YAMADA Group
Laboratory of Organic Chemistry,
Department of Chemistry, Tokyo Gakugei University
Quantification of Noncovalent Interactions at the Fullerene Surface
To investigate the nature and strength of Noncovalent interactions at the fullerene surface, molecular torsion balances consisting of C60 and organic moieties connected through a biphenyl linkage were synthesized. NMR and computational studies show that the unimolecular system remains in equilibrium between well‐defined folded and unfolded conformers owing to restricted rotation around the biphenyl C−C bond. The energy differences between the two conformers depend on the substituents and is ascribed to differences in the intramolecular noncovalent interactions between the organic moieties and the fullerene surface. Fullerenes favor interacting with the π‐faces of benzenes bearing electron‐donating substituents. The correlation between the folding free energies and corresponding Hammett constants of the substituents in the arene‐containing torsion balances reflects the contributions of the electrostatic interactions and dispersion force to face‐to‐face arene–fullerene interactions.
New Methods to Access Open-Cage Fullerenes
An open-cage fullerene bearing an eight-membered ring orifice has been synthesized in one pot by the reaction of C60 with propargylic phosphate in the presence of CuCl. The reaction cascade involves the transformation of the phosphate to the 1,3-dienyl phosphate, which enables the reaction with C60 by {4 + 2} cycloaddition to form the cyclohexene-annulated intermediate, and subsequent intramolecular syn-elimination of the phosphodiester affords the cyclohexadiene-annulated fullerene derivative as the precursor for the open-cage fullerene.
X-ray structure of the intermediate.
Transition-Metal-Catalyzed Functionalization of Fullerenes
The intriguing and valuable reactivity of easily accessible propargylic esters has recently been recognized in the context of transition-metal-catalyzed skeletal transformations. Specifically, propargylic esters can be transformed to diverse structures via a cascade of reaction steps with the aid of transition metals. Such a cascade reaction offers molecular complexity and diversity from simple structures. We envisage that transition-metal catalyzed cascade reactions provide a versatile and step-economical approach to the functionalization of fullerenes. We have demonstrated that transition-metal-catalyzed divergent reactions between C60 and propargylic esters allow easy access to formal {2 + 2} and {4 + 2} cycloadducts in reasonable yields, and that the production ratios depend on the metal catalyst used.
The single-step regio- and stereoselective platinum-catalyzed reactions of C60 with a series of 9-ethynyl-9H-fluoren-9-yl carboxylates afforded fullerene–fluorene dyads in their {2 + 2} cycloaddition forms. The presented reactions represent the first examples of the use of easily accessible fluorenyl carboxylates as fluorenylideneallene precursors. In addition, the single-crystal X-ray structure of one of the dyads reveals a distorted cyclobutane ring. Furthermore, the dyad forms a layered structure with close-packed arrays of C60 spheres in its crystals.
Functionalization of Fullerenes Using Photoaffinity Labeling Reagents
The photolysis of para-substituted 3-trifluoromethyl-3-phenyldiazirine, known as photoaffinity labeling (PAL) reagents in biochemistry, in the presence of C60 allowed easy access to functionalized methanofullerenes in reasonable yields. Among them, a succinimidyl-appended methanofullerene serves as an active site for the attachment of other molecules via amidation. We demonstrated that a biotin-appended methanofullerene was successfully synthesized by the amidation protocol with the corresponding amine under mild conditions. Standard HABA assays in DMSO revealed the binding ability of the biotin moiety to avidin.
Conjugation of Push–Pull Chromophores to Fullerenes
Conjugation of push-pull chromophores from cycloaddition-cycloreversion reaction (CA/CR reaction) to C60 led to axially chiral derivatives as a result of hindered rotation about the central C–C single bond of the push-pull buta-1,3-dienes. Their enantiomers could be isolated and the absolute configuration assigned. Upon heating these derivatives, an unprecedented rearrangement takes place, yielding new fullerene tetrakis adducts.
Electrochemical studies revealed that direct connection of the fullerene cages to push-pull motifs gives rise to ground state electronic interactions. On the other hand, when the two moieties were linked through pyrrolidine rings, no interactions occurred between the fullerene units and the push-pull chromophores in the ground states. Instead, an electron transfer proceeded upon light excitation giving the charge-separated states.
