Fusion of Photochromic Reaction and Synthetic Reaction: Photoassisted Cyclization to Highly Strained Chiral Azobenzenophanes

A method for synthesizing highly strained cyclic structures by combining photochromic and synthetic reactions is described. Tightly linked azobenzene–binaphthyl dyads (<i>R</i>)-<b>4</b> and (<i>R</i>)-<b>6</b> could not be obtained by conventional cyclization, but continuous application of photoirradiation, which induced (<i>E</i>)→(<i>Z</i>) isomerization of the azobenzene moiety, allowed the cyclization reaction to proceed, affording the desired chiral azobenzenophanes

Photoinversion of <i>Cisoid</i>/<i>Transoid</i> Binaphthyls

Axially chiral binaphthyl-azobenzene cyclic dyads in which the two moieties are connected by two linkers of different lengths were synthesized. In the case of benzylated-binaphthyl analogue <b>2b</b>, photoirradiation resulted in a dramatic change of the CD spectrum and optical rotation. Experimental and theoretical analyses indicated that the dihedral angle of the two naphthalene rings is strongly coupled to the azobenzene photoisomerization; <i>cis</i>-azobenzene induces a <i>transoid</i>-binaphthyl structure, while <i>trans</i>-azobenzene induces a <i>cisoid</i>-binaphthyl structure

Fusion of Photochromic Reaction and Synthetic Reaction: Photoassisted Cyclization to Highly Strained Chiral Azobenzenophanes

A method for synthesizing highly strained cyclic structures by combining photochromic and synthetic reactions is described. Tightly linked azobenzene–binaphthyl dyads (<i>R</i>)-<b>4</b> and (<i>R</i>)-<b>6</b> could not be obtained by conventional cyclization, but continuous application of photoirradiation, which induced (<i>E</i>)→(<i>Z</i>) isomerization of the azobenzene moiety, allowed the cyclization reaction to proceed, affording the desired chiral azobenzenophanes

In-Plane Aromaticity in Cycloparaphenylene Dications: A Magnetic Circular Dichroism and Theoretical Study

The electronic structures of [8]­cyclo­para­phenylene dication ([8]­CPP²⁺) and radical cation ([8]­CPP^•+) have been investigated by magnetic circular dichroism (MCD) spectroscopy, which enabled unambiguous discrimination between previously conflicting assignments of the UV–vis–NIR absorption spectral bands. Molecular orbital and nucleus-independent chemical shift (NICS) analysis revealed that [8]­CPP²⁺ shows in-plane aromaticity with a (4<i>n</i> + 2) π-electron system (<i>n</i> = 7). This aromaticity appears to be the origin of the unusual stability of the dication. Theoretical calculations further suggested that not only [8]­CPP²⁺ but also all [<i>n</i>]­CPP (<i>n</i> = 5–10) dications and dianions exhibit in-plane aromaticity

<i>N</i>‑Alkynylpyridinium Salts: Highly Electrophilic Alkyne–Pyridine Conjugates as Precursors of Cationic Nitrogen-Embedded Polycyclic Aromatic Hydrocarbons

We achieved the first synthesis of <i>N</i>-alkynylpyridinium salts, by reacting pyridines with alkynyl-λ³-iodanes. The <i>N</i>-alkynylpyridiniums exhibit highly electron-accepting character with extended π-conjugation. The electrophilic alkynyl groups were readily susceptible to Michael addition and 1,3-dipolar cycloaddition to afford various <i>N</i>-alkenylpyridiniums. Ring-fused pyridiniums were synthesized through intramolecular cyclization, demonstrating the utility of <i>N</i>-alkynylpyridiniums for the design of various electron-deficient cationic nitrogen-embedded polycyclic aromatic hydrocarbons with unique optical and electrochemical properties

“Naked” Lithium Cation: Strongly Activated Metal Cations Facilitated by Carborane Anions

Experimental and spectroscopic studies revealed unprecedented reactivity of a “naked” lithium cation with very weakly coordinating anions, including carborane anions. The superactivated lithium cation has greatly enhanced Lewis acidic character and mediates various organic reactions such as carbonyl-ene reaction, NBS-bromination of unactivated aromatics, and Friedel–Crafts alkylation, which are not promoted by conventional lithium salts. Chemical robustness of the counteranion also plays an important role in the chemistry of the strongly activated lithium cation

Near-Infrared Fluorescence from In-Plane-Aromatic Cycloparaphenylene Dications

Cycloparaphenylenes (CPPs) are hoop-shaped conjugated hydrocarbons corresponding to partial structures of fullerenes or armchair carbon nanotubes. Here, we examined the fluorescence properties of a series of [<i>n</i>]­cycloparaphenylene dications ([<i>n</i>]­CPP²⁺, <i>n</i> = 5–9), which have unique in-plane aromaticity. The fluorescence peak positions of the [<i>n</i>]­CPP²⁺s shifted to the longer-wavelength region with increasing ring size, reaching the near-infrared region for those with <i>n</i> > 5. The fluorescence quantum yield of [6]­CPP²⁺ was the highest among the [<i>n</i>]­CPP²⁺s examined in this study, and the value was on the same order as that of carbon nanotubes. The Stokes shifts of [<i>n</i>]­CPP²⁺s were smaller than those of neutral [<i>n</i>]­CPPs, which do not have in-plane aromaticity. Theoretical calculations indicate that [<i>n</i>]­CPP²⁺s undergo smaller structural changes upon S₀–S₁ transition than [<i>n</i>]­CPPs do, and this is responsible for the difference of the Stokes shift. Furthermore, molecular orbital analysis reveals that the S₀–S₁ transition of smaller [<i>n</i>]­CPP²⁺s has an electric-dipole-forbidden character due to HOMO → LUMO/HOMO → LUMO+1 mixing. The relatively high fluorescence quantum yield of [6]­CPP²⁺ is considered to arise from the balance between relatively allowed character and the dominant effect of energy gap

Conjugation between σ- and π‑Aromaticity in 1‑<i>C</i>‑Arylated Monocarba-<i>closo</i>-dodecaborate Anions

Conjugation between σ- and π-aromatic moieties in 1-<i>C</i>-arylated monocarba-<i>closo</i>-dodecaborate anion derivatives <b>2</b> has been identified by means of kinetic experimental studies combined with theoretical calculations. We found that the reaction rate of iodination at the 12-B vertex of the carborane anion cage was affected by distal substituents on the benzene ring connected at the antipodal carbon vertex. Hammett and Yukawa–Tsuno plots indicated that substantial resonance effects are involved. Density functional theory calculations enabled detailed interpretation of the electronic interaction

Overcoming the Low Reactivity of Aryl Chlorides: Amination via Reusable Polymeric Nickel–Iridium Dual Catalysis under Microwave and Visible Light

The amination of aryl chlorides was facilitated via dual activation using polymeric nickel and iridium catalysts under microwave and visible-light irradiation. For this, a polymeric iridium complex, poly-[Ir(ppy)2(dabpy)], was prepared as a stable and reusable photocatalyst, which was characterized by using nuclear magnetic resonance spectroscopy, elemental analysis, cyclic voltammetry, and UV–vis absorption–emission spectroscopy. The carbon–nitrogen bond-forming reaction of aryl chlorides and amines afforded up to a quantitative yield using a combination of poly-[Ir(ppy)2(dabpy)] and a polymeric nickel catalyst (P4VP-NiCl2), with only 0.03 mol % of iridium and 0.2 mol % of nickel sufficing to catalyze the reaction. Notably, the reaction displayed a broad substrate scope, accommodating primary, secondary, and aromatic amines, as well as both electron-rich and electron-deficient aryl chlorides. Furthermore, both the nickel and iridium catalysts could be recovered and reused multiple times without significant losses of activity. This approach can be used for the synthesis of several biologically active molecules

Aluminepin: Aluminum Analogues of Borepin and Gallepin

We report synthesis of dibenzoaluminepin as the first aluminepin, an aluminum analogue of borepin and gallepin. This compound contains one molecule of ethereal solvent on the Al atom, which adopts a tetrahedral geometry. The central 7-membered aluminepin ring has a boatlike conformation and was characterized by single-crystal X-ray diffraction, ¹H/¹³C NMR, and DFT studies. In addition, NICS, NBO, and theoretical calculations provide insight into the nature of the bonding and aromaticity of aluminepins