New highly soluble [6,6]-methanofullerene derivatives incorporating both α-keto and α, β-ester stabilized phosphorus ylides; synthesis, characterization, theoretical and biological studies

Abstract

Phosphonium salts [Ph2PCH2CH2PPh2CH2C(O)C6H4-m-R]Br (R = Br (1) and OCH3 (2)) were synthesized from the reaction of 1,1-bis(diphenylphosphino)ethane (dppe) and 2,3ʹ-dibromoacetophenone and 2-bromo-3′-methoxyacetophenone, respectively. Treatment with NEt3 gave the corresponding α-keto stabilized phosphorus ylides Ph2PCH2CH2PPh2C(H)C(O)C6H4-m-R (R = Br (3) and OCH3 (4)). Reaction of these ylides with C60 and dimethylacetylenedicarboxylate (DMAD) led to the formation of the new [6,6]-methanofullerene derivatives {C60.DMAD.Ph2PCH2CH2PPh2C(H)C(O)C6H4-m-R} (R = Br (5) and OCH3 (6)) bearing both α-keto and α, β-ester stabilized phosphorus ylides. These [6,6]-methanofullerene derivatives were highly soluble in conventional organic solvents such as toluene, chloroform, dimethyl sulfoxide (DMSO), N,N-dimethylformade (DMF), N-methyl-2-pyrrolidinone (NMP) and even partially soluble in ethanol and acetone. All compounds were characterized by IR, UV–Vis, ESI-MS, 1H, 13C, DEPT-135 and 31P NMR spectroscopic methods. The structure of the phosphine oxide derivative of compound 4 was characterized by single crystal X-ray crystallography. The electronic and molecular structure of compound 5 has been investigated at the BP86/def2-SVP level of theory. The nature of the CP bond in this compound was also analyzed with charge and energy decomposition methods. Compounds 5 and 6 were subjected to antioxidant activity screening by 2,2-diphenylpicrylhydrazyl (DPPH) free radical scavenging assay. The results from these experiments showed that the compounds possess strong antioxidant activity (IC50; 0.890 ± 0.004 to 1.081 ± 0.010 mg/ml).