of C02, that are thought to be homogeneous, particularly in non-aqueous solvents. The basic mechanism in this latter case is thought to be ... 

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