Diels-Alder reactions, electron-transfer sensitized

Cycloadditions only proceeding after electron transfer activation via the radical cation of one partner are illustrated by the final examples. According to K. Mizono various bis-enolethers tethered by long chains (polyether or alkyl) can be cyclisized to bicyclic cyclobutanes using electron transfer sensitizer like dicyanonaphthalene or dicyano-anthracene. Note that this type of dimerization starting from enol ethers are not possible under triplet sensitization or by direct irradiation. Only the intramolecular cyclization ci the silane-bridged 2>. s-styrene can be carried out under direct photolysis. E. Steckhan made use of this procedure to perform an intermolecular [4+2] cycloaddition of indole to a chiral 1,3-cyclohexadiene. He has used successfully the sensitizer triphenylpyrylium salt in many examples. Here, the reaction follows a general course which has been developed Bauld and which may be called "hole catalyzed Diels-Alder reaction". 

Wiest and co-workers have demonstrated that indole will participate in electron-transfer sensitized Diels-Alder reactions [14c, 41]. Irradiation of the electron-transfer sensitizer triphenylpyrylium tetrafluoroborate 21 in a methylene chloride solution containing indole, 1,3-cyclohexadiene and acetyl chloride yields the adduct 22 as an exo and endo mixture in a 1 3.3 ratio and a combined yield of 70% (Scheme 9) [14c,41a], Experiments are reported [14c, 41a] which demonstrate that the reaction proceeds by electron transfer from the indole to the excited sensitizer the indole radical cation produced then attacks the diene to give an intermediate 23 (Ri = = H) which, in conjunction with back-electron transfer from the... 

Indole has been shown to undergo a Diels-Alder reaction with both endo- and exo-cyclic 1,3-dienes, in which electron transfer catalysis is provided using a triarylpyrylium cation as a sensitizer (Equation (83)) <90SL275,9UOC1405,93TL639l>. The adducts are generally mixtures of isomers, with endo predominating in the case of cyclohexadienes. The indole radical cation is postulated as an intermediate. 

Five-ring naphthenes are more toxic than 6-rings, for example, cyclopentane is particularly toxic, probably because it is a precursor for the conversion of cyclopentadiene into polyaromatics by Diels-Alder chemistry. This reaction appears to have similar structure-sensitivity to hydrogenolysis and sulphiding.52 Barbier considers that there is electron transfer from Pt —> S. The composition of coke appears to be independent of feed composition.53... 

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