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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
Author(s) -
Schmittel Michael,
Wöhrle Clemens,
Bohn Ingo
Publication year - 1996
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.19960020820
Subject(s) - cycloaddition , chemistry , allene , steric effects , radical ion , reactivity (psychology) , photochemistry , diels–alder reaction , photoinduced electron transfer , electron transfer , stepwise reaction , medicinal chemistry , stereochemistry , organic chemistry , catalysis , kinetics , ion , reaction rate constant , medicine , physics , alternative medicine , pathology , quantum mechanics , order of reaction
Several novel electron‐rich aryl‐allenes 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°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 [3 + 2] 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.