182743-61-9Relevant academic research and scientific papers
The Aminium Salt and Photoinduced Electron Transfer Initiated Diels-Alder Cycloaddition of Electron-Rich Allenes: Evidence for a Stepwise Mechanism and the Importance of Steric and Electronic Effects for the Reactivity of Distonic Radical Cation Intermediates
Schmittel, Michael,Woehrle, Clemens,Bohn, Ingo
, p. 1031 - 1040 (2007/10/03)
Several novel electron-rich arylallenes have been synthesized, characterized, and used as dienophiles in the radical-cation-catalyzed cycloaddition with 1,2,3,4,5-pentamethylcyclopentadiene, which affords, in most cases, the Diels-Alder products in 5 min at 0 deg C with a high peri-, chemo-, facial, and stereoselectivity.In line with oxidation-potential considerations it is concluded that the electron-transfer-induced reaction proceeds along a pathway by cycloaddition of the diene radical cation to a neutral allene with a rather short chain length.The low cycloaddition yields from some of the allenes are interpreted as evidence for a stepwise mechanism involving distonic radical cations as key intermediates.We discuss the tendency of the distonic radical cations to undergo ring closure to the Diels-Alder-product radical cations in terms of enthalpy considerations, which, we suggest, offer a novel criterion for the design of stepwise radical cation reactions. - Keywords: allenes; cycloadditions; Diels-Alder reactions; mechanistic studies; radical cations
