Anion radicals from azo compounds

Section 15.6 contains tables of magnetic data obtained from anion radicals of azo compounds. This includes cyclic azo compounds which might also have been entered under the section on heterocycles as it was felt that this was the more natural classification. The data have been divided into subsections consising of aliphatic azoaUcenes benzo[c]cinnolines and azobenzene derivatives arylazophosphonates bidiazines andtet-razines. Where appropropriate, these subsections have been further subdivided, for instance, aliphatic azoaUcenes has been further divided into open chain azoaUcenes, cychc azoalkenes and azopolycycloalk-enes. 

The reactions of polymeric anions with appropriate azo-compounds or peroxides to form polymeric initiators provide other examples of anion-radical transformation (e.g. Scheme 7. 6). ""7i However, the polymeric azo and peroxy compounds have limited utility in block copolymer synthesis because of the poor efficiency of radical generation from the polymeric initiators (7.5.1). 

Azobenzenes, (29), and analogous heteroaromatic azo compounds, (30), are in aprotic solvents reduced in two sequential one-electron steps to the radical anion and the dianion [61-66]. Disproportionation of the radical anion to the dianion is favored by the presence of Li+ [67]. The dianion is considerably more basic than the radical anion, and the dianion is only stable in very dry nonacidic solvents [64, 65, 67, 68]. Both the dianion and the radical anion derived from (29) have been used as EGBs. The anion resulting from protonation of the dianion is less basic (by several pK units) than the dianion but more basic than... 

With compounds of rather negative reduction potential such as dimesityl-ketone or 2,2 -bipyridine the reaction produced only Ti(III) species in low amount. On the other hand, good n acceptors such as o-quinones or azo-con taining heterocycles yielded large amounts of Ti(IV) complexes of the anion radicals [89] according to Eq. (5) [88]. Similar Ti(IV) semiquinone complexes were reported from the reactions of photogenerated CpTiCl2 with o-quinones [90]. 

The anion BH, formed in Eq. (3), is thermodynamically a stronger base than B and will react with another molecule of substrate, as in Eq. (4). Each PB will therefore consume a total of two protons (and two electrons). An exception to the fast removal of BH by further reduction is sometimes found for radical anion oxygen bases derived from carbonyl compounds, as discussed in Sec. III.B.2. Radical anion EGBs are usually derived from aromatic systems such as aromatic hydrocarbons, A-heteroaromatic systems or azo-arenes. An example was given in Scheme 3 [3]. Radical anion EGBs are normally pro-... 

Electron-transfer initiation from other radical-anions, such as those formed by reaction of sodium with nonenolizable ketones, azomthines, nitriles, azo and azoxy compounds, has also been studied. In addition to radical-anions, initiation by electron transfer has been observed when one uses certain alkali metals in liquid ammonia. Polymerizations initiated by alkali metals in liquid ammonia proceed by two different mechanisms. In some systems, such as the polymerizations of styrene and methacrylonitrile by potassium, the initiation is due to amide ion formed in the system [Overberger et al., I960]. Such polymerizations are analogous to those initiated by alkali amides. Polymerization in other systems cannot be due to amide ion. Thus, polymerization of methacrylonitrile by lithium in liquid ammonia proceeds at a much faster rate than that initiated by lithium amide in liquid ammonia [Overberger et al., 1959]. The mechanism of polymerization is considered to involve the formation of a solvated electron ... 

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