Transition Metal-Catalyzed Radical Reactions of Organohalides

Abstract

Transition metal-mediated catalytic systems were studied for the development of free radical reactions of organohalides. In an effort to search for efficient alternatives for traditional organotin-mediated protocols (chapter 1), a nickel-catalyzed approach was developed for use in radical cyclization of unactivated alkyl halides (chapter 2). It was found that the use of a nickel/Pybox catalyst in conjunction with zinc as a stoichiometric reductant could promote the reaction efficiently. The new catalytic radical reaction could be performed with operational simplicity and high functional group tolerance without the need of slow addition of reagents that was essential for organotin-mediated protocols. The scope of the organohalide was expanded by introducing a visible-light harnessing photocatalysis (chapter 3). Cyclization and hydrodehalogenation reactions of organohalides proceeded efficiently under visible light irradiation by employing [Ir(ppy)2(dtbbpy)]PF6 as a catalyst and N,N-diisopropylethylamine as a reductant. The reaction is the first example of a visible-light-induced photocatalysis for reductive transformation of a broad range of alkyl, alkenyl and aryl halides, not limited to alkyl substrates with an activating group. Taking advantage of the capability of photoredox catalysis to mediate radical processes via single electron transfer, we investigated a nitrogen-centered radical mediated reaction (chapter 4). In the presence of fac-[Ir(dFppy)3] as a photocatalyst, visible light irradiation induced the formation of phthalimidyl N-radical through facile electron transfer to N-chlorophthalimide. The imidyl radical reacted with a wide range of aromatic substrates to afford the N-aryl compounds at room temperature, which represented a new approach for targeting aromatic amines.