Catalyst Regeneration in Transition-Metal-Mediated Atom-Transfer Radical Addition (ATRA) and Cyclization (ATRC) Reactions

Transition-metal-catalyzed atom-transfer radical addition (ATRA) and cyclization (ATRC) are considered fundamental reactions in organic chemistry for the formation of C-C bonds using free-radical means. Until recently, both processes were plagued by the large amounts of catalysts needed to achieve high selectivity towards the desired target compound (as high as 30 mol-%). The principal problem was the accumulation of the transition metal complex in the higher oxidation state as a result of unavoidable radical-radical termination reactions. In this article, recent advanced in the area of catalyst regeneration in transition-metal-mediated ATRA and ATRC reactions in the presence of free-radical diazo initiators or magnesium as reducing agents are reviewed. The role of the reducing agent in both systems is to continuously regenerate the activator (transition metal complex in the lower oxidation state) from the deactivator (transition metal complex in the higher oxidation state). As a result, ATRA and ATRC reactions can be conducted using very small concentrations of metal catalysts, making this methodology a greener alternative to currently available synthetic processes for such organic transformations.