Radical cation reactions cycloreversion

The electron transfer induced reaction of this diene system results in rapid [4 + 2]dimerization conversely, the dimer rapidly undergoes cycloreversion upon electron transfer. Both reactions result in strong CIDNP effects. The monomer polarization supports a radical cation with a spin density distribution like those of the butadiene or fulvene radical cations. The dimer polarization identifies a dimer radical cation with appreciable spin density only on two carbons of the dienophile fragment this species can only be the doubly linked radical cation D [135, 136], Significantly, a second dimer radical cation is implicated in a pulsed... 

Among the electron transfer induced reactions of cyclobutane systems, cycloreversions are the most prominent. These reactions are the reverse of the cycloadditions discussed in Sect. 4.1. The reactivity of the corresponding radical cations depends on their substitution pattern. We have mentioned the fast two-bond cycloreversion of quadicyclane radical cation as well as the ready ring closure of a tetracyclic system (3, Sect. 4.1). A related fragmentation of cis-, trans-, cis-1,2,3,4-tetraphenylcyclobutane (84) can be induced by pulse radiolysis of 1,2-dichloro-ethane solutions. This reaction produces the known spectrum of trans-stilbene radical cation (85) without a detectable intermediate and with a high degree of... 

Whereas a [2 + 2] pericyclic reaction is essentially forbidden in the ground state, a [2+1] open-shell reaction is feasible. In this respect, the radical cations detected in this context represent distinct stages of pericyclic, radical-cation catalyzed cycloaddi-tions/cycloreversions. In Fig. 7.11, three distinct stages, a tight (cyclobutane-like), an extended (bis ethene), and a trapezoid, of a hole- (or radical-cation) catalyzed cycloaddition/cycloreversion are presented in a schematic way. °... 

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

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,... 

The sensitizer dependency for the cycloreversion of tra/7, raf7 -2,3-diphenyl-4-methyloxetane has been studied. When chloranil is used as the sensitizer, the reaction proceeds via the radical cation of tr<3f7 -(3-methylstyrene, while with pyrylium salts the trans- iiXhQxvQ radical cation is involved.Other work in this area has examined the cycloreversion of the oxetanes (34) using (35) or chloranil as the sensitizers. ... 

Interpretation of the results from the product studies and nanosecond laser photolysis experiments led to the rate constants for each step in the overall dimerization reaction summarized in Scheme 4. The addition step is quite rapid, taking place with a rate constant of 1.5 x 10 M" s" , but formation of the cyclobutane radical cation is reversible, with a calculated rate constant of 8 x 10 s" for cycloreversion to regenerate the 4-methoxystyrene radical cation and neutral 4-methoxystyrene. The two other processes available to the intermediate cyclobutane radical cation are rearrangement to the hexalriene radical cation with a first-order rate constant of 2.5 x 10 s". and electron transfer with neutral 4-methoxy- styrene with a rate constant of 1.5 X 10 M s to generate the neutral cyelobutane and to regenerate the 4-methoxystyrene radical cation. 

Cycloreversion Reactions A cycloreversion reaction is the reverse of a cycloaddition reaction and leads to the formation of the starting reactants through the cleavage of two bonds in the ring [18], A typical example is the formation of C2H4+ and neutral C2H4 from the cyclobutane radical cation. As shown in reaction (6.37), this reaction proceeds through the intermediacy of a distonic ion. The radical cations of a variety of other four-membered cyclic compounds, such as cyclobutanones (3), diketene (4), oxetane (5), cyclobutylamine (6), and thiocyclobutane (7), are known to participate in cycloreversion reactions [27]. 

The cycloaddition of two ethylenes or the cycloreversion of cyclobutane is one of the textbook examples used in the illustration of the Woodward-Hoffman rules [20] of orbital symmetry. Studies on the cyclobutane radical cation [21,22] showed a low activation energy for the cycloaddition of an ethylene radical cation to ethylene, in remarkable contrast with the high activation energy for the corresponding neutral reaction [23]. The dissociation reaction of cyclobutane radical cation is endothermic. Although there is a cyclobutane ring in the pyrimidine dimer, its electronic structure is likely to be different from cyclobutane itself, because of the presence of the two pyrimidine rings. 

The cycloreversion reaction of the cyclobutane radical cation is highly endothermic at any level of the calculation [22], A transition state was determined between the cyclobutane radical cation (cB +) and the 7t complex cation intermediate, although no transition state was found between the... 

K complex cation intermediate and the infinite system (ethylene + ethylene( +)) [22]. In contrast, the cycloreversion reaction of the thymine dimer radical cation is exothermic at the CASSCF level of theory as shown in Table 6. Compared to the cB + case, the n conjugation in thymine is responsible for the greater stabilization of the monomer cation, thus making the reaction exothermic. To estimate the dynamic electron correlation effects, we carried out preliminary MP2 calculations for TTp-1 and T( +) T. The exothermicity was reduced to 5.0 kcal/mol. Since the number of the bonds in TTp-1 is larger than that in T( +) T, TTp-1 is much more stabilized by the dynamical electron correlation effects than T( +) T is. 

Takahashi et al. (1996) described another case of cation-radical cycloreversion. Benzocyclobu-tenols undergo ring opening induced by electron transfer to generate quinodimethide intermediates, which then tautomerize to benzophenones. The reaction proceeds on photoirradiation in the presence of tetracyanoanthracene (X > 350 nm). Yields (based on NMR (proton) analyses) are quantitative... 

Many rDA reactions are carried out at temperatures of 150 C or more in solution phase and often at temperatures of 400-600 C using the flash vapor pyrolysis (FVP) method individual conditions are referenced throughout the text. However, an accelerating effect by anionic, cationic and radical substimtion on either the dienophile or at the termini of the diene fragments has been predicted by Carpenter.Experimentally, this prediction has been substantiated only for anionic substitution. In 1967, Hart reported what is likely the first example of an oxyanion-accelerated rDA reaction. Both oxyanionic " and car-banionic substituents accelerate the cycloreversion reaction such that they proceed rapidly at room temperature (for example, equation 3). In addition, acid-catalyzed rDA reactions have been reported in which protonation effectively makes the dienophile fragment of the adduct more electron deficient. Grieco has utilized a room temperature retro aza DA reactitm useful for the N-methylation of dipeptides and amino acid derivatives (equatitm 4). ... 

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