Radical addition solvent effects

More pronounced solvent effects have been observed in special cases where substrates or products possess ionic character. Ito and Matsuda76 found a 35-fold reduction in the rate of addition of the arenethiyl radical 18 to cx-methylstyrene when the solvent was changed from dimelhylsulfoxide to cyclohexane. Rates for addition of other arenethiyl radicals do not show such a marked solvent dependence. The different behavior was attributed to the radical 18 existing partly in a zwitterionic quinonoid form (Scheme 1,7).77... 

AA sec acrylic acid abstraction sec hydrogen atom transfer abstraction v,v addition and micleophilicity 35 by aikoxy radicals 34-5, 124-5, 392 by alkoxycarbonyloxy radicals 103,127-8 by alkyl radicals 34 5, 113, 116 by f-amyloxy radicals 124 by arenethiyl radicals 132 by aryl radicals 35, 118 by benzovloxy radicals 35, 53, 120, 126 wilh MM a" 53, 120 by /-butovy radicals 35, 53, 55, 124 solvent effects 54, 55. 123 with alkenes 122 3 with ally I acrylates 122 wilh AMS 120, 123 wilh BMA 53, 123 with isopropenvl acetate 121 with MA 120 with MAN 121 with MMA 53, 55, 120.419 with VAc 121 with vinyl ethers 123... 

In contrast to the allyl system, where the reduction of an isolated double bond is investigated, the reduction of extensively delocalized aromatic systems has been in the focus of interest for some time. Reduction of the systems with alkali metals in aprotic solvents under addition of effective cation-solvation agents affords initially radical anions that have found extensive use as reducing agents in synthetic chemistry. Further reduction is possible under formation of dianions, etc. Like many of the compounds mentioned in this article, the anions are extremely reactive, and their intensive studies were made possible by the advancement of low temperature X-ray crystallographic methods (including crystal mounting techniques) and advanced synthetic capabilities. 

The relative proportions of unsaturated carbohydrate, sensitizer (usually acetone), and solvent may have a decided effect upon a photochemical addition reaction, as at least three competing processes (cycloaddition, radical addition, and energy transfer) are possible. The irradiation of 1 in the presence of 2-propanol and acetone provides an illustration (see Scheme 4). When a small proportion of sensitizer... 

The data compiled in Tables 6.15 and 6.16 indicate how a selection of methods perform in determining reaction barriers for methyl radical additions to a series of substituted alkenes. The experimental values with which comparisons are made in Tables 6.15 - 6.20 come from experiments in solution [40, 42, 45, 46] so there is the possibility of non-negligible solvent effects in some instances. 

Careful quantitative kinetic studies of the coupling steps of oKgomeric pyrroles and thiophenes have confirmed this mechanistic pattern [49]. In addition, quantum chemical studies reveal that the dimerization of two radical cations becomes perfectly feasible when solvent effects are included [50]. 

Workentin et al. (1994) described another interesting solvent effect on the competition between electron transfer and the addition reaction between organic cation-radicals and azides. TEE and AN were compared as solvents. In TEE, the cation-radicals of 4-methoxystyrene (R =R =H), P-methyl-4-methoxystyrene (R =Me, R =H), or p,p-dimethyl-4-methoxystyrene (R =R =Me) react with the azide ion according to the following equation ... 

Azonitriles are not susceptible to radical-induced decompositions (56) and their decomposition rates are not usually affected by other components of the environment. Cage recombination of the alkyl radicals occurs when azo initiators are used, and results in the formation of toxic tetrasubstituted succinonitrile derivatives (56). This can be a significant drawback to the use of azo initiators. In contrast to some organic peroxides, azonitrile decomposition rates show only minor solvent effects (54—56) and are not affected by transition metals, acids, bases, and many other contaminants. Thus azonitrile decomposition rates are predictable. Azonitriles can be used as thermal initiators for curing resins that contain a variety of extraneous materials since cure rates are not affected. In addition to curing of resins, azonitriles are used for polymerization of commercial vinyl monomers. 

In contrast, the need to evaluate the relative rates of competing radical reactions pervades synthetic planning of radical additions and cyclizations. Further, absolute rate constants are now accurately known for many prototypical radical reactions over wide temperature ranges.19,33 3S These absolute rate constants serve to calibrate a much larger body of known relative rates of radical reactions.33 Because rates of radical reactions show small solvent dependence, rate constants that are measured in one solvent can often be applied to reactions in another, especially if the two solvents are similar in polarity. Finally, because the effects of substituents near a radical center are often predictable, and because the effects of substituents at remote centers are often negligible, rate constants measured on simple compounds can often provide useful models for the reactions of complex substrates with similar substitution patterns. 

The addition of chloro azide CIN3 on the double bond of glycals proceeds by either an ionic or a radical mechanism depending on experimental conditions. Under UV irradiation, in solvents of low polarity and in the absence of oxygen, radical addition is predominantly regio- and stereoselective [51, 52]. The double bond reactivity is affected by the substituent at C-3 position and its inductive effect. Therefore, the presence of acetates lowers the reactivity, but azidosides are formed following Scheme 25. 

As would be expected for reactions with polar transition states, additions of per-fluoroalkyl radicals to alkenes are faster in CH3CN than in Freon 113 with the observed solvent effects being greater for additions to alkenes which are more electron-rich [70,117]. Table 10 provides comparisons of rates in the two solvents. 

Table 10. Solvent effects on the rates of addition of perfluoroalkyl radicals to styrene and pen-tafluorostyrene at 298 2°K [70,117]...

For example, the rate accelerations in acetonitrile relative to FI 13 for additions to styrene by CF3- and CF3CF2CF2- are a factor of 3.2 and 2.5, respectively, but for additions of these two radicals to pentafluorostyrene the solvent effects are only 1.3 and 2.1. For comparison, Salikhov and Fischer have found that the rate of addition of the nucleophilic fert-butyl radical to (electron-deficient) acrylonitrile (IP=10.9 eV) is also somewhat accelerated in more polar solvents, e.g.,fcadd(CH3CN)//cadd(c-C6H12) = 2.8[129],... 

In nonpolar solvents, exciplex formation is usually favored because of a small AG, and a favorable Coulombic term. The ions are likely to remain in intimate contact for a longer time, i.e., ion pairing is effective because of favorable Coulombic and solvent effects. That dissociation into solvent-separated is not likely for exciplexes formed in nonpolar solvents has been shown by extensive studies dealing with the photochemical additions of donor and acceptors. Reactions via exciplexes or CIP s frequently yield cycloadducts, whereas in polar solvents, coupling via substitution of radical ion pairs and other chemical reactions involving solvated radical ions may predominate [12]. 

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