(<i>Z</i>)‑Selective Hydroboration of Terminal Alkynes Catalyzed by a PSP–Pincer Rhodium Complex

A highly (Z)-selective hydroboration of terminal alkynes was achieved using a thioxanthene-based PSP–pincer rhodium catalyst. This hydroboration exhibited good chemoselectivity toward alkynes over carbonyl compounds such as ketones and aldehydes. The mechanistic studies indicated the involvement of rhodium–vinylidene intermediates, and the high (Z)-selectivity could be attributed to the rigid and electron-rich nature of the PSP–rhodium catalyst

Shining Visible Light on Reductive Elimination: Acridine–Pd-Catalyzed Cross-Coupling of Aryl Halides with Carboxylic Acids

Despite the recent tremendous progress on transition-metal/photoredox dual catalysis in organic synthesis, single transition-metal catalysis under visible-light irradiation, which can utilize light energy more efficiently, is still underdeveloped. Herein, we report the design of photosensitizing phosphinoacridine bidentate ligands for visible-light-induced transition-metal catalysis, expecting that the electron-accepting acridine moiety would create a highly reactive electron-deficient metal center toward reductive elimination via metal-to-ligand charge transfer (MLCT). Using these ligands, we have achieved a palladium-catalyzed cross-coupling reaction of aryl halides with carboxylic acids under visible-light irradiation. Electronic tuning of the phosphinoacridine ligands not only enabled the use of a variety of aryl halides as the coupling partner, including less reactive aryl chlorides, under blue light irradiation, but also realized the employment of lower-energy green and red light for the cross-coupling. Experimental mechanistic studies have proved that the reductive elimination of aryl esters is induced by photoirradiation of phosphinoacridine-ligated arylpalladium­(II) carboxylate complexes. The theoretical calculation suggests that the reductive elimination in the excited state is promoted by decreasing the electron density of the Pd center through photoinduced intramolecular electron transfer, i.e., MLCT, in the transition state owing to the electron-deficient acridine scaffold. This is a very rare example of photoinduced reductive elimination on palladium­(II) complexes

Palladium-Catalyzed Visible-Light-Driven Carboxylation of Aryl and Alkenyl Triflates by Using Photoredox Catalysts

A visible-light-driven carboxylation of aryl and alkenyl triflates with CO2 is developed by using a combination of Pd and photoredox catalysts. This reaction proceeds under mild conditions and can be applied to a wide range of substrates including acyclic alkenyl triflates

<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

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 N-alkynylpyridinium salts, by reacting pyridines with alkynyl-λ3-iodanes. The N-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 N-alkenylpyridiniums. Ring-fused pyridiniums were synthesized through intramolecular cyclization, demonstrating the utility of N-alkynylpyridiniums for the design of various electron-deficient cationic nitrogen-embedded polycyclic aromatic hydrocarbons with unique optical and electrochemical properties

<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 N-alkynylpyridinium salts, by reacting pyridines with alkynyl-λ3-iodanes. The N-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 N-alkenylpyridiniums. Ring-fused pyridiniums were synthesized through intramolecular cyclization, demonstrating the utility of N-alkynylpyridiniums for the design of various electron-deficient cationic nitrogen-embedded polycyclic aromatic hydrocarbons with unique optical and electrochemical properties

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

Unraveling the Electronic Structure of Azolehemiporphyrazines: Direct Spectroscopic Observation of Magnetic Dipole Allowed Nature of the Lowest π–π* Transition of 20π-Electron Porphyrinoids

Hemiporphyrazines are a large family of phthalocyanine analogues in which two isoindoline units are replaced by other rings. Here we report unambiguous identification of 20π-electron structure of triazolehemiporphyrazines (<b>1</b>, <b>2</b>) and thiazolehemiporphyrazine (<b>3</b>) by means of X-ray analysis, various spectroscopic methods, and density functional theory (DFT) calculations. The hemiporphyrazines were compared in detail with dibenzotetraazaporphyrin (<b>4</b>), a structurally related 18π-electron molecule. X-ray analysis revealed that tetrakis­(2,6-dimethylphenyloxy)­triazolehemiporphyrazine (<b>1b</b>) adopted planar geometry in the solid state. A weak absorption band with a pronounced vibronic progression, observed for all the hemiporphyrazines, was attributed to the lowest π–π* transition with the electric-dipole-forbidden nature. In the case of intrinsically chiral vanadyl triazolehemiporphyrazine (<b>2</b>), a large dissymmetry (<i>g</i>) factor was detected for the CD signal corresponding to the lowest π–π* transition with the magnetic-dipole-allowed nature. Molecular orbital analysis and NICS calculations showed that the azolehemiporphyrazines have a 20π-electron system with a weak paratropic ring current