Polar effects in radicals

Polar effects in radical reactions seem also not quite well understood. There are some attempts to divide radicals into electrophilic and nucleophilic classes (Pryor, 1966 Pryor et al., 1969 Johnston et al., 1966) resembling the pattern of ionic reactants, generally on the basis of the Hammett equation. This classification, however, seems to be alien to the nature of most carbon radicals. In addition reactions (2) the Hammett />-value is usually positive (radical addition to substituted nitrobenzenes Bartlett and Kwart, 1950, 1952 Sinitsyna and... 

Interaction of partially filled orbitals (i.e. singly occupied MOs-SOMOs) plays a significant role in radical chemistry. Depending on the relative energy of the SOMO, either the three-electron interaction with the HOMO or the one-electron interaction with the LUMO may become dominant, leading to polar effects in radical chemistry (Figure 2.35). 

I have been fascinated with unusual peptide natural products since my PhD days in the laboratory of Chris Easton at the University of Adelaide. At that stage, Chris s group was mainly a physical organic chemistry group studying, for example, polar effects in radical reactions of amino acids.I was one of... 

Salikhov K M, Molin Yu N, Sagdeev R Z and Buchachenko A L 1984 Spin Polarization and Magnetic Effects in Radical Reactions (Amsterdam Elsevier)... 

The mechanism of free-radical addition follows the pattern discussed in Chapter 14 (pp. 894-895). The method of principal component analysis has been used to analyze polar and enthalpic effect in radical addition reactions. A radical is generated by... 

Bernardi, F., Bottom, 1997, Polar Effect in Hydrogen Abstraction Reactions from Halo-Substituted Methanes by Methyl Radical A Comparison Between Hartree-Fock, Perturbation, and Density Functional Theories , J. Phys. Chem., 101, 1912. 

K. M. Salikhov.Y. N. Molin, R. Z. Sag-deev, A. L. Buchachenko, Spin Polarization and Magnetic Field Effects in Radical Reactions, Elsevier, Amsterdam, 1984. 

Another factor that influences the reactivity of two polar reactants, acylperoxyl radical with aldehyde, is the polar interaction of carbonyl group with reaction center in the transition state. Aldehydes are polar compounds, their dipole moments are higher than 2.5 Debye (see Section 8.1.1). The dipole moment of the acylperoxyl radical is about 4 Debye (/jl = 3.87 Debye for PhC(0)00 according to the quantum-chemical calculation [54]). Due to this, one can expect a strong polar effect in the reaction of peroxyl radicals with aldehydes. The IPM helps to evaluate the increment Ain the activation energy Ee of the chosen reaction using experimental data [1], The results of Acalculation are presented in Table 8.10. 

Two mechanisms have been proposed to explain the strong alternation tendency between electron-acceptor and electron-donor monomers. The polar effect mechanism (analogous to the polar effect in chain transfer—Sec. 3-6c-2) considers that interaction between an electron-acceptor radical and an electron-donor monomer or an electron-donor radical and... 

The polar effect was at first invoked to explain various directive effects observed in aliphatic systems. Methyl radicals attack propionic acid preferentially at the a-position, ka/kp = 7.8 (per hydrogen), whereas chlorine " prefers to attack at the /3-position, ka/kp = 0.03 (per hydrogen). In an investigation of f-butyl derivatives, a semiquanti-tative relationship was observed between the relative reactivity and the polar effect of the substituents, as evidenced by the pK, of the corresponding acid. In the case of meta- and / ara-substituted toluenes, it has been observed that a very small directive effect exists for some atoms or radicals. When treated by the Hammett relation it is observed that p = —0.1 for H , CeHs , P-CH3C6H4 and CHs . On the contrary, numerous radicals with an appreciable electron affinity show a pronounced polar effect in the reaction with the toluenes. Compilation of Hammett reaction constants and the type of substituent... 

Sphingomyehn, IR spectrophotometry, 683-4 Spin delocahzation, polar effects in decomposition, 903 Spin labels, free radicals, 665 Spin-lattice relaxation oxidized functional groups, 695 poly (methylstyrene peroxide), 709 Spin trapping artemisinin ESR, 1291 free radicals, 665... 

Ethyl peracetate was the first ester of a peroxy acid, and was characterized by Baeyer and Villiger in 1901. Kinetic studies of perester decomposition were reported by Blomquist and Ferris in 1951, and in 1958 Bartlett and Hiatt proposed that concerted multiple bond scission of peresters could occur when stabilized radicals were formed (equation 46). As noted below (equation 57), polar effects in perester decomposition are also significant. 

Reid DL, Armstrong DA, Rauk A, Nese C, Schuchmann MN, Westhoff U, von Sonntag C (2003) H-atom abstraction by C-centered radicals from cyclic and acyclic dipeptides. A theoretical and experimental study of reaction rates. Phys Chem Chem Phys 5 3278-3288 Roberts BP (1996) Understanding the rates of hydrogen abstraction reactions empirical, semi-em-pirical and ab initio approaches. J Chem Soc Perkin Trans 2 2719-2725 Russell GA (1973) Reactivity, selectivity, and polar effects in hydrogen atom transfer reactions. In Kochi JK (ed) Free radicals. Wiley, New York, pp 275-331 Russo-Caia C, Steenken S (2002) Photo- and radiation-chemical production of radical cations of methylbenzenes and benzyl alcohols and their reactivity in aqueous solution. Phys Chem Chem Phys 4 1478-1485... 

ItoO, Matsuda M (1979) Evaluation of addition rates of the thiyl radicals to vinyl monomers by flash photolysis. 2. Substituent effect on addition of substituted benzenethiyl radicals to methyl methacrylate or styrene. J Am Chem Soc 101 5732-5735 Ito O, Matsuda M (1981) Evaluation of addition rates of thiyl radicals to vinyl monomers by flash photolysis. 3. Polar effect in addition reactions of substituted benzenethiyl radicals. J Am Chem Soc 103 5871-5874... 

Ito O, Matsuda M (1982) Polar effects in addition reactions of benzenethiyl radicals to substituted styrenes and a-methylstyrenes determined by flash photolysis. J Am Chem Soc 104 1701-1703 Janata E, Veltwisch D, Asmus K-D (1980) Submicrosecond pulse radiolysis conductivity measurements in aqueous solutions. II. Fast processes in the oxidation of some organic sulphides. Ra-... 

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... 

Russell, G. A. Reactivity, Selectivity and Polar Effects in Hydrogen Atom Transfer Reactions. In Free Radicals Koch, J. K., Ed. Wiley New York, 1973 Vol. I, pp. 275-331. 

Comparison of the cyclic systems in Table 17 leads to the opposite conclusion, however the destabilization of the 1-norbornyl radical relative to the 1-adamantyl is less for the azo decompositions. Perhaps the mechanism of the azo decompositions of the more unreactive systems is different from that of, for example, the f-butyl azo compound (i.e. the rate determining step of the 1-norbomyl azo compound may be a one bond homolysis rather than the synchronous two bond fission of the f-butyl system312, 315)). Also, the smaller 1-norbornyl/1-adamantyl rate ratio for the f-butyl perester decompositions may be due to a greater influence of polar effects in these reactions 309a). This problem is under active investigation 309a). 

Russel GA. The rate of oxidation of aralkyl hydrocarbons. Polar effects in free radical reactions. J Am Chem Soc 1956 78 1047-1054. 

In contrast, because of similar electronegativities, the transition state for hydrogen transfer from benzene thiol to a perfluoro-n-alkyl radical should have little polar character. In confirming the important role of polar effects in these hydrogen transfer processes, a good Hammett correlation was observed for the reduction of n- c7f15i by a series of arene thiols [141,143]. 

Beranek I, Fischer H (1989) Polar Effects on Radical Addition Reactions An Ambiphilic Radical. In Minisci F (ed) Free Radicals in Synthesis and Biology. Kluwer, Amsterdam, p 303... 

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