Wei-Wei Yang

Electrochemical and H/D-Labeling Study of Oxazolino[60]Fullerene Rearrangement

Isomerization of an oxazoline cycle from a [6,6]- to [5,6]-junction on the C(60) sphere of dianionic [60]fullero-oxazoline (1(2-)) during a 1,4-addition is studied by electrochemistry and a stepwise addition of PhCH(2)Br and PhCD(2)Br. Cyclic voltammerty of the in situ generated 1(2-) shows a very unusual positive shift for the anodic peak corresponding to the oxidation of 1(2-), indicating that the C(60) cage of dianionic 1 bears only one unit negative charge due to the heterolytic cleavage of the C(60)-O bond. Further study with a stepwise addition of PhCH(2)Br and PhCD(2)Br, which are used to differentiate the aryl groups added at each step onto dianionic 1, shows explicitly there is an exclusive selectivity of the C-O bond for the ring-opening and ring-closure during the isomerization of the heterocycle. A reaction mechanism is proposed on the basis of the experimental results and computational calculations.

Reactions of anionic oxygen nucleophiles with C60 revisited.

Reactions of C(60) with oxygen nucleophiles of HO(-) and CH(3)O(-) are revisited in PhCN in the presence of PhCH(2)Br. Different from previous results that such reactions lead to the formation of complex mixtures, well-structured C(60) oxazolines are obtained when HO(-) is involved, while di- and tetraadducts with methoxy and benzyl addends are obtained when CH(3)O(-) is engaged. The reactions are followed by in situ vis-near-IR spectroscopy, which reveals further information for the reactions.

Formation of singly bonded PhCH2C60-C60CH2Ph dimers from 1,2-(PhCH2)HC60 via electroreductive C60-H activation.

Singly bonded PhCH(2)C(60)-C(60)CH(2)Ph dimers are generated via controlled-potential bulk electroreduction and electrooxidation of 1,2-(PhCH(2))HC(60). The reaction mixture was purified by HPLC, and the isolated fraction was characterized with single-crystal X-ray diffractions, (1)H and (13)NMR, MS, elemental analysis, and cyclic voltammetry. It was shown that the fraction consists of two HPLC-inseparable PhCH(2)C(60)-C(60)CH(2)Ph regioisomers, which are assigned as the meso and racemic regioisomers. The bulk electrolysis processes for the formation of the dimers were followed by in situ cyclic voltammetry and were further corroborated with an in situ voltammetric titration of 1,2-(PhCH(2))HC(60) with tetra-n-butylammonium hydroxide (TBAOH), on the basis of which a reaction mechanism is proposed.

Reaction of C602- with Organic Halides Revisited in DMF: Proton Transfer from Water to RC60- and Unexpected Formation of 1,2-Dihydro[60]fullerenes

The reactions of dianionic C(60) with organic halides, which have been studied extensively in benzonitrile (PhCN), are revisited in a different solvent medium, N,N-dimethylformamide (DMF), by using ArCH(2)Br (Ar = Ph, C(6)H(4)CH(3), C(6)H(4)Br). Interestingly, instead of the 1,4-R(2)C(60) adducts, which are the typical products when the reactions are carried out in PhCN, the 1,2-dihydro[60]fullerenes (1,2-HRC(60)) have been obtained as the major products in DMF, except for the case of o-CH(3)C(6)H(4)CH(2)Br probably due to the steric effect. The obtained 1,2-dihydro[60]fullerenes have been characterized with single-crystal diffraction, (1)H and (13)C NMR, high-resolution mass spectrometry (HRMS), and UV-vis. Further examination with deuterated reagents including PhCD(2)Br, DMF-d(7), and D(2)O has revealed that the fullerenyl hydrogen of 1,2-dihydrofullerenes originates from traces of water residue in DMF. When the reaction is re-examined in PhCN with the addition of an excessive amount of water, the yield of 1,2-dihydrofullerene increases significantly, but is still lower compared with that obtained in DMF, demonstrating a considerable solvent effect on the reactivity of C(60)(2-). A possible mechanism accounting for such a difference is proposed. The work has presented an alternative protocol for effective preparation of 1,2-dihydro[60]fullerenes, and may also provide clues toward a better understanding of the proton transfer process for anionic C(60)-mediated reducing reactions involving H(2)O.

Regioselective Oxazolination of C702– and Formation of cis-1 C70 Adduct with Respect to the Apical Pentagon

Oxazolination of C(70) has been achieved via the aerobic oxidation of C(70)(2-) in the presence of PhCN. Only one C(70) oxazoline regioisomer (1) is obtained, indicating that the oxazolination of C(70)(2-) occurs with an unusual regioselectivity. Further benzylation of 1(2-) with benzyl bromide leads to the formation of the first cis-1 C(70) derivative with respect to the apical pentagon (2), as shown by the X-ray single-crystal structure and various spectral characterizations. The structure of the obtained C(70) oxazoline (1) is resolved with H/D labeling benzylation and HMBC (heteronuclear multiple bond coherence) NMR on the basis of the structure of 2. The result shows that for compound 1, the O atom is selectively bonded to the C1, while the N atom is bonded to the C2 of C(70). The exhibited regioselectivity for the orientation of oxazolino group on C(70) is further rationalized with computational calculations, and a reaction mechanism for the oxazolination of C(70)(2-) is proposed.

Oxazolination of 1,4-(PhCH2)2C60: toward a better understanding of multiadditions of heteroaddends.

Multiadditions of heteroaddends to C(60) are achieved via the oxazolination reaction of 1,4-(PhCH(2))(2)C(60) with OH(-) and PhCN, which exhibit a unique regioselectivity regarding the addition sites of the heteroatoms.

A room-temperature fluorescence study of organofullerenes: cis-1 bisadduct with unusual blue-shifted emissions.

C(60) derivatives have shown enhanced fluorescent emissions with respect to C(60) due to the lowering of molecular symmetry and have demonstrated promising potentials as novel organoelectronic materials for application in light-emitting diodes. Previous work has indicated that the fluorescent properties of functionalized C(60) are mainly affected by the addition patterns, rather than the nature of addends. However, no report on the fluorescence of C(60)cis-1 bisadducts, one of the most favorable types of C(60) bisadducts, has appeared up to date. Herein, the fluorescent properties of two structurally related C(60)cis-1 bisadducts of fullerooxazolines, 1 and 2, are examined at room temperature. It shows that a significant difference exists for the fluorescent spectra of 1 and 2, where 1 displays a rather strong unusual blue-shifted emission band, even though the two compounds have the same addition pattern. Monoadducts bearing individual addends of 1 and 2, along with 1a and 1b, which have one PhCD(2)- positioned either next to the C(60)-N or C(60)-O bond, are also examined in order to gain a better understanding of such difference. The results indicate that the unusual blue-shifted emissions for 1 are likely to originate from the vibrational interactions of the addends, suggesting that the fluorescent emissions of C(60) derivatives can be tuned not only by the addition patterns, but also by the nature of the adducts. Density functional theory and time-dependent density functional theory calculations are performed to rationalize the experimental observations.

Reactions of C-70(2-) with Organic Halides Revisited: Unusual Magnetic Equivalence for the Diastereotopic Methylene Protons in 2,5-(PhCH2)(2)C-70

The reactions of C70(2-) with organic halides result in several isomeric products. However, the structures of the isomers have not been identified unambiguously and the reactions still remain elusive even though the first report of the reaction appeared more than ten years ago. Herein, the reactions of C70(2-) with ArCH2Br (Ar = Ph, o-, m-, and p-BrC6H4) are revisited. Two types of isomeric para-adducts of 2,5- and 7,23-(ArCH2)2C70 are obtained and characterized with the X-ray single-crystal diffractions, HRMS, (1)H and (13)C NMR, and UV-vis spectroscopies. An unusual singlet resonance instead of the typical AB quartet resonance is shown for the C2 diastereotopic methylene protons in (1)H NMR spectrum of 2,5-(PhCH2)2C70 recorded at ambient temperature. The unexpected magnetic equivalence is studied with the variable-temperature (VT) NMR and density functional theory (DFT) calculations. The results show that the greater local curvature in the C70 polar region is likely responsible for the unusual magnetic equivalence exhibited by the C2 diastereotopic methylene protons of 2,5-(PhCH2)2C70, indicating that under certain cases, it is improbable to elucidate the spectral data of fullerene derivatives with a less symmetrical carbon cage in analogy with those of the C60 derivatives.

Vis-Near-IR Spectroscopic and Time-Dependent DFT Study of Reduced Singly Bonded C60 Species.

Reduced fullerenes and fullerene derivatives exhibit intense absorptions in the vis-near-IR (vis-NIR) region. The absorptions are sensitive toward the addition pattern, number of addends, and oxidation state of the fullerene species and are used as an important benchmark for identifying anionic fullerene species. Similar absorptions are also shown for the reduced singly bonded C60 species, which are electronically different from reduced fullerene derivatives. However, much less work has been carried out on the vis-NIR spectroscopic study of the anionic singly bonded C60 species, likely due to the difficulty in controlled production of these species. Herein, we report the vis-NIR spectroscopic study of the mono- and dianions of the singly bonded C60 species (RC60(-), R(-)C60(-), and RC60(2-•)), which are selectively generated by controlled-potential bulk electrolysis (CPE) of the singly bonded C60 dimer and C60 oxazolino heterocycles. Time-dependent density functional theory (TD-DFT) calculations were performed to rationalize the experimental results.

Controlled Synthesis of C70 Equatorial Multiadducts with Mixed Addends from an Equatorial Diadduct: Evidence for an Electrophilic Carbanion

Controlled synthesis of the equatorial tetra-, hexa-, and octaorgano[70]fullerenes with mixed addends was achieved via the reaction of equatorial 7,23-Bn2C70 with MeO- and ArCH2Br. The products were structurally characterized by single crystal X-ray diffraction. The regioselectivity of the reaction was studied by in situ vis-NIR and Fukui function analysis. A surprising electrophilic triorgano[70]fullerene carbanion was shown, and an enhanced fluorescence was observed for the mixed octaadducts.