Selenium-Promoted Homocoupling Of Grignard Reagents

Homocoupling reactions of Grignard reagents are an easy and efficient access tosymmetrical di-aromatic and olefinic conjugated compounds. The potentialapplications of such compounds in optical materials, molecular devices, and organicconductors are well-recognized. Because of the significance and prevalence of thisclass of compounds, numerous symmetrical synthetic methods have been developedover the past few decades, but these methods still exhibit shortcomings of usingexpensive metal Pd, stoichiometric or poisonous oxidants. In this paper, in theabsence of oxidants, the formed Grignard reagents from bromohydrocarbons affordedcarbon-carbon coupling products promoted by certain amount of elemental seleniumvia formation of organo-magnesium selenate in one pot. The main research results areas follows:We discussed the amount of magnesium to prepare Grignard reagents andelemental selenium, reacting time, and the influence of catalysts and ligands （N, P）. Aseries of experiments were carried out to optimize the reaction conditions. Theexperimental procedure is as follows:Initiated by small amount of I2, hydrocarbon bromides （0.005mol） were reactedwith2.5equiv magnesium under nitrogen atmosphere in dry tetrahydrofuran at45℃for40min to give Grignard reagents. Then,30mol%oven dried selenium powder wasadded into the mixture to react for half an hour. Then,15mol%CoCl₂（bipy） andanother part of bromides （0.005mol） were added into the mixture to react for1hourunder reflux. High isolated yields of65-92%were obtained for total15differentbromides.