Hiroki Iida

Helical Polymers: Synthesis, Structures, and Functions

2.2.1. Polyisocyanates 6117 2.2.2. Polysilanes 6118 2.2.3. Polyacetylenes 6120 2.3. Foldamer-Based Helical Polymers 6124 2.3.1. Click Polymerization 6126 2.3.2. Ring-Closing Reaction 6126 2.4. Other Types of Helical Polymers 6127 2.4.1. π-Conjugated Helical Polymers 6127 2.4.2. Metallosupramolecular Helical Polymers 6128 2.5. Induced Helical Polymers 6130 2.5.1. Induced Helical Polyacetylenes 6130 2.5.2. Other Induced Helical Polymers and Oligomers 6132

Diastereo- and enantioselective hydrogenative aldol coupling of vinyl ketones: design of effective monodentate TADDOL-like phosphonite ligands.

We report the first enantioselective reductive aldol couplings of vinyl ketones, which were achieved through the design of a novel monodentate TADDOL-like phosphonite ligand. Specifically, hydrogenation of commercially available methyl vinyl ketone (MVK) or ethyl vinyl ketone (EVK) in the presence of aldehydes 1a-7a using cationic rhodium catalysts modified by chiral TADDOL-like phosphonite ligands AP-I and AP-IV produces aldol adducts 1b-7b and 1c-7c with excellent control of relative and absolute stereochemistry. The absolute stereochemical assignments of the aldol adducts are made in analogy to that determined for the 5-bromophthalimido derivative of aldol adduct 1b and the 2-bromo-5-nitrobenzoate of 3b, which were established by single-crystal X-ray diffraction analysis using the anomalous dispersion method.

Main-Chain Optically Active Riboflavin Polymer for Asymmetric Catalysis and Its Vapochromic Behavior

A novel optically active polymer consisting of riboflavin units as the main chain (poly-1) was prepared from naturally occurring riboflavin (vitamin B(2)) in three steps. The riboflavin residues of poly-1 were converted to 5-ethylriboflavinium cations (giving poly-2), which could be reversibly transformed into the corresponding 4a-hydroxyriboflavins (giving poly-2OH) through hydroxylation/dehydroxylation reactions. This reversible structural change was accompanied by a visible color change along with significant changes in the absorption and circular dichroism (CD) spectra. The nuclear Overhauser effect spectroscopy (NOESY) and CD spectra of poly-2 revealed a supramolecularly twisted helical structure with excess one-handedness through face-to-face stacking of the intermolecular riboflavinium units, as evidenced by the apparent NOE correlations between the interstrand riboflavin units and intense Cotton effects induced in the flavinium chromophore regions. The hydroxylation of poly-2 at the 4a-position proceeded in a diastereoselective fashion via chirality transfer from the induced supramolecular helical chirality assisted by the ribityl pendants, resulting in a 83:17 diastereomeric mixture of poly-2OH. The diastereoselectivity of poly-2 was remarkably higher than that of the corresponding monomeric model (64.5:35.5), indicating amplification of the chirality resulting from the supramolecular chirality induced in the stacked poly-2 backbones. The optically active poly-2 efficiently catalyzed the asymmetric organocatalytic oxidation of sulfides with hydrogen peroxide, yielding optically active sulfoxides with up to 60% enantiomeric excess (ee), whose enantioselectivity was higher than that catalyzed by the monomeric counterpart (30% ee). In addition, upon exposure to primary and secondary amines, poly-2 exhibited unique high-speed vapochromic behavior arising from the formation of 4a-amine adducts in the film.

Aerobic Reduction of Olefins by In Situ Generation of Diimide with Synthetic Flavin Catalysts

A versatile reducing agent, diimide, can be generated efficiently by the aerobic oxidation of hydrazine with neutral and cationic synthetic flavin catalysts 1 and 2. This technique provides a convenient and safe method for the aerobic reduction of olefins, which proceeds with 1 equiv of hydrazine under an atmosphere of O(2) or air. The synthetic advantage over the conventional gas-based method has been illustrated through high hydrazine efficiency, easy and safe handling, and characteristic chemoselectivity. Vitamin B(2) derivative 6 acts as a highly practical, robust catalyst for this purpose because of its high availability and recyclability. Association complexes of 1b with dendritic 2,5-bis(acylamino)pyridine 15 exhibit unprecedented catalytic activities, with the reduction of aromatic and hydroxy olefins proceeding significantly faster when a higher-generation dendrimer is used as a host pair for the association catalysts. Contrasting retardation is observed upon similar treatment of non-aromatic or non-hydroxy olefins with the dendrimer catalysts. Control experiments and kinetic studies revealed that these catalytic reactions include two independent, anaerobic and aerobic, processes for the generation of diimide from hydrazine. Positive and negative dendrimer effects on the catalytic reactions have been ascribed to the specific inclusion of hydrazine and olefinic substrates into the enzyme-like reaction cavities of the association complex catalysts.

Separation of enantiomers on diastereomeric right- and left-handed helical poly(phenyl isocyanide)s bearing L-alanine pendants immobilized on silica gel by HPLC

Diastereomeric left- and right-handed helical polyisocyanides composed of L-alanine-bound phenyl isocyanides were prepared. The helical polymers with a narrow molecular weight distribution were immobilized on a silica gel support via chemical bonding and their optical resolution abilities were evaluated as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC). The CSP prepared from the left-handed helical polyisocyanide resolved racemic cyclic ether and carbonyl compounds and cyclic dianilides and dibenzamides, whereas the right-handed helical polyisocyanide-based CSP showed a rather complementary chiral recognition ability and specifically resolved racemic metal acetylacetonate complexes, which were not separated on the former CSP at all. Additionally, the elution order of some enantiomers was reversed on the CSPs, thus indicating that the macromolecular helicity of the L-alanine-bound-polyisocyanides played a critical role in the enantioselectivity and elution order of the enantiomers.

Synthesis of functional poly(phenyl isocyanide)s with macromolecular helicity memory and their use as asymmetric organocatalysts

To develop a novel polymer-based asymmetric organocatalyst, a series of helical poly(phenyl isocyanide)s with functional pendant groups were prepared by modifying the side groups of the optically active helical poly(4-carboxyphenyl isocyanide) with a macromolecular helicity memory. Helical polyisocyanides partially modified with achiral amines, such as piperazine, maintained their chiral memory and enantioselectively catalyzed a direct aldol reaction. Although the enantioselectivity was low, the original helical poly(4-carboxyphenyl isocyanide) showed no catalytic activity. These results indicated that the macromolecular helicity of the modified polyisocyanides together with bifunctional amino and carboxy acid pendant residues arranged in a helical array along the polymer backbones plays an important role in the enantioselectivity.

Neutral Flavins: Green and Robust Organocatalysts for Aerobic Hydrogenation of Olefins

Various olefins can be hydrogenated quantitatively with neutral flavin 2 catalysts in the presence of 1-2 equiv of hydrazine under 1 atm of O(2). Vitamin B(2) derivative 2g acts as a highly efficient and robust catalyst for the present environmentally benign process producing water and nitrogen gas as the only waste products.

Catalytic Reductive Coupling of Alkenes and Alkynes to Carbonyl Compounds and Imines Mediated by Hydrogen

Catalytic transformations mediated by hydrogen or “hydrogenations” encompass a diverse range of environmentally benign processes, including large volume transformations of enormous socioeconomic impact, such as the Haber–Bosch process and the reduction of olefinic feedstocks. Despite considerable progress across diverse areas of catalytic hydrogenation, reductive C–C bond formations mediated by hydrogen have, until recently, been restricted to the incorporation of carbon monoxide, as illustrated by the Fischer–Tropsch reaction and alkene hydroformylation. In this account, the emerging family of hydrogen-mediated C–C bond formations beyond carbon monoxide coupling is reviewed. This new type of hydrogenation enables direct coupling of diverse π-unsaturated reactants to carbonyl compounds and imines under neutral condition with complete atom economy.

Synthesis and chiral recognition ability of helical polyacetylenes bearing helicene pendants

Novel polyacetylenes bearing an optically active or racemic [6]helicene unit as the pendant groups directly bonded to the main-chain (poly-1s) were prepared by the polymerisation of the corresponding acetylenes (1-rac, 1-P and 1-M) using a rhodium catalyst. The optically active polyacetylenes (poly-1-P and poly-1-M) formed a preferred-handed helical conformation biased by the optically active helicene pendants, resulting in the induced circular dichroism (ICD) in their π-conjugated polymer backbone regions. The optically active helical polymers, when employed as an enantioselective adsorbent, showed a high chiral recognition ability towards racemates, such as the monomeric [6]helicene and 1,1′-binaphthyl analogues, and enantioselectively adsorbed one of the enantiomers.

Guest‐Induced Unidirectional Dual Rotary and Twisting Motions of a Spiroborate‐Based Double‐Stranded Helicate Containing a Bisporphyrin Unit

Guest-Induced Unidirectional Dual Rotary and Twisting Motions of a Spiroborate-Based Double-Stranded Helicate Containing a Bisporphyrin Unit Influential guests : The intercalation of an electron-deficient aromatic guest (shown in red) between the two porphyrin rings of an optically active, porphyrin-linked double-stranded spiroborate helicate triggered rotary motion of the porphyrin rings in one direction in conjunction with a unidirectional twisting motion of the spiroborate helix. This system has potential for the development of chiralityresponsive molecular machines. Angewandte Chemie