Radical Cation [4+2] Cycloaddition of Non‐Conjugated Tetrasubstituted Alkenes by an FeCl₃/AgSbF₆ Co‐Initiator

Abstract

Radical cation [4+2] cycloaddition is an alternative strategy for constructing various six‐membered rings that cannot be easily accessed by thermal [4+2] cycloaddition. Here, we developed an FeCl3/AgSbF6 co‐initiator to promote radical cation [4+2] cycloaddition of non‐conjugated tetrasubstituted alkenes with 2,3‐dimethyl‐1,3‐butadiene. In the presence of 10 mol% of FeCl3 and 30 mol% of AgSbF6, the reaction proceeded smoothly in MeCN to provide the cycloadducts from tetrasubstituted alkenes having an electron‐rich aromatic group, which improved the yield. We demonstrate the efficiency of an FeCl3/AgSbF6 co‐initiator by comparing our finding to the results with previously reported iron(III) initiators in an investigation of the substrate scope. In addition, a kinetic study was conducted to elucidate the detailed reaction mechanism, in which the rate‐determining step can be facilitated by intramolecular single electron transfer.

Recent advances in radical-mediated intermolecular (4 + 2) cycloaddition

“…14 In addition to the electrocatalytic system, Ishihara and co-workers developed a FeCl 3 /AgSbF 6 co-catalysis method to promote radical cationic (4 + 2) cycloaddition by employing tetrasubstituted alkenes and 2,3-dimethyl-1,3-butadiene (Scheme 6b). 15 One year later, the group of You and Kang disclosed an interesting green light-driven radical cationic (4 + 2) cycloaddition of nucleophilic dienes and terminal chain alkenes by using Fe III (btz) 3 as a photocatalyst (Scheme 6c). 16 It was found that the ligand-to-metal charge-transfer transition state formed in this process had a high excited-state reduction potential for the selective oxidation of terminal styrene derivatives.…”