Chiral self-dimerization of vanadium complexes on a SiO2 surface for asymmetric catalytic coupling of 2-naphthol: structure, performance, and mechanism

Vanadium monomers with chiral tridentate Schiff-base ligands were supported on SiO(2) through a chemical reaction with surface silanols, where we found a new chirality creation by the self-dimerization of the vanadyl complexes on the surface. The chiral self-dimerization and the role of surface silanols in the self-assembly were investigated by means of X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), diffuse-reflectance ultraviolet/visible (DR-UV/VIS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR), electron spin resonance (ESR), and density functional theory (DFT) calculations. The surface vanadyl complexes had a distorted square-pyramidal conformation with a V=O bond. FT-IR spectra revealed that the Ph-O moiety of Schiff-base ligands was converted to Ph-OH by a surface-concerted reaction between the vanadium precursors and surface SiOH groups. The Ph-OH in an attached vanadyl complex interacted with a COO moiety of another vanadyl complex by hydrogen bonding to form a self-dimerized structure at the surface. The interatomic distance of V-V in the surface self-assembly was evaluated to be 0.40 +/- 0.05 nm by ESR after O(2) adsorption. The self-dimerized V structure on SiO(2) was modeled by DFT calculations, which demonstrated that two vanadium monomers with Ph-OH linked together by two hydrogen bonds and their V=O groups were directed opposite to each other. The surface self-dimerization of the vanadium precursors fixes the direction of the V=O bond and the plane of the Schiff-base ligand. Thus, a new chiral reaction field was created by two types of chirality: the chiral Schiff-base ligand and the chiral V center. We have also found that the chiral self-dimerized vanadyl complexes exhibit remarkable catalytic performance for the asymmetric oxidative coupling of 2-naphthol: 96% conversion, 100% selectivity to 1,1'-binaphthol (BINOL), and 90% enantiomeric excess (ee). Increasing the vanadium loading on SiO(2) caused a dramatic swell of enantioselectivity, and the maximum 90% ee was observed on the supported catalyst with the full coverage of the vanadyl complex (3.4 wt % vanadium). This value is equivalent to the maximum ee reported in homogeneous catalysis for the coupling reaction. Furthermore, the supported vanadium dimers were reusable without loss of the catalytic performance. To our knowledge, this is the first heterogeneous catalyst for the asymmetric oxidative coupling of 2-naphthol.     

NMR studies on the reaction between methacrylonitrile and trimethylaluminum

The reaction of MAN with AlMe₃ was studied by the ¹³C- and ¹H-NMR spectral measurement. In the presence of an excess of AlMe₃, it was found that a 1:1 complex of MAN and AlMe₃ is initially formed and subsequently dimethyl(α-methyl isopropylidene amino)aluminum is produced by the reaction of the complex with AlMe₃. The isolated product was pale yellow crystal (mp 39–40°C). The kinetics of the reaction were also examined.     

Some reactions of π-cyclopentadienylbis(triphenylphosphine) rhodium(I)

Reactions of π-cyclopentadienylbis(triphenylphosphine)rhodium(I) (I) with alkyl halides, olefins, acetylenes, carbon disulfide and elementary sulfur have been investigated. Methyl iodide gives the oxidative-addition product [πC₅H₅ Rh(PPh₃)₂CH₃]I but isopropyl iodide produces the alkyl substituted-cyclopentadienyl complex (π-i-C₃H₇C₅H₄)Rh(PPh₃)I₂. Under a nitrogen atmosphere, olefins and acetylenes give compounds of the composition π-C₅H₅ Rh(PPh₃)(L) (L = CH₂—CHCN, CH₂—CHCO₂CH₃, CH₃O₂—CCOO₂CH₃).In the presence of air, however, complexes of the composition π-C₅H₅Rh(L)₂ (L = CH₂—CHCN, CH₂—CHCO₂CH₃, CH₂—C(CH₃)CN) and π-C₅H₅Rh(L)₃ (L = CH₃O₂ CC—CCO₂ CH₃, PhC—CCO₂ CH₃) are formed. The reaction of carbon disulfide or sulfur with (I) also gives the compounds π-C₅H₅Rh(PPh₃)(L) (L = CS₂, CS₃, S₅).     

Effect of formaldehyde on the cationic polymerization of styrene

Polymerization of styrene has been carried out in the presence of formaldehyde at 30°C in benzene solution by using boron trifluoride etherate as a catalyst. The rate of polymerization in the initial stage was accelerated with addition of formaldehyde, while the steady-state rate of polymerization was retarded in the presence of formaldehyde. The acceleration for the rate of polymerization was found only in a short time from the beginning. The steady-state rate of polymerization followed the equation: where [C]₀ and [F]₀ are initial concentrations of catalyst and formaldehyde, [M] is the monomer concentration, and k₁, k₂, and k₃ are constants. It has been assumed that the chain-transfer reaction does not involve formaldehyde itself but rather the reaction products of formaldehyde, such as polystyrene having ethoxy or hydroxymethyl ends. The apparent chain-transfer constant for the added formaldehyde has been determined to be 1.63.     

NMR conformational analysis of biosynthetic precursor-type lipid A: monomolecular state and supramolecular assembly

The detailed conformational analysis of a single molecule of the tetraacyl biosynthetic precursor-type lipid A and its characteristic supramolecular assembly in aqueous SDS-micelles are described. Regular molecular arrangements were observed by detailed analysis of the NMR spectra of synthetically pure specimens, including regiospecifically 13C-labeled ones. NMR analysis of a biologically inactive precursor-type analogue with four shorter acyl chains demonstrated its conformational flexibility, indicating the importance of hydrophobic interactions for maintaining the conformation of such molecules.     

Four new 2-(2-phenylethyl)chromone derivatives from withered wood of Aquilaria sinensis

Four new chromone derivatives, 5-hydroxy-6-methoxy-2-(2-phenylethyl)chromone (1), 6-hydroxy-2-(2-hydroxy-2-phenylethyl)chromone (2), 8-chloro-2-(2-phenylethyl)-5,6,7-trihydroxy-5,6,7,8-tetrahydrochromone (3), 6,7-dihydroxy-2-(2-phenylethyl)-5,6,7,8-tetrahydrochromone (4) were isolated from the MeOH extract of withered wood of Aquilaria sinensis, together with seven known constituents of agarwood.     

Half-way coordination state of a butadienyl group on ruthenium. η-Allylic bonding with η-character or vice versa

The butadienyl complexes formed by the reaction of trans-(R¹)CH=CHCCR² (R¹, R² = SiMe₃, ^tBu, Me, Et) with RuCl(CO)H(PPh₃)₃ exhibit unique structures: instead of taking the 18-electron configuration of the metal by conventional η³-coordination of the butadienyl ligand, they shift significantly to the 16-electron η¹-coordination state.     

Highly versatile organostibine mediators for living radical polymerization

A new versatile method for conducting living radical polymerization has been developed in which organostibines induce consecutive group-transfer radical reactions with alkenes. The method has been successfully applied, for the first time, to the controlled polymerization of both conjugated and unconjugated vinyl monomers, and the desired polymers with predetermined molecular weight and low polydispersity index were obtained in excellent yields. This characteristic feature of this method is exemplified in the first synthesis of block copolymers composed of conjugated and unconjugated monomers, which would be of great importance as functional smart organic nanomaterials.