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[摘要]：Electron transfer catalysis is an effective method for the acceleration of Diels-Alder reactions between two substrates of similar electron d. The dependence of the selectivity of the Diels-Alder reaction between (R)-a-phellandrene and 4-methoxystyrene catalyzed by photoinduced electron transfer with tris(4-methoxyphenyl) pyrylium tetrafluoroborate is studied. Despite the fact that the radical ions involved are highly reactive species, complete regioselectivity favoring attack on the more highly substituted double bond is obsd. The endo/exo selectivity and the periselectivity between [4 + 2] and [2 + 2] cycloaddn. is found to be solvent-dependent. Stereochem. anal. showed that the periselectivity is correlated with the facial selectivity, with attack trans to the iso-Pr group leading to the [4 + 2] product and cis attack leading to the formation of the [2 + 2] product. A good correlation between the dielec. const. of the solvent and the endo/exo ratio is found, but more polar solvents lead to lower periselectivity. The effect of reactant and catalyst concns. is found to be smaller. These results are rationalized in the context of the relative stability of the ion-mol. complexes and the singly linked intermediate of the reaction. |
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