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We prepared bifunctional Mg(II) porphyrin catalysts 1 for the solvent-free synthesis of cyclic carbonates from epoxides and CO2. The activities of 1d, 1h, and 1i, which have Br(-), Cl(-), and I(-) counteranions, respectively, increased in the order 1i < 1h < 1d. Catalysts 1d and 1j-m, which bear four tetraalkylammonium bromide groups with different alkyl chain lengths, showed comparable but slightly different activities. Based on the excellent catalyst 1d, we synthesized Mg(II) porphyrin 1o with eight tetraalkylammonium bromide groups, which showed even higher catalytic activity (turnover number, 138,000; turnover frequency, 19,000 h(-1)). The catalytic mechanism was studied by using 1d. The yields were nearly constant at initial CO2 pressures in the 1-6 MPa range, suggesting that CO2 was not involved in the rate-determining step in this pressure range. No reaction proceeded in supercritical CO2, probably because the epoxide (into which the catalyst dissolved) dissolved in and was diluted by the supercritical CO2. Experiments with (18)O-labeled CO2 and D-labeled epoxide suggested that the catalytic cycle involved initial nucleophilic attack of Br(-) on the less hindered side of the epoxide to generate an oxyanion, which underwent CO2 insertion to afford a CO2 adduct; subsequent intramolecular ring closure formed the cyclic carbonate and regenerated the catalyst. Density functional theory calculations gave results consistent with the experimental results, revealing that the quaternary ammonium cation underwent conformational changes that stabilized various anionic species generated during the catalytic cycle. The high activity of 1d and 1o was due to the cooperative action of the Mg(II) and Br(-) and a conformational change (induced-fit) of the quaternary ammonium cation.

Title: Recent progress in catalytic conversions of carbon dioxide Development of new reactions and catalysts for the chemical fi xation of carbon dioxide (CO2) is needed to overcome its kinetic and thermodynamic stability. An overview is presented of organic and metal catalysts with excellent activity and selectivity developed for various chemical conversions of CO2.

We have developed a very active bifunctional porphyrin catalyst showing a high turnover number (TON = 103,000) for the synthesis of cyclic carbonates from CO(2) and epoxides under solvent-free conditions.

Highly active bifunctional diporphyrin and triporphyrin catalysts were synthesized through Stille coupling reactions. As compared with a porphyrin monomer, both exhibited improved catalytic activities for the reaction of CO2 with epoxides to form cyclic carbonates, because of the multiple catalytic sites which cooperatively activate the epoxide. Catalytic activities were carefully investigated by controlling temperature, reaction time, and catalyst loading, and very high turnover number and turnover frequency were obtained: 220 000 and 46 000 h(-1) , respectively, for the magnesium catalyst, and 310 000 and 40 000 h(-1) , respectively, for the zinc catalyst. Results obtained with a zinc/free-base hybrid diporphyrin catalyst demonstrated that the Br(-) ions on the adjacent porphyrin moiety also function as nucleophiles.

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A series of pyrenes sandwiched by axially chiral 1,1′-binaphthyls were synthesized. Among them, (R,R)-3 possessing 2-hydroxy-3,3′-dimethylbinaphthyls exhibited solvent-dependent inversion of the sign of circularly polarized luminescence (CPL) without a change in emission wavelength. The (−)-and (+)-CPL were detected in nonpolar and polar solvents, respectively, with g lum values of −0.012 and +0.012. This switching property originates from the inversion of excimer chirality caused by the presence or absence of intermolecular hydrogen bonds in the excited state. The CPL intensity was also changed by variation of the temperature and concentration.C ircularly polarized luminescence (CPL) organic dyes have attracted much attention because of their fascinating properties and behaviors. 1,2 Recently, for example, dipyrenyl-appended crown ethers and binaphthyls displaying intense CPL have been reported. 3 CPL dyes whose intensities are switchable by external stimuli, such as solvent, 4 temperature, 5 concentration, 6 UV irradiation, 7 photosensitizers, 8 metal ions, 9 and halogen ions, 10 have been recent topics of focus. These molecular switches widen the application range of CPL dyes. Although CPL switches with sign inversion, (+)/(−)-CPL, are still rare, 11 polymers showing CPL sign inversion are known. For example, Swager's group developed poly(p-phenylenevinylene) films whose CPL signs were dependent on the solvents used for spin coating, 11a while Suginome's group reported polyquinoxalines whose CPL signs were inverted by helical inversion induced by solvents. 11c These |g lum | values were less than 5 × 10 −3 , where the dissymmetry factor g lum is defined as 2(I L − I R )/(I L + I R ), in which I L and I R represent the intensities of the left and right CPL, respectively. Because of the limited number of examples, the development of small-sized CPL switches with sign inversion and high intensity is challenging.We have recently developed several axially chiral CPL dyes. 12 Among them, hexapyrenyl-appended quaternaphthyls 1 displayed a high |g lum | value of 0.037 (Figure 1). 12b The CPL was based on conformationally restricted intramolecular pyrene excimers, which allowed us to find the excimer chirality rule: right-and left-handed twist excimers exhibit (+)-and (−)-CPL, respectively. However, 1 is a large molecule synthesized by a laborious multistep synthesis. In addition, 1 and its analogues are not suitable for CPL switches because of the formation of rigid intramolecular excimers. Indeed, the CPL properties of 1 did not depend on the solvent, temperature, or concentration. We postulated that one pyrene moiety with chiral scaffolds might form an intermolecular excimer capable of responding to external stimuli. In this study, we designed and synthesized pyrenes (R,R)-2−8 sandwiched by two binaphthyls via ester linkers, an ether analogue (R,R)-9,

Helical carbazole-based BODIPY analogues were readily synthesized via aza [7]helicenes. The structures of azahelicene-incorporated BF 2 dyes were elucidated by x-ray diffraction analysis. DFT calculations revealed that the pconjugated system expanded from the helicene moiety to the BODIPY framework. The azahelicene-fused boron complexes showed the Cotton effects and the circularly polarized luminescence (CPL) in the visible region. Furthermore, an axially chiral binaphthyl group was attached to the helically chiral dyes, which enhanced the chiroptical properties.Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.