Free radicals reactive

No systematic study of the free radical reactivity of aminothiazole derivatives has yet been reported. Their behavior, however, may be extrapolated from the detailed work performed on other thiazoles (see Chapter III. Section IX. 1). 

The free-radical reactivity of thiazoles has been well studied with various radicals such as methyl, phenyl, substituted phenyl, cyclohexyl, and aromatic-heterocyclic, in nonpolar solvent or strong acids (180-182). 

D. Comparison of Free-Radical Reactivity with Theoretical Calculations... 

Free-radical reactivity of thiazole has been calculated by semiempirical methods, and results free valence and localization energy) have been compared with experimental data. For mono- and dimethylthiazoles the radical localization energy of the unsubstituted position may be correlated with the logarithm of experimental reactivity (180, 200). The value of the slope shows that a Wheland-type complex is involved in the transition state. 

Entries 4 and 5 point to another important aspect of free-radical reactivity. The data given illustrate that the observed reactivity of the chlorine atom is strongly influenced by the presence of benzene. Evidently, a complex is formed which attenuates the reactivity of the chlorine atom. This is probably a general feature of radical chemistry, but there are relatively few data available on solvent effects on either absolute or relative reactivity of radical intermediates. 

Although it does not concern us here, it should be mentioned that organoboron and organoaluminum compounds exhibit anionoid (Grignard) and free radical reactivity as well as their behavior as carbonium ion analogs. 

Free radicals reactive oxidizing agents bearing an unpaired electron. 

Davies AG, Roberts BP (1973) In Kochi JK (ed) Free Radicals (Reactive Intermediates in Organic Chemistry), vol 1. WUey, New York, p 547... 

Following the extensive investigation of bridgehead carbonium ion reactivities, which provides the most conclusive experimental evidence available for the preferred planarity of carbonium ions 187), similar studies of bridgehead free radical reactivity have been initiated. The results are equally instructive. 

Reactive metabolites include such diverse groups as epoxides, quinones, free radicals, reactive oxygen species, and unstable conjugates. Figure 8.2 gives some examples of activation reactions, the reactive metabolites formed, and the enzymes catalyzing their bioactivation. 

This similarity in reactivities probably derives from a fortuitous cancellation of substituent effects in 11. Fluorination increases chain stiffness and creates an unfavorable polarity mismatch between an electrophilic radical and an electron-poor double bond, but this is offset by the significant decrease of 7r-bond energy in 11. The vinyl ether 12 analog cyclizes about seven times faster than 11, which is consistent with the known lower 7r-bond energy and higher free-radical reactivity of perfluorovinyl ethers vs perfluoroalkenes [142]. 

Here q SN) is the electron population (not the charge) on atom k, etc. (see below). Note that/ is just the average of/j. and /k. The condensed Fukui functions measure the sensitivity to a small change in the number of electrons of the electron density at atom k in the LUMO (/ "), in the HOMO (fk ), and in a kind of intermediate orbital (/j° ) they provide an indication of the reactivity of atom k as an electrophile (reactivity toward nucleophiles), as a nucleophile (reactivity toward electrophiles), and as a free radical (reactivity toward radicals). 

Yoshida and coworkers described the control of free-radical reactivity during reduction by dynamic coordination pyridylethyl-substituted tin hydrides (123, 124) appear to selectively reduce alkyl iodides and bromides in preference to chlorides, as illustrated in equations 102 and 103805. Dumartin and his associates reported the immobilization of substrates required for the synthesis of 17a-(iodovinyl)estradiol through hydrostannylation with a polymer-supported tin hydride (equation 104)803. Baba and coworkers described the use of Bu2SnIH in the synthesis of nitrogen heterocycles (e.g. 125) (equation 105)804. 

These predictions are essentially confirmed by experience. Most free-radical reactivity ratios are measured by convention at temperatures near 60°C, and the effect of changes in conditions in the range 0-90" C is usually assumed to be negligible, compared to the experimental difficulties in detecting the elTects of slight variations in r or f2. 

Phenols are quite sensitive to oxidation. On the one hand, they are easily oxidized to quinols and on further oxidation with, for example, iron(III) chloride, chromic acid or silver(I) oxide give p-quinones. However, under one-electron transfer conditions the phenoxide anion is oxidized to the phenoxyl radical. This shows free radical reactivity on the oxygen atom and at the ortho and para positions (Scheme 4.20a). The phenoxyl radical may readily dimerize. This is exemplified by the formation of Pummerer s ketone from p-cresol (Scheme 4.20b). 

---