Selective reactions of transition-metal-generated radicals

In this microreview, we describe selective radical reactions in the presence of potassium permanganate, Mn(OAc)(3), and cerium(IV) ammonium nitrate (CAN), which were developed in our group during the last years. The in situ generation of manganese(III) by potassium permanganate results in a low concentration of the oxidant, and allowed the chemoselective synthesis of H-atom transfer products in good yields. Furthermore, tandem reactions were developed to afford bicyclooctanes in diastereomerically pure form. Manganese(III) acetate was the reagent of choice for the selective synthesis of oxidation products. Ultrasonic irradiation remarkably accelerated such radical reactions and the method was applied for the simple and selective synthesis of tetra-acceptor-substituted alkenes from commercially available malonates. Due to the mild reaction conditions, CAN was superior to Mn(OAc)(3) for applications in carbohydrate chemistry. Thus, 2-C-branched carbohydrates were synthesized from glycals in only one step and in excellent yields. The radical additions afford exclusively one regioisomer and exhibit a high degree of stereoselectivity. Finally, the anomeric radicals are trapped by CAN via a direct ligand transfer, which is important for the mechanistic understanding of transition-metal-mediated radical reactions. (C) 2002 Elsevier Science B.V. All rights reserved.