Masanori Takimoto

Nickel-catalyzed regio- and stereoselective synthesis of homoallylic alcohol derivatives from dienes and aldehydes

Abstract The reaction of 1,3-Dienes and aldehydes in the presence of Et 3 SiH using a catalytic amount of Ni(cod) 2 and PPh 3 gave homoallylic alcohol derivatives in a regio- and stereoselective manner.

Cu‐Catalyzed Formal Methylative and Hydrogenative Carboxylation of Alkynes with Carbon Dioxide: Efficient Synthesis of α,β‐Unsaturated Carboxylic Acids

The sequential hydroalumination or methylalumination of various alkynes catalyzed by different catalyst systems, such those based on Sc, Zr, and Ni complexes, and the subsequent carboxylation of the resulting alkenylaluminum species with CO2 catalyzed by an N-heterocyclic carbene (NHC)-copper catalyst have been examined in detail. The regio- and stereoselectivity of the overall reaction relied largely on the hydroalumination or methylalumination reactions, which significantly depended on the catalyst and alkyne substrates. The subsequent Cu-catalyzed carboxylation proceeded with retention of the stereoconfiguration of the alkenylaluminum species. All the reactions could be carried out in one-pot to afford efficiently a variety of α,β-unsaturated carboxylic acids with well-controlled configurations, which are difficult to construct by previously reported methods. This protocol could be practically useful and attractive because of its high regio- and stereoselectivity, simple one-pot reaction operation, and the use of CO2 as a starting material.

Scandium-catalyzed regio- and stereospecific methylalumination of silyloxy/alkoxy-substituted alkynes and alkenes.

Various alkynes and alkenes having a tethered ether group undergo methylalumination reactions with unprecedented regio- and stereoselectivity in the presence of a cationic half-sandwich alkylscandium species as a catalyst. The oxygen atom of the ether group plays an important role in controlling the selectivity, possibly by coordinating to the metal center. Even when a bulky tert-butyl(diphenyl)silyloxy group is used as the tether group, there is no loss of selectivity.

Remarkable regio-controlled effect of 1,3-diene as a ligand on nickel-promoted cyclization

In a nickel-promoted cyclization, 1,3-diene can serve as a ligand and it affects the reaction course. A s-cis conformation of 1,3-diene is necessary for coordination to the metal, and the highest selectivity (5-T/5-I=982) was obtained in the reaction of 1b with hydride nickel complex 4 using cyclopentadiene 2 2 as a ligand.

Copper‐Catalyzed Formal CH Carboxylation of Aromatic Compounds with Carbon Dioxide through Arylaluminum Intermediates

The C - H bond carboxylation of various aromatic compounds with CO2 was achieved by the deprotonative alumination with a mixed alkyl amido lithium aluminate compound iBu3 Al(TMP)Li followed by the NHC-copper-catalyzed carboxylation of the resulting arylaluminum species, which afforded the corresponding carboxylation products in high yield and high selectivity. In addition to benzene derivatives, heteroarenes such as benzofuran, benzothiophene, and indole derivatives are also suitable substrates. Functional groups such as Cl, Br, I, vinyl, amide, and CN could survive the reaction conditions. Some key reaction intermediates such as the copper aryl and isobutyl complexes and their carboxylation products were isolated and structurally characterized by X-ray crystallographic analyses, thus offering important information on the reaction mechanism.

Cu‐Catalyzed Alkylative Carboxylation of Ynamides with Dialkylzinc Reagents and Carbon Dioxide

Alkylative carboxylation of ynamides with CO2 and dialkylzinc reagents using a N-heterocyclic carbene (NHC)-copper catalyst has been developed. A variety of ynamides, both cyclic and acyclic, undergo this transformation under mild conditions to afford the corresponding α,β-unsaturated carboxylic acids, which contain the α,β-dehydroamino acid skeleton. The present alkylative carboxylation formally consists of Cu-catalyzed carbozincation of ynamides with dialkylzinc reagents with the subsequent nucleophilic carboxylation of the resulting alkenylzinc species with CO2 . Dialkylzinc reagents bearing a β-hydrogen atom such as Et2 Zn and Bu2 Zn still afford the alkylated products despite the potential for β-hydride elimination. This protocol would be a desirable method for the synthesis of highly substituted α,β- dehydroamino acid derivatives due to its high regio- and stereoselectivity, simple one-pot procedure, and its use of CO2 as a starting material.

Guanidinate ligated iridium(III) complexes with various cyclometalated ligands: synthesis, structure, and highly efficient electrophosphorescent properties with a wide range of emission colours

We report the synthesis, structure, and electrophosphorescence properties of a series of heteroleptic iridium(III) complexes with various cyclometalated (C⁁N) ligands based on the sterically demanding guanidinate ancillary ligand. The iridium(III) complexes contain two cyclometalated (C⁁N) ligands and one monoanionic guanidinate ancillary ligand [(NiPr)2C(NPh2)]. The reaction of the bis(C⁁N) iridium(III) chloride [(C⁁N)2Ir(μ-Cl)]2 with the lithium salt of guanidine ligand [Li{(NiPr)2C(NPh2)}] at 80 °C gave a 65–85% yield of the corresponding heteroleptic [(C⁁N)2Ir{(NiPr)2C(NPh2)}] complexes with several different cyclometalated (C⁁N) ligands such as 2-phenylpyridine (ppy) (1), 2-(2,4-diflurophenyl)pyridine (dfppy) (2), 2-(p-tolyl) pyridine (tpy) (3), benzoquinoline (bzq) (4), 2-phenylbenzoxazole (box) (5), 2-phenylbenzothiazole (btz) (6), 2-(2′-benzothienyl)pyridine (btp) (7) and 1-phenylisoquinoline (piq) (8). These heteroleptic cyclometalated (C⁁N) iridium(III) complexes showed intense absorption bands in the UV region, assignable to ligand-centered (π–π*) transitions and lower energy absorption bands that extended to the visible region are mainly derived from spin-forbidden ligand-centered (π–π*) transitions, as well as metal-to-ligand charge transfer (MLCT) transitions. These complexes also showed intense emissions at room temperature, leading to λmax values from green (λ = 505 nm) to a perfect red colour (λ = 655 nm) with quantum yields (Φ) of 0.18 to 0.64 and phosphorescence lifetimes of 0.78 to 5.80 μs. Organic light-emitting diodes (OLEDs) were fabricated by the use of these complexes as phosphorescent dopants in various concentrations (5–100%) in a N,N′-dicarbazolylbiphenyl (CBP) host. High current efficiency (ηc; up to 125 cd A−1) and power efficiency (ηp; up to 43.6 lm W−1) were observed at appropriate conditions. Because of the steric hindrance of guanidinate ancillary ligands, no significant intermolecular interactions were observed in these complexes, thus leading to the reduction of self-quenching and triplet–triplet (T–T) annihilation at high luminance/currents in OLEDs.

Substituent effect on the electroluminescence efficiency of amidinate-ligated bis(pyridylphenyl) iridium(III) complexes

This paper reports the synthesis, structure, and photophysical and electrophosphorescence properties of heteroleptic amidinate/bis(pyridylphenyl) iridium(III) complexes having different substituents on the nitrogen atoms of the amidinate ancillary ligands. The reaction of bis(pyridylphenyl) iridium(III) chloride [(ppy)2Ir(μ-Cl)]2 with the lithium salt of various amidinate ligands Li{(NR)(NR′)CPh} at 80 °C gave in 60–80% yields the corresponding heteroleptic bis(pyridylphenyl)/amidinate iridium(III) complexes having a general formula [(ppy)2Ir{(NR)(NR′)CPh}], where R = R′ = iPr (1), R = R′ = t-Bu (2), R = Et, R′ = t-Bu (3), and R = Et, R′ = (CH2)3N(CH3)2 (4). These heteroleptic iridium(III) complexes exhibited bright yellowish-green phosphorescence emission with moderate photoluminescence (PL) quantum yields (ΦPL = 0.16–0.34) and short phosphorescence lifetimes of 0.98–1.18 μs in toluene solution at room temperature. Organic light-emitting diodes (OLEDs) were fabricated by the use of these complexes as phosphorescent dopants in various concentrations (x = 5–100 wt%) in the 4,4′-N,N′-dicarbazolylbiphenyl (CBP) host. Because of the steric hindrance of the amidinate ligands, no significant intermolecular interaction was observed in these complexes, thus leading to the reduction of self-quenching and triple–triplet annihilation at high currents/luminance. A significant influence of the substituents in the amidinate ligands on the electroluminescence efficiency was observed. Among these complexes, complex (2), which contains the bulky t-butyl group on the amidinate nitrogen atoms, showed the highest current efficiency (ηc: up to 116 cd A−1), power efficiency (ηp: up to 72.2 lm W−1) and external quantum efficiency (ηext; up to 16.3%).

Regioselective Alkylative Carboxylation of Allenamides with Carbon Dioxide and Dialkylzinc Reagents Catalyzed by an N-Heterocyclic Carbene-Copper Complex.

The alkylative carboxylation of allenamide catalyzed by an N-heterocyclic carbene (NHC)-copper(I) complex [(IPr)CuCl] with CO2 and dialkylzinc reagents was investigated. The reaction of allenamides with dialkylzinc reagents (1.5 equiv) and CO2 (1 atm.) proceeded smoothly in the presence of a catalytic quantity of [(IPr)CuCl] to afford (Z)-α,β-dehydro-β-amino acid esters in good yields. The reaction is regioselective, with the alkyl group introduced onto the less hindered γ-carbon, and the carboxyl group introduced onto the β-carbon atom of the allenamides. The first step of the reaction was alkylative zincation of the allenamides to give an alkenylzinc intermediate followed by nucleophilic addition to CO2 . A variety of cyclic and acyclic allenamides were found to be applicable to this transformation. Dialkylzinc reagents bearing β-hydrogen atoms, such as Et2 Zn or Bu2 Zn, also gave the corresponding alkylative carboxylation products without β-hydride elimination. The present methodology provides an easy route to alkyl-substituted α,β-dehydro-β-amino acid ester derivatives under mild reaction conditions with high regio- and stereoselectivtiy.