On‐Surface Synthesis of Cumulene‐Containing Polymers via Two‐Step Dehalogenative Homocoupling of Dibromomethylene‐Functionalized Tribenzoazulene

Cumulene compounds are notoriously difficult to prepare and study because their reactivity increases dramatically with the increasing number of consecutive double bonds. In this respect, the emerging field of on‐surface synthesis provides exceptional opportunities because it relies on reactions on clean metal substrates under well‐controlled ultrahigh‐vacuum conditions. Here we report the on‐surface synthesis of a polymer linked by cumulene‐like bonds on a Au(111) surface via sequential thermally activated dehalogenative C?C coupling of a tribenzoazulene precursor equipped with two dibromomethylene groups. The structure and electronic properties of the resulting polymer with cumulene‐like pentagon–pentagon and heptagon–heptagon connections have been investigated by means of scanning probe microscopy and spectroscopy methods and X‐ray photoelectron spectroscopy, complemented by density functional theory calculations. Our results provide perspectives for the on‐surface synthesis of cumulene‐containing compounds, as well as protocols relevant to the stepwise fabrication of carbon–carbon bonds on surfaces.     

Nonclassical fullerenes with a heptagon violating the pentagon adjacency penalty rule

Nonclassical fullerenes with heptagon(s) and their derivatives have attracted increasing attention, and the studies on them are performing to enrich the chemistry of carbon. Density functional theory calculations are performed on nonclassical fullerenes C_n (n = 46, 48, 50, and 52) to give insight into their structures and stability. The calculated results demonstrate that the classical isomers generally satisfy the pentagon adjacency penalty rule. However, the nonclassical isomers with a heptagon are more energetically favorable than the classical ones with the same number of pentagon–pentagon bonds (B₅₅ bonds), and many of them are even more stable than some classical isomers with fewer B₅₅ bonds. The nonclassical isomers with the lowest energy are higher in energy than the classical ones with the lowest energy, because they have more B₅₅ bonds. Generally, the HOMO–LUMO gaps of the former are larger than those of the latter. The sphericity and asphericity are unable to rationalize the unique stability of the nonclassical fullerenes with a heptagon. The pyramidization angles of the vertices shared by two pentagons and one heptagon are smaller than those of the vertices shared by two pentagons and one hexagon. It is concluded that the strain in the fused pentagons can be released by the adjacent heptagons partly, and consequently, it is a common phenomenon for nonclassical fullerenes to violate the pentagon adjacent penalty rule. These findings are heuristic and conducive to search energetically favorable isomers of C_n, especially as n is 62, 64, 66, and 68, respectively. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Methane chemical vapor deposition on transition metal/GaAs samples – a possible route to Haeckelite carbon nanotubes?

We present a systematic study of atmospheric chemical vapor deposition growth of carbon nanotubes (CNTs) on patterned, transition metal/GaAs samples employing methane as the carbon feedstock. Controlled CNT growth was found to occur from the exposed metal‐semiconductor interface, rather than from the metal or semiconductor surfaces themselves. A fast sample loading system allowed for a minimization of the exposure to high temperatures, thereby preventing excessive sample damage. The optimum growth temperature for CrNi/GaAs interfaces is 700 °C (at a methane flow rate of 700 sccm). Possible growth scenarios involving the Ni–As–Ga system and its interaction with C is discussed. Raman spectroscopy of the CNTs revealed the presence of pentagon–heptagon defects. Closer analysis of the spectra points towards a mixture of so‐called Haeckelite CNTs. Copyright © 2011 John Wiley & Sons, Ltd.     

Diazulenorubicene as a Non-benzenoid Isomer of peri-Tetracene with Two Sets of 5/7/5 Membered Rings Showing Good Semiconducting Properties

Non-benzenoid polycyclic aromatic hydrocarbons (PAHs) have received a lot of attention because of their unique optical, electronic, and magnetic properties, but their synthesis remains challenging. Herein, we report a non-benzenoid isomer of peri-tetracene, diazulenorubicene (DAR), with two sets of 5/7/5 membered rings synthesized by a (3+2) annulation reaction. Compared with the precursor containing only 5/7 membered rings, the newly formed five membered rings switch the aromaticity of the original heptagon/pentagon from antiaromatic/aromatic to non-aromatic/antiaromatic respectively, modify the intermolecular packing modes, and lower the LUMO levels. Notably, compound 2 b (DAR-TMS) shows p-type semiconducting properties with a hole mobility up to 1.27 cm² V⁻¹ s⁻¹. Moreover, further extension to larger non-benzenoid PAHs with 19 rings was achieved through on-surface chemistry from the DAR derivative with one alkynyl group.     

Synthesis and Structure of LaSc2N@Cs(hept)‐C80 with One Heptagon and Thirteen Pentagons

The synthesis and single‐crystal X‐ray structural characterization of the first endohedral metallofullerene to contain a heptagon in the carbon cage are reported. The carbon framework surrounding the planar LaSc₂N unit in LaSc₂N@C_s(hept)‐C₈₀ consists of one heptagon, 13 pentagons, and 28 hexagons. This cage is related to the most abundant I_h‐C₈₀ isomer by one Stone–Wales‐like, heptagon/pentagon to hexagon/hexagon realignment. DFT computations predict that LaSc₂N@C_s(hept)‐C₈₀ is more stable than LaSc₂N@D_5h‐C₈₀, and suggests that the low yield of the heptagon‐containing endohedral fullerene may be caused by kinetic factors.     

Magnetism in Nonplanar Zigzag Edge Termini of Graphene Nanoribbons

Graphene nanoribbons (GNRs) and nanographenes synthesized by on-surface reactions using tailor-made molecular precursors offer an ideal playground for a study of magnetism towards nano-spintronics. Although the zigzag edge of GNRs has been known to host magnetism, the underlying metal substrates usually veil the edge-induced Kondo effect. Here, we report the on-surface synthesis of unprecedented, π-extended 7-armchair GNRs using 7-bromo-12-(10-bromoanthracen-9-yl)tetraphene as the precursor. Characterization by scanning tunneling microscopy/spectroscopy revealed unique rearrangement reactions leading to pentagon- or pentagon/heptagon-incorporated, nonplanar zigzag termini, which demonstrated Kondo resonances even on bare Au(111). Density functional theory calculations indicate that the nonplanar structure significantly reduces the interaction between the zigzag terminus and the Au(111) surface, leading to a recovery of the spin localization of the zigzag edge. Such a distortion of planar GNR structures offers a degree of freedom to control the magnetism on metal substrates.     

CH bond functionalization: emerging synthetic tools for natural products and pharmaceuticals

The direct functionalization of C? H bonds in organic compounds has recently emerged as a powerful and ideal method for the formation of carbon–carbon and carbon–heteroatom bonds. This Review provides an overview of C? H bond functionalization strategies for the rapid synthesis of biologically active compounds such as natural products and pharmaceutical targets.     

On-surface synthesis of porous graphene nanoribbons containing nonplanar [14]annulene pores

The precise introduction of nonplanar pores in the backbone of graphene nanoribbon represents a great challenge. Here, we explore a synthetic strategy toward the preparation of nonplanar porous graphene nanoribbon from a predesigned dibromohexabenzotetracene monomer bearing four cove-edges. Successive thermal annealing steps of the monomers indicate that the dehalogenative aryl-aryl homocoupling yields a twisted polymer precursor on a gold surface and the subsequent cyclodehydrogenation leads to a defective porous graphene nanoribbon containing nonplanar [14]annulene pores and five-membered rings as characterized by scanning tunneling microscopy and noncontact atomic force microscopy. Although the C–C bonds producing [14]annulene pores are not achieved with high yield, our results provide new synthetic perspectives for the on-surface growth of nonplanar porous graphene nanoribbons.     

Trifluoromethanesulfonic acid in organic synthesis

The review analyzes data published in the past decade on the use of trifluoromethanesulfonic acid (triflic acid, CF₃SO₃H, TfOH) in organic synthesis, in particular in electrophilic aromatic substitution (Friedel–Crafts) reactions, formation of carbon–carbon and carbon–heteroatom bonds, isomerizations, syntheses of carboand heterocyclic structures, and other reactions, as well as in natural and organometallic compounds chemistry. The high protonating power and low nucleophilicity makes trifluoromethanesulfonic acid capable of generating from organic molecules cationic species which can be detected by spectral methods (NMR, IR spectroscopy, etc.), and their transformations can be studied. Experimental simplicity and efficiency of reactions promoted by trifluoromethanesulfonic acid make it a convenient reagent for the synthesis of new organic compounds.     

Recent Advances in Organic Electrochemical C—H Functionalization

Organic electrochemistry has a rich history in organic synthesis and has been considered as a promising alternative to traditional chemical oxidants and reductants because it obviates the use of stoichiometric amount of dangerous and toxic reagents. In particular, the electrochemical C—H bonds functionalization is one of the most desirable approaches for the construction of carbon–carbon (C—C) and carbon–heteroatom (C—X) bonds. This review summarizes the substantial progress made in the last few years in C—H functionalization via organic electrochemistry. It is divided into sections on C—C, C—N, C—O, C—S, C–Halogen and C—P bond formation.     

Characteristic reactions of group 9 transition metal compounds in organic synthesis

Group 9 metal compounds in organic synthesis have two characteristic reactions. The first occurs because the group 9 metals have a high affinity to carbon–carbon or carbon–nitrogen π‐bonds. The first type of characteristic reactions in these group 9 metal compounds includes Pauson–Khand reactions, the Pauson–Khand‐type reactions ([2 + 2 + 1] cyclization), the other cyclizations and coupling reactions. The second occurs because the group 9 metals have a high affinity to carbonyl groups. The second type of characteristic reactions includes carbonylations such as hydroformylations, the carbonylations of methanol, amidocarbonylations and other carbonylations. The first characteristic reactions are applied for the synthesis of fine chemicals such as pharmaceuticals and agrochemicals. However, the second characteristic reactions are utilized not only for fine chemicals but also for important bulk commodity chemicals such as aldehydes, carboxylic acids and alcohols. Copyright © 2009 John Wiley & Sons, Ltd.     

On-Surface Synthesis of Porous Carbon Nanoribbons on Silver: Reaction Kinetics and the Influence of the Surface Structure

We report on the influence of the surface structure and the reaction kinetics in the bottom-up fabrication of porous nanoribbons on silver surfaces using low-temperature scanning tunneling microscopy. The porous carbon nanoribbons are fabricated by the polymerization of 1,3,5-tris(3-bromophenyl)benzene directly on the Ag surface using an Ullmann-type reaction in combination with dehydrogenative coupling reactions. We demonstrate the successful on-surface synthesis of porous nanoribbons on Ag(111) and Ag(100) even though the self-assemblies of the intermediate organometallic structures and covalently-linked polymer chains are different on both surfaces. Furthermore, we present the formation of isolated porous nanoribbons by kinetic control. Our results give valuable insights into the role of substrate-induced templating effects and the reaction kinetics in the on-surface synthesis of conformationally flexible molecules.     

Can azulene-like molecules function as substitution-free molecular rectifiers?

The feasibility of employing azulene-like molecules as a new type of high performance substitution-free molecular rectifier has been explored using NEGF-DFT calculation. The electronic transport behaviors of metal-molecule-metal junctions consisting of various azulene-like dithiol molecules are investigated, which reveals that the azulene-like molecules exhibit high conductance and bias-dependent rectification effects. Among all the substitution-free azulene-like structures, cyclohepta[b]cyclopenta[g]naphthalene exhibits the highest rectification ratio, revealing that the all fused aromatic ring structure and an appropriate separation between the pentagon and heptagon rings are essential for achieving both high conductance and high rectification ratio. The rectification ratio can be increased by substituting the pentagon ring with electron-withdrawing group and/or the heptagon ring with electron donating groups. Further increase of the rectification ratio may also be obtained by lithium adsorption on the pentagon ring. This work reveals that azulene-like molecules may be used as a new class of highly conductive unimolecular rectifiers.     

Three characteristic reactions of organocobalt compounds in organic synthesis

Organocobalt compounds in organic synthesis have three characteristic reactions. The first occurs because cobalt has a high affinity to carbon–carbon π‐bonds or carbon–nitrogen π‐bonds. The second occurs because cobalt has a high affinity to carbonyl groups. The third is due to cobalt easily tending to form square‐planar bipyramidal six‐coordination structures with four nitrogen atoms or two nitrogen atoms and two oxygen atoms at the square‐planar position, and to bond with one or two carbon atoms at the axial position. The first characteristic reactions are the representative reactions of organocobalt compounds with a mutually bridged bond between the two π‐bonds of acetylene and the cobalt–cobalt bond of hexacarbonyldicobalt. These are reactions with a Co₂(CO)₆ protecting group to reactive acetylene bond, the Nicholas reactions, the Pauson–Khand reactions ([2 + 2 + 1] cyclizations), [2 + 2 + 2] cyclizations, etc. These reactions are applied for the syntheses of many kinds of pharmaceutically useful compounds. The second reactions are carbonylations that have been used or developed as industrial processes such as hydroformylation for the manufacture of isononylaldehyde, and carbonylation for the production of phenylacetic acid from benzyl chloride. The third reactions are those reactions with the B₁₂‐type catalysts, and they have recently been used in organic syntheses and are utilized as catalysts for stereoselective syntheses. These reactions have been used as new applications for organic syntheses. Copyright © 2007 John Wiley & Sons, Ltd.     

Iridium‐Catalyzed Synthesis of Diaryl Ethers by Means of Chemoselective CF Bond Activation and the Formation of BF Bonds

Transition‐metal‐catalyzed C? F activation, in comparison with C? H activation, is more difficult to achieve and therefore less fully understood, mainly because carbon–fluorine bonds are the strongest known single bonds to carbon and have been very difficult to cleave. Transition‐metal complexes are often more effective at cleaving stronger bonds, such as C(sp²)? X versus C(sp³)? X. Here, the iridium‐catalyzed C? F activation of fluorarenes was achieved through the use of bis(pinacolato)diboron with the formation of the B? F bond and self‐coupling. This strategy provides a convenient method with which to convert fluoride aromatic compounds into symmetrical diaryl ether compounds. Moreover, the chemoselective products of the C? F bond cleavage were obtained at high yields with the C? Br and C? Cl bonds remaining.     

Rhenium-Catalyzed Construction of Polycyclic Hydrocarbon Frameworks by a Unique Cyclization of 1,n-Diynes Initiated by 1,1-Difunctionalization with Carbon Nucleophiles

A regioselective cyclization of 1,n-diynes under rhenium catalysis was developed on the basis of a rare type of 1,1-difunctionalization of terminal alkynes with carbon nucleophiles, followed by sequential addition reactions of the resulting alkenylrhenium species. The reaction provides an efficient approach to the synthesis of complex cyclopentane-fused bi- and tricycles and spirocycles, which are useful building blocks for the construction of essential frameworks of biologically active compounds as well as functional materials, from simple starting materials by the formation of up to six new carbon–carbon bonds in a single step. The reaction proceeds under neutral conditions and does not require external ligands or additives. The key to this reactivity is the unique activation mode of the rhenium carbonyl complex, which prefers to interact with heteroatoms in polar carbon–heteroatom bonds as well as nonpolar carbon–carbon unsaturated π bonds.     

Stability of Fullerene Cages： C30 to C70 and Their Dependence on Shared Pentagon Bonds

1 INTRODUCTION Since the discovery of buckministerfullerene (C60)[1], experimental and theoretical studies of the structure and properties of fullerenes have spread worldwide. Many experimental studies appeared on their syntheses[2], isolation[3], and characterization[4]. Theoretically, except studies on their chemical pro- perties[5], most of the work was concentrated on their interesting geometric[6～8] and electronic structures[9, 10]. For example, the ground state of C48 has C2 symme…     

Cyclic Hypervalent Iodine Reagents for Atom‐Transfer Reactions: Beyond Trifluoromethylation

Hypervalent iodine compounds are privileged reagents in organic synthesis because of their exceptional reactivity. Among these compounds, cyclic derivatives stand apart because of their enhanced stability. They have been widely used as oxidants, but their potential for functional‐group transfer has only begun to be investigated recently. The use of benziodoxol(on)es for trifluoromethylation (Togni's reagents) is already widely recognized, but other transformations have also attracted strong interest recently. In this Review, the development in the area since 2011 will be presented. After a short summary of synthetic methods to prepare benziodoxol(on)e reagents, their use to construct carbon–heteroatom and carbon–carbon bonds will be presented. In particular, the introduction of alkynes by using ethynylbenziodoxol(on)e (EBX) reagents has been highly successful. Breakthroughs in the introduction of alkoxy, azido, difluoromethyl, and cyano groups will also be described.     

C100 is Converted into C94Cl22 by Three Chlorination‐Promoted C2 Losses under Formation and Elimination of Cage Heptagons

Chlorination of the C₁₀₀(18) fullerene with a mixture of VCl₄ and SbCl₅ gives rise to branched skeletal transformations affording non‐classical (NC) C₉₄(NC1)Cl₂₂ with one heptagon in the carbon cage together with the previously reported C₉₆(NC3)Cl₂₀ with three heptagons. The three‐step pathway to C₉₄(NC1)Cl₂₂ starts with two successive C₂ losses of 5:6 C?C bonds to give two cage heptagons, whereas the third C₂ loss of the 5:5 C?C bond from a pentalene fragment eliminates one of the heptagons. Quantum‐chemical calculations demonstrate that the two unusual skeletal transformations—creation of a heptagon in C₉₆(NC3)Cl₂₀ through a Stone–Wales rearrangement and the presently reported elimination of a heptagon through C₂ loss—are both characterized by relatively low activation energy.     

Cross‐Coupling of α‐Carbonyl Sulfoxonium Ylides with C−H Bonds

The functionalization of carbon–hydrogen bonds in non‐nucleophilic substrates using α‐carbonyl sulfoxonium ylides has not been so far investigated, despite the potential safety advantages that such reagents would provide over either diazo compounds or their in situ precursors. Described herein are the cross‐coupling reactions of sulfoxonium ylides with C(sp²)−H bonds of arenes and heteroarenes in the presence of a rhodium catalyst. The reaction proceeds by a succession of C−H activation, migratory insertion of the ylide into the carbon–metal bond, and protodemetalation, the last step being turnover‐limiting. The method is applied to the synthesis of benz[c]acridines when allied to an iridium‐catalyzed dehydrative cyclization.