Rearrangements radical cation

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. 

The radical cations of fulvene systems are of interest, because steric and electronic factors might favor a perpendicular structure and because the energy difference between the respective cis and trans isomers are expected to be small. However, the chloranil photosensitized reaction resulted in CIDNP effects, indicating planar or slightly twisted structures. The Z- and E-2-tert-butyl-6-(dimethylamino)fulvene [20, R = — N(CH3)2] radical cations rearrange readily whereas di-/er/-butylfulvene [20, R = — C(CH3)3] showed no interconversion under comparable experimental conditions [160]. 

The barriers to radical cation rearrangements may be substantially reduced, and their reactions greatly accelerated this feature appears to be generally recognized (Sect. 4.4)... 

Oxgaard, J., Wiest, O. The Vinylcyclopropane Radical Cation Rearrangement and Related Reactions on the C5H8.bul.+ Hypersurface. J. Am. Chem. Soc. 1999,121,11531-11537. 

Oxgaard, J., Wiest, O. Substituent effects in the vinylcyclopropane radical cation rearrangement A computational road to a new synthetic tool. Eur. J. Org. Chem. 2003,1454-1462. 

Example 6 the toluene-cycloheptatriene radical cation rearrangement... 

Clark, T., The quadricyclane to norbomadiene radical cation rearrangement an ab initio and density functional study, Acta Chem. Scand., 51, 646-652, 1997. 

While these types of reactions are synthetically useful, the understanding of the reaction mechanism is still controversial. Thus, a clear mechanistic distinction between radical cation rearrangements and rearrangements promoted by acid catalysis, presumably generated during ET processes, for the above transformations is often very difficult to make. ... 

Radical cations generated in a mass spectrometer from aldehydes and ketones with y hydrogens undergo a rearrangement in which a y hydrogen is first transferred and a carbon-carbon bond is then cleaved, e.g. 

Fig. 8.9 Possible mechanisms of the bioluminescence reaction of dinoflagellate luciferin, based on the results of the model study (Stojanovic and Kishi, 1994b Stojanovic, 1995). The luciferin might react with molecular oxygen to form the luciferin radical cation and superoxide radical anion (A), and the latter deproto-nates the radical cation at C.132 to form (B). The collapse of the radical pair might yield the excited state of the peroxide (C). Alternatively, luciferin might be directly oxygenated to give C, and C rearranges to give the excited state of the hydrate (D) by the CIEEL mechanism. Both C and D can be the light emitter.

Diaminobiphenyl is formed by a completely different mechanism, though the details are not known. There is rate-determining breaking of the N—N bond, but the C—C bond is not formed during this step. The formation of the o-semidine also takes place by a nonconcerted pathway. Under certain conditions, benzidine rearrangements have been found to go through radical cations. 

Fortunately, for this solvent, the electron-capture centres give very broad e.s.r. features at 77 K, and hence the spectra for S + cations are readily distinguished. We know of no instance in which S + cations are not formed provided the ionization potential of S is less than that of the solvent. There are two complicating factors, one is unimolecular break-down or rearrangement of the radical cations, and the other is weak complexation with a solvent molecule. The latter is readily detected because specific interaction with one chlorine or one fluorine nucleus occurs, and the resulting hyperfine features are usually well-defined. 

Although the Capdevielle reaction for one-pot conversion of aldehydes to nitriles is a very convenient and widely applicable synthetic procedure, 3-substituted furoxans appear to be susceptible to rearrangement when substitutions with amine nucleophiles are attempted, even under relatively mild conditions (Scheme 29) <1999JOC8748>. The formation of the final product 107 in this reaction was explained via phenyl abstraction by carbamoyl radical cation from the second molecule of intermediate product 106 < 1999JOC8748>. 

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