Radical cation reaction

Interpretation of Radical Cation Reaction Patterns in Duplex DNA... 

The electrogenerated radical anions of aromatic hydrocarbons, e.g. DPA, rubrene, fluorene, can also act as reductants towards electro-chemically obtained radical cations which are derivatives of other aromatic compounds such as N,N-dimethyl-/>-phenylenediamine (Wurster s red) 150> (see Section VIII. B.). When a mixture of DPA and a halide such as 99 (DPACI2) or 100 is electrolysed, a bright chemiluminescence is observed the quantum yields are about two orders of magnitude higher than that of the DPA radical anion-radical cation reaction 153>. 

Oxidative Animation of Nitrones to a-Amino-Substituted Nitroxyl Radicals Similar to the oxidative methoxylation reaction, oxidative animation of 4H -imidazole TV-oxides, in amine saturated alcohol solutions, give stable nitroxyl (282), nitronyl nitroxyl (283), imino nitroxyl (284) and (285) radicals with the amino group at the a-carbon atom of the nitroxyl group (Scheme 2.107) (520, 521). The observed influence of substituents on the ratio of animation products at C2 and C5 atom is close to the ratio observed in the previously mentioned oxidative methoxylation reaction. It allows us to draw conclusions about the preference of the radical cation reaction route. 

The calculations of Pabon and Bauld on the hole-catalyzed ethylene dimerization were among the first ab initio studies on radical cation reactions [46]. 

Actually, the earliest derivative of a vinylcyclopropane radical cation was a serendipitous discovery. It was formed by an unusual hydrogen shift upon photo-induced electron transfer oxidation of tricyclo[4.1.0.0 ]heptane (26). This result has been questioned on the grounds that the same rearrangement was not observed in a Freon matrix. However, there is no basis for the assumption that radical cation reactions in frozen matrices at cryogenic temperatures should follow the same course as those at room temperature in fluid solution and in the presence of a radical anion, which is potentially a strong base. In several cases, matrix reactions have taken a decidedly different course from those in solution. For example, radiolysis of 8 in a Freon matrix generated the bicyclo[3.2.0]hepta-2,6-diene radical cation (27 ), or caused retro-Diels-Alder cleavage yet, the... 

Many of the reactions discussed are not suitable to establish such a relationship, either because of the general stereochemical course of the reaction type, or because of the inherent symmetry of the substrate. Any reaction in which the pyramidal sp hybridized cyclopropane centers are converted to planar sp hybridized ones will lose any memory of the radical cation chirality or stereochemistry, unless it is transferred to a new chiral center generated in the course of the conversion. On the other hand, there are several reaction types which, given appropriate substrates, may be used to probe the stereochemical course of a cyclopropane radical cation reaction. For example, several hydrogen migrations have shown elements of stereoselectivity. Similarly, oxygenation reactions may have the potential to reveal some degree of stereoselectivity. 

Radical Cation Reactions Relationships with Other Intermediates. 234... 

RADICAL CATION REACTIONS RELATIONSHIPS WITH OTHER INTERMEDIATES... 

The mechanistic aspects of aromatic121 and alkene122 radical cation reactions have been reviewed. A second review article covers the structure and properties of hydrocarbon radical cations, as revealed by low-temperature ESR and IR spectroscopy.123 A review of the reactivity of divalent phosphorus radical cations has appeared which discusses ionic and SET processes and their kinetics.124 The structure and reactivity of distonic radical cations have been reviewed, including experimental and calculated heats of formation, structures, reactivity, and mechanisms.122125... 

Cationic species are also formed when sulfides (Meissner et al. 1967 Adams 1970 Bonifacic et al. 1975a Janata et al. 1980 Hiller et al. 1981 Davies et al. 1984 Ramakrishna Rao et al. 1984) or thioureas (Wang et al. 1999 Schuchmann et al. 2000) react with OH. Especially stable are the dimeric radical cations [reactions (48) - (50)]. In the case of thiourea, the high stability of the dimeric radical cation may contribute to the driving force which leads, in acid solution, to its forma-... 

The phosphate-adduct radical is also formed, when the reaction is initiated by S04 [reaction (18)] in the presence of phosphate ions (Behrens et al. 1988). This may either be due to an Sn2 substitution reaction [reaction (19)] or a reaction of the phosphate ion with the radical cation [reaction (17)] formed either by an elimination of S042- plus H+ [reaction (20)] and subsequent protonation of the N(3)-centered radical [equilibrium (22)] or by S042- elimination [reaction (21)], as envisaged originally. The reaction of the radical cation with phosphate would then give rise to the observed radical [reaction (23)]. 

In the original mechanistic concept, a free radical cation, reaction (37), has been postulated (Catterall et al. 1992). Based on circumstantial evidence, the concomitant release of HS04- is now favored [reaction (36) Aravindakumar et al. 2003],... 

Analogous reactions are given by the minor radical cation [reactions (16) and (17)], and the secondary C(4 ) radicals can eliminate the other phosphate such as reaction (18) that is shown here as an example. Reduction and oxidation reactions depicted as examples in reactions (20) and (12) lead after release of unaltered bases [reactions (13) and (21)] to a series of dideoxypentoses and C(4 ) keto products, free and bound to DNA (Beesk et al. 1979 for the determination of these products see Chap. 13.2). 

Compared to the analogous reactions of the parent molecules, many radical cation reactions show a dramatic decrease in activation barriers, one of the most striking aspects of radical cation chemistry. Intuitively, this observation can be ascribed to the fact that the highest occupied molecular orbital (HOMO) of a radical cation is occupied by a single electron. As a result, the bond strength of one or more key bonds must be reduced and the bonds more easily decoupled. However, the barriers to some radical cation rearrangements appear to lie even lower than might be expected on the basis of this simple model. 

Several interesting rearrangements (stereomutations) were observed for a series of l-anisyl-2-vinylcyclopropanes (32, Ar = CgH -OCty. These reorganizations have been elaborated in greater detail than most other radical cation reactions. For this reason, we will consider them in somewhat greater detail, even though they were initiated primarily by chemical, rather than photochemical electron transfer. Treatment of the m-isomer (cis-32) with aminium or dioxygenyl salts at... 

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