Azine cation-radicals

The only azine cation-radical to have been characterized in solution by ESR seems to be that of 1,3,6,8-tetraazapyrene, (pyrimido[4,5,6-g/i]per-imidine) (103). The ESR spectrum of this radical is closely similar to that of 58, the corresponding (4n + 3)-electron radical, as would be expected.191 Sevilla has described cation-radicals from thymidine and various derivatives generated by photoionization in a glassy matrix.349... 

There is much interest at present in the reactivity of excited states of dioxygen and various oxygen species such as superoxide ion and the perhydroxyl radical, and hydroxyl radicals. The photooxidation of acetone azine proceeds via a free radical pathway initiated by electron transfer taking place from the azine to singlet oxygen ( 02). This generates superoxide and azine cation radical,... 

Recently a novel cation-radical-induced ring contraction of 3,6-dicyano-3,6-dimethylpiperid-azine (23) with thianthreniumyl perchlorate gave ra . -1,2-dimethylcyclobutane-l,2-dicarboni-trile (24) in 49% yield.83... 

Related compounds, cyclo[2.2.3]azine and l,2,3,4-dibenzocyclo[2.2.3]azine, also give ion radicals with peripheral 77-clcctron conjugate systems (Gerson et al. 1973 Mat-sumoto et al. 1996). The difference in conjugation between neutral molecules and their ion radicals can be additionally traced in the case of keto-enol tautomerizm. As a rule, enols are usually less stable than ketones. Under equilibrium conditions, enols exist only at very low concentration. However, the situation is different in the corresponding cation radicals, where gas-phase experiments have shown that enol cation radicals are usually more stable than their keto tautomers. This is due to the fact that enol cation radicals profit from allylic resonance stabilization that is not available to ketones (see Bednarek et al. 2001 and references therein) see Scheme 3-58 ... 

This Section deals with radicals which, although cationic, are (4n + 3) electron species and are thus isoelectronic with azine anion-radicals. They may, in principle, occur for any azine, monocyclic or polycyclic, which possesses at least two nitrogen atoms, for it is substitution at these heteroatoms which confers the positive charge, e.g., 17, the diconjugate acid of 22, and 37 (R = alkyl) its dialkyl derivative. Reviews which cover material pertinent to this Section are those of Forrester, Hay, and Thomson,4 of Nelsen,5 of Siskin8 and, most recently, of Bard, Ledwith, and Shine.11... 

In contrast to purines, the combination of azole and azine rings in indolizine gives rise to an electron-rich species as a consequence, only cation-radicals have been reported. The transient existence of a simple cation-radical is implied by the observation of oxidative dimerization of 1,2-dimethylindolizine on treatment with Fe(CN) " to give 179 (R1 = R2 = Me).593 The cation-radical of 179 (R1 = Ph R2 = Me) was synthesized by Colonna et al,461 This group also obtained the azaviolene cation-radical 180 (X = N) and related species.594,595... 

The synthesis and properties of polymeric thiazolobenzothiazolinone azines (e.g., 219) have been described oxidation leads to polymeric cation radicals. 

A simple model which correlates the HOMO and LUMO of cycl-[3,3,3]azine with the degenerate nonbonding orbitals of [12]annulene accounts for the ESR spectra of the radical cations and anions of 2a and some derivatives. This model also predicts the almost identical rc-spin population observed for the 2a-cation and the isoelectronic phenalenyl... 

The topological difference between cycl[3,3,3]azine (2) and cycl-[4,3,2]azine (85) follows from a comparison of the isoelectronic anions (86) and (87) using HMO calculations.108 Since the HOMO s of 86 and 87 are nonbonding, anions, cations, and radicals are of comparable stability. Charge density and bond orders of the cation and anion of 86 depend on the symmetry of the molecule the (positive or negative) charges are located in the starred positions (1,3. ..). There are three different bond orders, as shown in formula 86. 

An unusually easy conversion of A-alkylazinium cations into the uncharged azines occurs when the iodides (73) are treated with triethylamine at room temperature. The reaction pathway seems to involve a free radical intermediate (Scheme 27) [95MC104]. 

A similar study of the photooxidation of some spiropyrans and spironaphthox-azines indicates that the spiro and open forms of these dyes are singlet oxygen quenchers and that the colored form does not act as a sensitizer. A mechanism is proposed that involves the formation of a superoxide radical anion by photoinduced electron transfer to oxygen from a merocyanine form of the dye, followed by nucleophilic attack of the radical anion on the radical cation of the dye.174... 

A theoretical/NMR study of keto-enol tautomerism in 2-(2-methoxycarbony-lacetyl)pyrazine (277/278) and other similarly substituted azines has been undertaken the foregoing pyrazine exists in its enolic form (278) to the extent of 35% in deuterochloroform.411 l,4-Diacetyl-l,4-dihydropyrazine (279) gave the persistent radical cation (279) + on one-electron oxidation (cyclic voltammetry in MeCN— Bu4NC104).167... 

The hyperfine information for the series of azine radical donors requires deeper consideration. Gilbert et al. report detailed hyperfine information for unmethylated azine donors.However, the methylated donors provide a better model for application to the DCA species under consideration. Also, the Gilbert studies do not conclusively explain the experimental EPR data, noting a trend of reduced EPR resolution with donor heteroatom size, without conclusive explanation. We therefore have conducted a detailed study of the cw EPR for the oxidized methyl azine radical cation series (MePOZ , MePTZ" " , and MePSeZ ). 

With azines the situation is varied. In the radical cations of pyridine and diazines the semi-occupied orbital is largely confined to the nn orbital(s) (see Scheme 2, structure 2), while the radical cation is of the n type with monoazanaphthalenes, -phenanthrenes and -anthracenes. The situation might change with substitution. As an example, alkylpyridine radical cations are of the n type, like the parent compound, whereas for the 2,5-dimethyl, 2-chloro, and 2-bromo derivatives the structure is of the n type [13]. Likewise, with benzo[c]cinnoline the parent compound and its alkyl derivatives give an n radical cation, but with some dimethoxy derivatives a n structure is found [14] and a switch from n to 7t structure occurs also in passing from 1,2,4,5-tetrazine to its 3,6-diamino derivatives [15]. 

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