Radical cations 1,1-diphenylethylene

Diphenylethylene radical cation forms a dimeric product (90) via a 1,4-bifunctional-distonic intermediate (89 " ") that undergoes an alternative (1,6-) cycli-zation with participation of one phenyl group. 

Since the oxidation potential of 1,1-diphenylethylene is +1.8 V in acetonitrile formation of its radical cation should be thermodynamically permissible on... 

Addition to alkenes can be sensitized by both electron-donors and electron-acceptors, and it is most likely that the reactive species is the alkene radical anion or the alkene radical cation, respectively. 1,1-Diphenylethylene can be converted to the Markownikov addition product with methanol (2.48) using the electron-donating sensitizer t-methoxynaphthalene no added proton acid is needed. Using... 

Other olefinic substrates known to dimerize through photo-induced electron transfer sensitization include enamines (72), diarylethylenes (73-75), vinyl ethers (76), styrenes (77,78), and phenyl acetylenes (79). Alternate ring closures (besides cyclobutanes) are sometimes observed, probably via 1,4-radical cationic intermediates. For example, a tetrahydronaphthalene is formed from the radical cation of 1,1-diphenylethylene, eq. 

A particularly graphic example of the divergent chemical reactivity of common radical cationic intermediates generated under different experimental conditions is found in the chemistry of diphenylethylene. With this system, eqn 5,... 

A similar radical cation chain mechanism was suggested for the formation of 3,3,6,6-tetraphenyl-l,2-dioxane (16) upon chemical or photoinduced one-electron oxidation of 1,1-diphenylethylene [155, 156]. 

Alternatively, Lewis acids such as SbCl5 may initiate oligomerization directly by electron transfer from extremely reactive alkenes such as 1,1-diphenylethylene and 1,1 -di(p-methoxyphenyl)ethylene [28,143,144]. The dimeric tail-to-tail carbenium ion of 1,1-diphenylethylene shown in Eq. (32) was observed, and its formation explained by a radical cation intermediate. Because 1,1-diarylethylenes can not polymerize, only oligomerization was observed. 

The reaction of phenanthrene and unsaturated compounds, such as furan, 1,1-diphenylethylene and indene, in the presence of electron acceptors and a nucleophile leads to products incorporating the nucleophile (Majima et ai, 1981). Thus furan gives [96], Excitation generates the radical cation of phenanthrene (Phent) and the radical anion of 1,4-dicyanobenzene. Phent oxidises furan to its radical cation which reacts in the manner as shown in Scheme 16. 

Solution-phase studies are more important preparatively. Two main mechanisms seem to operate in solution. The first is attack of the radical cation of a heteroaromatic donor on a tz nucleophile, as happens in the arylation reactions reported above. Other examples include photochemical reactions in which the heterocycle participates as a donor—for example the formation of 2- and 3-(l,2-diphenylethyl)-pyrroles (yield 44 and 10 %, respectively) from the irradiation of ( )-stilbene in the presence of pyrrole, a reaction which evidence implies is initiated by SET from pyrrole [88]. 2-(2, 2 -Diphenylethyl)furans are cleanly formed on irradiation of the corresponding furans in the presence of 1,1-diphenylethylene and an electron-accepting sensitizer [89]. Likewise, irradiation of naphthalene and benzothiophene in the presence of pyrrole results in electron transfer from the latter and leads eventually to pyrrolyldihydronaphthalene or benzothiophene, 44, respectively (Scheme 29) [90]. 

It was discovered several decades ago that spontaneous thermal electron transfer from encapsulated aromatic organics to the zeolite framework is possible. Although the exact location of the electron on the framework was not determined, dehydration of the zeolite, the zeolite topology and the nature of the co-cations in the zeolite were found to be important in generating the radical species. NH4-Y zeolite rather than Na-Y was necessary for formation of radical cations of 1,1-diphenylethylene, triphenylamine, quinoline, perylene, aniline and p-phenylenediamine [126]. Recent studies have shown that stable, radical cations of a,(U-diphenylpolyenes can be formed thermally on activated Na ZSM-5 [127]... 

Besides cyclobutane formation, alternative ring closures are sometimes observed. One example is the 9,10-dicyanoanthracene sensitized dimerization of 1,1-diphenylethylene (169). The six-membered ring is formed via the 1,4-radical cation, which results from the addition of the free radical cation to diphenylethylene as indicated in Scheme 56, while the 1,4-biradical generated by back electron transfer from the radical ion pair yields tetraphen-ylcyclobutane (170) (Mattes and Farid, 1983). 

Under conditions that are not strictly nonaqueous, the oxidized dimer may be trapped by water, as was observed during the oxidation of 1,1-diphenylethylene catalyzed by the radical cation of dibenzo-1,4-dioxin [91]. The dimer dication upon reaction with water undergoes a 1,2-phenyl shift, resulting finally in 1,2,4,4-tetraphenyl-3-buten-l-one [Eq. (42)], reminiscent of the 1,2-shifts observed during anodic oxidation of 1-phenyl- and 1,4-diphenylnaphthalene in acidic dime thy Iformamide (DMF) [92]. 

Although the mechanism shown in Scheme 2 was satisfactory to explain the radical cation mediated dimerization of a variety of arylalkenes and vinyl ethers, other studies provided mechanistic evidence for additional reaction pathways. For example, early studies reported the formation of a [2 + 4] dimer, 1,1,4-triphenyl-1,2,3,4-tetrahydronaphthalene, in the ET-sensitized dimerization of 1,1-diphenylethylene and postulated a mechanism involving 1,6-cyclization of an initial 1,4-acyclic radical cation. Later work demonstrated that dimerization of this alkene... 

A variation of the reaction involved the use of the alkene itself as nucleophile. In this case, a radical cation dimer was formed by attack of the alkene radical cation by the neutral alkene, forming a distonic radical cation (Scheme 14.9, left part). With a-methylstyrene (17) as the alkene, a cychzation took place and the neutral radical resulting from the ensuing deprotonation coupled with the radical anion of the acceptor (in this case TCB), leading to the NOCAS adduct 18 as a diastereo-isomeric mixture in overall 90% yield [55]. The irradiation of aromatic nitriles in the presence of aUcenes may lead to different products, particularly when carried out in an apolar medium. As an example, 1,4-dicyanobenzene gave isoquinohnes by a [4-1-2]-cycloaddition with a cyano group through irradiation in the presence of diphenylethylenes in benzene via a polar exciplex [56]. 

The intramolecular coupling reaction does not occur when diphenylethylenes, that is, stilbene and its methyl derivatives, are electrolyzed under the same conditions (Stuart and Ohnesorge 1971). This difference in the anodic behavior of these substances was attributed to the low stability of the cation-radicals of stilbene and its methyl derivatives in comparison to the cation-radicals of tetraphenylethylene. The participation of the cation-radicals in the cyclization of tetraphenylethylene has been unequivocally proved (Svanholm et al. 1974, Steckhan 1977). 

If so, one may expect products to result from chemical bond formation between the cation-radical-anion-radical pair, which are both paramagnetic and of opposite charge. In the latter route, there is a precedent for the formation of dioxetane intermediates of stable olefin cation radicals [51], as in the characterization by Nelsen and coworkers of a dioxetane cation radical from adamantylidene cation radical [52]. If a dioxetane is formed, either in neutral form or as a cation radical, the Ti02 surface can function in an additional role, that is, as a Lewis acid catalyst, to induce decomposition of the dioxetane. Since no chemiluminescence could be observed in these reactions, apparently Lewis acid catalysis provides a nonradiative route for cleavage of this high-energy intermediate. That Ti02 can indeed function in this way can be demonstrated by independent synthesis of the dioxetane derived from 1,1-diphenylethylene, which does indeed decompose to benzophenone when it is stirred in the dark on titanium dioxide. 

Tetraphenylethylene is coupled to 9,10-diphenylphenanthrene (Table 7, number 3). The corresponding coupling with tri- and diphenylethylene was unsuccessful. It was proposed that the initially formed cation radical disproportionates to a dication that couples to the observed product [138]. 

One of the simplest routes to benzyne starts with aniline which is converted to benzenediazonium ion. Benzyne is liberated when this ion is deprotonated by acetate base in solvents such as benzene, Eq. (20). Detracting from this approach, however, are competing reactions in which phenyl radicals and phenyl cations are formed. Presently it is possible to minimize the importance of radical chain side reactions and thereby to favor benzyne formation by adding a radical trapping agent such as 1,1-diphenylethylene. 8S ... 

Fio. 15. 1,1-Diphenylethylene vapor adsorbed in vacuo. Curve 1 on dry silica-alumina catalyst curve 2 on a hydrated silica-alumina. Transmission spectra, from Webb (92). Band 1 is that of the carbonium ion, band 2 is that of the cation radical. 

The two alternatives relative to the interpretation of the 600 band have been resolved in favor of the olefin cation-radical (PhjO=CM8)t, when recently, after many attempts, an EPR signal has been detected for chemisorbed diphenylethylene (89). The depressing action of additionally adsorbed oxygen disproved any suspicion on its active role in producing the signal. 

A second criterion for assignment of the spectral band at 6050 A to a cation-radical resides in demonstrating that the corresponding anion-radical, having a similar spectrum, can be formed from this olefin. Anion-radicals of a number of diaryl-, and tetraarylolefins have been reported from the reaction of these hydrocarbons with alkali metals in diglyme or in tetrahydrofuran (119) however, these studies did not include 1,1-diphenylethylene. In order to obtain further confirmation of their... 

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