Synthesis and Properties of Cycloparaphenylene-2,5-pyridylidene: A Nitrogen-Containing Carbon Nanoring

The first synthesis of a nitrogen-containing cycloparaphenylene, cyclo[14]paraphenylene[4]2,5-pyridylidene ([14,4]CPPy), has been achieved. A palladium-catalyzed stepwise assembly of 2,2′-bipyridine, benzene, and L-shaped cyclohexane units, followed by NaHSO₄/<i>o</i>-chloranil-mediated aromatization, successfully provided [14,4]CPPy. While the absorption and fluorescence properties of [14,4]CPPy were somewhat similar to those of cycloparaphenylenes (λ_abs = 344 nm, ε = 7.3 × 10⁴ cm^–1 M^–1, λ_em = 427 nm, Φ_F = 0.80), it was found that [14,4]CPPy possesses an interesting halochromic property

Nickel-Catalyzed C–H/C–O Coupling of Azoles with Phenol Derivatives

The first nickel-catalyzed C–H bond arylation of azoles with phenol derivatives is described. The new Ni­(cod)₂/dcype catalytic system is active for the coupling of various phenol derivatives such as esters, carbamates, carbonates, sulfamates, triflates, tosylates, and mesylates. With this C–H/C–O biaryl coupling, we synthesized a series of privileged 2-arylazoles, including biologically active alkaloids. Moreover, we demonstrated the utility of the present reaction for functionalizing estrone and quinine

C–H Arylation of Phenanthrene with Trimethylphenylsilane by Pd/<i>o</i>‑Chloranil Catalysis: Computational Studies on the Mechanism, Regioselectivity, and Role of <i>o</i>‑Chloranil

The transition-metal-catalyzed C–H arylation of aromatic hydrocarbons represents a useful and ideal method for the production of biaryls and multiarylated aromatic compounds. We have previously reported the palladium-catalyzed direct C–H arylation of polycyclic aromatic hydrocarbons, such as phenanthrene, pyrene, and corannulene with various organosilicon, -borane, and -germanium compounds. In these reactions, <i>o</i>-chloranil proved to be an essential and unique promoter (stoichiometrically as an oxidant) and arylation occurred exclusively at the <i>K</i>-region. Herein, we report our mechanistic investigation of Pd/<i>o</i>-chloranil catalysis in C–H arylation of phenanthrene with trimethylphenylsilane by computational calculations. The results revealed that C–H arylation occurs through a sequence of transmetalation, carbometalation, and <i>trans</i>-β-hydrogen elimination steps. In addition, the triple role of <i>o</i>-chloranil as a ligand, oxidant, and base is also elucidated

Synthesis and Structural Features of Quadruple Helicenes: Highly Distorted π Systems Enabled by Accumulation of Helical Repulsions

Quadruple helicenes, bearing dithia[6]­helicene and [5]­helicene substructures, were prepared by a well-controlled Scholl reaction. The 4-fold helicity provides 9 stereoisomers including 4 pairs of enantiomers and 1 meso isomer. Among them, differently distorted structures of a propeller-shaped isomer (<b>QH-A</b>) and a saddle-shaped isomer (<b>QH-B</b>) were unambiguously determined by X-ray crystallography. Especially in the latter isomer, a proper accumulation of repulsions on the helical substructures twisted the naphthalene core to the limit (69.5°), the highest degree of twisting deformation per benzene unit (35.3° at the most). Photophysical and electrochemical studies showed a broadened HOMO–LUMO gap and a HOMO of <b>QH-B</b> lying lower compared to those of <b>QH-A</b>. These results together with the density functional theory (DFT) calculations have clearly demonstrated the electronic state dependency on the molecular geometry. Additionally, kinetic studies of the isomerization between these isomers using ¹H NMR, circular dichroism, and DFT calculations shed light on the interconversion pathways among the stereoisomers. The height of barriers in the inversion of a certain helical substructure may be affected by the neighboring helical substructures

Cyanation of Phenol Derivatives with Aminoacetonitriles by Nickel Catalysis

Generation of useful arylnitrile structures from simple aromatic feedstock chemicals represents a fundamentally important reaction in chemical synthesis. The first nickel-catalyzed cyanation of phenol derivatives with metal-free cyanating agents, aminoacetonitriles, is described. A nickel-based catalytic system consisting of a unique diphosphine ligand such as dcype or dcypt enables the cyanation of versatile phenol derivatives such as aryl carbamates and aryl pivalates. The use of aminoacetonitriles as a cyanating agent leads to an environmentally and easy-to-use method for arylnitrile synthesis

η⁶‑Cycloparaphenylene Transition Metal Complexes: Synthesis, Structure, Photophysical Properties, and Application to the Selective Monofunctionalization of Cycloparaphenylenes

The synthesis, structure, photophysical properties, and reactivity of cycloparaphenylenes (CPPs) coordinated to group 6 transition metal fragments are described. The η⁶-coordination of [9]­CPP or [12]­CPP with M­(CO)₆ (M = Cr, Mo, W) afforded the corresponding [<i>n</i>]­CPP-M­(CO)₃ complexes (<i>n</i> = 9, 12; M = Cr, Mo, W). In the ¹H NMR spectra of these complexes, characteristic upfield-shifted singlet signals corresponding to the four hydrogen atoms attached to the coordinated C₆H₄ ring of the CPPs were observed at 5.4–5.9 ppm. The complex [9]­CPP-Cr­(CO)₃ could be successfully isolated in spite of its instability. X-ray crystallographic analysis and computational studies of [9]­CPP-Cr­(CO)₃ revealed that chromium-CPP coordination occurs at the convex surface of [9]­CPP both in the solid state and in solution. TD-DFT calculations suggested that the emerging high-wavenumber absorption peak upon coordination of [9]­CPP to Cr­(CO)₃ should be assigned to a weak HOMO–LUMO transition. Moreover, by using the complex [9]­CPP-Cr­(CO)₃, a rapid and highly monoselective CPP functionalization has been achieved. The established one-pot method, consisting of complexation, deprotonation, nucleophilic substitution, and decomplexation steps, yielded silyl-, boryl-, and methoxycarbonyl-substituted CPPs in up to 93% yield relative to reacted starting material

Catalytic C–H Imidation of Aromatic Cores of Functional Molecules: Ligand-Accelerated Cu Catalysis and Application to Materials- and Biology-Oriented Aromatics

Versatile imidation of aromatic C–H bonds was accomplished. In the presence of copper bromide and 6,6′-dimethyl-2,2′-bipyridyl, a range of aromatics, such as polycyclic aromatic hydrocarbons, aromatic bowls, porphyrins, heteroaromatics, and natural products, can be imidated by <i>N</i>-fluorobenzenesulfonimide. A dramatic ligand-accelerated copper catalysis and an interesting kinetic profile were uncovered

Laterally π‑Extended Dithia[6]helicenes with Heptagons: Saddle-Helix Hybrid Molecules

A laterally π-extended dithia[6]­helicene <b>1</b>, representing an interesting saddle-helix hybrid molecule containing an unusual heptagon, has been synthesized by MoCl₅-mediated oxidative stitching of tetrakis­(thienylphenyl)­naphthalene precursor <b>2</b> involving reactive-site capping by chlorination and subsequent Pd-mediated dechlorination of tetrachlorinated intermediate <b>1-Cl</b>_<b>4</b>. Highly distorted, wide helical structures of dithia[6]­helicenes (<b>1</b> and <b>1-Cl</b>_<b>4</b>) were clarified by single-crystal X-ray diffraction analyses where heterochiral slipped π–π stacking was displayed in a one-dimensional fashion. Notably, theoretical studies on the thermodynamic behavior of <b>1</b> predicted an extraordinarily high isomerization barrier of 49.7 kcal·mol^–1, which enabled optical resolution and chiroptical measurements. Electronic structures of these huge helicenes were also examined by photophysical and electrochemical measurements

Synthesis and Properties of [9]Cyclo-1,4-naphthylene: A π-Extended Carbon Nanoring

The first synthesis of a π-extended carbon nanoring, [9]­cyclo-1,4-naphthylene ([9]­CN), has been achieved. Careful structure–property analyses uncovered a number of unique features of [9]­CN that are quite different from those of [9]­CPP, a simple carbon nanoring

All-Benzene Carbon Nanocages: Size-Selective Synthesis, Photophysical Properties, and Crystal Structure

The design and synthesis of a series of carbon nanocages consisting solely of benzene rings are described. Carbon nanocages are appealing molecules not only because they represent junction unit structures of branched carbon nanotubes, but also because of their potential utilities as unique optoelectronic π-conjugated materials and guest-encapsulating hosts. Three sizes of strained, conjugated [<i>n</i>.<i>n</i>.<i>n</i>]carbon nanocages (<b>1</b>, <i>n</i> = 4; <b>2</b>, <i>n</i> = 5; <b>3</b>, <i>n</i> = 6) were synthesized with perfect size-selectivity. Cyclohexane-containing units and 1,3,5-trisubstituted benzene-containing units were assembled to yield the minimally strained bicyclic precursors, which were successfully converted into the corresponding carbon nanocages via acid-mediated aromatization. X-ray crystallography of <b>1</b> confirmed the cage-shaped structure with an approximately spherical void inside the cage molecule. The present studies revealed the unique properties of carbon nanocages, including strain energies, size-dependent absorption and fluorescence, as well as unique size-dependency for the electronic features of <b>1</b>–<b>3</b>