Reactions of Some Cyclopropanes Activated by a Spiro-Linked Fluorene. Importance of Electronic Matching of a Reacting Partner in Thermal Cycloaddition with TCNE and Ring-Opening Dehydrogenation with DDQ

Article abstract of DOI:10.1021/jo00277a023

Cyclopropanes 1a-c substituted both by a fluorene group, linked in a spiro fashion, and by gem-dialkyl groups were found to be highly reactive in the reaction with a certain strongly electron-demanding unsaturated compound, such as TCNE and DDQ.Namely, 1 reacted with TCNE in the dark to give a 2 + ?²> type cycloadduct 5, whereas 1 was dehydrogenated by DDQ to afford 4 when one of the alkyl substituents is a methyl.The resulting 4 reacted with another molecule of DDQ to afford 10 ultimately.In contrast to 1, the corresponding diphenyl derivatives 2a-c were reluctant to undergo these reactions.Moreover, chloranil was unable to dehydrogenate spiro-activated 1 under similar conditions.It is thus concluded that the dark reaction occurs when the reagent and reactant match electronically in a donor-acceptor sense.Under the illumination of visible light, however, the same type of reactions was found to take place even with a mismatched pair.Namely, the reactions of 2a with TCNE, 2b or 2c with DDQ, and 1b, 1c, 2b, or 2c with chloranil were all brought about when the mixture was irradiated by a halogen lamp.In the chloranil dehydration, the expected diene remained intact in the product mixture.All these results suggest that the TCNE cycloaddition as well as the quinone dehydrogenation are initiated by a single-electron transfer (SET).The effect of alkyl substituents in 1 on the rate (reactivity sequence: 1a > 1b > 1c > 8) and the solvent polarity effect on the rate are indicative of the rate-controlling production of a radical ion pair, which would then react to give ultimate products.Possible routes for the observed products are proposed (Scheme I) and discussed.