Kyoko Nozaki

Catalytic Hydrogenation of Carbon Dioxide Using Ir(III)-Pincer Complexes

Catalytic hydrogenation of carbon dioxide in aqueous potassium hydroxide was performed using a newly synthesized isopropyl-substituted PNP-pincer iridium trihydride complex as a catalyst. Potassium formate was obtained with turnover numbers up to 3,500,000 and a turnover frequency of 150,000 h(-1), both of which are the highest values reported to date.

Coordination-Insertion Copolymerization of Fundamental Polar Monomers

numerous polyethylene or polypropylene compounds have been synthesized by metal-catalyzed coordination-insertion polymerization. On the other hand, organometallic catalysts * To whom correspondence should be addressed. E-mail: nozaki@chembio.t.utokyo.ac.jp. Akifumi Nakamura (left) was born in 1984 in Kanagawa, Japan. He received his B.S. degree in 2007 and M.S. degree in 2009 from the University of Tokyo under the guidance of Professor Kyoko Nozaki. During that time he joined Professor Keiji Morokuma’s group at Kyoto University as a visiting student. In 2009, he started his Ph.D. study at the University of Tokyo under the guidance of Professor Kyoko Nozaki. He is also a research fellow of the Japan Society for the Promotion of Science. His research interests include synthetic organic chemistry, organometallic chemistry, computational chemistry, and polymer chemistry.

Ortho -Phosphinobenzenesulfonate: A Superb Ligand for Palladium-Catalyzed Coordination–Insertion Copolymerization of Polar Vinyl Monomers

Ligands, Lewis bases that coordinate to the metal center in a complex, can completely change the catalytic behavior of the metal center. In this Account, we summarize new reactions enabled by a single class of ligands, phosphine-sulfonates (ortho-phosphinobenzenesulfonates). Using their palladium complexes, we have developed four unusual reactions, and three of these have produced novel types of polymers. In one case, we have produced linear high-molecular weight polyethylene, a type of polymer that group 10 metal catalysts do not typically produce. Secondly, complexes using these ligands catalyzed the formation of linear poly(ethylene-co-polar vinyl monomers). Before the use of phosphine-sulfonate catalysts, researchers could only produce ethylene/polar monomer copolymers that have different branched structures rather than linear ones, depending on whether the polymers were produced by a radical polymerization or a group 10 metal catalyzed coordination polymerization. Thirdly, these phosphine-sulfonate catalysts produced nonalternating linear poly(ethylene-co-carbon monoxide). Radical polymerization gives ethylene-rich branched ethylene/CO copolymers copolymers. Prior to the use of phosphine-sulfonates, all of the metal catalyzed processes gave completely alternating ethylene/carbon monoxide copolymers. Finally, we produced poly(polar vinyl monomer-alt-carbon monoxide), a copolymerization of common polar monomers with carbon monoxide that had not been previously reported. Although researchers have often used symmetrical bidentate ligands such as diimines for the polymerization catalysis, phosphine-sulfonates are unsymmetrical, containing two nonequivalent donor units, a neutral phosphine, and an anionic sulfonate. We discuss the features that make this ligand unique. In order to understand all of the new reactions facilitated by this special ligand, we discuss both the steric effect of the bulky phosphines and electronic effects. We provide a unified interpretation of the unique reactivity by considering of the net charge and the enhanced back donation in the phosphine-sulfonate complexes.

Chemistry of Boryllithium: Synthesis, Structure, and Reactivity

A series of lithium salts of boryl anion, boryllithiums, were synthesized and characterized by NMR spectroscopy and crystallographic analysis. In addition to the parent boryllithium compound 35a, structural modification of boryllithium, using saturated C-C and benzannulated C=C backbones in the five-membered ring and mesityl groups on the nitrogen atoms, also allowed generation of the corresponding boryllithium. The solid state structures of boryllithium showed that the boron-lithium bond is polarized where the boron atom is anionic in all (35a x DME)(2), 35a x (THF)(2), 35b x (THF)(2), and 35c x (THF)(2) when compared to the structures of hydroborane 38a-c and optimized free boryl anion opt-46a-c. Dissolution of the isolated single crystals of (35a x DME)(2) and 35a x (THF)(2) in THF-d(8) showed that the boron-lithium bond remained in solution and free DME or THF molecules were observed. Temperature-dependent (11)B NMR chemical shift changes of 35a were observed in THF-d(8) or methylcyclohexane-d(14), suggesting a change of chemical shift anisotropy around the boron center. The HOMO of opt-35a x (THF)(2) had a lone pair character on the boron atom, as observed for phenyllithium, whereas the HOMO of hydroborane 38a corresponds to the pi-orbital of the boron-containing five-membered heterocycle. The polarity of the B-Li bond, estimated by AIM analysis, was similar to that of alkyllithium. Boryllithiums 35a and 35b behave as a base or a boron nucleophile in reaction with organic electrophiles via deprotonation, S(N)2-type substitution, halogen-metal exchange or electron-transfer, 1,2-addition to a carbonyl group, and S(N)Ar reaction. In the case of the reaction with CO(2), intramolecular cyclization followed by CO elimination from borylcarboxylate anion and subsequent protonation gave hydroxyboranes 64a and 64b. The characters of the carbonyl groups in the borylcarbonyl compounds 60a, 60b, 61, 62, and 63a, which were obtained from the reaction of boryllithiums 35a and 35b, were investigated by X-ray crystallography, IR, and (13)C NMR spectroscopy to show that the boryl substituent weakened the C=O bond when compared to carbon substituted analogues.

Copolymerization of vinyl acetate with ethylene by palladium/alkylphosphine-sulfonate catalysts.

Coordination copolymerization of vinyl acetate (VAc) with ethylene, leading to linear copolymers that possess in-chain -CH(2)CH(OAc)- units, has been accomplished using novel palladium complexes bearing alkylphosphine-sulfonate ligands.

Facile Synthetic Route to Highly Luminescent Sila[7]helicene

A facile synthetic route to dimethylsila[7]helicene by using a Lewis acid catalyzed double-cyclization reaction for construction of the twisted two phenanthrene moieties is described. Sila[7]helicene exhibited a high fluorescence quantum yield and a realatively large g value (dissymmetric factor) of circularly polarized luminencence (CPL) for small molecules.

Syntheses of PBP Pincer Iridium Complexes: A Supporting Boryl Ligand

We synthesized a PBP pincer ligand precursor 2 and demonstrated its complexation with iridium(I) to afford the coordinatively unsaturated [PBP](hydrido)chloroiridium complex 3 via B-H oxidative addition. The reaction of 3 with carbon monoxide produced [PBP](hydrido)chloro(carbonyl)iridium 7. The longer Ir-Cl bond length of 7 than that of 8 revealed a stronger sigma-donor ability of the PBP ligand than that of PCP. Complex 3 was also converted to [PBP](ethylene)iridium(I) complex 9 by the reaction with LiTMP under an ethylene atmosphere.

Syntheses, Structures, and Reactivities of Borylcopper and -zinc Compounds: 1,4-Silaboration of an α,β-Unsaturated Ketone to Form a γ-Siloxyallylborane†

Carbanions are one of the most important reagents in synthetic organic chemistry. They act as a carbon nucleophile to form a carbon–carbon bond as a consequence of the highly polarized carbon–metal bonds (C M). The properties of polar carbon–metal bonds in a carbanionic species depend on the metallic counterpart, such as lithium, magnesium, copper, and zinc. The first two organometallic compounds directly attack the carbonyl group of a,b-unsaturated ketones, whereas the latter two react by 1,4-addition (conjugate addition). In contrast to the rich chemistry of carbanions, there is a limited number of examples of boryl anions. We recently reported the first synthesis of boryllithium 2a as a boron

Tetravalent Metal Complexes as a New Family of Catalysts for Copolymerization of Epoxides with Carbon Dioxide

New tetravalent metal complexes with a trianionic [ONNO]-tetradentate ligand and an ancillary chloride ligand were synthesized as catalysts for the copolymerization of epoxides with carbon dioxide (CO(2)). All of the titanium, zirconium, germanium, and tin complexes were found to copolymerize epoxides with CO(2). In particular, the copolymerization of propylene oxide with CO(2) gave the almost-completely alternating copolymers by using titanium or germanium complexes. These results are the first example of the copolymerization using tetravalent metal complexes as a main component of catalysts.

λ5-Phospha[7]helicenes: Synthesis, Properties, and Columnar Aggregation with One-Way Chirality†

Helical extension of p-conjugated systems is of great interest for the production of novel molecules and materials with unusual properties and applications. A typical example is helical polyacetylenes, in which continuous p conjugation is achieved through covalent bonds. Another example is the assembly of short helical fragments, such as helicenes. 4] For instance, helicenebisquinone derivatives were reported to form one-dimensional columnar aggregates with the aid of long alkyl side chains. Unique optical properties, such as high nonlinear optical susceptibility and circularly polarized luminescence, were reported for these aggregates. Single crystals would also be suitable for creating such architectures if the p-conjugated helical molecules are appropriately stacked. Nevertheless, our analysis of the Cambridge Crystallographic Database revealed that structures with one-dimensional columnar helicene packing that is driven by p–p stacking interactions are rather uncommon, and in the majority of structures with helicene packing, the packing is noncolumnar and mostly driven by CH-p interactions. We hypothesized that the use of dipole–dipole interactions in addition to inherent p–p stacking interactions might produce a one-dimensional columnar arrangement of helicenes. Dipole–dipole interactions have been used as part of systems that contain cooperative interactions for the formation of one-dimensional molecular arrangements. Theoretical studies on the corannulene dimer have also demonstrated that dipole–dipole interactions should be a significant part of the binding energy of bowl-shaped molecules. Previously, we have reported the synthesis of oxaand aza[7]helicenes 3 and 4 by using palladium-catalyzed reactions. These helicenes do not possess dipole-moment vectors that are parallel to their helical axes, and no onedimentional columnar stack was formed. Herein, we report the preparation of l-phospha[7]helicenes 1 and 2 as a new family of helicenes. 12] The phosphole oxide and phosphole sulfide moieties give these helicenes a dipole-moment vector that is parallel to their helical axes, and thus the columnar stacking of 1 and 2 was achieved. More importantly, a racemate of phosphole sulfide 2 crystallized with a unique packing motif: the columns with one dipole direction consist of a single enantiomer and the columns with the opposite dipole direction consist of the other enantiomer (Figure 1). l-Phospha[7]helicenes 1 and 2 were synthesized as shown in Scheme 1. A racemic 4,4’-biphenanthryl-3,3’-diyl bis(trifluoromethanesulfonate, rac-6), from which we previously synthesized aza[7]helicene, was selected as the starting compound. The palladium-catalyzed cross-coupling of rac-6 with ethyl phenylphosphinate gave a 46% yield of the monophosphrous compound rac-7 as a mixture of diastereomers (axial chirality and P-centered chirality). l-Phospha[7]helicene rac-9, which has a phosphole moiety, was obtained by the reduction of rac-7 with LiAlH4 [13c,14] and a subsequent palladium-catalyzed intramolecular P-arylation.