Chihiro Maeda

Recent progress in catalytic conversions of carbon dioxide

Chemical fixation of carbon dioxide (CO2), which is an inexpensive and renewable carbon source, is becoming more and more important. The development of both new reactions and new catalysts is needed to overcome the kinetic and thermodynamic stability of CO2. Organic and metal catalysts with unique and excellent activity and selectivity have been developed for various chemical conversions of CO2. In this perspective, we provide an overview of the recent progress in this field, classifying it into several categories, where each research is concisely summarized one by one using a single reaction scheme, a representative catalyst structure, and/or a catalytic cycle.

Bifunctional Porphyrin Catalysts for the Synthesis of Cyclic Carbonates from Epoxides and CO2: Structural Optimization and Mechanistic Study

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.

Bifunctional Catalysts Based on m‐Phenylene‐Bridged Porphyrin Dimer and Trimer Platforms: Synthesis of Cyclic Carbonates from Carbon Dioxide and Epoxides

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.

Highly Active and Robust Metalloporphyrin Catalysts for the Synthesis of Cyclic Carbonates from a Broad Range of Epoxides and Carbon Dioxide.

Bifunctional metalloporphyrins with quaternary ammonium bromides (nucleophiles) at the meta, para, or ortho positions of meso-phenyl groups were synthesized as catalysts for the formation of cyclic carbonates from epoxides and carbon dioxide under solvent-free conditions. The meta-substituted catalysts exhibited high catalytic performance, whereas the para- and ortho-substituted catalysts showed moderate and low activity, respectively. DFT calculations revealed the origin of the advantage of the meta-substituted catalyst, which could use the flexible quaternary ammonium cation at the meta position to stabilize various anionic species generated during catalysis. A zinc(II) porphyrin with eight nucleophiles at the meta positions showed very high catalytic activity (turnover number (TON)=240 000 at 120 °C, turnover frequency (TOF)=31 500 h(-1) at 170 °C) at an initial CO2 pressure of 1.7 MPa; catalyzed the reaction even at atmospheric CO2 pressure (balloon) at ambient temperature (20 °C); and was applicable to a broad range of substrates, including terminal and internal epoxides.

Synthesis of Carbazole-Based Selenaporphyrin via Annulation

Cu(I)-mediated alkoxylation of doubly 1,3-butadiyne-bridged carbazole dimer 1, followed by acid-catalyzed cyclization, provided furan-bridged carbazole dimer 3, while annulation reaction of 1 with selenium in the presence of hydrazine monohydrate provided selenophene-bridged carbazole dimer 5a. Oxidation of isophlorin 5a afforded carbazole-based selenaporphyrin 5b, which possessed distinct aromaticity and produced intensified and red-shifted absorption bands in the near-IR region.

Carbazole-Based Boron Dipyrromethenes (BODIPYs): Facile Synthesis, Structures, and Fine-Tunable Optical Properties

Carbazole-based BODIPYs were synthesized in three steps using an organometallic approach consisting of sequential Ir-catalyzed borylation, Suzuki-Miyaura coupling, and boron complexation. Various substituents were introduced into the carbazole moiety, and large substituent effects were confirmed by means of absorption spectroscopy, cyclic voltammetry, and DFT calculations. Dibenzocarbazoles were also converted into the corresponding BODIPYs.

Synthesis and characterization of novel fused porphyrinoids based on cyclic carbazole[2]indolones

The carbazole- and indolone-based porphyrinoids 3 and 4 were synthesized by stepwise transition-metal-catalyzed coupling reactions. Palladium metalation of 4 produced 4Pd, which exhibits near-infrared absorption.

Synthesis and Characterization of Carbazole-Based Expanded Thiaporphyrins

Expanded porphyrins, composed of more than four pyrroles or other equivalent subunits, have received considerable attention as a result of their unique properties. Of particular interest is the extended p conjugation of these macrocycles, as compared to regular porphyrins, which may lead to the application of such compounds as photosensitizers and near-infrared (NIR) dyes as well as in photodynamic therapy and nonlinear optical devices. The conjugation system within a porphyrin may be extended by the introduction of ethynyl groups or by the fusion of aromatic rings to the porphyrin core. However, such peripheral modifications have rarely been applied to expanded porphyrins. Carbazole-based materials have also been extensively studied due to their unique properties. They can be highly emissive and electron conducting and are also both chemically stable and capable of polymerization and metal-catalyzed cross-coupling. Because a carbazole is simply a benzene-fused pyrrole, the incorporation of carbazole units into fused porphyrins presents interesting possibilities. Despite the potential usefulness of such carbazole-based porphyrinoids, only a few examples have been reported to date. Recently, we reported a multiple annulation strategy, which allows the synthesis of novel porphyrinoids from 1,3-butadiyne-bridged cyclic carbazole oligomers (Scheme 1). Among these, the tetrabenzo-fused core-modified porphyrin 1 b exhibits distinct aromaticity and NIR absorption due to extended p conjugation over the entire macrocycle. Following this work, we investigated the ring expansion of such core-modified porphyrins in order to obtain expanded porphyrins with extended conjugation systems. Herein, we report the synthesis and characterization of oligothiophenebridged carbazoles as novel carbazole-based expanded thiaporphyrins. The synthetic procedures used in this work are shown in Scheme 2. A combination of both coupling and annulation reactions played a key role in the synthesis of these oligothiophene-bridged species. Bisthiophene-bridged carbazoles were synthesized through Stille coupling. A solution of 3,6di-tert-butyl-1,8-dibromocarbazole (9) and bis(tributylstannyl)bisthiophene in toluene in the presence of a catalytic amount of [Pd ACHTUNGTRENNUNG(PPh3)4] was heated to reflux for 12 h. After separation by gel permeation chromatography, the bisthiophene-bridged carbazole dimer 3 and the trimer 4 were obtained in 12 and 7.6 % yields, respectively. Trithiophenebridged carbazoles, however, were prepared in a manner similar to the synthesis of compound 1 a. The Glaser coupling reaction of 3,6-di-tert-butyl-1,8-bis(5-ethynylthien-2yl)carbazole (10) and a subsequent annulation reaction with Na2S [14] provided the trithiophene-bridged carbazole dimer 5 and the trimer 6. In addition, the carbazole dimers 7 and 8, incorporating both thiophene and bisthiophene or trithiophene linkages, were also prepared. The intramolecular cyclization of compounds 11 and 12 were achieved by Glaser coupling under dilute conditions, resulting in either compound 13 or compound 14, respectively. Finally, the annulation reactions of 13 and 14 with Na2S provided compounds 7 and 8. The structures of these compounds were characterized by both spectroscopic and X-ray diffraction analyses. The H NMR spectra of these macrocycles were fully consistent with their proposed structures, with the exception of compound 5. At room temperature, the NMR spectrum of this compound consists of rather broad, unresolved signals, whereas at 140 8C in [D2]tetrachloroethane, a single set of peaks is evident (see the Supporting Information). These re[a] M. Masuda, Dr. C. Maeda Department of Applied Chemistry Faculty of Science and Technology Keio University Kohoku-ku, Yokohama 223-8522 (Japan) Fax: (+81)45-566-1551 E-mail : cmaeda@applc.keio.ac.jp Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201202573. Scheme 1. Synthesis of the carbazole-based porphyrins 1a, 1b, and 2.

Effective π-Extension of Carbazole-Based Thiaporphyrins by Peripheral Phenylethynyl Substituents

Several tetrakis(phenylethynyl)- and (phenylethynylphenylethynyl)-substituted carbazole-based thiaporphyrins were synthesized. These π-extended porphyrins display remarkably intensified and red-shifted absorption bands in the NIR region up to 1126 nm due to perturbation by the phenylethynyl substituents.

Peripherally ethynylated carbazole-based core-modified porphyrins

Peripherally ethynylated carbazole-based core-modified porphyrins were synthesized by sequential metal-catalyzed coupling and annulation reactions. Experimental results and DFT calculations both confirm that the π-conjugated networks of the resulting porphyrins effectively delocalize over the entire macrocycle, including the ethynyl substituent groups.