Electron Transfer Initiated Diels-Alder Reactions

5 Electron Transfer Initiated Diels-Alder Reactions 

Furthermore, Fiirmeier and Metzger used the same procedure to study the electron transfer-initiated Diels-Alder reaction of isoprene and anethole, as well as the Diels-Alder dimerization of 1,3-cyclohexadiene. They unambiguously detected and characterized the respective transient radical cations of the dienophiles and of the Diels-Alder addition products. 

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Ftirmeier, S., Metzger, J.O. (2004) Detection of Transient Radical Cations in Electron Transfer-initiated Diels-Alder Reactions by Electrospray Ionization Mass Spectrometry. J. Am. Chem. Soc. 126 14485-14492. 

It is important to note that the reactions are fundamentally different from similar radical cation Diels-Alder reactions initiated with the use of a photochemical electron-transfer reaction [35, 36]. In photochemical reactions, a one-electron oxidation of the substrate leads to a cycloaddition that is then terminated by a back electron transfer . No net change is made in the oxidation state of the substrate. However, the reaction outlined in Scheme 13 involves a net two-electron oxidation of the substrate. Hence, the two pathways are complementary. 

Careful product and kinetic studies for selected electron-deficient alkenes, electron-rich dienes and vinyl-substituted aromatic systems have provided some clarification of the [2 + 2] versus [2 + 2] cycloaddition issue. The thermodynamically favored product can often be anticipated on structural grounds. Reactions of TCNE with vinyl-substituted benzenoid aromatics, protoporphorins or heteroaromaticsgive [2 + 2] products, but for some styrenes the [2 + 4] addition may be kinetically favored. p-Methoxystyrene and TCNE react to form a charge-transfer complex which leads reversibly to the Diels-Alder product, and eventually to the finally isolated [2 + 2] adduct. An isomer of di-cyclopentadiene shows the same pattern, with the initially formed Diels-Alder adduct giving rise to a [2 + 2] adduct. 

The data for the reactions of four substituted styrene radical cations with selected dienes are summarized in Table 8. - As discussed above for the reaction of styrene radical cations with nucleophiles, the interpretation of these data is complicated by the possibility that two competing reactions are responsible for the observed quenching of the radical cation. One of these is electron transfer from the alkene to the styrene radical cation to generate the neutral styrene and the radical cation of the alkene (Eq. 29). In this case, the quenching rate constant is that for electron transfer, and does not provide any information on the kinetics for the initial addition, although the secondary radical calion/neutral pair may in some cases lead to adduct formation. The other reaction is addition of the alkene to the radical cation to generate an adduct radical cation that is the precursor of the final cyclobutanation and Diels-Alder products (Eq. 30). 

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