Efficient and Selective Debromination of vic-Dibromides to Alkenes Using CoCl2·6H2O/Indium System

The combination of halogenation and reductive dehalo-genation is a useful step in the double bond protection-deprotection strategy.1 Reductive debromination of vic-dibromides with a variety of different agents has been the subject of many reports. However, most methods lack the desired chemoselectivity when other reducible functional groups are present and often require prolonged reaction times, or involve harsh reaction conditions.2 As a result, there is always considerable interest in the search for more efficient and selective procedures for the debromination of vic-dibromides. The reduction of CoCl2 to low-valent cobalt species and the synthetic utility of such species are well documented in the literature.3 Generally, reducing agents, such as zinc and magnesium, are used for the reduction of CoCl2. Because indium and zinc closely resemble each other in several aspects, including first ionization, we considered that a combination of CoCl2·6H2O with indium could facilitate the reductive debromination of vic-dibromides under mild conditions. As in the case of zinc, the reduction potential of indium is not highly negative (In: Eo, In+3/In = −0.345 V; Zn: Eo, Zn+2/Zn = −0.763 V): thus, indium is not sensitive to water and does not form oxides readily in air. In recent years, indium metal has been the subject of active interest because of its unique properties such as low toxicity and high stability in water and air compared to other metals.4 In connection with our interest in exploring the utility of low-valent metal reagents for organic transformations,5 we herein wish to report an efficient and chemoselective method for the debromination of vic-dibromides 1 to alkenes 2 using CoCl2·6H2O/indium system at room temperature. The reaction can be generalized as shown by eq. (1). Recently, we reported the deoxygenation of various amine N-oxides using a CoCl2·6H2O/indium system