It is challenging to design non-ionic organic bases that match the basicity of inorganic bases. There are examples of highly basic nitrogen or phosphorus atoms in organic compounds. Non-ionic carbon can also act as a base: compounds such as carbodiphosphoranes (R₃P−C−PR₃) have two lone electron pairs that are mostly localized at the carbon atom. Using very electron-rich PR₃ substituents could increase the already high basicity of the central carbon atom in these compounds.

Jörg Sundermeyer, University of Marburg, Germany, and colleagues have synthesized a new generation of superbasic carbodiphosphoranes. The team used electron-rich pyrrolidine, tetramethylguanidine, or hexamethylphosphazene as electron-rich substituents. They first prepared cationic precursors, which were deprotonated using bases such as potassium bis(trimethylsilyl)amide (KHMDS) or sodium amide (NaNH₂).

The resulting carbodiphosphoranes’ basicity was determined using NMR titration experiments. The tetramethylguanidinyl-substituted product (pictured) is orders of magnitude more basic than the strongest uncharged carbon superbase known so far, H₂C=P(2,4,6-(MeO)₃-C₆H₂)₂Ph. According to the researchers, the compounds could be useful in organic superbase catalysis.

• Design of Non-Ionic Carbon Superbases: Second Generation Carbodiphosphoranes,
  Sebastian Ullrich, Borislav Kovačević, Björn Koch, Klaus Harms, Jörg Sundermeyer,
  Chem. Sci. 2019.
  https://doi.org/10.1039/c9sc03565f