Radicals selective coupling

Stable radicals can show selectivity for particular radicals. For example, nitroxides do not trap oxygcn-ecntcrcd radicals yet react with carbon-ccntcrcd radicals by coupling at or near diffusion controlled rates.179,184 This capability was utilized by Rizzardo and Solomon181 to develop a technique for characterizing radical reactions and has been extensively used in the examination of initiation of radical polymerization (Section 3.5.2.4). In contrast DPPH, w hile an efficient... 

The polymerization mechanism of phenols is described as follows. The phenol is adsorbed on the electrode surface and accumulated in the diffusion layer. The adsorbed phenol undergoes one-electron oxidation to the phenoxy radical on the electrode surface. The concentrated phenoxy radical is coupled with each other at p-position to form the dimer, and the dimer repeats the electro-oxidation and coupling. The phenoxy radical is assumed to he adsorbed or oriented upon the electrode surface thus resulted in the selective coupling reaction. 

The anodic coupling of aryl ethers is reviewed in Ref. [180]. Aryl ethers are more selectively coupled than phenols for the following reasons The carbon-oxygen coupling is made impossible and the ortho-coupling and the oxidation to quinones become more difficult. A mixture of triflu-oroacetic acid (TFA) and dichloromethane proved to be the most suitable electrolyte [181]. TFA enhances the radical cation stability and suppresses the nucle-ophilicity of water. Of further advantage is the addition of alumina or trifluo-roacetic anhydride [182]. Table 12 compiles representative examples of the aryl ether coupling. 

The thermodynamic driving force notwithstanding, the coupling of two different radicals is not an especially practical preparative method to form a new bond. This is because the preparation of precursors that directly decompose to radicals is rarely convenient, because disproportionation can often compete effectively with recombination, and especially because chemoselectivity (that is, the selective-coupling of two different radicals to the exclusion of self-coupling) is difficult to achieve if all coupling reactions occur at the same rate (the diffusion-controlled limit). 

Among the other possible Gif products shown in Scheme 1, the oxidative transformation of pyridine to bipyridines and hydroxypyridines is of rather limited use, because of the formation of isomers that are not amenable to facile separation. Nevertheless, the reaction has precedence in the historic synthesis [35] of bipyridines via radical pyridine coupling mediated by FeCl3 in the presence of Zn, but has been superseded by more selective coupling reactions that enable synthesis of unsymmetrically substituted bipyridines [36]. 

A wide series of oxidants, spanning from TiCLj to iodine, has been used in the oxidative homocoupling of chiral 3-arylpropionic acid derivatives aimed at the preparation of lignans. The /f,/f-selectivity in the reactivity of 34 has been explained by a radical coupling mechanism (equation 20). The initially formed lithium (Z)-enolate may transform into the titanium enolate 35, which undergoes oxidation to the radical intermediate 36 via a single electron transfer process. The iyw-Z-type radicals 36 couple each other at the less hindered S-side si face) to give the R,/f-isomers 37 stereoselectively. 

Santus R, Patterson LK, Filipe P, Morlire P, Hug GL, Fernandes A, Mazire J-C. (2001) Redox reactions of the urate radical/urate couple with the superoxide radical anion, the tryptophan neutral radical and selected flavonoids in neutral aqueous solutions Free Rad Res 35 129-136. 

An oxidation using a nickel hydroxide electrode is shown in 15.8.433 Electrochemistry is also a way to produce radicals and anions. The hydrodimerization of acrylonitrile to adiponitrile just mentioned may involve the coupling of free radicals. The coupling of carbonyl compounds, such as p lolualdehyde, to form pinacols with up to 100% selectivity, by way of free radicals, can be done electrically.434 Anions can also be formed electrochemically and used in situ, as in example (15.9).435... 

Substantial progresses have been shown in the past years as most of the experimental k, values are now explained on the basis of a theoretical approach. All these works allow to (i) describe the transition state, (ii) explain the role of the enthalpy/ polar effects in the addition properties in a very large set of radical/monomer couples, (iii) interpret the large scale of values for the addition rate constants, (iv) study the reversibility of the thiyl radical addition, and (v) design radicals exhibiting both a high reactivity and a low selectivity. 

The experiments were conducted directly in the cavity of the EPR spectrometer [94]. Other reactions of note summarized in Table I include the decarboxylation of carboxylic acids ( 90% selectivity) [75, 88] the dehydrogenation of organonitriles, equation (24) [88], and the radical cross coupling observed when solutions of alkanes and ethers are simultaneously irradiated in the presence of Q4W10O32, equation (25) [75,90]. In equation... 

The Huisgen [3 + 2] cycloaddition between organic azides and alkynes and the radical-mediated coupling of thiols and alkenes ( click reaaions) are both selective and highly efficient coupling processes.These reactions have had a vast impact in polymer chemistry since they allow multiple site modifications of polymers in high yields, and typically do not interfere with other functional groups. However, use of these reactions for modification of polypeptides has only been demonstrated recently. 

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