Copper-catalyzed atom transfer radical addition (ATRA) and cyclization (ATRC) reactions in the presence of environmentally benign ascorbic acid as a reducing agent

Abstract

In recent years, copper-catalyzed atom transfer radical addition (ATRA) has emerged as a viable organic procedure for the formation of carbon-carbon bonds starting from alkyl halides and alkenes. Studies have primarily focused on the use of free-radical initiators to regenerate the copper(I) complex or activator in situ. Although these initiators led to a significant decrease in the amount of metal catalyst, they were much less effective for highly active alkenes that readily undergo free-radical polymerization. In this study, the non-radical reducing agent ascorbic acid (commonly known as Vitamin C) was effectively employed resulting in TONs as high as 15,200 in the homogenous ATRA of polyhalogenated compounds to α-olefins, and enabling selective monoadduct formation for highly active alkenes such as acrylonitrile (TONs as high as 11,800). As low as 7-20 mol% of ascorbic acid relative to substrate was sufficient for all ATRA and ATRC reactions examined. Further, product isolations for all selected syntheses were quite facile and nearly quantitative, requiring only simple liquid-liquid extraction techniques. Reactions in the presence of ascorbic acid were also examined kinetically via (1)H NMR and UV/Vis spectroscopy. A half order rate dependence on reducing agent concentration was observed, but the first order kinetic plots became nonlinear as the concentration of ascorbic acid was increased. Finally, the use of ascorbic acid circumvented otherwise necessary purging techniques, successfully furthering the utility of these reactions in organic synthesis.