Synthesis and Electrochemistry of Electronegative Spiroannelated Methanofullerenes: Theoretical Underpinning of the Electronic Effect of Addends and a Reductive Cyclopropane Ring-Opening Reaction

Regioselective ortho C-H insertion of N-nitrosoanilines with naphthoquinone carbenes

A Rh(III)-catalyzed ortho C-H migratory insertion of N-nitrosoanilines with naphthoquinone carbenes has been developed. The products were obtained in good yields under mild reaction conditions. Diverse elaborations of the products were explored. This method is valuable for the synthesis of biarylamines and their derivatives.

Preparation of π-extended fullerene derivatives through addition of phenylenediamine to open-cage fullerene derivatives

Open-cage fullerenes with a quinoxaline moiety on the rim of the orifice showed evident π-system extension effect on the NMR and UV-Vis spectra.

Rhodium-catalyzed reaction of diazoquinones with allylboronates to synthesize allylphenols

A rhodium-catalyzed reaction of diazoquinones and allylboronates was developed, which provides access to a range of substituted allylphenols under mild conditions.

A palladium-catalyzed Barluenga cross-coupling - reductive cyclization sequence to substituted indoles

A short and flexible synthesis of substituted indoles employing two palladium-catalyzed reactions, a Barluenga cross-coupling of p-tosylhydrazones with 2-nitroarylhalides followed by a palladium-catalyzed, carbon monoxide-mediated reductive cyclization has been developed. A one-pot, two-step methodology was further developed, eliminating isolation and purification of the cross-coupling product. This was accomplished by utilizing the initially added 0.025 equivalents of bis(triphenylphosphine)palladium dichloride, thus serving a dual role in the cross-coupling and the reductive cyclization. It was found that addition of 1,3-bis(diphenylphosphino)propane and carbon monoxide after completion of the Barluenga reaction afforded, in most cases, significantly better overall yields.

[2 + 1] Cycloaddition reactions of fullerene C60 based on diazo compounds

The most common variant of fullerene core functionalization is the [2 + 1] cycloaddition process. Of these, reactions leading to methanofullerenes are the most promising. They are synthesized in two main reactions: nucleophilic cyclopropanation according to the Bingel method and thermal addition of diazo compounds. This present review summarizes the material on the synthesis of monofunctionalized methanofullerenes - analogues of [60]PCBM - based on various diazo compounds. The main cyclopropanating agents for the synthesis of monosubstituted methanofullerenes, the optimal conditions and the mechanism of the [2 + 1] cycloaddition, as well as the practical application of the target products are analyzed.

Recent quinone diazide based transformations via metal–carbene formation

Recent advancements in versatile synthetic transformations using quinone diazide based metal carbenes have been summarized.

Palladium-catalyzed domino spirocyclization of [60]fullerene: synthesis of diverse [60]fullerene-fused spiro[4,5]/[5,5] derivatives

Herein a new, general and practical method for the spirocyclization of [60]fullerene through a palladium-catalyzed domino Heck/C-H activation reaction is presented. A wide range of novel [60]fullerene-fused spirocyclic derivatives can be easily and flexibly synthesized with a broad substrate scope and excellent functional-group tolerance. A plausible mechanism involving an alkyl Pd(ii) species as a key intermediate has been proposed.

Synthesis and Crystal Structure of Li+@Fluoreno[60]fullerene: Effect of Encapsulated Lithium Ion on Electrochemistry of Spiroannelated Fullerene

The reaction of [Li⁺@C₆₀]TFSI^- (TFSI = bis(trifluoromethanesulfonyl)imide) with 9-diazofluorene directly produced a [6,6]-adduct of lithium-ion-containing fluoreno[60]fullerene, [6,6]-[Li⁺@C₆₀(fluoreno)]TFSI^-, which was crystallographically characterized. Cyclic voltammetry of the compound showed a reversible one-electron reduction wave at -0.51 V (vs Fc/Fc⁺) and an irreversible reduction wave for the second electron. The latter was attributed to opening of the three-membered ring due to strong stabilization of the resulting sp³-carbanion by the encapsulated Li⁺ and formation of a 14π-electron aromatic fluorenyl anion.

Transition-metal-catalyzed divergent functionalization of [60]fullerene with propargylic esters

Transition-metal-catalyzed divergent reactions between C₆₀ and propargylic esters allow easy access to formal [2 + 2] and [4 + 2] cycloadducts.

Cp*Rh(iii) and Cp*Ir(iii)-catalysed redox-neutral C–H arylation with quinone diazides: quick and facile synthesis of arylated phenols

Cp*Rh(iii)- and Cp*Ir(iii)-catalysed direct C–H arylation with quinone diazides provides a facile and redox-neutral access to arylated phenols.

Quinone diazides for olefin functionalization

The utility of quinone diazides in materials science is vast and well-documented, yet this potentially useful motif has languished in the annals of organic synthesis. Herein we show that modern tools of catalysis can be employed with free or suitably masked quinone diazides to unleash the power of these classic diazo compounds in the context of both inter- and intramolecular olefin cyclopropanation.

First synthesis and aggregation behaviour of periconjugated triazoliumfullerene

Triazoliumfullerene was first prepared by the [3+2] cycloaddition of in situ generated 1,3-diaza-2-azoniaallene with fullerene and was characterized by the dispersed positive charge over the fullerene sphere associated with periconjugation. This new type of amphiphilic fullerene tended to form vesicles and crystalline aggregates after the casting of THF or MeOH solutions.

Switchable conductance in functionalized carbon nanotubes via reversible sidewall bond cleavage

We propose several covalent functionalizations for carbon nanotubes that display switchable on/off conductance in metallic tubes. The switching action is achieved by reversible control of bond-cleavage chemistry in [1 + 2] cycloadditions via the sp(3) ⇌ sp(2) rehybridization that it induces; this leads to remarkable changes of conductance even at very low degrees of functionalization. Reversible bond-cleavage chemistry is achieved by identifying addends that provide optimal compensation between the bond-preserving through-space π orbital interactions with the tube against the bond-breaking strain energy of the cyclopropane moiety. Several strategies for real-time control, based on redox or hydrolysis reactions, cis-trans isomerization or excited-state proton transfer are proposed. Such designer functional groups would allow for the first time direct control of the electrical properties of metallic carbon nanotubes, with extensive applications in nanoscale devices.

New diarylmethanofullerene derivatives and their properties for organic thin-film solar cells

A number of diarylmethanofullerene derivatives were synthesized. The cyclopropane ring of the derivatives has two aryl groups substituted with electron-withdrawing and -donating groups, the latter with long alkyl chains to improve solubility in organic solvents, an important property in processing cells. First reduction potentials of most derivatives were less negative than that of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), which is possibly ascribed to their electron-withdrawing nature. Organic thin-film photovoltaic cells fabricated with poly(3-hexylthiophene) (P3HT) as the electron-donor and diarylmethanofullerene derivatives as the electron-acceptor material were examined. The {(methoxycarbonyl)phenyl[bis(octyloxy)phenyl]methano}fullerene showed power conversion efficiency as high as PCBM, but had higher solubility in a variety of organic solvents than PCBM. The V(oc) value was higher than that of PCBM, which is derived from the electron-donating (octyloxy)phenyl group, possibly raising the LUMO level. Photovoltaic effects of the devices fabricated with the derivatives having some electron-withdrawing groups were also examined.

Ring-Opening Reaction of Cyclopropanated [60]Fullerenes: Unexpected Transformation of Methano[60]fullerenes Having an Electron-Donating Group on the Methano-Bridge Carbon

A series of novel transformations of [60]fullerene derivatives were found, starting from methano[60]fullerenes with an electron-donating group on the methano-bridge carbon. Aminomethano[60]fullerenes, in situ generated by the treatment of their trifluoromethanesulfonic acid salts with a base, were readily converted into 1-acyl-1,2-dihydro[60]fullerenes via the ring opening of the cyclopropane moiety. The aldehyde/ketones thus obtained were easily hydrolyzed to give 1,2-dihydro[60]fullerene in the presence of hydroxide anions.

Synthesis, Spectroscopic and Nonlinear Optical Properties of Multiple [60]Fullerene-Oligo(p-phenylene ethynylene) Hybrids

A series of multiple [60]fullerene terminated oligo(p-phenylene ethynylene) (OPE) hybrid compounds has been synthesized through a newly developed in situ ethynylation method. Structural and magnetic shielding properties of the highly unsaturated carbon-rich C(60) and OPE scaffolds were characterized by 1D and 2D NMR spectroscopic analyses. Electronic interactions between the [60]fullerenes and the OPE backbones were investigated by UV/Vis spectroscopic and cyclic voltammetry (CV) experiments. Our studies clearly show that although the multiple [60]fullerene groups are connected via pi-conjugated OPE frameworks, they present diminutive electronic interactions in the ground state, and the electronic behavior of the [60]fullerene cages are only affected by the OPE backbones through modest inductive effects. Interestingly, sizable third-order nonlinear optical (NLO) responses (gamma) and enhanced two-photon absorption (TPA) cross-sections (sigma((2))) were determined for the multifullerene-OPE hybrid 31 relative to its OPE precursor from differential optical Kerr effect (DOKE) experiments. Such enhanced NLO performance is presumably due to the occurrence of periconjugation and/or charge transfer effects in the excited state. In addition, comparatively strong excited-state absorption was observed and characterized for OPE pentamer 12. Thus, the use of such fullerene-derivatized conjugated oligomers aids the quest for molecules with large third-order NLO and TPA properties.

Evidence for the Formation of Singly Bonded Dimers During the Reductive Electrochemistry of Methanofullerenes

Four methanofullerene derivatives, with phosphonate or sulfone groups attached to a C(60) core through a Bingel procedure, were synthesized to probe their stability upon electrolytic reduction. Derivatives 1 and 2 are the most stable upon electroreduction and do not exhibit retro-cyclopropanation reactions until more than three electrons per C(60) derivative are transferred. The cyclopropane ring is then removed and C(60)(>CH(2))(n) (n=1-3) products result from reactions of the trianion of C(60) with the solvent, CH(2)Cl(2). The situation with diphosphonate 3 or phosphonatecarboxylate 4 is dramatically different. For 3, quantitative retro-cyclopropanation occurs when 2.8 e(-) per molecule are transferred. In the case of 4, when more than two electrons per molecule are transferred, there is evidence of the reversible formation of a very stable intermediate, which is oxidized at a potential 500 mV more positive than the first fullerene-based reduction of the parent compound. Electrolysis of a simple C(70)-Bingel monoadduct (5) also exhibits the formation of a similar intermediate. On the basis of cyclic voltammetry, ESR spectroscopy, and MALDI analysis of products, the intermediate observed during the electrolysis of compounds 4 and 5 is assigned to a dimeric structure.

Rearrangement of the cyclohexadiene derivatives of C60 to bis(fulleroid) and bis(methano)fullerene: structure, stability, and mechanism

The cyclohexadiene derivative of C(60) rearranges photochemically to bis(fulleroid) (two [6,5] open structure) and bis(methano)fullerene (two [6,6] closed structure). During this process, a [6,5] open/[6,6] closed intermediate is observed. The isolated intermediate undergoes photochemical rearrangement to bis(fulleroid) and bis(methano)fullerene. On the other side, it undergoes retrorearrangement to the starting material in the dark. The structure and energetics of these C(60) derivatives have been studied at the AM1, PM3, RHF, and B3LYP levels of theory. It is found that bis(fulleroid) bearing four tert-butoxycarbonyl substituents is 5.8 kcal/mol (B3LYP) more stable than the corresponding bis(methano)fullerene. The isolated intermediate having the [6,5] open/[6,6] closed structure is 6.7 kcal/mol more favorable than the previously proposed two [6,5] closed intermediate, and the formation of this compound is well explained by the di-pi-methane rearrangement. (13)C NMR calculation at the B3LYP level reproduced the experimental chemical shifts with very good accuracy for each molecular system. Theoretical studies mainly at the unrestricted B3LYP level on singlet and triplet state potential energy surfaces on fullerene derivatives support the di-pi-methane rearrangement mechanism. The previously proposed symmetrical [4+4]/[2+2+2] and the novel proposed unsymmetrical di-pi-methane pathways may coexist during the reaction.