Hexadienes radical cations

At least one of the hexadiene radical cations (20 +) can be viewed as a species containing spin and charge in two separate (though equivalent) molecular fragments. This separation can be enforced in systems of lower symmetry in this context, we mention two radical cations derived from l,l-diaryl-2-methylenecyclopropane (23), and 6-methoxy-l,2,3,4,6-pentamethyl-5-methylenebicyclo [2.2.0]hex-2-ene (25). 

Radical cations derived from a variety of hexadiene systems constitute an interesting family of intermediates, since they are related to the potential mechanistic extremes of the Cope rearrangement. The electrocyclic reaction of a hexadiene radical cation has three mechanistic extremes a) addition precedes cleavage (associative mechanism) b) cleavage preceeds addition (dissociative mechanism) c) addition and cleavage occur in coordinated fashion (concerted mechanism). To date, radical cations corresponding to all three mechanistic extremes have been characterized. This illustrates remarkable differences between... 

The first structure type to be established for a hexadiene radical cation was one in which cleavage is achieved without bonding, i.e. a representative of the dissociative mechanism. Dicyclopentadiene and several derivatives can be oxidized to radical cations (137) in which one of the bonds linking the monomer units is cleaved. The unique spin density distribution of 137 is reflected in an unmistakable polarization pattern [386-389]. 

Another structural possibility for a hexadiene radical cation arises, when the two allylic moieties are linked in pairwise fashion to two- or three-carbon spacers. This structure type can be approached by oxidation of molecules such as semibullvalene [391-393] or barbaralane [394]. In the resulting radical cations, the two allylic moieties are held in close proximity model considerations suggest a non-bonding C—C distance of 2.2-2.3 A, considerably closer than for the previously discussed structure type. At this distance, a moderately strong interaction of the twin moieties cannot be excluded. Accordingly, we assigned cyclic conjugated structures to radical cations derived from semibullvalene (-> 138 cf. 

In Scheme 1, the radical cations of the linear hexadienes and some cyclic isomers are contrasted. The heats of formation, AHr, as determined from the heats of formation of the species involved, as well as the heats of formation of the isomeric radical cations themselves clearly reveal the favourable stability of the cyclic isomers and/or fragment ions. Thus, instead of the linear pentadienyl cation (3), the cyclopenten-3-yl cation (2) is eventually formed during the loss of a methyl radical from ionized 1,3-hexadiene (1). Since 1,2-H+ shifts usually have low energy requirements (5-12 kcalmol-1), interconversion of the linear isomers, e.g., 4, and subsequent formation of the cyclic isomers, in particular of the ionized methylcyclopentenes 5 and 6, can take place easily on the level of the... 

For example, EPR evidence showed that cyclohexane-1,4-diyl, generated by radiolysis of hexadiene, rearranged to cyclohexene radical cation. Similarly, ant/-5-methylbicyclo[2.1.0]-pentane radical cation (33) rearranged to 1-methylcy-clopentene radical cation (34) via a 1,2-shift of the syn-5-hydrogen. ... 

Radical cations derived from 1,5-hexadiene systems illustrate major differences between the potential surfaces of radical cations and neutral precursors. On the precursor potential surface, the states of intermediate geometry are saddle points (transition structures), but pronounced minima (Fig. 6.14) on the radical cation potential surface. 

The third radical cation structure type is the cyclohexane-1,4-diyl radical cation (22 +) derived from 1,5-hexadiene. The free electron spin is shared between two carbons, which may explain the blue color of the species ( charge resonance). Four axial p and two a protons are strongly coupled (a = 1.19 mT, 6H). + ... 

In some cases, the structures of oxygenation products have been crucial for assigning the structures of unusual radical cations. Eor example, the endo-peroxides (83 and 85) support the structures assigned to radical cations (24 and 84 ) derived from l,l-diaryl-2-methylenecyclopropane (23) and 2,5-diaryl-l,5-hexadiene, respectively.Time-resolved spectroscopic data suggest that 83 is generated by coupling of triplet biradical (24 ) with (triplet) molecular oxygen. 

The fluorescence of DCA is also quenched efficiently by 2,5-diphenyl-l,5-hexadiene with a nearly diffusion-limited rate constant in MeCN (1.1 x 10lodm3 mol-1 s ), since the photoinduced electron transfer from the diene ( ° = 1.70 V vs. SCE) to DCA (E ed = 1.91 V vs. SCE) [170] is exergonic [184], The photoinduced electron transfer induces Cope rearrangement of the diene via the cyclohexane-1,4-radical cation intermediate. In... 

Radical Cations Derived From Hexadiene Systems. 221... 

The third radical cation structure type for hexadiene systems is formed by radical cation addition without fragmentation. Two hexadiene derivatives were mentioned earlier in this review, allylcyclopropene (Sect. 4.4) [245] and dicyclopropenyl (Sect. 5.3) [369], The products formed upon electron transfer from either substrate can be rationalized via an intramolecular cycloaddition reaction which is arrested after the first step (e.g. -> 133). Recent ESR observations on the parent hexadiene system indicated the formation of a cyclohexane-1,4-diyl radical cation (141). The spectrum shows six nuclei with identical couplings of 11.9G, assigned to four axial p- and two a-protons (Fig. 29) [397-399]. The free electron spin is shared between two carbons, which may explain the blue color of the sample ( charge resonance). At temperatures above 90 K, cyclohexane-1,4-diyl radical cation is converted to that of cyclohexene thus, the ESR results do not support a radical cation Cope rearrangement. 

Two of the structure types established for hexadiene-derived radical cations, viz., 137 and 141, feature two different sites, in which spin and charge are located (and stabilized). To date several structure types have been described that meet this general description the following section contains a few brief comments on some representative examples. 

In the photoreaction of 2-cyanonaphthalene with 2,3-dimethyl-2-butene in methanol, McCullough proposed that a key step of this photoreaction is a radical coupling between the radical anion of 2-cyanonaphthalene and the radical cation of 2,3-dimethyl-2-butene [170]. The zwitter ionic intermediate thus produced reacts then with methanol (Scheme 63). In the photoreaction of 9-cyanophenanthrene with 2,3-dimethyl-2-butene in methanol, Pac proposed that a nucleophile adds first to the radical cation of the alkene to produce the radical species which then reacts with the radical anion of 9-cyanophenanthrene [171]. Protonation of the anion gives the solvent-incorporated adduct (Scheme 64). In the photoreaction of l-cyano-2,3-diphenylethene with 2,5-dimethyl-2,4-hexadiene in methanol, Lewis proposed that a key step of this photoreaction is a... 

Many studies used radiation chemistry to produce the radical and radical cations and anions of various dienes in order to measure their properties. Extensive work was devoted to the radical cation of norbomadiene in order to solve the question whether it is identical with the cation radical of quadricyclane . Desrosiers and Trifunac produced radical cations of 1,4-cyclohexadiene by pulse radiolysis in several solvents and measured by time-resolved fluorescence-detected magnetic resonance the ESR spectra of the cation radical. The cation radical of 1,4-cyclohexadiene was produced by charge transfer from saturated hydrocarbon cations formed by radiolysis of the solvent. In a similar system, the radical cations of 1,3- and 1,4-cyclohexadiene were studied in a zeolite matrix and their isomerization reactions were studied. Dienyl radicals similar to many other kinds of radicals were formed by radiolysis inside an admantane matrix. Korth and coworkers used this method to create cyclooctatrienyl radicals by radiolysis of bicyclo[5.1.0]octa-2,5-diene in admantane-Di6 matrix, or of bromocyclooctatriene in the same matrix. Williams and coworkers irradiated 1,5-hexadiene in CFCI3 matrix to obtain the radical cation which was found to undergo cyclization to the cyclohexene radical cation through the intermediate cyclohexane-1,4-diyl radical cation. 

In Scheme 1, the radical cations of the linear hexadienes and some cyclic isomers are contrasted. The heats of formation, as determined from the heats of formation of... 

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