Bismuth provides catalytic surprises

A research team led by Berthold Hoge of Bielefeld University has gone into a little-used corner of the periodic table to develop a new set of highly active catalysts—bismuth perfluoroalkylphosphinates—that could be useful in industrial organic synthesis (Chem. Eur. J. 2016, DOI: 10.1002/chem.201604914). Although bismuth is a rare heavy metal, and the heaviest stable element in the periodic table to boot, it’s relatively inexpensive as a by-product of copper, lead, and tin mining. And unlike most heavy metals, its salts have remarkably low toxicity—most are less toxic than table salt. With those attributes, chemists are increasingly attracted to bismuth salts as Lewis acid catalysts to replace corrosive and toxic industrial catalysts such as boron trifluoride. Some examples such as bismuth triflate, Bi(SO₃CF₃)₃, have already been successfully tested. But they are moisture sensitive. Hoge and his team found that switching to fluoroalkylphosphinates creates air- and moisture-stable compounds without altering the Lewis acidity. To make bismuth(III) catalysts, the researchers treated commercially available Bi(C₆H₅)₃ with perfluoroalkyl­phosphinic acid, (C₂F₅)₂P(O)OH. To make an unprecedented bismuth(V) catalyst, they treated Bi(C₆H₅)₃Cl₂ with Ag[(C₂F₅)₂PO₂]. Hoge and his colleagues showed that these bismuth phosphinates are versatile in mediating a range of carbon-carbon bond-forming reactions, including Friedel-Crafts alkylation and the Diels-Alder reaction.