Synthesis and chemistry of fullerene derivatives bearing phosphorus substituents. Unusual reaction of phosphines with electron-deficient acetylenes and C₆₀

A unique method to introduce phosphorus substituents onto C₆₀ is based on the reaction of phosphines with acetylenes and C₆₀. Treatment of C₆₀ with phosphines (PR₃) and electron-deficient acetylenes (A) in toluene at ambient temperature gave fullerene derivatives (1, R = C₆H₅ and A = MeO₂CC≡CCO₂Me; 2, R = C₆D₅ and A = MeO₂CCΓCO₂Me; 3, R = C₆H₅ and A = EtO₂CC≡CCO₂Et; 4, R = p-CH₃C₆H₅ and A = MeO₂CC≡CCO₂Me; 6, R = C₆H₅ and A = trans-MeO₂CC≡CCH=CHCO₂Me) consisting of a phosphorus ylide group and a cyclopropane ring on the fullerene moiety in good to excellent yields. The structures of these ylide derivatives are determined on the basis of their spectral data and single-crystal X-ray diffraction measurements. All these ylides show temperature-dependent NMR spectra that can be rationalized on the basis of interchange between two Z,E isomers and restricted rotation of the substituents on the fullerene moiety. Based on the known chemistry of phosphines and acetylenes, we propose a mechanism to account for the formation of these ylide derivatives. Phosphine ylides 1 and 4 readily undergo protonation and decarboxylation in the presence of acid to give phosphonium salts 7 and 8, respectively.