The Structure-Reactivity Relationship of Ion Pairs In Photoinduced Electron-Transfer

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We previously found that the photoinduced electron-transfer (PET) reaction of exo, cis-1,4-diary1-2,3-dimethylbicyclo[2.2.0]hexane was strongly sensitive to solvent polarity and the nature of cyanoaromatic sensitizers. "Lose" ion-radical pairs such as SSIP and FI favored a concerted cycloreversion to give E,E-1,8-dimethy1-3,6-diary-2,6-octadiene while E,Z-1,8-dimethy1-3,6-diary1-2,6-octadiene was formed predominantly from a "tight" ion-radical pair such CIP in a stepwise manner. For the generalization of the relationship between a structure of an ion-radical pair and products/mechanisms, we investigated the PET reactions of 2,2-diarylspiropentanes, which competitively afforded 1-(diarylmethylene) cyclobutane(I) and 2,2-diary1-1methylene-cyclobutane (II) in a concerted and stepwise mechanism, respectively. The ratio of [I : II] increased with an increse in solvent polarity and a decrease in negative charge density of anion radicals of cyanoaromatic sensitizers. Those facts are compatible with the facts observed in the PET reactions of exo, cis-1,4-diaryl1-2,3-dimethylbicyclo[2.2.0]hexane, showing that a structure of an ion-radical pairs significantly changes a reaction mechanism and then products.