Tethering Three Radical Cascades for Controlled Termination of Radical Alkyne <i>peri</i>-Annulations: Making Phenalenyl Ketones without Oxidants
Chaowei Hu, Leah Kuhn, Favour D. Makurvet, Erica S. Knorr, Xinsong Lin, Rahul Kisan Kawade, Frédéric Mentink‐Vigier, Kenneth Hanson, Igor V. Alabugin
Abstract
Although Bu 3 Sn-mediated radical alkyne peri -annulations allow access to phenalenyl ring systems, the oxidative termination of these cascades provides only a limited selection of the possible isomeric phenalenone products with product selectivity controlled by the intrinsic properties of the new cyclic systems. In this work, we report an oxidant-free termination strategy that can overcome this limitation and enable selective access to the full set of isomerically functionalized phenalenones. The key to preferential termination is the preinstallation of a “weak link” that undergoes C–O fragmentation in the final cascade step. Breaking a C–O bond is assisted by entropy, gain of conjugation in the product, and release of stabilized radical fragments. This strategy is expanded to radical exo-dig cyclization cascades of oligoalkynes, which provide access to isomeric π-extended phenalenones. Conveniently, these cascades introduce functionalities ( i.e., Bu 3 Sn and iodide moieties) amenable to further cross-coupling reactions. Consequently, a variety of polyaromatic diones, which could serve as phenalenyl-based open-shell precursors, can be synthesized.