Approaching a possible stepwise/concerted mechanistic crossover point in the cation radical cycloadditions of cis- and trans-anethole

The Diels-Alder cycloadditions of the cis- and trans-anethole cation radica ls to cyclopenta-1,3-diene have carefully been examined and are found to pr oduce sharply different stereochemical results. The cis-anethole cation rad ical adds via a distinctly stepwise mechanism, yielding comparable amounts of all four diastereoisomeric Diels-Alder adducts. In contrast, the trans-a nethole cation radical yields only trans adducts, with the endo isomer pred ominating. None of the cis adducts could be detected, and the percentage of cis adducts formed, if any, is significantly less than 0.1%, This result s tands in contrast to the additions of aryl cis- and trans-propenyl ethers t o the same diene, in which both geometric alkene isomers yield all four add ucts in comparable amounts. Consequently, if trans-anethole also reacts by a stepwise mechanism, the rate of cyclization in the intermediate distonic cation radical must be of the order of 1000 times the rate of bond rotation . Alternatively, although it is less likely, the trans cation radical could react Ilia a concerted reaction path.