Borinium Cation Is A Stellar Lewis Acid

Japanese researchers have developed a simple approach to creating highly electron deficient borinium cations, R₂B⁺, by stabilizing boron with only a pair of aryl substituents. The linear dimesitylborinium cation prepared by Yoshiaki Shoji of Tokyo Institute of Technology and coworkers exhibits extraordinary reactivity and could offer a new tool for exploring Lewis acid-promoted chemical reactions (Nat. Chem. 2014, DOI: 10.1038/nchem.1948). Boron, with only six valence electrons in its neutral, three-coordinate state, is electron deficient and readily accepts a pair of electrons from a Lewis base partner to form stable adducts. Chemists have learned how to push boron further into electron debt by eliminating a bond to create two-coordinate borinium ions in which boron has only four valence electrons. Consensus among researchers has been that bulky electron-donating partners, such as amines, are necessary to make stable borinium ions, but they do so at the expense of neutralizing reactivity. Shoji and coworkers figured out how to use silicon reagents to strip fluorine from commercially available dimesitylfluoroborane to make borinium cations balanced by noncoordinating carborane or borate counterions. Dimesitylborinium retains its Lewis acidity, accepting a pair of electrons from normally unreactive carbon dioxide in an unusual reaction that ends with both the borinium ion and CO₂ split apart.