Reactions with Electrophilic Radicals

Although much less well developed than the Minisci reaction, substitution with electrophihc radicals can be used in some cases to achieve selective reaction in electron-rich heterocycles.  

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Thiols react more rapidly with nucleophilic radicals than with electrophilic radicals. They have very large Ctr with S and VAc, but near ideal transfer constants (C - 1.0) with acrylic monomers (Table 6.2). Aromatic thiols have higher C,r than aliphatic thiols but also give more retardation. This is a consequence of the poor reinitiation efficiency shown by the phenylthiyl radical. The substitution pattern of the alkanethiol appears to have only a small (<2-fokl) effect on the transfer constant. Studies on the reactions of small alkyl radicals with thiols indicate that the rate of the transfer reaction is accelerated in polar solvents and, in particular, water.5 Similar trends arc observed for transfer to 1 in S polymerization with Clr = 1.4 in benzene 3.6 in CUT and 6.1 in 5% aqueous CifiCN.1 In copolymerizations, the thiyl radicals react preferentially with electron-rich monomers (Section 3.4.3.2). 

Accordingly, many reactions can be performed on the sidewalls of the CNTs, such as halogenation, hydrogenation, radical, electrophilic and nucleophilic additions, and so on [25, 37, 39, 42-44]. Exhaustively explored examples are the nitrene cycloaddition, the 1,3-dipolar cycloaddition reaction (with azomethinylides), radical additions using diazonium salts or radical addition of aromatic/phenyl primary amines. The aryl diazonium reduction can be performed by electrochemical means by forming a phenyl radical (by the extrusion of N2) that couples to a double bond [44]. Similarly, electrochemical oxidation of aromatic or aliphatic primary amines yields an amine radical that can be added to the double bond on the carbon surface. The direct covalent attachment of functional moieties to the sidewalls strongly enhances the solubility of the nanotubes in solvents and can also be tailored for different... 

Due to the highly exothermic nature of the process, the replacement of primary, secondary and tertiary hydrogens upon reaction with electrophilic fluorine atoms is not as selective as for other radicals. For example, early work by Tedder [30,34], showed that the order of selectivity follows the usual pattern, i. e. tert > sec > prim, but the relative selectivity of fluorine atoms is less than chlorine atoms (Table 2). 

We consider as dihydro derivatives those rings which contain either one or two 5p3-hybridized carbon atoms. According to this definition, all reactions of the aromatic compounds with electrophiles, nucleophiles or free radicals involve dihydro intermediates. Such reactions with electrophiles afford Wheland intermediates which usually easily lose H+ to re-aromatize. However, nucleophilic substitution (in the absence of a leaving group such as halogen) gives an intermediate which must lose H and such intermediates often possess considerable stability. Radical attack at ring carbon affords another radical which usually reacts further rapidly. In this section we consider the reactions of isolable dihydro compounds it is obvious that much of the discussion on the aromatic heterocycles is concerned with dihydro derivatives as intermediates. 

Heberger K, Lopata A (1998) Assessment of nudeophilicity and electrophilicity of radicals, and of polar and enthalpy effects on radical addition reactions. J Org Chem 63 8646-8653 HerakJN, Behrens G (1986) Formation and structure of radicals from D-riboseand 2-deoxy-D-ribose by reactions with SO4 radicals in aqueous solution. An "in-situ" electron spin resonance study. Z Naturforsch 41c 1062-1068... 

O. Hammerich, M. F. Nielsen, The Competition Between the Dimerization of Radical Anions and Their Reactions with Electrophiles, Acta Chem. Scand. 1998, 52, 831-857. 

The loss of n energy that hinders the reaction of these carbanions with the phenyl radical diminishes with electrophilic radicals, favouring the coupling. 

The formation of ring systems by the anionic cyclization of olefinic alkyl, aryl and vinyl-lithiums is an interesting synthetic transformation that provides a regiospecific and highly stereoselective route to five-membered carbocycles and heterocycles99. Most importantly, it is possible to functionalize the initially formed cyclization product by a tandem reaction with electrophiles, a reaction that is not generally possible in the case of radical cyclizations. 

Radical ions are created in solution by chemically or electrochemically induced electron transfer to or from a conjugated ir-system. Even if these ions are thermodynamically stable they are only of limited persistence since they are susceptible to reactions with electrophiles and nucleophiles or undergo other processes like dimerization or electron-transfer induced bond cleavage [9, 10]. Pairs of radical anions and radical cations can also be formed by electron transfer between neutral donors and acceptors either in the ground state or upon photochemical excitation [11, 12]. 

These species remain in solution or they are adsorbed and undergo usually irreversible follow-up reactions with electrophiles e to yield finally the product P. This conventional type of electron transfer is called extrinsic in Fig. 3(a). However, the reactivity of radical ions is weakened by using large aromatic molecules, and in the absence of any strong electrophilic or nucleophilic reaction partner the counterions merely associate, and the dissolved radical ion salt accumulates. One can even go a step further using a relatively nonpolar solvent, the radical ion salt is electrodeposited as a salt layer. Examples of this so-called electrocrystallization are furnished by naphthalene [113] or perylene [114]. In both cases, a 2 1 molecular charge stoichiometry is established, e.g., [(perylene)2] A (cf. [115,116]). An application in batteries is conceivable, according to... 

Tin-carbon bonds can be broken by reaction with electrophiles (e.g. protic acids, Lewis acids, halogens), nucleophiles (e.g. RLi), or free radicals (e.g. succimidyl, t-butoxyl), or with certain transition metal (particularly palladium) compounds. Fragmentation can also be induced through the radical cations which are formed by electron transfer. 

Reactivity (General Topics, Reactions with Electrophiles and Oxidants, Reactions with Nucleophiles and Reducing Agents, Reactions toward Free Radicals, Carbenes, etc., Reactions with Cyclic Transition State, Reactivity of Substituents, Heterocycles as Intermediates in Organic Synthesis). 

Another variation of Table 3.12 is to change the olefin, while keeping the radical constant. Table 3.14 shows some results for the reaction with OH radical, X = 7.5 eV and p — 5.7 eV. As expected, the least electrophilic olefins react the fastest. As the large values of AA suggest, there is no energy barrier for most of these reactions. Instead, electron transfer must be increasing the frequency of collision, or the duration of the collision. Only tetrachloroethylene has an appreciable energy barrier of 2.6 kcal/mol. This shows up as a reduction in the rate due to steric hindrance. 

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