Studies on Tetracyanocyclopentadienides in Synthetic Organic Chemistry

Tetra- and pentacyanocyclopentadienides (TCCP and PCCP) are superacid conjugate bases stabilized by the mesomeric and inductive effects of cyano groups and the aromaticity of the cyclopentadienide ring. We have developed an efficient synthetic method for various TCCP and PCCP using tetracyanothiophene and sulfones, followed by substituent transformations. We conducted a series of application studies using our synthetic method for tetra- and pentacyanocyclopentadienides. NanoGoblin, the doll-shaped ion pair of TCCP, catalyzed methanolysis of the cyclic acetal in the anthropomorphic molecule NanoKid. The cyano groups of TCCP acted as hindrances to rotation of CAr-O bonds in 3,3′-BINOL esters. TCCP exhibited high lipophilicity, which was utilized in anionic phase transfer reactions and the ion-pair extraction of quaternary ammonium cations. Lastly, the weakly basic nature of PCCP was helpful in the Au/Ag-catalyzed cyclization-3-aza-Cope-Mannich cascade, enabling us to achieve the total synthesis of cephalotaxine.