ISSN:
0947-6539
Keywords:
allenes
;
cycloadditions
;
Diels-Alder reactions
;
mechanistic studies
;
radical cations
;
Chemistry
;
General Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
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.
Additional Material:
5 Tab.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/chem.1460020818
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