Alkyl radicals disproportionation

Consider now the alkyl radical disproportionation reactions. In these reactions the attacking radical abstracts a hydrogen atom from the radical attacked to form an alkane molecule and an olefin molecule. If the disproportionation involves two different radicals, two reaction paths are possible ... 

Table 10. Rate constants (in cm mol s ) and activation energies of alkyl radical disproportionation reactions [232]...

Most organic free radicals have very short lifetimes, but certain structural features enhance stability. Radicals without special stabilization rapidly dimerize or disproportionate. The usual disproportionation process for alkyl radicals involves transfer of a hydrogen from the carbon P to the radical site, leading to formation of an alkane and an alkene ... 

Entry 3 has only alkyl substituents and yet has a significant lifetime in the absence of oxygen. The tris(/-butyl)methyl radical has an even longer lifetime, with a half-life of about 20 min at 25°C. The steric hindrance provided by the /-butyl substituents greatly retards the rates of dimerization and disproportionation of these radicals. They remain highly reactive toward oxygen, however. The term persistent radicals is used to describe these species, because their extended lifetimes have more to do with kinetic factors than with inherent stability." Entry 5 is a sterically hindered perfluorinated radical and is even more long-lived than similar alkyl radicals. 

Although these reactions are thus closely related to the acyl-alkyl diradical disproportionation to ketenes, the stereospecificity of (55) -> (56) and (57) -> (58) shows that these hydroxyketones cannot proceed through free radicals capable of rotating about single bonds prior to the intramolecular hydrogen... 

The cleavage of alkyl aryl sulfones by sodium amalgam and alcohols65 probably proceeds also through the intermediacy of a radical anion, followed by splitting to the arylsulfinate anion and an alkyl radical. Both the sulfinate anion and the disproportionation products of the radical have been observed. 

Homolytic aromatic substitution often requires high temperatures, high concentrations of initiator, long reaction times and typically occurs in moderate yields.Such reactions are often conducted under reducing conditions with (TMSlsSiH, even though the reactions are not reductions and often finish with oxidative rearomatization. Reaction (68) shows an example where a solution containing silane (2 equiv) and AIBN (2 equiv) is slowly added (8h) in heated pyridine containing 2-bromopyridine (1 equiv) The synthesis of 2,3 -bipyridine 75 presumably occurs via the formation of cyclohexadienyl radicals 74 and its rearomatization by disproportionation with the alkyl radical from AIBN. ... 

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 recombination and the disproportionation of alkyl radicals play an important role in many other chain reactions, for example, pyrolysis, photolysis, and radiolysis of organic... 

The alkyl radicals possess reducing activity as well. In the disproportionation reaction, one alkyl radical reacts as an acceptor and another as a donor of the hydrogen atom. This is the reason for the reducing action of alkyl radicals in reactions with peroxides. 

The following explanation accounts for the facts. Alkyl radicals react with phenoxyl radicals by two parallel reactions, namely, recombination and disproportionation. 

Pletcher and associates [155, 159, 160] have studied the electrochemical reduction of alkyl bromides in the presence of a wide variety of macrocyclic Ni(II) complexes. Depending on the substrate, the mediator, and the reaction conditions, mixtures of the dimer and the disproportionation products of the alkyl radical intermediate were formed (cf. Section 18.4.1). The same group [161] reported that traces of metal ions (e.g., Cu2+) in the catholyte improved the current density and selectivity for several cathodic processes, and thus the conversion of trichloroacetic acid to chloroacetic acid. Electrochemical reductive coupling of organic halides was accompanied several times by hydrodehalogena-tion, especially when Ni complexes were used as mediators. In many of the reactions examined, dehalogenation of the substrate predominated over coupling [162-165]. 

Note that the disproportionation reaction involves intermolecu-lar hydrogen abstraction. Due to steric factors, tertiary alkyl radicals undergo disproportionation rather than radical combination. Radicals forming addition products with compounds containing C=C bonds ... 

The efficiency of product formation in solution is also controlled by the stabilities of the radicals. Stable radicals such as tertiary alkyl radicals or benzyl radicals lead to efficient decarbonylation in solution. Because of steric factors involving bulky groups, tertiary radicals tend to preferentially undergo disproportionation rather than radical combination and so the quantum yield of the products formed by disproportionation exceeds that of the radical combination product. 

Primary [21] and secondary [22] alkyl iodides are reduced in a stepwise fashion at mercury cathodes to form alkyl radicals and alkyl carbanions the alkyl radicals undergo coupling and disproportionation as well as interaction with the electrode to yield diorganomercury compounds, and... 

Steric constraints dictate that reactions of organohalides catalysed by square planar nickel complexes cannot involve a cw-dialkyl or diaryl Ni(iii) intermediate. The mechanistic aspects of these reactions have been studied using a macrocyclic tetraaza-ligand [209] while quantitative studies on primary alkyl halides used Ni(n)(salen) as catalyst source [210]. One-electron reduction affords Ni(l)(salen) which is involved in the catalytic cycle. Nickel(l) interacts with alkyl halides by an outer sphere single electron transfer process to give alkyl radicals and Ni(ii). The radicals take part in bimolecular reactions of dimerization and disproportionation, react with added species or react with Ni(t) to form the alkylnickel(n)(salen). Alkanes are also fonned by protolysis of the alkylNi(ii). 

Fig. 4. Relation between the statistical corrected ratio of rates of disproportionation and dimerization of alkyl radicals and their Es-constants Ig = -0.48 Eg - 1.73 (r = 0.9901)...

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