Alkoxy radicals polarity

For f alkoxy radicals, polar and aromatic solvents favor abstraction over addition, and p-scission over either addition or abstraction (3.4,2.1,1), Addition, abstraction and P-scission have quite different Arrhenius parameters. As a further example the temperature dependence of the rate constants for addition of cumyloxy radicals to styrene, abstraction from isopropylbenzene, and P-scission to give methyl radicals is shown in Figure 3.1. Low temperatures favor abstraction over addition and both of these reactions over P-scission,... 

Minato ct a/.1(12 proposed that the transition state for disproportionation has polar character while that for combination is neutral. The finding that polar solvents enhance kJkK for ethyl170 and /-butyl radicals (Section 2.5.3.5), the very high kjktc seen for alkoxy radicals with a-hydrogens,171 and the trend in kJkK observed for reactions of a scries of fluoroalkyl radicals (Scheme 1.13, Table 1.7) have been explained in these terms.141102... 

Various /-alkoxy radicals may be formed by processes analogous to those described for /-butoxy radicals. The data available suggest that their propensities for addition vs abstraction are similar.72 However, rate constants for [3-scission of /-alkoxy radicals show marked dependence on the nature of substituents a to oxygen (Figure 3.6).210 420,421 Polar, steric and thermodynamic factors are all thought to play a part in favoring this trend.393... 

For the primary and secondary a-alkoxy radicals 24 and 29, the rate constants for reaction with Bu3SnH are about an order of magnitude smaller than those for reactions of the tin hydride with alkyl radicals, whereas for the secondary a-ester radical 30 and a-amide radicals 28 and 31, the tin hydride reaction rate constants are similar to those of alkyl radicals. Because the reductions in C-H BDE due to alkoxy, ester, and amide groups are comparable, the exothermicities of the H-atom transfer reactions will be similar for these types of radicals and cannot be the major factor resulting in the difference in rates. Alternatively, some polarization in the transition states for the H-atom transfer reactions would explain the kinetic results. The electron-rich tin hydride reacts more rapidly with the electron-deficient a-ester and a-amide radicals than with the electron-rich a-alkoxy radicals. 

In the Barton reaction, an alkyl nitrite is converted into an alcohol-oxime. The nitrite is photoexcited to a 1,2-diradical. It fragments to NO and an alkoxy radical (RO-), and the latter abstracts H- from the nearest C-H bond. The resulting alkyl radical then combines with NO to give a nitroso compound, which then tautomerizes to the oxime, probably by a polar stepwise mechanism. The Barton reaction has been used for the remote functionalization of hydrocarbons, especially steroids. 

The subsequent transformations of alkoxy radicals containing strong C-0 bonds at the surface of V-Mo- catalyst with weakly bonded oxygen atoms yields preferentially formation of full CB products with some hydrogen evolution, while alkyl radicals stabilized on acid sites at the surface of RLH catalysts as a result of C-H bonds polarization in the strong field of metal-... 

Hydroxy and alkoxy radicals, obtained from hydrogen peroxide or alkyl peroxides were used 57,58), Bond dissociation energies and polar effects preferentially promote the hydrogen abstraction from the C—H bond. However a hydrogen abstraction from the N—H bond would result without consequences because it would give rise to an electrophilic nitrogen-centered radical, unreactive towards proto-nated heteroaromatic bases. 

Only one example of A, iV-dialkyl diazonium salt was studied, but it provided key information on the mechanism of the solvolysis step in the radical-polar crossover scheme [10]. Thus, the A -allyl substrate 27 was reacted in moist acetone and afforded the primary alcohol 28. Since previous primary substrates, for example la, had afforded tetrathiafulvalenium salts 2a which did not undergo solvolysis, this showed that the A -allyl group was important in exerting a neighboring group effect, assisting the solvolysis of salt 29 via delocalized cation 30. Since ort/m-alkoxy... 

The reaction of alkoxy radicals with triethyl phosphite apparently is sufficiently rapid to swamp out their known rapid conversion to alcohols or ketones, since such products were not detected. It would be interesting to know whether unsymmetrical or less sterically hindered dialkyl peroxides could be induced to interact via a polar mechanism. 

Other classes of radicals have also been examined with TRIR methods. The rate constant for P-scission of alkoxy radicals (Scheme 2.6) was found to be enhanced in polar solvents by monitoring the rate of production of the product carbonyl compound [74]. The decarboxlyation of peroxyesters [75], the reactivity of azidyl radicals [76], and chlorine atom abstraction reactions [77] have also been investigated. 

A considerably higher ratio of disproportionation occurs with more polar radicals such as primary or secondary alkoxy radicals and halogen alkyl radicals. The predominance of disproportionaticHi over coml ation for pdar radicals could be explained by electrostatic r ulsion of the radicals with electric dipoles functioning against dimerization. The polar structure of radu facilitates the proper orientation at the formation of an activated complex of disproprationation. 

Deeble DJ, von Sonntag C (1992) Decarboxylation of 3,4-dihydroxymandelic acid induced by the superoxide radical anion a chain reaction. Int J Radiat Biol 62 105 Deeble DJ, Parsons BJ, Phillips GO (1987) Evidence for the addition of the superoxide anion to the anti- oxidant -propyl gallate in aqueous solution. Free Rad Res Commun 2 351-358 Deeble DJ, Parsons BJ, Phillips GO, Schuchmann H-P, von Sonntag C (1988) Superoxide radical reactions in aqueous solutions of pyrogallol and n-propyl gallate the involvement of phenoxyl radicals. A pulse radiolysis study. Int J Radiat Biol 54 179-193 Denisov ET, Denisova TG (1993) The polar effect in the reaction of alkoxy and peroxy radicals with alcohols. Kinet Catal 34 738-744... 

The susceptibility to hydrogen abstraction of C-H bonds a to ethereal oxygen, which often reflects thermochemical, polar, and stereoelectronic effects, enables selective functionalization of ethereal a-C-H bonds [1, 2]. The resulting a-alkoxy-alkyl radicals stabilized by conjugative electron delocalization between the oxygen... 

The opposite polarity in the organotin reagent R3Sn5+X5 can be exploited in, for example, the reaction of a tin alkoxide with an vinyl ester,83 the addition of an alkoxy- or amino-tin compound to a ketene,84 the reaction of a silyl enolate with a tin chloride,85 or the addition of an alkyltin hydride under non-radical conditions to an a,P-unsaturated carbonyl compound.86... 

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