1,1,3,3-Tetratriflylpropene

George Olah was a Nobel Prize–winning chemist at Case Western Reserve University (Cleveland) and the University of Southern California (Los Angeles) who passed away in March 2017. He was awarded the prize for his research on carbocations—stable organic cations—that he generated with the aid of superacids. Developing superacids was as significant as the carbocation studies.

Superacids have Hammett acidity values (H₀) that are greater negative numbers than the –12 for 100% sulfuric acid. James Bryant Conant at Harvard University coined the term in 1927, when he prepared “magic acid” (SbF₅–FSO₃H) with H₀ = –19.2. The strongest superacid to date is fluoroantimonic acid (SbF₆^–H₂F⁺) with H₀ = –31.3.

Traditional superacids that contain elements such as antimony and fluorine are toxic and difficult to handle, so chemist have developed a series of C–H superacids that contain trifluoromethanesulfonyl (triflyl, Tf, or CF₃SO₂) groups to produce their acidity. The strongest of these is tritriflylmethane (Tf₃CH), with a pK_a (similar to H₀) value of ≈–16.4, depending on the solvent used. C–H superacids are widely used as organic catalysts in C–C bond forming reactions such as Friedel–Crafts acylation and Mukaiyama aldol addition.

Benjamin List and colleagues at the Max Planck Institute for Coal Research (Mülheim an der Ruhr, Germany) postulated that C–H acids with more electron-withdrawing triflyl groups than Tf₃CH might be even better catalysts. They synthesized 1,1,3,3-tetratriflylpropene (TTP), a four-Tf molecule that, when ionized, delocalizes the negative charge throughout the anion.

Although the pK_a value of TTP (–15.2) is no stronger than that of Tf₃CH, it has better catalytic activity in classical reactions such as Friedel–Crafts. The authors continue to expand the scope of reactions for which TTP is useful; and they believe that “TTP may be useful for applications beyond organocatalysis, such as in electrolytes and ionic liquids, and as a weakly coordinating anion.”