Tetratriflylpropene: A new organic superacid is in town

When chemists need a superstrong acid, they often reach for triflic acid, CF₃SO₃H; triflimide, (CF₃SO₂)₂NH; or perhaps tris(triflyl)methane, (CF₃SO₂)₃CH. These acids and the salts made from them are sought after as electrolytes in fuel cells, stabilizers of reactive species, and as catalysts. In seeking to diversify the strong organic acid family, researchers led by Denis Höfler and Benjamin List of the Max Planck Institute for Kohlenforschung have designed a new C–H acid: 1,1,3,3-tetratriflylpropene (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201609923). Creating a new organic superacid typically starts with designing the corresponding anion, strategically locating electron-withdrawing substituents adjacent to a target C–H bond in such a way as to delocalize the negative charge to reduce its basicity. The triflyl group is often used because it’s one of the most electron-withdrawing groups known, and including more triflyl groups typically increases the acidity of C–H bonds. The researchers were looking to create a new acid with more than three electron-withdrawing groups and to keep the molecule symmetrical for distributing the negative charge. That led them to try installing four triflyl groups on propene, which they did in a two-step method starting from commercially available bis(triflyl)methane. The team found that tetratriflylpropene has the highest catalytic activity among other triflyl compounds tested for challenging Mukaiyama aldol reactions, Friedel-Crafts acylations, and other reactions.