Electron transfer processes cycloadducts

DP) and leads (Equation 4.5) to the formation of an anthracene cation radical as a result of the single-electron transfer process. The resulting ion-radical pair [AN, DP is the critical intermediate that subsequently evolves to cycloadduct (AD). 

A patent has been lodged dealing with the photochemical synthesis of the thiolactones (82) from the thioacids (83). The cycloadduct (84) is the sole product from the irradiation of 1,1-diethoxyethene with biacetyl in non-polar solvents. The authors suggest that an electron transfer process is operative and that the cycloaddition therefore proceeds via the zwitterion (85). A study of chiral induction in the photochemical synthesis of oxetanes using the chiral... 

Akasaka et al. reported that the photoexcited Ceo generates [2 + 3] cycloadducts with the disiUranes, such as 1,1,2,2-tetramesityl-l,2-disilirane via the electron-transfer process (Scheme 2) [61]. It is interesting to note that the adduct formation was not observed by the thermal reactions in the dark. The photoadduct formation occurs in the nonpolar toluene, suggesting that the exciplex is a plausible intermediate. When the reaction proceeds in benzonitrile, the generated radical cation of the disiUrane forms 1 1 or 1 2 adducts with benzonitrile in this case, Cgg acts as a photocatalyst... 

In the presence of oxygen or in air-saturated solutions, 11 is the major product (Scheme 6.9) [56, 59]. The formation of the cycloadduct follows a multistep process that probably includes several inter- or intramolecular electron-transfer steps. Suggestions include the electron transfer to singlet oxygen as an important part of the mechanism [58]. One possible mechanism is shown in Scheme 6.9. Oxygen plays the role of both an electron- and a proton-acceptor. 

The coupling of the ion pair to form the cycloadduct is a very fast process as judged by the ultrashort lifetimes found for various arene ion radicals (t a 20 ps, see inset in Figure 10). However, the low quantum efficiencies (3>c < 0.01) [114, 161] of the cycloadditions indicate that the predominant decay pathway of the ion radical pair in Eq. 34 is not the coupling step kc), but back electron transfer (k-Ex) to restore the original EDA complex. Quantitative evaluation of kinetics and quantum efficiencies yields the relatively fast cycloaddition rate constants of kc 10 s characteristic for highly exergonic bond formation. 

Photochemical addition reactions may also occur as electron-transfer reactions involving a radical ion pair. An illustrative example is the photochemical reaction of 9-cyanophenanthrene (154) with 2,3-dimethyl-2-butene, which, in nonpolar solvents, gives good yields of a [2 + 2] cycloadduct via a singlet exciplex, while in polar solvents radical ions are formed in the primary photochemical process. The olefin radical cation then undergoes deprotonation to yield an allyl radical or suffers nucleophilic attack by the solvent to produce a methoxy alkyl radical. Coupling of these radicals with... 

The toluene-soluble fraction consists of a major product that has been identified as a CgQ-TEA monocycloadduct 7V-ethyl-rra/w-2, 5 -dimethyl-pyrrol-idino[3, 4 l,2][60]fullerene (14) by use of matrix-assisted laser desorption ionization mass spectroscopy and NMR methods [71]. It is interesting that the photochemical reaction actually results in the formation of a cycloadduct. This is unique to the fuilerene system because there have been no reports of cycloadducts in reactions involving nonfullerene acceptors [119-122], such as rra/is-stilbene. In the context of the classical photoinduced electron transfer-proton transfer mechanism [124], a two-step process for the formation of the cycloadduct has been proposed [71]. 

The cycloaddition of l,l-bis(2-thienyl)ethylene (prepared in situ from the ethanol (169) with strong electron acceptors such as TCNE or ddq has been shown to proceed via a radical ion pair formed by electron transfer (Scheme 29) <90H(3i)i873>. The reaction with TCNE is rapid and quantitative, taking place at room temperature in 15 min. With ddq, the initial product (170) is further oxidized to (171). When the ethanol (169) is heated alone in the dark at 150°C, it generates the aromatized cycloadduct (173) in 70% yield. Other minor products possibly result from a radical process <91CB1203>. On the other hand, irradiation of the alcohol (169) generates l,l-bis(2-thienyl)ethylene cleanly, which is subsequently transformed to the cycloadduct (172). A radical cation may be implicated in this photochemical [4 + 2] cycloaddition also. 

Biradical reversion appears to be a much more important process in enone additions to electron-deficient alkenes. Thus, Schuster and co-workers found that, in the reaction of 3-methylcyclohexenone (3-MCH) with Z- and E-l,2-dicyanoethene (maleo- and fumaronitrile), isomerization of the alkenes accompanies formation of cycloadducts. Based upon quantum yields for aU processes and the rate constants for quenching of the enone triplet by these alkenes, Schuster et al. concluded that alkene isomerization occurred by reversion of 1,4-biradical intermediates (i.e., a Schenck-type mechanism) rather than by triplet-triplet energy transfer from the enone to the alkenes, although the latter was a distinct possibility due to the relatively low triplet energies of these particular alkenes.The full significance of biradical reversion as a critical factor in affecting the course of enone photocycloadditions is discussed below. 

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