Uncovering the On-Pathway Reaction Intermediates for Metal-Free Atom Transfer Radical Addition to Olefins through Photogenerated Phenalenyl Radical Anion

Photoredox catalysis has emerged as a powerful approach for atom transfer radical addition (ATRA) while generating the desired intermediates that are otherwise difficult to access with traditional methods. However, photoredox ATRA with organic photosensitizers is rare, giving an opportunity for metal-free routes to complex organic synthesis. Here, we demonstrate exquisite control of product selectivity for addition reactions of in situ generated carbenes to form either cyclopropanated or corresponding hydroalkylation products with a phenalenyl-based photocatalyst through H/I-ATRA. A combination of steady-state and time-resolved spectroscopic tools identified the transient formation of the reactive phenalenyl radical anion that drives photocatalysis. Kinetic isotope measurements along with radical trapping methods mapped the complete mechanistic cycle for the cyclopropanation as well as hydroalkylation reaction of styrene-based hydrocarbons. Our multicomponent H/I-ATRA methodology with photogenerated phenalenyl radical anion shows high functional group tolerance encompassing biologically active molecules.