Quinodimethanes radical cations

Radical anion reactions have been treated less thoroughly than radical cations, although the same principles should apply. Bauld et al. [13] discussed the benzocyclobutene to o-quinodimethane (3 4) rearrangement because... 

A special case are the radical cations of o- and p-quinodimethane which have low-lying non-Koopmans excited states. Due to mixing between A- and B-type configurations (cf Figure 27) these states sometimes show up as additional weak bands (so-called satellite bands) in the PE spectra and thus represent rare examples of cases where the number of PE bands exceeds the number of occupied MOs in the range of tt-ionizations. In the case of o-quinodimethane and derivatives, the positions of these states were confirmed by the EA spectra of the corresponding radical cations whereas the same was not done for p-quinodimethanes. 

The interconversion of butadiene radical cations and ionized cyclobutene represents a model case for a formal pericyclic process. Much work has been invested to study not only the distinguishability of these isomers and their derivatives by mass spectrometry, but also to check the role of orbital symmetry in the ionic species. Hass has addressed the latter problem in depth in a review on pericyclic reactions in radical cations in both the gas and condensed phases and no further survey on the papers mentioned there will be given here. The topic pertains also to the ring-opening of ionized benzocyclobutene to ionized ortho-quinodimethane (cf Section V) and various otha- phenyl-, methyl- and carboxy-substituted derivatives. In this context, we restrict ourselves hwe mentioning that an upper limit of 7 kcalmol only has been detemined by CE mass spectrometry for the activation barrier of the cycloreversion of the parent cyclobutene radical cations. The energy requirement for the cycloreversion of ionized 1- and 3-substituted cyclobutenes were found, by experiment, to be markedly different. Obviously, dissociation of the (in a sense bis-allylic) strained C—C bond is much more facile when the substituent is at C-3,... 

For electrodes based on conducting organic charge-transfer salts such as TTF + TCNQ (a complex of the radical cation of tetrathiafulvalene and the radical anion tetracyano-p-quinodimethane) or NMP +TCNQ " (N-methylphenaziniumtetracyano-p-quinodimethane), direct [155, 169] and mediated [154] electron transfer mechanisms have been described. In analogy with the theory of outer-sphere electron transfer [170], Kulys and co-workers [118,171] have developed a mathematical model which permits to evaluate the depth of the active site of some oxidoreductases from the steric requirements of inorganic redox couples (Table 14-4). 

Chemical oxidation of the TTF groups in compounds 34 and 35 has been achieved by reaction with an excess of iodine in dichloromethane solution, leading to new low-energy absorptions in the UV/visible spectra which are diagnostic of TTF cation radicals the broad absorption at = 830 nm for the iodide salt of 35 suggests the formation of aggregated TTF species. A charge transfer complex formed by 35 and tetracyano-p-quinodimethane (TCNQ) has been isolated as an insoluble black powder. The stoichiometry is (35), (TCNQ)3 (i.e. 8 TTF units 3... 

Takahashi and co-authors (1996) described another case of cation radical cycloreversion. Benzocyclobutenols undergo ring opening induced by electron transfer to generate quinodimethane intermediates, which then tautomerize to benzenoids. The reaction proceeds upon irradiation in the presence of tetracyanoanthracene (X > 350 nm). Yields (based on PMR analyses) are quantitative (Scheme 6-42). 

L. R. Melby, Substituted quinodimethans VIII. Salts derived from the 7,7,8,8-tetracyanoquinodimethan anion-radical and benzologues of quaternary pyrazinium cations, Can J. Chem. 43 1448-1453 (1965). 

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